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Optical Materials

Research Groups:

  • Colloidal Materials (web)
  • Multifunctional Optical Materials (web)

Profesores de Investigación

Míguez García, Hernán Ruy

954 48 95 81

Grupo de Investigación:
Materiales Ópticos Multifuncionales

Ocaña Jurado, Manuel

954 48 95 33

Grupo de Investigación:
Materiales Coloidales

Científicos Titulares

Becerro Nieto, Ana Isabel

954 48 95 45

Grupo de Investigación:
Materiales Coloidales

Calvo Roggiani, Mauricio

954 48 96 51

Grupo de Investigación:
Materiales Ópticos Multifuncionales

Núñez Alvarez, Nuria

954 48 96 21

Grupo de Investigación:
Materiales Coloidales

Investigadores/Doctores Contratados

Galisteo López, Juan Francisco

954 48 95 00 + ext. 90 92 48

Grupo de Investigación:
Materiales Ópticos Multifuncionales

Lozano Barbero, Gabriel

954 48 96 52

Grupo de Investigación:
Materiales Ópticos Multifuncionales

Becarios Predoctorales

Anaya Martín, Miguel

954 48 95 00 + ext. 90 92 35

Grupo de Investigación:
Materiales Ópticos Multifuncionales

Esteso Carrizo, Victoria

954 48 95 00 + ext. 90 92 35

Grupo de Investigación:
Materiales Ópticos Multifuncionales

González Mancebo, Daniel

954 48 95 00 + ext. 90 92 39

Grupo de Investigación:
Materiales Coloidales

Miranda Muñoz, José María

954 48 95 00 + ext. 90 92 35

Grupo de Investigación:

Rubino, Andrea

954 48 95 93

Grupo de Investigación:
Materiales Ópticos Multifuncionales

Personal Contratado

Castillo Flores, Lucía T.

954 48 95 93

Grupo de Investigación:
Materiales Ópticos Multifuncionales

Gutiérrez Lázaro, M. Carmen

954 48 95 00 + ext. 90 92 35

Grupo de Investigación:
Materiales Ópticos Multifuncionales

Nanophosphor-based photonic materials for next generation light-emitting devices NANOPHOM

Research head: Gabriel S. Lozano Barbero
Period: 12-12-2016 / 31-03-2022
Financial source: European Commission STARTING GRANT
Code: H2020-ERC-STG/0259
Research group:

Abstract [+]

Energy-efficient and environmentally friendly light sources are an essential part of the global strategy to reduce the worldwide electricity consumption. Light-emitting diodes (LEDs) emerge as a key alternative to conventional lighting, due to their high power-conversion efficiency, long lifetime, fast switching, robustness, and compact size. Nonetheless, their implementation in the consumer electronic industry is hampered by the limited control over brightness, colour quality and directionality of LED emission that conventional optical elements relying on geometrical optics provide.

This project exploits new ways of controlling the emission characteristics of nanophosphors, surpassing the limits imposed by conventional optics, through the use of nanophotonic concepts. The development of reliable and scalable nanophosphor-based photonic materials will allow ultimate spectral and angular control over the light emission properties, addressing the critical shortcomings of current LEDs. The new optical design of these devices will be based on multilayers, surface textures and nano-scatterers of controlled composition, size and shape, to attain large-area materials possessing photonic properties that will enable a precise management of the visible radiation.

Nanophom will significantly advance our comprehension of fundamental phenomena like the formation of photonic modes in complex optical media to which light can couple, as well as advancing the state of the art of high-efficiency solid-state lighting devices.

PhoLED – Photonic Nanostructures for Light-Emitting Devices

Research head: Hernán Míguez García
Period: 1-09-2015 / 31-08-2017
Financial source: Unión Europea
Code: EU144490_01 Marie Curie Actions
Research group: Dongling Geng

Abstract [+]

This project has received funding from the European Union’s H2020 Programme for research, technological development and demonstration under grant agreement no 657434.
The PhoLED project seeks to largely surpass the optical performance of state-of-the-art light emitters devised for illumination applications and contribute to solve some of the main technical limitations that the current technology presents. This project aims at integrating novel optical nanostructures and emitters, such as colloidal quantum dots or nanophosphors, to yield the next generation of light-emitting devices in which full spectral and angular control over the emission properties will be possible. The approach focuses on the development of: i) new synthetic routes to achieve efficient nanophosphors, and ii) preparation and processing strategies, based on surface textures and colloidal scatterers, to attain large area optical nanostructures possessing photonic properties that will allow a precise control on the intensity, angular distribution and color quality of light emission. Results achieved within this project will provide significant advance both in the comprehension of fundamental phenomena as well as in the development of versatile solid-state lighting devices of optimized efficiency, aiming to overcome technical barriers and maximize performance. The project’s outcome is twofold: a substantial expansion of the preparation of optical nanostructures to control light-mater interaction, and the practical realization of nanostructured lightemitting devices with unprecedented properties.

Advanced optical materials for efficient optoelectronic devices

Research head: Hernán Míguez García / Manuel Ocaña Jurado
Period: 1-01-2015 / 31-12-2017
Financial source: Ministerio de Economía y Competitividad
Code: MAT2014-54852-R
Research group: Ana Isabel Becerro Nieto, Nuria Núñez Alvarez, Mauricio E. Calvo Roggiani, Gabriel Lozano Barbero, Juan Francisco Galisteo López, Miguel Anaya Martin

Abstract [+]

The MODO project will focus on the development of optical materials to optimize the performance of optoelectronic devices such as solar cells or light emitting devices, thereby improving their energy conversion efficiency. The main objective of this proposal is to increase their performance by controlling light absorption and emission processes occurring in the materials composing these devices. This will be achieved through the design and integration of photonic nanostructures whose properties are also compatible with the manufacture and operation requirements of these systems, such as thermal, chemical and mechanical stability, durability, ease of processing and scale-up.

Integration of Photonic Nanostructures in Flexible Dye Solar Cells

Research head: Hernán Míguez García
Period: 1-07-2014 / 30-06-2016
Financial source: Unión Europea
Code: FP7-PEOPLE-2013-IIF Marie Curie Actions
Research group: Yuelong Li

Abstract [+]

It is the main goal of this project to bring to the host institution and the European Research Area the knowledge and technology to prepare current record flexible dye sensitized photovoltaic devices, previously developed by the candidate in South Korea and then the USA, in order to be able to further improve them, while endowing them with semi-transparency, using stretchable and bendable optical materials. The candidate has demonstrated that several key materials and processes provide better
performance of bendable dye solar cells, i.e., enhanced efficiency and flexibility, by allowing the preparation of electrodes in which the electron diffusion length is longer and charge collection efficiency is consequently enhanced. However, highly efficient dye solar cells are opaque as a consequence of the particular diffuse scattering design employed to improve light absorption, which limits their application in building or automotive integrated photovoltaics. This proposal seeks to solve such drawback by
introducing photonic nanostructures in different configurations, yielding both light harvesting enhancement and preserving transparency, hence placing Europe at the forefront of research in this specific area within the field of renewable energy. This final goal will be attempted through different approaches, each one challenging from the materials science perspective. Preparation of such highly efficient and transparent devices will combine the flexible solar cell processing tools previously developed by the candidate with the versatile optical material preparation techniques pioneered by the host institution. More specifically, integration of novel porous flexible photonic structures into the light harvesting layer, use of flexible mirrors attached to the back of the counter-electrode, and designed distribution of scatterers will be employed to
reach the target.

Highly optimized unit for a sustainable enhanced solar system HOUSESS

Research head: Hernán Míguez García
Period: 03-02-2014 / 31-12-2017
Financial source: Ministerio de Economía y Competitividad
Code: RTC-2014-2333-3 (Programa Retos)
Research group: Juan Francisco Galisteo López, José María Miranda Muñoz

Abstract [+]

The aim of the project is the design, development, prototyping and validation of a hybrid photovoltaic-thermosolar system that allows the storage and manageability of the generated solar energy. This integrated system will generate electricity at lower costs than standard thermosolar technology.

The hybrid system consists of a parabolic cylinder system and a low concentration photovoltaic solar receiver. Between these two components a dichroic filter is placed, which receives the reflected light from the parabolic cylinder primary mirror and allows the selective separation of the solar spectrum, letting pass a portion of the light to the photovoltaic receiver and reflecting the rest to the thermal tube receiver. Said dichroic filter sends to the photovoltaic receiver photons with wavelengths which are more efficiently absorbed by the solar cell. The thermal part of the system also shows the ability to controllably deliver power, allowing energy storage for its use in the most suitable moment of the day.

Control of the Optical Emission and Absorption properties of Nanomaterials Integrated in Multifunctional Porous Photonic Structures

Research head: Hernán R. Míguez García
Period: 01-01-2012 / 31-12-2014
Financial source: Ministerio de Ciencia e Innovación
Code: MAT2011-23593
Research group: Nuria Nuñez Alvarez, Mauricio Calvo Roggiani, Carmen López López, Sonia Rodríguez Liviano, Manuel Ocaña Jurado, Silvia Colodrero Pérez, José Raúl Castro Smirnov

Abstract [+]

In this project the modifications of both optical emission and absorption of nano-materials of different sort (rare earth doped nanoparticles, semiconductor quantum dots, metallic nanoparticles, and films of organic dyes of nanometer dimensions) that occur when they are embedded in different types of photonic structures will be investigated. Both fundamental and applied aspects of the subject will be analysed. Efforts will be mainly focused on materials of current technological interest for solar cells, sensors and light emitting devices. From the applied point of view, this project finds its motivation in the possibility that photonic structures offer of modifying absorption and emission processes in a controlled manner so that they can be inhibited or amplified depending on the specific goal pursued. Particularly, we seek to put into practice these concepts to generate new designs of more efficient solar cells, capable of harvesting a larger amount of the incident radiation, and in the development of films for sensing devices responsive to modifications of different kind, such as presence of targeted molecules, variations of ambient gas pressure, etc... Also, more efficient or controlled light extraction from light emitting devices is sought after. The development of small prototype devices to prove the novel concepts under research is also an objective of this grant proposal.
In its more fundamental aspect, our project aims at deepening our knowledge of the interaction between light and matter in systems in which there exists a strong dispersion and anisotropy of the dielectric constant, and in which it is possible to attain very low photon propagation speeds. For this analysis, we will employ different types of porous photonic structures, such as one-dimensional and three-dimensional photonic crystals, as well as disordered assemblies of particles, as hosts in which a wide range of organic and inorganic nanomaterials will be integrated in different configurations and whose absorption and emission will be experimentally and theoretically studied.
Although this project has a fundamental character due to the nature of the prepara-tion techniques and complex optical properties we seek to analyze, it is our aim to continue generating and transferring intellectual property based on the novel concepts, properties and designs which are the subject of our research.


Polymer-Inorganic Flexible Nanostructured Films for the Control of Light (POLIGHT)

Research head: Hernán R. Míguez García
Period: 01-01-2012 / 30-11-2017
Financial source: Unión Europea
Code: 307081
Research group:

Abstract [+]

The POLIGHT project will focus on the integration of a series of inorganic nanostruc-tured materials possessing photonic or combined photonic and plasmonic properties into polymeric films, providing a significant advance with respect to current state of the art in flexible photonics. These highly adaptable films could act either as passive UV-Vis-NIR selective frequency mirrors or filters, or as matrices for light absorbing or optically active species capable of tailoring their optical response. The goal of this project is two-fold. In one aspect, the aim is to fill a currently existing hole in the field of materials for radiation protection, which is the absence of highly flexible and adaptable films in which selected ranges of the electromagnetic spectrum wavelengths can be sharply blocked or allowed to pass depending on the different foreseen applications. In another, the POLIGHT project seeks to go one step beyond in the integration of absorbing and emitting nanomaterials into simple flexible polymeric matrices by including hierarchically structured photonic lattices that provide fine tuning of the optical properties of these hybrid ensembles. This will be achieved by means of enhanced matter-radiation interactions that result from field localization effects at specific resonant modes. The opportunity arises as a result of the recent development of a series of robust inorganic photonic structures that present interconnected porous networks susceptible of hosting polymers and thus inheriting their mechanical properties.

Flexible hybrid nanostructures for applications as ultraviolet, visible and near infrared filters

Research head: Hernán Míguez García
Period: 03-02-2010 / 03-02-2013
Financial source: Junta de Andalucía
Code: FQM6090
Research group: Mauricio Calvo Roggiani, Agustín Mihi Cervelló, Silvia Colodrero Pérez, Nuria Hidalgo Serrano, Gabriel Lozano Barbero, Olalla Sánchez Sobrado

Abstract [+]

This project aims at developing radiation filters and screens in the shape of films and capable of blocking or selecting ultraviolet (UV), visible (Vis) or near infrared (NIR) radiation within well-defined spectral ranges. Biocompatibility, flexibility and specific adhesive proper-ties will be sought after in order to make these films usable to protect all types of ill, wounded or burnt skin. The aim is to fill a currently existing hole in the field of skin phototherapy based on the healing properties of UV-Vis-NIR light, which is the absence of biocompatible patches in which selected ranges of the electromagnetic spectrum wavelengths can be sharply blocked or allowed to pass depending on the needs of the patient. For clinical cases that so required, an integral approach to skin photo-healing will be taken, devising materials that allow therapeutic wavelengths to reach the skin while blocking harmful ones and providing the controlled topical release of substances that have a beneficial effect on the skin. This project is based on a new series of novel prototype materials that have recently been developed in the group headed by the applicant in the Institute of Materials Science of Seville.

Applications of photonic crystals in solar cells: power conversion efficiency enhancement though optical absorption amplification

Research head: Hernán R. Míguez García
Period: 14-01-2009 / 13-01- 2012
Financial source: Junta de Andalucía
Code: P08-FQM-03579 (Proyecto de Excelencia)
Research group: Manuel Ocaña Jurado, Mauricio Calvo Roggiani, Nuria Nuñez, Agustín Mihi, Gabriel Lozano, Silvia Colodrero, Nuria Hidalgo, Olalla Sánchez Sobrado

Abstract [+]

Porous photonic crystals introduced in heterojunction solar cells allow to enhance sig-nificantly their photovoltaic performance by increasing the light harvested by the device. This concept, pioneered by the multifunctional optical materials group, have lead to highly efficient and transparent dye solar cells that preserve their potential application as window modules, one of their main added values. The concepts proposed in this project are not only interesting from a fundamental point of view in photonics and energy conversion, but also of clear relevance for building integrated photovoltaics.

Control of Optical Emission and Absorption Properties of Nanomaterials in Photonic Crystals

Research head: Hernán R. Míguez García
Period: 01-01-2009 / 31-12-2011
Financial source: Ministerio de Ciencia e Innovación
Code: MAT2008-02166
Research group: Manuel Ocaña Jurado, Mauricio Calvo Roggiani, Nuria Nuñez, Agustín Mihi, Gabriel Lozano, Silvia Colodrero, Nuria Hidalgo, Olalla Sánchez

Abstract [+]

In this project the modifications of both optical emission and absorption of nano-materiales of different sort (rare earth doped nanoparticles, semiconductor quantum dots, and films of organic dyes of nanometer dimensions) that occur when they are embedded in a photonic crystal structure. Both fundamental and applied aspects of the subject will be ana-lysed, efforts being focused on materials of current technological interest. From the applied point of view, this project finds its motivation in the possibility that photonic crystal offer of modifying those absorption and emission processes in a controlled manner so that they can be inhibited or amplified depending on the specific goal pursued. Particularly, we seek to put into practice these concepts to generate new designs of more efficient solar cells, capable of harvesting a larger amount of the incident radiation, and in the development of films for sensing devices sensitive to modifcations of different kind, such as presence of targeted molecules, variations of ambient gas pressure, etc... In its more fundamental aspect, our project aims at deepening our knowledge of the interaction between light and matter in systems in which there exists a strong dispersion and anisotropy of the dielectric constant, and in which it is possible to attain very low photon propagation speeds. For this analysis, we will employ photonic crystals with three dimensional order as hosts in which a wide range of organic and inorganic nanomaterials will be integrated in different configurations and whose absorption and emission will be experimentally and theoretically studied.


Flexible and Adaptable Light-Emitting Coatings for Arbitrary Metal Surfaces based on Optical Tamm Mode Coupling

Jiménez-Solano, A.; Galisteo-López, J.; Míguez, H.
Advanced Optical Materials, 6 (2018) 1700560


This study demonstrates a design that maximizes the power radiated into free space from a monolayer of nanoemitters embedded in a flexible distributed Bragg reflector conformably attached to a metal surface. This is achieved by positioning the light source at the precise depth within the multilayer for which optical Tamm states provide enhanced quantum yield and outcoupling efficiency, which are combined to optimize the luminous power radiated by the surface of the ensemble. This approach, based on the adhesion of flexible multilayer stacks onto metal surfaces with an arbitrary curvature, is versatile and permits the realization of spectrally narrow monodirectional or self-focusing light-emitting surfaces.

January, 2018 | DOI: 10.1002/adom.201700560


Fluorescent Humidity Sensors Based on Photonic Resonators

Szendrei, K; Jimenez-Solano, A; Lozano, G; Lotsch, BV; Miguez, H
Advanced Optical Materials, 5 (2017) 1700663


Among the different approaches to humidity sensing available, those based on fluorescent signals are gathering a great deal of attention due to their fast response and versatility of detection and design. So far, all proposals have focused on the use of luminescent probes whose emission is either triggered or inhibited by the presence of water that reacts or alters their chemical environment, hence inducing the signal change. Here, a novel concept in fluorescent humidity sensing based on combining stimuli-responsive photonic resonators with molecular fluorescent probes is introduced. The resonator is assembled from humidity-swellable antimony phosphate nanosheets embedding a planar light-emitting probe, whose emission is dramatically modified by the changes that ambient humidity causes in its photonic environment. Guided by "in silico" optical design of the resonator architecture and subsequent experimental realization, two embodiments of fluorescent photonic humidity sensors featuring turn-on and turn-off detection schemes are presented. The interplay between the luminescent properties of an emitter and its photonic environment implies a fundamental advantage as the emitters are not chemically altered during the detection process. At the same time, it paves the way toward a new generation of photonic humidity sensors which can conveniently be interfaced with common fluorescence detection schemes.

December, 2017 | DOI: 10.1002/adom.201700663

Local Disorder and Tunable Luminescence in Sr1–x/2Al2–xSixO4 (0.2 ≤ x ≤ 0.5) Transparent Ceramics

Fernandez-Carrion, AJ; Al Saghir, K; Veron, E; Becerro, AI; Porcher, F; Wisniewsld, W; Matzen, G; Fayon, F; Allix, M
Inorganic Chemistry, 56 (2017) 14446-14458


Eu-doped Sr1–x/2Al2–xSixO4 (x = 0.2, 0.4, and 0.5) transparent ceramics have been synthesized by full and congruent crystallization from glasses prepared by aerodynamic levitation and laser-heating method. Structural refinements from synchrotron and neutron powder diffraction data show that the ceramics adopt a 1 × 1 × 2 superstructure compared to the SrAl2O4 hexagonal polymorph. While the observed superstructure reflections indicate a long-range ordering of the Sr vacancies in the structure, 29Si and 27Al solid-state NMR measurements associated with DFT computations reveal a significant degree of disorder in the fully polymerized tetrahedral network. This is evidenced through the presence of Si–O–Si bonds, as well as Si(OAl)4 units at remote distances of the Sr vacancies and Al(OAl)4 units in the close vicinity of Sr vacancies departing from local charge compensation in the network. The transparent ceramics can be doped by europium to induce light emission arising from the volume under UV excitation. Luminescence measurements then reveal the coexistence of Eu2+ and Eu3+ in the samples, thereby allowing tuning the emission color depending on the excitation wavelength and suggesting possible applications such as solid state lighting.

December, 2017 | DOI: 10.1021/acs.inorgchem.7b01881

Microemulsion-Mediated Synthesis and Properties of Uniform Ln:CaWO4 (Ln = Eu, Dy) Nanophosphors with Multicolor Luminescence for Optical and CT Imaging

Laguna, M; Nuñez, NO; Garcia, FJ; Corral, A; Parrado-Gallego, A; Balcerzyk, M; Becerro, AI; Ocaña, M
European Journal of Inorganic Chemistry, 44 (2017) 5158-5168


A new room-temperature method has been developed that yields, for the first time in the literature, uniform and well-dispersed CaWO4 nanospindles. This method is based on the use of microemulsions consisting of aqueous solutions of Ca2+ and WO42- precursors, cyclohexane as the organic medium, Triton X-100 as the surfactant, and n-octanol as the cosurfactant. We show that the formation of uniform nanospindles requires a restrictive set of experimental conditions. These particles crystallize into the tetragonal CaWO4 phase and emit blue-green luminescence when excited by UV radiation. The reported method is also useful for doping the CaWO4 spindles with Eu3+ or Dy3+ cations, resulting in multicolor emissions (red for Eu3+; white for Dy3+). The luminescence is much stronger when excited through a WO42--Ln(3+) (Ln = Eu or Dy) energy-transfer band than through the f-f transition bands of the Ln(3+) cations. Interestingly, because of the white luminescence associated with the Dy:CaWO4 nanophosphor, it might be useful for LED technologies. Luminescence dynamics and energy-transfer efficiency have been analyzed to determine the optimum phosphors. Finally, the Eu-doped CaWO4 nanospindles also showed excellent X-ray attenuation efficacy, which confers double functionality to this material as both a luminescence bioprobe and as a contrasting agent for X-ray computed-tomography.

December, 2017 | DOI: 10.1002/ejic.201700650

Materials chemistry approaches to the control of the optical features of perovskite solar cells

Calvo, Mauricio E.
Journal of Materials Chemistry A, 5 (2017) 20561-20578


Perovskite solar cells have revolutionized the field of photovoltaics. Apart from their impressive photo-conversion efficiencies, the ease of their fabrication – principally when carried out by solution processing – has permitted addition of new functionalities to the photovoltaic cell. Some of these features are related to the optical properties. In this review, the different materials chemistry approaches that allow controlling the spectral absorption of ABX3 perovskite layers and the changes that they produce in the visual aspect of the solar cell will be covered. These modifications can be done either by playing with the composition of the precursors or by integrating different types of nanostructures. Spectral bandgap tuning, semitransparency, color and enhancement of light absorption are examples of how these modifications operate in the core of ABX3 perovskite solar cells. These optical features bring benefits in terms of photo-conversion efficiencies or else in the aesthetical integration of perovskite solar cells with architectonic elements for building integrated photovoltaics. Additionally, surface passivation approaches are covered to show its effect over the photo-conversion efficiency and stability of the cell.

October, 2017 | DOI: 10.1039/c7ta05666d

Facile Synthesis of Hybrid Organic–Inorganic Perovskite Microcubes of Optical Quality Using Polar Antisolvents

Li, Yuelong; Galisteo-Lopez, Juan F.; Calvo, Mauricio E.; Miguez, Hernan
ACS Applied Materials & Interfaces, 9 (2017) 35505-35510


Herein, we demonstrate a synthetic approach producing highly crystalline methylammonium lead bromide perovskite (MAPbBr3) microcubes at room temperature by injecting a perovskite precursor solution into an environmentally friendly antisolvent (isopropyl alcohol). Confirmed by X-ray and electron diffraction, as well as electron microscopy, these MAPbBr3 microcubes are single crystals and have perfect cubic structure, with sizes varying between 1 and 15 μm depending on the synthesis conditions. The stoichiometry of the MAPbBr3 crystal is proven by energy-dispersive X-ray analysis. Finally, optical analysis carried out by means of laser scanning confocal microscopy evidences how the crystalline quality of the microcubes translates into a homogeneous photoluminescence throughout the cube volume.

