The properties of the nitrogen-vacancy center in milled chemical vapor deposition nanodiamonds

TL;DR

This study demonstrates that ball milling of CVD diamonds can produce nanodiamonds with nitrogen-vacancy centers exhibiting bulk-like spin coherence properties, suitable for scalable quantum sensing applications.

Contribution

It introduces an optimized ball milling process for CVD diamonds to create nanodiamonds with consistent NV center properties, matching bulk diamond spin coherence times.

Findings

CVD FNDs have lower NV- charge state contribution than HPHT FNDs.

CVD FNDs exhibit longer PL lifetime compared to HPHT FNDs.

CVD FNDs show bulk-like T1 spin relaxation times (~4 ms).

Abstract

Fluorescent nanodiamonds (FNDs) containing negatively charged nitrogen-vacancy (NV-) centers are vital for many emerging quantum sensing applications from magnetometry to intracellular sensing in biology. However, developing a scalable fabrication method for FNDs hosting color centers with consistent bulk-like photoluminescence (PL) and spin coherence properties remains a highly desired but unrealized goal. Here, we investigate optimized ball milling of single-crystal diamonds produced via chemical vapor deposition (CVD) and containing 2 ppm of substitutional nitrogen and 0.3 ppm of NV- to achieve this goal. The NV charge state, PL lifetime, and spin properties of bulk CVD diamond samples are directly compared to milled CVD FNDs and commercial high-pressure high-temperature (HPHT) FNDs. We find that on average, the relative contribution of the NV- charge state to the total NV PL is lower and the NV PL lifetime is longer in CVD FNDs compared to HPHT FNDs, both likely due to the lower Ns0 concentration in CVD FNDs. The CVD bulk and CVD FNDs on average show similar average T1 spin relaxation times of 3.2 $\pm$ 0.7 ms and 4.7 $\pm$ 1.6 ms, respectively, compared to 0.17 $\pm$ 0.01 ms for commercial HPHT FNDs. Our results demonstrate that ball milling of CVD diamonds enables the large-scale fabrication of NV ensembles in FNDs with bulk-like T1 spin relaxation properties.