An STM application of the surface diffusion metal ion source: Li nanostructures on Pt

Abstract

First application of the new Lithium Surface Diffusion Metal Ion Source in the Scanning Tunneling Microscope (STM) mode under UHV conditions is reported. By applying 5-lops voltage pulses to the Li-covered W tip placed within tunneling range, single Li hillocks (4Ox4Onm) as well as Li nanostructures are deposited on the chosen area of the Pt(II0) sample in a well-reproducible way. The topography and evolution of the created Li-nanoformations can be immediately probed using the same t@ in the STM imuging mode. The Surface Diffusion Metal Ion Source (SDMIS), a novel type of solid-state ion source, was recently presented [ 11. In this device ions are generated directly on a field emitter surface via field desorption from the chemisorbed state and the supply of the working substance (e.g. Li) occurs via surface diffusion from the multilayer deposit on the shank of the tip. Metal ions, field desorbed from the top of the tip, form a nearly monoenergetic ion flux [2] at constant (on a nanoscale) geometric dimensions. The working mechanism of the Li-SDMIS (field-desorption from the solid state) differs essentially from that of the traditional liquid metal ion sources (LMIS), where ions are formed far outside of the emitter surface. A huge difference in the degree of localization of the ionization process in LMIS, illustrated by the width of energy distribution (FWHM of about 4.5eV for LMIS [3] and 0.25eV for Li-SDMIS [2], respectively, is measured) emphasize the principal difference between the Li-SDMIS and a LMIS. Since no geometric changes, like liquid metal cone foimation [4], take place near the surface of the SDMIS at the instant the ion current is switched on, it is challenging to use such a nanometersized source in an STM mode (placing the Li-covered tip within the tunneling gap). We here report a first realization of such an STM mode of the Li-SDMIS under UHV conditions. Experiments were performed in the homemade UHV-STM combined with a field electron microscope (FEM), both operated at room temperature. A [ 1 101-oriented tungsten STM tip covered by Li multilayer (deposited in UHV conditions using commercial SAES Getters Li source) was used as a Li-solid-state emitter. The details of the preparation of the field emitter tip as a long lasting source of the field desorbed Li ions can be found elsewhere [5,6].