Mechanisms and energetic of atomic processes in surface diffusion

Surface diffusion is a subject of basic importance for understanding mass transport phenomena in surface and nano science. In the particle aspect of surface diffusion of single atoms and simple molecules, information of interest is the detail atomic mechanisms and the activation energy of various atomic processes, and also the binding energy of atoms at different surface sites. In the absence of an external force, atoms will perform random walk without a preferred direction. When an atom is subjected to an external force, or when a chemical potential gradient exists, it will move preferentially in the direction of the force, or in the direction of decreasing chemical potential, thus the random walk becomes directional. Using atomic resolution microscopy, it is now possible to observe random walk diffusion of atoms, molecules and atomic clusters directly as well as to study the dynamic behavior of atoms as perturbed by the electronic interactions of the surface in great detail. Here, methods of studying quantitatively the particle aspect of surface diffusion and how it affects the dynamic behavior of the surface are very briefly reviewed.     

Role of unstable directions in the equilibrium and aging dynamics of supercooled liquids

The connectivity of the potential energy landscape in supercooled atomic liquids is investigated through a calculation of the instantaneous normal modes spectrum and a detailed analysis of the unstable directions in configuration space. We confirm the hypothesis that the mode-coupling critical temperature is the T at which the dynamics crosses over from free to activated exploration of configuration space. We also observe changes in the local connectivity of configuration space sampled during aging, following a temperature jump from a liquid to a glassy state.     

General theory of microscopic dynamical response in surface probe microscopy: from imaging to dissipation

We present a general theory of atomistic dynamical response in surface probe microscopy when two solid surfaces move with respect to each other in close proximity, when atomic instabilities are likely to occur. These instabilities result in a bistable potential energy surface, leading to temperature dependent atomic scale topography and damping (dissipation) images. The theory is illustrated on noncontact atomic force microscopy and enables us to calculate, on the same footing, both the frequency shift and the excitation signal amplitude for tip oscillations. We show, using atomistic simulations, how dissipation occurs through reversible jumps of a surface atom between the minima when a tip is close to the surface, resulting in dissipated energies of 1.6 eV. We also demonstrate that atomic instabilities lead to jumps in the frequency shift that are smoothed out with increasing temperature.     

压电式纳米发电机的原理和潜在应用

纤锌矿结构氧化锌纳米线具有半导体性能和压电效应。用导电的原子力显微镜探针针尖去弯曲竖直生长的氧化锌纳米线，在纳米线的内部和外部分别造成压缩和拉伸，这种独特结构导致了弯曲纳米线的内外表面产生反极性的极化电荷，借助半导体性质的氧化锌纳米线和其金属尖部的肖特基势垒将电能暂时储存在氧化锌纳米线内，并可用导电的原子力显微镜探针接通这一电源，向外界输电，从而完美地实现了在纳米尺度上把机械能转化为电能。该纳米发电机的发电效率可以达到17％-30％，此项重要的科学发现将为自发电式纳米器件奠定物理基础。文章介绍了它的工作原理和潜在应用。     

Nonstochastic behavior of atomic surface diffusion on Cu(111) down to low temperatures

Atomic diffusion is usually understood as a succession of random, independent displacements of an adatom over the surface's potential energy landscape. Nevertheless, an analysis of molecular dynamics simulations of self-diffusion on Cu(111) demonstrates the existence of different types of correlations in the atomic jumps at all temperatures. Thus, the atomic displacements cannot be correctly described in terms of a random walk model. This fact has a profound impact on the determination and interpretation of diffusion coefficients and activation barriers.     

Mechanism of contrast formation in atomic force microscopy in water

We use computer modeling to investigate the mechanism of atomic-scale corrugation in frequency-modulation atomic force microscopy imaging of inorganic surfaces in solution. Molecular dynamics simulations demonstrate that the forces acting on a microscope tip result from the direct interaction between a tip and a surface, and forces entirely due to the water structure around both tip and surface. The observed force depends on a tip structure and is a balance between largely repulsive potential energy changes as the tip approaches and the entropic gain when water is sterically prevented from occupying sites near the tip and surface.     

