125 research outputs found
CO adsorption on Pt induced Ge nanowires
Using density functional theory, we investigate the possible adsorption sites
of CO molecules on the recently discovered Pt induced Ge nanowires on Ge(001).
Calculated STM images are compared to experimental STM images to identify the
experimentally observed adsorption sites. The CO molecules are found to adsorb
preferably onto the Pt atoms between the Ge nanowire dimer segments. This
adsorption site places the CO in between two nanowire dimers, pushing them
outward, blocking the nearest equivalent adsorption sites. This explains the
observed long-range repulsive interaction between CO molecules on these Pt
induced nanowires.Comment: 12 pages, 10 figure
Models and Simulations in Material Science: Two Cases Without Error Bars
We discuss two research projects in material science in which the results
cannot be stated with an estimation of the error: a spectro- scopic
ellipsometry study aimed at determining the orientation of DNA molecules on
diamond and a scanning tunneling microscopy study of platinum-induced nanowires
on germanium. To investigate the reliability of the results, we apply ideas
from the philosophy of models in science. Even if the studies had reported an
error value, the trustworthiness of the result would not depend on that value
alone.Comment: 20 pages, 2 figure
Formation of Pt induced Ge atomic nanowires on Pt/Ge(001): a DFT study
Pt deposited onto a Ge(001) surface gives rise to the spontaneous formation
of atomic nanowires on a mixed Pt-Ge surface after high temperature annealing.
We study possible structures of the mixed surface and the nanowires by total
energy (density functional theory) calculations. Experimental scanning
tunneling microscopy images are compared to the calculated local densities of
states. On the basis of this comparison and the stability of the structures, we
conclude that the formation of nanowires is driven by an increased
concentration of Pt atoms in the Ge surface layers. Surprisingly, the atomic
nanowires consist of Ge instead of Pt atoms.Comment: 4 pages, 3 figure
Reply to 'Comment on "Extending Hirshfeld-I to bulk and periodic materials" '
The issues raised in the comment by T.A. Manz are addressed through the
presentation of calculated atomic charges for NaF, NaCl, MgO, SrTiO and
LaCeO, using our previously presented method for calculating
Hirshfeld-I charges in Solids [J. Comput. Chem.. doi: 10.1002/jcc.23088]. It is
shown that the use of pseudo-valence charges is sufficient to retrieve the full
all-electron Hirshfeld-I charges to good accuracy. Furthermore, we present
timing results of different systems, containing up to over atoms,
underlining the relatively low cost for large systems. A number of theoretical
issues is formulated, pointing out mainly that care must be taken when deriving
new atoms in molecules methods based on "expectations" for atomic charges.Comment: 7 pages, 2 Tables, 2 figure
Extending Hirshfeld-I to bulk and periodic materials
In this work, a method is described to extend the iterative Hirshfeld-I
method, generally used for molecules, to periodic systems. The implementation
makes use of precalculated pseudo-potential based charge density distributions,
and it is shown that high quality results are obtained for both molecules and
solids, such as ceria, diamond, and graphite. The use of such grids makes the
implementation independent of the solid state or quantum chemical code used for
studying the system. The extension described here allows for easy calculation
of atomic charges and charge transfer in periodic and bulk systems.Comment: 11 pages, 4 Tables, 5 Figures, pre-referee draft only, much extended
post referee version only available at publishe
The Formation of Self-Assembled Nanowire Arrays on Ge(001): a DFT Study of Pt Induced Nanowire Arrays
Nanowire (NW) arrays form spontaneously after high temperature annealing of a
submonolayer deposition of Pt on a Ge(001) surface. These NWs are a single atom
wide, with a length limited only by the underlying beta-terrace to which they
are uniquely connected. Using ab-initio density functional theory (DFT)
calculations we study possible geometries of the NWs and substrate. Direct
comparison to experiment is made via calculated scanning tunneling microscope
(STM) images. Based on these images, geometries for the beta-terrace and the
NWs are identified, and a formation path for the nanowires as function of
increasing local Pt density is presented. We show the beta-terrace to be a
dimer row surface reconstruction with a checkerboard pattern of Ge-Ge and Pt-Ge
dimers. Most remarkably, comparison of calculated to experimental STM images
shows the NWs to consist of germanium atoms embedded in the Pt-lined troughs of
the underlying surface, contrary to what was assumed previously in experiments.Comment: 6 pages, 4 figures, E-proceedings of 2009 MRS spring Meetin
Assigning probabilities to non-Lipschitz mechanical systems
We present a method for assigning probabilities to the solutions of initial
value problems that have a Lipschitz singularity. To illustrate the method, we
focus on the following toy-example: , , and
, where the dots indicate derivatives to time and
. This example has a physical interpretation as a mass in a
uniform gravitational field on a dome of particular shape; the case with
is known as Norton's dome. Our approach is based on (1) finite
difference equations, which are deterministic, (2) a uniform prior on the phase
space, and (3) non-standard analysis, which involves infinitesimals and which
is conceptually close to numerical methods from physical praxis. This allows us
to assign probabilities to the solutions of the initial value problem in the
original, indeterministic model.Comment: 13 figure
Can Europium Atoms form Luminescent Centres in Diamond: A combined Theoretical-Experimental Study
The incorporation of Eu into the diamond lattice is investigated in a
combined theoretical-experimental study. The large size of the Eu ion induces a
strain on the host lattice, which is minimal for the Eu-vacancy complex. The
oxidation state of Eu is calculated to be 3+ for all defect models considered.
In contrast, the total charge of the defect-complexes is shown to be negative
-1.5 to -2.3 electron. Hybrid-functional electronic-band-structures show the
luminescence of the Eu defect to be strongly dependent on the local defect
geometry. The 4-coordinated Eu substitutional dopant is the most promising
candidate to present the typical Eu3+ luminescence, while the 6-coordinated
Eu-vacancy complex is expected not to present any luminescent behaviour.
Preliminary experimental results on the treatment of diamond films with
Eu-containing precursor indicate the possible incorporation of Eu into diamond
films treated by drop-casting. Changes in the PL spectrum, with the main
luminescent peak shifting from approximately 614 nm to 611 nm after the growth
plasma exposure, and the appearance of a shoulder peak at 625 nm indicate the
potential incorporation. Drop-casting treatment with an electronegative polymer
material was shown not to be necessary to observe the Eu signature following
the plasma exposure, and increased the background luminescence.Comment: 12 pages, 7 figures, 5 table
DFT study of Pt-induced Ge(001) reconstructions
Pt deposited on a Ge(001) surface spontaneously forms nanowire arrays. These
nanowires are thermodynamically stable and can be hundreds of atoms long. The
nanowires only occur on a reconstructed Pt-Ge-surface where they fill the
troughs between the dimer rows on the surface. This unique connection between
the nanowires and the underlying substrate make a thorough understanding of the
latter necessary for understanding the growth of the nanowires. In this paper
we study possible surface reconstructions containing 0.25 and 0.5 of a
monolayer of Pt. Comparison of calculated STM images to experimental STM images
of the surface reconstruction reveal that the Pt atoms are located in the top
layer, creating a structure with rows of alternating Pt-Ge and Ge-Ge dimers in
a c(4x2) arrangement. Our results also show that Pt atoms in the second or
third layer can not be responsible for the experimentally observed STM images.Comment: 16 pages, 13 figure
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