Synthesis of sub-millimeter single-crystal grains of aligned hexagonal boron nitride on an epitaxial Ni film

Hexagonal boron nitride (h-BN), an insulating two-dimensional (2D) layered material, has attracted increasing interest due to its electrical screening effect, high-temperature-resistant gas barrier properties, and other unique applications. However, the presence of grain boundaries (GBs) in h-BN is a hindrance to obtain these properties. Here, we demonstrate the epitaxial growth of monolayer h-BN by chemical vapor deposition (CVD) on Ni(111) thin films deposited on c-plane sapphire. The Ni(111) films showed higher thermal stability than Cu(111) and Cu-Ni(111) alloy films, allowing us to perform CVD growth at a high temperature of 1100 degrees C. This resulted in an increase of the h-BN grain sizes to up to 0.5 millimeter, among the highest reported so far, and in a well-defined triangular grain shape. Low-energy electron microscopy (LEEM) revealed the epitaxial relationship between h-BN and the underlying Ni(111) lattice, leading to a preferential alignment of the h-BN grains. Both the large grain size and the alignment are expected to facilitate the synthesis of h-BN with a low density of GBs. We also found that the addition of N-2 gas during the CVD improves the crystalline shape of the h-BN grains, changing from an irregular, truncated to a sharp triangle. The growth behavior of monolayer h-BN is further discussed in terms of the dependences on growth temperature and pressure, as well as on the structural evolution of the Ni metal catalyst. Our findings not only help understand the h-BN growth mechanism but also offer a new route to grow high-quality, monolayer h-BN films.