Hard, transparent, sp3-containing 2D phase formed from few-layer graphene under compression


Luiz G. Pimenta Martins, Diego L. Silva, Jesse S. Smith, Ang-Yu Lu, Cong Su, Marek Hempel, Connor Occhialini, Xiang Ji, Ricardo Pablo, Rafael S. Alencar, Alan C.R. Souza, Alysson A. Pinto, Alan B. de Oliveira, Ronaldo J. C. Batista, Tomás Palacios, Mário S. C. Mazzoni, Matheus J. S. Matos, Riccardo Comin, Jing Kong, and Luiz G. Cançado. 2020. “Hard, transparent, sp3-containing 2D phase formed from few-layer graphene under compression.” Carbon.


Despite several theoretically proposed two-dimensional (2D) diamond structures, experimental efforts to obtain such structures are in initial stage. Recent high-pressure experiments provided significant advancements in the field, however, expected properties of a 2D-like diamond such as sp3 content, transparency and hardness, have not been observed together in a compressed graphene system. Here, we compress few-layer graphene samples on SiO2/Si substrate in water and provide experimental evidence for the formation of a quenchable hard, transparent, sp3-containing 2D phase. Our Raman spectroscopy data indicates phase transition and a surprisingly similar critical pressure for two-, five-layer graphene and graphite in the 4-6 GPa range, as evidenced by changes in several Raman features, combined with a lack of evidence of significant pressure gradients or local non-hydrostatic stress components of the pressure medium up to ≈ 8 GPa. The new phase is transparent and hard, as evidenced from indentation marks on the SiO2 substrate, a material considerably harder than graphene systems. Furthermore, we report the lowest critical pressure (≈ 4 GPa) in graphite, which we attribute to the role of water in facilitating the phase transition. Theoretical calculations and experimental data indicate a novel, surface-to-bulk phase transition mechanism that gives hint of diamondene formation.