October, 2017 | DOI: 10.1021/acsami.7b08431

HoF3 and DyF3 Nanoparticles as Contrast Agents for High-Field Magnetic Resonance Imaging

Gonzalez-Mancebo, Daniel; Becerro, Ana I.; Rojas, T. Cristina; Garcia-Martin, Maria L.; de la Fuente, Jesus M.; Ocana, Manuel
Particle & particle systems characterization, 34 (2017) art. 1700116


Clinical contrast agents (CAs) currently used in magnetic resonance imaging (MRI) at low fields are less effective at high magnetic fields. The development of new CAs is mandatory to improve diagnostic capabilities of the new generation of high field MRI scanners. The purpose of this study is to synthesize uniform, water dispersible LnF3 (Ln = Ho, Dy) nanoparticles (NPs) and to evaluate their relaxivity at high magnetic field (9.4 T) as a function of size and composition. Two different types of HoF3 NPs are obtained by homogeneous precipitation in ethylene glycol at 120 °C. The use of holmium acetate as holmium precursor leads to rhombus-like nanoparticles, while smaller, ellipsoid-like nanoparticles are obtained when nitrate is used as the holmium salt. To explain this behavior, the mechanism of formation of both kinds of particles is analyzed in detail. Likewise, rhombus-like DyF3 nanoparticles are prepared following the same method as for the rhombus-like HoF3 nanoparticles. We have found, to the best of knowledge, the highest transverse relaxivity values at 9.4 T described in the literature for this kind of CAs. Finally, the LnF3 NPs have shown negligible cytotoxicity for C6 rat glioma cells for concentrations up to 0.1 mg mL−1.

October, 2017 | DOI: 10.1002/ppsc.201700116

Europium-doped NaGd(WO4)(2) nanophosphors: synthesis, luminescence and their coating with fluorescein for pH sensing

Laguna, M; Escudero, A; Nuñez, NO; Becerro, AI; Ocaña, M
Dalton Transactions, 46 (2017) 11575-11583


Uniform Eu-doped NaGd(WO4)(2) nanophosphors with a spherical shape have been synthesized for the first time by using a wet chemistry method based on a homogeneous precipitation process at low temperature (120 degrees C) in ethylene glycol/water mixtures. The obtained nanoparticles crystallized into the tetragonal structure and presented polycrystalline character. The europium content in such phosphors has been optimized through the analysis of the luminescence dynamics (lifetime measurements). By coating the Eu3+-doped wolframate based nanoparticles with fluorescein through a layer-by-layer (LbL) approach, a wide range (4-10) ratiometric pH-sensitive sensor has been developed, which uses the pH insensitive emission of Eu3+ as a reference.


September, 2017 | DOI: 10.1039/c7dt01986f

Rare earth based nanostructured materials: synthesis, functionalization, properties and bioimaging and biosensing applications

Escudero, Alberto; Becerro, Ana I.; Carrillo-Carrion, Carolina; Nunez, Nuria O.; Zyuzin, Mikhail V.; Laguna, Mariano; Gonzalez-Mancebo, Daniel; Ocana, Manuel; Parak, Wolfgang J.
Nanophotonics, 6 (2017) 881-921


Rare earth based nanostructures constitute a type of functional materials widely used and studied in the recent literature. The purpose of this review is to provide a general and comprehensive overview of the current state of the art, with special focus on the commonly employed synthesis methods and functionalization strategies of rare earth based nanoparticles and on their different bioimaging and biosensing applications. The luminescent (including downconversion, upconversion and permanent luminescence) and magnetic properties of rare earth based nanoparticles, as well as their ability to absorb X-rays, will also be explained and connected with their luminescent, magnetic resonance and X-ray computed tomography bioimaging applications, respectively. This review is not only restricted to nanoparticles, and recent advances reported for in other nanostructures containing rare earths, such as metal organic frameworks and lanthanide complexes conjugated with biological structures, will also be commented on.

September, 2017 | DOI: 10.1515/nanoph-2017-0007

Crystal structure, NIR luminescence and X-ray computed tomography of Nd3+:Ba0.3Lu0.7F2.7 nanospheres

Gonzalez-Mancebo, D; Becerro, AI; Cantelar, E; Cusso, F; Briat, A; Boyer, D; Ocana, M
Dalton Transactions, 46 (2017) 6580-6587


Uniform, hydrophilic 50 nm diameter Nd3+-doped Ba0.3Lu0.7F2.7 nanospheres are synthesized at 120 degrees C using a singular one-pot method based on the use of ethylene glycol as solvent, in the absence of any additive. The composition and crystal structure of the undoped material are analyzed in detail using ICP and XRD, which reveals a BaF2 cubic crystal structure that is able to incorporate 70 mol% of Lu ions. This finding contrasts with the reported phase diagram of the system, where the maximum solubility is around 30 mol% Lu. XRD proves as well that the Ba0.3Lu0.7F2.7 structure is able to incorporate Nd3+ ions up to, at least 10 mol%, without altering the uniform particles morphology. The Nd-doped particles exhibit near-infrared luminescence when excited at 810 nm. The maximum emission intensity with the minimum concentration quenching effect is obtained at 1.5% Nd doping level. X-ray computed tomography experiments are carried out on powder samples of the latter composition. The sample significantly absorbs X-ray photons, thus demonstrating that the Nd3+-doped Ba0.3Lu0.7F2.7 nanospheres are good candidates as contrast agents in computed tomography.

August, 2017 | DOI: 10.1039/c7dt00453b

Photonic Tuning of the Emission Color of Nanophosphor Films Processed at High Temperature

Geng, Dongling; Lozano, Gabriel; Calvo, Mauricio E.; Nunez, Nuria O.; Becerro, Ana I.; Ocana, Manuel; Miguez, Hernan
Advanced Optical Materials, 5 (2017) art. 1700099


Photonics offers new possibilities to tailor the photoluminescence process in phosphor-converted light emitting diodes. Herein, it is demonstrated that the emission color of thin layers of rare-earth doped nanocrystals can be strongly modulated in tunable spectral ranges using optical resonators specifically designed to this end. GdVO4:Dy3+ nanoparticles of controlled size and shape are synthesized using a solvothermal method with which highly transparent nanophosphor thin films are prepared. This paper designs and fabricates optical multilayers, which are transparent in the UV and resonant at the frequencies where the Dy3+ ions emit, to prove that the color coordinates of this emitter can be tuned from green to blue or yellow with unprecedented precision. Key to the achievement herein reported is the careful analysis of the structural and optical properties of thin nanophosphor layers with the processing temperature in order to achieve efficient photoluminescence while preserving the transparency of the film. The results open a new path for fundamental and applied research in solid-state lighting in which photonic nanostructures allow controlling the emission properties of state-of-the-art materials without altering their structure or chemical composition.

July, 2017 | DOI: 10.1002/adom.201700099

Design and Realization of a Novel Optically Disordered Material: A Demonstration of a Mie Glass

Miranda-Munoz, Jose M.; Lozano, Gabriel; Miguez, Hernan
Advanced Optical Materials, 5 (2017) art. 1700025


Herein, a diffusive material presenting optical disorder is introduced, which represents an example of a Mie glass. Comprising spherical crystalline TiO2 nanoparticles randomly dispersed in a mesoporous TiO2 matrix, it is proved that the scattering of light in this inhomogeneous solid can be predicted in an unprecedented manner from single-particle considerations employing Mie theory. To that aim, a study of the dependence of the key parameters employed is performed to describe light propagation in random media, i.e., the scattering mean free path and the transport mean free path, as a function of the size and concentration of the spherical inclusions based on a comparison between experimental results and analytical calculations. It is also demonstrated that Mie glasses enable enhanced fluorescence intensity due to a combined absorptance enhancement of the excitation light combined with an improved outcoupling of the emitted light. The method offers the possibility to perform a deterministic design for the realization of a light diffuser with tailor-made scattering properties.

May, 2017 | DOI: 10.1002/adom.201700025

Diverse Applications of Nanomedicine

Pelaz, Beatriz; Alexiou, Christoph; Alvarez -Puebla, Ramon A.; Alves, Frauke; Andrews, Anne M.; Ashraf, Sumaira; Balogh, Lajos P.; Ballerini, Laura; Bestetti, Alessandra; Brendel, Cornelia; Bosi, Susanna; Carril, Monica; Chan, Warren C. W.; Chen, Chunying
ACS Nano, 11 (2017) 2312-2381


The design and use of materials in the nanoscale size range for addressing medical and health-related issues continues to receive increasing interest. Research in nanomedicine spans a multitude of areas, including drug delivery, vaccine development, antibacterial, diagnosis and imaging tools, wearable devices, implants, high-throughput screening platforms, etc. using biological, nonbiological, biomimetic, or hybrid materials. Many of these developments are starting to be translated into viable clinical products. Here, we provide an overview of recent developments in nanomedicine and highlight the current challenges and upcoming opportunities for the field and translation to the clinic.

March, 2017 | DOI: 10.1021/acsnano.6b06040

Aperiodic Metal-Dielectric Multilayers as Highly Efficient Sunlight Reflectors

Alberto Jiménez-Solano; Miguel Anaya; Mauricio E. Calvo; Mercedes Alcon-Camas; Carlos Alcañiz; Elena Guillén; Noelia Martínez; Manuel Gallas; Thomas Preussner; Ramón Escobar-Galindo; Hernán Míguez
Advanced Optical Materials, 5 (2017) 1600833


The optimum reflection of the solar spectrum at well-defined incident directions as well as its durability in time are, both, fundamental requirements of the optics of thermosolar and photovoltaic energy conversion systems. The stringent high performance needed for these applications implies that, almost exclusively, second face mirrors based on silver are employed for this purpose. Herein, the possibility to develop solar mirrors using other metals, such as copper and aluminum, is theoretically and experimentally analyzed. It is found that reflectors based on these inexpensive metals are capable of reflecting the full solar spectrum with efficiencies comparable to that of silver-based reflectors. The designs herein proposed are based on aperiodic metal-dielectric multilayers whose optimized configuration is chosen employing a code based on a genetic algorithm that allows selecting the best one among 108 tested reflectors. The use of metals with wider spectral absorption bands is compensated by the use of multilayered designs in which metal absorption is almost suppressed, as the analysis of the electric field intensity distribution demonstrates. The feasibility of the proposed mirrors is demonstrated by their actual fabrication by large area deposition techniques amenable for mass production.

March, 2017 | DOI: 10.1002/adom.201600833

Strong Quantum Confinement and Fast Photoemission Activation in CH3NH3PbI3 Perovskite Nanocrystals Grown within Periodically Mesostructured Films

Miguel Anaya; Andrea Rubino; Teresa Cristina Rojas; Juan Francisco Galisteo-López; Mauricio Ernesto Calvo; Hernán Míguez
Advanced Optical Materials


In this Communication, a synthetic route is demonstrated to obtain stabilized MAPbI3 nanocrystals embedded in thin metal oxide films that display well-defined and adjustable quantum confinement effects over a wide range of 0.34 eV. Mesostructured TiO2 and SiO2 films displaying an ordered 3D pore network are prepared by evaporation-induced self-assembly of a series of organic supramolecular templates in the presence of metal oxide precursors. The pores in the inorganic films obtained after thermal annealing are then used as nanoreactors to synthesize MAPbI3crystallites with narrow size distribution and average radius comprised between 1 and 4 nm, depending on the template of choice. Both the static and dynamic photoemission properties of the ensemble display features distinctive of the regime of strong quantum confinement. Photoemission maps demonstrate that the spectral and intensity properties of the luminescence extracted from the perovskite quantum dot loaded films are homogeneous over squared centimeters areas. At variance with their bulk counterparts, constant emission intensity is reached in time scales at least four orders of magnitude shorter.

March, 2017 | DOI: 10.1002/adom.201601087

Morphology control of uniform CaMoO4 microarchitectures and development of white light emitting phosphors by Ln doping (Ln = Dy3+, Eu3+)

Laguna, Mariano; Nuñez, Nuria O.; Becerro, Ana I.; Ocaña, Manuel
Crystengcomm, 19 (2017) 1590-1600


A very simple synthesis procedure based on precipitation reactions at moderate temperature (120 degrees C) from solutions containing calcium nitrate and sodium molybdate, using mixed solvents (polyols and water) has been developed, which produces uniform tetragonal CaMoO4 microarchitectures with different morphologies (peanuts, cocoons, spindles and spheres) composed of self-assembled entities. The morphology and crystal size of such assemblies could be tuned by a simple change of the nature of the components of the solvent mixture or their volumetric ratio in such a mixture. All particles presented similar excitation and emission spectra arising from a charge transfer process within the MoO4 2-groups. The emitted light presented a bluish-green color and its intensity was higher for the spindle-type particles. This synthesis procedure was also suitable for doping peanut-like CaMoO4 architectures with Eu3+ or Dy3+ cations up to a 1% molar ratio (Ln/Ln + Ca), without altering their morphology or crystalline structure. The so prepared phosphors emitted an intense red (Eu-doped) or greenish (Dy-doped) light when excited through the MoO42- group excitation band, indicating the presence of an energy transfer process from such groups to the Ln(3+) cations. Finally, a white light emitting phosphor with chromaticity coordinates x = 0.335 and y = 0.365 and a correlated color temperature of 5407 K was developed by codoping peanut-type CaMoO4 particles with suitable amounts of Dy3+ (0.35%) and Eu3+ (0.15%) cations, which could find applications in white light emitting diodes.

March, 2017 | DOI: 10.1039/c6ce02611g

Comprehensive and Systematic Analysis of the Immunocompatibility of Polyelectrolyte Capsules

Zyuzin, MV; Diez, P; Goldsmith, M; Carregal-Romero, S; Teodosio, C; Rejman, J; Feliu, N; Escudero, A; Almendral, MJ; Linne, U; Peer, D; Fuentes, M; Parak, WJ
Bioconjugate Chemistry, 28 (2017) 556-564


The immunocompability of polyelectrolyte capsules synthesized by layer-by-layer deposition has been investigated. Capsules of different architecture and composed of either non-degradable or biodegradable polymers, with either positively or negatively charged outer surface, and with micrometer size, have been used, and the capsule uptake by different cell lines has been studied and quantified. Immunocompatibility studies were performed with peripheral blood mononuclear cells (PBMCs). Data demonstrate that incubation with capsules, at concentrations relevant for practical applications, did not result in a reduced viability of cells, as it did not show an increased apoptosis. Presence of capsules also did not result in an increased expression of TNF-α, as detected with antibody staining, as well as at mRNA level. It also did not result in increased expression of IL-6, as detected at mRNA level. These results indicate that the polyelectrolyte capsules used in this study are immunocompatible.

February, 2017 | DOI: 10.1021/acs.bioconjchem.6b00657

Electron injection and scaffold effects in perovskite solar cells

M. Anaya, W. Zhang, B. Clasen Hames, Y. Li, F. Fabregat-Santiago, M.E. Calvo, H.J. Snaith, H. Míguez, I. Mora-Seró
Journal of Materials Chemistry C, 5 (2017) 634-644


In spite of the impressive efficiencies reported for perovskite solar cells (PSCs), key aspects of their working principles, such as electron injection at the contacts or the suitability of the utilization of a specific scaffold layer, are not yet fully understood. Increasingly complex scaffolds attained by the sequential deposition of TiO2 and SiO2 mesoporous layers onto transparent conducting substrates are used to perform a systematic characterization of both the injection process at the electron selective contact and the scaffold effect in PSCs. By forcing multiple electron injection processes at a controlled sequence of perovskite–TiO2 interfaces before extraction, interfacial injection effects are magnified and hence characterized in detail. An anomalous injection behavior is observed, the fingerprint of which is the presence of significant inductive loops in the impedance spectra with a magnitude that correlates with the number of interfaces in the scaffold. Analysis of the resistive and capacitive behavior of the impedance spectra indicates that the scaffolds could hinder ion migration, with positive consequences such as lowering the recombination rate and implications for the current–potential curve hysteresis. Our results suggest that an appropriate balance between these advantageous effects and the unavoidable charge transport resistive losses introduced by the scaffolds will help in the optimization of PSC performance.

January, 2017 | DOI: 10.1039/C6TC04639H


Three-Dimensional Optical Tomography and Correlated Elemental Analysis of Hybrid Perovskite Microstructures: An Insight into Defect-Related Lattice Distortion and Photoinduced Ion Migration

Galisteo-Lopez, JF; Li, YL; Miguez, H
Journal of Physical Chemistry Letters, 7 (2016) 5227-5234


Organic lead halide perovskites have recently been proposed for applications in light-emitting devices of different sorts. More specifically, regular crystalline microstructures constitute an efficient light source and fulfill the geometrical requirements to act as resonators, giving rise to waveguiding and optical amplification. Herein we show three-dimensional laser scanning confocal tomography studies of different types of methylammonium lead bromide microstructures which have allowed us to dissect their photoemission properties with a precision of 0.036 mu m(3). This analysis shows that their spectral emission presents strong spatial variations which can be attributed to defect-related lattice distortions. It is also largely enhanced under light exposure, which triggers the migration of halide ions away from illuminated regions, eventually leading to a strongly anisotropic degradation. Our work points to the need for performing an optical quality test of individual crystallites prior to their use in optoelectronics devices and provides a means to do so.

December, 2016 | DOI: 10.1021/acsjpclett.6b02456

Optical sensing by integration of analyte-sensitive fluorophore to particles

Carrillo-Carrion, C; Escudero, A; Parak, WJ
TrAC Trends in Analytical Chemistry, 84 (2016) 84-85


Analyte-sensitive fluorophores are a common tool in analytical chemistry. In case they are conjugated to the surface of colloidal nanoparticles new or improved applications are possible. An overview of the potential of such fluorophore-particle conjugates is given by means of several examples. First, using pH-sensitive fluorophores attached to particles are a helpful tool for investigating particle uptake by cells, as they can indicate whether particles are in the neutral slightly alkaline extracellular medium, or in acidic intracellular vesicles after endocytosis. Second, relating to lifetime-based methodologies, the fluorescence resonance energy transfer between fluorophores attached to quantum dots leads to longer lifetimes, improving their performance and expanding the possibilities of methods, such as lifetime imaging for in vivo applications. It also can be exploited for multiplexing approaches, in which the effective lifetime of the fluorophores can be tuned, allowing thus for the detection of several analytes based on temporal discrimination. Attention is focused to these three areas of application, because they are among the most reported in recent literature, and therefore of particular interest.

November, 2016 | DOI: 10.1016/j.trac.2016.05.001

Multifunctional Eu-doped NaGd(MoO4)(2) nanoparticles functionalized with poly(L-lysine) for optical and MRI imaging

M. Laguna; N.O. Nuñez; V. Rodríguez; E. Cantelar, G. Stepien, M.L. García, J.M. de la Fuente; M. Ocaña
Dalton Transactions, 45 (2016) 16354-16365


A method for the synthesis of non-aggregated and highly uniform Eu3+ doped NaGd(MoO4)(2) nanoparticles is reported for the first time. The obtained particles present tetragonal structure, ellipsoidal shape and their size can be varied by adjusting the experimental synthesis parameters. These nanoparticles, which were coated with citrate anions and functionalised with PLL, have also been developed in order to improve their colloidal stability in physiological medium (2-(N-morpholino) ethanesulfonic acid, MES). A study of the luminescent dynamics of the samples as a function of the Eu doping level has been conducted in order to find the optimum nanophosphors, whose magnetic relaxivity and cell viability have also been evaluated for the first time for this system, in order to assess their suitability as multifunctional probes for optical (in vitro) and magnetic bioimaging applications.

November, 2016 | DOI: 10.1039/c6dt02663j

Quantitative uptake of colloidal particles by cell cultures

Feliu, N; Huhn, J; Zyuzin, MV; Ashraf, S; Valdeperez, D; Masood, A; Said, AH; Escudero, A; Pelaz, B; Gonzalez, E; Duarte, MAC; Roy, S; Chakraborty, I; Lim, ML; Sjoqvist, S; Jungebluth, P; Parak, WJ
Science of the Total Environment, 568 (2016) 819-828


The use of nanotechnologies involving nano-and microparticles has increased tremendously in the recent past. There are various beneficial characteristics that make particles attractive for a wide range of technologies. However, colloidal particles on the other hand can potentially be harmful for humans and environment. Today, complete understanding of the interaction of colloidal particles with biological systems still remains a challenge. Indeed, their uptake, effects, and final cell cycle including their life span fate and degradation in biological systems are not fully understood. This is mainly due to the complexity of multiple parameters which need to be taken in consideration to perform the nanosafety research. Therefore, we will provide an overview of the common denominators and ideas to achieve universal metrics to assess their safety. The review discusses aspects including how biological media could change the physicochemical properties of colloids, how colloids are endocytosed by cells, how to distinguish between internalized versus membrane-attached colloids, possible correlation of cellular uptake of colloids with their physicochemical properties, and how the colloidal stability of colloids may vary upon cell internalization. In conclusion three main statements are given. First, in typically exposure scenarios only part of the colloids associated with cells are internalized while a significant part remain outside cells attached to their membrane. For quantitative uptake studies false positive counts in the form of only adherent but not internalized colloids have to be avoided. pH sensitive fluorophores attached to the colloids, which can discriminate between acidic endosomal/lysosomal and neutral extracellular environment around colloids offer a possible solution. Second, the metrics selected for uptake studies is of utmost importance. Counting the internalized colloids by number or by volume may lead to significantly different results. Third, colloids may change their physicochemical properties along their life cycle, and appropriate characterization is required during the different stages.

October, 2016 | DOI: 10.1016/j.scitotenv.2016.05.213

Microstructural, spectroscopic, and antibacterial properties of silver-based hybrid nanostructures biosynthesized using extracts of coriander leaves and seeds

Luna, C; Barriga-Castro, ED; Gomez-Trevino, A; Nuñez, NO; Mendoza-Resendez, R
International Journal of Nanomedicine, 11 (2016) 4787-4798


Coriander leaves and seeds have been highly appreciated since ancient times, not only due to their pleasant flavors but also due to their inhibitory activity on food degradation and their beneficial properties for health, both ascribed to their strong antioxidant activity. Recently, it has been shown that coriander leaf extracts can mediate the synthesis of metallic nanoparticles through oxidation/reduction reactions. In the present study, extracts of coriander leaves and seeds have been used as reaction media for the wet chemical synthesis of ultrafine silver nanoparticles and nanoparticle clusters, with urchin-and tree-like shapes, coated by biomolecules (mainly, proteins and polyphenols). In this greener route of nanostructure preparation, the active biocompounds of coriander simultaneously play the roles of reducing and stabilizing agents. The morphological and microstructural studies of the resulting biosynthesized silver nanostructures revealed that the nanostructures prepared with a small concentration of the precursor Ag salt (AgNO3 =5 mM) exhibit an ultrafine size and a narrow size distribution, whereas particles synthesized with high concentrations of the precursor Ag salt (AgNO3 =0.5 M) are polydisperse and formation of supramolecular structures occurs. Fourier transform infrared and Raman spectroscopy studies indicated that the bioreduction of the Ag- ions takes place through their interactions with free amines, carboxylate ions, and hydroxyl groups. As a consequence of such interactions, residues of proteins and polyphenols cap the biosynthesized Ag nanoparticles providing them a hybrid core/shell structure. In addition, these biosynthesized Ag nanomaterials exhibited size-dependent plasmon extinction bands and enhanced bactericidal activities against both Gram-positive and Gram-negative bacteria, displaying minimal inhibitory Ag concentrations lower than typical values reported in the literature for Ag nanoparticles, probably due to the synergy of the bactericidal activities of the Ag nanoparticle cores and their capping ligands.

