  Chemical functionalization of diamond surfaces by hydrogen is an important
method for controlling the charge state of near-surface fluorescent color
centers, an essential process in fabricating devices such as diamond
field-effect transistors and chemical sensors, and a required first step for
realizing families of more complex terminations through subsequent chemical
processing. In all these cases, termination is typically achieved using
hydrogen plasma sources which can etch or damage the diamond as well as
deposited materials or embedded colour centers. This work explores alternative
methods for lower-damage hydrogenation of diamond surfaces, specifically the
annealing of diamond samples in high-purity, non-explosive mixtures of nitrogen
and hydrogen gas, and the exposure of samples to microwave hydrogen plasmas in
the absence of intentional stage heating. The effectiveness of these methods
are characterized by x-ray photoelectron spectroscopy, and comparison of the
results to density-functional modelling of the surface hydrogenation energetics
implicates surface oxygen ligands as the primary factor limiting the
termination quality of annealed samples. Finally, photoluminescence
spectroscopy is used to verify that both the annealing and reduced sample
temperature plasma methods are non-destructive to near-surface ensembles of
nitrogen-vacancy centers, in stark contrast to plasma treatments which use
heated sample stages.
