208 research outputs found
The influence of surface stress on the equilibrium shape of strained quantum dots
The equilibrium shapes of InAs quantum dots (i.e., dislocation-free, strained
islands with sizes >= 10,000 atoms) grown on a GaAs (001) substrate are studied
using a hybrid approach which combines density functional theory (DFT)
calculations of microscopic parameters, surface energies, and surface stresses
with elasticity theory for the long-range strain fields and strain relaxations.
In particular we report DFT calculations of the surface stresses and analyze
the influence of the strain on the surface energies of the various facets of
the quantum dot. The surface stresses have been neglected in previous studies.
Furthermore, the influence of edge energies on the island shapes is briefly
discussed. From the knowledge of the equilibrium shape of these islands, we
address the question whether experimentally observed quantum dots correspond to
thermal equilibrium structures or if they are a result of the growth kinetics.Comment: 7 pages, 8 figures, submitted to Phys. Rev. B (February 2, 1998).
Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Density-functional study of hydrogen chemisorption on vicinal Si(001) surfaces
Relaxed atomic geometries and chemisorption energies have been calculated for
the dissociative adsorption of molecular hydrogen on vicinal Si(001) surfaces.
We employ density-functional theory, together with a pseudopotential for Si,
and apply the generalized gradient approximation by Perdew and Wang to the
exchange-correlation functional. We find the double-atomic-height rebonded D_B
step, which is known to be stable on the clean surface, to remain stable on
partially hydrogen-covered surfaces. The H atoms preferentially bind to the Si
atoms at the rebonded step edge, with a chemisorption energy difference with
respect to the terrace sites of >sim 0.1 eV. A surface with rebonded single
atomic height S_A and S_B steps gives very similar results. The interaction
between H-Si-Si-H mono-hydride units is shown to be unimportant for the
calculation of the step-edge hydrogen-occupation. Our results confirm the
interpretation and results of the recent H_2 adsorption experiments on vicinal
Si surfaces by Raschke and Hoefer described in the preceding paper.Comment: 13 pages, 8 figures, submitted to Phys. Rev. B. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Adlayer core-level shifts of random metal overlayers on transition-metal substrates
We calculate the difference of the ionization energies of a core-electron of
a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a
fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using
density-functional-theory. We analyze the initial-state contributions and the
screening effects induced by the core hole, and study the influence of the
alloy composition for a number of noble metal-transition metal systems. Data
are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x)
Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our
analysis clearly indicates the importance of final-state screening effects for
the interpretation of measured core-level shifts. Calculated deviations from
the initial-state trends are explained in terms of the change of inter- and
intra-atomic screening upon alloying. A possible role of alloying on the
chemical reactivity of metal surfaces is discussed.Comment: 4 pages, 2 figures, Phys. Rev. Letters, to appear in Feb. 199
Identifying the Azobenzene/Aniline reaction intermediate on TiO2-(110) : a DFT Study
Density functional theory (DFT) calculations, both with and without dispersion corrections, have been performed to investigate the nature of the common surface reaction intermediate that has been shown to exist on TiO2(110) as a result of exposure to either azobenzene (C6H5N═NC6H5) or aniline (C6H5NH2). Our results confirm the results of a previous DFT study that dissociation of azobenzene into two adsorbed phenyl imide (C6H5N) fragments, as was originally proposed, is not energetically favorable. We also find that deprotonation of aniline to produce this surface species is even more strongly energetically disfavored. A range of alternative surface species has been considered, and while dissociation of azobenzene to form surface C6H4NH species is energetically favored, the same surface species cannot form from adsorbed aniline. On the contrary, adsorbed aniline is much the most stable surface species. Comparisons with experimental determinations of the local adsorption site, the Ti–N bond length, the molecular orientation, and the associated C 1s and N 1s photoelectron core level shifts are all consistent with the DFT results for adsorbed aniline and are inconsistent with other adsorbed species considered. Possible mechanisms for the hydrogenation of azobenzene required to produce this surface species are discussed
The role of electronic correlation in the Si(100) reconstruction: a quantum Monte Carlo study
Recent low-temperature scanning tunneling experiments have challenged the
generally accepted picture of buckled silicon dimers as the ground state
reconstruction of the Si(100) surface. Together with the symmetric dimer model
of the surface suggested by quantum chemistry calculations on small clusters,
these findings question our general understanding of electronic correlations at
surfaces and its proper description within density functional theory. We
present quantum Monte Carlo calculations on large cluster models of the
symmetric and buckled surface, and conclude that buckling remains energetically
more favorable even when the present-day best treatment of electronic
correlation is employed.Comment: 5 pages, Revtex, 10 figure
First-principle study of excitonic self-trapping in diamond
We present a first-principles study of excitonic self-trapping in diamond.
