42 research outputs found
Band gap control via tuning of inversion degree in CdInS spinel
Based on theoretical arguments we propose a possible route for controlling
the band-gap in the promising photovoltaic material CdInS. Our
\textit{ab initio} calculations show that the experimental degree of inversion
in this spinel (fraction of tetrahedral sites occupied by In) corresponds
approximately to the equilibrium value given by the minimum of the theoretical
inversion free energy at a typical synthesis temperature. Modification of this
temperature, or of the cooling rate after synthesis, is then expected to change
the inversion degree, which in turn sensitively tunes the electronic band-gap
of the solid, as shown here by accurate screened hybrid functional
calculations.Comment: In press in Applied Physics Letters (2012); 4 pages, 2 figures, 1
tabl
Analysis of SnS2 hyperdoped with V proposed as efficient absorber material
Intermediate-band materials can improve the photovoltaic efficiency of solar cells through the absorption of two subband-gap photons that allow extra electron-hole pair formations. Previous theoretical and experimental findings support the proposal that the layered SnS2 compound, with a band-gap of around 2 eV, is a candidate for an intermediate-band material when it is doped with a specific transition-metal. In this work we characterize vanadium doped SnS2 using density functional theory at the dilution level experimentally found and including a dispersion correction combined with the site-occupancy-disorder method. In order to analyze the electronic characteristics that depend on geometry, two SnS2 polytypes partially substituted with vanadium in symmetry-adapted non-equivalent configurations were studied. In addition the magnetic configurations of vanadium in a SnS2 2H-polytype and its comparison with a 4H-polytype were also characterized. We demonstrate that a narrow intermediate-band is formed, when these dopant atoms are located in different layers. Our theoretical predictions confirm the recent experimental findings in which a paramagnetic intermediate-band material in a SnS2 2H-polytype with 10% vanadium concentration is obtained
Estudio de la microporosidad estrecha de carbones activados obtenidos de semilla de palma por activación quÃmica con KOH
Se estudió el desarrollo de los microporos por vÃa quÃmica en carbones que se obtuvieron a partir de semillas
de la palma veitchia merriillii, utilizando en todos los casos una baja relación KOH/carbonizado y flujo de nitrógeno. El
procedimiento de obtención fue realizado mediante dos pasos, carbonización y activación, en este último, se utilizaron
temperaturas de activación crecientes de 873, 973, 1 073 y 1 173 K respectivamente. La caracterización fue llevada a cabo
mediante las isotermas de adsorción de CO2 a 295 K, en un equipo de vidrio pyrex y varias válvulas de teflón, acoplado a
un sistema Edward de alto vacÃo, el que también consta de un transductor de presión. Los resultados se compararon con
los publicados por Dubinin para los carbones comerciales tipo tamiz molecular CMS4A y CMS5A y con carbones microporosos
con diferentes porcentajes de quemado o burn off, obtenidos también por Dubinin a partir de cascarón de coco.
Por otra parte, se estudió la influencia de la temperatura de activación y de la relación KOH/carbonizado en todos los
resultados. Las conclusiones en este trabajo indican de manera general que los carbones activados obtenidos presentan
volúmenes de microporos y microporosidades estrechas tan o más desarrolladas que los carbones obtenidos del mismo
material activado por vÃa fÃsica y también mayores que los demás carbones obtenidos por Dubinin con los que fueron
comparados, por lo que pueden ser utilizados como material de partida para la obtención de carbones tipo tamiz molecula
Engineering the electronic bandgaps and band edge positions in carbon-substituted 2D boron nitride: a first-principles investigation
Modification of graphene to open a robust gap in its electronic spectrum is essential for its use in field effect transistors and photochemistry applications. Inspired by recent experimental success in the preparation of homogeneous alloys of graphene and boron nitride (BN), we consider here engineering the electronic structure and bandgap of C2xB1−xN1−x alloys via both compositional and configurational modification. We start from the BN end-member, which already has a large bandgap, and then show that (a) the bandgap can in principle be reduced to about 2 eV with moderate substitution of C (x < 0.25); and (b) the electronic structure of C2xB1−xN1−x can be further tuned not only with composition x, but also with the configuration adopted by C substituents in the BN matrix. Our analysis, based on accurate screened hybrid functional calculations, provides a clear understanding of the correlation found between the bandgap and the level of aggregation of C atoms: the bandgap decreases most when the C atoms are maximally isolated, and increases with aggregation of C atoms due to the formation of bonding and anti-bonding bands associated with hybridization of occupied and empty defect states. We determine the location of valence and conduction band edges relative to vacuum and discuss the implications on the potential use of 2D C2xB1−xN1−x alloys in photocatalytic applications. Finally, we assess the thermodynamic limitations on the formation of these alloys using a cluster expansion model derived from first-principles
Material de banda intermedia basado en un compuesto semiconductor de tipo calcogenuro de estaño
Material de banda intermedia basado en un compuesto
semiconductor de tipo calcogenuro de estaño.