Force probing of protein crystals: An atomic force microscopy study

The atomic force microscope (AFM) was used for measuring force-distance curves on horse spleen ferritin crystals in liquid environment. In the region of the approach curve which corresponds to tip-surface contact, discrete jumps were recorded, as predicted by molecular dynamics simulations in the case of low tip-sample interaction. The observed jumps can be related to the removal of individual molecules from the surface by the AFM tip. A simple steric model, which takes into account tip and ferritin molecule size, can explain the displacements observed with excellent agreement. The elemental force jump resulting from the approach curves is a direct measure of the force required to remove a single molecule from the crystal face. We discuss the conditions under which the cantilever potential energy difference along the elemental force step provides the energy of extraction of a single molecule. The estimate of the intermolecular binding energy turns out to be in good agreement with the value calculated independently from the surface free energy of ferritin crystals. Received 10 February 2000 and Received in final form 4 May 2000     

Nonlinear response from the perspective of energy landscapes and beyond

The paper discusses the nonlinear response of disordered systems. In particular we show how the nonlinear response can be interpreted in terms of properties of the potential energy landscape. It is shown why the use of relatively small systems is very helpful for this approach. For a standard model system we check which system sizes are particular suited. In case of the driving of a single particle via an external force the concept of an effective temperature helps to scale the force dependence for different temperature on a single master curve. In all cases the mobility increases with increasing external force. These results are compared with a stochastic process described by a 1d Langevin equation where a similar scaling is observed. Furthermore it is shown that for different classes of disordered systems the mobility can also decrease with increasing force. The results can be related to the properties of the chosen potential energy landscape. Finally, results for the crossover from the linear to the nonlinear conductivity of ionic liquids are presented, inspired by recent experimental results in the Roling group. Apart from a standard imidazolium-based ionic liquid we study a system which is characterized by a low conductivity as compared to other ionic liquids and very small nonlinear effects. We show via a real space structural analysis that for this system a particularly strong pair formation is observed and that the strength of the pair formation is insensitive to the application of strong electric fields. Consequences of this observation are discussed.     

Network topology of a potential energy landscape: a static scale-free network

Here we analyze the topology of the network formed by the minima and transition states on the potential energy landscape of small clusters. We find that this network has both a small-world and scale-free character. In contrast to other scale-free networks, where the topology results from the dynamics of the network growth, the potential energy landscape is a static entity. Therefore, a fundamentally different organizing principle underlies this behavior: The potential energy landscape is highly heterogeneous with the low-energy minima having large basins of attraction and acting as the highly connected hubs in the network.     

Probing the non-pairwise interactions between CO molecules moving on a Cu(111) surface

The coverage dependent dynamics of CO on a Cu(111) surface are studied on an atomic scale using helium spin-echo spectroscopy. CO molecules occupy top sites preferentially, but also visit intermediate bridge sites in their motion along the reaction coordinate. We observe an increase in hopping rate as the CO coverage grows; however, the motion remains uncorrelated up to at least 0.10 monolayers (ML). From the temperature dependence of the diffusion rate, we find an effective barrier of 98 ± 5 meV for diffusion. Thermal motion is modelled with Langevin molecular dynamics, using a potential energy surface having adsorption sites at top and bridge positions and the experimental data are well represented by an adiabatic barrier for hopping of 123 meV. The sites are not degenerate and the rate changes observed with coverage are modelled successfully by changing the shape of the adiabatic potential energy surface in the region of the transition state without modifying the energy barrier. The results demonstrate that sufficient detail exists in the experimental data to provide information on the principal adsorption sites as well as the energy landscape in the region of the transition state.     

Fractionation of polydisperse colloid with acousto-optically generated potential energy landscapes

The motion of colloidal particles on a periodic optical potential energy landscape in the presence of an external driving force may result in particle separation. In contrast to recent methods of holographic or interferometric generation of such landscapes, we use an acousto-optic deflector to create two-dimensional landscapes. We present what is believed to be the first experimental realization of fractionation with simultaneous sorting of four different sizes of colloidal microparticle into laterally separated parallel laminar streams.     

Strain field due to self-interstitial impurity in Ni

The embedded-atom method have been applied to study the strain field produced by the self-interstitial impurity at the octahedral site in Ni. The calculation have been carried out consistently on the basis of discrete lattice theory, using Kanzaki method. The atomic force constants are evaluated using Wills and Harrison interatomic potential. The dynamical matrix and external force are evaluated considering the interaction up to first nearest neighbors. The atomic displacements are tabulated up to 20NN’s. These displacements are of oscillatory nature and of decreasing magnitude with NN’s distance. The physical properties such as self-interstitial formation energy and volume change calculated using atomic displacements are in accordance with the earlier studies.     

