Chemisorption of OH on the H-terminated Si(0 0 1) surface

In order to reveal the etching mechanism of the H-terminated Si(0 0 1) surfaces in ultrapure water, first-principles molecular dynamics simulations of H-terminated Si(0 0 1) surfaces interacting with OH molecules were carried out on the basis of density-functional theory. It was confirmed that the interaction between two OH molecules and the surface silicon atom on the step edge breaks the Si–Si back-bond and initiates the etching process.     

Dynamics of fast dislocations

Plastic deformation of crystalline solids at ultra-high strain rates may involve dislocations moving at supersonic speeds, the feasibility of which has been demonstrated via molecular dynamics simulation. The motion of these dislocations in a crystal depends on the defects they encounter, which may slow, or even pin, them down. Recently, we have conducted a series of investigations on the dynamics of transonic dislocations during their interactions with other dislocations, small voids and small interstitial loops, using the molecular dynamics method. The results indicate that a transonic dislocation will be slowed down to subsonic speed by a subsonic dislocation in front of it, and that approaching dislocations at sufficiently high velocities may not form a stable dipole. Small defects, like voids and interstitial clusters, on the other hand, will only temporarily slow down a segment of the transonic dislocation, which absorbs the interstitial loop by forming jogs, and sweeps the void into a few smaller defects of vacancy type. Upon release from the clusters, this segment of dislocation regains speed and becomes transonic again. In view of the possible important role played by high-speed dislocations during high-speed deformation, and from the point of scientific interest, we summarize this series of investigations, and discuss their implications in the present paper.     

Large-scale molecular dynamics simulations of hyperthermal cluster impact

Using multimillion-atom classical molecular dynamics simulations, we have studied the impact dynamics of solid and liquid spherical copper clusters (10–30 nm radius) with a solid surface, at velocities ranging from 100 m/s to 2 km/s. The resulting shock, jetting, and fragmentation processes are analyzed, demonstrating three distinct mechanisms for fragmentation. At early times, shock-induced ejection and hydrodynamic jetting produce fragments in the normal and tangential directions, respectively, while sublimation (evaporation) from the shock-heated solid (liquid) surface produces an isotropic fragment flux at both early and late times.     

Relaxation processes in a glassy polymer containing methanol molecules

The interaction of small guest molecules with the molecular degrees of freedom of a glassy polymer matrix is investigated, i.e. (i) the dynamics of these guest molecules and (ii) their effect on the secondary ‘β’ and main ‘’ relaxations of the host polymeric matrix. The system considered here is the glassy poly(methyl metacrylate) modified by introduction and desorption of methanol. The dynamics of the system is observed by means of low-frequency mechanical and wide band dielectric spectroscopies. The presence of small molecules in the PMMA glassy matrix induces several effects, namely (i) a strong relaxation peak develops at low temperature (near 120 K at 1 Hz and called _m in the following), (ii) the strength of the β relaxation is increased while the temperature of the maximum shifts towards the low temperatures, (iii) a sharp peak appears superimposed on the β peak, and finally (iv) the relaxation shifts towards the low temperatures. Dielectric and mechanical spectroscopies results are in agreement and make it possible to capture the dynamical behavior in a wide frequency range (eight decades). The experimental results are explained on the basis of physical concepts recently introduced in the physics of glassy matter: cooperativity and nanoheterogeneity. In particular, the low-temperature relaxation process _m is attributed to cooperative motions in methanol clusters which form in the nanoheterogeneous polymeric matrix, in agreement with small angle X-rays scattering and low-frequency Raman scattering observations recently reported.     

Surface self-diffusion of adatom on Pt cluster with truncated octahedron structure

Surface diffusion of single Pt adatom on Pt cluster with truncated octahedron structure is investigated through a combination of molecular dynamics and nudged elastic band method. Using an embedded atom method to describe the atomic interactions, the minimum energy paths are determined and the energy barriers for adatom diffusion across and along step are evaluated. The diffusion of adatom crossing step edge between {111} and {100} facets has a surprisingly low barrier of 0.03 eV, which is 0.12 eV lower than the barrier for adatom diffusion from {111} to neighboring {111} facet. Owing to the small barrier of adatom diffusion across the step edge between {111} and {100} facets, the diffusion of adatom along the step edge cannot occur. The molecular dynamics simulations at low temperatures also support these results. Our results show that mass transport will prefer step with {100} microfacet and the Pt clusters can have only {111} facets in epitaxial growth.     

