IMAGING OF COLLOIDAL GOLD ON GRAPHITE BY SCANNING TUNNELING MICROSCOPY - ISOLATED PARTICLES, AGGREGATES, AND ORDERED ARRAYS

Gold colloidal suspensions have been deposited onto highly oriented pyrrolitic graphite and analyzed by using a scanning tunneling microscope (STM). The size distribution of individual particles measured with the STM (16 +/- 1.5 nm) agreed well with measurements made by transmission electron microscopy. STM images of colloidal aggregates containing up to 15 individual particles could be obtained under a wide variety of experimental conditions. The quality of these images was extremely sensitive to the type of aggregating agent used, with ionic salts giving better results than molecular species. Under our colloidal deposition conditions we often observed highly ordered hexagonal patterns with a periodicity of 75-150 nm, extending over areas as large as 150 x 150 nm2. Preliminary evidence suggests that this hexagonal lattice is associated with the graphite substrate itself, and not with an ordered, two-demensional array of colloidal particles.