La invención se refiere a compuestos formados mediante
la introducción, dentro de un semiconductor de partida
que es de tipo calcogenuro de estaño tetravalente octaédricamente
coordinado, de un elemento de transición
en posición octaédrica, para la fabricación de materiales
o dispositivos para aplicaciones fotónicas. El elemento
de transición genera una banda intermedia parcialmente
ocupada separada de las de valencia y conducción del
semiconductor de partida, según resulta de cálculos mecanocuánticos.
Esto posibilita obtener, por absorción de
dos fotones de energÃa inferior a la anchura de la banda
prohibida del semiconductor de partida, un resultado
equivalente al que se consigue absorbiendo un fotón de
energÃa superior a dicha anchura en ausencia de banda
intermedia. Usando el material de la invención se obtiene
un mayor rendimiento y mejores prestaciones en diversos
dispositivos de tipo fotovoltaico, fotocatalÃtico, fotoelectroquÃmico,
optoelectrónico o de conversión fotónica.Peer reviewedConsejo Superior de Investigaciones CientÃficas (España), Universidad Politécnica de MadridA1 Solicitud de patentes con informe sobre el estado de la técnic
New generation of more efficient solar energy materials: Quantum modeling and experimental realizations
Trabajo presentado la XXV IUPAP Conference on Computational Physics, celebrada en Moscú (Rusia) del 20 al 24 de agosto de 2013.The intermediate band (IB) solar cell has been proposed as a novel device structure which could enhance photovoltaic efficiency, thanks to the cooperation of two sub-bandgap energy photons which allows the full excitation of an electron across the whole bandgap of a semiconductor. To realize this principle we have proposed in recent years, on the basis of quantum mechanical calculations, several materials where a metal or heavy element substitutes an atom in a known semiconductor with appropriate band gap, creating inside the band gap a partially filled band. This new intermediate band, allows the absorption of low energy photons inside the gap increasing the photo-current and also maintaining the photo-voltage. We present here compounds derived from different families of sulphides semiconductors, mainly spinels thin film and layered compounds:
- In2S3 and other sulphides containing octahedral In. The V-doped In2S3 material is particularly promising. We have synthesized it in nanocrystalline form and shown that its optical absorption spectrum has the features predicted by quantum calculations3 . Recent photocatalytic tests made with it show that the V dopant extends its spectral response down to the IR range without increasing recombination.
- Octahedral SnIV layered sulphide and other similar compounds show also, according to theoretical modeling the formation of an IB with the desired characteristics when transition metals are introduced at Sn sites. The Van der Waals cleavage plane (0001) of these layered semiconductors is characterized by hexagonal arrays of close packed chalcogenide ions which are covalently bound within X-M-X sandwiches. This is an ideal substrate to study fundamental aspects of the metal/semiconductor interaction. The experimental synthesis of such sulphide show optical absorption spectra matching again the expectations for an IB material4 .
