No abstract

No abstract

Fast-moving nanoscale systems offer the tantalizing possibility for rapid processing of materials, energy or information 1. Frictional forces can easily dominate the motion of these systems, yet whereas nanomechanical techniques, such as atomic force microscopy, are widely used to measure static nanoscale friction 2 , they are too slow to measure the kinetic friction crucial for short-timescale motion. Here, we report measurements of frictional damping for a prototypical nanoscale system: benzene on a graphite surface, driven by thermal motion. Spin-echo spectroscopy is used to measure the picosecond time dependence of the motion of single benzene molecules, indicating a type of atomic-scale continuous Brownian motion not previously observed. Quantifying the frictional coupling between moving molecules and the surface, as achieved in these measurements, is important for the characterization of phononically driven nanomechanical tools. The data also provide a benchmark for simulations of nanoscale kinetic friction and demonstrate the applicability of the spin-echo technique. Recent nanoscale linear motors have shown that phonon flow, from hot to cold regions, can drive a section of nanotube along a coaxial 'track' 1,3 , in a reversal of the normal frictional process. Speeds are potentially ∼10 8 µm s −1 , enabling atomic-scale motion on picosecond timescales 1. Measurements of nanoscale friction have been carried out with friction force microscopy (FFM), using the remarkable ability of scanning probe techniques to manipulate objects and measure small forces 4. FFM is, however, limited by the instrument's inertia to ∼10 µm s −1 (ref. 5), and there is a need for much faster probes to study the kinetic friction associated with nanoelectromechanical systems. As the moving parts are not loaded laterally or normally in the conventional sense, one approach is to study the dynamics of molecules directly as they move over a surface. For example, Krim et al. 6 used a quartz-crystal microbalance to study the friction of layers of molecules and fluorescence correlation spectroscopy 7 provides diffusion information in the time domain. However, neither of these techniques, nor any other, offers spatial information on an atomic scale together with picosecond time resolution, as in the present work. Here, we demonstrate an alternative approach, carrying out in effect a nanotribological measurement of kinetic friction in the single-molecule limit. Friction and the coupling to surface phonons is deduced for a model nanoscale system from the way individual molecules move on the atomic scale as they are pushed around on picosecond timescales by thermal excitation. Our work is also driven by the knowledge that macroscale friction is fundamentally a microscopic phenomenon 8 , because real surfaces contact only at microscopic asperities, and that energy dissipation, just as in nanoscale systems, is often dominated by the creation of phonons 4. Simple theoretical models of lateral motion of surface species

Quasielastic helium atom scattering provides direct evidence for a fully mobile two-dimensional gas of Xe atoms on Pt(111) at a coverage of Q Xe 0.017 and a temperature of T s 105 K. Molecular dynamics simulations of the experimental data show that the frictional coupling rate between the Xe atoms and substrate is less than 0.25 ps 21 and the lateral potential corrugation for single Xe atoms is less than 9.6 meV, which is only 40% of the value previously determined at high coverage.[S0031-9007 (99)09358-8] PACS numbers: 68.35.Fx, 05.10.Gg, 61.18.BnThe phase diagrams of adsorbate species on single crystal surfaces have been extensively studied for many systems [1]. The adsorbates can form solid, liquid, or gas phase analogs. Whereas the solid phase is accessible to many surface science techniques, these same techniques are largely insensitive to the motion that characterizes the liquid and gas phases. In the 1950's, Van Hove and Vineyard developed a formalism to describe the "quasielastic" energetic broadening of the elastic signal induced when neutrons are scattered from moving target species in crystals, bulk liquids, and gases [2][3][4]. They identified a number of classes of dynamical behavior, each with a characteristic dependence of the quasielastic peak width on the scattering momentum transfer. Helium atom scattering has been shown to be well suited for analogous experiments on single crystal surfaces because of its unique sensitivity, even to single atoms [5]. Using quasielastic helium atom scattering (QHAS), the microscopic jump diffusion of isolated atoms and molecules has been studied on metal surfaces [5,6] and recently even under conditions approaching a liquid phase [7]. The present paper reports the first experimental QHAS observation of the linear dependence of the peak width on the parallel momentum transfer for a two-dimensional ideal gas. While adsorbate "gas" phases have been observed before [8], it has not been possible to distinguish between a lattice gas, in which adatoms occupy specific adsorption sites and move by jump diffusion, and a fully mobile two-dimensional gas, where atoms run virtually unimpeded across the surface.In the dilute adsorbate phases, the helium atoms are scattered from isolated particles which all have the same atomic form factor F͑DK, DE͒ (where DK and DE are the surface parallel momentum and energy transfers of the helium atoms, respectively), so the kinematic scattering approximation can be used. Therefore, the scattered amplitude is given as the product of a form factor and a structure factor S͑DK, DE͒ [9]: I͑DK, DE͒ jF͑DK, DE͒j 2 jS͑DK, DE͒j 2 .(1)If F can be assumed to be a slowly varying function of DK, the quasielastic peak shape for a particular momen-tum transfer is given by jS͑DK, DE͒j 2 around DE 0 [9]. Moreover, at low coverages, the Vineyard approximation [3] may be used, jS͑DK, DE͒j 2~G s ͑DK, DE͒, where G s is the Fourier transform of the self-paircorrelation function G s ͑R, t͒ [the probability of finding a particle at ͑R, t͒ that was at R 0 at t 0 [9...

