Spin defects in hexagonal boron nitride produced by helium ion bombardment

July 24, 2023

(Nanowerk News) Physicists from the National University of Singapore (NUS) have developed a method using a focused beam of helium ions to create a defect array in hexagonal boron nitride (hBN) that can potentially be used for magnetic sensing applications.

Hexagonal boron nitride (hBN) is a two-dimensional (2D) material consisting of boron and nitrogen atoms arranged in a hexagonal lattice structure. This exhibits unique properties for applications in quantum sensing. Many types of defects have been found in hBN and one of them, the negatively charged boron vacancies (VB), is of particular interest because it has spin properties that make it valuable for quantum sensing applications.

In this study, a high-energy helium ion beam generated at the accelerator facility at the Center for Ion Beam Applications (CIBA) in the Department of Physics, NUS was used to irradiate hBN flakes to produce VB optical center (shown schematically in Figure 1a). The ability to focus the ion beam to nano-sized dots and to scan the beam spatially allows patterned optical emitting arrays to be fabricated with high precision. Figure 1b shows the photoluminescence map of the irradiated regions in the hBN flake. Figure 1: (a) Schematic of the vacancy of negatively charged boron (VB) optical defect centers in hexagonal boron nitride (hBN), (b) V array photoluminescence mapB resulting optical defects in hBN flakes. (c) Experimental setup to perform Optically Detected Magnetic Resonance (ODMR) measurements. (d) The ODMR spectrum of VB defects showing decreased resonance as measured with zero and 10 mT magnetic fields applied. (Image: National University of Singapore)

The work is the result of a collaboration between a research team led by Associate Professor Andrew BETTIOL and a team led by Associate Professor Goki EDA#, both from the Department of Physics, NUS. VB the optical defect center generated through experiments carried out by the research team, exhibits some interesting properties when exposed to microwave energy.

This study is published in the journal Advanced Optical Materials (“High Sensitivity Spin Defect in hBN Created by High Energy He Beam Irradiation”).

A spectroscopic technique known as Optically Detected Magnetic Resonance (ODMR) is used to sense tiny magnetic fields in the experiment. This technique combines the principles of magnetic resonance and optical spectroscopy to study the properties of paramagnetic materials and their interactions with electromagnetic radiation. The experimental step scheme used is shown in Figure 1c. First, a green laser is used to excite VB the center of the defect thus emits light at a wavelength of about 810 nm, which is in the near-infrared part of the electromagnetic spectrum. A copper antenna is then used to generate a specific microwave frequency near the hBN sample. This microwave energy initializes the defect into a spin state which results in a reduction in the intensity of the light emitted by the defect. The microwave frequency is adjusted until a decrease in light intensity is detected. This occurs at around 3.48 GHz, where a double decrease in the photoluminescence intensity is observed (shown in Figure 1d). After the microwave resonance frequency is found, the sensor is ready to be used to detect magnetic fields.

Prof Bettiol said, “Using this unique property exhibited by hBN, a small magnetic field that occasionally occurs in biological systems or in magnetic materials will shift the resonant frequency and this will cause the light emission from the sensor to be restored. The light from VB the optical defect center provides a means of optically detecting the local magnetic field.”

Prof Eda added, “hBN is a versatile material that can be easily integrated into on-chip devices. Our demonstration to fabricate spin defects in hBN with high precision is an important step toward realizing on-chip magnetic sensors.”

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