An overview of these systems, including results obtained on them using high level, state-of-the-art quantum calculation methods will be presented. Experimental results obtained for such novel IB materials matching in all cases the theoretical predictions.Peer Reviewe
More Efficient Photovoltaic Materials by Metal Substitution in Sulphides Compounds: Theoretical Predictions and Experimental Results
Trabajo presentado en EUROMAT: European Congress and Exhibition on advanced materials and processes, celebrado en Sevilla (España) del 8 al 13 de septiembre de 2013.The introduction of a properly chosen transition metal at high concentration into an octahedral semiconductor provides in-gap delocalized and partially occupied levels required by the Intermediate Band (IB) concept. We have verified with accurate DFT calculations including Van der Waals forces, that layered semiconductors as SnS2 or ZnIn2S4 can provide this situation when an octahedral cation in their structure is partially substituted by an element such as vanadium. The Van der Waals cleavage plane (0001) of these layered semiconductors is characterized by hexagonal arrays of close packed chalcogenide ions which are covalently bound within X-M-X sandwiches. This is an ideal substrate to study fundamental aspects of the metal/semiconductor interaction. Experimental work made via wet chemistry methods verifies that new absorption features appear in the optical absorption spectrum which matches the predicted DFT-based theoretical absorption results. Moreover, simple photocatalytic tests with these two materials, show spectral responses which evidence in both cases a very substantial ability to use sub-bandgap photons to drive electron and hole transfers at the material interface with scarce efficiency degradation by recombination effects. The results show that these materials have the appropriate characteristics for building photovoltaic devices of boosted efficiency using the whole range of the visible light spectrumPeer Reviewe
New generation of materials for more efficient solar energy use: Quantum modeling and experimental realizations
Trabajo presentado en el Simposio en EnergÃa y Sostenibilidad; XXXIV Reunión Bienal de la Real Sociedad Española de FÃsica, celebrado en Valencia (España) del 15 al 19 de julio de 2013.The intermediate band (IB) solar cell has been proposed as a novel device structure which could enhance photovoltaic efficiency, thanks to the cooperation of two sub-bandgap energy photons which allows the full excitation of an electron across the whole bandgap of a semiconductor. To realize this principle we have proposed in recent years, on the basis of quantum mechanical calculations, several materials where a metal or heavy element substitutes an atom in a known semiconductor with appropriate band gap, creating inside the band gap a partially filled band. This new intermediate band, allows the absorption of low energy photons inside the gap increasing the photo-current and also maintaining the photo-voltage. We present here compounds derived from different families of sulphides semiconductors, mainly spinels thin film and layered compounds:
- In2S3 and other sulphides containing octahedral In. The V-doped In2S3 material is particularly promising. We have synthesized it in nanocrystalline form and shown that its optical absorption spectrum has the features predicted by quantum calculations3 . Recent photocatalytic tests made with it show that the V dopant extends its spectral response down to the IR range without increasing recombination.
- Octahedral SnIV layered sulphide and other similar compounds show also, according to theoretical modeling the formation of an IB with the desired characteristics when transition metals are introduced at Sn sites. The Van der Waals cleavage plane (0001) of these layered semiconductors is characterized by hexagonal arrays of close packed chalcogenide ions which are covalently bound within X-M-X sandwiches. This is an ideal substrate to study fundamental aspects of the metal/semiconductor interaction. The experimental synthesis of such sulphide show optical absorption spectra matching again the expectations for an IB material4 .
An overview of these systems, including results obtained on them using high level, state-of-the-art quantum calculation methods will be presented. Experimental results obtained for such novel IB materials matching in all cases the theoretical predictions.Peer Reviewe
Separation of an aqueous mixture of 6-kestose/sucrose with zeolites: A molecular dynamics simulation
Extra-large pore zeolites are a small subset (21) among the whole list of 253 zeolites available. The discovery of new low-glycemic sugars is very attractive as new healthy additives in the food field. This is the case of the 6-kestose. In the present case, it appears in a mixture in aqueous solution together with sucrose, the separation of the mixture being necessary. For this, we have focused on using certain zeolites with adequate pore sizes that allow the separation of this mixture, considering that since the molecular size of 6-kestose is greater than sucrose, it is necessary to promote the sorption of the latter, so that the first can be purified. After a computational screening of micropores of the 253 IZA zeolites, 11 zeolites were selected. Of these, 3 extra-large pore zeolites (AET, DON, ETR) have been proposed, which were analyzed in-depth through a molecular dynamics study considering the external surface. The results show that DON presents the most promising theoretical results for a selective sucrose/6-kestose separation.We thank MICINN of Spain for funding through projects RTI2018-101784-B-I00, RTI2018-101033-B-I00, SEV-2016-0683 as well as ASIC-UPV and CESGA for computational facilities. IBL and PGI gratefully acknowledge CSIC for a JAE-Intro fellowship. AM thanks Generalitat Valenciana for the predoctoral fellowship GRISOLIAP/2019/084