High-Field ¹³C Dynamic Nuclear Polarization in Nanodiamond

Hyperpolarization in diamond via dynamic nuclear polarization (DNP) has attracted a lot of interest for various applications as diamond exhibits extremely long ¹³C spin relaxation times and has endogenous polarizing agents in the form of crystalline defects. Diamond DNP in polycrystalline samples has been shown to decrease with particle size, and significant ¹³C enhancements have only been observed in micrometer- or sub-micrometer-sized particles. Nanometer-sized samples have shown enhancements of less than 10 even at DNP-friendly conditions of low field (∼3.3 kG) and low temperature. The lack of efficient DNP in nanodiamond was attributed to high density of unpaired electron spins at surface dangling bonds and has been a technical bottleneck for applications such as an imaging agent for magnetic resonance imaging. In this study, we were able to achieve a ¹³C NMR enhancement of over 100 with nanodiamonds of 25 nm median size at a moderately low temperature (20 K) with our home-built DNP spectrometer at 9.2 T (ωₑ ≈ 260.5 GHz). The high-field electron paramagnetic resonance (EPR) spectrum at 20 K shows a broad line width comparable to the ¹³C NMR frequency (99.5 MHz). This renders cross effect or thermal mixing to be effective at high fields.