220 research outputs found
Photoabsorption Spectra of Na_n+ clusters: Thermal Line-Broadening Mechanisms
Photoabsorption cross sections of small sodium cluster cations (Na,
n=3,5,7 and 9) were calculated at various temperatures with the time-dependent
local-density-approximation (TDLDA) in conjunction with ab initio molecular
dynamics simulations, yielding spectra that agree with measured ones without
ad-hoc line broadening or renormalization. Three thermal line-broadening
mechanisms are revealed: (I) lifting of level degeneracies caused by
symmetry-breaking ionic motions, (II) oscillatory shifts of the entire spectrum
caused by breathing vibrations, and (III) cluster structural isomerizations.Comment: 4 pages,2 figures, to appear in Phys. Rev. Let
Molecular-dynamics simulation of thin-film growth by energetic cluster impact
Langevin-molecular-dynamics simulations of thin-film growth by energetic cluster impact were carried out. The impact of a Mo 1043 cluster on a Mo(001) surface was studied for impact energies of 0.1, 1, and 10 eV/atom using the Finnis-Sinclair many-body potential. The characteristics of the collision range from a soft touchdown at 0.1 eV/atom, over a flattening collision at 1 eV/atom, to a meteoric impact at 10 eV/atom. The highest energy impact creates a pressure of about 100 GPa in the impact zone and sends a strong shock wave into the material. The cluster temperature reaches a maximum of 596 K for 0.1 eV/atom, 1799 K for 1 eV/atom, and 6607 K for 10 eV/atom during the first ps after the touchdown. For energies of 1 and 10 eV/atom the cluster recrystallizes after 20 ps. The consecutive collision of 50 Mo 1043 clusters with a Mo(001) surface at T=300 K was simulated for the three impact energies. The formation of a porous film is calculated for clusters impinging with low kinetic energy, while for the clusters with the highest energy a dense mirrorlike film is obtained, in good agreement with experiment
Understanding of the phase transformation from fullerite to amorphous carbon at the microscopic level
We have studied the shock-induced phase transition from fullerite to a dense
amorphous carbon phase by tight-binding molecular dynamics. For increasing
hydrostatic pressures P, the C60-cages are found to polymerise at P<10 GPa, to
break at P~40 GPa and to slowly collapse further at P>60 GPa. By contrast, in
the presence of additional shear stresses, the cages are destroyed at much
lower pressures (P<30 GPa). We explain this fact in terms of a continuum model,
the snap-through instability of a spherical shell. Surprisingly, the relaxed
high-density structures display no intermediate-range order.Comment: 5 pages, 3 figure
Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface
Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001)
surface F-center (FC) was studied by spin-density-functional molecular dynamics
simulations. The incident clusters are steered by an attractive "funnel"
created by the FC, resulting in adsorption of the cluster, with one of its
atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their
gas-phase structures, while for N>6 surface-commensurate isomers are
energetically more favorable. Adsorbed clusters with N > 3 are found to remain
magnetic at the surface.Comment: 5 pages, 2 figs, Phys.Rev.Lett., accepte
Optical response of two-dimensional electron fluids beyond the Kohn regime: strong non-parabolic confinement and intense laser light
We investigate the linear and non-linear optical response of two-dimensional
(2D) interacting electron fluids confined by a strong non-parabolic potential.
We show that such fluids may exhibit higher-harmonic spectra under realistic
experimental conditions. Higher harmonics arise as the electrons explore
anharmonicities of the confinement potential (electron-electron interactions
reduce this non-linear effect). This opens the possibility of controlling the
optical functionality of such systems by engineering the confinement potential.
Our results were obtained within time-dependent density-functional theory,
employing the adiabatic local-density approximation. A classical hydrodynamical
model is in good agreement with the quantum-mechanical results.Comment: 4 pages, 4 figure
Ageing of a Microscopic Sliding Gold Contact at Low Temperatures
Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300 K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170 K. Below 170 K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact
Novel Approaches towards Highly Selective Self-Powered Gas Sensors
The prevailing design approaches of semiconductor gas sensors struggle to overcome most of their current limitations such as poor selectivity, and high power consumption. Herein, a new sensing concept based on devices that are capable of detecting gases without the need of any external power sources required to activate interaction of gases with sensor or to generate the sensor read out signal. Based on the integration of complementary functionalities (namely; powering and sensing) in a singular nanostructure, self-sustained gas sensors will be demonstrated. Moreover, a rational methodology to design organic surface functionalization that provide high selectivity towards single gas species will also be discussed. Specifically, theoretical results, confirmed experimentally, indicate that precisely tuning of the sterical and electronic structure of sensor material/organic interfaces can lead to unprecedented selectivity values, comparable to those typical of bioselective processes. Finally, an integrated gas sensor that combine both the self-powering and selective detection strategies in one single device will also be presented. © 2015 Published by Elsevier Ltd.Peer ReviewedPostprint (published version
The shrinking instability of toroidal liquid droplets in the Stokes flow regime
We analyze the stability and dynamics of toroidal liquid droplets. In
addition to the Rayleigh instabilities akin to those of a cylindrical droplet
there is a shrinking instability that is unique to the topology of the torus
and dominates in the limit that the aspect ratio is near one (fat tori). We
first find an analytic expression for the pressure distribution inside the
droplet. We then determine the velocity field in the bulk fluid, in the Stokes
flow regime, by solving the biharmonic equation for the stream function. The
flow pattern in the external fluid is analyzed qualitatively by exploiting
symmetries. This elucidates the detailed nature of the shrinking mode and the
swelling of the cross-section following from incompressibility. Finally the
shrinking rate of fat toroidal droplets is derived by energy conservation.Comment: 6 pages, 7 figure
Electronic shell structure and chemisorption on gold nanoparticles
We use density functional theory (DFT) to investigate the electronic
structure and chemical properties of gold nanoparticles. Different structural
families of clusters are compared. For up to 60 atoms we optimize structures
using DFT-based simulated annealing. Cluster geometries are found to distort
considerably, creating large band gaps at the Fermi level. For up to 200 atoms
we consider structures generated with a simple EMT potential and clusters based
on cuboctahedra and icosahedra. All types of cluster geometry exhibit
jellium-like electronic shell structure. We calculate adsorption energies of
several atoms on the cuboctahedral clusters. Adsorption energies are found to
vary abruptly at magic numbers. Using a Newns-Anderson model we find that the
effect of magic numbers on adsorption energy can be understood from the
location of adsorbate-induced states with respect to the cluster Fermi level.Comment: 14 pages, 18 figure
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