  The production of high-quality hexagonal boron nitride (h-BN) is essential
for the ultimate performance of two-dimensional (2D) materials-based devices,
since it is the key 2D encapsulation material. Here, a decisive guideline is
reported for fabricating high-quality h-BN on transition metals: It is crucial
to exclude carbon from h-BN related process. Otherwise carbon prevails over
boron and nitrogen due to its larger binding energy, thereupon forming graphene
on metals after high-temperature annealing. We demonstrate the surface
reaction-assisted conversion from h-BN to graphene with high-temperature
treatments. The pyrolysis temperature Tp is an important quality indicator for
h-BN/metals. When the temperature is lower than Tp, the quality of h-BN layer
is improved upon annealing. While the annealing temperature is above Tp, in
case of carbon-free conditions, the h-BN disintegrates and nitrogen desorbs
from the surface more easily than boron, eventually leading to clean metal
surfaces. However, once the h-BN layer is exposed to carbon, graphene forms on
Pt(111) in the high-temperature regime. This not only provides an indispensable
principle (avoid carbon) for fabricating high-quality h-BN materials on
transition metals, but also offers a straightforward method for the surface
reaction-assisted conversion from h-BN to graphene on Pt(111).
