Electronic Spin Relaxation and Room Temperature NMR DNP in Microcrystalline HPHT Diamond Particles

Crystalline micro- and nanosized HPHT diamond particles are attracting significant research interest because of the unique combination of mechanical, physical, chemical, and, most importantly, quantum spin properties. The latter properties provide for unmatched opportunities for developing biosensors and quantum information systems. The latter two applications capitalize on quantum physics of optical centers and, primarily, the Nitrogen Vacancy (NV) center. The quantum state of the NV center can be initialized, manipulated with external electromagnetic fields, and then measured with high fidelity due to exceptionally long spin relaxation times even at room temperature. Future progress in this technology calls for fabricating and characterizing ND particles with suitable spin properties. Here we present experimental studies of electronic spin relaxation in microcrystalline HPHT diamond as a function of particle sizes and the processing conditions. Electronic <i>T</i>₁ and <i>T</i>₂ relaxation were measured by pulsed EPR at X- (9.5 GHz, 0.33 T magnetic field), Q-band (34 GHz, 1.2 T magnetic field), and W-band (94 GHz, 3.4 T magnetic field) at 295 and 77 K. The data clearly demonstrate a heterogeneous distribution of electronic relaxation properties. Continuous wave EPR also reveals the presence of spin clusters. We then investigated microcrystalline diamond particles for room temperature dynamic nuclear polarization (DNP) at 300 MHz ¹H NMR frequency by observing static natural abundance ¹³C signal with mm-wave field on and off and compared that with a single crystal HPHT diamond. Significant DNP enhancements were observed for all the samples with &gt;1500-fold enhancement of ¹³C natural abundance NMR signal measured for a single crystal at full incident millimeter wave power. Commercial diamond lapping films such as 3M 666XW incorporating diamond particles were also investigated.