Molecular dynamics study on vanadium pentoxide

We have studied bulk structure and three low-index surfaces of V₂O₅ using molecular dynamics (MD) simulation. The calculated infra-red (IR) absorption bands of V₂O₅ bulk structure are consistent with the experimental result. The (0 0 1) surface was calculated to be the most stable, small energy difference between the (0 0 1) surface and bulk corresponds their similarity. Atoms with small coordination relax much more than bulk like atoms, they undergo vertical as well as lateral relaxations in order to compensate the missing bonds at the top layer. The driving force which determines the direction of relaxation seems to be the improvement of local environments of the top layer atoms. The vanadyl oxygens exposed to the (0 0 1) and (0 1 0) as well as (1 0 0) surfaces seem to act as active sites in the oxidation process of hydrocarbons.     

Computer Simulation of Small Noncrystalline Silica Clusters

The semiclassical molecular dynamics simulation method proposed earlier for studying ionic–covalent oxide systems is applied to small noncrystalline silica clusters. The internal energy comprises the contributions from the ionization energy of silicon, electron affinity of oxygen, Coulombic interaction, repulsion between ionic shells, and covalent interaction. The ionic charges are computed by minimizing the potential energy at each simulation step. Taking into account the intermediate type of bonding improves agreement with experimental density and energy data. The computed excess surface energy of silica clusters is almost independent of cluster size and is in reasonable agreement with the experimentally determined surface tension of liquid silica. At a distance of several nanometers, the interaction between silica clusters is very weak.     

Anharmonic effects on Be(0 0 0 1): A molecular dynamics study

Using molecular dynamics simulations and a modified analytic embedded atom method (MAEAM), the anharmonic effects of Be(0 0 0 1) surface have been studied in the temperature range from 0 K to 1400 K. The temperature dependence of the interlayer separation, mean square vibrational displacement, phonon frequencies and phonon line width, and layer structure factor are calculated. The obtained results for temperature dependence of interlayer separation and mean square displacement show that the anharmonic effects are small in the temperature range from 0 K to 1100 K. The calculated layer order parameters indicate that Be(0 0 0 1) surface loses its long-range translational order, but do not premelt up to 50 K below the bulk melting point. The surface disordering may result from strongly contracted c/a ratio of Be.     

Thermodynamic properties of silver nanoclusters

Icosahedral models of magic number silver clusters have been constructed using molecular dynamics simulations with the Doyama-Kogure potential. We have calculated the energy, heat capacity, entropy, Gibbs energy change, and excess surface energy as functions of cluster size and temperature. The vibrational contributions to the heat capacity and entropy are weak functions of cluster size (N), and the statistical degeneracy is lifted starting at 250–300 K. The surface energy density of the silver clusters is size-independent down to N = 13. The saturated vapor pressure over the clusters has been estimated. The calculated thermodynamic properties of fcc silver agree well with standard thermodynamic data.     

An atomic-scale study of hydrogenated silicon cluster deposition on a crystalline silicon surface

Controlled deposition of clusters on solid surfaces has attracted lots of attention in recent years, because of its potential application to tailoring the desired electronic properties of the resulting surfaces. We have carried out an atomic-scale study to understand the deposition mechanism. The molecular dynamics approach based on a modified Tersoff potential is used to simulate the deposition mechanism of hydrogenated silicon clusters on a crystalline silicon surface in detail. The important factors governing the deposition process such as impact energy and substrate temperature, are investigated for the hydrogenated silicon cluster Si₂₉H₂₄ on a H-terminated Si(100)-(2x1) surface.     

Formation of nanometer-sized surface platinum oxide clusters on a stepped Pt(557) single crystal surface induced by oxygen: a high-pressure STM and ambient-pressure XPS study

We studied the oxygen-induced restructuring process on a stepped Pt(557) single crystal surface using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at O(2) pressures up to 1 Torr. HP-STM has revealed that nanometer-sized clusters are created on Pt(557) at 1 Torr of O(2) and at room temperature. These clusters are identified as surface Pt oxide by AP-XPS. The appearance of clusters is preceded by the formation of 1D chain structures at the step edges. By using a Pt(111) surface as a reference, it was found that the step sites are the nucleation centers for the formation of surface oxide clusters. These surface oxide clusters disappear and the stepped structure is restored on Pt(557) after evacuating O(2) to 10(-8) Torr. Changes in the surface oxide concentration in response to variations in the O(2) gas pressure are repeatable for several cycles. Our results that small clusters are initiated at step sites at high pressures demonstrate the importance of performing in situ characterization of stepped Pt catalysts under reaction conditions.     

