Absorption and luminescence of hydrogen and oxygen passivated silicon quantum dots

Silicon (Si) quantum dots (QDs) passivated with oxygen and hydrogen of size 1 nm in diameter are prepared by wet chemical route and electrochemical route respectively. The optical measurements reveal the strong absorption feature around 4.7 eV and weak absorption at 3.4 eV for oxygen passivated Si QDs. Hydrogen passivated Si QDs of the same size show absorption at 4.9 eV. Both the oxygen and hydrogen passivated Si QDs show broad luminescence around 3.9 and 3.8 eV. Films of these QDs, when coated on crystalline silicon solar cells, show an increase in the efficiency of the solar cell by 12 %. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2788

Surface Kinetics and Generation of Different Terms in a Conservative Growth Equation

A method based on the kinetics of adatoms on a growing surface under epitaxial growth at low temperature in (1+1) dimensions is proposed to obtain a closed form of local growth equation. It can be generalized to any growth problem as long as diffusion of adatoms govern the surface morphology. The method can be easily extended to higher dimensions. The kinetic processes contributing to various terms in the growth equation (GE) are identified from the analysis of in-plane and downward hops. In particular, processes corresponding to the (h -> -h) symmetry breaking term and curvature dependent term are discussed. Consequence of these terms on the stable and unstable transition in (1+1) dimensions is analyzed. In (2+1) dimensions it is shown that an additional (h -> -h) symmetry breaking term is generated due to the in-plane curvature associated with the mound like structures. This term is independent of any diffusion barrier differences between in-plane and out of-plane migration. It is argued that terms generated in the presence of downward hops are the relevant terms in a GE. Growth equation in the closed form is obtained for various growth models introduced to capture most of the processes in experimental Molecular Beam Epitaxial growth. Effect of dissociation is also considered and is seen to have stabilizing effect on the growth. It is shown that for uphill current the GE approach fails to describe the growth since a given GE is not valid over the entire substrate.Comment: 14 pages, 7 figure

An ab-initio study of silicon adsorption on metallic surfaces (Au/Ag): Novel perspective to explore chemical bonding

We report a first-principle investigation of the structure and electronic properties of small Sin (n = 1−6,9) clusters deposited on the Au(111) and Ag(111) surfaces. The calculations were performed using a plane wave based pseudopotential method under the framework of density functional theory. The results reveal the preference of Si atom to be adsorbed on the h.c.p. site of the metal (111) surfaces with strong binding energy. We study monolayer (ML) deposition as well as the cluster deposition on both the surfaces. The clusters introduce interlayer forces in the adsorbate. Based on PDOS (projected density of states) analysis it is found that Si atoms acquire charges from the Au/Ag surface. The binding energies are consistent with the known cohesive energy of Ag and Au silicides. The planar Sin cluster deposition on metal surfaces show that Au provides an adjustable surface with relatively strong Au-Si interaction while Ag-Si relatively weak interaction leading to dimerization of Si. The strong bonding with the surface atoms is a result of p-d hybridization. Some of the 3-D clusters show shape distortions after deposition on metal surfaces. This leads to internal stresses after deposition. A statistical parameter is defined over PDOS. It helps to measure the state delocalization in energy. Implications of the Si-Metal interaction on the initial stages of growth are discussed

Oxygen impact on quantum confinement effect for silicon clusters in different size regimes: ab initio investigations

The phenomena where the parameter like optical gap, HOMO-LUMO gap etc of a cluster will behave like a monotonic function with respect to its size is called quantum confinement effect. Here we are dealing with clusters having number of silicon atoms as 10, 16, 19, 20, 35, 54 and 78 which are representatives of different size regime clusters. For each clusters we have experimented with a number of isomers and the results we are showing are only of the stable most isomers. Then for each clusters we are capping them with oxygen atoms and calculating their optical response using DFT based calculations. We also calculate their HOMO-LUMO gap with and without oxygen atoms. The main essence of this work is mainly revolving around the variation of optical gap as well as the HOMO-LUMO gap for all the clusters with respect to their size and also whether oxygen insertion can induce any changes in quantum confinement effect. Also for Si20, Si19, Si16 and Si10 we have calculating the optical spectra with a variation of inserted oxygen on the surface of those clusters. All the calculation are performed using Vienna ab initio simulation package (VASP) and Car Parrinello molecular dynamics (CPMD) for geometry optimization. The optical spectra of the clusters are performed by real space PARSEC code and time dependent density functional theory implemented RGWBS code which takes into account the many body effects using GW and BSE equations