Surface chemistry and density distribution influence on visible luminescence of silicon quantum dots: an experimental and theoretical approach.

The impact of the surface reconstruction of the density distribution and photoluminescence of silicon quantum dots (QDs) embedded in a silicon oxide matrix (SiOx) has been studied. Annealing treatments carried out on the as-deposited samples provoked the effusion of hydrogen species. Moreover, depending on the surrounding density and coalescence of QDs, they resulted in a change in the average size of the particles depending on the initial local environment. The shift in the luminescence spectra all over the visible region (blue, green and red) shows a strong dependence on the resultant change in the size and/or the passivation environment of QDs. Density functional theoretical (DFT) calculations support this fact and explain the possible electronic transitions (HOMO-LUMO gap) involved. Passivation in the presence of oxygen species lowers the band gap of Si29 and Si35 nanoclusters up to 1.7 eV, whereas, surface passivation in the environment of hydrogen species increases the band gap up to 4.4 eV. These results show a good agreement with the quantum confinement model described in this work and explain the shift in the luminescence all over the visible region. The results reported here offer vital insight into the mechanism of emission from silicon quantum dots which has been one of the most debated topics in the last two decades. QDs with multiple size distribution in different local environments (band gap) observed in this work could be used for the fabrication of light emission diodes (LEDs) or shift-conversion thin films in third generation efficient tandem solar cells for the maximum absorption of the solar spectrum in different wavelength regions.

Spontaneous, Solvent-Free, Polymer-Templated, Solid-Solid Transformation of Thin Metal Films into Nanoparticles.

Metal nanoparticles have unusual optical, electronic, sensing, recognition, catalytic, and therapeutic properties. They are expected to form the basis of many of the technological and biological innovations of this century. A prerequisite for future applications using nanoparticles as functional entities is control of the shape, size, and homogeneity of these nanoparticles and of their interparticle spacing and arrangement on surfaces, between electrodes, or in devices. Here, we demonstrate that thin films of gold, silver, and copper sputter-deposited onto the surface of an organic polymer poly[[1,1':4',1″-terphenyl]-4,4″-diyl(2-bromo-1-carboxyethylidene)] (PTBC) undergo spontaneous solid-solid transformation into nanoparticles. Furthermore, we show that, by varying the thickness of the films, the volume-to-surface ratio of the polymer substrate, and the amount of plasticizer, it is possible to control the rate of transformation and the morphology of the nanoparticles formed. PTBC containing Au nanoparticles was found to enhance the cell adhesion and proliferation. To the best of our knowledge, our findings constitute the first experimental evidence of spontaneous, room-temperature, solid-solid transformation of metal films sputtered onto the surface of an organic polymeric substrate into nanoparticles (crystals).

Experimental and theoretical rationalization of the growth mechanism of silicon quantum dots in non-stoichiometric SiN x : role of chlorine in plasma enhanced chemical vapour deposition.

Silicon quantum dots (Si-QDs) embedded in an insulator matrix are important from a technological and application point of view. Thus, being able to synthesize them in situ during the matrix growth process is technologically advantageous. The use of SiH2Cl2 as the silicon precursor in the plasma enhanced chemical vapour deposition (PECVD) process allows us to obtain Si-QDs without post-thermal annealing. Foremost in this work, is a theoretical rationalization of the mechanism responsible for Si-QD generation in a film including an analysis of the energy released by the extraction of HCl and the insertion of silylene species into the terminal surface bonds. From the results obtained using density functional theory (DFT), we propose an explanation of the mechanism responsible for the formation of Si-QDs in non-stoichiometric SiN x starting from chlorinated precursors in a PECVD system. Micrograph images obtained through transmission electron microscopy confirmed the presence of Si-QDs, even in nitrogen-rich (N-rich) samples. The film stoichiometry was controlled by varying the growth parameters, in particular the NH3/SiH2Cl2 ratio and hydrogen dilution. Experimental and theoretical results together show that using a PECVD system, along with chlorinated precursors it is possible to obtain Si-QDs at a low substrate temperature without annealing treatment. The optical property studies carried out in the present work highlight the prospects of these thin films for down shifting and as an antireflection coating in silicon solar cells.

