  Hexagonal boron nitride sheets have been noted especially for their enhanced
properties as substrates for sp2 carbon-based nanodevices. To evaluate whether
such enhanced properties would remain under various realistic conditions, we
investigate the structural and electronic properties of semiconducting carbon
nanotubes on perfect and defective hexagonal boron nitride sheets under an
external electric field as well as with a metal impurity, using density
functional theory. We verify that the use of a perfect hexagonal boron nitride
sheet as a substrate indeed improves the device performances of carbon
nanotubes, compared with the use of conventional substrates such as SiO2. We
further show that the hexagonal boron nitride even with some defects can
perform better performance as a substrate. Our calculations, on the other hand,
also suggest that some defective boron nitride layers with a monovacancy and a
nickel impurity could bring about poor device behaviors since the imperfections
impair electrical conductivity due to residual scattering under an applied
electric field.
