INVESTIGATION OF NANOPARTICLES IN HIGH-RESOLUTION SCANNING ELECTRON-MICROSCOPY (SEM) AND LOW-VOLTAGE SEM BY DIGITAL IMAGE-ANALYSIS

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 subs trates by thermal evaporation in high-vacuum or in an inert gas atmosp here. Imaging of the particles was performed in a high resolution scan ning 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 diff erent beam voltages V-o (0.5-30 kV). The micrographs were digitally re corded and analyzed with an image processing system operated on-line t o the HRSEM. Grey-value line profiles and densitometric quantities of single particles, as well as the contrast between particle and substra te, changed with V-o. The results for tin-particles on a bulk carbon s ubstrate are shown. In all cases considered, only positive contrasts, i.e., particles looking brighter than the substrate, were obtained. Th e 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 imag es simultaneously recorded with transmitted electrons; however, Au-par ticles did not show that difference. This effect may be qualitatively explained by SE resulting from decaying plasmons.