Vanadium-Doped In and Sn Sulphides: Photocatalysts able to use the whole visible light spectrum

Using photocatalysis for energy applications depends, more than for environmental purposes or selective chemical synthesis, on converting as much of the solar spectrum as possible; the best photocatalyst, titania, is far from this. Many efforts are pursued to use better that spectrum in photocatalysis, by doping titania or using other materials (mainly oxides, nitrides and sulphides) to obtain a lower bandgap, even if this means decreasing the chemical potential of the electron-hole pairs. Here we introduce an alternative scheme, using an idea recently proposed for photovoltaics: the intermediate band (IB) materials. It consists in introducing in the gap of a semiconductor an intermediate level which, acting like a stepstone, allows an electron jumping from the valence band to the conduction band in two steps, each one absorbing one sub-bandgap photon. For this the IB must be partially filled, to allow both sub-bandgap transitions to proceed at comparable rates; must be made of delocalized states to minimize nonradiative recombination; and should not communicate electronically with the outer world. For photovoltaic use the optimum efficiency so achievable, over 1.5 times that given by a normal semiconductor, is obtained with an overall bandgap around 2.0 eV (which would be near-optimal also for water phtosplitting). Note that this scheme differs from the doping principle usually considered in photocatalysis, which just tries to decrease the bandgap; its aim is to keep the full bandgap chemical potential but using also lower energy photons. In the past we have proposed several IB materials based on extensively doping known semiconductors with light transition metals, checking first of all with quantum calculations that the desired IB structure results. Subsequently we have synthesized in powder form two of them: the thiospinel In2S3 and the layered compound SnS2 (having bandgaps of 2.0 and 2.2 eV respectively) where the octahedral cation is substituted at a â?10% level with vanadium, and we have verified that this substitution introduces in the absorption spectrum the sub-bandgap features predicted by the calculations. With these materials we have verified, using a simple reaction (formic acid oxidation), that the photocatalytic spectral response is indeed extended to longer wavelengths, being able to use even 700 nm photons, without largely degrading the response for above-bandgap photons (i.e. strong recombination is not induced) [3b, 4]. These materials are thus promising for efficient photoevolution of hydrogen from water; work on this is being pursued, the results of which will be presented.

Interband absorption of photons by extended states in intermediate band solar cells

This paper considers sub-bandgap photon absorption in an InAs/GaAs quantum dot matrix. Absorption coefficients are calculated for transitions from the extended states in the valence band to confined states in the conduction band. This completes a previous body of work in which transitions between bound states were calculated. The calculations are based on the empirical k·p Hamiltonian considering the quantum dots as parallelepipeds. The extended states may be only partially extended?in one or two dimensions?or extended in all three dimensions. It is found that extended-to-bound transitions are, in general, weaker than bound-to-bound transitions, and that the former are weaker when the initial state is extended in more coordinates. This study is of direct application to the research of intermediate band solar cells and other semiconductor devices based on light absorption in semiconductors nanostructured with quantum dots.

Comparative analysis of road financing approaches in Europe and the United States

Road infrastructure has a remarkable economic and social impact on society. This is why road financing has always drawn the attention of policymakers, especially when resources available for government spending become scarce. Nations exhibit differing approaches to dealing with road transportation financing. In the United States, the current system of road funding has been called into question because some regard it as insufficient to meet the amounts now required for road expenditures. By contrast, in most European countries, road charges are very high, but these revenues are not allocated for the funding of roads. This paper analyzes the balance between charging for the use of and expenditure on the road sector in the United States and compares the American policy with those of several European countries (Germany, United Kingdom, France, Spain, and Switzerland). To that end, a methodology is defined to calculate the annual amount of fee charges levied on light and heavy vehicles in the selected countries in order to compare those charges with annual road expenditures. The results show that road charges in America are noticeably lower than those paid in Europe. Additionally, the research concludes that in Europe, road-generated revenues exceed road expenditures in all the countries studied, so road charges actually subsidize other policies. By contrast, in the United States, the public sector subsidizes the road system in order to maintain the current level of expenditure.

