Unconventional MBE Strategies from Computer Simulations for Optimized Growth Conditions

We investigate the influence of step edge diffusion (SED) and desorption on Molecular Beam Epitaxy (MBE) using kinetic Monte-Carlo simulations of the solid-on-solid (SOS) model. Based on these investigations we propose two strategies to optimize MBE growth. The strategies are applicable in different growth regimes: During layer-by-layer growth one can exploit the presence of desorption in order to achieve smooth surfaces. By additional short high flux pulses of particles one can increase the growth rate and assist layer-by-layer growth. If, however, mounds are formed (non-layer-by-layer growth) the SED can be used to control size and shape of the three-dimensional structures. By controlled reduction of the flux with time we achieve a fast coarsening together with smooth step edges.Comment: 19 pages, 7 figures, submitted to Phys. Rev.

Electrical and spectroscopic analysis in nanostructured SnO2: "Long-term" resistance drift is due to in-diffusion

A model for conductance in n-type non-degenerate semiconductors is proposed and applied to polycrystalline SnO2 used as a gas sensor. Particular attention is devoted to the fundamental mechanism of Schottky barrier formation due to surface states in nanostructured grains. Electrical and absorption infra-red spectroscopic analysis constitutes strong evidence for oxygen diffusion into the tin oxide grains. The model is then extended to include oxygen in- and out-diffusion. Thus, it is possible to explain the long-term resistance drift in oxygen for fully depleted grained samples in terms of tunneling through the double barrier

Tunneling through surface barrier and modified mass action law in nanostructured metal oxide semiconductors

The surface barrier of metal oxide semiconductors was studied, under spherical symmetry, for grains smaller than the depletion width, w the condition which makes the material properly nanostructured. Thus, it was possible to explain the lower observed surface barrier due to the unpinning of Fermi level with respect to coarser grained materials. The model allowed us to explain the different behavior of conductance in gas of two sets of sensors with grains having two well distinct characteristic radii (Rw). In the first case (R<w), the overlapping of barriers can take place in the presence of a gas, an effect which strongly affects the tunneling contribution to conductivity with respect to the thermionic one, at the same temperature. The behavior of conductance in the presence of gases and the effect of temperature have been explained through the mechanism of barrier modulation through gas chemisorption assuming that the density of vacancies can only be modified by interstitial oxygen diffusion in and out of the nanograins. The apparent contradiction of the density of vacancies dependence on the field is finally solved

Novel CO2 and CO gas sensor based on nanostructured Sm 2O3 hollow microspheres

Sm2O3 microspheres were prepared by the coprecipitation method, using samarium nitrate and formic acid. The size of the microspheres was in the range 1-6 μm; however, extensive fragmentation was observed. An outstanding improvement of the microstructure was achieved by using an aqueous solution of pectin. In this case, hollow and nanoporous microspheres, having little fragmentation, were produced. The evolution of the crystal structure with calcination temperature revealed the formation of single-phase cubic Sm2O3, at 600 °C. The effects of the calcination temperature and the concentration of samarium nitrate, on the microstructure of Sm2O3, were investigated. For the gas sensing characterization, thick films were prepared with the as-prepared Sm 2O3 microspheres. The gas sensing performance of Sm 2O3 sensor devices was enhanced when gold electrodes were used. Transient impedance measurements revealed a reproducible and reliable detection of carbon dioxide and carbon monoxide at 400°C. The increase of the applied frequency produced stable and noiseless graphs. Furthermore, quantitative detection of the test gases was revealed by impedance and polarization measurements. © 2014 Elsevier B.V

Novel communication scheme based on chaotic Rossler circuits

Sm2O3 microspheres were prepared by the coprecipitation method, using samarium nitrate and formic acid. The size of the microspheres was in the range 1-6 ?m; however, extensive fragmentation was observed. An outstanding improvement of the microstructure was achieved by using an aqueous solution of pectin. In this case, hollow and nanoporous microspheres, having little fragmentation, were produced. The evolution of the crystal structure with calcination temperature revealed the formation of single-phase cubic Sm2O3, at 600 °C. The effects of the calcination temperature and the concentration of samarium nitrate, on the microstructure of Sm2O3, were investigated. For the gas sensing characterization, thick films were prepared with the as-prepared Sm 2O3 microspheres. The gas sensing performance of Sm 2O3 sensor devices was enhanced when gold electrodes were used. Transient impedance measurements revealed a reproducible and reliable detection of carbon dioxide and carbon monoxide at 400°C. The increase of the applied frequency produced stable and noiseless graphs. Furthermore, quantitative detection of the test gases was revealed by impedance and polarization measurements. " 2014 Elsevier B.V.",,,,,,"10.1016/j.snb.2014.06.038",,,"http://hdl.handle.net/20.500.12104/43239","http://www.scopus.com/inward/record.url?eid=2-s2.0-84904169760&partnerID=40&md5=7c5e92223ad26f3a3d88c503cd365283",,,,,,,,"Sensors and Actuators, B: Chemical",,"122

Gas in-diffusion contribution to impedance in tin oxide thick films

The ac electrical resistance and capacitance of SnO2 thick films under vacuum and air atmosphere were analyzed using the Cole–Cole plot. To fit the experimental results, a simple circuit model that considers a capacitance and a resistance in parallel was employed. An explanation for the resistance variation considering spherical grains with different characteristics is proposed. A careful analysis of the resulting depletion layers and doping levels gives evidence for gas diffusion into the grain

Conduction mechanisms in epitaxial NiO Graphene gas sensors

Integrated, highly sensitive and reversible sensor devices for toxic and hazardous gases in environmental pollution monitoring can be realized with graphene-based materials. Here we show that, single layer graphene grown on SiC can be utilized to implement sensor devices being extremely sensitive towards NO2 showing an n-type response. A second type of sensor with an added NiO layer on top of the single layer graphene changed its response to p-type but did not reduce its sensitivity. We show that the conduction switch from n-type to p-type was not a consequence of an alteration of the graphene layer but is found to be an effect of the NiO layer. We find that the NiO leads to lowering of the Fermi level to a point that a crossing of the Dirac Point in the graphene switched the conduction type. These sensors were tested in the 100 ppb NO2 regime, showing good response and a detection limit extrapolated to be below 1 ppb. This new NiO/graphene/SiC configuration can be an attractive p-type sub-ppb sensor platform for NO2 and related gases