High-frequency EPR of surface impurities on nanodiamond

Diamond is a fascinating material, hosting nitrogen-vacancy (NV) defect centers with unique magnetic and optical properties. There have been many reports that suggest the existence of paramagnetic impurities near surface of various kinds of diamonds. Electron paramagnetic resonance (EPR) investigation of mechanically crushed nanodiamonds (NDs) as well as detonation NDs revealed g$\sim$2 like signals that are attributed to structural defects and dangling bonds near the diamond surface. In this presentation, we investigate paramagnetic impurities in various sizes of NDs using high-frequency (HF) continuous wave (cw) and pulsed EPR spectroscopy [1]. Strong size dependence on the linewidth of HF cw EPR spectra reveals the existence of paramagnetic impurities in the vicinity of the diamond surface. We also study the size dependence of the spin-lattice and spin-spin relaxation times ($T_1$ and $T_2$) of single substitutional nitrogen defects in NDs Significant deviations from the temperature dependence of the phonon-assisted $T_1$ process were observed in the ND samples, and were attributed to the contribution from the surface impurities. [1] F. H. Cho, V. Stepanov, R. D. Akiel, X. Zhang, and S. Takahashi, submitted (2016).