A. Hernández-Hernández

Physical properties of CdTe:Cu films grown at low temperature by pulsed laser deposition

CdTe:Cu films were grown by pulsed laser deposition on Corning glass slides at a substrate temperature of 300 °C. The thin films were grown using CdTe and Cu2Te powders, varying the Cu2Te concentration from 3 to 10 wt. %. The structural, compositional, optical, and electrical properties were analyzed as a function of the nominal copper concentration. X-ray diffraction shows that films have CdTe cubic phase. The compositional analysis indicates that CdTe:Cu films grown with lower Cu content have Te excess, on the other hand, films with higher Cu content have Te deficiencies. The electrical measurements showed that CdTe:Cu films grown with low Cu content present lowest resistivity.

Study of the interplay between N-graphene defects and small Pd clusters for enhanced hydrogen storage via a spill-over mechanism.

The hydrogen spill-over mechanism was studied by applying Density Functional Theory. We used small palladium clusters to act as the catalyst supported on the substrate (comprised of pyridinic and pyrrolic nitrogen doped graphene), in order to study hydrogen dissociation, migration and diffusion. Charge transfer and strong binding between the catalyst and the substrate lead to dissociated states of H2 and prevent clusters from detaching and coalescing. In dissociated cases of H2 on Pd clusters, energy barriers below 0.6 eV were found. Likewise, concerning hydrogen migration from the catalyst to the substrate, energy barrier values of 0.8 eV (pyridinic defect) and 0.5 eV (pyrrolic defect) were apparent in the case of the Pd4 cluster at full hydrogen saturation. This indicates that hydrogen dissociation and migration may occur spontaneously at room temperature. This result shows that the interaction between the defects and the small metal clusters may explain the role that defects play in hydrogen migration from the catalyst to the substrate. Subsequently, it was found that thermal desorption does not limit chemisorbed hydrogen diffusion on the substrate. This work may thus help to determine experimental strategies with the capacity to enhance hydrogen storage.

Structural and optical properties of Cu-doped CdTe films with hexagonal phase grown by pulsed laser deposition

Cu-doped CdTethin films were prepared by pulsed laser deposition on Corning glass substrates using powders as target. Films were deposited at substrate temperatures ranging from 100 to 300 °C. The X-ray diffraction shows that both the Cu-doping and the increase in the substrate temperature promote the presence of the hexagonal CdTe phase. For a substrate temperature of 300 °C a CdTe:Cu film with hexagonal phase was obtained. Raman and EDS analysis indicate that the films grew with an excess of Te, which indicates that CdTe:Cu films have p-type conductivity.

Synthesis of visible light emitting self assembled Ge nanocrystals embedded within a SiO2 matrix

As-grown light emitting self-assembled Ge nanocrystals (Ge-NCs) embedded in a SiO2 matrix were produced via a sequential deposition process of SiO2/Ge/SiO2 layers employing a reactive radio frequency sputtering technique. Obtained Ge-NCs show a crystallographic phase, the proportion, size, quality, and specific orientation of which are determined by the oxygen partial pressure. Photoluminescence (PL) spectra indicate that the size distribution of Ge-NCs is reduced and centered on about 8 nm when higher oxygen partial pressure is employed; the formation of Ge-NCs is corroborated by transmission electron microscopy measurements, and their sizes are consistent with estimates from PL measurements. Resistivity measurements are explained by a near neighbors hopping process, with specific features depending on the Ge-NCs’ size. The features of PL and resistivity measurements indicate that there is no appreciable dependence of the number of interfacial defects on the oxygen partial pressure.

Influence of plasma parameters and substrate temperature on the structural and optical properties of CdTe thin films deposited on glass by laser ablation

In the pulsed laser deposition of thin films, plasma parameters such as energy and density of ions play an important role in the properties of materials. In the present work, cadmium telluride thin films were obtained by laser ablation of a stoichiometric CdTe target in vacuum, using two different values for: substrate temperature (RT and 200 °C) and plasma energy (120 and 200 eV). Structural characterization revealed that the crystalline phase can be changed by controlling both plasma energy and substrate temperature; which affects the corresponding band gap energy. All the thin films showed smooth surfaces and a Te rich composition.

Study of the structure, optical properties, surface morphology and topology of ZnO thin films grown by sol–gel on silicon substrates

Polycrystalline visible light photoluminescent zinc oxide (ZnO) thin films have been grown on silicon substrates by the dip immersion sol?gel technique at low sintering temperature. The optical properties of the films were analyzed by UV?vis spectroscopy. The structural properties were measured with x-ray diffraction. The results show that the films are polycrystalline and have hexagonal wurtzite structure. The estimated band gap of the samples by UV?vis absorption spectroscopy is in good agreement with the value measured by UV?vis reflectance spectroscopy and is near 3.2 eV. The surface topography was observed with atomic force microscopy. The morphology of the samples was analyzed by scanning electron microscopy. Energy dispersive x-ray spectroscopy was used to make chemical composition measurements, the results indicate that the sample is stoichiometric. The photoluminescence (PL) spectroscopy shows a strong band in the blue?green region of the spectrum, which makes these films interesting to optoelectronic applications.

Stoichiometric 6H-SiC thin films deposited at low substrate temperature by laser ablation

Silicon carbide thin films were grown by laser ablation on silicon substrates at different deposition temperatures using SiC powders as target material. The structural, morphological, compositional, and optical properties were studied as a function of the deposition temperature. The 6H-SiC crystalline phase was observed by Raman spectroscopy, x-ray diffraction, and transmission electron diffraction without the presence of any other polytype. In the room temperature photoluminescence spectra, a broad band was observed in the visible region which suggests that these films can have applications on silicon based optoelectronics.

Changes to the dissociation barrier of H2 due to buckling induced by a chemisorbed hydrogen on a doped graphene surface

AbstractAn effectiveway of enhancing hydrogen storage on adsorbent materials can be induced by the hydrogen spill-over mechanism, although to date there is no general consensus which satisfactorily explains the mechanism. In this work, a possible reaction path to explain hydrogen adsorption is shown. Density-functional calculations were used to study the dissociation of molecular hydrogen near to a stressed region, as a consequence of chemisorbed hydrogen at the graphene-nitrogen surface. We found that as a result of the buckling induced by the chemisorbed hydrogen, the dissociation barrier of molecular hydrogen diminished by 0.84 eV. The chemisorbed hydrogen is the final state in the spill-over mechanism on a graphene-nitrogen decorated with palladium clusters. This effect helps to create hydrogen nanoislands that may change the diffusion and detrapping of H. An electronic structure analysis suggests that these systems occasionally present metallic or semiconductor behavior. Graphical AbstractHydrogen dissociation and adsorption process via buckling defect

Temperature dependent transport study of the SiOx/Ge/SiOx system

The transport properties of the SiOx/Ge/SiOx system are studied using the van der Pauw technique as function of temperature in the range from 35 K to 150 K for two representative samples grown by Radio Frequency sputtering under different conditions. It is found that variable range hopping conduction explains the temperature dependence of the resistivity. For both samples, the nearest neighbor hopping conduction process explains the temperature dependence of the resistivity in the range between 66 K and 150 K. For the sample with the roughest surface, Efros-Shkovskiis variable range hopping process explains better the results below 66 K, while for the other one, a combination of Motts variable range hopping in two and three dimensions explain better the results in the same temperature range.