  Nitrogen-doped ultrananocrystalline diamond (N-UNCD) is a form of diamond
electrode with near-infrared photoresponsivity, making it well suited for
physiological applications. N-UNCD's photoresponsivity is strongly influenced
by its surface. While it is known that oxygen treatment provides a higher
photoresponsivity, a better understanding of its surface processes is needed to
tailor the material for optical neuromodulation. This work examines the impact
of various oxygen treatment methods, with aim of creating oxygen rich surfaces
with different chemical and structural properties. Surface characterisation
methods along with electrochemical and photoelectrochemical measurements and
modelling were used to investigate the films. It was found that oxygen furnace
annealing resulted in orders of magnitude improvement in the near-infrared
photoresponsivity, to 3.75 +/- 0.05 uA/W. This translates to an approximate 200
times increase in the photocurrent compared to the untreated surface. This
enhancement in photocurrent is largely due to the changes in the chemical
species present at the surface. The photocurrent is estimated to be sufficient
for extra-cellular stimulation of brain neurons within the safe optical
exposure limit, positioning N-UNCD as an excellent candidate to be used in
next-generation photoelectrodes for photobiomodulation.