September, 2016 | DOI: 10.2147/IJN.S105166

Optical analysis of CH3NH3SnxPb1−xI3 absorbers: a roadmap for perovskite-on-perovskite tandem solar cells

Anaya, M.; Correa-Baena, J.P.; Lozano, G.; Saliba, M.; Anguita, P.; Roose, B.; Abate, A.; Steiner, U.; Gratzel, M.; Calvo, M.E.; Hagfeldt, A.; Míguez, H.
Journal lf Materials Chemistry A, 4 (2016) 11214-11221


Organic–inorganic perovskite structures in which lead is substituted by tin are exceptional candidates for broadband light absorption. Herein we present a thorough analysis of the optical properties of CH3NH3SnxPb1−xI3 films, providing the field with definitive insights about the possibilities of these materials for perovskite solar cells of superior efficiency. We report a user's guide based on the first set of optical constants obtained for a series of tin/lead perovskite films, which was only possible to measure due to the preparation of optical quality thin layers. According to the Shockley–Queisser theory, CH3NH3xPb1−xI3 compounds promise a substantial enhancement of both short circuit photocurrent and power conversion efficiency in single junction solar cells. Moreover, we propose a novel tandem architecture design in which both top and bottom cells are made of perovskite absorbers. Our calculations indicate that such perovskite-on-perovskite tandem devices could reach efficiencies over 35%. Our analysis serves to establish the first roadmap for this type of cells based on actual optical characterization data. We foresee that this study will encourage the research on novel near-infrared perovskite materials for photovoltaic applications, which may have implications in the rapidly emerging field of tandem devices.

August, 2016 | DOI: 10.1039/C6TA04840D

Luminescent Rare-earth-based Nanoparticles: A Summarized Overview of their Synthesis, Functionalization, and Applications

Escudero, A; Carrillo-Carrion, C; Zyuzin, MV; Parak, WJ
Topics in current chemistry, 374 (2016) Article number 48


Rare-earth-based nanoparticles are currently attracting wide research interest in material science, physics, chemistry, medicine, and biology due to their optical properties, their stability, and novel applications. We present in this review a summarized overview of the general and recent developments in their synthesis and functionalization. Their luminescent properties are also discussed, including the latest advances in the enhancement of their emission luminescence. Some of their more relevant and novel biomedical, analytical, and optoelectronic applications are also commented on.

August, 2016 | DOI: 10.1007/s41061-016-0049-8

Modified emission of extended light emitting layers by selective coupling to collective lattice resonances

Ramezani, Mohammad; Lozano, Gabriel; Verschuuren, Marc A.; Gomez-Rivas, Jaime
Physical Review B, 94 (2016) 12


We demonstrate that the coupling between light emitters in extended polymer layers and modes supported by arrays of plasmonic particles can be selectively enhanced by accurate positioning of the emitters in regions where the electric field intensity of a given mode is maximized. The enhancement, which we measure to reach up to 70%, is due to the improved spatial overlap and coupling between the optical mode and emitters. This improvement of the coupling leads to a modification of the emission spectrum and the luminous efficacy of the sample.

August, 2016 | DOI: 10.1103/PhysRevB.94.125406

Cellular Viscosity in Prokaryotes and Thermal Stability of Low Molecular Weight Biomolecules

Cuecas, A; Cruces, J; Galisteo-Lopez, JF; Peng, XJ; Gonzalez, JM
Biophysical Journal, 111 (2016) 875–882


Some low molecular weight biomolecules, i.e., NAD(P)H, are unstable at high temperatures. The use of these biomolecules by thermophilic microorganisms has been scarcely analyzed. Herein, NADH stability has been studied at different temperatures and viscosities. NADH decay increased at increasing temperatures. At increasing viscosities, NADH decay rates decreased. Thus, maintaining relatively high cellular viscosity in cells could result in increased stability of low molecular weight biomolecules (i.e., NADH) at high temperatures, unlike what was previously deduced from studies in diluted water solutions. Cellular viscosity was determined using a fluorescent molecular rotor in various prokaryotes covering the range from 10 to 100°C. Some mesophiles showed the capability of changing cellular viscosity depending on growth temperature. Thermophiles and extreme thermophiles presented a relatively high cellular viscosity, suggesting this strategy as a reasonable mechanism to thrive under these high temperatures. Results substantiate the capability of thermophiles and extreme thermophiles (growth range 50–80°C) to stabilize and use generally considered unstable, universal low molecular weight biomolecules. In addition, this study represents a first report, to our knowledge, on cellular viscosity measurements in prokaryotes and it shows the dependency of prokaryotic cellular viscosity on species and growth temperature.

August, 2016 | DOI: 10.1016/j.bpj.2016.07.024

Confinement and surface effects on the physical properties of rhombohedral-shape hematite (alpha-Fe2O3) nanocrystals

Luna, C; Cuan-Guerra, AD; Barriga-Castro, ED; Nunez, NO; Mendoza-Resendez, R
Materials Research Bulletin, 80 (2016) 44-52


Morphological, microstructural and vibrational properties of hematite (alpha-Fe2O3) nanocrystals with a rhombohedral shape and rounded edges, obtained by forced hydrolysis of iron(III) solutions under a fast nucleation, have been investigated in detail as a function of aging time. These studies allowed us to propose a detailed formation mechanism and revealed that these nanocrystals are composed of four {104} side facets, two {110} faces at the edges of the long diagonal of the nanocrystals and two {-441} facets as the top and bottom faces. Also, the presence of nanoscopic pores and fissures was evidenced. The vibrational bands of such nanocrystals were shifted to lower frequencies in comparison with bulk hematite ones as the nanocrystal size was reduced due to phonon confinement effects. Also, the indirect and direct transition band gaps displayed interesting dependences on the aging time arising from quantum confinementand surface effects 

August, 2016 | DOI: 10.1016/j.materresbull.2016.03.029

Unbroken Perovskite: Interplay of Morphology, Electro-optical Properties, and Ionic Movement

Correa-Baena, JP; Anaya, M; Lozano, G; Tress, W; Domanski, K; Saliba, M; Matsui, T; Jacobsson, TJ; Calvo, ME; Abate, A; Gratzel, M; Miguez, H; Hagfeldt, A
Advanced Materials, 28 (2016) 5031-5037


Hybrid organic-inorganic perovskite materials have risen up as leading components for light-harvesting applications. However, to date many questions are still open concerning the operation of perovskite solar cells (PSCs). A systematic analysis of the interplay among structural features, optoelectronic performance, and ionic movement behavior for FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3) PSCs is presented, which yield high power conversion efficiencies up to 20.8%.

July, 2016 | DOI: 10.1002/adma.201600624

Metallic nanostructures for efficient LED lighting

Lozano, G; Rodriguez, SRK; Verschuuren, MA; Rivas, JG
Light: Science and Applications, 5 (2016) e16080


Light-emitting diodes (LEDs) are driving a shift toward energy-efficient illumination. Nonetheless, modifying the emission intensities, colors and directionalities of LEDs in specific ways remains a challenge often tackled by incorporating secondary optical components. Metallic nanostructures supporting plasmonic resonances are an interesting alternative to this approach due to their strong light-matter interaction, which facilitates control over light emission without requiring external secondary optical components. This review discusses new methods that enhance the efficiencies of LEDs using nanostructured metals. This is an emerging field that incorporates physics, materials science, device technology and industry. First, we provide a general overview of state-of-the-art LED lighting, discussing the main characteristics required of both quantum wells and color converters to efficiently generate white light. Then, we discuss the main challenges in this field as well as the potential of metallic nanostructures to circumvent them. We review several of the most relevant demonstrations of LEDs in combination with metallic nanostructures, which have resulted in light-emitting devices with improved performance. We also highlight a few recent studies in applied plasmonics that, although exploratory and eminently fundamental, may lead to new solutions in illumination.

June, 2016 | DOI: 10.1038/lsa.2016.80

Synthesis and functionalization of monodisperse near-ultraviolet and visible excitable multifunctional Eu3+, Bi3+:REVO4 nanophosphors for bioimaging and biosensing applications

Escudero, Alberto; Carrillo-Carrion, Carolina; Zyuzin, Mikhail V.; Ashraf, Sumaira; Hartmann, Raimo; Nunez, Nuria O.; Ocana, Manuel; Parak, Wolfgang J.
Nanoscale, 8 (2016) 12221-12236


Near-ultraviolet and visible excitable Eu- and Bi-doped NPs based on rare earth vanadates (REVO4, RE = Y, Gd) have been synthesized by a facile route from appropriate RE precursors, europium and bismuth nitrate, and sodium orthovanadate, by homogeneous precipitation in an ethylene glycol/water mixture at 120 °C. The NPs can be functionalized either by a one-pot synthesis with polyacrylic acid (PAA) or by a Layer-by-Layer approach with poly(allylamine hydrochloride) (PAH) and PAA. In the first case, the particle size can also be tuned by adjusting the amount of PAA. The Eu- Bi-doped REVO4 based nanophosphors show the typical red luminescence of Eu(III), which can be excited through an energy transfer process from the vanadate anions, resulting in a much higher luminescence intensity in comparison to the direct excitation of the europium cations. The incorporation of Bi into the REVO4 structure shifts the original absorption band of the vanadate anions towards longer wavelengths, giving rise to nanophosphors with an excitation maximum at 342 nm, which can also be excited in the visible range. The suitability of such nanophosphors for bioimaging and biosensing applications, as well as their colloidal stability in different buffer media of biological interest, their cytotoxicity, their degradability at low pH, and their uptake by HeLa cells have been evaluated. Their suitability for bioimaging and biosensing applications is also demonstrated.

June, 2016 | DOI: 10.1039/C6NR03369E

Plasmonic Nanoparticles as Light-Harvesting Enhancers in Perovskite Solar Cells: A User’s Guide

Carretero-Palacios, S.; Jiménez-Solano, A.; Míguez, H.
ACS Energy Letters, 1 (2016) 323-331


In this Perspective we discuss the implications of employing metal particles of different shape, size, and composition as absorption enhancers in methylammonium lead iodide perovskite solar cells, with the aim of establishing some guidelines for the future development of plasmonic resonance-based photovoltaic devices. Hybrid perovskites present an extraordinarily high absorption coefficient which, as we show here, makes it difficult to extrapolate concepts and designs that are applied to other solution-processed photovoltaic materials. In addition, the variability of the optical constants attained from perovskite films of seemingly similar composition further complicates the analysis. We demonstrate that, by means of rigorous design, it is possible to provide a realistic prediction of the magnitude of the absorption enhancement that can be reached for perovskite films embedding metal particles. On the basis of this, we foresee that localized surface plasmon effects will provide a means to attain highly efficient perovskite solar cells using films that are thinner than those usually employed, hence facilitating collection of photocarriers and significantly reducing the amount of potentially toxic lead present in the device.

June, 2016 | DOI: 10.1021/acsenergylett.6b00138

A panchromatic modification of the light absorption spectra of metal-organic frameworks

Otal, E. H.; Kim, M. L.; Calvo, M. E.; Karvonen, L.; Fabregas, I. O.; Sierra, C. A.; Hinestroza, J. P.
Chemical Communications, 52 (2016) 6665-6668


The optical absorption of UiO-66–NH2 MOF was red-shifted using a diazo-coupling reaction. The modifications performed with naphthols and aniline yielded reddish samples, and the modifications with diphenylaniline yielded dark violet ones. The photocatalytic activity of these modified MOFs was assessed for methylene blue degradation, showing a good performance relative to traditional TiO2. The degradation performance was found to correlate with the red shift of the absorption edge. These findings suggest potential applications of these materials in photocatalysis and in dye sensitized solar cells.

May, 2016 | DOI: 10.1039/c6cc02319c

Solution processed high refractive index contrast distributed Bragg reflectors

Anaya, M; Rubino, A; Calvo, ME; Miguez, H
Journal of Materials Chemistry C, 4 (2016) 4532-4537


We have developed a method to alternate porous and dense dielectric films in order to build high refractive index contrast distributed Bragg reflectors (DBRs) capable of reflecting very efficiently in a targeted spectral range employing a small number of layers in the stack. Porous layers made of SiO2 nanoparticles and compact sol–gel processed TiO2 layers are sequentially deposited. The key to the preservation of porosity of every other layer during the deposition process is the use of a sacrificial layer of polystyrene that prevents the infiltration of the interstitial voids between nanoparticles with the homogeneous solution of TiO2 precursors. Our approach allows preparing a series of DBRs operating along the whole visible spectral range. Reflectance values as high as 90% are achieved from only seven layers. The particular distribution of porosity along one direction gives rise to an interesting interplay between the optical properties of the system and the vapor pressure in the surrounding atmosphere, which we foresee could be put into practice in gas sensing devices.

May, 2016 | DOI: 10.1039/C6TC00663A

Transparent polycrystalline SrREGa3O7 melilite ceramics: potential phosphors for tuneable solid state lighting

Boyer, M; Carrion, AJF; Ory, S; Becerro, AI; Villette, S; Eliseeva, SV; Petoud, S; Aballea, P; Matzen, G; Allix, M
Journal of Materials Chemistry C, 15 (2016) 3238-3247


Full and congruent crystallization from glass is applied to the SrREGa3O7 melilite family (RE = Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Y). This innovative process enables the synthesis of polycrystalline ceramics exhibiting high transparency both in the visible and near infrared regions, despite tetragonal crystal structures and micrometer scale grain sizes. Moreover, glass crystallization provides an original route to synthesize new crystalline phases which are not accessible via a classic solid state reaction, as demonstrated for SrYbGa3O7 and SrTmGa3O7. To illustrate the potential optical applications of such materials, SrGdGa3O7 transparent polycrystalline ceramics are doped with Dy3+ or Tb3+/Eu3+ in order to generate white light emission under UV excitation. It is foreseen that such transparent melilite ceramic phosphors, prepared via a cost-effective process, can be successfully used in solid state lighting devices of considerable technological interest.

April, 2016 | DOI: 10.1039/C6TC00633G

Effect of temperature variations on equilibrium distances in levitating parallel dielectric plates interacting through Casimir forces

Esteso, V; Carretero-Palacios, S; Miguez, H
Journal of Applied Physics, 119 (2016) 144301


We study at thermal equilibrium the effect of temperature deviations around room temperature on the equilibrium distance (d(eq)) at which thin films made of Teflon, silica, or polystyrene immersed in glycerol levitate over a silicon substrate due to the balance of Casimir, gravity, and buoyancy forces. We find that the equilibrium nature (stable or unstable) of d(eq) is preserved under temperature changes, and provide simple rules to predict whether the new equilibrium position will occur closer to or further from the substrate at the new temperature. These rules depend on the static permittivities of all materials comprised in the system (epsilon((m))(0)) and the equilibrium nature of d(eq). Our designed dielectric configuration is excellent for experimental observation of thermal effects on the Casimir force indirectly detected through the tunable equilibrium distances (with slab thickness and material properties) in levitation mode.

April, 2016 | DOI: 10.1063/1.4945428

High-Throughput Fabrication of Resonant Metamaterials with Ultrasmall Coaxial Apertures via Atomic Layer Lithography

Yoo, D; Nguyen, NC; Martin-Moreno, L; Mohr, DA; Carretero-Palacios, S; Shaver, J; Peraire, J; Ebbesen, TW; Oh, SH
Nano Letters, 16 (2016) 2040-2046


We combine atomic layer lithography and glancing angle ion polishing to create wafer-scale metamaterials composed of dense arrays of ultrasmall coaxial nanocavities in gold films. This new fabrication scheme makes it possible to shrink the diameter and increase the packing density of 2 nm-gap coaxial resonators, an extreme subwavelength structure first manufactured via atomic layer lithography, both by a factor of 100 with respect to previous studies. We demonstrate that the nonpropagating zeroth-order Fabry-Perot mode, which possesses slow light-like properties at the cutoff resonance, traps infrared light inside 2 nm gaps (gap volume similar to lambda(3)/10(6)). Notably, the annular gaps cover only 3% or less of the metal surface, while open-area normalized transmission is as high as 1700% at the epsilon-near-zero (ENZ) condition. The resulting energy accumulation alongside extraordinary optical transmission can benefit applications in nonlinear optics, optical trapping, and surface-enhanced spectroscopies. Furthermore, because the resonance wavelength is independent of the cavity length and dramatically red shifts as the gap size is reduced, large-area arrays can be constructed with lambda(resonance) >> period, making this fabrication method ideal for manufacturing resonant metamaterials.

March, 2016 | DOI: 10.1021/acs.nanolett.6b00024

Integration of Photonic Crystals into Flexible Dye Solar Cells: A Route toward Bendable and Adaptable Optoelectronic Devices Displaying Structural Color and Enhanced Efficiency

Li, YL; Calvo, ME; Miguez, H
Advanced Optical Materials, 4 (2016) 464-471


Herein is presented what is believed to be the first example of integration of photonic structures in a flexible optoelectronic device. The resulting devices may be designed to display any color in the visible range and, simultaneously, present enhanced power conversion efficiency as a consequence of the increased light harvesting caused by the colored back reflection. The achievement results from the incorporation of nanoparticle-based multilayers with photonic crystal properties that are modified to be compatible with the chemical and physical processing of flexible nanocrystalline titania electrodes of dye solar cells. The photovoltaic performance of these colored flexible cells remains unaltered after one hundred bending cycles, thus showing the high-mechanical stability of the ensemble. These devices reunite most characteristics required for building integration or for the construction of solar window panes, such as light weight, stability upon bending, adaptability, and color. This work may trigger promising applications of these highly adaptable and versatile photonic crystals in other flexible devices.

March, 2016 | DOI: 10.1002/adom.201500547

Maximized performance of dye solar cells on plastic: a combined theoretical and experimental optimization approach

Li, Yuelong; Carretero-Palacios, Sol; Yoo, Kicheon; Kim, Jong Hak; Jimenez-Solano, Alberto; Lee, Chul-Ho; Miguez, Hernan; Ko, Min Jae
Energy & Environmental Science, 9 (2016) 2061-2071


We demonstrate that a combined optimization approach based on the sequential alternation of theoretical analysis and experimental realization gives rise to plastic supported dye solar cells for which both light harvesting efficiency and electron collection are maximized. Rationalized configurations with optimized light trapping and charge extraction are realized to achieve photoanodes on plastic prepared at low temperature, showing a power conversion efficiency of 8.55% and a short circuit photocurrent of 16.11 mA cm−2, unprecedented for plastic based dye solar cell devices. Furthermore, the corresponding fully flexible designs present stable mechanical properties after several bending cycles, displaying 7.79% power conversion efficiency, an average broadband internal quantum efficiency above 90%, and a short circuit photocurrent of 15.94 mA cm−2, which is the largest reported value for bendable cells of this sort to date.

February, 2016 | DOI: 10.1039/C6EE00424E

Photophysical Analysis of the Formation of Organic–Inorganic Trihalide Perovskite Films: Identification and Characterization of Crystal Nucleation and Growth

Anaya, M; Galisteo-Lopez, JF; Calvo, ME; Lopez, C; Miguez, H
Journal of Physical Chemistry C, 120 (2016) 3071-3076


In this work we demonstrate that the different processes occurring during hybrid organic–inorganic lead iodide perovskite film formation can be identified and analyzed by a combined in situ analysis of their photophysical and structural properties. Our observations indicate that this approach permits unambiguously identifying the crystal nucleation and growth regimes that lead to the final material having a cubic crystallographic phase, which stabilizes to the well-known tetragonal phase upon cooling to room temperature. Strong correlation between the dynamic and static photoemission results and the temperature-dependent X-ray diffraction data allows us to provide a description and to establish an approximate time scale for each one of the stages and their evolution. The combined characterization approach herein explored yields key information about the kinetics of the process, such as the link between the evolution of the defect density during film formation, revealed by a fluctuating photoluminescence quantum yield, and the gradual changes observed in the PbI2-related precursor structure.

February, 2016 | DOI: 10.1021/acs.jpcc.6b00398

Efficient bifacial dye-sensitized solar cells through disorder by design

Miranda-Munoz, JM; Carretero-Palacios, S; Jimenez-Solano, A; Li, YL; Lozano, G; Miguez, H
Journal of Materials Chemistry A, 4 (2016) 1953-1961


Herein we realize an optical design that optimizes the performance of bifacial solar cells without modifying any of the usually employed components. In order to do so, dielectric scatterers of controlled size and shape have been successfully integrated in the working electrodes of dye-sensitized solar cells (DSSCs), resulting in bifacial devices of outstanding performance. Power conversion efficiencies (PCEs) as high as 6.7% and 5.4% have been attained under front and rear illumination, respectively, which represent a 25% and a 33% PCE enhancement with respect to an 8 μm-thick standard solar cell electrode using platinum as the catalytic material. The remarkable bifacial character of our approach is demonstrated by the high rear/front efficiency ratio attained, around 80%, which is among the largest reported for this sort of device. The proposed optimized design is based on a Monte Carlo approach in which the multiple scattering of light within the cell is fully accounted for. We identified that the spherical shape of the scatterers is the key parameter controlling the angular distribution of the scattering, the most efficient devices being those in which the inclusions provide a narrow forward-oriented angular distribution of the scattered light.

January, 2016 | DOI: 10.1039/C5TA10091G

Gold-Based Nanomaterials for Applications in Nanomedicine

Ashraf, S; Pelaz, B; del Pino, P; Carril, M; Escudero, A; Parak, WJ; Soliman, MG; Zhang, Q; Carrillo-Carrion, C
Light-Responsive Nanostructured Systems for Applications in Nanomedicine, 370 (2016) 169-202


In this review, an overview of the current state-of-the-art of gold-based nanomaterials (Au NPs) in medical applications is given. The unique properties of Au NPs, such as their tunable size, shape, and surface characteristics, optical properties, biocompatibility, low cytotoxicity, high stability, and multifunctionality potential, among others, make them highly attractive in many aspects of medicine. First, the preparation methods for various Au NPs including functionalization strategies for selective targeting are summarized. Second, recent progresses on their applications, ranging from the diagnostics to therapeutics are highlighted. Finally, the rapidly growing and promising field of gold-based theranostic nano-platforms is discussed. Considering the great body of existing information and the high speed of its renewal, we chose in this review to generalize the data that have been accumulated during the past few years for the most promising directions in the use of Au NPs in current medical research.

January, 2016 | DOI:

Ligand-Free Synthesis of Tunable Size Ln:BaGdF5 (Ln = Eu3+ and Nd3+) Nanoparticles: Luminescence, Magnetic Properties, and Biocompatibility

Becerro, AI; Gonzalez-Mancebo, D; Cantelar, E; Cusso, F; Stepien, G; de la Fuente, JM; Ocana, M
Langmuir, 32 (2016) 411-420


Bifunctional and highly uniform Ln:BaGdF5 (Ln = Eu3+ and Nd3+) nanoparticles have been successfully synthesized using a solvothermal method consisting of the aging at 120 degrees C of a glycerol solution containing the corresponding Lanthanide acetylacetonates and butylmethylimidazolium tetrafluoroborate. The absence of any surfactant in the synthesis process rendered hydrophilic nanospheres (with tunable diameter from 45 nm 85 nm, depending on the cations concentration of the starting solution) which are suitable for bioapplications. The particles are bifunctional because they showed both optical and magnetic properties due to the presence of the optically active lanthanides (Eu3+ in the visible and Nd3+ in the NIR regions of the electromagnetic spectrum) and the paramagnetic gadolinium ion, respectively. The luminescence decay curves of the nanospheres doped with different amounts of Eu3+ and Nd3+ have been recorded in order to determine the optimum dopant concentration in each case, which turned out to be 5% Eu3+ and 0.5% Nd3+. Likewise, proton relaxation times were measured at 1.5 T in water suspensions of the optimum particles found in the luminescence study. The values obtained suggested that both kinds of particles could be used as positive contrast agents for MRI. Finally, it was demonstrated that both the 5% Eu3+ and 0.5% Nd3+-doped BaGdF5 nanospheres showed negligible cytotoxicity for VERO cells for concentrations up to 0.25 mg mL(-1).

