Impact of irradiation conditions on the magnetic field sensitivity of spin defects in hBN nano flakes

TL;DR

The study addresses how irradiation conditions affect magnetic field sensing using V_B- centers in hBN nanoflakes. By employing helium FIB implantation across a wide fluence range and characterizing with PL, PODMR, and Raman spectroscopy, it demonstrates that spin and lattice properties are preserved up to $10^{14}$ ions/cm$^2$, establishing an optimal AC sensitivity around $1 μT/√Hz$ and a DC sensitivity near $30 μT/√Hz$ at that fluence. The results reveal a trade-off: higher fluence increases V_B- brightness but degrades spin coherence via instantaneous diffusion and lattice damage, while defect migration across implanted boundaries is negligible, confirming localization of defects. These insights enable design of patterned, scalable 2D quantum sensor platforms with controlled defect placement and known sensitivity limits.

Abstract

We study $V_{\mathrm{B}}^-$ centres generated by helium focused ion beam (FIB) irradiation in thin ($\sim$70 nm) hBN nanoflakes, in order to investigate the effect of implantation conditions on the key parameters that influence the magnetic field sensitivity of $V_{\mathrm{B}}^-$ quantum sensors. Using a combination of photoluminescence, optically detected magnetic resonance, and Raman spectroscopy, we examine the competing factors of maximising signal intensity through larger $V_{\mathrm{B}}^-$ concentration against the degradation in spin coherence and lattice quality observed at high ion fluences. Our results indicate that both the $V_{\mathrm{B}}^-$ spin properties and hBN lattice parameters are largely preserved up to an ion fluence of $10^{14}$ ions/cm$^2$, and beyond this significant degradation occurs in both. At the optimal implantation dose, an AC magnetic sensitivity of $\sim 1\,μ\mathrm{T}/\sqrt{\mathrm{Hz}}$ is achieved. Using the patterned implantation enabled by the FIB, we find that $V_{\mathrm{B}}^-$ centres and the associated lattice damage are well localised to the implanted regions. This work demonstrates how careful selection of fabrication parameters can be used to optimise the properties of $V_{\mathrm{B}}^-$ centres in hBN, supporting their application as quantum sensors based on 2D materials.

Figures (4)

• Figure 1: Fabrication and optical characterization of $V_{\rm B}^-$ centres in hBN nanoflakes. a) AFM (left) and fluorescence confocal image (right) of a sample irradiated with an ion fluence of $10^{14}$ cm$^{-2}$. The interfaces between implanted and unimplanted regions are visible in the confocal image. b) PL spectra of unimplanted and implanted samples, with the latter showing characteristic $V_{\rm B}^-$ emissions centered around $\sim 820$ nm. To aid comparison of spectral features, the PL spectra of samples 1e12 and 1e13 have been scaled in amplitude by a factor of 10 and 2, respectively. c) Relative fluorescence intensity (calculated from PL spectra) of different samples with dotted line representing linear increase in counts with ion fluence. d) Pulsed ODMR spectra (left) along with fitted data for sample 1e14 (right) using the theoretical model (for details, see SI).
• Figure 2: Spin properties of $V_{\rm B}^-$ centres as a function of ion fluence. a) PODMR contrast (solid circles) and transverse strain splitting parameter ($E$, open circles). b) Spin-lattice relaxation time, T$_{1}$. (c) The decoherence time T$_{2}$ measured by Hahn echo, including various limiting factors: a decoherence rate from the nuclear spin bath (independent of ion fluence), and contributions from instantaneous diffusion (ID) calculated using the model described in the text. d) Estimated AC and DC magnetic sensitivity for the different samples.
• Figure 3: Raman spectra for different ion-implanted hBN samples and one unimplanted sample. Inset shows the variation in the linewidth of the hBN peak (1365 cm$^{-1}$).
• Figure 4: PL and Raman studies around the boundary between implanted and pristine areas for samples 1e14 and 1e15, respectively. a) Normalized PL signal from V$_{B}^{-}$ centres along a line. b) Raman peak intensity ratio I$_{V_{B}}$/I$_{hBN}$ (normalized w.r.t. maxima). c) The hBN peak width(FWHM) along the line.