[show abstract][hide abstract] ABSTRACT: The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are -1.1 and -1.4 eV, respectively, in good agreement with recent calculations and experiments.
Physical Chemistry Chemical Physics 04/2010; 12(13):3203-9. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: A cavity ring-down spectrometer designed to investigate optical properties of size-selected clusters on surfaces under ultrahigh vacuum (UHV) conditions is presented. Clusters are produced using a laser vaporization cluster source with typical size-selected cluster currents of about 100 pA. The size of the deposition area can be controlled by means of a focusing octopole. Using the UHV compatible mirror exchanger, it is possible to have up to ten ring-down cavities and to adjust them while in vacuum. With ten cavities it is possible to cover a continuous spectral range as broad as 600 nm. The sensitivity of the method is shown to be about 5 ppm, which is two orders of magnitude better than commercial techniques. The optical spectra of small Ni <sub>N</sub> clusters ( N=7 , 10, and 20) supported at amorphous SiO <sub>2</sub> in the range between 417 and 669 nm are presented. Simple Mie theory calculations and oscillator-strength sum rule are in good agreement with the experimental data. The method proved to be sensitive enough for detection of defect sites and therefore makes it possible to study the interaction between the clusters and different surface defects. Furthermore the cryogenic sample holder can be used to perform cavity ring-down spectroscopy at low temperatures.
Journal of Applied Physics 01/2009; · 2.21 Impact Factor