January, 2016 | DOI: 10.1021/acs.langmuir.5b03837

Deposition of silica protected luminescent layers of Eu:GdVO4 nanoparticles assisted by atmospheric pressure plasma jet

Moretti, E; Pizzol, G; Fantin, M; Enrichi, F; Scopece, P; Nunez, NO; Ocana, M; Benedetti, A; Polizzi, S
Thin Solid Films, 598 (2016) 88-94


Eu:GdVO4 nanophosphors with an average size of 60 nm, synthesized by a facile solvothermal method, were deposited on monocrystalline silicon wafers by a spray-coating technique with artworks anti-counterfeiting applications in mind. Atmospheric pressure plasma jet (APPJ) was used to deposit a silica-based layer on top of the nanometric luminescent layer, in order to improve its adhesion to the substrate and to protect it from the environment. The nanophosphors were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Coating composition was investigated by Fourier transform infrared spectroscopy (FT-IR) and its morphology was characterized by scanning electron microscopy (FEG-SEM). The film thickness was evaluated by means of ellipsometry and adhesion was estimated by a peeling test. Luminescent properties of the nanophosphors deposited and fixed on silicon wafers were also measured. The whole layer resulted well-adhered to the silicon substrate, transparent and undetectable in the presence of visible light, but easily activated by UV light source.

January, 2016 | DOI: 10.1016/j.tsf.2015.11.061


Design and realization of transparent solar modules based on luminescent solar concentrators integrating nanostructured photonic crystals

Jimenez-Solano, A; Delgado-Sanchez, JM; Calvo, ME; Miranda-Munoz, JM; Lozano, G; Sancho, D; Sanchez-Cortezon, E; Miguez, H
Progress in Photovoltaics, 23 (2015) 1785-1792


Herein, we present a prototype of a photovoltaic module that combines a luminescent solar concentrator integrating one-dimensional photonic crystals and in-plane CuInGaSe2 (CIGS) solar cells. Highly uniform and wide-area nanostructured multilayers with photonic crystal properties were deposited by a cost-efficient and scalable liquid processing amenable to large-scale fabrication. Their role is to both maximize light absorption in the targeted spectral range, determined by the fluorophore employed, and minimize losses caused by emission at angles within the escape cone of the planar concentrator. From a structural perspective, the porous nature of the layers facilitates the integration with the thermoplastic polymers typically used to encapsulate and seal these modules. Judicious design of the module geometry, as well as of the optical properties of the dielectric mirrors employed, allows optimizing light guiding and hence photovoltaic performance while preserving a great deal of transparency. Optimized in-plane designs like the one herein proposed are of relevance for building integrated photovoltaics, as ease of fabrication, long-term stability and improved performance are simultaneously achieved.

November, 2015 | DOI: 10.1002/pip.2621

Adaptable Ultraviolet Reflecting Polymeric Multilayer Coatings of High Refractive Index Contrast

Smirnov, JRC; Ito, M; Calvo, ME; Lopez-Lopez, C; Jimenez-Solano, A; Galisteo-Lopez, JF; Zavala-Rivera, P; Tanaka, K; Sivaniah, E; Miguez, H
Advanced Optical Materials, 3 (2015) 1633-1639


A synthetic route is demonstrated to build purely polymeric nanostructured multilayer coatings, adaptable to arbitrary surfaces, and capable of efficiently blocking by reflection a targeted and tunable ultraviolet (UV) range. Reflection properties are determined by optical interference between UV light beams reflected at the interfaces between polystyrene layers of different porosity and hence refractive index. As no dopant absorber intervenes in the shielding effect, polymer degradation effects are prevented. Alternated porosity results from the modulation of photochemical effects at the few tens of nanometers length scale, combined with the collective osmotic shock induced during the processing of the precursor diblock copolymer film. Experimental evidence of the application of this method to coat rough surfaces with smooth and conformal UV protecting films is provided.

October, 2015 | DOI: 10.1002/adom.201500209

Single-step fabrication process of 1-D photonic crystals coupled to nanocolumnar TiO2 layers to improve DSC efficiency

Gonzalez-Garcia, L; Colodrero, S; Miguez, H; Gonzalez-Elipe, AR
Optics Express, 23 (2015) A1642-A1650


The present work proposes the use of a TiO2 electrode coupled to a one-dimensional photonic crystal (1DPC), all formed by the sequential deposition of nanocolumnar thin films by physical vapor oblique angle deposition (PV-OAD), to enhance the optical and electrical performance of DSCs while transparency is preserved. We demonstrate that this approach allows building an architecture combining a non-dispersive 3 µm of TiO2 electrode and 1 µm TiO2-SiO2 1DPC, both columnar, in a single-step process. The incorporation of the photonic structure is responsible for a rise of 30% in photovoltaic efficiency, as compared with a transparent cell with a single TiO2 electrode. Detailed analysis of the spectral dependence of the photocurrent demonstrates that the 1DPC improves light harvesting efficiency by both back reflection and optical cavity modes confinement within the TiO2 films, thus increasing the overall performance of the cell.

October, 2015 | DOI: 10.1364/OE.23.0A1642

Full solution processed mesostructured optical resonators integrating colloidal semiconductor quantum dots

Calvo, ME; Hidalgo, N; Schierholz, R; Kovacs, A; Fernandez, A; Bellino, MG; Soler-Illia, GJAA; Miguez, H
Nanoscale, 7 (2015) 16583-16589


Herein we show a solution based synthetic pathway to obtain a resonant optical cavity with embedded colloidal semiconductor quantum dots (CSQDs). The optical cavity pore network, surrounded by two dense Bragg mirrors, was designed ad hoc to selectively host the quantum dots, while uncontrolled infiltration of those in the rest of the layered structure was prevented. Coupling between the optical resonant modes of the host and the natural emission of the embedded nanoparticles gives rise to the fine tuning of the luminescence spectrum extracted from the ensemble. Our approach overcomes, without the need for an encapsulating agent and exclusively by solution processing, the difficulties that arise from the low thermal and chemical stability of the CSQDs. It opens the route to achieving precise control over their location and hence over the spectral properties of light emitted by these widely employed nanomaterials. Furthermore, as the porosity of the cavity is preserved after infiltration, the system remains responsive to environmental changes, which provides an added value to the proposed structure.

September, 2015 | DOI: 10.1039/C5NR03977K

Synergistic strategies for the preparation of highly efficient dye-sensitized solar cells on plastic substrates: combination of chemical and physical sintering

Li, Y; Yoo, K; Lee, DK; Kim, JY; Son, HJ; Kim, JH; Lee, CH; Miguez, H; Ko, MJ
RSC Advances, 5 (2015) 76795-76803


Preparation of well-interconnected TiO2 electrodes at low temperature is critical for the fabrication of highly efficient dye-sensitized solar cells (DSCs) on plastic substrates. Herein we explore a synergistic approach using a combination of chemical and physical sintering. We formulate a binder-free TiO2 paste based on “nanoglue” as the chemical sintering agent, and use it to construct a photoelectrode on plastic by low-temperature physical compression to further improve the connectivity of TiO2 films. We systematically investigated the factors affecting the photovoltaic performance and found the conditions to achieve electron diffusion lengths as long as 25 μm and charge collection efficiencies as high as 95%, as electrochemical impedance spectroscopy measurements indicate. We apply this approach to obtain a DSC deposited on plastic displaying 6.4% power conversion efficiency based on commercial P25 titania particles.

September, 2015 | DOI: 10.1039/C5RA10290A

Biocompatible Films with Tailored Spectral Response for Prevention of DNA Damage in Skin Cells

Nunez-Lozano, R; Pimentel, B; Castro-Smirnov, JR; Calvo, ME; Miguez, H; de la Cueva-Mendez, G
Advanced Healthcare Materials, 4 (2015) 1944-1948


A hybrid nanostructured organic–in­organic biocompatible film capable of efficiently blocking a preselected range of ultraviolet light is designed to match the genotoxic action spectrum of human epithelial cells. This stack protects cultured human skin cells from UV-induced DNA lesions. As the shielding mechanism relies exclusively on reflection, the secondary effects due to absorption harmful radiation are prevented

August, 2015 | DOI: 10.1002/adhm.201500223

Uniform Poly(acrylic acid)-Functionalized Lanthanide-Doped LaVO4 Nanophosphors with High Colloidal Stability and Biocompatibility

Nunez, NO; Zambrano, P; Garcia-Sevillano, J; Cantelar, E; Rivera-Fernandez, S; de la Fuente, JM; Ocana, M
European Journal of Inorganic Chemistry, 27 (2015) 4546-4554


Ln-doped (Ln = Eu or Nd) LaVO4 nanoparticles functionalized with poly(acrylic acid) (PAA) were prepared from lanthanide and vanadate precursors in the presence of PAA by a simple one-pot method that consists of a homogeneous precipitation reaction in ethylene glycol/water at a moderate temperature (120 degrees C). The size of the nanoparticles could be modified in the 40-70 nm range by adjusting the amount of PAA added. The effects of the Eu and Nd contents of these nanomaterials on theirs optical properties (emission intensity and lifetime) were also analyzed to find the optimum nanophosphors. Finally, the nanoparticles showed negligible cytotoxicity for Vero cells at concentrations up to 0.05 mgmL(-1) and a high colloidal stability in physiological buffer solutions; therefore, they satisfy the most important requirements for in vitro biotechnological applications.

August, 2015 | DOI: 10.1002/ejic.201500265

Absorption Enhancement in Organic-Inorganic Halide Perovskite Films with Embedded Plasmonic Gold Nanoparticles

Carretero-Palacios, S; Calvo, ME; Miguez, H
Journal of Physical Chemistry C, 119 (2015) 18635-18640


We report on the numerical analysis of solar absorption enhancement in organic-inorganic halide perovskite films embedding plasmonic gold nanoparticles. The effect of particle size and concentration is analyzed in realistic systems in which random particle location within the perovskite film and the eventual formation of dimers are also taken into account. We find a maximum integrated solar absorption enhancement of similar to 10% in perovskite films of 200 nm thickness and similar to 6% in 300 nm films, with spheres of radii 60 and 90 nm, respectively, in volume concentrations of around 10% in both cases. We show that the presence of dimers boosts the absorption enhancement up to,similar to 12% in the thinnest films considered. Absorption reinforcement arises from a double contribution of plasmonic near-field and scattering effects, whose respective weight can be discriminated and evaluated from the simulations.

July, 2015 | DOI: 10.1021/acs.jpcc.5b06473

BaGa4O7, a new A3BC10O20 crystalline phase: synthesis, structural determination and luminescence properties

Boyer, Marina; Veron, Emmanuel; Becerro, Ana Isabel; Porcher, Florence; Suchomel, Matthew R.; Matzen, Guy; Allix, Mathieu
CrystEngComm, 17 (2015) 6127-6135


The synthesis, structural determination and luminescence properties of a new barium gallate, BaGa4O7, are reported. This crystalline material can uniquely be obtained by direct cooling from the molten state. The crystallographic structure was determined using a combination of electron, synchrotron and neutron powder diffraction data. BaGa4O7 crystallizes in the monoclinic I2/mspace group with a = 15.0688(1) Å, b = 11.7091(1) Å, c = 5.1429(2) Å and β = 91.0452(2)° and can be described as an original member of the A3BC10O20 family. Atypical for this A3BC10O20structural framework, BaGa4O7 is found to contain exclusively divalent and trivalent cations. In order to maintain overall electroneutrality, disordered defect-type partial substitution of gallium and oxygen ions on barium sites occurs within the pentagonal channels of BaGa4O7. Thanks to the flexibility of this structural framework, BaGa4O7 can be heavily doped with europium and thus is shown to exhibit strong orange-red luminescence emission at 618 nm under 393 nm excitation.

July, 2015 | DOI: 10.1039/C5CE01101A

Environmental Effects on the Photophysics of Organic-Inorganic Halide Perovskites

Galisteo-Lopez, JF; Anaya, M; Calvo, ME; Miguez, H
Journal of Physical Chemistry Letters, 6 (2015) 2200-2205


The photophysical properties of films of organic-inorganic lead halide perovskites under different ambient conditions are herein reported. We demonstrate that their luminescent properties are determined by the interplay between photoinduced activation and darkening processes, which strongly depend on the atmosphere surrounding the samples. We have isolated oxygen and moisture as the key elements in each process, activation and darkening, both of which involve the interaction with photogenerated carriers. These findings show that environmental factors play a key role in the performance of lead halide perovskites as efficient luminescent materials.

May, 2015 | DOI: 10.1021/acs.jpclett.5b00785

Fine Tuning the Emission Properties of Nanoemitters in Multilayered Structures by Deterministic Control of their Local Photonic Environment

Alberto Jiménez-Solano, Juan Francisco Galisteo-López and Hernán Míguez
Small, 11 (2015) 2727-2732


Deterministic control on the dynamics of organic nanoemitters is achieved through precise control of its photonic environment. Resonators are fabricated by a combination of spin- and dip-coating techniques, which allows placement of the emitters at different positions within the sample, thus acting as a probe of the local density of states.

May, 2015 | DOI: 10.1002/smll.201402898

An Optically Controlled Microscale Elevator Using Plasmonic Janus Particles

Nedev, S; Carretero-Palacios, S; Kuhler, P; Lohmuller, T; Urban, AS; Anderson, LJE; Feldmann, J
ACS Photonics, 2 (2015) 491-496


In this article, we report how Janus particles, composed of a silica sphere with a gold half-shell, can be not only stably trapped by optical tweezers but also displaced controllably along the axis of the laser beam through a complex interplay between optical and thermal forces. Scattering forces orient the asymmetric particle, while strong absorption on the metal side induces a thermal gradient, resulting in particle motion. An increase in the laser power leads to an upward motion of the particle, while a decrease leads to a downward motion. We study this reversible axial displacement, including a hysteretic jump in the particle position that is a result of the complex pattern of a tightly focused laser beam structure above the focal plane. As a first application we simultaneously trap a spherical gold nanoparticle and show that we can control the distance between the two particles inside the trap. This photonic micron-scale “elevator” is a promising tool for thermal force studies, remote sensing, and optical and thermal micromanipulation experiments.

March, 2015 | DOI: 10.1021/ph500371z

Biosynthesis of silver fine particles and particles decorated with nanoparticles using the extract of Illicium verum (star anise) seeds

Luna, Carlos; Chavez, V. H. G.; Diaz Barriga-Castro, Enrique; Nunez, Nuria O.; Mendoza-Resendez, Raquel
Spectrochimica Acta Part A-Molecular and Biomolecular Spectroscopy, 141 (2015) 43-50


Given the upsurge of new technologies based on nanomaterials, the development of sustainable methods to obtain functional nanostructures has become an imperative task. In this matter, several recent researches have shown that the biodegradable natural antioxidants of several plant extracts can be used simultaneously as reducing and stabilizing agents in the wet chemical synthesis of metallic nanoparticles, opening new opportunities to design greener synthesis. However, the challenge of these new techniques is to produce stable colloidal nanoparticles with controlled particle uniformity, size, shape and aggregation state, in similar manner than the well-established synthetic methods. In the present work, colloidal metallic silver nanoparticles have been synthesized using silver nitrate and extracts of Illicium verum (star anise) seeds at room temperature in a facile one-step procedure. The resulting products were colloidal suspensions of two populations of silver nanoparticles, one of them with particle sizes of few nanometers and the other with particles of tens of nm. Strikingly, the variation of the AgNO3/extract weight ratio in the reaction medium yielded to the variation of the spatial distribution of the nanoparticles: high AgNO3/extract concentration ratios yielded to randomly dispersed particles, whereas for lower AgNO3/extract ratios, the biggest particles appeared coated with the finest nanoparticles.-This biosynthesized colloidal system, with controlled particle aggregation states, presents plasmonic and SERS properties with potential applications in molecular sensors and nanophotonic devices. 

March, 2015 | DOI: 10.1016/j.saa.2014.12.076

Highly Efficient Perovskite Solar Cells with Tunable Structural Color

W. Zhang, M. Anaya, G. Lozano, M.E. Calvo, M.B. Johnston, H. Míguez, H.J. Snaith
Nano Letters, 15 (2015) 1698-1702


The performance of perovskite solar cells has been progressing over the past few years and efficiency is likely to continue to increase. However, a negative aspect for the integration of perovskite solar cells in the built environment is that the color gamut available in these materials is very limited and does not cover the green-to-blue region of the visible spectrum, which has been a big selling point for organic photovoltaics. Here, we integrate a porous photonic crystal (PC) scaffold within the photoactive layer of an opaque perovskite solar cell following a bottom-up approach employing inexpensive and scalable liquid processing techniques. The photovoltaic devices presented herein show high efficiency with tunable color across the visible spectrum. This now imbues the perovskite solar cells with highly desirable properties for cladding in the built environment and encourages design of sustainable colorful buildings and iridescent electric vehicles as future power generation sources.

February, 2015 | DOI: 10.1021/nl504349z

Sunlight Absorption Engineering for Thermophotovoltaics: Contributions from the Optical Design

Miguez, H
ChemSusChem, 8 (2015) 786-788


Nowadays, solar thermophotovoltaic systems constitute a platform in which sophisticated optical material designs are put into practice with the aim of achieving the long sought after dream of developing an efficient energy conversion device based on this concept. Recent advances demonstrate that higher efficiencies are at reach using photonic nanostructures amenable to mass production and scale-up.

February, 2015 | DOI: 10.1002/cssc.201403361

Nanolevitation Phenomena in Real Plane-Parallel Systems Due to the Balance between Casimir and Gravity Forces

Esteso, V; Carretero-Palacios, S; Miguez, H
Journal of Physical Chemistry C, 119 (2015) 5663-5670


We report on the theoretical analysis of equilibrium distances in real plane-parallel systems under the influence of Casimir and gravity forces at thermal equilibrium. Due to the balance between these forces, thin films of Teflon, silica, or polystyrene in a single-layer configuration and immersed in glycerol stand over a silicon substrate at certain stable or unstable positions depending on the material and the slab thickness. Hybrid systems containing silica and polystyrene, materials which display Casimir forces and equilibrium distances of opposite nature when considered individually, are analyzed in either bilayer arrangements or as composite systems made of a homogeneous matrix with small inclusions inside. For each configuration, equilibrium distances and their stability can be adjusted by fine-tuning of the volume occupied by each material. We find the specific conditions under which nanolevitation of realistic films should be observed. Our results indicate that thin films of real materials in plane-parallel configurations can be used to control suspension or stiction phenomena at the nanoscale.

February, 2015 | DOI: 10.1021/jp511851z

Up-conversion in Er3+/Yb3+ co-doped LaPO4 submicron-sized spheres

Garcia-Sevillano, J.; Cantelar, E.; Cusso, F.; Ocana, M.
Optical Materials, 41 (2015) 104-107


Er3+/Yb3+ co-doped materials have been extensively used for imaging in biomedical applications using either visible up-converted (UC) or near-infrared (NIR) emissions. The UC spectrum is composed mainly by two Erbium emissions in the green (2H11/2:4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) spectral range, while the NIR spectrum includes Er3+ (λ ∼ 1.5 μm, 4I13/2 → 4I15/2) and Er3+/Yb3+ (λ ∼ 980 nm, 2F5/2 → 2F7/2(Yb3+):4I11/2 → 4I15/2 (Er3+)) transitions; which relative intensities are dependent on several physical parameters. In the present work, we present the preparation and optical characterization of Er3+/Yb3+ co-doped LaPO4 submicron-sized spheres. The luminescence (CW and pulsed) characteristics, after different post-annealing treatments, are studied. It is found that such treatments strongly increment the emission efficiency, possibly due to the suppression of residual impurities. After calcination at 1100 °C the material behaves as an excellent UC and NIR–NIR wavelength converter.

February, 2015 | DOI: 10.1016/j.optmat.2014.10.022

Flexible Distributed Bragg Reflectors from Nanocolumnar Templates

Calvo, ME; Gonzalez-Garcia, L; Parra-Barranco, J; Barranco, A; Jimenez-Solano, A; Gonzalez-Elipe, AR; Miguez, H
Advanced Optical Materials, 3 (2015) 171-175


A flexible distributed Bragg reflector is made by the infiltration of a nanocolumnar array with polydimethyl siloxane oligomers. The high optical reflectance displayed by the final material is a direct consequence of the high refractive index contrast of the columnar layers whereas the structural stability is due to the polymer properties.

January, 2015 | DOI: 10.1002/adom.201400338

Optical Description of Mesostructured Organic-Inorganic Halide Perovskite Solar Cells

Anaya, M; Lozano, G; Calvo, ME; Zhang, W; Johnston, MB; Snaith, HJ; Miguez, H
Journal of Physical Chemistry Letters, 6 (2015) 48-53


Herein we describe both theoretically and experimentally the optical response of solution-processed organic–inorganic halide perovskite solar cells based on mesostructured scaffolds. We develop a rigorous theoretical model using a method based on the propagation of waves in layered media, which allows visualizing the way in which light is spatially distributed across the device and serves to quantify the fraction of light absorbed by each medium comprising the cell. The discrimination between productive and parasitic absorption yields an accurate determination of the internal quantum efficiency. State-of-the-art devices integrating mesoporous scaffolds infiltrated with perovskite are manufactured and characterized to support the calculations. This combined experimental and theoretical analysis provides a rational understanding of the optical behavior of perovskite cells and can be beneficial for the judicious design of devices with improved performance. Notably, our model justifies the presence of a solid perovskite capping layer in all of the highest efficiency perovskite solar cells based on thinner mesoporous scaffolds.

December, 2014 | DOI: 10.1021/jz502351s

Quick synthesis, functionalization and properties of uniform, luminescent LuPO4-based nanoparticles

Becerro, AI; Ocana, M
RSC Advances, 44 (2015) 34517-34524


The aim of this study was to find a surfactant-free method for the synthesis of uniform Eu:LuPO4nanophosphors which are able to form stable colloidal suspensions in aqueous media. Uniform, ovoid Eu-doped LuPO4 fluorescent nanoparticles were obtained after aging for 30 minutes at 180 °C a butylene glycol solution containing, exclusively, lutetium acetate, europium acetate and H3PO4. XRD and digital diffraction patterns of HRTEM images suggested that the particles were single crystalline in nature with the c-axis of the unit cell parallel to the long particle axis. The luminescence study revealed that the optimum doping level was 5 molar%. The latter particles (85 nm × 40 nm dimensions) were functionalized with polyacrylic acid and their colloidal stability in two different biological buffers was demonstrated to persist for at least 15 days.

