Nitrogen-Terminated Milled Nanodiamond Surfaces by Plasma Exposure

This study investigates surface modifications of hydrogen-terminated milled nanodiamond (H-MND) drop-cast films by microwave (MW) and radio frequency (RF) nitrogen plasma exposures. The RF(N2) damaging plasma treatment results in the highest nitrogen adsorption (∼7.5 at. %) followed by MW(N2) (∼4.2 at. %) and RF(N2) nondamaging (∼3.8 at. %) plasma treatments. Upon MW(N2) plasma exposure, nitrogen predominantly adsorbs in C–N/C═N and NH states, whereas RF(N2) treatments result in mixed C–N/C═N, C≡N, and NH states, as revealed by electron spectroscopy. Crystalline edges strongly influence N, H, and O adsorption onto MND and act as active adsorption sites. The NH(ads) concentration is notably higher on MND surfaces compared to that on poly/single-crystalline surfaces, likely favored by the additional bonding configuration of hydrogen on the MND. NH(ads) species desorbed upon vacuum annealing in the 500–700 °C range, leaving CN adsorbed species onto the MND surfaces. The nitrogen and oxygen concentration monotonically decreases with annealing temperatures from 300 to 1000 °C. Upon high-temperature annealing, partial recovery of the MND surfaces occurs, depending on plasma exposure conditions. This study may be critical in all ex situ applications influenced by the near-surface physicochemical and electronic properties of nitrogen-terminated MND surfaces, such as NV centers in nanocrystals.