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

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 sp(3) 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, sp(3)-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 approximate 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 (approximate to 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. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Researchers successfully compressed few-layer graphene samples in water to form a hard, transparent, sp(3)-containing 2D phase. Raman spectroscopy data showed a similar critical pressure for the new phase and graphene/graphite, as well as a lack of evidence of significant pressure gradients or non-hydrostatic stress components.