  Hexagonal boron nitride (h-BN) has attracted significant attention due to its
superior properties as well as its potential as an ideal dielectric layer for
graphene-based devices. The h-BN films obtained via chemical vapor deposition
in earlier reports are always polycrystalline with small grains due to high
nucleation density on substrates. Here we report the successful synthesis of
large single-crystal h-BN grains on rational designed Cu-Ni alloy foils. It is
found that the nucleation density can be greatly reduced to 60 per mm2 by
optimizing Ni ratio in substrates. The strategy enables the growth of
single-crystal h-BN grains up to 7,500 um2, about 2 orders larger than that in
previous reports. This work not only provides valuable information for
understanding h-BN nucleation and growth mechanisms, but also gives an
effective alternative to exfoliated h-BN as a high-quality dielectric layer for
large-scale nano-electronic applications.
