Optimization of carbon irradiation parameters for creating spin defects in hexagonal boron nitride

An increasing number of luminescent defects, commonly known as color centers, have been observed in wide-bandgap materials. Among these, optically active spin defects are attracting growing attention owing to their advantages in quantum sensing, offering high spatial resolution and sensitivity at room temperature.^1–10 In particular, negatively charged nitrogen vacancy color centers in diamond^3–6 and negatively charged silicon vacancy and the neutral divacancy color centers in silicon carbide^7–9 have opened up a wide range of applications in condensed matter physics, energy engineering, materials science,and biology. For instance, spin defects in diamond and silicon carbide can be used to determine the structure and dynamics of biomolecules^3,6 and the magnetism of various materials at the nanoscale.^5,9 However, it is challenging to fabricate near-surface spin defects in diamond and the associated quantum sensing devices at low cost.^10,11

Recently, multiple color centers have been found in hexagonal boron nitride (hBN). Experimental and theoretical investigations indicate that vacancies, antisite defects, carbon- or oxygen-doped defects, or complexes of these point defects might serve as single-photon emitters or spin defects at room temperature.^12–18 Currently, only negatively charged boron vacancy (⁠$VB−$⁠) color centers in hBN have been accurately characterized, which has been done by combining photoluminescence (PL) and optically detected magnetic resonance (ODMR) spectra with density functional theory (DFT) calculations.^19–24 The ensemble of $VB−$ color centers possess spin S = 1 and display broad emission in the range of 700–1000 nm, which can be applied for the measurement of magnetic and electric fields, temperature, pressure, strain, radiofrequency signals, and paramagnetic spins in liquids at room temperature.¹³ Owing to the weak van der Waals interactions between adjacent hBN layers, hBN can be readily exfoliated into a desired thickness, and $VB−$ spin defects in atomically thin flakes are stable.²⁵ This allows ultrathin hBN sensors based on color centers to be positioned close to the surface of a probed sample at the atomic scale, particularly facilitating integration into two-dimensional materials or devices. Nevertheless, the low photoluminescent quantum yield,²³ the wide ODMR linewidth induced by strong hyperfine interaction between $VB−$ electron spins and nuclear spins,²⁴ and the short spin coherence time^32,33 of $VB−$ centers limit the sensitivity of such sensors.

To improve the sensing sensitivity of $VB−$ ensembles, plasmonic cavities have recently been employed to enhance photoluminescence (PL) intensity.¹³ Additionally, $VB−$ defects in isotopically purified h¹⁰B¹⁵N nanosheets exhibit well-resolved hyperfine structure and narrower ODMR linewidth.²⁴ Advanced dynamical decoupling techniques have also been utilized to extend the coherence time of $VB−$ centers,^32,33 which is essential for the detection of radiofrequency signals and coherence control of spin qubits.¹³ Besides, optimizing fabrication methods is a simpler and more direct strategy for enhancing the sensing sensitivity of $VB−$ ensembles. Initially, thermal neutron irradiation was employed to create $VB−$ centers;¹⁹ however, ion irradiation has now become the most commonly used method, owing to its high efficiency and low cost. For example, energetic ions such as hydrogen,^26–29 helium,^30–39 carbon,^34,40–43 and nitrogen^34,44–47 are frequently used to create $VB−$ centers for spin property research and quantum sensing applications. Nevertheless, the optimal irradiation dose required for different ions with varying energies, as well as the irradiation parameters such as the incident angle, need to be further investigated.

In this study, we examine the effects of ion irradiation parameters on the magnetic sensitivity and spin coherence time of $VB−$ spin defects in hBN. For 30 keV carbon ion irradiation, we obtain an optimal irradiation dose of 4 × 10¹³ ions/cm². Notably, we discover that the incident angle of energetic ions plays a crucial role in influencing the depth of $VB−$ ensembles—this factor has typically been ignored in previous investigations. Continuous ODMR spectra reveal that the room-temperature magnetic sensitivity reaches 36.0 $μT/Hz$ under a 6.28 mT bias magnetic field. Furthermore, the results of pulsed ODMR measurements indicate that the spin–lattice relaxation time (∼14 μs) and spin coherence time (∼100 ns) are relatively long. These results are essential for optimizing the irradiation process to enhance the spin properties of $VB−$ in hBN and for the development of nanoscale quantum sensors.

