  The conversion of graphene into diamond is a new way for preparing ultrathin
diamond film without pressure. Herein, we investigated the transformation
mechanism of surface-hydrogenated bilayer graphene (SHBG) into
surface-hydrogenated single-layer diamond (SHSLD) crystal, inserting fifteen
kinds of single metal atoms without any pressure, by using the systematical
first-principles calculations. Compared with the configuration without metal
atom, SHBG can be transformed into SHSLD spontaneously in thermodynamics under
the action of single metal atom, and its formation energy can even decrease
from 0.82 eV to -5.79 eV under the action of Hf atom. According to our results,
the outer electron orbits and atomic radius of metal atom are two important
factors that affect the conversion. For the phase transition to occur, the
metal atom needs to have enough empty d orbitals, and the radius of the metal
atom is in the range of 0.136-0.159 nm. Through further analysis, we find that
the p orbitals of carbon atoms and d orbital of metal atom in SHBG will be
strongly hybridized, thereby promoting the conversion. The results supply
important significance to experimentally prepare diamond without pressure
through hydrogenated graphene.