December, 2014 | DOI: 10.1039/C5RA05305F

Template-free synthesis and luminescent properties of hollow Ln:YOF (Ln = Eu or Er plus Yb) microspheres

Martinez-Castro, E; Garcia-Sevillano, J; Cusso, F; Ocana, M
Journal of Alloys and Compounds, 619 (2015) 44-51


A method for the synthesis of hollow lanthanide doped yttrium oxyfluoride (YOF) spheres in the micrometer size range with cubic structure based on the pyrolysis at 600 degrees C of liquid aerosols generated from aqueous solutions containing the corresponding rare earth chlorides and trifluoroacetic acid has been developed. This procedure, which has been used for the first time for the synthesis of YFO based materials, is simpler and advantageous when compared with other methods usually employed for the production of hollow spheres since it does not require the use of sacrificial templates. In addition, it is continuous, which is desirable because of practical reasons. The procedure is also suitable for doping the YOF spheres with europium cations resulting in down converting red phosphors when activated with UV light, or for co-doping with both Er3+ and Yb3+ giving rise to up-converting phosphors, which emit intense red light under near infrared (NIR) irradiation. Because of their optical properties and hollow architecture, the developed materials may find applications in optoelectronic devices and biotechnology. 

December, 2014 | DOI: 10.1016/j.jallcom.2014.09.023


One-Step Synthesis and Polyacrylic Acid Functionalization of Multifunctional Europium-Doped NaGdF4 Nanoparticles with Selected Size for Optical and MRI Imaging

Nunez, NO; Garcia, M; Garcia-Sevillano, J; Rivera-Fernandez, S; de la Fuente, JM; Ocana, M
European Journal of Inorganic Chemistry, 35 (2014) 6075-6084


Multifunctional Eu:NaGdF4 nanospheres functionalized with polyacrylic acid (PAA) polymer have been prepared for the first time by a simple one-pot method that consists of a homogeneous precipitation reaction at 120 °C. The size of the nanospheres, which were polycrystalline and crystallized into a hexagonal structure, could be altered in the 60–95 nm range by adjusting the amount of polyacrylic acid added. The effects of Eu content and particle size of these nanomaterials on their optical properties (emission intensity and lifetime) as well as on their relaxivity (r1 and r2) values were also analyzed to find the optimum system for optical bioimaging and as a positive contrast agent for magnetic resonance imaging (MRI) applications. Finally, such optimum nanoparticles showed negligible cytotoxicity for Vero cells for concentrations up to 0.5 mg mL–1 and a high colloidal stability in 2-morpholinoethanesulfonic acid solutions, thereby satisfying the most important requirements for their use in biotechnological applications.

November, 2014 | DOI: 10.1002/ejic.201402690

Morphological and structural behavior of TiO2 nanoparticles in the presence of WO3: crystallization of the oxide composite system

Kubacka, A; Iglesias-Juez, A; di Michiel, M; Becerro, AI; Fernandez-Garcia, M
Physical Chemistry Chemical Physics, 16 (2014) 19540-19549


Composite TiO2-WO3 oxide materials were prepared by a single pot microemulsion method and studied during calcination treatments under dry air in order to analyze the influence of tungsten on the behavior of the dominant titania component. To this end, the surface and bulk morphological and structural evolution of the solid precursors was studied using X-ray diffraction and infrared spectroscopy. In the calcination process, differences in the dominant titania component behavior appeared as a function of the W/Ti atomic ratio of the precursor. First, the crystallization of the anatase phase is affected by tungsten through an effect on the primary particle size growth. Furthermore, such an effect also influences the anatase to rutile phase transformation. The study provides evidence that the W-Ti interaction develops differently for a low/high W/Ti atomic ratio below/above 0.25 affecting fundamentally the above-mentioned anatase primary particle size growth process and the subsequent formation of the rutile phase and showing that addition of tungsten provides a way to control morphology and phase behavior in anatase-based oxide complex materials.

August, 2014 | DOI: 10.1039/c4cp02181a

Multidirectional Light-Harvesting Enhancement in Dye Solar Cells by Surface Patterning

Lopez-Lopez, C; Colodrero, S; Jimenez-Solano, A; Lozano, G; Ortiz, R; Calvo, ME; Miguez, H
Advanced Optical Materials, 2 (2014) 879-884


One dimensional gratings patterned on the surface of nanocrystalline titania electrodes are used as a light harvesting strategy to improve the overall performance of dye solar cells under both frontal and rear illumination conditions. A soft-lithography-based micromoulding approach is employed to replicate a periodic surface relief pattern onto the surface of the electrode, which is later sensitized with a dye. As the patterned surface acts as an optical grating both in reflection and transmission modes, its effect is to increase the light path of diffracted beams within the absorbing layer when it is irradiated either from the electrode or the counter electrode for a broad range of angles of incidence on each surface. Full optical and photovoltaic characterization demonstrates not only the optical quality of the patterned surfaces but also the multidirectional character of the enhancement of light harvesting and conversion efficiency. The approach herein presented thus permits to preserve the operation of the cell when irradiated from its two faces while increasing its overall power conversion efficiency. This feature is a key advantage over other light harvesting efficiency enhancing methods, such as the deposition of a back diffuse scattering layer, in which the performance of the cell under illumination from one of its sides is enlarged at the expense of reducing the output under reverse irradiation conditions.

August, 2014 | DOI: 10.1002/adom.201400160

Tailor-made directional emission in nanoimprinted plasmonic-based light-emitting devices

Lozano, G; Grzela, G; Verschuuren, MA; Ramezani, M; Rivas, JG
Nanoscale, 6 (2014) 9223-9229


We demonstrate an enhanced and tailor-made directional emission of light-emitting devices using nanoimprinted hexagonal arrays of aluminum nanoparticles. Fourier microscopy reveals that the, luminescence of the device is not only determined by the material properties of the organic dye molecules but is also strongly influenced by the coherent scattering resulting from periodically arranged metal nanoparticles. Emitters can couple to lattice-induced hybrid plasmonic-photonic modes sustained by plasmonic arrays. Such modes enhance the spatial coherence of an emitting layer, allowing the efficient beaming of the emission along narrow angular and spectral ranges. We show that tailoring the separation of the nanoparticles in the array yields an accurate angular distribution of the emission. This combination of large-area metal nanostructures fabricated by nanoimprint lithography and light-emitting devices is beneficial for the design and optimization of solid-state lighting systems.

July, 2014 | DOI: 10.1039/c4nr01391c

New Single-Phase, White-Light-Emitting Phosphors Based on delta-Gd2Si2O7 for Solid-State Lighting

Fernandez-Carrion, AJ; Ocana, M; Garcia-Sevillano, J; Cantelar, E; Becerro, AI
Journal of Physical Chemistry C, 118 (2014) 18035-18043


Two new white-light (WL)-emitting phosphors (δ-Gd2Si2O7:Dy and δ-Gd2Si2O7:Eu,Tb) have been synthesized by the sol–gel method. The Gd-Ln3+ (Ln3+= Dy3+, Tb3+, Eu3+) energy-transfer band has been used to excite both phosphors, which provides an enhancement of the Ln3+ emissions. First, WL was generated from δ-Gd2Si2O7:xDy thanks to the particular ratio of the blue and yellow emissions observed in all three compositions, which had chromatic coordinates of x = 0.30, y = 0.33 and CCT values of between 7077 and 6721 K. The decay curves of the main transitions of Dy3+ showed a maximum lifetime value for δ-Gd2Si2O7:0.5%Dy, which is, therefore, the most efficient doping level. Second, a broad spectral range, single-phase, WL-emitting phosphor was generated by codoping δ-Gd2Si2O7 with Tb3+ and Eu3+. The composition δ-Gd2Si2O7:0.3%Eu3+;0.8%Tb3+ showed CIE coordinates well inside the ideal WL region of the CIE diagram and a CCT value of 5828 K. The single-phase WL-emitting phosphors presented in this paper are promising materials to be used in white solid-state lighting systems and field-emission displays due to the advantages provided both by Gd3+ ions and by the high thermal and chemical stabilities of the rare earth disilicate matrix.

July, 2014 | DOI: 10.1021/jp505524g

Bifunctional, Monodisperse BiPO4-Based Nanostars: Photocatalytic Activity and Luminescent Applications

Becerro, AI; Criado, J; Gontard, LC; Obregon, S; Fernandez, A; Colon, G; Ocana, M
Crystal Growth & Design, 14 (2014) 3319-3326


Monodisperse, monoclinic BiPO4 nanostars have been synthesized by a homogeneous precipitation reaction at 120 °C through controlled release of Bi3+ cations from a Bi–citrate chelate, in a mixture of glycerol and ethylene glycol, using H3PO4 as the phosphate source. The set of experimental conditions necessary to obtain uniform nanoparticles is very restrictive, as the change in either the polyol ratio or the reactant concentrations led to ill-defined and/or aggregated particles. The morphology of the particles consists of a starlike, hierarchical structure formed by the ordered arrangement of nanorod bundles. Transmission electron tomography has revealed that the nanostars are not spherical but flattened particles. Likewise, Fourier transform infrared spectroscopy and thermogravimetry have shown that the synthesized nanostars are functionalized with citrate groups. The mechanism of formation of the nanostars has been analyzed to explain their morphological features. The as-synthesized BiPO4 nanostars exhibit an efficient photocatalytic performance for the degradation of Rhodamine B. Finally, it has been demonstrated that the stars can be Eu3+-doped up to 2 mol % without any change in the particle morphology or symmetry, and the doped samples show emission in the orange-red region of the visible spectrum after ultraviolet excitation. These experimental observations make this material a suitable phosphor for biotechnological applications.

June, 2014 | DOI: 10.1021/cg500208h

Synthesis and luminescence of uniform europium-doped bismuth fluoride and bismuth oxyfluoride particles with different morphologies

A. Escudero; E. Moretti; M. Ocaña
CrysEngComm, 16 (2014) 3274-3283


Facile synthesis routes have been developed for the preparation of uniform cubic bismuth fluoride and bismuth oxyfluoride particles. The synthesis methods are based on homogeneous precipitation reactions at 120 °C in solutions of bismuth nitrate and sodium tetrafluoroborate precursors in polyol-based solvents. Both the nature of the solvent and the heating modes (conventional or microwave-assisted heating) have a remarkable effect on the morphology and crystallinity of the resulting particles. Thus, polycrystalline spheres of α-BiF3 with a mean diameter ranging from 1.2 to 2 μm could be obtained by heating solutions with the appropriate reagent concentrations in a mixture of ethylene glycol and glycerol (1 : 1 by volume) using a conventional oven, whereas octahedral single crystals of α-BiOyF3−2y with mean edges ranging from 250 nm to 920 nm precipitated when using a diethylene glycol–water mixture (8 : 2 in volume) as solvent and a microwave reactor for heating. To explain these different morphological and structural features, the mechanism of formation of such particles was investigated. Both kinds of particles were also doped with Eu3+, and both the morphological and luminescence properties of the resulting materials were evaluated. It was found that the luminescence intensity of the europium-doped α-BiOyF3−2y nanoparticles was higher than that of the europium-doped α-BiF3 sub-micrometric spheres, which was associated with the higher crystallinity of the former. Moreover, the presence of oxygen in the europium-doped α-BiOyF3−2y samples permits the excitation of the europium cations through an Eu–O energy transfer process, which results in a much higher luminescence intensity with respect to that corresponding to the direct excitation of the europium cations. Finally, the effect of the amount of dopant on the luminescence properties of the phosphors was also evaluated.

March, 2014 | DOI: 10.1039/C3CE42462F

Dye sensitized solar cells as optically random photovoltaic media

Galvez, FE; Barnes, PRF; Halme, J; Miguez, H
Energy & Environmental Science, 6 (2014) 1260-1266


In order to enhance optical absorption, light trapping by multiple scattering is commonly achieved in dye sensitized solar cells by adding particles of a different sort. Herein we propose a theoretical method to find the structural parameters (particle number density and size) that optimize the conversion efficiency of electrodes of different thicknesses containing spherical inclusions of diverse composition. Our work provides a theoretical framework in which the response of solar cells containing diffuse scattering particles can be rationalized. Optical simulations are performed by combining a Monte Carlo approach with Mie theory, in which the angular distribution of scattered light is accounted for. Several types of scattering centers, such as anatase, gold and silver particles, as well as cavities, are considered and their effect compared. Estimates of photovoltaic performance, insight into the physical mechanisms responsible for the observed enhancements, and guidelines to improve the cell design are provided. We discuss the results in terms of light transport in weakly disordered optical media and find that the observed variations between the optimum scattering configurations attained for different electrode thicknesses can be understood as the result of the randomization of the light propagation direction at different depths within the active layer. A primary conclusion of our study is that photovoltaic performance is optimised when the scattering properties of the film are adjusted so that the distance over which incident photons are randomized is comparable to the thickness of the film. This simple relationship could also be used as a design rule to attain the optimum optical design in other photovoltaic materials.

January, 2014 | DOI: 10.1039/C3EE42587H

Fully stable numerical calculations for finite one-dimensional structures: Mapping the transfer matrix method

Luque-Raigon, JM; Halme, J; Miguez, H
Journal of Quantitative Spectroscopy and Radiative Transfer, 134 (2014) 9-20


We design a fully stable numerical solution of the Maxwell's equations with the transfer matrix method (TMM) to understand the interaction between an electromagnetic field and a finite, one-dimensional, non-periodic structure. Such an exact solution can be tailored from a conventional solution by choosing an adequate transformation between its reference systems, which induces a mapping between its associated TMMs. The paper demonstrates theoretically the numerical stability of the TMM for the exact solution within the framework of Maxwell's equations, but the same formalism can efficiently be applied to resolve other classical or quantum linear wave-propagation interaction in one, two, and three dimensions. This is because the formalism is exclusively built up for an in depth analysis of the TMM's symmetries.

January, 2014 | DOI: 10.1016/j.jqsrt.2013.10.007

Nanometer-Scale Precision Tuning of 3D Photonic Crystals Made Possible Using Polyelectrolytes with Controlled Short Chain Length and Narrow Polydispersity

Wang, Z; Calvo, ME; Masson, G; Arsenault, AC; Peiris, F; Mamak, M; Miguez, H; Manners, I; Ozin, GA
Advanced Materials Interfaces, 1 (2014) Art. 1300051


Nanometer-scale tuning of the optical properties of prefabricated photonic crystals is achieved via layer-by-layer assembly of polyelectrolytes in the interstitial spaces of the photonic lattice. The key to the approach is using polyelectrolytes with controlled short chain lengths. This ensures they do not block the air voids, thereby maintaining uniform coating and thus precise and reproducible optical 

January, 2014 | DOI: 10.1002/admi.201300051

Panchromatic porous specular back reflectors for efficient transparent dye solar cells

Lopez-Lopez, C; Colodrero, S; Miguez, H
Physical Chemistry Chemical Physics, 16 (2014) 663-668


A panchromatic specular reflector based dye solar cell is presented herein. Photovoltaic performance of this novel design is compared to that of cells in which standard diffuse scattering layers are integrated. The capability of the proposed multilayer structures to both emulate the broad band reflection of diffuse scattering layers of standard thickness (around 5 microns) and give rise to similarly high light harvesting and power conversion efficiencies, yet preserving the transparency of the device, is demonstrated. Such white light reflectors are comprised of stacks of different porous optical multilayers, each one displaying a strong reflection in a complementary spectral range, and are designed to leave transmittance unaltered in a narrow red-frequency range in which the sensitized electrode shows negligible absorption, thus allowing us to see through the cell. The reflectance bandwidth achieved is three times as broad as the largest bandwidth previously achieved using any photonic structure integrated into a dye solar cell.

December, 2013 | DOI: 10.1039/C3CP53939C

Synthesis of antibacterial silver-based nanodisks and dendritic structures mediated by royal jelly

Mendoza-Resendez, R; Gomez-Trevino, A; Barriga-Castro, ED; Nunez, NO; Luna, C
RSC Advances, 4 (2014) 1650-1658



The one-step preparation of silver nanoparticles and dendritic structures mediated by aqueous royal jelly solutions has been investigated for the first time. It has been found that royal jelly (RJ) is a complex organic matrix that can be simultaneously used as a reducing and stabilizing agent in the chemical synthesis of colloidal silver-based nanostructures from aqueous AgNO3 solutions, without the requirement of additional reagents or heating sources to initiate the oxidation–reduction reactions. The resulting product consisted of very fine single-crystal disks of Ag and silver 4,4′-dimethyldiazoaminobenzene (a triazenic compound). Both kinds of particles tended to coalesce and form supramolecular dendritic structures, the AgNO3/RJ weight ratio chosen in the synthesis being a key parameter to control the crystal growth and the microstructural properties of the Ag nanodisks. Data obtained from Fourier transform infrared and Raman spectroscopy analysis indicated that these nanostructures are coated by RJ biomolecules (residues of proteins and carbohydrates). In vitro biological assays showed that these nanostructures exhibit a promising enhanced antibacterial activity against both Gram-positive and Gram-negative bacteria.

December, 2013 | DOI: 10.1039/C3RA45680C


Symmetry analysis of the numerical instabilities in the transfer matrix method

Luque-Raigon, JM; Halme, J; Miguez, H; Lozano, G
Journal of Optics, 15 (2013) 125719


This paper discusses the numerical exponential instability of the transfer matrix method (TMM) in the framework of the symmetry formalism. This numerical weakness is attributed to a series of increasingly extreme exponentials that appear in the TMM when it is applied to geometries involving total internal reflection (TIR) or very high absorption. We design a TMM formalism that identifies the internal symmetries of the multilayer geometry. These symmetries suggest particular transformations of a reference system in the TMM that improve its ill-conditioned exponentials. To illustrate the numerical improvements, we present examples with calculations of electric fields.

November, 2013 | DOI: 10.1088/2040-8978/15/12/125719

Crystal Structures and Photoluminescence across the La2Si2O7–Ho2Si2O7 System

Fernandez-Carrion, AJ; Allix, M; Ocana, M; Garcia-Sevillano, J; Cusso, F; Fitch, AN; Suard, E; Becerro, AI
Inorganic Chemistry, 52 (2013) 13469-13479


The La2Si2O7−Ho2Si2O7 system displays a solid solubility region of G-(La,Ho)2Si2O7 which extends to the La0.6Ho1.4Si2O7 composition. Compositions richer in Ho3+ show a two-phase domain (G+δ), while δ-(La,Ho)2Si2O7 is the stable phase for Ho3+ contents higher than La0.2Ho1.8Si2O7. A preferential occupation of Ho for the RE2 site of the G-unit cell is observed. Luminescence measurements have shown that the lifetimes remain unchanged in the range 0.5% < [Ho3+] < 10%, and only above this value does concentration quenching become operative.

October, 2013 | DOI: 10.1021/ic401867c

Synthesis of metallic silver nanoparticles and silver organometallic nanodisks mediated by extracts of Capsicum annuum var. aviculare (piquin) fruits

Mendoza-Resendez, R; Nunez, NO; Barriga-Castro, ED; Luna, C
RSC Advances, 43 (2013) 20765-20771


Silver-based nanostructures were prepared through reduction/oxidation reactions of aqueous silver nitrate solutions mediated by extracts of red fruits of the piquin pepper (Capsicum annuum var. aviculare) at room temperature. Detailed morphological and microstructural studies using X-ray diffraction, conventional and high-resolution transmission electron microscopy and selected area electron diffraction revealed that the product was constituted by three kinds of nanoparticles. One of them was composed of twinned metallic silver nanoparticles with a size of few nanometers. Other kind of particles was ultrafine disk-like single crystals of silver 4,4′-dimethyldiazoaminobenzene, being in our best knowledge the first time that this compound is reported in the form of nanoparticles. Both kinds of nanoparticles experienced processes of self-assembly and subsequent grain growth to form the third kind of nanoparticles. Such resulting nanostructures are monocrystalline and flattened metallic silver nanoparticles that have diameters around tens of nanometers, the [112] direction perpendicular to the particle plane, and are coated by a surface organometallic layer and residues of biomolecules. The ultraviolet-visible spectrum of the biosynthesized product showed a surface plasmon resonance (SPR) extinction band with an absorbance maximum at around 400 nm, thereby confirming the presence of fine Ag particles. Studies carried out by Fourier transform infrared spectroscopy indicated that the principal active compounds responsible of the reduction of the Ag ions are proteins and capsaicin (through the amino groups) and phenolic compounds (through hydroxyl groups).

October, 2013 | DOI: 10.1039/C3RA43524E

Perfectly Transparent Sr3Al2O6 Polycrystalline Ceramic Elaborated from Glass Crystallization

Alahrache, S; Al Saghir, K; Chenu, S; Veron, E; Meneses, DD; Becerro, AI; Ocana, M; Moretti, F; Patton, G; Dujardin, C; Cusso, F; Guin, JP; Nivard, M; Sangleboeuf, JC; Matzen, G; Allix, M
Chemistry of Materials, 25 (2013) 4017-4024


The highly visible and infrared (up to 6 mu m) transparent Sr3Al2O6 polycrystalline ceramic was obtained by full crystallization of the corresponding glass composition. The glass synthesis and the direct congruent crystallization processes are described, and the material transparency is discussed in light of its microstructure. This new transparent ceramic exhibits a high density (i.e., complete absence of porosity) and micrometer-scale crystallites with very thin grain boundaries. These microstructural characteristics, inherent to the preparation method, minimize light scattering and demonstrate the advantages of this synthesis route compared to the high-pressure process used for the few reported transparent polycrystalline materials. This Sr3Al2O6 ceramic shows a H = 10.5 GPa hardness, a E-r = 150 GPa reduced elasticity modulus, and a 9.6 x 10(-6) K-1 thermal expansion coefficient. Such a transparent strontium aluminate ceramic opens the way to a wide range of applications, especially photonics when doped by various doping agents. As examples, the luminescence of Sr3Al2O6:Eu3+ and Sr3Al2O6:Er3+, which show strong emissions in the visible and infrared ranges, respectively, is presented. Moreover, the Sr3Al2O6:Ce3+ material was found to exhibit scintillation properties under X-ray excitation. Interestingly, the analogous Sr3Ga2O6 transparent polycrystalline ceramic material could equally be prepared using the same elaboration method, although its hygroscopicity prevents the preservation of its high transparency under normal conditions. The establishment of the key factors for the transparency of this economical and innovative synthesis method should enable the prediction of new classes of technologically relevant transparent ceramics.

September, 2013 | DOI: 10.1021/cm401953d

Monoclinic–Tetragonal Heterostructured BiVO4 by Yttrium Doping with Improved Photocatalytic Activity

Usai, S; Obregon, S; Becerro, AI; Colon, G
Journal of Physical Chemistry C, 117 (2013) 24479-24484


Yttrium-doped BiVO4 has been synthesized by means of a surfactant free hydrothermal method having good photoactivities under sun-like excitation for the degradation of Methylene Blue (MB). From the structural and morphological characterization it has been stated that the presence of Y3+ induces the progressive stabilization of the tetragonal phase and the slight higher surface area values. By following the tetragonal cell parameters, the substitutional incorporation of Y3+ into the BiVO4 tetragonal lattice might be considered. Best photocatalytic performances were attained for the samples with Y3+ content of 3.0 at. % for which the MB degradation rate constant appears 2-fold higher. Furthermore, photoactivities for visible-light-driven O2 evolution demonstrate that the photocatalytic performance of the best Y-doped system (initial rate of O2 evolution, 285 μmol g–1 h–1) was more than 5 times that of undoped m-BiVO4 (initial rate of O2 evolution, 53 μmol g–1 h–1). The occurrence of Y3+ doping and a monoclinic–tetragonal heterostructured BiVO4 system induces the higher photocatalytic activities. PL analysis provides a clear evidence of the lower charge carriers recombination in heterostructured yttrium-doped systems.

