Reflection electron energy-loss spectroscopy and imaging for surface studies in transmission electron microscopes.

A review is given on the techniques and applications of high-energy reflection electron energy-loss spectroscopy (REELS) and reflection electron microscopy (REM) for surface studies in scanning transmission electron microscopes (STEM) and conventional transmission electron microscopes (TEM). A diffraction method is introduced to identify a surface orientation in the geometry of REM. The surface dielectric response theory is presented and applied for studying alpha-alumina surfaces. Domains of the alpha-alumina (012) surface initially terminated with oxygen can be reduced by an intense electron beam to produce Al metal; the resistance to beam damage of surface domains initially terminated with Al+3 ions is attributed to the screening effect of adsorbed oxygen. Surface energy-loss near-edge structure (ELNES), extended energy-loss fine structure (EXELFS), and microanalysis using REELS are illustrated based on the studies of TiO2 and MgO. Effects of surface resonances (or channeling) on the REELS signal-to-background ratio are described. The REELS detection of a monolayer of oxygen adsorption on diamond (111) surfaces is reported. It is shown that phase contrast REM image content can be significantly increased with the use of a field emission gun (FEG). Phase contrast effects close to the core of a screw dislocation are discussed and the associated Fresnel fringes around a surface step are observed. Finally, an in situ REM experiment is described for studying atomic desorption and diffusion processes on alpha-alumina surfaces at temperatures of 1,300-1,400 degrees C.