  Nanodiamond (ND) hosting nitrogen-vacancy (NV) centers is a promising
platform for quantum sensing applications. Sensitivity of the applications
using NV centers in NDs is often limited due to presence of paramagnetic
impurity contents near the ND surface. Here, we investigate near-surface
paramagnetic impurities in NDs. Using high-frequency (HF) electron paramagnetic
resonance spectroscopy, the near-surface paramagnetic impurity within the shell
of NDs is probed and its g-value is determined to be 2.0028(3). Furthermore, HF
electron-electron double resonance-detected nuclear magnetic resonance
spectroscopy and a first principle calculation show that a possible structure
of the near-surface impurity is the negatively charged vacancy V-. The
identification of the near-surface impurity by the present investigation
provides a promising pathway to improve the NV properties in NDs and the
NV-based sensing techniques.
