Biaxial strain effect in the room temperature formation of 2D diamond from graphene stacks

Discovering innovative low-energy pathways to obtain room-temperature functional materials and novel methods to control their phase transitions presents far-reaching challenges. Conventional hydrostatic pressure and modern strain engineering are key practical tools to achieve tuning of material phases at both bulk and nanoscale levels. Here, we reveal the pivotal role of biaxial strain in the formation of 2D diamond-like sp${}^3$ carbon from few-layer graphene (FLG) stacks at room temperature and pressure of 7 GPa. By employing in situ resonance Raman and optical absorption spectroscopies, utilizing van der Waals heterostructures (hBN/FLG) on different substrates (SiO₂/Si and diamond) placed in a water environment under high-pressure, we unveil the key physical influence of in-plane biaxial strain induced by substrate compression along with the chemical interaction at the graphene-ice interface. Ab initio molecular dynamics simulations corroborate the role of both water and biaxial strain in locally stabilizing interlayer sp${}^3$ carbon structures. This breakthrough not only advances nanodiamond technology but also establishes biaxial strain engineering as a promising tool to explore novel phases of 2D layered materials, enabling advanced functionalities at room temperature and near ambient conditions.