# Investigation of Near-Surface Defects of Nanodiamonds by High-Frequency EPR and DFT Calculation

## TL;DR

This study uses high-frequency EPR and DFT calculations to identify near-surface paramagnetic impurities in nanodiamonds, revealing the negatively charged vacancy V- as a key defect affecting quantum sensing performance.

## Contribution

It combines advanced spectroscopic techniques with computational modeling to identify the structure of near-surface impurities in nanodiamonds, a novel approach in this context.

## Key findings

- Near-surface paramagnetic impurity g-value determined as 2.0028(3)
- HF EPR and DFT suggest the impurity is the negatively charged vacancy V-
- Identification of impurity offers pathways to enhance NV-based sensing

## Abstract

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.