94 research outputs found
Premelting of Thin Wires
Recent work has raised considerable interest on the nature of thin metallic
wires. We have investigated the melting behavior of thin cylindrical Pb wires
with the axis along a (110) direction, using molecular dynamics and a
well-tested many-body potential. We find that---in analogy with cluster
melting---the melting temperature of a wire with radius is lower
than that of a bulk solid, , by . Surface melting
effects, with formation of a thin skin of highly diffusive atoms at the wire
surface, is observed. The diffusivity is lower where the wire surface has a
flat, local (111) orientation, and higher at (110) and (100) rounded areas. The
possible relevance to recent results on non-rupturing thin necks between an STM
tip and a warm surface is addressed.Comment: 10 pages, 4 postscript figures are appended, RevTeX, SISSA Ref.
131/94/CM/S
Melting Point and Lattice Parameter Shifts in Supported Metal Nanoclusters
The dependencies of the melting point and the lattice parameter of supported
metal nanoclusters as functions of clusters height are theoretically
investigated in the framework of the uniform approach. The vacancy mechanism
describing the melting point and the lattice parameter shifts in nanoclusters
with decrease of their size is proposed. It is shown that under the high vacuum
conditions (p<10^-7 torr) the essential role in clusters melting point and
lattice parameter shifts is played by the van der Waals forces of
cluster-substrate interation. The proposed model satisfactorily accounts for
the experimental data.Comment: 6 pages, 3 figures, 1 tabl
Diffusion of gold nanoclusters on graphite
We present a detailed molecular-dynamics study of the diffusion and
coalescence of large (249-atom) gold clusters on graphite surfaces. The
diffusivity of monoclusters is found to be comparable to that for single
adatoms. Likewise, and even more important, cluster dimers are also found to
diffuse at a rate which is comparable to that for adatoms and monoclusters. As
a consequence, large islands formed by cluster aggregation are also expected to
be mobile. Using kinetic Monte Carlo simulations, and assuming a proper scaling
law for the dependence on size of the diffusivity of large clusters, we find
that islands consisting of as many as 100 monoclusters should exhibit
significant mobility. This result has profound implications for the morphology
of cluster-assembled materials
Preparation, stabilization and characterization of TiO2 on thin polyethylene films (LDPE): Photocatalytic applications
An innovative way to fix preformed nanocrystalline TiO2 on low-density polyethylene film (LDPE-TiO2) is presented. The LDPE-TiO2 film was able to mediate the complete photodiscoloration of Orange II using about seven times less catalyst than a TiO2 suspension and proceeded with a photonic efficiency of ~0.02. The catalyst shows photostability over long operational periods during the photodiscoloration of the azo dye Orange II. The LDPE-TiO2 catalyst leads to full dye discoloration under simulated solar light but only to a 30% TOC reduction since long-lived intermediates generated in solution seem to preclude full mineralization of the dye. Physical insight is provided into the mechanism of stabilization of the LDPE-TiO2 composite during the photocatalytic process by X-ray photoelectron spectroscopy (XPS). The adherence of TiO2 on LDPE is investigated by electron microscopy (EM) and atomic force microscopy (AFM). The thickness of the TiO2 film is seen to vary between 1.25 and 1.69 mm for an unused LDPE-TiO2 film and between 1.31 and 1.50 mmfor a sample irradiated 10h during Orange II discoloration pointing out to a higher compactness of the TiO2 film after the photocatalysis
Spontaneous alloying in binary metal microclusters - A molecular dynamics study -
Microcanonical molecular dynamics study of the spontaneous alloying(SA),
which is a manifestation of fast atomic diffusion in a nano-sized metal
cluster, is done in terms of a simple two dimensional binary Morse model.
Important features observed by Yasuda and Mori are well reproduced in our
simulation. The temperature dependence and size dependence of the SA phenomena
are extensively explored by examining long time dynamics. The dominant role of
negative heat of solution in completing the SA is also discussed. We point out
that a presence of melting surface induces the diffusion of core atoms even if
they are solid-like. In other words, the {\it surface melting} at substantially
low temperature plays a key role in attaining the SA.Comment: 15 pages, 12 fgures, Submitted to Phys.Rev.
Thermodynamics of tin clusters
We report the results of detailed thermodynamic investigations of the
Sn cluster using density-functional molecular dynamics. These
simulations have been performed over a temperature range of 150 to 3000 K, with
a total simulation time of order 1 ns. The prolate ground state and low-lying
isomers consist of two tricapped trigonal prism (TTP) units stacked end to end.