From oxygen adsorption to the growth of thin oxides on silicon surfaces

Oxidation of silicon surfaces at relatively low temperatures is shown to go through several activated steps, in the form of configurations inert to further uptake of oxygen. Starting from room temperature adsorption, different configurations of oxygen atoms adsorbed on and in the Si(1 1 1) and Si(0 0 1) surfaces are found, with history and/or coverage dependent energy barriers connecting them. From well below to slightly above an effective oxide coverage of a monolayer, clustering of up to three oxygen atoms around one single silicon atom has been predicted for the Si(0 0 1) surface to represent one such energy minimum; this model is confirmed here experimentally. These and other clusters are shown to agglomerate into silicon dioxide islands before coalescing into a contiguous, inert layer upon higher oxygen supplies. Another problem addressed here is the presence of molecular adsorbates in the oxidation reaction path, an issue which is still debated in the literature. For the Si(1 1 1) surface a molecular, charged oxygen species has earlier been found at temperatures up to room temperature, but not for the Si(0 0 1) surface. This is confirmed in the present experiments, and new data for this state shows that it is highly mobile until quenched at a critical oxygen coverage. It is not the initial state of oxygen on silicon, and therefore not the precursor for atomic insertion of oxygen; rather, it is found to co-exists with atomic oxygen inserted in back-bonds, at a certain, low coverage regime in which parts of the Si(1 1 1) surface are still ordered.     

Adsorption of gases on Pt/Ni(1 1 1) systems

The chemisorptive properties of bimetallic systems could be sometimes very different in comparison with those of the pure metallic components. The (1 1 1) face of Pt₅₀–Ni₅₀ alloys has shown a clear decrease in their chemisorptive capacity when compared with Pt(1 1 1) or Ni(1 1 1). In the present work, we study the adsorption of the H atom as well as CO and benzene molecules on a Pt/Ni(1 1 1) surface. Our approach was based on a semiempirical molecular orbital method in the cluster approximation. The binding energy of different molecules decreases significantly with respect to the pure metal surfaces, in agreement with the available experimental information. These results can be related and explained taking into consideration the electronic changes of the Pt states in the overlayer.     

Energetics of Ge addimers on the Si(1 0 0) and Ge(1 0 0) surfaces: a comparative study

The adsorption energetics of Ge dimers on the (1 0 0) surfaces of Ge and Si has been investigated using the first-principles molecular dynamics method. Four high-symmetry configurations have been considered and fully relaxed. The most stable configuration for Ge dimers on Si(1 0 0) is found to be in the trough between two surface dimer rows, oriented parallel to the substrate Si dimers. These results are consistent with recent experimental studies of the system using the scanning tunneling microscopy (STM), and help to clarify some existing controversies on the interpretation of the STM images. In contrast, for Ge dimers on Ge(1 0 0), the most stable configuration is on top of the substrate dimer row.     

Simulation Analysis and Experiment Study of Nanocutting with AFM Probe on the Surface of Sapphire Substrate by Using Three Dimensional Quasi-steady Molecular Statics Nanocutting Model

The three-dimensional quasi-steady molecular statics nanocutting model is used by this paper to carry out simulation analysis of nanocutting of sapphire in order to explore the effects of conical tools with different tip radii of probe and straight-line cutting at different cutting depths, on cutting force. Meanwhile, this paper uses a cutting tool of atomic force microscopy (AFM) with a probe tip similar to a semisphere to conduct nanocutting experiment of sapphire substrate. Furthermore, from the experimental results of nanocutting sapphire substrate, this paper innovatively proposes the theoretical model and equation that the specific down force energy (SDFE) during nanocutting by using AFM probe as the nanocutting tool, is approximately a constant value. This paper uses three-dimensional quasi-steady molecular statics nanocutting model to simulate calculation and obtain nanocutting down force. It is compared with the down force calculated by SDFE theoretical equation proposed for verification. As a result, the down force obtained by the paper's simulation is very close to the down force calculated by SDFE theory. Therefore, it can be verify that the three-dimensional quasi-steady molecular statics nanocutting theoretical model used by this paper is feasible. The SDFE proposed by this paper is defined as equating to down force energy dividing the removed volume of down press of the workpiece by the AFM probe. From the experimental data and the calculation results, it is found that the values of SDFE under different down force actions are almost close to a constant value. The three-dimensional quasi-steady molecular statics nanocutting sapphire workpiece model is to find the trajectory of each atom of the sapphire workpiecs being cut whenever the diamond cutter goes forward one step. It uses the optimization search method to solve the force equilibrium equation of the Morse force in the X, Y and Z directions when each atom moves a small distance, so as to find the new movement position of each atom, and step by step calculates the behavior during cutting.     