Bulk hBN with a thickness of several tens of micrometers was purchased from HQ Graphene. It was then carefully cleaved into several flakes, each exceeding 1 µm in thickness. These hBN flakes were subsequently adhered to silicon substrates by conductive tape for vertical implantation or affixed to a triangular silver frame for oblique implantation, as shown in Fig. 1(a). Carbon ion irradiation experiments were conducted using the 320 kV Platform for Multidisciplinary Research with Highly Charged Ions at the Institute of Modern Physics, CAS. The kinetic energy of carbon ions was fixed at 30 keV, and the corresponding irradiation dose rate was 1.43 × 10¹² ions/(cm² s). The depth distributions of carbon atoms and irradiation-induced vacancies, as simulated by the SRIM software package,⁴⁸ are presented in Figs. 1(b) and 1(c). To create a moderate concentration of boron vacancy defects in hBN, the irradiation dose was set at 1 × 10¹³, 4 × 10¹⁴, and 1 × 10¹⁴ ions/cm². Additionally, a carbon-implanted hBN flake with a dose of 1 × 10¹⁴ ions/cm² at an angle of 30° was annealed for 1 h in an argon atmosphere at a temperature of 1297 K.

After 30 keV carbon irradiation, the Raman and PL spectra of carbon-implanted hBN samples were measured using a confocal Raman spectrometer (Horiba iHR550) with a high-numerical-aperture (100×, NA = 0.9) microscope objective, under 532 nm laser excitation. The ODMR spectra of the carbon-implanted hBN flakes were obtained by sweeping the microwave frequency, and the total PL counts from 650 to 1000 nm were detected by an avalanche photodiode (APD). Our ODMR experiments were conducted using custom-built equipment from Guoyi Quantum (Hefei) Technology Co. Ltd. (Hefei, China), as shown in Fig. 2(a). A confocal microscope objective (60×, NA = 0.7) was employed to focus a 520 nm excitation laser onto the sample. The PL signal from the sample was separated from excitation light by a dichroic mirror, and the residual laser light was shielded by a 650 nm long-pass filter before arriving at the APD. To manipulate the spin states of the $VB−$ ensemble, the radiofrequency (RF) signal from the microwave source was amplified using a microwave amplifier, and then delivered to the hBN sample through a 20 μm copper wire. The hBN sample and copper wire were fixed on a printed circuit board, which was mounted on a piezoelectric nano-positioning stage. For the pulsed ODMR experiments, a multichannel arbitrary sequence generator (ASG) was used to control the excitation laser and RF signal, as well as the triggering of the APD. A permanent magnet mounted on a translation stage was used to provide an external magnetic field that could be tuned by adjusting the distance between the magnet and the hBN sample.

Figure 2(b) illustrates the atomic structure and energy level structure of $VB−$ defects, which possess a spin triplet (S = 1) with a quantization axis that is perpendicular to the hBN crystal. The energy levels of $VB−$ defects are all located in the wide bandgap of hBN, and the defect levels with ground and excited states allow laser absorption and PL emission. Consequently, the origin of the PL spectra can be explained by the energy level structure of $VB−$ defects in hBN. Moreover, the defect levels with ground and excited states are all spin triplets with m_s = 0 and ±1. There is zero-field splitting (ZFS) D between m_s = 0 and m_s = ±1 due to spin–spin interaction, which can be excited by microwaves, and there exists a tiny splitting E between m_s = −1 and m_s = +1, even in the absence of an external field, due to the local electric field surrounding charged defects.^25,32,49 The PL intensity decreases under green laser excitation when the electrons around $VB−$ defects are excited by the resonant microwaves from m_s = 0 to m_s = ±1. This is because the electrons in the excited m_s = ±1 states can relax to ground states by nonradiative transition through metastable states. As a result, we can record the ratio of the spin-dependent PL intensity change of $VB−$ defects under laser excitation as the microwave frequency is varied, thus obtaining ODMR spectra.

