Electron Optical Investigation of Thin Films of Lithium on Tantalum, Platinum, and Carbon Substrates

Abstract

Thin films of lithium when evaporated onto well-polished, chemically cleaned, and outgassed Ta and Pt substrates formed uniformly thick layers (in the range 240–1600 Å) suitable for the p-n reaction in the Van de Graaff generator. A thin carbon coating, (about 300 Å thick) evaporated upon the Li film from a heated carbon filament immediately following the deposition of lithium on Ta and Pt substrates, insulated chemically the lithium from the air with its water vapor content and also provided carbon replicas of the Li films surface for electron microscopy. Proton bombardment of the carbon-coated Li films had no visible effect on the carbon coating nor on the Ta or Pt substrate. However, after a proton beam bombardment of 10 μA-min at 1.88 MeV, the Li films gave evidence of some initial damage effect (growth of grain size plus surface unevenness) which appeared to anneal away with subsequent bombardment. Orientation effects were observed in Li films deposited upon carbon substrates (in turn deposited on nickel lectromesh grids) in a special evaporator, attached to and forming a part of the vacuum system of the electron microscope (RCA type EMU-2B) in order to avoid contamination of the Li with the laboratory atmosphere. Electron diffraction patterns revealed the following: (1) random orientation for the thinner (240 Å, approximately) Li films and (2) preferred orientation of the (110) and (200) planes nearly perpendicular to the surface for the thicker (1600 Å, approximately) films.