Gold as an intruder in ZnO nanowires.

Several techniques for obtaining ZnO nanowires (ZnO NWs) have been reported in the literature. In particular, vapour-liquid-solid (VLS) with Au as a catalyst is widely used. During this process, Au impurities in the ZnO NWs can be incorporated accidentally, and for this reason we named these impurities as intruders. It is thought that these intruders may produce interesting alterations in the electronic characteristics of nanowires. In the experiment, it is not easy to detect either Au atoms in these nanowires, or the modification that intruders produce in different electrical, optical and other properties. For this reason, in this density functional theory investigation, the effect of Au intruders on ZnO NWs is analysed. Au extended (thread) and point defects (atoms replacing Zn or O, or Au interstitials) are used to simulate the presence of gold atoms. Optimised geometries, band-gaps and density of states indicate that the presence of small amounts of Au drastically modifies the electronic states of ZnO NWs. The results reported here clearly indicate that small amounts of Au have a strong impact on the electronic properties of ZnO NWs, introducing states in the band edges that may promote transitions in the visible spectral region. The presence of Au as an intruder in ZnO NWs enhances the potential use of this system for photonic and photovoltaic applications.

Polymorphous silicon thin films obtained by plasma-enhanced chemical vapor deposition using dichlorosilane as silicon precursor.

Polymorphous silicon thin films (pm-Si) have been deposited from mixtures of dichlorosilane and hydrogen, using argon as the diluting gas by plasma-enhanced chemical vapor deposition. The deposition conditions were chosen to simultaneously obtain both Si nanocrystallites and an amorphous silicon matrix in the as-grown samples. High resolution transmission electron microscopy studies show the crystallinity of Si domains whose dimensions are in the interval of 2-14 nm. The surface passivation state of the silicon nanocrystals was inferred from Fourier transform infrared spectroscopy analysis. Two optical absorption edges, corresponding to the amorphous matrix and the Si nanocrystals, were observed for all the pm-Si thin films. Intense visible photoluminescence was observed for the as-grown samples. The possibility of using these thin films for the down-conversion effect in silicon solar cells is discussed.

Photoluminescence of as-grown silicon nanocrystals embedded in silicon nitride: influence of atomic hydrogen abundance.

Silicon nanocrystals embedded in silicon nitride films were grown by direct plasma enhanced chemical vapor deposition at 300 degrees C, using mixtures of SiH2Cl2/NH3/H2/Ar. The films composition and chemical stability was tested by Fourier Transform Infrared Spectroscopy and Rutherford Backscattering Spectroscopy. The influence of hydrogen abundance during the deposition process on the photoluminescence of as-grown samples was studied as a function of the radiofrequency power and hydrogen dilution flow rate. In situ Optical Emission Spectroscopy allowed the diagnostic of the species in the plasma region and their general trends as a function of the radiofrequency power. The changes in the hydrogen content and silicon incorporation to the film as a function of the radiofrequency power were discussed in terms of silicon nanocrystals formation and growth in the silicon nitride matrix. The photoluminescence emission from the as-grown samples was found to red-shift with increasing hydrogen abundance. This observation is consistent with the increase in silicon content associated to nc-Si of larger size. On the other hand, the photoluminescence intensity was observed to decrease for very high radiofrequency powers and hydrogen dilutions. High Resolution Transmission Electron Microscopy confirmed the presence of silicon nanocrystals embedded in the amorphous silicon nitride matrix and allowed the correlation between the nanocrystals size and the photoluminescence emission energy using the quantum confinement model.

Growth of silicon nanoclusters on different substrates by plasma enhanced chemical vapor deposition.