Helium-3 spin-echo (3HeSE) is a powerful, new experimental technique for studying dynamical phenomena at surfaces with ultra-high energy resolution. Resolution is achieved by using the 3He nuclear spin as an internal timer, to enable measurement of the energy changes of individual atoms as they scatter. The technique yields a measurement of surface correlation in reciprocal space and real time, and probes the nanometre length scales and picosecond to nanosecond timescales that are characteristic of many important atomistic processes. In this article we provide an introductory description of the 3HeSE technique for quasi-elastic scattering measurements and explain how it can be used to obtain unique insights into the motion of adsorbates. We illustrate the technique by reviewing recent measurements, starting with simple hopping and then showing how correlations, arising from adsorbate interactions, can be observed. The final measurements demonstrate how the absence of such correlations, when expected, are used to question the conventional description that attributes the coverage dependence of surface processes entirely to pairwise forces between adsorbates. The emphasis throughout is on the characteristic signatures of adsorbate motion that can be seen in the data, without recourse to a detailed theoretical analysis. Numerical simulations using the Langevin equation are used to illustrate generic behaviour and to provide a quantitative analysis of the experiment.

No abstract

An understanding of hydrogen diffusion on metal surfaces is important not only for its role in heterogeneous catalysis and hydrogen fuel cell technology but also because it provides model systems where tunneling can be studied under well-defined conditions. Here we report helium spin–echo measurements of the atomic-scale motion of hydrogen on the Ru(0001) surface between 75 and 250 K. Quantum effects are evident at temperatures as high as 200 K, while below 120 K we observe a tunneling-dominated temperature-independent jump rate of 1.9 × 109 s–1, many orders of magnitude faster than previously seen. Quantum transition-state theory calculations based on ab initio path-integral simulations reproduce the temperature dependence of the rate at higher temperatures and predict a crossover to tunneling-dominated diffusion at low temperatures. However, the tunneling rate is underestimated, highlighting the need for future experimental and theoretical studies of hydrogen diffusion on this and other well-defined surfaces.

The diffusion of sodium adatoms on a Cu(00l) surface has been studied with quasielastic helium atom scattering. A jump mechanism was found with activation energy 51 meV, jump length 2.56 A, and jump attempt frequency vo 0.53 THz. The adatom vibrational frequency parallel to the surface is measured to be 1.23 THz. The data are interpreted with the aid of a realistic molecular-dynamics simulation which reveals that vo is governed by the rate of energy exchange between the adatoms and the substrate, and not, as transition state theory would predict, by the vibrational frequencies of the adatom.PACS numbers: 79.20.Rf The two-dimensional diffusion of atoms on surfaces is an important elementary process in surface chemistry and provides an ideal model system for studying activated processes [1]. In the experiments to be reported here we have used helium atom scattering to identify a singlejump diffusion mechanism, measure the activation energy, and even determine the related vibrational frequencies of adatoms diffusing on a well defined single-crystal surface. The present detailed results make it possible to carry out a direct comparison with a realistic moleculardynamics simulation of the diffusing adatoms' motion which confirms the interpretation of the data and provides information on the role of energy dissipation and multiple jumps.The technique used here relies on the fact that helium atoms have large diffuse scattering cross sections from defects on a crystal surface [2], It has been shown both theoretically [3,4] and experimentally [5] that the diffuse "elastic" scattering from mobile defects has an energy broadening due to the finite residence time of the defect at a particular surface site. From the dependence of this broadening on the scattering wave vector it is possible to determine the microscopic surface diffusion mechanism. The first such experiments were performed on a premelting lead surface [5] just below the melting point where neither the concentration of the diffusing atoms nor the surface structure are well defined [6]. In this new experiment a known small concentration of sodium adatoms (0=0.1) was evaporated onto a clean Cu(001) surface and the angular distributions of elastically scattered helium atoms were used to show that the sodium adatoms are widely separated and interact only weakly with each other by long-range dipole-dipole repulsive forces [7] of the type predicted by Kohn and Lau [8]. The diffusion measurements were made over a range of temperatures far below the bulk copper melting point, where only relatively small substrate vibrations are expected.The theory of the quasielastic scattering phenomena is described elsewhere [3][4][5]9,10]. If diffusion proceeds by single jumps with jump vectors {j} and mean jump frequencies {VJ}, then the quasielastic peak shape is a Lorentzian with an energy FWHM A£(AK)-2ft2>j[l-cos(AK-j)], (1) j where AK is the component of the scattering wave vector parallel to the surface [10]. The periodic behavior of AE with AK arises because if, for a part...