High temperature annealing enhanced diamond 13C hyperpolarization at room temperature

Abstract

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 13 C 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 13 C 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 13 C hyperpolarization in diamond particles -especially electron irradiation and annealing conditions that drive NVcenter formation. We discover surprisingly that diamond annealing at elevated temperatures close to 1720 • C 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 NVelectron relaxation time and coherence time, as well as the reduction of paramagnetic content, and an increase in 13 C 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, 13 C hyperpolarized, nanodiamonds and pathways for their use as multi-modal (optical and MRI) imaging and hyperpolarization agents.

Cross-relaxation studies with optically detected magnetic resonances in nitrogen-vacancy centers in diamond in an external magnetic field

“…Inside the diamond crystals exist a wide range of paramagnetic defects. Among these defects are substitutional nitrogen (P1 center), which occurs when one of the 12 C atoms is replaced by a nitrogen atom 14 N, or point-like defects, which can occur, for example, when one of the carbon 12 C isotopes is replaced by a carbon 13 C. NV centers can be used as efficient sensors that are already inside the diamond crystal to study these defects [21][22][23], which are interesting not only by themselves but for the role they play in the hyperpolarization effects of the nuclear spin of nitrogen. Initial hyperpolarization of the nitrogen nucleus through optically pumped NV electrons have a broad range of applications [24], but can be influenced by other point-like defects in the diamond crystal lattice [25].…”