MINIMIZING TRANSMISSION ELECTRON-MICROSCOPY BEAM DAMAGE DURING THE STUDY OF SURFACE-REACTIONS ON SODIUM-CHLORIDE

Electron beam damage is a significant limitation for transmission elec tron microscopy (TEM) studies of beam-sensitive samples. An approach f or studying surface reactions on alkali halide crystals using 200 kV T EM is presented. Experiments were designed to monitor the reaction of NaCl crystals with HNO3 gas followed by water vapor to form solid NaNO 3. During beam damage experiments, TEM micrographs record structural c hanges to both NaCl and NaNO3, including dislocation loops, void forma tion, and decomposition. Sample decomposition can be successfully mini mized by a combination of commonly used techniques: (1) focusing the b eam adjacent to the area of interest, (2) lowering the electron densit y, (3) choosing to image larger (micrometer- versus submicrometer-size d) alkali halide crystals, and (4) lowering temperature by the use of a liquid nitrogen tooling stage. From these results, additional studie s were desired that monitored sequential experiments. Sensitive microm eter-sized sodium chloride single crystals before and after exposure t o nitric acid vapor and water vapor and the subsequent growth of submi crometer-sized sodium nitrate single crystals could then be successful ly imaged using TEM.