  Understanding the emergent electronic structure in twisted atomically thin
layers has led to the exciting field of twistronics. However, practical
applications of such systems are challenging since the specific angular
correlations between the layers must be precisely controlled and the layers
have to be single crystalline with uniform atomic ordering. Here, we suggest an
alternative, simple and scalable approach where nanocrystalline two-dimensional
(2D) film on three-dimensional (3D) substrates yield
twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron
nitride (h-BN) thin films are directly grown on high in-plane lattice
mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the
twist-dependent structure-property correlations. Concurrently, nanocrystalline
h-BN thin film shows strong non-linear second-harmonic generation and ultra-low
cross-plane thermal conductivity at room temperature, which are attributed to
the twisted domain edges between van der Waals stacked nanocrystals with random
in-plane orientations. First-principles calculations based on time-dependent
density functional theory manifest strong even-order optical nonlinearity in
twisted h-BN layers. Our work unveils that directly deposited 2D
nanocrystalline thin film on 3D substrates could provide easily accessible
twist-interfaces, therefore enabling a simple and scalable approach to utilize
the 2D-twistronics integrated in 3D material devices for next-generation
nanotechnology.
