Towards a better understanding of the structure of diamanoïds and diamanoïd/graphene hybrids

Abstract

Hot-filament process was recently employed to convert, totally or partially, few-layer graphene (FLG) with Bernal stacking into crystalline sp3–C layers at low pressure. The result reported earlier relies on Raman spectroscopy and Fourier transform infrared microscopy. As soon as the number of graphene layers in the starting FLG is higher than 2-3, the sp2–C to sp3–C conversion is partial only, due to the prevalent Bernal stacking sequence. We report new evidences confirming the sp2–C to sp3–C conversion from electron diffraction at low energy, Raman spectroscopy and Density Functional Theory (DFT) calculations. Partial sp2–C to sp3–C conversion generates couples of twisted, superimposed coherent domains (TCD), supposedly because of stress relaxation, which are evidenced by electron diffraction and Raman spectroscopy. TCDs come with the occurrence of a twisted bilayer graphene feature located at the interface between the upper diamanoïd domain and the non-converted graphenic domain underneath, as evidenced by a specific Raman signature consistent with the literature. DFT calculations show that Raman T peak originates from a combination of the sp3–C stretching mode of a sp3–C layer with the optical out-of-plane mode of a graphene layer; both layers being sandwiched between a highly hydrogenated sp3–C surface and the underneath unconverted graphene layer(s).