  Methods of optical dynamic nuclear polarization (DNP) open the door to the
replenishable hyperpolarization of nuclear spins, boosting their NMR/MRI
signature by orders of magnitude. Nanodiamond powder rich in negatively charged
Nitrogen Vacancy (NV) defect centers has recently emerged as one such promising
platform, wherein 13C nuclei can be hyperpolarized through the optically pumped
defects completely at room temperature and at low magnetic fields. Given the
compelling possibility of relaying this 13C polarization to nuclei in external
liquids, there is an urgent need for the engineered production of highly
"hyperpolarizable" diamond particles. In this paper, we report on a systematic
study of various material dimensions affecting optical 13C hyperpolarization in
diamond particles -- especially electron irradiation and annealing conditions
that drive NV center formation. We discover surprisingly that diamond annealing
at elevated temperatures close to 1720C have remarkable effects on the
hyperpolarization levels, enhancing them by upto 36-fold over materials
annealed through conventional means. We unravel the intriguing material origins
of these gains, and demonstrate they arise from a simultaneous improvement in
NV electron relaxation time and coherence time, as well as the reduction of
paramagnetic content, and an increase in 13C relaxation lifetimes. Overall this
points to significant recovery of the diamond lattice from radiation damage as
a result of the high-temperature annealing. Our work suggests methods for the
guided materials production of fluorescent, 13C hyperpolarized, nanodiamonds
and pathways for their use as multi-modal (optical and MRI) imaging and
hyperpolarization agents.
