501 research outputs found
Work functions of self-assembled monolayers on metal surfaces
Using first-principles calculations we show that the work function of noble
metals can be decreased or increased by up to 2 eV upon the adsorption of
self-assembled monolayers of organic molecules. We identify the contributions
to these changes for several (fluorinated) thiolate molecules adsorbed on
Ag(111), Au(111) and Pt(111) surfaces. The work function of the clean metal
surfaces increases in this order, but adsorption of the monolayers reverses the
order completely. Bonds between the thiolate molecules and the metal surfaces
generate an interface dipole, whose size is a function of the metal, but it is
relatively independent of the molecules. The molecular and bond dipoles can
then be added to determine the overall work function.Comment: 5 pages, 2 figure
Surface Dipoles and Work Functions of Alkylthiolates and Fluorinated Alkylthiolates on Au(111)
We study the dipole formation at the surface formed by -CH3 and -CF3
terminated shortchain alkyl-thiolate monolayers on Au(111). In particular, we
monitor the change in work function upon chemisorption using density functional
theory calculations. We separate the surface dipole into two contributions,
resulting from the gold-adsorbate interaction and the intrinsic dipole of the
adsorbate layer, respectively. The two contributions turn out to be
approximately additive. Adsorbate dipoles are defined by calculating dipole
densities of free-standing molecular monolayers. The gold-adsorbate interaction
is to a good degree determined by the Au-S bond only. This bond is nearly
apolar and its contribution to the surface dipole is relatively small. The
surface dipole of the self-assembled monolayer is then dominated by the
intrinsic dipole of the thiolate molecules. Alkyl-thiolates increase the work
function of Au(111), whereas fluorinated alkyl-thiolates decrease it.Comment: 24 pages, 5 figures, 4 table
Structural studies of phosphorus induced dimers on Si(001)
Renewed focus on the P-Si system due to its potential application in quantum
computing and self-directed growth of molecular wires, has led us to study
structural changes induced by P upon placement on Si(001)-. Using
first-principles density functional theory (DFT) based pseudopotential method,
we have performed calculations for P-Si(001) system, starting from an isolated
P atom on the surface, and systematically increasing the coverage up to a full
monolayer. An isolated P atom can favorably be placed on an {\bf M} site
between two atoms of adjacent Si dimers belonging to the same Si dimer row. But
being incorporated in the surface is even more energetically beneficial due to
the participation of the {\bf M} site as a receptor for the ejected Si. Our
calculations show that up to 1/8 monolayer coverage, hetero-dimer structure
resulting from replacement of surface Si atoms with P is energetically
favorable. Recently observed zig-zag features in STM are found to be consistent
with this replacement process. As coverage increases, the hetero-dimers give
way to P-P ortho-dimers on the Si dimer rows. This behavior is similar to that
of Si-Si d-dimers but are to be contrasted with the Al-Al dimers, which are
found between adjacent Si dimers rows and in a para-dimer arrangement. Unlike
Al-Si system P-Si does not show any para to ortho transition. For both systems,
the surface reconstruction is lifted at about one monolayer coverage. These
calculations help us in understanding the experimental data obtained using
scanning tunneling microscope.Comment: To appear in PR
First-principles study of the interaction and charge transfer between graphene and metals
Measuring the transport of electrons through a graphene sheet necessarily
involves contacting it with metal electrodes. We study the adsorption of
graphene on metal substrates using first-principles calculations at the level
of density functional theory. The bonding of graphene to Al, Ag, Cu, Au and
Pt(111) surfaces is so weak that its unique "ultrarelativistic" electronic
structure is preserved. The interaction does, however, lead to a charge
transfer that shifts the Fermi level by up to 0.5 eV with respect to the
conical points. The crossover from p-type to n-type doping occurs for a metal
with a work function ~5.4 eV, a value much larger than the work function of
free-standing graphene, 4.5 eV. We develop a simple analytical model that
describes the Fermi level shift in graphene in terms of the metal substrate
work function. Graphene interacts with and binds more strongly to Co, Ni, Pd
and Ti. This chemisorption involves hybridization between graphene -states
and metal d-states that opens a band gap in graphene. The graphene work
function is as a result reduced considerably. In a current-in-plane device
geometry this should lead to n-type doping of graphene.Comment: 12 pages, 9 figure
Real space finite difference method for conductance calculations
We present a general method for calculating coherent electronic transport in