Self assembled and ordered group III nitride nanocolumnar structures for light emitting applications

El objetivo de este trabajo es un estudio profundo del crecimiento selectivo de nanoestructuras de InGaN por epitaxia de haces moleculares asistido por plasma, concentrandose en el potencial de estas estructuras como bloques constituyentes en LEDs de nueva generación. Varias aproximaciones al problema son discutidas; desde estructuras axiales InGaN/GaN, a estructuras core-shell, o nanoestructuras crecidas en sustratos con orientaciones menos convencionales (semi polar y no polar). La primera sección revisa los aspectos básicos del crecimiento auto-ensamblado de nanocolumnas de GaN en sustratos de Si(111). Su morfología y propiedades ópticas son comparadas con las de capas compactas de GaN sobre Si(111). En el caso de las columnas auto-ensambladas de InGaN sobre Si(111), se presentan resultados sobre el efecto de la temperatura de crecimiento en la incorporación de In. Por último, se discute la inclusión de nanodiscos de InGaN en las nanocolumnas de GaN. La segunda sección revisa los mecanismos básicos del crecimiento ordenado de nanoestructuras basadas en GaN, sobre templates de GaN/zafiro. Aumentando la relación III/V localmente, se observan cambios morfológicos; desde islas piramidales, a nanocolumnas de GaN terminadas en planos semipolares, y finalmente, a nanocolumnas finalizadas en planos c polares. Al crecer nanodiscos de InGaN insertados en las nanocolumnas de GaN, las diferentes morfologias mencionadas dan lugar a diferentes propiedades ópticas de los nanodiscos, debido al diferente carácter (semi polar o polar) de los planos cristalinos involucrados. La tercera sección recoge experimentos acerca de los efectos que la temperatura de crecimiento y la razón In/Ga tienen en la morfología y emisión de nanocolumnas ordenadas de InGaN crecidas sobre templates GaN/zafiro. En el rango de temperaturas entre 650 y 750 C, la incorporacion de In puede modificarse bien por la temperatura de crecimiento, o por la razón In/Ga. Controlar estos factores permite la optimización de la longitud de onda de emisión de las nanocolumnas de InGaN. En el caso particular de la generación de luz blanca, se han seguidos dos aproximaciones. En la primera, se obtiene emisión amarilla-blanca a temperatura ambiente de nanoestructuras donde la región de InGaN consiste en un gradiente de composiciones de In, que se ha obtenido a partir de un gradiente de temperatura durante el crecimiento. En la segunda, el apilamiento de segmentos emitiendo en azul, verde y rojo, consiguiendo la integración monolítica de estas estructuras en cada una de las nanocolumnas individuales, da lugar a emisores ordenados con un amplio espectro de emisión. En esta última aproximación, la forma espectral puede controlarse con la longitud (duración del crecimiento) de cada uno de los segmentos de InGaN. Más adelante, se presenta el crecimiento ordenado, por epitaxia de haces moleculares, de arrays de nanocolumnas que son diodos InGaN/GaN cada una de ellas, emitiendo en azul (441 nm), verde (502 nm) y amarillo (568 nm). La zona activa del dispositivo consiste en una sección de InGaN, de composición constante nominalmente y longitud entre 250 y 500 nm, y libre de defectos extendidos en contraste con capas compactas de InGaN de similares composiciones y espesores. Los espectros de electroluminiscencia muestran un muy pequeño desplazamiento al azul al aumentar la corriente inyectada (desplazamiento casi inexistente en el caso del dispositivo amarillo), y emisiones ligeramente más anchas que en el caso del estado del arte en pozos cuánticos de InGaN. A continuación, se presenta y discute el crecimiento ordenado de nanocolumnas de In(Ga)N/GaN en sustratos de Si(111). Nanocolumnas ordenadas emitiendo desde el ultravioleta (3.2 eV) al infrarrojo (0.78 eV) se crecieron sobre sustratos de Si(111) utilizando una capa compacta (“buffer”) de GaN. La morfología y eficiencia de emisión de las nanocolumnas emitiendo en el rango espectral verde pueden ser mejoradas ajustando las relaciones In/Ga y III/N, y una eficiencia cuántica interna del 30% se deriva de las medidas de fotoluminiscencia en nanocolumnas optimizadas. En la siguiente sección de este trabajo se presenta en detalle el mecanismo tras el crecimiento ordenado de nanocolumnas de InGaN/GaN emitiendo en el verde, y sus propiedades ópticas. Nanocolumnas de InGaN/GaN con secciones largas de InGaN (330-830 nm) se crecieron tanto en sustratos GaN/zafiro como GaN/Si(111). Se encuentra que la morfología y la distribución espacial del In dentro de las nanocolumnas dependen de las relaciones III/N e In/Ga locales en el frente de crecimiento de las nanocolumnas. La dispersión en el contenido de In entre diferentes nanocolumnas dentro de la misma muestra es despreciable, como indica las casi identicas formas espectrales de la catodoluminiscencia de una sola nanocolumna y del conjunto de ellas. Para las