Detection of Nuclear Quadrupolar Resonance of a nanoscale 2D material by optimized nitrogen vacancy ensembles in diamond.

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) of bulk quantum materials has provided significant insight into magnetic phenomena such as quantum phase criticality, magnetism, and superconductivity. With the emergence of nanoscale 2-D materials with magnetic phenomena, inductively detected NMR is not sufficiently sensitive to detect the small nuclear density in these materials. The nitrogen-vacancy center in diamond (NV) has shown great promise in bringing the spectroscopic capabilities of NQR to the nanoscale. Single NV spins provide excellent spatial resolution but can be unstable and maybe less sensitive to NMR than NV ensembles. However, NV NMR detection of 2D materials has yet to be demonstrated with NV ensembles. Furthermore, the optimal NV layer for NMR detection will require an optimized depth calibration which trades off sample-NV standoff (closer to the diamond surface) and higher activation/better spin properties (further from diamond surface).

In this work, we prepare six near-surface NV ensembles, determine their depths by 19F NMR, and determine their NMR sensitivities using a home-built fluorescence microscope with a 40 μm excitation diameter. We use one of our most sensitive NV ensembles for detection of 11B NQR in hBN exfoliated onto the diamond surface.  To prepare our near surface NV ensembles, we implant nitrogen at energies from 1.5 to 7 keV.  We experimentally demonstrate that minimizing sample sensor stand-off minimizes measurement time to achieve SNR = 3, t(SNR=3).  We then exfoliated 100nm thick flakes of hBN on a diamond and measured the NQR signal intensity of 11B with CPMGXY8-N, to demonstrate the sensitivity of these samples. We estimate these NV ensembles should have shorter t(SNR=3) than a single NV for 11B in hBN for excitation diameters as small as 8 μm.

SAND2021-13251 A