Multiscale Imaging and Tip-Scratch Studies Reveal Insight into the Plasma Oxidation of Graphite

Abstract

The plasma oxidation process of highly oriented pyrolytic graphite (HOPG) has been investigated through a combination of multiscale (micrometric to atomic) imaging by atomic force and scanning tunneling microscopies (AFM/STM) and STM tip-scratching of the HOPG substrate. Complementary information was obtained by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Repetitive imaging of the same HOPG location following a series of consecutive plasma treatments allowed an accurate determination of the plasma etch rates along both the a and c crystallographic directions of the graphite lattice. The etch rates were typically in the range of a few nm per min along the a axis, and the equivalent of 1-6 graphene layers per min along the c axis. The results pointed to the existence of two main plasma etching regimes, related to short (<20-30 min) and long (> or =30 min) treatment times. This was inferred not only from the measured plasma etch rates but also from the observation of fundamental differences in the atomic-scale surface structure of the plasma-treated HOPG samples, and from the general mechanical behavior of the materials under the action of the AFM tip. In particular, atomic-scale STM imaging suggested a change from a defected, but essentially graphitic, surface in the first regime to an amorphous carbon surface in the second regime. Together with AFM and STM, Raman spectroscopy and XPS provided a consistent picture of the surface structure and chemistry of the plasma-modified HOPG in the two regimes. The implications of these results as well as the possible mechanism that drives the plasma etching process in the two regimes are discussed.

Characterization of gold nanoclusters deposited on HOPG by atmospheric plasma treatment

“…In the present case, the C 1s spectra ( Fig. 1(d)) were curve-fitted with the inclusion of a peak attributed to C-C bonding in defect-rich regions, shifted by ∼1 eV from the main peak located at a binding energy of 284.4 eV, [18] which is assigned to C-C bonding in defect-free graphite. [18] The contribution of carbon atoms bound to oxygen was accounted for by the inclusion of two peaks in the curve-fitted spectra, located at binding energies of 286.9 and 289.4 eV.…”

“…Other authors attribute this line to carbon atoms in localized sp 2 bonding as a result of bond breaking, which is the case of carbon atoms at graphene edges (in Ref. [18]). About the oxygen species, the O1s spectrum was curvefitted into two peaks (Fig.…”

“…The high-resolution C1s spectra of graphitic materials are known to be influenced by the presence of carbon atoms bound both to other carbon atoms in defect-rich regions and oxygen atoms in different configurations (oxygen functional groups; in Ref. [18]). In the present case, the C 1s spectra ( Fig.…”

“…1(d)) were curve-fitted with the inclusion of a peak attributed to C-C bonding in defect-rich regions, shifted by ∼1 eV from the main peak located at a binding energy of 284.4 eV, [18] which is assigned to C-C bonding in defect-free graphite. [18] The contribution of carbon atoms bound to oxygen was accounted for by the inclusion of two peaks in the curve-fitted spectra, located at binding energies of 286.9 and 289.4 eV. [18] The presence of the two last species should be reasonably considered as the HOPG sample was exposed during plasma treatment to a strong oxidising atmosphere due to the injection of water droplets.…”

“…[18] The contribution of carbon atoms bound to oxygen was accounted for by the inclusion of two peaks in the curve-fitted spectra, located at binding energies of 286.9 and 289.4 eV. [18] The presence of the two last species should be reasonably considered as the HOPG sample was exposed during plasma treatment to a strong oxidising atmosphere due to the injection of water droplets. Indeed, optical emission spectrometry of the postdischarge (not shown) reveals the presence of atomic oxygen lines at 777 and 852 nm.…”