The Anomalous Formation of Irradiation Induced Nitrogen-Vacancy Centers in 5-Nanometer-Sized Detonation Nanodiamonds

TL;DR

This study reveals that nitrogen-vacancy centers in 5 nm nanodiamonds form through a unique self-annealing process during electron irradiation, resulting in higher NV$^-$ concentrations than larger nanodiamonds, with implications for quantum sensing.

Contribution

It demonstrates that NV$^-$ centers can form in 5 nm nanodiamonds at room temperature without annealing, a process not observed in larger particles, and explains this via size-dependent vacancy migration.

Findings

NV$^-$ centers form at irradiation in 5 nm DNDs without annealing.

NV$^-$ concentration in 5 nm DNDs exceeds that in 20 nm nanodiamonds.

Irradiation increases NV$^-$ concentration 12.5 times without saturation.

Abstract

Nanodiamonds containing negatively charged nitrogen-vacancy (NV$^-$) centers are versatile room-temperature quantum sensors in a growing field of research. Yet, knowledge regarding the NV-formation mechanism in very small particles is still limited. This study focuses on the formation of the smallest NV$^-$-containing diamonds, 5 nm detonation nanodiamonds (DNDs). As a reliable method to quantify NV$^-$ centers in nanodiamonds, half-field signals in electron paramagnetic resonance (EPR) spectroscopy are recorded. By comparing the NV$^-$ concentration with a series of nanodiamonds from high-pressure high-temperature (HPHT) synthesis (10 - 100 nm), it is shown that the formation process in 5 nm DNDs is unique in several aspects. NV$^-$ centers in DNDs are already formed at the stage of electron irradiation, without the need for high-temperature annealing. The effect is explained in terms of "self-annealing", where size and type dependent effects enable vacancy migration close to room temperature. Although our experiments show that NV$^-$ concentration generally increases with particle size, remarkably, the NV$^-$ concentration in 5 nm DNDs surpasses that of 20 nm-sized nanodiamonds. Using Monte Carlo simulations, we show that the ten times higher substitutional nitrogen concentration in DNDs compensates the vacancy loss induced by the large relative particle surface. Upon electron irradiation at a fluence of $1.5 \times 10 ^{19}$ e$^-$/cm$^2$, DNDs show a 12.5-fold increment in the NV$^-$ concentration with no sign of saturation. These findings can be of interest for the creation of defects in other very small semiconductor nanoparticles beyond NV-nanodiamonds as quantum sensors.