89 research outputs found
Fast coarsening in unstable epitaxy with desorption
Homoepitaxial growth is unstable towards the formation of pyramidal mounds
when interlayer transport is reduced due to activation barriers to hopping at
step edges. Simulations of a lattice model and a continuum equation show that a
small amount of desorption dramatically speeds up the coarsening of the mound
array, leading to coarsening exponents between 1/3 and 1/2. The underlying
mechanism is the faster growth of larger mounds due to their lower evaporation
rate.Comment: 4 pages, 4 PostScript figure
An ancient pathway combining carbon dioxide fixation with the generation and utilization of a sodium ion gradient for ATP synthesis
Synthesis of acetate from carbon dioxide and molecular hydrogen is considered to be the first carbon assimilation pathway on earth. It combines carbon dioxide fixation into acetyl-CoA with the production of ATP via an energized cell membrane. How the pathway is coupled with the net synthesis of ATP has been an enigma. The anaerobic, acetogenic bacterium Acetobacterium woodii uses an ancient version of this pathway without cytochromes and quinones. It generates a sodium ion potential across the cell membrane by the sodium-motive ferredoxin:NAD oxidoreductase (Rnf). The genome sequence of A. woodii solves the enigma: it uncovers Rnf as the only ion-motive enzyme coupled to the pathway and unravels a metabolism designed to produce reduced ferredoxin and overcome energetic barriers by virtue of electron-bifurcating, soluble enzymes
Low-Energy Electron Microscopy Studies of Interlayer Mass Transport Kinetics on TiN(111)
In situ low-energy electron microscopy was used to study interlayer mass
transport kinetics during annealing of three-dimensional (3D) TiN(111) mounds,
consisting of stacked 2D islands, at temperatures T between 1550 and 1700 K. At
each T, the islands decay at a constant rate, irrespective of their initial
position in the mounds, indicating that mass is not conserved locally. From
temperature-dependent island decay rates, we obtain an activation energy of
2.8+/-0.3 eV. This is consistent with the detachment-limited decay of 2D TiN
islands on atomically-flat TiN(111) terraces [Phys. Rev. Lett. 89 (2002)
176102], but significantly smaller than the value, 4.5+/-0.2 eV, obtained for
bulk-diffusion-limited spiral step growth [Nature 429, 49 (2004)]. We model the
process based upon step flow, while accounting for step-step interactions, step
permeability, and bulk mass transport. The results show that TiN(111) steps are
highly permeable and exhibit strong repulsive temperature-dependent step-step
interactions that vary between 0.003 and 0.076 eV-nm. The rate-limiting process
controlling TiN(111) mound decay is surface, rather than bulk, diffusion in the
detachment-limited regime.Comment: 26 pages, 5 figure
Atomic step motion during the dewetting of ultra-thin films
We report on three key processes involving atomic step motion during the
dewetting of thin solid films: (i) the growth of an isolated island nucleated
far from a hole, (ii) the spreading of a monolayer rim, and (iii) the zipping
of a monolayer island along a straight dewetting front. Kinetic Monte Carlo
results are in good agreement with simple analytical models assuming
diffusion-limited dynamics.Comment: 7 pages, 5 figure
Dissociation of O2 molecules on strained Pb(111) surfaces
By performing first-principles molecular dynamics calculations, we
systematically simulate the adsorption behavior of oxygen molecules on the
clean and strained Pb(111) surfaces. The obtained molecular adsorption
precursor state, and the activated dissociation process for oxygen molecules on
the clean Pb surface are in good agreements with our previous static
calculations, and perfectly explains previous experimental observations [Proc.
Natl. Acad. Sci. U.S.A. 104, 9204 (2007)]. In addition, we also study the
influences of surface strain on the dissociation behaviors of O2 molecules. It
is found that on the compressed Pb(111) surfaces with a strain value of larger
than 0.02, O2 molecules will not dissociate at all. And on the stretched
Pb(111) surfaces, O2 molecules become easier to approach, and the adsorption
energy of the dissociated oxygen atoms is larger than that on the clean Pb
surface
Numerical test of the damping time of layer-by-layer growth on stochastic models
We perform Monte Carlo simulations on stochastic models such as the
Wolf-Villain (WV) model and the Family model in a modified version to measure
mean separation between islands in submonolayer regime and damping time
of layer-by-layer growth oscillations on one dimension. The
stochastic models are modified, allowing diffusion within interval upon
deposited. It is found numerically that the mean separation and the damping
time depend on the diffusion interval , leading to that the damping time is
related to the mean separation as for the WV model
and for the Family model. The numerical results are in
excellent agreement with recent theoretical predictions.Comment: 4 pages, source LaTeX file and 5 PS figure
How metal films de-wet substrates - identifying the kinetic pathways and energetic driving forces
We study how single-crystal chromium films of uniform thickness on W(110)
substrates are converted to arrays of three-dimensional (3D) Cr islands during
annealing. We use low-energy electron microscopy (LEEM) to directly observe a
kinetic pathway that produces trenches that expose the wetting layer. Adjacent
film steps move simultaneously uphill and downhill relative to the staircase of
atomic steps on the substrate. This step motion thickens the film regions where
steps advance. Where film steps retract, the film thins, eventually exposing
the stable wetting layer. Since our analysis shows that thick Cr films have a
lattice constant close to bulk Cr, we propose that surface and interface stress
provide a possible driving force for the observed morphological instability.
Atomistic simulations and analytic elastic models show that surface and
interface stress can cause a dependence of film energy on thickness that leads
to an instability to simultaneous thinning and thickening. We observe that
de-wetting is also initiated at bunches of substrate steps in two other
systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are
converted into patterns of unidirectional stripes as the trenches that expose
the wetting layer lengthen along the W[001] direction. Finally, we observe how
3D Cr islands form directly during film growth at elevated temperature. The Cr
mesas (wedges) form as Cr film steps advance down the staircase of substrate
steps, another example of the critical role that substrate steps play in 3D
island formation
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