September, 2013 | DOI: 10.1021/jp409170y

Crystal Structure and Luminescent Properties of Eu3+-Doped A-La2Si2O7 Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis

Fernandez-Carrion, Alberto J.; Ocana, Manuel; Florian, Pierre; Garcia-Sevillano, Jorge; Cantelar, Eugenio; Fitch, Andrew N.; Suchomel, Matthew R.; Becerro, Ana I.
Journal of Physical Chemistry C, 117 (2013) 20876-20886


Pure A-La2Si2O7 powder has been synthesized through a spray pyrolysis method followed by calcination at 1100 degrees C for 15 h. The crystallographic structure, refined from the synchrotron powder diffraction pattern of the sample, showed tetragonal symmetry with space group P4(1), a = 6.83565(1) angstrom, and c = 24.84133(1) angstrom. The Si-29 and La-139 NMR spectra have been described here for the first time in the literature and could be simulated with four Si and four La resonances, respectively, in good agreement with the presence of four Si and four La crystallographic sites in the unit cell. The same synthesis method was 2 successful for the synthesis of Eu3+-doped A-La2Si2O7 (%Eu = 3-40). The analysis of the unit cell volumes indicated that Eu3+ replaces La3+ in the unit cell for all Eu3+ substitution levels investigated. However, anomalous diffraction data indicated that the La/Eu substitution mechanism was not homogeneous, but Eu much prefers to occupy the RE3 sites. The Eu-doped A-La2Si2O7 phosphors thus synthesized exhibited a strong orange-red luminescence after excitation at 393 nm. Lifetime measurements indicated that the optimum phosphor was that with an Eu3+ content of 20%, which showed a lifetime of 2.3 ms. The quantum yield of the latter was found to be 12% at 393 nm excitation. These experimental observations together with the high purity of the phase obtained by the proposed spray pyrolysis method make this material an excellent phosphor for optoelectronic applications.

September, 2013 | DOI: 10.1021/jp407172z

Small Particle-Size Talc Is Associated with Poor Outcome and Increased Inflammation in Thoracoscopic Pleurodesis

Arellano-Orden, E; Romero-Falcon, A; Juan, JM; Jurado, MO; Rodriguez-Panadero, F; Montes-Worboys, A
Respiration, 86 (2013) 201-209


Rationale: Talc is very effective for pleurodesis, but there is concern about complications, especially acute respiratory distress syndrome. Objectives: It was the aim of this study to investigate if talc with a high concentration of small particles induces greater production of cytokines, and if pleural tumor burden has any influence on the local production and spillover of cytokines to the systemic circulation and eventual complications. Methods: We investigated 227 consecutive patients with malignant effusion submitted to talc pleurodesis. One hundred and three patients received ‘small-particle talc' (ST; containing about 50% particles <10 µm) and 124 received ‘large-particle talc' (with <20% particles <10 µm). Serial samples of both pleural fluid and blood were taken before and 3, 24, 48 and 72 h after thoracoscopy. Also, mesothelial cells were stimulated with both types of talc in vitro. Measurements and Results: Interleukin-8, tumor necrosis factor-α, vascular endothelial growth factor, basic fibroblast growth factor and thrombin-antithrombin complex were measured in all samples. Early death (<7 days after talc) occurred in 8 of 103 patients in the ST and in 1 of 124 in the ‘large-particle talc' group (p = 0.007). Patients who received ST had significantly higher proinflammatory cytokines in pleural fluid and serum after talc application, and also in supernatants of the in vitro study. Pleural tumor burden correlated positively with proinflammatory cytokines in serum, suggesting that advanced tumor states induce stronger systemic reactions after talc application. Conclusions: ST provokes a strong inflammatory reaction in both pleural space and serum, which is associated with a higher rate of early deaths observed in patients receiving it.

August, 2013 | DOI: 10.1159/000342042

Surface modified Eu:GdVO4 nanocrystals for optical and MRI imaging

Nuñez, Nuria O.; Rivera, Sara; Alcantara, David; de la Fuente, Jesus M.; Garcia-Sevillano, Jorge; Ocaña, Manuel
Dalton Transactions, 42 (2013) 10725-10734


A facile solvothermal route has been developed for the preparation of europium doped gadolinium orthovanadate nanoparticles ([similar]70 nm) with tetragonal structure, based on a homogenous precipitation reaction at 120 °C from rare earth precursors (yttrium nitrate and europium nitrate) and sodium orthovanadate solutions using an ethylene glycol–water mixture as the solvent. The effects of the doping level on the luminescence properties were evaluated in order to find the optimum nanophosphors. These nanocrystals were successfully functionalized with amino (two step process) and carboxylate (one-pot process) groups provided by amino-dextran polymers (AMD) and polyacrylic acid (PAA), respectively. It was found that while the luminescent properties of both kinds of functionalized systems were similar, the colloidal stability of the PAA-modified sample was higher, because of which, it was selected to study their cytotoxicity and magnetic properties (relaxivity and phantom analyses) to assess their potentiality as multifunctional probes for both “in vitro” optical biolabels and negative contrast agents for magnetic resonance imaging.

July, 2013 | DOI: 10.1039/C3DT50676B

Thermal Expansion of Rare-Earth Pyrosilicates

Fernandez-Carrion, AJ; Allix, M; Becerro, AI
Journal of the American Ceramic Society, 96 (2013) 2298-2305


The use of RE2Si2O7 materials as environmental barrier coatings (EBCs) and in the sintering process of advanced ceramics demands a precise knowledge of the coefficient of thermal expansion of the RE2Si2O7. High-temperature X-ray diffraction (HTXRD) patterns were collected on different RE2Si2O7 polymorphs, namely A, G, α, β, γ, and δ, to determine the changes in unit cell dimensions. RE2Si2O7 compounds belonging to the same polymorph showed, qualitatively, very similar unit cell parameters behavior with temperature, whereas the different polymorphs of a given RE2Si2O7 compound exhibited markedly different thermal expansion evolution. The isotropy of thermal expansion was demonstrated for the A-RE2Si2O7 polymorph while the rest of polymorphs exhibited an anisotropic unit cell expansion with the biggest expansion directed along the REOx polyhedral chains. The apparent bulk thermal expansion coeficcients (ABCTE) were calculated from the unit cell volume expansion for each RE2Si2O7 compound. All compounds belonging to the same polymorph exhibited similar ABCTE values. However, the ABCTE values differ significantly from one polymorph to the other. The highest ABCTE values correspond to A-RE2Si2O7 compounds, with an average of 12.1 × 10−6 K−1, whereas the lowest values are those of β- and γ-RE2Si2O7, which showed average ABCTE values of ~4.0 × 10−6 K−1.

June, 2013 | DOI: 10.1111/jace.12388

Selective UV Reflecting Mirrors Based on Nanoparticle Multilayers

Smirnov, JRC; Calvo, ME; Miguez, H
Advanced Functional Materials, 23 (2013) 2805-2811


A new type of nanostructured selective ultraviolet (UV) reflecting mirror is presented. Periodic porous multilayers with photonic crystal properties are built by spin-coating-assisted layer-by-layer deposition of colloidal suspensions of nanoparticles of ZrO2 and SiO2 (electronic band gap at λ < 220 nm). These optical filters are designed to block well-defined wavelength ranges of the UVA, UVB, and UVC regions of the electromagnetic spectrum while preserving transparency in the visible. The shielding against those spectral regions arises exclusively from optical interference phenomena and depends only on the number of stacked layers and the refractive index contrast between them. In addition, it is shown that the accessible pore network of the as-deposited multilayer allows preparing thin, flexible, self-standing, transferable, and adaptable selective UV filters by polymer infiltration, without significantly losing reflectance intensity, i.e., preserving the dielectric contrast. These films offer a degree of protection comparable to that of traditional ones, without any foreseeable unwanted secondary effects, such as photodegradation, increase of local temperature or, as is the case for organic absorbers, generation of free radicals, all of which are caused by light absorption.

May, 2013 | DOI: 10.1002/adfm.201202587

Resonant Photocurrent Generation in Dye-Sensitized Periodically Nanostructured Photoconductors by Optical Field Confinement Effects

Anaya, M; Calvo, ME; Luque-Raigon, JM; Miguez, H
Journal of the American Chemical Society, 135 (2013) 7803-7806


Herein we show experimental evidence of resonant photocurrent generation in dye-sensitized periodically nanostructured photoconductors, which is achieved by spectral matching of the sensitizer absorption band to different types of localized photon modes present in either periodic or broken symmetry structures. Results are explained in terms of the calculated spatial distribution of the electric field intensity within the configurations under analysis.

April, 2013 | DOI: 10.1021/ja401096k

Energy transfer efficiency in YF3 nanocrystals: Quantifying the Yb3+ to Tm3+ infrared dynamics

Quintanilla, M; Nuñez, NO; Cantelar, E; Ocaña, M; Cussó, F
Journal of Applied Physics, 113 (2013) 174308 (6 pages)


In this work, we report on the determination of the infrared Yb3+ → Tm3+ energy transfer efficiency in YF3:Yb3+/Tm3+ nanocrystals through the study of Yb3+ dynamics. The obtained results are compared to those previously reported in macrocrystals to analyze possible changes related to size reduction. Luminescence lifetimes are much shorter in the nanoparticles than in bulk samples, a behavior that can be related to Yb3+ → Yb3+ migration and the enhanced surface/volume ratio of the nanoparticles. On the other hand, Yb3+ → Tm3+ energy transfer macroparameter remains unaltered, demonstrating that spectroscopic intrinsic parameters such as radiative and non-radiative probabilities are not affected by size reduction. Finally, a formula that describes Yb3+ lifetime dependence with Yb3+ and Tm3+ concentration is proposed, considering both the effects produced by migration between Yb3+ ions and energy transfer from Yb3+ to Tm3+ ions.

April, 2013 | DOI: 10.1063/1.4803540

Angular response of photonic crystal based dye sensitized solar cells

López López, C.; Colodrero, S.; Calvo, M.E. and Míguez, H.
Energy & Environmental Science, 6 (2013) 1260-1266


Herein we report an experimental analysis of the performance of photonic crystal based dye solar cells (PC-DSCs) as the incident light angle moves away from the normal with respect to the cell surface. Nanoparticle multilayers operating at different wavelength ranges were coupled to the working electrode of a dye solar cell for this study. The interplay between optical and photovoltaic properties with the incident light angle is discussed. We demonstrate that an efficiency enhancement is attained for PC-DSCs at all angles measured, and that rational design of the photonic crystal back mirror leads to a reduction of the photocurrent losses related to the tilt angle of the cell, usually labeled as cosine losses. Angular variations of the cell transparency are also reported and discussed. These angular properties are relevant to the application of these solar devices in building integrated photovoltaics as potential window modules.

March, 2013 | DOI: 10.1039/C3EE23609A

Ionic Liquid Mediated Synthesis and Surface Modification of Multifunctional Mesoporous Eu:GdF3 Nanoparticles for Biomedical Applications

Rodriguez-Liviano, S; Nunez, NO; Rivera-Fernandez, S; de la Fuente, JM; Ocana, M
Langmuir, 29 (2013) 3411-3418


A procedure for the synthesis of multifunctional europium(III)-doped gadolinium(III) fluoride (Eu:GdF3) nanoparticles (85 nm) with quasispherical shape by precipitation at 120 °C from diethylene glycol solutions containing lanthanide chlorides and an ionic liquid (1-Butyl, 2-methylimidazolium tetrafluoroborate) as fluoride source has been developed. These nanoparticles were polycrystalline and crystallized into a hexagonal structure, which is unusual for GdF3. They were also mesoporous (pore size = 3.5 Å), having a rather high BET surface area (75 m2 g–1). The luminescent and magnetic (relaxivity) properties of the Eu:GdF3 nanoparticles have been also evaluated in order to assess their potentiality as “in vitro” optical biolabels and contrast agent for magnetic resonance imaging. Finally, a procedure for their functionalization with aspartic-dextran polymers is also reported. The functionalized Eu:GdF3 nanoparticles presented negligible toxicity for Vero cells, which make them suitable for biotecnological applications.

February, 2013 | DOI: 10.1021/la4001076

LaPO4:Er microspheres with high NIR luminescent quantum yield

Garcia-Sevillano, J; Cantelar, E; Justo, A; Ocana, M; Cusso, F
Materials Chemistry and Physics, 138 (2013) 666-671


Er-doped LaPO4 microspheres have been synthesized by spray pyrolysis and the near infrared (NIR) properties have been characterized. It has been found that, following an adequate post-annealing treatment, the emission properties are remarkably improved. The NIR luminescence intensity is highly enhanced and its decay time increases to a value almost coincident with the reported radiative lifetime, which implies that the quantum yield approaches η ≈ 100%. This improvement in luminescence characteristics is probably related to the suppression of residual OH− radicals, that otherwise act as NIR luminescence quenchers, and to the increase in material's crystallinity.

February, 2013 | DOI: 10.1016/j.matchemphys.2012.12.036

A Novel 3D Architecture of GdPO4 Nanophosphors: Multicolored and White Light Emission

Becerro, AI; Rodriguez-Liviano, S; Fernandez-Carrion, AJ; Ocaña, M
Crystal Growth & Design, 55 (2013) 454-460


Homogeneous monoclinic GdPO4 particles composed of three intersecting lance-shaped crystals forming a penetration twin have been synthesized following a very restrictive, simple, and fast (10 min) method consisting of the hydrothermal reaction of gadolinium acetylacetonate with H3PO4 in a mixture of ethylene glycol and water at 180 °C. Slightly increasing the amount of water in the solvent mixture leads to hexagonal rodlike GdPO4·0.5H2O nanoparticles, whereas the variation of the Gd source, PO4 source, aging temperature, and polyol type gave rise to heterogeneous particles. The synthesis procedure is also suitable for the preparation of Eu3+-, Tb3+-, and Dy3+-doped GdPO4 particles with the same morphology and crystalline structure as the undoped materials. The effect of the doping level on the luminescent properties of the twinlike nanophosphors was evaluated, finding optimum doping levels of 5, 5, and 1% for the Eu3+-, Tb3+-, and Dy3+-doped materials, respectively. The twinlike GdPO4 nanophosphors were found to be more efficient than the rodlike GdPO4 ones in terms of emission intensity. Finally, a solid-state single-phase white-light-emitting nanophosphor has been fabricated for the first time in this system by triply doping the GdPO4 twined particles with appropriate concentrations of Eu3+, Tb3+, and Dy3+ and exciting through the Gd–Ln energy-transfer band at 273 nm. In addition to this energy transfer band, other energy charge transfer processes among the three dopants (Eu3+, Tb3+, and Dy3+) have been observed in the triply doped material.

January, 2013 | DOI: 10.1021/cg301023k

Microwave-Assisted Synthesis of Biocompatible Europium-Doped Calcium Hydroxyapatite and Fluoroapatite Luminescent Nanospindles Functionalized with Poly(acrylic acid)

Escudero, A; Calvo, ME; Rivera-Fernandez, S; de la Fuente, JM; Ocana, M
Langmuir, 29 (2013)


Europium-doped calcium hydroxyapatite and fluoroapatite nanophosphors functionalized with poly(acrylic acid) (PAA) have been synthesized through a one-pot microwave-assisted hydrothermal method from aqueous basic solutions containing calcium nitrate, sodium phosphate monobasic, and PAA, as well as sodium fluoride in the case of the fluoroapatite particles. In both cases a spindlelike morphology was obtained, resulting from an aggregation process of smaller subunits which also gave rise to high specific surface area. The size of the nanospindles was 191 (32) × 40 (5) nm for calcium hydroxyapatite and 152 (24) × 38 (6) nm for calcium fluoroapatite. The luminescent nanoparticles showed the typical red luminescence of Eu3+, which was more efficient for the fluoroapatite particles than for the hydroxyapatite. This is attributed to the presence of OH– quenchers in the latter. The nanophosphors showed negligible toxicity for Vero cells. Both PAA-functionalized nanophosphors showed a very high (up to at least 1 week) colloidal stability in 2-(N-morpholino)ethanesulfonic acid (MES) at pH 6.5, which is a commonly used buffer for physiological pH. All these features make both kinds of apatite-based nanoparticles promising tools for biomedical applications, such as luminescent biolabels and tracking devices in drug delivery systems.

January, 2013 | DOI: 10.1021/la304534f

Synthesis and Properties of Multifunctional Tetragonal Eu:GdPO4 Nanocubes for Optical and Magnetic Resonance Imaging Applications

Rodriguez-Liviano, S; Becerro, AI; Alcantara, D; Grazu, V; de la Fuente, JM; Ocana, M
Inorganic Chemistry, 52 (2013) 647-654


A simple and fast (7 min) procedure for synthesis of gadolinium phosphate nanocubes (edge = 75 nm) based on the microwave-assisted heating at 120 °C of gadolinium acetylacetonate and phosphoric acid solutions in buthylene glycol is reported. These nanocubes were highly crystalline and crystallized into a tetragonal structure, which has not been ever reported for pure gadolinium phosphate. Determination of such crystal structure has been carried out here for the first time in the literature by means of powder X-ray diffraction. The developed synthesis procedure was also successful for preparation of multifunctional europium(III)-doped the gadolinium phosphate nanocubes, which were nontoxic for cells and exhibited strong red luminescence under UV illumination and high transverse relaxivity (r2) values. These properties confer them potential applications as biolabels for in vitro optical imaging and as negative contrast agent for magnetic resonance imaging.

December, 2012 | DOI: 10.1021/ic3016996

Synthesis and functionalization of biocompatible Tb:CePO4 nanophosphors with spindle-like shape

Rodriguez-Liviano, S; Aparicio, FJ; Becerro, AI; Garcia-Sevillano, J; Cantelar, E; Rivera, S; Hernandez, Y; de la Fuente, JM; Ocana, M
Journal of Nanoparticle Research 15 (2013) 15:1402


Monoclinic Tb:CePO4 nanophosphors with a spindle-like morphology and tailored size (in the nanometer and micrometer range) have been prepared through a very simple procedure, which consists of aging, at low temperature (120 °C), ethylene glycol solutions containing only cerium and terbium acetylacetonates and phosphoric acid, not requiring the addition of surfactants or capping agents. The influence of the heating mode (conventional convection oven or microwave oven) and the Tb doping level on the luminescent, structural and morphological features of the precipitated nanoparticles have also been analyzed. This study showed that microwave-assisted heating resulted in an important beneficial effect on the luminescent properties of these nanophosphors. Finally, a procedure for the functionalization of the Tb:CePO4 nanoparticles with aspartic-dextran is also reported. The functionalized nanospindles presented negligible toxicity for Verocells, which along with theirs excellent luminescent properties, make them suitable for biomedical applications.

December, 2012 | DOI: 10.1007/s11051-012-1402-7


Revealing Structural Detail in the High Temperature La2Si2O7–Y2Si2O7 Phase Diagram by Synchrotron Powder Diffraction and Nuclear Magnetic Resonance Spectroscopy

Fernandez-Carrion, AJ; Allix, M; Florian, P; Suchomel, MR; Becerro, AI
Journal of Physical Chemistry C, 116 (2012) 21523-21535


High resolution synchrotron powder XRD, 89Y CPMG NMR, and 139La MAS NMR spectroscopy have been applied to eventually draw the phase diagram of the La2Si2O7–Y2Si2O7 system. The diagram presents a solid solubility region of G-(La,Y)2Si2O7, which extends to the La0.9Y1.1Si2O7 composition at any temperature of this study. Compositions richer in Y show two-phase domains, with G + α at T < 1450 °C and G + δ at T > 1450 °C. The Y-rich extreme is more complex, showing two solid solution regions of δ- and γ-(La,Y)2Si2O7 polymorphs which appear with increasing Y content, respectively. It is interesting to note that the La for Y substitution mechanism in the G-(La,Y)2Si2O7 polymorph is not homogeneous, but a preferential occupation of Y for the RE2 site is observed. Finally, the 89Y and 139La isotropic chemical shift values in G-(La,Y)2Si2O7 have been described here for the first time and assigned to the different RE crystallographic sites of the unit cell.

September, 2012 | DOI: 10.1021/jp305777m

Characterization of Mesoporous Thin Films by Specular Reflectance Porosimetry

Hidalgo, N; Lopez-Lopez, C; Lozano, G; Calvo, ME; Miguez, H
Langmuir, 28 (2012) 13777-13782


The pore size distribution of mesoporous thin films is herein investigated through a reliable and versatile technique coined specular reflectance porosimetry. This method is based on the analysis of the gradual shift of the optical response of a porous slab measured in quasi-normal reflection mode that occurs as the vapor pressure of a volatile liquid varies in a closed chamber. The fitting of the spectra collected at each vapor pressure is employed to calculate the volume of solvent contained in the interstitial sites and thus to obtain adsorption–desorption isotherms from which the pore size distribution and internal and external specific surface areas are extracted. This technique requires only a microscope operating in the visible range attached to a spectrophotometre. Its suitability to analyze films deposited onto arbitrary substrates, one of the main limitations of currently employed ellipsometric porosimetry and quartz balance techniques, is demonstrated. Two standard mesoporous materials, supramolecularly templated mesostructured films and packed nanoparticle layers, are employed to prove the concept proposed herein.

September, 2012 | DOI: 10.1021/la3025793

Optical interference for the matching of the external and internal quantum efficiencies in organic photovoltaic cells

Betancur, R; Martinez-Otero, A; Elias, X; Romero-Gomez, P; Colodrero, S; Miguez, H; Martorell, J
Solar Energy Materials and Solar Cells, 104 (2012) 87-91


We demonstrate experimentally that an appropriate combination of the layer thicknesses in an inverted P3HT:PCBM cell leads to an optical interference such that the EQE amounts to 91% of IQE. We observe that reflectivity between layers is minimized in a wavelength range of more than 100 nm. In that range the EQE closely matches the IQE. The role played by the optical interference in improving the performance of the fabricated solar cells is confirmed by EQE calculated numerically using a model based on the transfer matrix method. Additionally, we observed that a similar cell with an active material 1.7 times thicker exhibited a lower PCE. The poor photon harvesting in the later cell configuration is attributed to an EQE that amounts only to 72% of the IQE.

August, 2012 | DOI: 10.1016/j.solmat.2012.04.047

Introducing structural colour in DSCs by using photonic crystals: interplay between conversion efficiency and optical properties

Colonna, D; Colodrero, S; Lindstrom, H; Di Carlo, A; Miguez, H
Energy & Environmental Science, 5 (2012) 8238-8243


Herein we analyze experimentally the effect that introducing highly reflecting photonic crystals, operating at different spectral ranges, has on the conversion efficiency of dye sensitized solar cells. The interplay between structural colour and cell performance is discussed on the basis of the modified spectral response of the photogenerated current observed and the optical characterization of the cells. We demonstrate that, with the approach herein discussed, it is possible to achieve relatively high efficiencies using thin electrodes while preserving transparency. At the same time, the appearance of the device can be controllably modified, which is of relevance for their potential application in building integrated photovoltaics (BIPV) as window modules.

July, 2012 | DOI: 10.1039/c2ee02658a

Structural and kinetic study of phase transitions in LaYSi2O7

Fernandez-Carrion, AJ; Escudero, A; Suchomel, MR; Becerro, AI
Journal of the European Ceramic Society, 32 (2012) 2477-2486


Phase transitions in LaYSi 2O 7 have been investigated as a function of temperature using XRD, NMR and TEM. Previously described empirical crystal structure guidelines based on average cation radius in rare-earth RE 2Si 2O 7-type disilicates predict a stable tetragonal A-LaYSi 2O 7 polymorph at temperatures below 1500°C. This study demonstrates that A-LaYSi 2O 7 is not thermodynamically stable at these temperatures and suggests that guidelines based on average cation size do not accurately describe the equilibrium behaviour of this silicate system. The A to G-type polymorph transition is extremely sluggish; complete transformation requires ~250h at 1200°C, and more than 3 weeks of calcination at 1100°C. This observation is important when this type of material is used as environmental barrier coating (EBC) of advanced ceramics. Analysis of XRD and TEM data reveal complete substitution of Y and La on the rare-earth cation sites in both LaYSi 2O 7 polymorphs, but indicate preferential site occupancies in the G-type polymorph.