As shown in Fig. 3(a), the PL spectra of carbon-implanted hBN flakes indicate that only $VB−$ color centers can be produced directly. The characteristic PL spectra of the $VB−$ ensemble span from 700 to 1000 nm, with a peak at 818 nm. The PL intensity reaches its maximum when the implantation dose is in the range of 4 × 10¹³–1 × 10¹⁴ ions/cm². Moreover, an oblique impact at 30° incident angle reduces the PL intensity when the implantation dose is fixed at 1 × 10¹⁴ ions/cm²; a schematic of the incident angle can be seen in Fig. 1(a). Monte Carlo simulations [Figs. 1(b) and 1(c)] indicate that implantation at 30° decreases the depth distribution of carbon atoms and vacancies by ∼14 nm, and thus oblique implantation may lead to excessive point defects and even defect clusters, which further decrease the $VB−$ density. A recent study³² has also found that only a small portion of the irradiation-induced boron vacancy defects are negatively charged state $VB−$ spin defects. Figure 3(b) presents Raman spectra of carbon-implanted hBN flakes, which exhibit two obvious peaks at 449 and 1295 cm⁻¹ attributed to irradiation-induced defects, as well as the characteristic phonon mode E_2g at 1366 cm⁻¹.^37,38

After 30 keV carbon ion implantation with a dose of 4 × 10¹³ ions/cm², the total PL intensity of the $VB−$ ensemble can reach 5.5 × 10⁶ photons/s under 520 nm laser excitation with a power of 5.6 mW, as shown in Fig. 4(a). The integral PL intensity I of $VB−$ spin defects increases with increasing laser power P, and the experimental data are well fitted by $I=Isat/1+Psat/P$⁠, where P_sat is the saturation laser power and I_sat is the saturation PL count rate.³⁰ 

Figure 5(a) shows that the integral PL count of an hBN flake irradiated with a dose of 1 × 10¹⁴ ions/cm² reaches 6.8 × 10⁶ photons/s under 520 nm laser excitation with a power of 5.6 mW, slightly higher than the count for an hBN flake irradiated with a dose of 4 × 10¹³ ions/cm². This is consistent with the PL spectra presented in Fig. 3(a). However, for the hBN flake irradiated with a dose of 1 × 10¹⁴ ions/cm², the ODMR contrast is only 4.7% in the absence of an external magnetic field, and 2.5% under a 9.6 mT bias magnetic field, which are much lower than those for the hBN flake irradiated with a dose of 4× 10¹³ ions/cm². As a result, the optimal dose for creating $VB−$ spin defects in hBN is 4 × 10¹³ ions/cm² using 30 keV carbon ion irradiation. Additionally, the incident angle is crucial, since it influences the depth distribution and density of vacancy defects, as illustrated in Figs. 1(c) and 3(a).

We also conducted pulsed ODMR measurements to evaluate the spin coherence properties of $VB−$ spin defects in carbon-irradiated hBN with an optimal dose of 4 × 10¹³ ions/cm². The Rabi oscillation shown in Fig. 6(a) demonstrates that the spin states of a $VB−$ ensemble can be coherently manipulated and read using an excitation laser and microwave pulses at room temperature. Moreover, this approach can also be used to evaluate the spin dephasing time by fitting the Rabi oscillation with $A+B⁡exp−τ/T2*cos2πfRτ+φ$⁠, where A and B are fitting constants, f_R is the Rabi frequency, τ is the length of the microwave pulse, and the fitting spin dephasing time $T2*$ is ∼30 ns.

We employed two kinds of pulses to estimate the spin–lattice relaxation time T₁, as shown in Figs. 6(b) and 6(c). At room temperature, T₁ is ∼14 μs, which is comparable to the value for hBN samples irradiated with thermal neutrons^52,53 or other ions.^32–34, Figure 6(d) shows that the spin coherence time of $VB−$ spin defects induced by 30 keV carbon implantation with the optimal dose is ∼102 ns, which is similar to previously measured values.^32,33,43,52 These pulsed ODMR measurements are significant for enabling more complex sensing protocols, such as the detection of paramagnetic spins in liquids and RF signals.¹³ 

A comparison between different irradiation parameters and the spin properties of the $VB−$ ensemble in hBN flakes is presented in Table I. The fabrication methods and parameters significantly influence spin properties, including ODMR contrast, sensitivity, and coherence properties. It is noteworthy that microcavities can substantially enhance ODMR contrast and sensitivity,¹³ although this is not included in Table I. Additionally, for a given irradiated hBN sample, the ODMR contrast and sensitivity also depend on the microwave waveguide configuration, as well as the powers of the microwave and laser. For instance, for a $VB−$ ensemble in hBN flakes with a thickness of 100 nm created by 10 keV carbon ion implantation at a given dose, the ODMR contrast can be improved from 1.5% to 20% by substituting a 10 μm-wide gold waveguide by a gold coplanar waveguide with a width of 50 μm.^40,43 Thermal neutrons and MeV electrons can penetrate through hBN flakes with a thickness of a few micrometers, producing an almost uniform density of $VB−$ defects. The depth distributions of $VB−$ defects in ion-irradiated hBN flakes are dependent on ion species, energy, and incident angle, as estimated by SRIM⁴⁸ simulations.

