Crystal Growth of hBN from Ni and Ni-Cr Solutions at High N₂ pressure

Single crystals of hexagonal BN have been grown from solutions on the surface of Ni-Cr and pure Ni metals under a pressure of N₂ gas of 1000-1500 bar. The transparent and colorless hBN crystals obtained in this study exhibited high structural quality and uniformity as confirmed by micro-Raman mapping, showing less than 8 cm^-1 widths of the high-frequency E_2g Raman peaks corresponding to the intralayer vibrations of the B and N atoms in hBN. Optical absorption measurements indicated the indirect character of electronic transitions in bulk hBN. Due to the solubility of nitrogen increased by a factor of 30-40 by compression, it was possible to grow significantly thicker (up to 30 μm) hBN crystals from pure Ni solvent than by a similar approach at atmospheric pressure. The addition of chromium to the solution seriously disrupted the hBN crystallization process, as at elevated N₂ pressure efficient synthesis of CrN occurred at the same time. The results of the crystallization experiments are discussed in the context of the thermodynamic properties of III-N compounds and the effect of the metal solvent on the conditions of thermal stability of BN concerning its components. To further assess the quality of the crystals and evaluate their potential for applications, thin flakes exfoliated from the grown hBN crystals were applied in graphene-based devices. Remarkably high carrier mobility, exceeding 21.2 m²·V^-1·s^-1 at 230 K for both electrons and holes, was observed in magnetotransport studies on hBN-encapsulated graphene transistors.