Clustering and Heterogeneous P1 Distributions in Diamond Govern DNP Mechanisms at 6.9 and 13.8 T

Nitrogen substitutions (P1 centers) in diamonds were shown to provide significant ¹³C hyperpolarization at room temperature and high fields, with the presence of P1 clusters hypothesized to be essential. We compare the 6.9 and 13.8 T ¹³C dynamic nuclear polarization (DNP) and electron paramagnetic resonance spectra of two single-crystal diamonds: one with a high P1 concentration (∼20 ppm) and abundant P1 clusters, and another with a low P1 concentration (∼3.5 ppm) and no clusters. The first exhibits a 540 ± 180 nm ¹³C NMR enhancement with broad and narrow DNP features, whereas the second displays only weak, sharp DNP peaks after prolonged irradiation. The dependence of ¹³C polarization buildup rate on millimeter-wave frequency distinguishes solid-effect (SE) from cross-effect (CE), confirming P1 clusters are essential for efficient high-field DNP. Moreover, we show that in the high-P1-concentration diamond, the SE and CE polarize distinct pools of ¹³C nuclei, demonstrating DNP's ability to map the spatial distribution of diamond defects.