Our calculation provides evidence for self-trapping of the 1s core exciton and
gives a coherent interpretation of recent experimental X-ray absorption and
emission data. Self-trapping does not occur in the case of a single valence
exciton. We predict, however, that self-trapping should occur in the case of a
valence biexciton. This process is accompanied by a large local relaxation of
the lattice which could be observed experimentally.Comment: 12 pages, RevTex file, 3 Postscript figure
Formation and stability of self-assembled coherent islands in highly mismatched heteroepitaxy
We study the energetics of island formation in Stranski-Krastanow growth
within a parameter-free approach. It is shown that an optimum island size
exists for a given coverage and island density if changes in the wetting layer
morphology after the 3D transition are properly taken into account. Our
approach reproduces well the experimental island size dependence on coverage,
and indicates that the critical layer thickness depends on growth conditions.
The present study provides a new explanation for the (frequently found) rather
narrow size distribution of self-assembled coherent islands.Comment: 4 pages, 5 figures, In print, Phys. Rev. Lett. Other related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Pseudopotential study of binding properties of solids within generalized gradient approximations: The role of core-valence exchange-correlation
In ab initio pseudopotential calculations within density-functional theory
the nonlinear exchange-correlation interaction between valence and core
electrons is often treated linearly through the pseudopotential. We discuss the
accuracy and limitations of this approximation regarding a comparison of the
local density approximation (LDA) and generalized gradient approximations
(GGA), which we find to describe core-valence exchange-correlation markedly
different. (1) Evaluating the binding properties of a number of typical solids
we demonstrate that the pseudopotential approach and namely the linearization
of core-valence exchange-correlation are both accurate and limited in the same
way in GGA as in LDA. (2) Examining the practice to carry out GGA calculations
using pseudopotentials derived within LDA we show that the ensuing results
differ significantly from those obtained using pseudopotentials derived within
GGA. As principal source of these differences we identify the distinct behavior
of core-valence exchange-correlation in LDA and GGA which, accordingly,
contributes substantially to the GGA induced changes of calculated binding
properties.Comment: 13 pages, 6 figures, submitted to Phys. Rev. B, other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Equilibrium shapes and energies of coherent strained InP islands
The equilibrium shapes and energies of coherent strained InP islands grown on
GaP have been investigated with a hybrid approach that has been previously
applied to InAs islands on GaAs. This combines calculations of the surface
energies by density functional theory and the bulk deformation energies by
continuum elasticity theory. The calculated equilibrium shapes for different
chemical environments exhibit the {101}, {111}, {\=1\=1\=1} facets and a (001)
top surface. They compare quite well with recent atomic-force microscopy data.
Thus in the InP/GaInP-system a considerable equilibration of the individual
islands with respect to their shapes can be achieved. We discuss the
implications of our results for the Ostwald ripening of the coherent InP
islands. In addition we compare strain fields in uncapped and capped islands.Comment: 10 pages including 6 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Super-resolution imaging of C-type lectin spatial rearrangement within the dendritic cell plasma membrane at fungal microbe contact sites
Dendritic cells express DC-SIGN and CD206, C-type lectins (CTLs) that bind a variety of pathogens and may facilitate pathogen uptake for subsequent antigen presentation. Both proteins form punctate membrane nanodomains (~80 nm) on naïve cells. We analyzed the spatiotemporal distribution of CTLs following host-fungal particle contact using confocal microscopy and three distinct methods of cluster identification and measurement of receptor clusters in super-resolution datasets: DBSCAN, Pair Correlation and a custom implementation of the Getis spatial statistic. Quantitative analysis of confocal and super-resolution images demonstrated that CTL nanodomains become concentrated in the contact site relative to non-contact membrane after the first hour of exposure and established that this recruitment is sustained out to 4 h. DC-SIGN nanodomains in fungal contact sites exhibit a 70% area increase and a 38% decrease in interdomain separation. Contact site CD206 nanodomains possess 90% greater area and 42% lower interdomain separation relative to non-contact regions. Contact site CTL clusters appear as disk-shaped domains of approximately 150–175 nm in diameter. The increase in length scale of CTL nanostructure in contact sites suggests that the smaller nanodomains on resting membranes may merge during fungal recognition, or that they become packed closely enough to achieve sub-resolution inter-domain edge separations of <30 nm. This study provides evidence of local receptor spatial rearrangements on the nanoscale that occur in the plasma membrane upon pathogen binding and may direct important signaling interactions required to recognize and respond to the presence of a relatively large pathogen
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