  Discovering innovative methods to understand phase transitions, modify phase
diagrams, and uncover novel synthesis routes poses significant and far-reaching
challenges. In this study, we demonstrate the formation of nanodiamond-like sp3
carbon from few-layer graphene (FLG) stacks at room temperature and relatively
low transition pressure (~7.0 GPa) due to chemical interaction with water and
physical biaxial strain induced by substrate compression. By employing
resonance Raman and optical absorption spectroscopies at high-pressure on FLG
systems, utilizing van der Waals heterostructures (hBN/FLG) on different
substrates (SiO2/Si and diamond), we originally unveiled the key role of
biaxial strain. Ab initio molecular dynamics simulations corroborates the
pivotal role of both water and biaxial strain in locally stabilizing sp3 carbon
structures at the graphene-ice interface. This breakthrough directly enhances
nanodiamond technology but also establishes biaxial strain engineering as a
promising tool to explore novel phases of 2D nanomaterials.
