High-Field ¹³C Dynamic Nuclear Polarization in Nanodiamond

Abstract

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.