Oxidation experiments using a particular grade of highly oriented pyrolytic graphite have allowed observation of large numbers of both monolayer and multilayer etch pits on the same samples, formed under identical conditions. Scanning tunneling microscopy was used to measure pits produced after various etch times, temperatures, and O2 pressures. From these data pit growth rates, activation energies, and reaction orders were derived. Although multilayer pits were observed to grow over 3 times faster than monolayer pits in air, both types of pits had the same activation energy. Multilayer etch pits were sometimes observed to form at screw dislocations in the graphite but were also seen in the absence of such defects. The experimentally determined reaction rates and activation energies were not consistent with a direct reaction of edge-carbon atoms with atmospheric oxygen, but instead suggest a chain reaction or preequilibrium process. A mechanism for oxidation of multilayer pits involving reaction of partially oxidized sites on adjacent graphite layers is suggested.

Individual fluorescent polystyrene nanospheres (<10-100-nm diameter) and individual fluorescently labeled DNA molecules were dispersed on mica and analyzed using time-resolved fluorescence spectroscopy and atomic force microscopy (AFM). Spatial correlation of the fluorescence and AFM measurements was accomplished by (1) positioning a single fluorescent particle into the near diffraction-limited confocal excitation region of the optical microscope, (2) recording the time-resolved fluorescence emission, and (3) measuring the intensity of the excitation laser light scattered from the apex of an AFM probe tip and the AFM topography as a function of the lateral position of the tip relative to the sample substrate. The latter measurements resulted in concurrent high-resolution (approximately 10-20 nm laterally) images of the laser excitation profile of the confocal microscope and the topography of the sample. Superposition of these optical and topographical images enabled unambiguous identification of the sample topography residing within the excitation region of the optical microscope, facilitating the identification and structural characterization of the nanoparticle(s) or biomolecule(s) responsible for the fluorescence signal observed in step 2. These measurements also provided the lateral position of the particles relative to the laser excitation profile and the surrounding topography with nanometer-scale precision and the relationship between the spectroscopic and structural properties of the particles. Extension of these methods to the study of other types of nanostructured materials is discussed.