  Nearly two-dimensional diamond, or diamane, is coveted as ultrathin
$sp^3$-carbon film with unique mechanics and electro-optics. The very thinness
($~h$) makes it possible for the surface chemistry, e.g. adsorbed atoms, to
shift the bulk phase thermodynamics in favor of diamond, from multilayer
graphene. Thermodynamic theory coupled with atomistic first principles
computations predicts not only the reduction of required pressure
($p/p_{\infty}>1-h_0/h$), but also the nucleation barriers, definitive for the
kinetic feasibility of diamane formation. Moreover, the optimal adsorbent
chair-pattern on a bilayer graphene results in a cubic diamond lattice, while
for thicker precursors the adsorbent boat-structure tends to produce hexagonal
diamond (lonsdaleite), if graphene was in AA` stacking to start with. As
adsorbents, H and F are conducive to diamond formation, while Cl appears
sterically hindered.
