Detection of paramagnetic spins with an ultrathin van der Waals quantum sensor

TL;DR

This paper demonstrates the use of ultrathin hexagonal boron nitride as a 2D quantum sensor capable of detecting paramagnetic spins through spin relaxation measurements, offering a versatile and sensitive tool for chemical and biological analysis.

Contribution

It introduces a novel 2D quantum sensor based on hBN defects for detecting paramagnetic spins, overcoming limitations of bulk sensors.

Findings

Detection of Gd$^{3+}$ ions via T1 quenching in hBN

Successful spin measurements on solution-suspended hBN

Ultrathin hBN exhibits measurable spin relaxation properties

Abstract

Detecting magnetic noise from small quantities of paramagnetic spins is a powerful capability for chemical, biochemical, and medical analysis. Quantum sensors based on optically addressable spin defects in bulk semiconductors are typically employed for such purposes, but the 3D crystal structure of the sensor inhibits the sensitivity by limiting the proximity of the defects to the target spins. Here we demonstrate the detection of paramagnetic spins using spin defects hosted in hexagonal boron nitride (hBN), a van der Waals material which can be exfoliated into the 2D regime. We first create negatively charged boron vacancy (V$_{\rm B}^-$) defects in a powder of ultrathin hBN nanoflakes ($<10$~atomic monolayers thick on average) and measure the longitudinal spin relaxation time ($T_1$) of this system. We then decorate the dry hBN nanopowder with paramagnetic Gd$^{3+}$ ions and observe a clear $T_1$ quenching, under ambient conditions, consistent with the added magnetic noise. Finally, we demonstrate the possibility of performing spin measurements including $T_1$ relaxometry using solution-suspended hBN nanopowder. Our results highlight the potential and versatility of the hBN quantum sensor for a range of sensing applications, and pave the way towards the realisation of a truly 2D, ultrasensitive quantum sensor.