July, 2012 | DOI: 10.1016/j.jeurceramsoc.2012.03.009

Effect of nanostructured electrode architecture and semiconductor deposition strategy on the photovoltaic performance of quantum dot sensitized solar cells

Samadpour, M; Gimenez, S; Boix, PP; Shen, Q; Calvo, ME; Taghavinia, N; Zad, AI; Toyoda, T; Miguez, H; Mora-Sero, I
Electrochimica Acta, 75 (2012) 139-147


Here we analyze the effect of two relevant aspects related to cell preparation on quantum dot sensitized solar cells (QDSCs) performance: the architecture of the TiO 2 nanostructured electrode and the growth method of quantum dots (QD). Particular attention is given to the effect on the photovoltage, V oc, since this parameter conveys the main current limitation of QDSCs. We have analyzed electrodes directly sensitized with CdSe QDs grown by chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR). We have carried out a systematic study comprising structural, optical, photophysical and photoelectrochemical characterization in order to correlate the material properties of the photoanodes with the functional performance of the manufactured QDSCs. The results show that the correspondence between photovoltaic conversion efficiency and the surface area of TiO 2 depends on the QDs deposition method. Higher V oc values are systematically obtained for TiO 2 morphologies with decreasing surface area and for cells using CBD growth method. This is systematically correlated to a higher recombination resistance of CBD sensitized electrodes. Electron injection kinetics from QDs into TiO 2 also depends on both the TiO 2 structure and the QDs deposition method, being systematically faster for CBD. Only for electrodes prepared with small TiO 2 nanoparticles SILAR method presents better performance than CBD, indicating that the small pore size disturb the CBD growth method. These results have important implications for the optimization of QDSCs.Elect

June, 2012 | DOI: 10.1016/j.electacta.2012.04.087

Novel approaches to flexible visible transparent hybrid films for ultraviolet protection

Calvo, ME; Smirnov, JRC; Miguez, H
Journal of Polymer Science Part B-Polymer Physics, 50 (2012) 945-956


Herein, we present an overview of the most recent achievements and innovations regarding the development of flexible visible transparent films for selective ultraviolet (UV) shielding, with focus on those based on hybrid inorganic-organic materials. The main synthetic paths used nowadays to ensure a high degree of protection are reviewed. Polymers containing organic UV absorbing molecules, hybrid mixtures of polymers and nanoparticles, and the recently introduced series of structures displaying structural color, are identified as the three main types of materials used for this purpose. The use of biocompatible and flexible films to achieve spectrally selective UV protection can find applications in a wide diversity of fields such as photo-treatment of skin diseases, food and beverage packing, and storage of cosmetics. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 In this review, the different approaches taken to obtain flexible and transparent films that block ultraviolet radiation based on the use of hybrid materials are covered. The synthetic pathways that lead to films that can shield against UV radiation either by absorption or by interference are described.

June, 2012 | DOI: 10.1002/polb.23087

Aluminum solubility in TiO2 rutile at high pressure and experimental evidence for a CaCl2-structured polymorph

Escudero, A; Langenhorst, F; Muller, WF
American Mineralogist, 97 (2012) 1075-1082


Aluminum incorporation into TiO 2 has been studied in the TiO 2-Al 2O 3 system as a function of pressure at temperatures of 900 and 1300 °C using commercial Al 2TiO 5 nanopowder as starting material. A new orthorhombic TiO 2 polymorph with the CaCl 2 structure has been observed in the recovered samples synthesized from 4.5 to 7 GPa and 900 °C and from 2.5 to 7 GPa at 1300 °C. The phase transition to the α-PbO 2 type TiO 2 phase takes place between 7 and 10 GPa at both temperatures. Two mechanisms of Al incorporation in TiO 2 rutile have been observed in the recovered samples. The substitution of Ti 4+ by Al 3+ on normal octahedral sites is dominant at lower pressures. High pressure induces the incorporation of Al 3+ into octahedral interstices of the rutile structure, which is responsible for an orthorhombic distortion of the TiO 2 rutile structure and gives rise to a (110) twinned CaCl 2 type structure. This phase is probably a result of temperature quench at high pressure. Aluminum solubility in TiO 2 increases with increasing pressure. TiO 2 is able to accommodate up to 9.8 wt% Al 2O 3 at 7 GPa and 1300 °C. Temperature has a large effect on the aluminum incorporation in TiO 2, especially at higher pressures. High pressure has a strong effect on both the chemistry and the microstructure of Al-doped TiO 2. Enhanced aluminum concentration in TiO 2 rutile as well as TiO 2 grains with a microstructure consisting of twins are a clear indication of high-pressure conditions.

June, 2012 | DOI: 10.2138/am.2012.4049

Integration of Gold Nanoparticles in Optical Resonators

Jimenez-Solano, A; Lopez-Lopez, C; Sanchez-Sobrado, O; Luque, JM; Calvo, ME; Fernandez-Lopez, C; Sanchez-Iglesias, A; Liz-Marzan, LM; Miguez, H
Langmuir, 28 (2012) 9161-9167


The optical absorption of one-dimensional photonic crystal based resonators containing different types of gold nanoparticles is controllably modified by means of the interplay between planar optical cavity modes and localized surface plasmons. Spin-casting of metal oxide nanoparticle suspensions was used to build multilayered photonic structures that host (silica-coated) gold nanorods and spheres. Strong reinforcement and depletion of the absorptance was observed at designed wavelength ranges, thus proving that our method provides a reliable means to modify the optical absorption originated at plasmonic resonances of particles of arbitrary shape and within a wide range of sizes. These observations are discussed on the basis of calculations of the spatial and spectral dependence of the optical field intensity within the multilayers.

May, 2012 | DOI: 10.1021/la300429k

Chromium incorporation into TiO2 at high pressure

Escudero, A; Langenhorst, F
Journal of Solid State Chemistry, 190 (2012) 61-67


Chromium incorporation into TiO 2 up to 3 GPa at 1300 °C and 900 °C has been studied by XRD as well as TEM. A CaCl 2 type TiO 2 polymorph has been observed in the quenched samples from high pressure. Two different mechanisms of solubility occur in the recovered samples. Chromium replaces titanium on normal octahedral sites but it also occupies interstitial octahedral sites, especially in the samples recovered from higher pressures. Interstitial chromium is responsible for an orthorhombic distortion of the TiO 2 rutile structure in the quenched samples and gives rise to a (1 1 0) twinned CaCl 2-structured polymorph. This phase is very likely the result of temperature quench at high pressure. The formation of this phase is directly related to the chromium content of the TiO 2 grains. Chromium solubility in TiO 2 increases with increasing the synthesis pressure. TiO 2 is able to accommodate up to 15.3 wt% Cr 2O 3 at 3 GPa and 1300 °C, compared to 5.7 wt% at atmospheric pressure at the same temperature.

May, 2012 | DOI: 10.1016/j.jssc.2012.01.041

Effect of diffuse light scattering designs on the efficiency of dye solar cells: An integral optical and electrical description

Galvez, FE; Kemppainen, E; Miguez, H; Halme, J
Journal of Physical Chemistry C, 116 (2012) 11426-11433


Herein, we present an integral optical and electrical theoretical analysis of the effect of different diffuse light scattering designs on the performance of dye solar cells. Light harvesting efficiencies and electron generation functions extracted from optical numerical calculations based on a Monte Carlo approach are introduced in a standard electron diffusion model to obtain the steady-state characteristics of the different configurations considered. We demonstrate that there is a strong dependence of the incident photon to current conversion efficiency, and thus of the overall conversion efficiency, on the interplay between the value of the electron diffusion length considered and the type of light scattering design employed, which determines the spatial dependence of the electron generation function. Other effects, like the influence of increased photoelectron generation on the photovoltage, are also discussed. Optimized scattering designs for different combinations of electrode thickness and electron diffusion length are proposed.

April, 2012 | DOI: 10.1021/jp2092708

Incorporation of Si into TiO2 phases at high pressure

Escudero, A; Langenhorst, F
American Mineralogist, 97 (2012) 524-531


Silicon incorporation in TiO 2 phases at increasing pressures until 20 GPa at 1300 °C has been studied by XRD and TEM. Rutile is the stable Si-doped TiO 2 phase until at least 7 GPa, transforming into α-PbO 2 structured TiO2 between 7 and 10 GPa. The further transformation to the TiO 2 polymorph with the baddeleyite structure, akaogiite, has not been observed on the quenched samples. XRD and TEM-EDX data suggest that the Si-doped TiO 2 akaogiite polymorph is non-quenchable and reverts to a-PbO2 structured TiO 2 when releasing the pressure. This transformation gives rise to α-PbO 2 structured TiO 2 grains decorated with p fringes stacking faults. Silicon solubility in TiO 2 phases increases with increasing the synthesis pressure until 16 GPa, implying the substitutional solid solution to be the mechanism of solubility. The influence of the dopants on the stability of the rutile and the α-PbO2 structured TiO 2 has also been analyzed.

March, 2012 | DOI: 10.2138/​am.2012.3941

Efficient Transparent Thin Dye Solar Cells Based on Highly Porous 1D Photonic Crystals

Colodrero, S; Forneli, A; Lopez-Lopez, C; Pelleja, L; Miguez, H; Palomares, E
Advanced Functional Materials, 22 (2012) 1303-1310


A working electrode design based on a highly porous 1D photonic crystal structure that opens the path towards high photocurrents in thin, transparent, dye-sensitized solar cells is presented. By enlarging the average pore size with respect to previous photonic crystal designs, the new working electrode not only increases the device photocurrent, as predicted by theoretical models, but also allows the observation of an unprecedented boost of the cell photovoltage, which can be attributed to structural modifications caused during the integration of the photonic crystal. These synergic effects yield conversion efficiencies of around 3.5% by using just 2 mu m thick electrodes, with enhancements between 100% and 150% with respect to reference cells of the same thickness.

February, 2012 | DOI: 10.1002/adfm.201102159

Synthesis and Structure Resolution of RbLaF4

Rollet, AL; Allix, M; Veron, E; Deschamps, M; Montouillout, V; Suchomel, MR; Suard, E; Barre, M; Ocana, M; Sadoc, A; Boucher, F; Bessada, C; Massiot, D; Fayon, F
Inorganic Chemistry, 51 (2012) 2272-2282


The synthesis and structure resolution of RbLaF4 are described. RbLaF4 is synthesized by solid-state reaction between RbF and LaF3 at 425 degrees C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (Pnma, a = 6.46281(2) angstrom, b = 3.86498(1) angstrom, c = 16.176:29(4) angstrom, Z = 4). One-dimensional Rb-87, La-139, and F-19 MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the F-19 resonances is performed on the basis of both F-19-La-139 J-coupling multiplet patterns observed in a heteronudear DQ-filtered J-resolved spectrum and F-19-Rb-87 HMQC MAS experiments. DFT calculations of both the F-19 isotropic chemical shieldings and the Rb-87, La-139 electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF4 and luminescence properties of Eu-doped LaRbF4 are investigated.

February, 2012 | DOI: 10.1021/ic202301e

Enhanced diffusion through porous nanoparticle optical multilayers

Lopez-Lopez, C; Colodrero, S; Raga, SR; Lindstrom, H; Fabregat-Santiago, F; Bisquert, J; Miguez, H
Journal of Materials Chemistry, 22 (2012) 1751-1757


Herein we demonstrate improved mass transport through nano-particle one-dimensional photonic crystals of enhanced porosity. Analysis is made by impedance spectroscopy using iodine and ionic liquid based electrolytes and shows that newly created large pores and increased porosity improve the diffusion of species through the photonic crystal. This achievement is based on the use of a polymeric porogen (polyethylene glycol), which is mixed with the precursor suspensions used for the deposition of nanoparticle TiO2 and SiO2 layers and then eliminated to generate a more open interconnected void network, as confirmed by specular reflectance porosimetry. A compromise between pore size and optical quality of these periodic structures is found.

January, 2012 | DOI: 10.1039/c1jm15202e

Microwave-Assisted Synthesis and Luminescence of Mesoporous REDoped YPO4 (RE = Eu, Ce, Tb, and Ce plus Tb) Nanophosphors with Lenticular Shape

Rodriguez-Liviano, S; Aparicio, FJ; Rojas, TC; Hungria, AB; Chinchilla, LE; Ocana, M
Crystal Growth and Design, 12 (2012) 635-645


Mesoporous tetragonal RE:YPO 4 nanophosphors (RE = Eu, Ce, Tb, and Ce + Tb) with a lenticular morphology, narrow size distribution, and high surface area have been prepared by an homogeneous precipitation procedure consisting of aging, at low temperature (80-120 °C) in a microwave oven, ethylene glycol solutions containing only yttrium acetylacetonate and phosphoric acid. This synthesis method involves important advantages such as its simplicity, rapidness (reaction time = 7 min), and high reaction yields. The mechanism of nanoparticle growth has been also addressed finding that the lenticular nanoparticles are formed through an ordered aggregation of smaller entities, which explains their porosity. In all cases, the doping levels were systematically varied in order to optimize the nanophosphors luminescence. All optimum nanophosphors presented a high luminescence quantum yield (QY). In particular, for the Eu and Tb doped systems, the obtained QY values (60% for Eu and 80% for Tb) were the highest so far reported for this kind of nanomaterial. The morphological, microstructural, and luminescent properties of these nanophosphors and their dispersibility in water make them suitable for biomedical applications.

January, 2012 | DOI: 10.1021/cg201358c

Collective osmotic shock in ordered materials

Paul Zavala-Rivera, Kevin Channon, Vincent Nguyen, Easan Sivaniah, Dinesh Kabra, Richard H. Friend, S. K. Nataraj, Shaheen A. Al-Muhtaseb, Alexander Hexemer, Mauricio E. Calvo & Hernan Miguez
Nature Materials, 11 (2012) 53–57


Osmotic shock in a vesicle or cell is the stress build-up and subsequent rupture of the phospholipid membrane that occurs when a relatively high concentration of salt is unable to cross the membrane and instead an inflow of water alleviates the salt concentration gradient. This is a well-known failure mechanism for cells and vesicles (for example, hypotonic shock) and metal alloys (for example, hydrogen embrittlement). We propose the concept of collective osmotic shock, whereby a coordinated explosive fracture resulting from multiplexing the singular effects of osmotic shock at discrete sites within an ordered material results in regular bicontinuous structures. The concept is demonstrated here using self-assembled block copolymer micelles, yet it is applicable to organized heterogeneous materials where a minority component can be selectively degraded and solvated whilst ensconced in a matrix capable of plastic deformation. We discuss the application of these self-supported, perforated multilayer materials in photonics, nanofiltration and optoelectronics.

December, 2011 | DOI: 10.1038/nmat3179


Porous one dimensional photonic crystals: novel multifunctional materials for environmental and energy applications

Mauricio E. Calvo, Silvia Colodrero, Nuria Hidalgo, Gabriel Lozano, Carmen López-López, Olalla Sánchez-Sobrado and Hernán Míguez
Energy and Environmental Science, 4 (2011) 4800-4812


In recent times, several synthetic pathways have been developed to create multilayered materials of diverse composition that combine accessible porosity and optical properties of structural origin, i.e., not related to absorption. These materials possess a refractive index that varies periodically along one direction, which gives rise to optical diffraction effects characteristic of Bragg stacks or one-dimensional photonic crystals (1DPCs). The technological potential of such porous optical materials has been demonstrated in various fields related to energy and environmental sciences, such as detection and recognition of targeted biological or chemical species, photovoltaics, or radiation shielding. In all cases, improved performance is achieved as a result of the added functionality porosity brings. In this review, a unified picture of this emerging field is provided.

November, 2011 | DOI: 10.1039/C1EE02081A

Synthesis, through pyrolysis of aerosols, of YIn1−xMnxO3 blue pigments and their efficiency for colouring glazes

M. Ocaña, J.P. Espinós, J.B. Carda
Dyes and Pigments, 91 (2011) 501-507


Mn-doped YInO3 blue pigments have been synthesised at a much lower temperature (1100 °C) than that required by the traditional solid state method (1400 °C). The developed procedure, which is based on the pyrolysis at 600 °C of aerosols generated from aqueous solutions of Y, In and Mn nitrates followed by an annealing treatment at 1100 °C, yields spherical pigments particles with heterogeneous size in the optimum range required for ceramic applications (<10 μm). The amount of Mn introduced in the YInO3 matrix has been systematically varied in order to evaluate the effects of the Mn content on the colour properties of the pigments. It has been found that the optimum pigment composition (bluer colour with the lowest Mn content) is given by the formula YIn0.90Mn0.10O 3. The technological performance of these YIn1-xMn xO3 blue pigments has also been evaluated by testing their efficiency for colouring ceramic glazes of different composition (boracic and plumbic) and properties, aiming to find a less toxic alternative for the Co-based pigments commonly used by the ceramic industry.

November, 2011 | DOI: 10.1016/j.dyepig.2011.03.009

Angular emission properties of a layer of rare-earth based nanophosphors embedded in one-dimensional photonic crystal coatings

Sánchez-Sobrado, O., Yacomotti, A.M., Calvo, M.E., Martínez, O.E., Ocaña, M., Núñez, N., Levenson, J.A., Míguez, H.
Applied Physics Letters, 99 (2011) Article number 051111


The angular properties of light emitted from rare-earth based nanophosphors embedded in optical resonators built in one-dimensional photonic crystal coatings are herein investigated. Strong directional dependence of the photoluminescence spectra is found. Abrupt angular variations of the enhancement caused by the photonic structure and the extraction power are observed, in good agreement with calculated polar emission patterns. Our results confirm that the optical cavity favors the extraction of different wavelengths at different angles and that integration of nanophosphors within photonic crystals provides control over the directional emission properties that could be put into practice in phosphorescent displays.

July, 2011 | DOI: 10.1063/1.3619814

Porous supramolecularly templated optical resonators built in 1D photonic crystals

Hidalgo, N., Calvo, M.E., Bellino, M.G., Soler-Illia, G.J.A.A., Míguez, H.
Advanced Functional Materials, 21 (2011) 2534-2540


A synthetic route to attain photonic multilayers that presents controlled porosity only at the middle-layer level is shown. The spectral resonance associated with this porous layer shows strong sensitivity to the presence of vapors adsorbed or condensed within the void network, providing a potentially relevant material for gas detection. The importance of the interplay between pore and probe-molecule diameters is studied and its implications in size-selective detection are discussed. Controlled porosity: Photonic multilayers that presents controlled porosity only at the middle-layer level are herein introduced. The optical response of the ensemble shows strong sensitivity to the presence of vapors adsorbed or condensed within the void network, providing a potentially relevant material for gas detection.

June, 2011 | DOI: 10.1002/adfm.201002486

Interplay of Resonant Cavity Modes with Localized Surface Plasmons: Optical Absorption Properties of Bragg Stacks Integrating Gold Nanoparticles

Olalla Sánchez-Sobrado, Gabriel Lozano, Mauricio E. Calvo, Ana Sánchez-Iglesias, Luis M. Liz-Marzán, Hernán Míguez
Advanced Materials, 23 (2011) 2108-2112


A procedure to prepare porous photonic crystal resonators containing gold nanoparticles is reported. The optical absorption of the ensemble, resulting from the excitation of the localized surface plasmon of the metallic beads, is finely tuned by a gradual shift of the cavity mode. This is achieved by infiltration of the void network with different guest compounds.

April, 2011 | DOI: 10.1002/adma.201004401

Analysis of artificial opals by scanning near field optical microscopy

Barrio, J; Lozano, G; Lamela, J; Lifante, G; Dorado, LA; Depine, RA; Jaque, F; Miguez, H
J. Appl. Phys., 109 (2011) 083514(5 pages)


Herein we present a detailed analysis of the optical response of artificial opal films realized employing a near-field scanning optical microscope in collection and transmission modes. Near-field patterns measured at the rear surface when a plane wave impinges on the front face are presented with the finding that optical intensity maps present a clear correlation with the periodic arrangement of the outer surface. Calculations based on the vector Korringa-Kohn-Rostoker method reproduce the different profiles experimentally observed as well as the response to the polarization of the incident field. These observations constitute the first experimental confirmation of the collective lattice resonances that give rise to the optical response of these three dimensional periodic structures in the high-energy range.

March, 2011 | DOI: 10.1063/1.3573777

Tuning from blue to magenta the up-converted emissions of YF3:Tm3+/Yb3+ nanocrystals

Quintanilla, M; Nunez, NO; Cantelar, E; Ocana, M; Cusso, F
Nanoscale, 3 (2011) 1046-1052


Monodisperse YF3:Tm3+/Yb3+ nanocrystals have been synthesized to explore the visible up-converting properties under near infrared (975 nm) excitation. It has been found that the nanoparticles exhibit intense red up-converted emissions, in addition to the characteristic UV and blue Tm3+-bands. It is demonstrated that, by carefully selecting Tm3+ and Yb3+ contents, the relative intensity of the different emissions can be changed producing an overall emission colour that can be tuned from blue to magenta.

February, 2011 | DOI: 10.1039/c0nr00676a

A facile single-step procedure for the synthesis of luminescent Ln 3+:YVO4 (Ln = Eu or Er + Yb)-silica nanocomposites

Ocana, M; Cantelar, E; Cusso, F
Materials Chemistry and Physics, 125 (2011) 224-230


A simple and single-step method for the production of Ln-doped YVO 4 nanocrystals and their simultaneous encapsulation in a silica network based on the pyrolysis of liquid aerosols at 800 °C is reported. The procedure is illustrated for Yb,Er:YVO4-silica nanocomposites consisting of spherical particles, which present up-converted green luminescence after IR excitation whose efficiency increased on annealing up to 1000 °C due to the release of impurities (adsorbed water, and residual anions). XPS spectroscopy and TEM observations revealed that the surface of the composite particles was enriched in silica, which would facilitate their functionalisation required to use them in biological applications. The procedure can also be used to prepare other rare earth doped systems as illustrated for the case of Eu-doped YVO4/silica having down-converted red luminescence.

December, 2010 | DOI: 10.1016/j.matchemphys.2010.09.011


Porous One-Dimensional Photonic Crystal Coatings for Gas Detection

Hidalgo, N; Calvo, ME; Colodrero, S; Miguez, H
IEEE Sensors Journal, 10 (2010) 1206-1212


Herein, we present an overview of recent progress on the development of different types of porous 1-D photonic crystal coatings which are optically responsive to gas pressure changes in the environment. Modification of the surrounding vapor pressure gives rise to adsorption and condensation phenomena within the porous networks of the photonic crystal building blocks, varying their refractive index and hence their optical features. This effect can be put into practice to precisely detect and monitor changes in the ambient through the spectral shift of either the photonic bandgap of the structure or of some other optical features. Our results demonstrate the potential of these optical coatings as new materials for gas sensing devices.

