Electron spin properties of boron vacancies in hexagonal boron nitride single crystals created by neutron irradiation
Principal Investigator
- Name:
- James Edgar
- Email:
- [email protected]
- Phone:
- (208) 526-6918
Team Members:
| Name: | Institution: | Expertise: | Status: |
|---|---|---|---|
| Vincent Jacques | University of Montpellier, France | Optical and magnetic resonance spectroscopies | Faculty |
| Evan Glaser | Naval Research Laboratory | Optical and magnetic resonance spectroscopies | Other |
| Andrew Yates | Naval Research Laboratory | Optical and magnetic resonance spectroscopies | Other |
Experiment Details:
- Experiment Title:
- Electron spin properties of boron vacancies in hexagonal boron nitride single crystals created by neutron irradiation)
- Hypothesis:
- The optimum neutron fluence to create boron vacancies in hexagonal boron nitride single crystals to maximize the electron spin properties for quantum sensing is between 3.6E15 n-cm^-2 and 7.20E16 n-cm^-2.
- Work Description:
- The negatively charged boron vacancy in hexagonal boron nitride (hBN) is a promising defect for the quantum sensing of temperature, pressure, magnetic fields, and electric fields. Its potential applications is in the same space as quantum defects in diamond, but because hBN is a two-dimensional material, its defects remain active at its surface, so they can be placed in closer proximity to its target, thus increasing its signal and spatial resolution. Neutron irradiation is an effective method for creating boron vacancies in hBN and unlike alternative techniques such as ion implantation in which the defects are confined to the near sample surface, neutron irradiation can produce a uniform distribution of boron vacancies. In our studies, the electron spin state of boron vacancies is determined by optically detected magnetic resonance (ODMR). With OMDR, we spatially mapped the magnetic fields of a magnetic two dimensional material, CrTe2. More recently, we proved that hBN crystal flakes enriched in the boron-10 and nitrogen-15 isotopes produces a clearer signal than that of hBN with the natural distribution of boron and nitrogen isotopes. The lower nuclear spin of nitrogen-15 compared to natural nitrogen-14 results in a more clearly defined hyperfine structure, which can lead to a greater device sensitivity. OMDR has proven to be much more sensitive to boron vacancies in h10B15N that we anticipated. While this is a good outcome, it means that under our previous RTE, the hBN fluences we requested were too high. Under our previous RTE, h10B15N samples received neutron fluences between 1.4E16 n-cm^-2 to 8.6E17 n-cm^-2. Our expectation was that the higher the density of boron vacancies, the greater the photoluminescence from the defect. While the photoluminescence intensity did indeed increase initially with the fluence, it drastically decreased at the highest fluence. Furthermore, the spin relaxation time (T1) and spin coherence time (T2) decreased significantly over the entire fluence range. Therefore it appears that the best neutron fluence for creating quantum sensors is either slightly above or slightly below the lowest fluence we previously tested. Under the previous RTE, the hBN samples were neutron-irradiated for 1, 10, and 60 hours at the nuclear reactor at Ohio State University operated at a power of 300 kW. For the new RTE, we propose to irradiate samples for the equivalence of 0.25 h, 0.5 h, 0.75 h, 1.5 h, 2.0 h, 2.5 h, 3.0 h, 4.0 h, and 5.0 h at 300 kW of power in a thermal neutron flux environment on the order of 10E12 neutrons·cm-2·s-1. The two lowest fluences will be performed by irradiating at lower reactor powers, that is for 0.75 h at 100 kW and 1.0 h at 150 kW. The resulting thermal neutron fluence will range between 3.6E15 n/cm2 to 7.2E16 n/cm2. Each irradiation will be done on multiple h10B15N flakes ranging in diameter from 0.5 mm to 3 mm, with a total irradiated area of 36 mm2. The flake thicknesses range from 10 to 20 microns. As a means to further optimized the spin properties of the boron vacancies, thermal annealing will be performed on some samples. The thermal annealing studies will begin with the conditions identified by Suzuki et al, as producing the most favorable quantum sensing properties, annealing between 500 °C and 600 °C for 5 minutes and 30 minutes. Once irradiated, the hBN flakes will be thoroughly characterized. These samples will be shared with Vincent Jacques, an expert with optically detected magnetic resonance (ODMR), from the University Montpellier, France. Similar samples will be shared with Evan Glaser, an expert on electron paramagnetic resonance (EPR) and Andrew Yeats (also ODMR) both at the Naval Research Laboratory, Washington, DC. In our preliminary studies, a strong ODMR signal was detected from the h10B15N crystals that was irradiated for just 1.0 hr at 300 kW. EPR measurements will be taken at 6K with a 9.5 GHz spectrometer, with a microwave power of 2 mW and a 1-2 Gauss modulation amplitude and a 100 kHz field modulation. ODMR measurements will be taken using a confocal microscope with a laser excitation at 532 nm at cryogenic temperatures and variable magnetic fields strengths and microwave frequencies. The effects of annealing on the structure of the defects will be studied by high resolution transmission electron microscopy via a user facility award at Brookhaven National Laboratory. The proposed study will permit us to accurately establish the best fluence for fabricating quantum detectors based on hBN.
Project Summary
The negatively charged boron vacancy (VB-) in hexagonal boron nitride (hBN) has tremendous potential for quantum sensing applications, since its spin state is optically addressable. It is under intense development worldwide, for quantum sensing of static magnetic fields, temperature, pressure, and nuclear spins. It may also be used in quantum computing, sensing, networks, and communications. Because hBN is a two-dimensional van der Waal material, it has potential advantages over 3D materials such as the NV center in diamond, including better light collection, easier incorporation into heterostructures with other 2D materials, and integrating into nano-photonics.
