High-pressure Raman study of graphene: a spectroscopic evidence for the diamondol

Abstract

In this dissertation, the e ect of high pressures on double transferred CVD graphene sitting on a Teflon substrate were investigated via Raman spectroscopy in a diamond anvil cell (DAC) using water as the pressure transmitting media (PTM). The Raman spectra were acquired with both 488 nm and 532 nm excitation light sources. Initially, a detailed study of the electronic and vibrational properties of graphene, as well as the interaction between them, was performed. Then, an investigation on the origin of the G band in graphene was carried out, as well as the perturbations to this band caused by strain and doping. A numerical value for the slope of G band frequency with pressure (@!G=@P) of 5 cm1=GPa was obtained via a combination of theoretical analysis and experimental data. This value agrees with experimental observations for free-standing graphene samples subjected to high pressures. The results provide strong evidence of a phase transition from a sp2 to a sp2 sp3 mixed structure at 5 6 GPa, reversible upon pressure release. The main evidences were the blueshift of the G band frequency (!G) and the decrease of G bands full width at half maximum (G) with increasing excitation energy, and the change of r´egime of the !G P curve. Based on the experimental data, we propose the formation of diamondol, a 2D hydroxylated diamond consisting of two layers of sp3 carbon. We hypothesized that the top layer was hydroxylated at 5 6 GPa, followed by a detachment of the diamondol from the Teflon substrate at 9:5 GPa, which in turn led to the hydroxylation of the bottom layer. This hypothesis is consistent with the sp3 content of the system inferred from the variation of !G and G data, and with the fact that graphene was found to be detached from the Teflon substrate in some regions after the DAC was opened and the water evaporated. To test the diamondol hypothesis, mono and double layer CVD graphene sitting on a Teflon substrate were compressed using water and Nujol (mineral oil) as PTMs, respectively. No dispersion of the G band, change in the !G P curve or any net decrease in G with excitation energy were observed for both systems, corroborating with the diamondol hypothesis. The rise of a new band at 620 cm1 under 1 GPa and above was also observed, and possible origins for this band were proposed. If the diamondol hypothesis is confirmed, this work will open up directions for synthesizing new 2D materials with remarkable properties via high pressure experimental routes.