Chemical vapor deposition growth of uniform multilayer hexagonal boron nitride driven by structural transformation of a metal thin film

Hexagonal boron nitride (h-BN), in particular, multilayer h-BN, has played an important role in the research of two-dimensional materials by enabling the observation of their intrinsic and excellent physical properties via effective screening of the influences from the surrounding environment. However, it is still difficult to synthesize high-quality multilayer h-BN in large scale, and its growth mechanism is not clearly understood. Here, we investigated the chemical vapor deposition (CVD) growth of multilayer h-BN using thin Ni−Fe films that are deposited on sapphire substrates with different crystal planes. The Ni−Fe film on r-plane sapphire was found to produce a uniform multilayer h-BN sheet whose surface coverage is much higher than those on c- and a-plane sapphire. Electron backscatter diffraction and X-ray diffraction investigations revealed that the uniform segregation of multilayer h-BN on Ni−Fe/r-sapphire occurs simultaneously with a drastic structural change of the Ni−Fe thin film from polycrystalline to face-centered cubic (fcc (111)) structure. On the other hand, the Ni−Fe films on c- and a-plane sapphire possessed the fcc (111) structure even before the reaction with borazine feedstock and did not show such structural change. The unique crystallographic change of the Ni−Fe thin film associated with uniform h-BN segregation was further supported by the studies using spinel substrates. Our work indicates the essential role of dynamic evolution of the crystal structure of the thin-film metal catalyst in h-BN growth, highlighting the importance of the controlling crystallographic properties of the metal catalyst for the synthesis of high-quality and uniform multilayer h-BN films.