We report an atomic force microscopy study of the early stages of growth of silicon nanoclusters formed on different substrates by plasma-enhanced chemical vapor deposition, using dichlorosilane (SiH2CI2) and hydrogen (H2) as reactive gases. (100) n-type single crystalline silicon, fused silica, amorphous silicon nitride and corning glass, were used as substrates for the growth of the nanoclusters, which were formed at low substrate temperature (200 degrees C). The diameter, height and number density of the clusters were controlled by the deposition time and pressure. It was found that not only the plasma conditions but also the surface characteristics of the substrate influence the cluster density, shape, and size. For the ordered silicon surface and the amorphous fused silica, the nanoclusters result oval in shape and exhibit preferential growth along the surface. When deposited over amorphous silicon nitride and corning glass, the density of nanoclusters increases and there is a tendency toward columnar growth since the diameter of the nanoclusters tends to decrease. We conclude that although the specific features of the nanoclusters originate from the chlorine chemistry introduced by the SiHxCly deposition precursor and the chemical stability of chlorine-terminated surfaces under hydrogen plasma, the surface quality and roughness also plays an important role on the nucleation and mobility of the species. The combination of both effects gives rise to the different nanostructured growths observed.

Use of polyethyleneglycol for porcine islet cryopreservation.

The aim of this work was to determine whether polyethylene glycol 20000 Da (PEG) could be used as protective agent in porcine islet cryopreservation. Cryopreservation was performed on 1-wk cultured pig islets and consisted in an overnight storage in liquid nitrogen. In a first set of experiments, we compared the in vitro function of PEG-cryopreserved islets to that of porcine islets cryopreserved under the standard procedure using dimethylsulfoxide (DMSO), by incubating the islets over 45 min in Krebs buffer containing either 2.8 or 10 mmol/L glucose. Insulin secretion of both types of islets reached a maximum at day 10 postthawing and had stimulation indices above 2 up to 3 wk after thawing. PEG-cryopreserved islets secreted more insulin than DMSO-treated islets and showed glucose-dependency insulin secretion in a 0-16.6 mmol/L glucose range. We also established that PEG-cryopreserved islets were as functional in vitro as nonfrozen tissue and that they could reverse experimental diabetes of the mouse for longer periods of time than noncryopreserved islets (p < 0.005 3 wk after transplantation) when implanted in the peritoneal cavity, being immunoprotected in a semipermeable hollow fiber. PEG can, therefore, be considered as a suitable cryoprotective compound for porcine islet storage.

Zinc intervention on macrophages and lymphocytes response.

Normal zinc levels are essential for the development and maintenance of immune functions; Zn deficiency is detrimental to the embryo and offspring of experimental animals, especially concerning immune development. It is known that Zn supplementation improves immune responses. To further explore the relation between Zn administration and the metal in vitro effects, we studied zinc (500 mg/l) supplementation impact on lymphocytes and macrophages and zinc in vitro effects, in BALB/c mice supplemented from gestation to lactation. Results show a significant increase in proliferation (assessed by 3H incorporation) in lymphocytes exposed to Zn (0.1 mM) in vitro, in 3-wk-old mice; this effect is annulled when the supplementation period is lengthened, indicating saturation of the mechanisms involved in zinc induced stimulation. Macrophages functional capacity assessed by erythrophagocytosis was also improved by Zn supplementation and furthermore by the in vitro exposure to the metal, in mice 3 wk old, this was also depressed by Zn accumulation due to the supplementation period extension (9 weeks). Results show an improvement in the immune parameters analysed due to zinc supplementation and to zinc in vitro exposure. Results also suggest the accumulation of zinc as a result of prolonged supplementation periods, suppresses the cells response to zinc in vitro.

Photoluminescence Study of Gallium Nitride Thin Films Obtained by Infrared Close Space Vapor Transport.