nanocolumnas de InGaN/GaN crecidas sobre GaN/Si(111) y emitiendo en el rango espectral verde, la eficiencia cuántica interna aumenta hasta el 30% al disminuir la temperatura de crecimiento y aumentar el nitrógeno activo. Este comportamiento se debe probablemente a la formación de estados altamente localizados, como indica la particular evolución de la energía de fotoluminiscencia con la temperatura (ausencia de “s-shape”) en muestras con una alta eficiencia cuántica interna. Por otro lado, no se ha encontrado la misma dependencia entre condiciones de crecimiento y efiencia cuántica interna en las nanoestructuras InGaN/GaN crecidas en GaN/zafiro, donde la máxima eficiencia encontrada ha sido de 3.7%. Como alternativa a las nanoestructuras axiales de InGaN/GaN, la sección 4 presenta resultados sobre el crecimiento y caracterización de estructuras core-shell de InGaN/GaN, re-crecidas sobre arrays de micropilares de GaN fabricados por ataque de un template GaN/zafiro (aproximación top-down). El crecimiento de InGaN/GaN es conformal, con componentes axiales y radiales en el crecimiento, que dan lugar a la estructuras core-shell con claras facetas hexagonales. El crecimiento radial (shell) se ve confirmado por medidas de catodoluminiscencia con resolución espacial efectuadas en un microscopio electrónico de barrido, asi como por medidas de microscopía de transmisión de electrones. Más adelante, el crecimiento de micro-pilares core-shell de InGaN se realizó en pilares GaN (cores) crecidos selectivamente por epitaxia de metal-orgánicos en fase vapor. Con el crecimiento de InGaN se forman estructuras core-shell con emisión alrededor de 3 eV. Medidas de catodoluminiscencia resuelta espacialmente indican un aumento en el contenido de indio del shell en dirección a la parte superior del pilar, que se manifiesta en un desplazamiento de la emisión de 3.2 eV en la parte inferior, a 3.0 eV en la parte superior del shell. Este desplazamiento está relacionado con variaciones locales de la razón III/V en las facetas laterales. Finalmente, se demuestra la fabricación de una estructura pin basada en estos pilares core-shell. Medidas de electroluminiscencia resuelta espacialmente, realizadas en pilares individuales, confirman que la electroluminiscencia proveniente del shell de InGaN (diodo lateral) está alrededor de 3.0 eV, mientras que la emisión desde la parte superior del pilar (diodo axial) está alrededor de 2.3 eV. Para finalizar, se presentan resultados sobre el crecimiento ordenado de GaN, con y sin inserciones de InGaN, en templates semi polares (GaN(11-22)/zafiro) y no polares (GaN(11-20)/zafiro). Tras el crecimiento ordenado, gran parte de los defectos presentes en los templates originales se ven reducidos, manifestándose en una gran mejora de las propiedades ópticas. En el caso de crecimiento selectivo sobre templates con orientación GaN(11-22), no polar, la formación de nanoestructuras con una particular morfología (baja relación entre crecimiento perpedicular frente a paralelo al plano) permite, a partir de la coalescencia de estas nanoestructuras, la fabricación de pseudo-templates no polares de GaN de alta calidad. ABSTRACT The aim of this work is to gain insight into the selective area growth of InGaN nanostructures by plasma assisted molecular beam epitaxy, focusing on their potential as building blocks for next generation LEDs. Several nanocolumn-based approaches such as standard axial InGaN/GaN structures, InGaN/GaN core-shell structures, or InGaN/GaN nanostructures grown on semi- and non-polar substrates are discussed. The first section reviews the basics of the self-assembled growth of GaN nanocolumns on Si(111). Morphology differences and optical properties are compared to those of GaN layer grown directly on Si(111). The effects of the growth temperature on the In incorporation in self-assembled InGaN nanocolumns grown on Si(111) is described. The second section reviews the basic growth mechanisms of selectively grown GaNbased nanostructures on c-plane GaN/sapphire templates. By increasing the local III/V ratio morphological changes from pyramidal islands, to GaN nanocolumns with top semi-polar planes, and further to GaN nanocolumns with top polar c-planes are observed. When growing InGaN nano-disks embedded into the GaN nanocolumns, the different morphologies mentioned lead to different optical properties, due to the semipolar and polar nature of the crystal planes involved. The third section reports on the effect of the growth temperature and In/Ga ratio on the morphology and light emission characteristics of ordered InGaN nanocolumns grown on c-plane GaN/sapphire templates. Within the growth temperature range of 650 to 750oC the In incorporation can be modified either by the growth temperature, or the In/Ga ratio. Control of these factors allows the optimization of the InGaN nanocolumns light emission wavelength. In order to achieve white light emission two approaches are used. First