Surface chemistry and bonding configuration of ultrananocrystalline diamond surfaces and their effects on nanotribological properties

We present a comprehensive study of surface composition and nanotribology for ultrananocrystalline diamond (UNCD) surfaces, including the influence of film nucleation on these properties. We describe a methodology to characterize the underside of the films as revealed by sacrificial etching of the underlying substrate. This enables the study of the morphology and composition resulting from the nucleation and initial growth of the films, as well as the characterization of nanotribological properties which are relevant for applications including micro-/nanoelectromechanical systems. We study the surface chemistry, bonding configuration, and nanotribological properties of both the topside and the underside of the film with synchrotron-based x-ray absorption near-edge structure spectroscopy to identify the bonding state of the carbon atoms, x-ray photoelectron spectroscopy to determine the surface chemical composition, Auger electron spectroscopy to further verify the composition and bonding configuration, and quantitative atomic force microscopy to study the nanoscale topography and nanotribological properties. The films were grown on $\mathrm{Si}{\mathrm{O}}_2$ after mechanically polishing the surface with detonation synthesized nanodiamond powder, followed by ultrasonication in a methanol solution containing additional nanodiamond powder. The $s{p}^2$ fraction, morphology, and chemistry of the as-etched underside are distinct from the topside, exhibiting a higher $s{p}^2$ fraction, some oxidized carbon, and a smoother morphology. The nanoscale single-asperity work of adhesion between a diamond nanotip and the as-etched UNCD underside is far lower than for a silicon-silicon interface ($59.2\pm 2$ vs $826\pm 186\mathrm{mJ}∕{\mathrm{m}}^2$, respectively). Exposure to atomic hydrogen dramatically reduces nanoscale adhesion to $10.2\pm 0.4\mathrm{mJ}∕{\mathrm{m}}^2$, at the level of van der Waals’ interactions and consistent with recent ab initio calculations. Friction is substantially reduced as well, demonstrating a direct link between the surface chemistry and nanoscale friction. The proposed mechanism, supported by the detailed surface spectroscopic analysis, is the elimination of reactive (e.g., ${\mathrm{C}}^{*}$), polar (e.g., $\mathrm{C}\mathrm{O}$), and $\pi$-bonded $(\mathrm{C}\mathrm{C})$ surface groups, which are replaced by fully saturated, hydrogen-terminated surface bonds to produce an inert surface that interacts minimally with the contacting counterface.

Figure 1

Schematic illustrating the methodology used to characterize UNCD topside and underside surfaces using various spectroscopic techniques.

Figure 2

[(a)–(c)] SEM images of the topside, as-etched underside, and H-terminated underside surfaces of the UNCD film.

Figure 3

(Color) [(a)–(c)] Topographic AFM images of the topside, as-etched underside, and H-terminated underside surfaces of the UNCD film.

Figure 4

$\mathrm{C}1s$ XPS spectra of the UNCD film from (a) the as-etched underside and (b) the underside surfaces after H termination.

Figure 5

Auger C $KLL$ fine structure obtained on the reference sample single crystal diamond, UNCD topside, as-etched underside, and underside surfaces after H termination, respectively.

Figure 6

(a) $\mathrm{C}1s$ TEY XANES spectra of the UNCD taken on topside, as-etched underside, and underside surfaces after the hydrogen plasma treatment. The inset shows magnified view of the pre-edge features in the spectra.

Figure 7

(a) Work of adhesion between a diamond-coated tip and UNCD underside surfaces before and after H termination. Results for a silicon tip making contact with a single crystal silicon (111) wafer (both the tip and sample have a native oxide) are included for comparison. (b) TEM images of the diamond-coated tip used for adhesion measurements, demonstrating no measurable change after the measurements.

Figure 8

(a) Nanoscale friction force experienced by a diamond-coated tip scanning against the UNCD underside surface before and after H termination, measured at zero externally applied load. (b) TEM images of the diamond-coated tip used for this experiment, demonstrating no measurable wear.