Raman spectroscopy study of sp2 to sp3 transition in bilayer graphene under high pressures

Understanding the transition from sp2 to sp3 bonding between graphene layers remains a challenge for the development of thin diamond films. Herein, in situ Raman spectroscopy is used to trace the structural evolution of bilayer graphene (BLG) films under high pressures applied by a diamond anvil cell. When water is used as the pressure transmission medium (PTM), the G peak of BLG disappears at ∼37 GPa, accompanied by vanishing of the 2D peak after pressure release, together considered as evidence of sp2–sp3 transition in BLG under high pressures. In contrast, BLG in silicone oil PTM or single layer graphene in water PTM does not experience such changes in Raman spectra with pressure, suggesting that water is critical for the formation of sp3 bonding under high pressures, e.g., by providing functional groups on the surface of BLG.