June, 2011 | DOI: 10.1109/JSEN.2010.2043525

Versatility and multifunctionality of highly reflecting Bragg mirrors based on nanoparticle multilayers

Olalla Sánchez-Sobrado, Mauricio E. Calvo and Hernán Míguez
Journal of Materials Chemistry, 20 (2010) 8240-8246


The use of both supported and flexible self-standing nanoparticle-based one dimensional photonic crystal films as effective frequency selective filters in the UV-vis-NIR is herein evaluated. The requirements to achieve a flat spectral response at the desired frequency range are analyzed and a synthetic route to realize materials with such properties presented. Strict control over the structural parameters yields multilayers in which the opening or closing of higher order photonic band gaps can be devised, thus leading to films capable of blocking the UV and NIR ranges simultaneously. Furthermore, the physico-chemical properties of the mirror can be modified to yield either moisture-repelling or, on the contrary, environmentally responsive optical filters. These materials present a great potential to be used as versatile and multifunctional optical elements.

September, 2010 | DOI: 10.1039/c0jm01508c

Uniform YF3:Yb,Er up-conversion nanophosphors of various morphologies synthesized in polyol media through an ionic liquid

Nuñez, NO; Quintanilla, M; Cantelar, E; Cusso, F; Ocaña, M
Journal of Nanoparticle Research, 12 (2010) 2553-2565


We describe a facile procedure for the synthesis at low temperature (120 A degrees C) of water-dispersible uniform YF3:Yb,Er up-conversion nanophosphors of various morphologies (rhombic and spheroidal) by homogeneous precipitation in polyol solutions containing different lanthanide salts and an ionic liquid (1-butyl, 2-methylimidazolium tetrafluoroborate) as fluoride source. It is shown that the shape of the obtained nanoparticles is mainly determined by the nature of both, the polyol and the lanthanide precursors, which also affects to their colloidal stability in water suspensions. These morphological differences are explained on the basis of a different mechanism of particle formation. The efficiency of the up-conversion processes in the synthesized rhombic and spheroidal nanoparticles is also comparatively analyzed and the observed differences are justified on the basis of the different impurities incorporated to the nanophosphors during their synthesis process.

August, 2010 | DOI: 10.1007/s11051-009-9824-6

Citrate mediated synthesis of uniform monazite LnPO(4) (Ln = La, Ce) and Ln:LaPO4 (Ln = Eu, Ce, Ce plus Tb) spheres and their photoluminescence

Nuñez, NO; Liviano, SR; Ocaña, M
Journal of Colloid and Interface Science, 349 (2010) 484-491


A simple method for the synthesis of spherical LaPO4 (monazite) particles with narrow size distribution and tailored size in the 150-500 nm range is reported. The procedure is based on a homogeneous precipitation process at low temperature (120 degrees C) from solutions containing La3+, citrate and phosphate ions under a very restrictive set of experimental conditions, which involves the use of La nitrate, citric acid and phosphoric acid as precursors and ethylene glycol as solvent. The growth mechanism of the spheres was investigated aiming at explaining the differences in particle size and shape observed when varying the experimental conditions. The applicability of this method for the synthesis of spherical particles of other lanthanide (Ce, Tb, Eu) phosphates is also analyzed. Finally, it is shown that the developed procedure can be used to dope the lanthanum phosphate particles with lanthanide cations, which resulted in spherical phosphors as illustrated for the Eu-doped, Ce-doped and Ce, Tb codoped systems, whose luminescent properties are also evaluated.

August, 2010 | DOI: 10.1016/j.jcis.2010.05.079

TiO2-SiO2 one-dimensional photonic crystals of controlled porosity by glancing angle physical vapour deposition

Gonzalez-Garcia, L; Lozano, G; Barranco, A; Miguez, H; Gonzalez-Elipe, AR
Journal of Materials Chemistry, 20 (2010) 6408-6412


Herein we present a synthetic route to attain porous one-dimensional photonic crystals of high optical quality. The method employed, based on the alternate deposition of TiO2 and SiO2 porous layers by glancing angle physical vapour deposition, yields a highly accessible interconnected pore network throughout the entire multilayer structure. Furthermore, it allows a strict control over the average size and density of the interstitial sites, which results in the precise tuning of the refractive index of the individual layers and thus of the optical response of the ensemble. The controlled environmental response of the multilayer is confirmed by the optical monitoring of the infiltration of liquids of different refractive index.

July, 2010 | DOI: 10.1039/C0JM00680G

Flexible, Adhesive, and Biocompatible Bragg Mirrors Based on Polydimethylsiloxane Infiltrated Nanoparticle Multilayers

Calvo, ME, Miguez, H
Chemistry of Materials, 22 (2010) 3909-3915


Herein we present a series of self-standing, flexible, and biocompatible optical interference filters obtained through infiltration and polymerization of an elastomer (polydimethylsiloxane) in a porous Bragg mirror prepared by alternating deposition of layers of TiO2 and SiO2 nanoparticles. The method proposed yields the uniform filling of the nanopores of the multilayer by the polymer, which allows lifting off the hybrid structure as long as the ensemble is cooled to temperatures below the glass transition of the polymer. This multifunctional material combines the optical properties of the periodic nanoporous multilayer and the structural and physicochemical characteristics of polydimethylsiloxane. Experimental demonstrations of their potential use as flexible and adhesive UV-protecting filters, as well as of light, highly-efficient conformal back reflectors to enhance the efficiency of photovoltaic devices are provided.

June, 2010 | DOI: 10.1021/cm1001016

Anomalous group velocity at the high energy range of a 3D photonic nanostructure

Botey, M; Martorell, J; Dorado, LA; Depine, RA; Lozano, G; Miguez, H
Optics Express, 18 (2010) 15682-15690


We report on a study of electromagnetic waves propagation in thin periodically ordered photonic nanostructures in the spectral range where the light wavelength is on the order of the lattice parameter. The vector KKR method we use allows us to determine the group index from finite photonic structures including extinction providing confirmation of recently emerged results. We show that for certain frequencies the group velocity of opal slabs can either be superluminal or approach zero depending on the crystal thickness and the unavoidable presence of losses. In some cases, group velocity can be negative. Such behavior can be clearly attributed to the finite character of the three-dimensional structure and reproduces previously reported experimental observations. Calculations show that contrary to the predictions of extraordinary group velocity reductions for infinite periodic structures, the group velocity of real opals may exhibit strong fluctuations at the high energy range. Hence, a direct identification between the calculated anomalous group velocities, for an actual opal film, and the predicted propagating low dispersion modes for an ideal infinite ordered structure seems difficult to establish.

June, 2010 | DOI: 10.1364/OE.18.015682

Gallium Arsenide Infiltration of Nanoporous Multi layers: A Route to High-Dielectric-Contrast One-Dimensional Photonic Crystals

Sanchez-Sobrado, O; Thomas, K; Povey, I; Pemble, ME; Miguez, H
Small, 6 (2010) 1283-1287


Periodic multilayers of wide photonic bandgap and high reflectance in the visible and near infrared regions are fabricated. Optical properties show that reflectance intensities close to 90% are reached for stacks of only six layers, as well as gap-to-midgap ratios of 50%. The optical response of the hybrid ensemble can be accurately tuned through the number of infiltration cycles performed.

May, 2010 | DOI: 10.1002/smll.200902190

Environmentally responsive nanoparticle-based luminescent optical resonators

Sanchez-Sobrado, O; Calvo, ME; Nunez, N; Ocana, M; Lozano, G; Miguez, H
Nanoscale, 2 (2010) 936-941


In this work, we demonstrate that optical resonators built using all-nanoparticle-based porous building blocks provide a responsive multifunctional matrix, totally different emission spectra being attained from the same embedded luminescent nanophosphors under varying environmental conditions. We show a clear correlation between modifications in the ambient surroundings, the induced changes of the resonant modes, and the resulting variations in the emission response. The method is versatile and allows nanophosphors of arbitrary shape to be integrated in the cavity. By precise control of the spectral features of the optical resonances, luminescence is strongly modulated in selected and tuneable wavelength ranges. Applications in the fields of sensing and detection are foreseen for these materials.

May, 2010 | DOI: 10.1039/b9nr00338j

Angular dependence of the intensity of light beams diffracted by colloidal crystals

Lozano, G; Mazzaferri, JE; Dorado, LA; Ledesma, S; Depine, RA; Miguez, H
Journal of the Optical Society of America B-Optical Physics, 27 (2010) 1394-1399


An experimental and theoretical analysis of the angular dependence of the diffracted light beams emerging from three-dimensional colloidal photonic crystals is herein presented. Diffracted beams are identified according to their associated reciprocal-lattice vectors, and their intensities are obtained as a function of the zenithal and azimuthal incidence angles. Significant changes in the beam intensities are observed for large zenithal incidence angles as the azimuthal angle is varied. This phenomenon is related to the excitation of new resonant modes inside the photonic crystal which cannot be observed under normal incidence conditions.

May, 2010 | DOI: 10.1364/JOSAB.27.001394

Theoretical Analysis of the Performance of One-Dimensional Photonic Crystal-Based Dye-Sensitized Solar Cells

Lozano, G; Colodrero, S; Caulier, O; Calvo, ME; Miguez, H
Journal of Physical Chemistry C, 114 (2010) 3681-3687


A simple analytical model that allows designing one-dimensional photonic crystal based dye sensitized solar cells of optimized performance, accounting for the actual optical features of the device, is herein presented. Based on the theoretical description of the effect of coupling such Bragg mirrors to the light harvesting electrode, recently reported experimental values of the spectral dependence of incident photon to current conversion efficiency attained for such Structures are fairly reproduced and rationalized. A thorough analysis or them in terms of the interplay between the effect of the electrode thickness and the characteristics of the Bragg reflection, such as intensity, spectral position, and width, is provided. Predictions on the maximum enhancement factors expected for realistic Structures are also presented.

February, 2010 | DOI: 10.1021/jp9096315

Conformal Growth of Organic Luminescent Planar Defects within Artificial Opals

Aparicio, FJ; Lozano, G; Blaszczyk-Lezak, I; Barranco, A; Miguez, H
Chemistry of Materials, 22 (2010) 379-385


Herein, we present the result of combining, for the first time, the techniques of colloidal self-assembly and plasma-enhanced chemical vapor deposition to create a novel, high-quality, purely Organic active photonic crystal structure of controlled optical properties. We show a fast. reliable, and accurate procedure to introduce two-dimensional luminescent organic defect layers within artificial polystyrene opals via a versatile room-temperature remote plasma deposition process. This method is gentle enough to allow highly coil formal growth on polystyrene microspheres without altering their morphology or the ordered arrangement that they form. The luminescent organic layer behaves both as all optical dopant, causing the opening of transmission windows within the forbidden frequency interval of the lattice, and as an optically active material, whose emission call be tailored by the photonic environment.

December, 2009 | DOI: 10.1021/cm902819x


Mesostructured Thin Films as Responsive Optical Coatings of Photonic Crystals

Hidalgo, N; Calvo, ME; Miguez, H
Small, 5 (2009) 2309-2315


A synthetic route is presented to attain high-optical-quality multilayered structures that residtfront coupling ordered n7esoporous tilaniuni oxide thin films to the surface of a dense one-dimensional photonic crystal. Such architectures present spectrally well-defined photon resonant modes localized in the outer coating that finely respond to physicochemically induced modifications of its pore volume. The potential of these porous coatings in detection of environmental changes through variations of the photonic response of the ensemble is demonstrated by performing isothermal optical reflectance measurements under controlled vapor-pressure conditions.

September, 2009 | DOI: 10.1002/smll.200900411

Optical Analysis of the Fine Crystalline Structure of Artificial Opal Films

Lozano, G; Dorado, LA; Schinca, D; Depine, RA; Miguez, H
Langmuir, 25 (2009) 12860-12864


Herein, we present a detailed analysis of the structure of artificial opal films. We demonstrate that, rather than the generally assumed face centered cubic lattice of spheres, opal films are better approximated by rhombohedral assemblies of distorted colloids. Detailed analysis of the optical response in a very wide spectral range (0.4 ≤ a/λ ≤ 2, where a is the conventional lattice constant), as well as at perpendicular and off-normal directions, unambiguously shows that the interparticle distance coincides very approximately with the expected diameter only along directions contained in the same close-packed plane but differs significantly in directions oblique to the [111] one. A full description of the real and reciprocal lattices of actual opal films is provided, as well as of the photonic band structure of the proposed arrangement. The implications of this distortion in the optical response of the lattice are discussed.

September, 2009 | DOI: 10.1021/la903077r

Synthesis by pyrolysis of aerosols and ceramic application of Cr-doped CaYAlO4 red–orange pigments

Lyubenova, TS; Carda, JB; Ocana, M
Journal of the European Ceramic Society, 29 (2009) 2193-2198


The synthesis of red–orange Cr-doped YCaAlO4 pigments has been improved (softer thermal conditions and lower environmental impact) and optimised by using the pyrolysis of aerosols method. We also study the crystallochemical features of the Cr chromophore with special emphasis on its oxidation state which has not been yet clarified, finding that Cr(III) and Cr(IV) species are present in the octahedral and interstitial tetrahedral sites of the YCaAlO4 lattice, respectively. Finally, the applicability of this system as ceramic pigment was tested using conventional industrial glazes. A change from orange to pink shades was detected after glaze firing, which is mainly attributed to the Cr3+ to Cr4+ oxidation.

July, 2009 | DOI: 10.1016/j.jeurceramsoc.2009.01.020

Light generation at the anomalous dispersion high energy range of a nonlinear opal film

Botey, M; Maymo, M; Molinos-Gomez, A; Dorado, L; Depine, RA; Lozano, G; Mihi, A; Miguez, H; Martorell, J
Optics Express, 17 (2009) 12210-12216


We study experimentally and theoretically light propagation and generation at the high energy range of a close-packed fcc photonic crystal of polystyrene spheres coated with a nonlinear material. We observe an enhancement of the second harmonic generation of light that may be explained on the basis of amplification effects arising from propagation at anomalous group velocities. Theoretical calculations are performed to support this assumption. The vector KKR method we use allows us to determine, from the linear response of the crystal, the behavior of the group velocity in our finite photonic structures when losses introduced by absorption or scattering by defects are taken into account assuming a nonzero imaginary part for the dielectric constant. In such structures, we predict large variations of the group velocity for wavelengths on the order or smaller than the lattice constant of the structure, where an anomalous group velocity behavior is associated with the flat bands of the photonic band structure. We find that a direct relation may be established between the group velocity reduction and the enhancement of a light generation processes such as the second harmonic generation we consider. However, frequencies for which the enhancement is found, in the finite photonic crystals we use, do not necessarily coincide with the frequencies of flat high energy bands.

June, 2009 | DOI: 10.1364/OE.17.012210

M-Doped Al2TiO5 (M=Cr, Mn, Co) Solid Solutions and their Use as Ceramic Pigments

Dondi, M; Lyubenova, TS; Carda, JB; Ocaña, M
Journal of the American Ceramic Society, 92 (2009) 1972-1980


New ceramic pigments based on the tialite (Al2TiO5) structure, doped with Co (pink), Cr (green), or Mn (brown), were prepared through the pyrolysis of aerosols followed by calcination of the obtained powders at 1400°C. The expected decomposition of Al2TiO5 into a mixture of Al2O3 and TiO2 on refiring was inhibited by Cr-doping and also by co-doping with Mg the Mn- or Co-doped samples. Microstructure and phase evolution during pigment preparation were monitored by scanning electron microscopy and XRPD. Unit cell parameters of tialite were determined by Rietveld refinement of the X-ray diffraction patterns, revealing in all cases the formation of solid solutions where the solubility of dopants in the Al2TiO5 lattice followed the trend Co<Mn<Cr. The valence state and possible location of dopants in the tialite lattice were investigated by X-ray photoelectron spectra and diffuse reflectance spectroscopies, which suggested the presence of Cr3+ ions in a large interstitial site of the tialite lattice with a distorted octahedral geometry, and of Mn3+ and Co2+ ions in the Al3+ octahedral sites of the tialite lattice in the former case, and in both Al3+ and Ti4+ octahedral sites in the latter. Testing the ceramic glazes assessed the technological behavior of pigments, which found that the color stability was reasonably good for the Mn-doped tialite and the Cr-doped pigment, although the latter suffered a small loss of green hue. The Co-doped pigment was found to be not stable in glazes, undergoing a cobalt-leaching effect.

May, 2009 | DOI: 10.1111/j.1551-2916.2009.03172.x

Molding with nanoparticle-based one-dimensional photonic crystals: a route to flexible and transferable Bragg mirrors of high dielectric contrast

Calvo, ME; Sobrado, OS; Lozano, G; Miguez, H
Journal of Materials Chemistry, 19 (2009) 3144-3148


Self-standing, flexible Bragg mirror films of high refractive index contrast and showing intense and wide Bragg peaks are herein presented. Nanoparticle-based one-dimensional photonic crystals are used as templates to infiltrate a polymer, which provides the multilayer with mechanical stability while preserving the dielectric contrast existing in the mold. Such films can be lifted off the substrate and used to coat another surface of arbitrary shape.

April, 2009 | DOI: 10.1039/B902090J

Porous One-Dimensional Photonic Crystals Improve the Power-Conversion Efficiency of Dye-Sensitized Solar Cells

Colodrero, S; Mihi, A; Haggman, L; Ocana, M; Boschloo, G; Hagfeldt, A; Miguez, H
Advanced Materials, 21 (2009) 764-770


A device for solar-energy conversion was introduced in which a porous and highly reflecting 1D photonic crystal (1D PC) was coupled to a dye-sensitized nanocrystals anatase (NC-TiO2) electrode. The results show that the transparency of the PC-based dye-sensitized solar cells (DSSC) in the visible range of the electromagnetic spectrum is very similar to that of the reference cell. The multilayer whose photonic bandgap has a larger overlap with the absorption band of the ruthenium dye, gives rise to a larger enhancement of the photocurrent. It is also seen that the porous 0.5μm thick PC, whose deleterious effect is compensated by the large increment in photocurrent. The spectral photoelectric response of the cell clearly shows the effect that coupling to a PC has on the current photogenerated in the dye-sensitized electrode.

January, 2009 | DOI: 10.1002/adma.200703115

Control over the Structural and Optical Features of Nanoparticle-Based One-Dimensional Photonic Crystals

Calvo, ME; Sanchez-Sobrado, O; Colodrero, S; Miguez, H
Langmuir, 25 (2012) 2443-2448


Herein we present a detailed analysis of the effect of the spin-coating protocol over the optical properties of nanoparticle-based one-dimensional photonic crystals. Based on these results, we provide a reliable synthetic route to attain high-quality porous multilayers in which the effect of imperfections is minimized and whose Bragg diffraction can be precisely tuned over the entire visible and near-infrared spectrum. We present a systematic study of the effect of the acceleration ramp and final rotation speed over the structural and optical quality of these materials. This allows us to relate the structural variations observed with the different relative importance of fluid flow and solvent evaporation on the thinning of each layer in the stack for the different deposition conditions employed.

January, 2009 | DOI: 10.1021/la8030057

Towards a full understanding of the growth dynamics and optical response of self-assembled photonic colloidal crystal films

Lozano, GS; Dorado, LA; Depine, RA; Miguez, H
Journal of Materials Chemistry, 19 (2009) 185-190


Recent advances in the comprehension of the growth dynamics of colloidal crystal films opens the door to rational design of experiments aiming at fabricating lattices in which the density of intrinsic defects is minimized. Since such imperfections have a dramatic effect on scattered light of wavelength smaller than the lattice constant, the evaluation of the experimental optical response at those energy ranges, based on the comparison to rigorous calculations, is identified as the most sensitive guide to accurately evaluate the progress towards the actual realization of defect-free colloidal crystals.

December, 2008 | DOI: 10.1039/b811955d

Experimental Demonstration of the Mechanism of Light Harvesting Enhancement in Photonic-Crystal-Based Dye-Sensitized Solar Cells

Colodrero, S; Mihi, A; Anta, JA; Ocaña, M; Miguez, H
Journal of Physical Chemistry C, 113 (2012) 1150-1154


Herein, we report an experimental analysis of the photogenerated current of very thin and uniform dye-sensitized nanocrytalline titanium oxide (nc-TiO2) electrodes coupled to high-quality one-dimensional photonic crystals. The effect of well-defined optical absorption resonances are detected both in optical spectroscopy and photogenerated current experiments, a clear correspondence between them being established. Our study demonstrates that light trapping within absorbing electrodes is responsible for the absorption enhancement that has previously been reported and unveils the mechanism behind it. We prove that this effect improves significantly the power conversion efficiency of very thin electrodes.

December, 2008 | DOI: 10.1021/jp809789s

Sonia Rodríguez Liviano
Nanopartículas uniformes luminiscentes basadas en tierras raras para aplicaciones biomédicas

Director/s: Manuel Ocaña Jurado
Universidad de Sevilla
Wednesday, 13 September, 2017

Alberto Jiménez Solano
Diseño de la absorción y emisión ópticas de nanomateriales en entornos fotónicos controlados

Director/s: Juan Francisco Galisteo López y Hernán R. Míguez García
Universidad de Sevilla
Friday, 23 June, 2017

José Raúl Castro Smirnov
Nanoestructuras Ópticas Híbridas Flexibles para Aplicaciones como Filtros de Radiación Ultravioleta

Director/s: Hernán Míguez García y Mauricio Calvo Roggiani
Universidad de Sevilla
Friday, 13 February, 2015

Alberto José Fernández Carrión
Disilicatos de tierras raras simples y sus soluciones sólidas: Estructura cristalina y propiedades físicas

Director/s: Ana Isabel Becerro Nieto
Universidad de Sevilla
Wednesday, 28 May, 2014

M. Carmen López López
Nanostructured Optical Materials for Dye Solar Cells

Director/s: Hernán Míguez García
Universidad de Sevilla
Wednesday, 18 December, 2013

Silvia Colodrero Pérez
Porous One Dimensional Photonic Crystals for Enhanced Photovoltaic Performance of Dye Solar Cells

Director/s: Hernán Míguez García
Universidad de Sevilla
Friday, 21 June, 2013
PDF icon Doctoral Thesis Silvia.pdf

Nuria Hidalgo Serrano
Aplicaciones de láminas con mesoestructura controlada en cristales fotónicos

Director/s: Hernán Míguez García
Universidad de Sevilla
Wednesday, 19 December, 2012

Olalla Sánchez-Sobrado
Optical Absorption and Emission of Nanomaterials Integrated in One-Dimensional Photonic Crystals

Director/s: Hernán Miguez García
Universidad de Sevilla
Friday, 23 September, 2011
PDF icon Tesis-olallaSanchez.pdf

Marco Mantovani
Transformación de silicatos laminares en presencia de fluidos alcalinos. Escenarios naturales y artficiales

Director/s: Ana Isabel Becerro Nieto
Universidad de Sevilla
Friday, 18 March, 2011
PDF icon Tesis Marco 2011.pdf

Gabriel Lozano Barbero
Análisis del crecimiento y las propiedades ópticas en alta energía de cristales fotónicos coloidales

Director/s: Hernán R. Míguez García
Universidad de Sevilla
Friday, 17 December, 2010
PDF icon Tesis G Lozano.pdf

Elena López Navarrete
Pigmentos cerámicos de tonalidad roja: síntesis mediante pirólisis de aerosoles y estudio cristaloquímico

Director/s: Manuel Ocaña Jurado
Universidad de Sevilla
Monday, 13 December, 2010

Agustín Mihi Cervelló
Preparación, Caracterización y Modelización de Cristales Fotónicos Coloidales para Aplicaciones en Células Solares

Director/s: Hernán Míguez García
Universidad de Sevilla
Thursday, 10 July, 2008
PDF icon Tesis A Mihi.pdf

Raúl Pozas Bravo
Nanopartículas aciculares de Fe-Co protegidas mediante recubrimientos inorgánicos

Director/s: Manuel Ocaña Jurado y Carlos J. Serna
Universidad de Sevilla
Wednesday, 6 April, 2005