Synergetic interplay between pressure and surface chemistry for the conversion of sp2-bonded carbon layers into sp3-bonded carbon films

The effects of the interplay between pressure and surface chemistry on the transformation of few-layer graphene into an sp3-bonded carbon film were investigated with first-principles density functional theory calculations including ab initio molecular dynamics. N2H4, H2O, and He were each considered as a candidate pressure medium. Compared with the bulk graphite, the surface chemistry overwhelmingly governed the conversion energetics for nanometer-thick graphene layers. A hydrogen-donating medium reduced the required conversion pressure compared with an inert one; the conversion pressure obtained by using N2H4 was 40% of the corresponding pressure obtained with He. We suggest that pressurizing the cell through hydrogen-donating pressure media has the advantage from the surface chemistry by concentrating hydrogen atoms on carbon surfaces.