yellow-white light emission can be obtained at room temperature from nanostructures where the InGaN region is composition-graded by using temperature gradients during growth. In a second approach the stacking of red, green and blue emitting segments was used to achieve the monolithic integration of these structures in one single InGaN nanocolumn leading to ordered broad spectrum emitters. With this approach, the spectral shape can be controlled by changing the thickness of the respective InGaN segments. Furthermore the growth of ordered arrays of InGaN/GaN nanocolumnar light emitting diodes by molecular beam epitaxy, emitting in the blue (441 nm), green (502 nm), and yellow (568 nm) spectral range is reported. The device active region, consisting of a nanocolumnar InGaN section of nominally constant composition and 250 to 500 nm length, is free of extended defects, which is in strong contrast to InGaN layers (planar) of similar composition and thickness. Electroluminescence spectra show a very small blue shift with increasing current, (almost negligible in the yellow device) and line widths slightly broader than those of state-of-the-art InGaN quantum wells. Next the selective area growth of In(Ga)N/GaN nanocolumns on Si(111) substrates is discussed. Ordered In(Ga)N/GaN nanocolumns emitting from ultraviolet (3.2 eV) to infrared (0.78 eV) were then grown on top of GaN-buffered Si substrates. The morphology and the emission efficiency of the In(Ga)N/GaN nanocolumns emitting in the green could be substantially improved by tuning the In/Ga and total III/N ratios, where an estimated internal quantum efficiency of 30 % was derived from photoluminescence data. In the next section, this work presents a study on the selective area growth mechanisms of green-emitting InGaN/GaN nanocolumns and their optical properties. InGaN/GaN nanocolumns with long InGaN sections (330-830nm) were grown on GaN/sapphire and GaN-buffered Si(111). The nanocolumn’s morphology and spatial indium distribution is found to depend on the local group (III)/N and In/Ga ratios at the nanocolumn’s top. A negligible spread of the average indium incorporation among different nanostructures is found as indicated by similar shapes of the cathodoluminescence spectra taken from single nanocolumns and ensembles of nanocolumns. For InGaN/GaN nanocolumns grown on GaN-buffered Si(111), all emitting in the green spectral range, the internal quantum efficiency increases up to 30% when decreasing growth temperature and increasing active nitrogen. This behavior is likely due to the formation of highly localized states, as indicated by the absence of a complete s-shape behavior of the PL peak position with temperature (up to room temperature) in samples with high internal quantum efficiency. On the other hand, no dependence of the internal quantum efficiency on the growth conditions is found for InGaN/GaN nanostructures grown on GaN/sapphire, where the maximum achieved efficiency is 3.7%. As alternative to axial InGaN/GaN nanostructures, section 4 reports on the growth and characterization of InGaN/GaN core-shell structures on an ordered array of top-down patterned GaN microrods etched from a GaN/sapphire template. Growth of InGaN/GaN is conformal, with axial and radial growth components leading to core-shell structures with clear hexagonal facets. The radial InGaN growth (shell) is confirmed by spatially resolved cathodoluminescence performed in a scanning electron microscopy as well as in scanning transmission electron microscopy. Furthermore the growth of InGaN core-shell micro pillars using an ordered array of GaN cores grown by metal organic vapor phase epitaxy as a template is demonstrated. Upon InGaN overgrowth core-shell structures with emission at around 3.0 eV are formed. With spatially resolved cathodoluminescence, an increasing In content towards the pillar top is found to be present in the InGaN shell, as indicated by a shift of CL peak position from 3.2 eV at the shell bottom to 3.0 eV at the shell top. This shift is related to variations of the local III/V ratio at the side facets. Further, the successful fabrication of a core-shell pin diode structure is demonstrated. Spatially resolved electroluminescence measurements performed on individual micro LEDs, confirm emission from the InGaN shell (lateral diode) at around 3.0 eV, as well as from the pillar top facet (axial diode) at around 2.3 eV. Finally, this work reports on the selective area growth of GaN, with and without InGaN insertion, on semi-polar (11-22) and non-polar (11-20) templates. Upon SAG the high defect density present in the GaN templates is strongly reduced as indicated by TEM and a dramatic improvement of the optical properties. In case of SAG on non-polar (11-22) templates the formation of nanostructures with a low aspect ratio took place allowing for the fabrication of high-quality, non-polar GaN pseudo-templates by coalescence of the nanostructures.