Imaging of atomic clouds outside the surfaces of gold crystals by electron microscopy

Abstract

The structure of small metal crystals has been intensively examined by high-resolution electron microscopy (HREM). In particular, multiply-twinned gold and silver crystals have been characterized using the profile-imaging method^1,2 at atomic resolution, and reconstructed metal surfaces observed². Crystal structure images of large gold clusters consisting of 55 gold atoms arranged in a cubeoctahedron have been recorded³ using 2.5 Å resolution, and crystal growth, row by row, on a {111} surface has been documented⁴ using a low-light-level silicon-intensified target television (TV) camera and video system with an on-line image processor. Direct imaging of rearrangements of atomic columns on extended gold surface⁵ established that profile imaging can provide information about surface self-diffusion. The motion of surface atoms, recorded with a real-time video tape-recorder (VTR) system, and the formation of surface atom steps on {100} surfaces, although not {111}, has also been reported recently⁶. Dynamic HREM observations at TV rate showing defect motion in gold⁷ and CdTe (refs. 8, 9) has given information on ‘in-lattice’ rearrangements of columns of atoms. We report here surface profile images recorded with the electron beam along the 〈110〉 direction with spatial resolution of ∼2.0 Å which reveal changes in occupancy of the atom columns often within periods of less than 0.1 s. Surfaces with several adjacent atom columns involved in rapid structural changes frequently interacted with a cloud of atoms extending out to 9 Å off the crystal and changes in shape and density of the clouds were recorded. Although these clouds have already been described^6,10, the present work is the first to analyse these events properly and to describe them in detail. The motion of atomic columns and the existence of atom clouds revealed here may have important consequences for crystal growth, surface science and catalysis studies.