quantum wires and tunnel junctions. It is based upon a real space high order
finite difference representation of the single particle Hamiltonian and wave
functions. Landauer's formula is used to express the conductance as a
scattering problem. Dividing space into a scattering region and left and right
ideal electrode regions, this problem is solved by wave function matching (WFM)
in the boundary zones connecting these regions. The method is tested on a model
tunnel junction and applied to sodium atomic wires. In particular, we show that
using a high order finite difference approximation of the kinetic energy
operator leads to a high accuracy at moderate computational costs.Comment: 13 pages, 10 figure
Microfossils from the late Mesoproterozoic - early Neoproterozoic Atar/EI Mreiti Group, Taoudeni Basin, Mauritania, northwestern Africa
The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/EI Mreiti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreiti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of organic-rich laminae interpreted as benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a circular opening interpreted as a sophisticated circular excystment structure (a pylome), and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well dated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreiti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicates that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy
First principles modelling of magnesium titanium hydrides
Mixing Mg with Ti leads to a hydride Mg(x)Ti(1-x)H2 with markedly improved
(de)hydrogenation properties for x < 0.8, as compared to MgH2. Optically, thin
films of Mg(x)Ti(1-x)H2 have a black appearance, which is remarkable for a
hydride material. In this paper we study the structure and stability of
Mg(x)Ti(1-x)H2, x= 0-1 by first-principles calculations at the level of density
functional theory. We give evidence for a fluorite to rutile phase transition
at a critical composition x(c)= 0.8-0.9, which correlates with the
experimentally observed sharp decrease in (de)hydrogenation rates at this
composition. The densities of states of Mg(x)Ti(1-x)H2 have a peak at the Fermi
level, composed of Ti d states. Disorder in the positions of the Ti atoms
easily destroys the metallic plasma, however, which suppresses the optical
reflection. Interband transitions result in a featureless optical absorption
over a large energy range, causing the black appearance of Mg(x)Ti(1-x)H2.Comment: 22 pages, 9 figures, 4 table
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Sporulenes, Heptaprenyl Metabolites from Bacillus subtilis Spores
Sporulene, a C35-terpenoid hydrocarbon with an unusual pentacyclic structure, is produced by Bacillus subtilis during sporulation.Earth and Planetary SciencesMolecular and Cellular Biolog
Ab-initio calculation of the electronic and optical excitations in polythiophene: effects of intra- and interchain screening
We present an calculation of the electronic and optical excitations of an
isolated polythiophene chain as well as of bulk polythiophene. We use the GW
approximation for the electronic self-energy and include excitonic effects by
solving the electron-hole Bethe-Salpeter equation. The inclusion of interchain
screening in the case of bulk polythiophene drastically reduces both the
quasi-particle band gap and the exciton binding energies, but the optical gap
is hardly affected. This finding is relevant for conjugated polymers in
general.Comment: 4 pages, 1 figur
First-principles study of the dipole layer formation at metal-organic interfaces
We study the dipole layer formed at metal-organic interfaces by means of
first-principles calculations. Interface dipoles are monitored by calculating
the work function change of Au, Ag, Al, Mg and Ca surfaces upon adsorption of a
monolayer of PTCDA (3,4,9,10-perylene-tetra-carboxylic-di-anhydride), perylene
or benzene molecules. Adsorption of PTCDA leads to pinning of the work function
for a range of metal substrates. It gives interface dipoles that compensate for
the difference in the clean metal work functions, leading to a nearly constant
work function. In contrast, adsorption of benzene always results in a decrease
of the work function, which is relatively constant for all metal substrates.
Both effects are found in perylene, where adsorption on low work function
metals gives work function pinning, whereas adsorption on high work function
metals gives work function lowering. The work function changes upon adsorption
are analyzed and interpreted in terms of two competing effects. If the molecule
and substrate interact weakly, the molecule pushes electrons into the surface,
which lowers the work function. If the metal work function is sufficiently low
with respect to the unoccupied states of the molecule, electrons are donated
into these states, which increases the binding and the work function.Comment: 18 pages, 12 figures, 5 table
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