Heteroepitaxial (111) Diamond Quantum Sensors with Preferentially Aligned Nitrogen-Vacancy Centers for an Electric Vehicle Battery Monitor

A platform for heteroepitaxial (111) chemical vapor deposition (CVD) diamond quantum sensors with preferentially aligned nitrogen vacancy (NV) centers on a large substrate is developed, and its operation as an electric vehicle (EV) battery monitor is demonstrated. A self-standing heteroepitaxial CVD diamond film with a (111) orientation and a thickness of 150 µm is grown on a non-diamond substrate and subsequently separated from it. The high uniformity and crystallinity of the (111)-oriented diamond is confirmed. A 150-µm thick NV-diamond layer is then deposited on the heteroepitaxial diamond. The T₂ value measured by confocal microscopy is 20 µs, which corresponds to substitutional nitrogen defect concentration of 8 ppm. The nitrogen-vacancy concentration and T₂^* are estimated to be 0.05 ppm and 0.05 µs by continuous wave optically detected magnetic resonance (CW-ODMR) spectroscopy in a fiber-top sensor configuration. In a gradiometer, where two sensors are placed on both sides of the busbar, the noise floor is 17 nT/Hz^0.5 in the frequency range of 10–40 Hz without magnetic shielding. The Allan deviation of the magnetic field noise in the laboratory is below 0.3 µT, which corresponds to a busbar current of 10 mA, in the accumulation time range of 10 ms to 100 s.