In this project, VB- defects will be created in hBN single crystals by neutron irradiation, and their optical and spin properties characterized. Neutron irradiation creates boron vacancies by the transmutation of the boron-10 isotope into lithium and alpha particles. By using single crystals, the properties of the point defects may be studied without interference from grain boundaries and dislocations, which are typically found in hBN powders and composites. Furthermore, we will use hBN crystals enriched in boron-10 (>99%), to achieve a higher concentration of boron vacancies, and nitrogen-15 (>99%), to minimize the background noise. This will increase the sensitivity of hBN by increasing the quantum coherence time in comparison to hBN with the natural distribution of boron isotopes.
hBN crystals will be irradiated for four different neutron fluences, ranging from 5.0E15 n-cm^-2 to 5.0E16 n-cm^-2 corresponding to times of 0.5 h, 1.0 h, 2.0 h, and 5.0 h at a thermal neutron flux of 4E12 neutrons·cm-2·s-1. Our previous measurements on neutron irradiated h10B15 established that this fluence range is probably the most optimal. The optical/spin properties of the neutron irradiated hBN will be determined by electron spin resonance (EPR) and optically detected magnetic resonance (ODMR). The optical and magnetic properties and quantum coherence time of the defects will be determined as a function of the neutron dose. The fluence that produces a strong, easily detected signal with a long quantum coherence time is best. Confocal imaging of the ODMR response will also provide spatial resolution of these properties, shedding light on the effects of local defects and microstructures on the spin properties of these materials. The irradiation can be completed in less than two months. The ESR and ODMR measurements can each be completed in six weeks. These experiments will establish control and the properties of the VB- defect, and its suitability for quantum information science applications. Both magnetic resonance techniques can also reveal and track other defects that may arise under the varying neutron fluence conditions. Such defects could potentially play important roles in the hBN sensing behavior. Furthermore, this project will provide material for additional fundamental property measurements, such as the effect of neutron irradiation on hBN’s thermal conductivity and mechanical properties. It will also provide the material needed for quantum sensors based on and incorporating crystalline hBN.
In this project, VB- defects will be created in hBN single crystals by neutron irradiation, and their optical and spin properties characterized. Neutron irradiation creates boron vacancies by the transmutation of the boron-10 isotope into lithium and alpha particles. By using single crystals, the properties of the point defects may be studied without interference from grain boundaries and dislocations, which are typically found in hBN powders and composites. Furthermore, we will use hBN crystals enriched in boron-10 (>99%), to achieve a higher concentration of boron vacancies, and nitrogen-15 (>99%), to minimize the background noise. This will increase the sensitivity of hBN by increasing the quantum coherence time in comparison to hBN with the natural distribution of boron isotopes.
hBN crystals will be irradiated for four different neutron fluences, ranging from 5.0E15 n-cm^-2 to 5.0E16 n-cm^-2 corresponding to times of 0.5 h, 1.0 h, 2.0 h, and 5.0 h at a thermal neutron flux of 4E12 neutrons·cm-2·s-1. Our previous measurements on neutron irradiated h10B15 established that this fluence range is probably the most optimal. The optical/spin properties of the neutron irradiated hBN will be determined by electron spin resonance (EPR) and optically detected magnetic resonance (ODMR). The optical and magnetic properties and quantum coherence time of the defects will be determined as a function of the neutron dose. The fluence that produces a strong, easily detected signal with a long quantum coherence time is best. Confocal imaging of the ODMR response will also provide spatial resolution of these properties, shedding light on the effects of local defects and microstructures on the spin properties of these materials. The irradiation can be completed in less than two months. The ESR and ODMR measurements can each be completed in six weeks. These experiments will establish control and the properties of the VB- defect, and its suitability for quantum information science applications. Both magnetic resonance techniques can also reveal and track other defects that may arise under the varying neutron fluence conditions. Such defects could potentially play important roles in the hBN sensing behavior. Furthermore, this project will provide material for additional fundamental property measurements, such as the effect of neutron irradiation on hBN’s thermal conductivity and mechanical properties. It will also provide the material needed for quantum sensors based on and incorporating crystalline hBN.
Relevance
The proposed research will advance the DOE’s Office of Nuclear Energy mission to provide energy that is efficiently produced, reliable, and safe. Establishing the fundamental basic science on the quantum properties of defects in hexagonal boron nitride has the potential to lead to technologies that support this mission. One example is the development of quantum-based, sensitive neutron detectors. These can provide in-pile instrumentation and neutron flux measurement in advanced reactors and monitoring the stockpiles of nuclear materials. A second technology would take advantage of the ability of defects (the boron vacancy) in hBN to be quantum sensors, made possible by the ability to detect their spin state optically. They also present good opportunities for quantum science and engineering, including quantum computing, which is useful for efficient nuclear physics calculations. Defect engineering in hBN may lead to the development of solid-state, quantum devices that can operate at room temperature. In contrast, most quantum information science is currently based on expensive technologies that operate at cryogenic temperatures. This project would be leveraged by on-going research to grow large, high quality, hexagonal boron nitride single crystals, supported by the Office of Naval Research. The unique boron-10 and nitrogen-15 enriched hBN crystals produced under the ONR project will greatly enhance and improve this DOE NSUF proposal by making signals clearer and with finer structure by reducing competing background noise from other types of defects such as grain boundaries.
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About Us
The Nuclear Science User Facilities (NSUF) is the U.S. Department of Energy Office of Nuclear Energy's only designated nuclear energy user facility. Through peer-reviewed proposal processes, the NSUF provides researchers access to neutron, ion, and gamma irradiations, post-irradiation examination and beamline capabilities at Idaho National Laboratory and a diverse mix of university, national laboratory and industry partner institutions.
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