The Impact of Boron and Nitrogen Isotopes on the Properties of Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) is a two-dimensional dielectric material prized for its potential electronic, optoelectronic, and nanophotonic device applications. Here we describe how its properties are affected by changing its boron and nitrogen isotopes. hBN consists of 20% ¹⁰B and 80% ¹¹B, and 99.6% ¹⁴N and 0.4% ¹⁵N when it is synthesized from sources with the natural distribution of isotopes (hBN^nat). Isotopic disorder is greatly reduced when hBN is synthesized using an isotopically enriched boron source, either ¹⁰B or ¹¹B. Consequently, the phonon polariton propagation length and lifetimes are increased by factors of eight and four times in monoisotopic hBN crystals. Due to reduced phonon scattering, the thermal conductivity is also increased, by as much as 50% to 650 Wm^-1K^-1 for h¹⁰BN at room temperature. Reducing the density of nuclear spin density motivates the synthesis of hBN with the ¹⁵N isotope: ¹⁴N and ¹⁵N have nuclear spins of 1 and ½ respectively. Thus, the DC sensitivity of the boron vacancy V_B^- to magnetic fields is increased by a factor of four with h¹⁰B¹⁵N over hBN^nat.<br/><br/>These hBN crystals with controlled isotope concentrations were grown at atmospheric pressure from molten metal solutions of nickel, chromium, and enriched boron in contacted with nitrogen gas at 1550 °C. By slow cooling (1-4 °C/h), hBN is synthesized and its crystals precipitate on the surface of the metal. This technique produces high quality hBN crystals, as indicated by their narrow E_2g Raman peak widths (&lt;3 cm^-1 for h¹⁰BN) and strong photoluminescence intensities at energies great than 5.75 eV. This is a versatile, scalable technique for making high quality, large area, hBN single crystals with low residual impurity concentrations.