Scanning Microscopy


Small particles (Cu, Ag, In, Sn, Au, also MgO and NaCl) were prepared in the diameter range from 1 nm to 100 nm on different conductive substrates by thermal evaporation in high-vacuum or in an inert gas atmosphere. Imaging of the particles was performed in a high resolution scanning electron microscope (HRSEM) that can also be operated at low beam voltages of a few hundred volts. This mode of operation is called low voltage SEM (LVSEM). Scanning electron micrographs were taken at different beam voltages VO (0.5-30 kV). The micrographs were digitally recorded and analyzed with an image processing system operated on-line to the HRSEM. Grey-value line profiles and densitometric quantities of single particles, as well as the contrast between particle and substrate, changed with VO. The results for tin-particles on a bulk carbon substrate are shown. In all cases considered, only positive contrasts, i.e., particles looking brighter than the substrate, were obtained. The main contrast producing mechanism is, therefore, assigned to effects that include the particle's geometrical properties of size, shape and surface. Sn-, In-, and Ag-particles, imaged in the secondary electron (SE) mode showed significantly larger particle diameters, as did images simultaneously recorded with transmitted electrons; however, Au-particles did not show that difference. This effect may be qualitatively explained by SE resulting from decaying plasmons.

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