Three-body interaction and phonon dispersion relations in aluminium

Based on an ab initio cohesive energy calculation and a model of three-body interaction, the pair potential can be calculated using the Möbius inversion theorem in the theory of numbers. Then the atomic force constants and the phonon dispersion for A1 are evaluated both with and without three-body interaction. Compared with experiments, the results show that taking the three-body interaction into account considerably improves the dispersions. Contrary to previous work, the method for calculating the atomic force constants and phonon dispersions presented here is simple, with only two adjustable parameters.     

High resolution imaging of contact potential difference using a novel ultrahigh vacuum non-contact atomic force microscope technique

An ultrahigh vacuum scanning Kelvin probe force microscope (UHV SKPM) based on the gradient of electrostatic force was developed using the technique of a UHV non-contact atomic force microscope (NC-AFM) capable of atomic level imaging, and used for simultaneous observation of contact potential difference (CPD) and NC-AFM images. The CPD images with a potential resolution of less than 10 meV were observed in the UHV SKPM, demonstrating an atomic level resolution. The change of potential corresponding to the charges on the insulated surface of polypropylene have been observed in UHV SKPM. We also demonstrated a reliable method to obtain the CPD from the bias voltage dependence curves of the frequency shift in all of the scanning area. The results are consistent with comparing the barrier height images in that the work functions of adatoms are greater than the work function of corner holes.     

Saddles in the energy landscape: extensivity and thermodynamic formalism

We formally extend the energy landscape approach for the thermodynamics of liquids to account for saddle points. By considering the extensive nature of macroscopic potential energies, we derive the scaling behavior of saddles with system size, as well as several approximations for the properties of low-order saddles (i.e., those with only a few unstable directions). We then cast the canonical partition function in a saddle-explicit form and develop, for the first time, a rigorous energy landscape approach capable of reproducing trends observed in simulations, in particular, the temperature dependence of the energy and fractional order of sampled saddles.     

氢分子离子势能函数的修正

利用近似有心力场原子能量的计算方法修正氢分子离子的势能函数，重新计算了氢分子离子的势能数值，其理论曲线和实验曲线基本相符。     

Spatially selective melting and evaporation of nanosized gold particles

We have developed an atomic force microscope-tip-based concept to pattern metallic nanoparticles on substrates. This new process has the potential to control the assembly of nanometer sized particles by combining their unique optical and thermophysical properties and is a flexible and low energy method of patterning at the nanoscale. The proof of concept is detailed by preliminary experimental work showing selective melting and evaporation of groups of 50 and 100 nm gold spherical particles.     

A dynamical transition of a chain of charged particles in a 2D substrate potential

The transport of a chain of charged particles with a transverse degree of freedom is investigated in a 2D asymmetric potential. Here, the energy of the periodic driving force is converted into motion in the vertical direction. The analysis exhibits a transition from stick-slip motion to periodic oscillation. The chain velocity can be controlled to an optimized value by adjusting system parameters, such as the amplitude and frequency of the periodic force. The existence of a resonance platform indicates resonance between the motion of the chain and the periodic force as coupling strength increases adiabatically. The atomic configuration and the transverse degree of freedom also play key roles in the control.     

Single molecule unzipping of coiled coils: sequence resolved stability profiles

We use a high resolution atomic force microscopy technique to mechanically unzip and rezip single coiled-coil proteins. This allows us to read off the complete stability profile of the protein turn by turn. We investigated three coiled coils with different length as well as a point mutation and find force fluctuations between 9 and 15 pN that can be directly related to the amino-acid sequences. An equilibrium model previously applied to DNA fully describes the mechanical unzipping process including free-energy contributions of the individual turns and seed formation energy.     

Energy landscape of fullerene materials: a comparison of boron to boron nitride and carbon

Using the minima hopping global geometry optimization method on the density functional potential energy surface we show that the energy landscape of boron clusters is glasslike. Larger boron clusters have many structures which are lower in energy than the cages. This is in contrast to carbon and boron nitride systems which can be clearly identified as structure seekers. The differences in the potential energy landscape explain why carbon and boron nitride systems are found in nature whereas pure boron fullerenes have not been found. We thus present a methodology which can make predictions on the feasibility of the synthesis of new nanostructures.