1,721,039 research outputs found
Density of states at the Si(111)-(2×1) surface: A study of the clean and H-covered surface
No full textThe electronic structure of the Si (111)-(2×1) surface is analyzed by comparing different reconstruction models. The changes of state density induced by chemisorption of hydrogen are investigated for the different surface geometries
Pairing of Hydrogen Atoms on the Si(100)-2×1 Surface: The Role of Interactions Among Dimers
No full textLocal-density-functional calculations of the pairing of H atoms on the (100)-2×1 surface of Si are presented. We find that significant interactions between neighboring dimers along a row are present, favoring configurations where H-unpaired dimers are in the same row and have the H-atoms all on the same side. We discuss how these effects can contribute to the effective H-pairing energies determined experimentally
Electron transfer with core-level excitations at hybrid interfaces
No full textElectron core-level spectroscopies have emerged as effective tools to investigate several aspects of the hybrid interface between organic molecules and a substrate. In particular, resonant photoemission spectroscopy can measure interfacial electron transfer times down to the femtosecond timescale. Furthermore, the strong perturbation induced by the core hole opens up the several questions on how the properties of the interface are modified, calling for a theoretical description of the core-excited system.
We adopt a theoretical framework based on density-functional theory (DFT), where the excitation is introduced explicitly in the core-level occupation of an atom in a molecule, to investigate the electronic structure and electron transfer from/to organic molecules adsorbed on metal, semimetal, and semiconducting substrates.
The perturbing potential lowers the energy of the molecular orbitals. Focusing on the lowest-unoccupied (LUMO), a filling of the core-excited LUMO* by substrate electrons may occur within the core-hole lifetime, as found for molecules on metals where the adsorption angle is also shown to influence the electron transfer rate [1,2]. In the case of a semimetal graphene substrate, a spin-polarized LUMO* pinned at the Fermi level can be determined for physisorbed molecules. In that case electron transfer would be suppressed given the low density of states of unsupported graphene at that energy, but still possible for graphene supported on a metal [3]. For molecules adsorbed on a semiconductor, the LUMO* may form a bound exciton in the gap [4]. Here, we found especially interesting to consider the influence of thermal motion on the energy-level alignment and the absorption coefficient [5,6].
References
[1] D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, submitted.
[2] A. Baby, G. Fratesi, S.R. Vaidya, L.L. Patera, C. Africh, L. Floreano, G.P. Brivio, J. Phys. Chem. C 119 (2015) 3624.
[3] A. Ravikumar, A. Baby, H. Lin, G.P. Brivio, and G. Fratesi, Scientific Reports 6 (2016) 24603.
[4] G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118 (2014) 8775
[5] H. Lin, G. Fratesi, S. Selçuk, G.P. Brivio, and A. Selloni, J. Phys. Chem. C, 120 (2016) 3899.
[6] M. Muller, D. Sànchez-Portal, H. Lin, G. Fratesi, G.P. Brivio, and A. Selloni, in preparation
Hydrogen-induced Dereconstruction of Si(111)-2x1 from First-Principles Molecular Dynamics
No full textWe have studied the dereconstruction of Si(111)2 x 1 upon hydrogen adsorption by means of first-principles total energy and molecular dynamics calculations. We find that the (1 x 1) bulk-terminated surface structure becomes energetically favored over the (2 x 1), starting from a coverage theta approximately 0.3. This is consistent with LEED results showing that the dereconstruction is completed at theta approximately 0.5. A path for this conversion has been determined via direct simulation at theta = 0.5. This has a rather low energy barrier, approximately 0.10 - 0.15 eV/(surface atom), and should therefore occur with significant probability during hydrogenation of the surface
A DFT Study of Cysteine Adsorption on Au(111)
No full textThe adsorption of the cysteine amino acid (H-SCbetaH2-CalphaH-NH2-COOH) on the (111) surface of gold is studied by means of periodic density functional calculations. Results for different adsorption sites and molecular configurations show that chemisorption involving S(thiolate)-Au bonds on Au(111) is favored by starting with either cysteine or cystine gas-phase molecular precursors. In the most stable adsorption configuration, the sulfur headgroup sits at the bridge site between two surface An atoms, and the S-C-beta bond is tilted by 57degrees with respect to the surface normal, whereas the in-plane orientation of the molecular backbone plays a secondary role. The analysis of the electronic properties shows that the hybridization of the p-like S states with the d-like Au states produces both bonding and antibonding occupied orbitals, and the process is well described by a model for the interaction of localized orbitals with narrow-band dispersive electron states. The bonding orbitals well below the Fermi level contribute to the strong chemisorption of cysteine on gold. The calculated sulfur-projected density of states allows us to locate the cysteine molecular orbitals with respect to the system Fermi level, which gives a measure of the injection barrier at the molecule/electrode junction
Lifetime for resonant tunneling in a transverse magnetic field
No full textWe present a numerical study of the time-dependent resonant tunneling of a Gaussian wave packet through a double-barrier structure in a transverse magnetic field. From the decay rate of the charge trapped in the quantum well, we obtain the field dependence of the lifetime τ0 of the resonant level. τ0 is found to exhibit an oscillatory behavior with increasing magnetic field. This effect is explained as due to the field-induced hybridization of the resonant level with the interfacial Landau states corresponding to the semiclassical skipping orbits at the barrier interfaces. An experiment which should allow the observation of such effects in real heterostructures is suggested
beta-phase of Pb on Ge(111): the competing roles of electronic bonding and thermal fluctuations
No full textWe have studied the low temperature phases of Pb/Ge(111) at Pb coverages THETA = 1/3, 1, and 4/3 using ab initio molecular dynamics. Stable phases are found to occur at THETA = 1/3 and 4/3. For the low-coverage phase, we confirm the model of Pb atoms adsorbed at T4 sites of the Ge(111) substrate. For the high THETA phase and very low temperature, we determine a structure characterized by the presence of covalently bonded Pb chains parallel to [110BAR] substrate directions. At room temperature, fluctuations are found to disrupt long-range Pb chains and restore the threefold symmetry of the ideal Ge(111). Our results agree well with x-ray diffraction and STM data
Time-dependent tunneling of electron wave packets in a transverse magnetic field
No full textWe present a numerical study of the time-dependent tunneling of a Gaussian wave packet through a thick barrier in the presence of a transverse magnetic field. The time evolution of the charge density, current density, and total current are described in detail. The field dependence of the effective barrier transmission coefficient is studied and found to agree well with semiclassical results. The wave-packet center-of-mass delay time is found to decrease with increasing field strength and appears to tend to the zero-field phase-delay time
Luminescence line-shape analysis of the electron-hole plasma in direct-gap Ga-Al-As: Random-phase-approximation approach
No full textThe luminescence line shapes of an electron-hole plasma in Ga-Al-As have been calculated within the random-phase approximation (RPA). Different models have been tested for the spectral weight function. The fully self-consistent RPA is found to overestimate the weight of the plasmon sideband and to underestimate the energy-gap narrowing. An approach which neglects plasmon replicas instead yields luminescence spectra in excellent agreement with experiment over a wide range of plasma densities and temperatures. A further model is also introduced to estimate an upper bound for the weight of plasmon sidebands in the experimental spectra. We used both static and dynamical phonon screening. We found that the former screening correctly describes luminescence up to densities such that the plasmon energy is of the order of the longitudinal-optical-phonon energ
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