Glycine adsorption on silicon (001)

In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the dissociative adsorption process of glycine on the silicon surface. Our total energy calculations indicate that the chemisorption of the molecule is as follow. The gas phase NH2-C2H2-OOH adsorbs molecularly to the electrophilic surface Si atom and then dissociates into NH2-C2H2-OO and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms respectively, with an energy barrier corresponding to a thermal activation that is smaller than the usual growth temperature, indicating that glycine molecules will be observed in their dissociated states at room temperature. This picture is further support by our calculated vibrational modes for the considered adsorbed species

Bilipid Membrane Phase Characterization by Reflectance Anisotropy Spectroscopy (RAS)

In this work we propose the use of experimental and theoretical reflectance anisotropy spectra (RAS) as a new tool to identify structural and dynamical aspects of the bilipid membrane and its various constituent molecules. The role of geometric details at the atomic level and macroscopic quantities, such as the membrane curvature and tilt for the different gel phases, in the theoretical RAS spectra (using Kohn-Sham density functional theory (KS-DFT)) are presented. Then the results are compared to the experimentally measured spectra taken from other techniques.FAPESPCNPqCAPE

Adsorption of C2H2-C2O3 on Si(001)

The adsorption of maleic anhydride on the Si(001) surface was investigated using the first-principles pseudopotential formalism. Our results indicate that C2H2-C2O3 adsorbs through a [2+2] cycloaddition of the C-C bond to the dangling bonds of Si-Si dimers, as observed for other small hydrocarbons. According to our calculations, the adsorption of a second maleic anhydride molecule will preferentially occur on first-neighbours Si-Si dimer sites in either the dimer chain our dimer row, i.e. considering a (4×2) reconstruction, C2H2-C2O3 can either form a linear chain or a zig-zag chain, depending on the growth conditions. Although both structures are found to be stable from the energetic point of view, only our STM theoretical images for the linear chain are comparable to available experimental data

A theoretical study of acrylonitrile adsorption on Si(001)

The present work is a comparative study of possible adsorption structures of the conjugated molecule acrylonitrile on Si(001) employing the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory. In the recent literature it is proposed the interaction of acrylonitrile with Si(001) through a cycloaddition reaction of the cyano group, in competition with the bounding of the two outer atoms of the molecule skeleton with the Si dimer in cross-dimer and cross-trench geometries; between other geometries like which correspond the reaction of the C = C bond with a Si dimer. Starting from a large number of configurations our calculations favor the planar cycloaddition through the terminal N and C atoms on the Si dimer. In this way we explain the electronic and vibrational features obtained experimentally

CH3CN on Si(001) : adsorption geometries and electronic structure

In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of acetonitrile on the silicon surface. Our first-principles calculations indicate that CH3CN adsorbs via a [2+2] cycloaddition reaction through the CºN group with an adsorption energy around 35 kcal/mol, close to the 30 kcal/mol estimated by Tao and co-workers. The electronic structure and the surface states calculated for the adsorbed system are also discussed

Surface properties of CdS nanoparticles

With a view to contribute to the understanding the surface effects on optical properties process, and its hole in the electronic properties of the nanoparticles, CdS based nanoparticles are characterised by different experimental techniques and the experimental results compared to density functional theory calculations. Our results indicate that cubic CdS nanoparticles present a strong structural deformation, hexagonal reconstructed structures preserve their lattice behaviour. Both cubic and hexagonal CdS nanoparticles are S-terminated after relaxation, even when mildly Cd-rich nanoparticles are considered. A broad peak observed in our PL measurements is interpreted as an experimental evidence of the surface related peak observed around 1.8 eV in our calculated DOS for the hexagonal relaxed structure

Sulfur Radicals as Tethers for the Adsorption of Aromatic Molecules on Silicon Surface

In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of benzenethiol and diphenyl disulfide with the silicon (001) surface. A direct comparison of different adsorption structures with Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) allow us to identify that benzenethiol and diphenyl disulfide dissociatively adsorb on the silicon surface. In addition, theoretically obtained data suggests that the C6H5SH:Si(001) presents a higher Schottky barrier height contact when compared to other similar aromatic molecules.FAPESPCNP

The role of carbon impurities on the Si(001)-c(4 x 4) surface reconstruction: Theoretical calculations

In this work we employ the state-of-the-art pseudopotential method, within a generalized gradient approximation to the density functional theory, combined with a recently developed method for the calculation of HREELS spectra to study a series of different proposed models for carbon incorporation on the silicon (001) surface. A fully discussion on the geometry, energetics and specially the comparison between experimental and theoretical STM images and electron energy loss spectra indicate that the Si(100)-c(4 x 4) is probably induced by Si-C surface dinners, in agreement with recent experimental findings. (C) 2009 Elsevier B.V. All rights reserved.Brazilian Agencies FAPESP and CNPqFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Comparative study of the adsorption and dissociation of vinylacetic acid and acrylic acid on silicon (001)

In this work, we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of acrylic acid (AAc) and vinylacetic acid (VAA) on the silicon surface. Our total energy calculations support the proposed experimental process, as it indicates that the chemisorption of the molecule is as follows: The gas phase VAA (AAc) adsorbs molecularly to the electrophilic surface Si atom and then dissociates into H(2)C = CH - COO and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The activation energy for both processes correspond to thermal activations that are smaller than the usual growth temperature. In addition, the electronic structure, calculated vibrational modes, and theoretical scanning tunneling microscopy images are discussed, with a view to contribute to further experimental investigations

Semiconductor nanoparticle modeling via density functional theory

In this work, a 2.0 nm nanoparticle (low limit synthesized system) is compared to possible simplified models: passivated clusters, small (1.3 nm) nanoparticles and sets of plane surfaces. Our density functional theory results suggest that even when geometric aspects are properly described by the simplifications considered, electronic properties might be very different, especially when edge atoms are not properly taken into account in the nanoparticle`s modeling. In addition, we propose a protocol that might help future theoretical descriptions of nanoparticles.FAPESPFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPqCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)CAPE