The ionic specific heat, calculated via a multihistogram fit, shows a small
peak around 500 K and a shoulder around 850 K. The main peak occurs around 1200
K, about 700 K higher than the bulk melting temperature, but significantly
lower than that for Sn. The main peak is accompanied by a sharp change
in the prolate shape of the cluster due to the fusion of the two TTP units to
form a compact, near spherical structure with a diffusive liquidlike ionic
motion. The small peak at 500 K is associated with rearrangement processes
within the TTP units, while the shoulder at 850 K corresponds to distortion of
at least one TTP unit, preserving the overall prolate shape of the cluster. At
all temperatures observed, the bonding remains covalent.Comment: Latex File and EPS Figures. 18 pages,11 Figures. Submitted to Phys.
Rev.
Growth of nanostructures by cluster deposition : a review
This paper presents a comprehensive analysis of simple models useful to
analyze the growth of nanostructures obtained by cluster deposition. After
detailing the potential interest of nanostructures, I extensively study the
first stages of growth (the submonolayer regime) by kinetic Monte-Carlo
simulations. These simulations are performed in a wide variety of experimental
situations : complete condensation, growth with reevaporation, nucleation on
defects, total or null cluster-cluster coalescence... The main scope of the
paper is to help experimentalists analyzing their data to deduce which of those
processes are important and to quantify them. A software including all these
simulation programs is available at no cost on request to the author. I
carefully discuss experiments of growth from cluster beams and show how the
mobility of the clusters on the surface can be measured : surprisingly high
values are found. An important issue for future technological applications of
cluster deposition is the relation between the size of the incident clusters
and the size of the islands obtained on the substrate. An approximate formula
which gives the ratio of the two sizes as a function of the melting temperature
of the material deposited is given. Finally, I study the atomic mechanisms
which can explain the diffusion of the clusters on a substrate and the result
of their mutual interaction (simple juxtaposition, partial or total
coalescence...)Comment: To be published Rev Mod Phys, Oct 99, RevTeX, 37 figure
Atomic-resolution imaging of surface and core melting in individual size-selected Au nanoclusters on carbon
Experimental and theoretical data sets for the Nature Communications paper "Atomic-resolution imaging of surface and core melting in individual size-selected Au nanoclusters on carbon". Data structure and formats are described in the README.docx file
FCC-ee: The Lepton Collider: Future Circular Collider Conceptual Design Report Volume 2
In response to the 2013 Update of the European Strategy for Particle Physics, the Future Circular Collider (FCC) study was launched, as an international collaboration hosted by CERN. This study covers a highest-luminosity high-energy lepton collider (FCC-ee) and an energy-frontier hadron collider (FCC-hh), which could, successively, be installed in the same 100 km tunnel. The scientific capabilities of the integrated FCC programme would serve the worldwide community throughout the 21st century. The FCC study also investigates an LHC energy upgrade, using FCC-hh technology. This document constitutes the second volume of the FCC Conceptual Design Report, devoted to the electron-positron collider FCC-ee. After summarizing the physics discovery opportunities, it presents the accelerator design, performance reach, a staged operation scenario, the underlying technologies, civil engineering, technical infrastructure, and an implementation plan. FCC-ee can be built with today’s technology. Most of the FCC-ee infrastructure could be reused for FCC-hh. Combining concepts from past and present lepton colliders and adding a few novel elements, the FCC-ee design promises outstandingly high luminosity. This will make the FCC-ee a unique precision instrument to study the heaviest known particles (Z, W and H bosons and the top quark), offering great direct and indirect sensitivity to new physics
HE-LHC: The High-Energy Large Hadron Collider: Future Circular Collider Conceptual Design Report Volume 4
In response to the 2013 Update of the European Strategy for Particle Physics (EPPSU), the Future Circular Collider (FCC) study was launched as a world-wide international collaboration hosted by CERN. The FCC study covered an energy-frontier hadron collider (FCC-hh), a highest-luminosity high-energy lepton collider (FCC-ee), the corresponding 100 km tunnel infrastructure, as well as the physics opportunities of these two colliders, and a high-energy LHC, based on FCC-hh technology. This document constitutes the third volume of the FCC Conceptual Design Report, devoted to the hadron collider FCC-hh. It summarizes the FCC-hh physics discovery opportunities, presents the FCC-hh accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation. Combining ingredients from the Large Hadron Collider (LHC), the high-luminosity LHC upgrade and adding novel technologies and approaches, the FCC-hh design aims at significantly extending the energy frontier to 100 TeV. Its unprecedented centre-of-mass collision energy will make the FCC-hh a unique instrument to explore physics beyond the Standard Model, offering great direct sensitivity to new physics and discoveries
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