METHODSIn Scanning Tunneling Microscopy (STM) a tip is brought so close to the surface to be imaged that a tunneling current flows at suitable tip potential. The tip position is adjusted by a piezoelectric element. The surface is scanned by the tip, the vertical distance of which is typically controlled through the tunneling current by a feedback loop. In this constant current mode the tip distance (given by the piezo voltage) is controlled such that the current remains constant. The piezo voltage (height) is then the image forming signal. Other modes of operation (e.g. constant height mode) are possible. By varying the tip potential, Scanning Tunneling Spectroscopy (STS) is possible. Such a spectrum determines the density of electronic states around the Fermi level. In particular, STS enables distinction between metals and semiconductors and reveals the band gap of the latter. STM or STS have been applied to small particles in few cases only, and former studies mainly refer to island-like structures produced by vacuum deposition on surfaces (e.g. K. Sattler, 1991). K. Sattler et al. (1994) have also applied STM to nanophase materials. In the present study we investigated the possibilities of characterizing particles from an aerosol generation process (gas evaporation) by STM and STS. The particles are essentially spherical and expected to exhibit less interaction with the substrate than those produced by vacuum deposition. Particles in the nm regime were formed by evaporation from a heated metal wire in ultra-pure Helium evaporating from the surface of liquid Helium. The particles were size characterized by mobility analysis and deposited on a highly oriented pyrolytic graphite (HOPG) substrate. In particular, beryllium (Be) and silver (Ag) particles were produced and analyzed in an STM under ambient air. Be forms a monoatomic oxide layer, which prevents from substantial oxidation. Since Be could have oxidized during the generation process by possible oxygen impurities, another sample was prepared by vapor deposition in vacuum for comparison: A tantalum boat, previously outgassed at high temperature, was filled with pieces of beryllium and heated in the vicinity of a HOPG surface. Be atoms hitting the surface adhere to it and grow to form small islands or particles on this surface. The sample was transferred to a vacuum STM without air contact.RESULTSOn all samples, particles were disturbed by the tunneling tip. Fig. 1 a) shows an initially particle covered region swept clean by a number of scans. In fig 1 b) the former left rim of the image is centered, and accumulation of particles by action of the tip becomes visible. It must be assumed that some particles attach to the tip, forming a rather loose cluster there, which impedes sharp imaging. Atomic resolution was rarely acheivable on particle covered substrates, especially if particles larger than 2 nm were present. Single particles of 2nm in diameter or less, however, always adhered to certain sites of the HOPG, mainly at edges or in ditches, where they could be imaged. Agglomerates, composed of extremely small primary particles could be resolved. STS spectra of 1-2 nm particles were taken for the Be samples. The one for the aerosol generated sample is shown in fig. 3. The vacuum generated sample showed a similar spectrum, indicating pureness of the former one. The clear occurrence of a gap indicates that the particles examined are not metallic but semiconducting. This is surprising, because theory predicts a metal-semiconductor transition at smaller size for Be. Further studies are in progress to verify that the particles examined are indeed Be. In any case, these results demonstrate the potential of STM and STS in characterization of nanoparticles. For Ag and Be the methods are applicable to particles 2 nm in diameter or smaller using HOPG as a substrate. Other substrates should be tested in view of possible stronger particle adherence.