The study of the adsorption and diffusion of heavy atoms on light element substrates by means of the atomic resolution stem

Abstract The feasibility of using a high resolution scanning transmission electron microscope (STEM) to study the adsorption and diffusion of heavy atoms on low atomic number substrates is investigated. It is shown that the study of surface phenomena on an atomic scale with the STEM can be complementary to existing techniques since it is capable of observing substrate—adsorbate systems not easily accessible by other methods. Two types of measurements which can be performed at present have been demonstrated. They are: (a) the measurement of the spacing distribution of adatoms (or admolecules) deposited onto substrates; and (2) the direct measurement of the diffusion of individual adatoms, admolecules or clusters on substrates. Our initial experiments have utilized thin evaporated carbon films as substrates and solvent evaporated heavy atom containing molecules as adsorbates because of the ease in preparation of such samples. However, the method can be extended to any substrate that can be made of small enough mass thickness and any adsorbate that can be visualized. We have: (1) found that admolecules deposited onto substrates at low concentrations (from 7 × 10 −2 /A 2 to 5 × 10 −3 /A 2 ) exhibit preferential spacings with respect to one another (as opposed to being randomly distributed); and (2) measured the thermal motion of silver and uranium atoms deposited from dilute solutions of silver cyanide and uranyl chloride on carbon substrates and determined the diffusion coefficient and activation energy for diffusion for the individual adatoms (at room temperature). In addition, we have shown for the uranium samples that within our experimental error the effects of electron-beam interactions on the spacing distributions and the observed frequency of motion are small and do not predominate.