Cluster diffusion and surface morphological transitions on Pt (111) via reptation and concerted motion

Embedded-atom molecular dynamics simulations were used to follow the diffusion dynamics of compact Pt clusters with up to 19 atoms on Pt (111) surfaces. The results reveal a novel cluster diffusion mechanism, involving successive shear translations of adjacent subcluster regions, which give rise to reptation, a snake-like gliding motion. We show that for compact clusters with 4 to 6 atoms, this mechanism competes energetically with that of island diffusion through concerted motion. However, as the cluster size increases from >7 to 20 atoms, reptation becomes the energetically favored diffusion mechanism. The concerted shear motion of subcluster regions, leading to reptation, is also shown to play a significant role in dendritic-to-compact morphological transitions of Pt islands.     

The Electronic Structures and Photographic Properties of Nanometer-sized Silver Clusters

The electronic structures and spectroscopic properties of nanometer-sized silver clusters Ag_n (n = 1 to 15) have been examined in the framework of self-consistent-field local density theory SCC-DV-X_α. The results show that there is a quantum size effect in Agn clusters, which can be observed as a red shift in the absorption threshold and semiconductor-conductor transition with increasing n. Based on the molecular orbital energies we studied the microscopical mechanisms of the transition point from lower efficiency step to higher efficiency step in latent image growth. The results also show that there are positive charges on the surface silver atoms of some large Ag" clusters, which is favourable for the latent image particles to adsorb developer anion in development.     

On the possible mechanism of crystal growth from melt under zero gravity conditions

A possible mechanism of the growth of semiconductor single crystals from melt under zero gravity conditions is considered. The results of computer simulation performed by the molecular dynamics method for a thin layer of melt on a single crystal surface are presented. The main characteristics of the melt component dynamics suggest a mechanism whereby the crystal grows to a significant extent due to the attachment of atoms or small atomic clusters, which accounts for the perfection of the crystal structure being grown.     

Ground state structures and properties of small hydrogenated silicon clusters

We present results for ground state structures and properties of small hydrogenated silicon clusters using the Car-Parrinello molecular dynamics with simulated annealing. We discuss the nature of bonding of hydrogen in these clusters. We find that hydrogen can form a bridge like Si-H-Si bond connecting two silicon atoms. We find that in the case of a compact and closed silicon cluster hydrogen bonds to the silicon cluster from outside. To understand the structural evolutions and properties of silicon cluster due to hydrogenation, we have studied the cohesive energy and first excited electronic level gap of clusters as a function of hydrogenation. We find that first excited electronic level gap of Sin and SinH fluctuates as function of size and this may provide a first principle basis for the short-range potential fluctuations in hydrogenated amorphous silicon. The stability of hydrogenated silicon clusters is also discussed.     

Loss due to transverse thermoelastic currents in microscale resonators

In order to understand the loss mechanisms associated with microscale resonators we have studied the importance of thermoelastic (TE) damping due to transverse thermal currents. In the work presented here, we study this damping mechanism as it applies to structures involving torsional vibration, or in general possessing a non-trivial mode shape such as those associated with microelectromechanical systems (MEMS) devices. A model of thermoelastic dissipation is presented that is based on the observation that the resonant modes of elastic structures almost always contain some flexural component. We determine a flexural energy participation factor and apply this to Zener’s model for damping of a simple reed in pure bending. Predictions agree well with internal friction measurements for a macroscale single-crystal silicon double paddle oscillator (300 μm thick) at temperatures from 130 to 300 K. The approach has also been successfully applied to predict microscale oscillator (1.5 μm thick) internal friction measurements at room temperature. Our results indicate that the internal friction arising from this mechanism is strong and can be quite significant for silicon-based MEMS (Q<10⁴) and persists down to 50 nm scale structures even for nominally torsional or even slightly asymmetric compressional devices which one might conclude have no loss. The importance of the thermoelastic mechanism is examined as a function of material properties. From this perspective, diamond possesses desirable thermal expansivity and diffusivity that is examined with our model.