Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons, and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors. Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons. Rather, metallic surfaces should support only conventional surface plasmons-higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy and sub-wavelength optics, but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.

the contact width, and L is the nanotube length. These quantities are dif®cult to measure independently and calculations are modeldependent. As a guide, we apply a JKR model for the contact of cylinders 25 and ®nd a contact width of 3 nm for a tube radius of 13.5 nm. Our measurements of 0.006 N m -1 for the friction force per unit length is then consistent with a shear stress of 2 3 10 6 Pa. This can be compared with a value of 5 3 10 6 Pa, as inferred from AFM tip/graphite measurements 16 . To compare rolling and sliding in a single tube, we can calculate the force (4 nN for L 590 nm) and that would be needed to slide tube B, which in fact rolls. Finally, we note that the area under the lateral force trace is a direct measure of energy loss in rolling. For tube B, we measure an energy loss of 8 6 3 3 10 2 16 J per revolution. The sliding energy loss expected for this distance (85 nm) can be calculated using the frictional force of 4 nN, yielding 3 3 10 2 16 J.When we compare our lateral force measurements for sliding and rolling cases, we ®nd that the stick peaks in rolling are higher than the lateral force needed to sustain sliding, and that the energy cost for rolling is larger than that of the sliding cases. Why should the nanotubes roll? We speculate that, owing to the size and surface features of the rolling nanotubes, a stick peak for sliding in side-on pushing might exist that is larger than the threshold for rolling. Atomic-scale substrate interactions may also play a roll as we have observed this characteristic rolling only on graphite. Rolling behaviour has been accompanied by a preferential, threefold, inplane orientation that indicates intimate nanotube/graphite contact, and perhaps lattice registry. Rolling may occur only when both the nanotube and the underlying graphite have long-range order. In these cases that there may be a barrier for sliding which is larger than that for rolling and may preclude the direct measurement of sliding friction 7 . M

The surface structure of Bi͑111͒ was investigated by low-energy electron diffraction ͑LEED͒ intensity analysis for temperatures between 140 and 313 K and by first-principles calculations. The diffraction pattern reveals a ͑1 ϫ 1͒ surface structure and LEED intensity versus energy simulations confirm that the crystal is terminated with a Bi bilayer. Excellent agreement is obtained between the calculated and measured diffraction intensities in the whole temperature range. The first interlayer spacing shows no significant relaxation at any temperature while the second interlayer spacing expands slightly. The Debye temperatures deduced from the optimized atomic vibrational amplitudes for the two topmost layers are found to be significantly lower than in the bulk. The experimental results for the relaxations agree well with those of our first-principles calculation.

Contrary to previous reports we show that the acoustic surface plasmon (ASP) exists also at noble-metal surfaces, thus demonstrating the generality of this phenomenon in the presence of partially filled Shockley surface states. Angle-resolved high-resolution electron energy loss spectroscopy measurements and calculations of the surface loss function indicate that for Cu(111) the ASP is a sharp feature up to a loss energy of about 0.4 eV. The dispersion is indeed linear (acoustic) with a slope (sound velocity) of (4.33 ± 0.33) eVÅ in good agreement with recent theoretical predictions. The ASP can play important roles down to the meV regime, precluded to ordinary surface plasmons, for electron, phonon and adsorbate dynamics, as well as chemical reactions and advanced microscopies.

A key challenge in thin-film growth is controlling structure and composition at the atomic scale. We have used spatially resolved electron scattering to measure how the three-dimensional composition profile of an alloy film evolves with time at the nanometer length scale. We show that heterogeneity during the growth of Pd on Cu(001) arises naturally from a generic step-overgrowth mechanism relevant in many growth systems.

We present a combined experimental and theoretical study of molecular methanethiol ͑CH 3 SH͒ adsorption on the reconstructed Au͑111͒ surface in the temperature range between 90 and 300 K in UHV. We find that the simplest thiol molecules form two stable self-assembled monolayer ͑SAM͒ structures that are created by distinct processes. Below 120 K, a solid rectangular phase, preserving the herringbone reconstruction, emerges from individual chains of spontaneously formed dimers. At higher adsorption temperatures below 170 K, a close-packed phase forms via dissociative CH 3 SH adsorption and the formation of Au adatoms that are not incorporated into the SAM. We show that the combination of a strong substrate-mediated interaction with nondissociative dimerization and temperature activated removal of the Au͑111͒ reconstruction drives the largescale assembly of molecular CH 3 SH into two distinct phases.

We have measured the structure and chemical composition of ultrathin Pd films on Cu͑001͒ using lowenergy electron microscopy. We determine their local stoichiometry and structure, with 8.5 nm lateral spatial resolution, by quantitatively analyzing the scattered electron intensity and comparing it to dynamical scattering calculations, as in a conventional low-energy electron diffraction ͑LEED͒-IV analysis. The average t-matrix approximation is used to calculate the total atomic scattering matrices for this random substitutional alloy. As in the traditional LEED analysis, the structural and compositional parameters are determined by comparing the computed diffraction intensity of a trial structure to that measured in experiment. Monte Carlo simulations show how the spatial and compositional inhomogeneity can be used to understand the energetics of Cu-Pd bonding.

We have used in situ low-energy electron microscopy (LEEM) to correlate the atomic and electronic structure of graphene films on polycrystalline Ni with nm-scale spatial resolution. Spatially resolved electron scattering measurements show that graphene monolayers formed by carbon segregation do not support the π-plasmon of graphene, indicating strong covalent bonding to the Ni. Graphene bilayers have the Bernal stacking characteristic of graphite and show the expected plasmon loss at 6.5 eV. The experimental results, in agreement with first-principles calculations, show that the π-band structure of free-standing graphene appears only in films with a thickness of at least two layers and demonstrate the sensitivity of the plasmon loss to the electronic structure.