Prospects for Ultralow-Mass Nuclear Magnetic Resonance using Spin Defects in Hexagonal Boron Nitride

TL;DR

This paper explores the potential of boron vacancy defects in hexagonal boron nitride as a new nanoscale NMR sensor, aiming to overcome limitations of diamond NV centers by leveraging hBN's surface properties for improved sensitivity and resolution.

Contribution

It introduces boron vacancy defects in hBN as a promising alternative for ultralow-mass NMR, detailing experimental designs, measurement protocols, and performance comparisons with existing NV-based systems.

Findings

Boron vacancies in hBN can exist within 1 nm of the surface without degradation.

Proposed protocols enable NMR measurements at nano- and micron-scales.

Expected performance surpasses traditional NV-NMR in certain regimes.

Abstract

Optically active quantum defects in solids, such as the nitrogen vacancy (NV) center in diamond, are a leading modality for micron-scale and nanoscale (ultralow-mass) nuclear magnetic resonance (NMR) spectroscopy and imaging under ambient conditions. However, the spin and optical properties of NV centers degrade when closer than about 10 nm from the diamond surface, limiting NMR sensitivity as well as spectral and spatial resolution. Here we outline efforts to develop an alternative nanoscale NMR sensor using the negatively charged boron vacancy ($V_B^-$) in hexagonal boron nitride (hBN). As a van der Waals material, hBN's surface is free from dangling bonds and other sources of paramagnetic noise that degrade the performance of near surface NVs, allowing stable $V_B^-$ defects to exist $\sim1\,$nm from the material surface. We discuss the properties of boron vacancies as they apply to narrowband (AC) magnetic field sensing and outline experimental designs optimized for this system. We propose measurement protocols for $V_B^-$ NMR for both statistically and uniformly polarized samples at the nano- and micron-scales, including relevant pulse sequences, sensitivity calculations, and sample confinement strategies; and compare the expected performance to NV-NMR. We estimate back-action effects between the $V_B^-$ electronic spins and the sample nuclear spins at the nanoscale; and account for unconventional diffusion dynamics in the flow-restricted nanoscale regime, calculating its effects on the expected $V_B^-$ NMR signal. Lastly, we identify potential sample targets and operational regimes best suited for both nanoscale and micron-scale $V_B^-$ NMR.