The coherence time of $VB−$ defects is limited by the nuclear spin bath of the hBN crystal, and also is influenced by the irradiation parameters. A spin coherence time of 102 ns is consistent with previously reported values.^32,33,43,52 Coherence times of the order of microseconds have also been observed;^30,53 however, the contrast of the spin-echo signal in these studies was quite weak, and experimental data at shorter time scales were not presented.⁵² The external magnetic field and advanced dynamical decoupling sequences can be used to extend the coherence time of $VB−$ defects,^32,33,43 although this is not included in Table I. According to Table I, the optimal irradiation parameters are crucial for enhancing the spin properties. Our optimal carbon ion parameters demonstrate a relatively good ODMR contrast and coherence time compared with other irradiation parameters when a similar microwave waveguide is used.

To create carbon-related color centers in a carbon-irradiated hBN flake with a dose of 1 × 10¹⁴ ions/cm² at a 30° incident angle, a thermal annealing experiment was performed at 1297 K in an argon atmosphere for 1 h. Figure 7 shows that the additional Raman peaks at 449 and 1295 cm⁻¹ observed in Fig. 3(b), along with the characteristic PL spectra of the $VB−$ ensemble, have disappeared,^37,38 which indicates that the most of the irradiation-induced $VB−$ defects were repaired after the thermal annealing. Besides, the annealed hBN flake exhibits a PL spectrum with a peak at ∼580 nm, which is very similar to recent experimental results.⁵⁶ According to previous theoretical calculations, the PL signal with peak at ∼580 nm may originate from a carbon trimer or tetramer.^57–59 In this work, we have focused on $VB−$ ensembles in hBN created by 30 keV carbon ion implantation.

We have investigated the optimal ion irradiation parameters on the basis of PL intensity, ODMR contrast, magnetic sensitivity, and spin coherence time of $VB−$ spin defects in hBN. For 30 keV carbon ion irradiation, we have identified an optimal dose of 4 × 10¹³ ions/cm². Importantly, we have found that the angle of energetic ions has a significant impact on the depth and density distributions of the $VB−$ ensemble—a factor often overlooked in earlier studies. Our continuous ODMR spectra have demonstrated a room-temperature magnetic sensitivity of 36.0 $μT/Hz$ under a 6.28 mT bias magnetic field, and the pulsed ODMR results have shown that the spin–lattice relaxation time and spin coherence time are relatively long. Additionally, we have found that it is possible to generate carbon-related color centers in bulk hBN flakes by combining 30 keV carbon implantation with annealing at 1297 K for 1 h. These results are crucial for refining the irradiation process to enhance the spin properties of $VB−$ in hBN and for advancing the development of atomic-scale quantum sensors utilizing this van der Waals semiconductor.

This work was supported by the National Key Research and Development Program Project (2024YFF0726104), Key Laboratory of Modern Optical Technologies of the Education Ministry of China, Soochow University (Grant No. KJS2135), a China Postdoctoral Science Foundation Funded Project (Grant No. 2024M751236), and the Jiangxi Provincial Natural Science Foundation (Grant No. 20232BAB211030).

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Fei Ren received his Master’s degree in Nuclear Technology and Applications from Lanzhou University in 2018. He is currently pursuing his doctorate at the State Key Laboratory of Precision Measuring Technology and Instruments, within the School of Precision Instrument and Opto-Electronics Engineering at Tianjin University, China. His research interests include quantum sensing based on spin defects in hexagonal boron nitride, and atomistic simulations of point defects in semiconductors.

Zongwei Xu is a Professor at Tianjin University and a Doctoral Supervisor. His research interests include ultrafast energy beam (ion and laser) processing, Raman and photoluminescence spectroscopy characterization, wide-bandgap semiconductor devices, microcutting tools, and nanocutting technology. He is the Chairman of the first Sino-German Symposium on Defect Engineering in SiC Device Manufacturing–Atomistic Simulations, Characterization and Processing and the Principal Investigator of the 2021–2023 Mobility Programme of the Sino-German Center for Research Promotion (M-0396).

Yiyuan Wu is a Lecturer and Master’s Supervisor at the East China University of Technology. He obtained his doctoral degree in Particle Physics and Nuclear Physics from Lanzhou University in 2020. His research interests include the development of novel semiconductor neutron detectors, and radiation effects on nuclear materials and semiconductors.