Search

In this work, the electronic structures of quantum dots (QDs) of nine direct band gap semiconductor materials belonging to the group II-VI and III-V families are investigated, within the empirical tight-binding framework, in the effective bond orbital model. This methodology is shown to accurately describe these systems, yielding, at the same time, qualitative insights into their electronic properties. Various features of the bulk band structure such as band-gaps, band curvature, and band widths around symmetry points affect the quantum confinement of electrons and holes. These effects are identified and quantified. A comparison with experimental data yields good agreement with the calculations. These theoretical results would help quantify the optical response of QDs of these materials and provide useful input for applications. [ABSTRACT FROM AUTHOR]

The structure of wurtzite and zinc blende InAs-GaAs (001) core-shell nanowires grown by molecular beam epitaxy on GaAs (001) substrates has been investigated by transmission electron microscopy. Heterowires with InAs core radii exceeding 11 nm, strain relax through the generation of misfit dislocations, given a GaAs shell thickness greater than 2.5 nm. Strain relaxation is larger in radial directions than axial, particularly for shell thicknesses greater than 5.0 nm, consistent with molecular statics calculations that predict a large shear stress concentration at each interface corner. [ABSTRACT FROM AUTHOR]

We report on selective area growth of InAs and GaAs quantum dots (QDs) on GaAs through ∼20 nm SiO2 windows prepared by block copolymer lithography. We discuss the mechanisms of growth through these masks, highlighting the variation of the resulting morphology (dot size, spacing, uniformity, and areal density) as a function of growth parameters. We have obtained highly uniform arrays of InAs and GaAs QDs with mean diameters and areal densities of 20.6 nm and 1×1011 cm-2, respectively. We have also investigated the optical characteristics of these QDs as a function of temperature and drawn correlations between the optical response and their crystalline quality. [ABSTRACT FROM AUTHOR]

It has been established that deviations from stoichiometry during the growth of ZnSe crystals result in point defects, which influence its electronic properties. We report on detailed photoluminescence results and their systematic analysis for ZnSe nanowires. We studied photoluminescence from vapor-phase grown undoped ZnSe nanowires grown under excess Zn conditions, and in particular the dependence on excitation intensity. Luminescence spectra were characterized by strong near-band-edge luminescence with negligible deep-level emission. We observed excitonic emission at 2.794 eV related to the neutral donor at VSe. The binding energy of the exciton was found to be 7 meV, and that of the donor was 35 meV. Two donor-acceptor pair transitions at 2.714 and 2.686 eV were also observed, which can be related to the defect complexes of native defects with other native defects or with common unintentional shallow donors and acceptors. The ionization energies of both donors were 27 meV, whereas those of the acceptors were 102 and 139 meV, respectively. [ABSTRACT FROM AUTHOR]

In order to realize the full potential of nanowires for optical applications, it is essential to synthesize nanowires that can emit predominantly via band to band or band edge (BE) transitions. However, many compound semiconductor nanowires, irrespective of the method of their growth, contain a high density of native defects; these result in competing deep defect (DD) related emission, limiting their utility for optoelectronic device applications. The concentration of these native defect states depends on the gas phase stoichiometry. In this work, we report on the influence of gas phase stoichiometry on the structural and optical properties of single crystal zinc selenide (ZnSe) nanowires. We find that nanowires grown under stoichiometric conditions contain such defect states with associated weak BE emission and strong DD emission. However, nanowires grown under Zn-rich conditions were characterized by photoluminescence spectra dominated by strong BE emission while those grown under Se-rich conditions showed strong DD related emission. Hence, it is necessary to develop a strategy for enhancing the BE emission while simultaneously quenching the DD emission. We demonstrate a technique of postgrowth treatment that can effectively perform this function, and using this strategy the ratio of the BE/DD emission can be increased by a factor of several thousands, at least an order of magnitude higher than previously reported values. This reveals BE dominated photoluminescence in these nanowires and makes these nanowires suitable for developing future optoelectronic devices. [ABSTRACT FROM AUTHOR]

A method for quantum control of coherence in an open quantum computing system is presented. The approach was applied to finding a suitable control sequence for maintaining coherence in a silicon-based nuclear spin quantum computing system, subject to Bose-type environmental noise, and under the Born-Markovian and rotating wave approximations. It is shown that the evolution operator remains decoherence free while the three control pulses are being applied. [ABSTRACT FROM AUTHOR]

The selective area growth of InAs quantum dots (QDs) on GaAs through 20 nm Si[O.sub.2] windows prepared by block copolymer lithography is described. Highly uniform array of InAs and GaAs QDs is obtained with mean diameters and areal densities.