Photoluminescence (PL) studies in GaN thin films grown by infrared close space vapor transport (CSVT-IR) in vacuum are presented in this work. The growth of GaN thin films was done on a variety of substrates like silicon, sapphire and fused silica. Room temperature PL spectra of all the GaN films show near band-edge emission (NBE) and a broad blue and green luminescence (BL, GL), which can be seen with the naked eye in a bright room. The sample grown by infrared CSVT on the silicon substrate shows several emission peaks from 2.4 to 3.22 eV with a pronounced red shift with respect to the band gap energy. The sample grown on sapphire shows strong and broad ultraviolet emission peaks (UVL) centered at 3.19 eV and it exhibits a red shift of NBE. The PL spectrum of GaN films deposited on fused silica exhibited a unique and strong blue-green emission peak centered at 2.38 eV. The presence of yellow and green luminescence in all samples is related to native defects in the structure such as dislocations in GaN and/or the presence of amorphous phases. We analyze the material quality that can be obtained by CSVT-IR in vacuum, which is a high yield technique with simple equipment set-up, in terms of the PL results obtained in each case.

Concentración de fluor en orina en tres escuelas rurales y análisis de las tres marcas comerciales de sal de cocina del Municipio de la Unión, Antioquia / Fluoride concentration in urine in 3 rural schools and analysis of three commercial salt brands in the municipality of the Union in Colombia

Durante mucho tiempo se ha utilizado la fluorización del agua y de la sal como medidas preventivas masivas. En Colombia desde 1.984 se esta adicionando el flúor a la sal de cocina.Este estudio describe las concentraciones de flúor en la orina de una población rural. Igualmente, hace el análisis de muestras de sal de cocina recogidas en la misma zona.La muestra fue de 64 niños, entre los 6 y 11 años, que se dividieron en dos subgrupos: de 6 a 8 años y de 9 a 11 años. Se recolectaron dos muestras: en la mañana y después del almuerzo. Se estudiaron mediante el método directo para análisis de flúor con electrodo específico. Para las muestras de sal se utilizó el método del electrodo del ión específico.El flúor esta llegando a la comunidad estudiada. En todos los grupos hubo un incremento en la concentración de flúor en el grupo de 6 a 8 años después del almuerzo.La concentración de flúor en la sal vario en todas las marcas...

Estudio descriptivo de los desórdenes traumáticos acumulativos en los trabajadores del Complejo Industrial de Barrancabermeja, septiembre 1996-agosto 1997 / Descriptive study of cumulative trauma disorders in workers of the Industrial Complex in Barrancabermeja, September 1996-August 1997

Los desórdenes traumáticos acumulativos (DTA) son los desórdenes músculo-esqueléticos relacionados con el trabajo como resultado del sobreuso de alguna parte del cuerpo por repetición o movimientos forzados requeridos en muchas ocupaciones, y que constituyen uno de los motivos de consulta médica más frecuentes en la población trabajadora. El presente estudio tuvo como objetivo describir el comportamiento de estos transtornos en trabajadores vinculados a término indefinido en la Gerencia Complejo Barrancabermeja (GCB) de Ecopetrol en Bucaramanga, entre septiembre de 1996 y agosto de 1997. En 2.775 trabajadores la prevalencia de DTA fue de 15,86 por ciento, con un compromiso principalmente de la región lumbar (prevalencia de 9,87 por ciento) el hombro (3,03 por ciento) y el cuello (2,34 por ciento), se destaca la escasa frecuencia del síndrome de túnel carpiano (0,54 por ciento). El riesgo DTA que se aumenta con la edad y con la antiguedad en la empresa, fue significativamente mayor para los trabajadores de nómina convencional en comparación con los directivos. No se observaron diferencias por género. Por dicha causa se presentó un índice de ausencia de 17,1 por millón de horas programadas, con un costo promedio por trabajador convencional de $1.092.792. Al comparar la problemática por patología lumbar con estudios realizados anteriormente en la empresa, se destaca una disminución en el número de consultas promedio, y se resalta que la consulta repetitiva por esta causa no es problema relevante en la empresa. Como conclusión, el estudio brinda información sobre la gran magnitud de la problemática de los DTA en las empresa y la necesidad de implantar un sistema de vigilancia epidemiológica que facilite el control de los factores de riesgo ergonómicos asociados con este tipo de transtornos