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

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.

The outcome of molecule–surface collisions can be modified by pre-aligning the molecule; however, experiments accomplishing this are rare because of the difficulty of preparing molecules in aligned quantum states. Here we present a general solution to this problem based on magnetic manipulation of the rotational magnetic moment of the incident molecule. We apply the technique to the scattering of H2 from flat and stepped copper surfaces. We demonstrate control of the molecule's initial quantum state, allowing a direct comparison of differences in the stereodynamic scattering from the two surfaces. Our results show that a stepped surface exhibits a much larger dependence of the corrugation of the interaction on the alignment of the molecule than the low-index surface. We also demonstrate an extension of the technique that transforms the set-up into an interferometer, which is sensitive to molecular quantum states both before and after the scattering event.

Spin-echo measurements offer a major step forward in the technique of helium atom surface scattering. The experimental energy transfer resolution has been improved by several orders of magnitude, and has resulted in an expansion in the range of surface systems which can be studied. We describe the measurements within a double Fourier transform formalism, which by using appropriately selected ‘tilted projections’ for measurement allows elastic, inelastic and quasi-elastic measurements to be treated on an equal footing. We illustrate the wide scope of application for helium spin-echo with measurements from each of these areas.

3He spin-echo measurements are used to follow the picosecond motion of sodium atoms on a copper (001) substrate. 2D correlated motion arising from repulsive adsorbate interactions is observed for coverages as low as 0.04 ML. At coverages greater than 0.05 ML there is a pronounced onset of motion perpendicular to the surface. The perpendicular motion is thermally activated and seems related to the basic translational hopping diffusion process. The correlated motion is modeled successfully using a molecular dynamics simulation and a dipolelike lateral interaction. A simple model which relates the apparent height of the atom with its local coverage is shown to reproduce the experimental observations.

Like dihydrogen, water exists as two spin isomers, ortho and para, with the nuclear magnetic moments of the hydrogen atoms either parallel or antiparallel. The ratio of the two spin isomers and their physical properties play an important role in a wide variety of research fields, ranging from astrophysics to nuclear magnetic resonance (NMR). Unlike ortho and para H(2), however, the two water isomers remain challenging to separate, and as a consequence, very little is currently known about their different physical properties. Here, we report the formation of a magnetically focused molecular beam of ortho-water. The beam we formed also had a particular spin projection. Thus, in the presence of holding magnetic fields, the water molecules are hyperpolarized, laying the foundation for ultrasensitive NMR experiments in the future.

We demonstrate two approaches that use the recently developed helium spin-echo technique to measure surface potential energy landscapes. For helium-lithium fluoride (100), we use the selective adsorption phenomenon to obtain the complete experimental band structure of atoms in a corrugated surface potential. For carbon monoxide-copper (001), we measure the diffusion-induced energy broadening in the scattered helium beam and extract properties of the adsorbate-substrate potential. The measurements are made possible by the resolution of our new spectrometer, which improves on existing resolution by three orders of magnitude. We show that it is possible to produce benchmark energy landscapes to assist evaluation and development of first-principles theory in the problematic van der Waals/weak chemisorption regime.