  Due to the absence of matching carbon atoms at honeycomb centers with carbon
atoms in adjacent graphene sheets, theorists predicted that a sliding process
is needed to form AA, AB, or ABC stacking when directly converting graphite
into sp3 bonded diamond. Here, using twisted bilayer graphene, which naturally
provides AA and AB stacking configurations, we report the ultrafast formation
of a transient 2D diamond-like structure (which is not observed in aligned
graphene) under femtosecond laser irradiation. This photo-induced phase
transition is evidenced by the appearance of new bond lengths of 1.94A and
3.14A in the time-dependent differential pair distribution function using MeV
ultrafast electron diffraction. Molecular dynamics and first principles
calculation indicate that sp3 bonds nucleate at AA and AB stacked areas in
moire pattern. This work sheds light on the direct graphite-to-diamond
transformation mechanism, which has not been fully understood for more than 60
years.
