Properties of Neutron Irradiated Hexagonal Boron Nitride

James H. Edgar
Kansas State University – Lawrence, Kansas, USA
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Neutron irradiation has potential advantages for creating boron vacancies in hexagonal boron nitride for quantum sensing applications. The boron-10 isotope has one of the strongest interactions with thermal neutrons of any isotope of any element, roughly 3750 barns, compared to 1-2 barns for the isotopes of most elements. Consequently, boron compounds enriched in the boron-10 isotope are often used in nuclear energy production and medical treatments. Upon interaction with neutrons, the boron-10 isotope undergoes transmutation producing high energy lithium-7 ions and alpha particles, as well as producing negatively charged boron vacancies (V_B– ). We focused on hBN enriched with both the boron-10 and nitrogen-15 isotopes, ie h¹⁰B¹⁵N. Enriching in boron-10 results in a stronger interaction than that of natural boron, which is 20% ¹⁰B and 80% ¹¹B, while the lower nuclear spin of ¹⁵N (I=1/2) compared to ¹⁴N (I=1) improve the spin properties of the VB – centers. To determine the optimum thermal neutron fluences to create boron vacancies, a set of h¹⁰B¹⁵N crystals, grown at atmospheric pressure from a molten mixture of nickel and chromium (at 1550 °C) were irradiated over the range 3.6×10¹⁵ neutrons/cm² to 8.6×10¹⁷ neutrons/cm² . New peaks appeared in the Raman spectra at 1320 cm^-1 and in the photoluminescence spectra at 820 nm. Their intensity increased with the thermal neutron fluence. The c-lattice constant of the h¹0B¹⁵N crystals increases with fluence, due to the creation of defects and interstitial clusters caused by the knock-on collisions from the energetic particles created during nuclear transmutation. Stacking faults were apparent in transmission electron microscopic images for samples receiving the highest fluence. Annealing at 500 °C and 600 °C for 5 or 30 minutes in either air or nitrogen reduced the Raman peak at 1320 cm-1 while a new peak formed in the PL spectra at 620 nm to 650 nm. However, this annealing did not significantly change the longitudinal spin relaxation time or the spin coherence time.

Email: edgarjh@ksu.edu