The full details of the synthesis and characterization of highly metallized polyferrocenylsilanes (PFSs) with pendant cobalt, molybdenum, and nickel clusters through macromolecular complexation of carbon-carbon triple bonds in acetylide-substituted PFSs is reported. The studies suggest that the highly metallized polymers may be useful in the fabrication of patterned arrays of alloy nanoparticles for both materials science and catalytic applications.

Ferroelectric Ba0.7Sr0.3TiO3 films are fabricated using a PbO-doped barium strontium titanate sol-gel precursor. The findings show that doping in excess of 20 at. % PbO is found to markedly decrease film porosity, while enhancing crystallization by decreasing the annealing temperature required for the formation of the perovskite phase form above 650 degree C to 500 degree C.

Presents information on a study which described the evolution of a model of a two-dimensional quantum cellular neural network (QCNN). Approximate polarization states; QCNN driven by an external magnetic field; Evolution of the density operator; Conclusions.

Presents a study which developed models for ab initio molecular orbital calculations for stimulating the main characteristics of reconstructed GaAs (100) surfaces. Theoretical background; Computational approach; Results and discussion.

Polymer encapsulants are an essential component in photovoltaic (PV) devices, providing mechanical support, optical coupling, and electrical and physical isolation. However, moisture ingress into the module can degrade these polymers and subsequently the performance of the device. In this paper, we report experimental measurements of the temporal evolution of moisture content in ethylene‐vinyl acetate (EVA) encapsulant in a double‐glass PV module. Using physical properties of EVA as determined by water vapour transmission rate measurements, we simulate diffusion of water into the module using a finite element model. The model accounts for realistic geometry of our module and is used to simulate accelerated test conditions and outdoor operation in geographic locations. Using the calculated results, we propose two schemes using the accelerated test results to understand the behaviour of modules operating in humid climates. Finally, we show that the time needed to reach the saturation water concentration can be increased by as much as a factor of two by reducing the initial water content in EVA films. Moisture ingress through encapsulant polymers is a major source of performance degradation in photovoltaic (PV) modules. We provide accurate numerical modelling of moisture ingress validated by experimental measurements using embedded humidity sensors. Location specific climate data are used to predict moisture content under operating conditions during the life‐time of PV modules, and the importance of controlling initial water concentration in the encapsulant is highlighted. [ABSTRACT FROM AUTHOR]

Nanowire-based field-effect transistors (FETs) have demonstrated considerable promise for a new generation of chemical and biological sensors. Indium arsenide (InAs), by virtue of its high electron mobility and intrinsic surface accumulation layer of electrons, holds properties beneficial for creating high performance sensors that can be used in applications such as point-of-care testing for patients diagnosed with chronic diseases. Here, we propose devices based on a parallel configuration of InAs nanowires and investigate sensor responses from measurements of conductance over time and FET characteristics. The devices were tested in controlled concentrations of vapour containing acetic acid, 2-butanone and methanol. After adsorption of analyte molecules, trends in the transient current and transfer curves are correlated with the nature of the surface interaction. Specifically, we observed proportionality between acetic acid concentration and relative conductance change, off current and surface charge density extracted from subthreshold behaviour. We suggest the origin of the sensing response to acetic acid as a two-part, reversible acid-base and redox reaction between acetic acid, InAs and its native oxide that forms slow, donor-like states at the nanowire surface. We further describe a simple model that is able to distinguish the occurrence of physical versus chemical adsorption by comparing the values of the extracted surface charge density. These studies demonstrate that InAs nanowires can produce a multitude of sensor responses for the purpose of developing next generation, multi-dimensional sensor applications. [ABSTRACT FROM AUTHOR]

This paper presents a new hybrid passive power device, denoted as the hybrid transformer, which is an improved alternative to the classical two-winding high-frequency transformer (100's kHz–MHz). The hybrid transformer is comprised of a coupled inductor topology, that provides load–source matching, denoted as the mutual branch topology, and a dielectric isolation device denoted as the multilayer capacitive isolation device. The main focus of this paper is on the design and operation of the mutual branch topology. Simulation results are shown to highlight the improved efficiency of the hybrid transformer over a conventional high-frequency transformer. Finally, the efficiency of the hybrid transformer is compared with that of a classical two-winding transformer using the same resonant converter, highlighting the improved efficiency of the hybrid transformer (up to 10% improved efficiency for light loads). [ABSTRACT FROM AUTHOR]