Lattice vibrations of single and multi-layer isotopologic graphene

Isotope-substituted materials, isotopologues, have been crucial in understanding various scientific problems. In this work, we report full Raman spectroscopic investigation of C-13 graphene, its multilayers and isotopologic bilayers. All samples were mechanically exfoliated or artificially-stacked using C-13-enriched bulk crystals that were grown by the high-pressure and high-temperature (HPHT) method. Frequencies of graphitic G, 2D and combination peaks showed a linear relation between two isotopologues of C-12 and C-13, which led to precise optical determination of isotopic purity in agreement with mass spectrometric results. Single to eight-layered graphene exhibited correspondence in spectral features between isotopologues: thickness-dependent progressions of intensities and line shapes, and excitation energy-dependent dispersion of 2D frequency. Line shape analyses of stacking-sensitive 2D revealed spatial distribution of stacking domains and dominance of ABC-polytype unlike natural or kish graphite. Using isotopologic bilayers of C-12/C-13, the change in Fermi velocity induced by vdW interlayer interactions was separately quantified from strain and charge density. This work also presents direct evidence that the puzzling thermally activated doping is induced by hole dopants located at the graphene-substrate interface. (C) 2018 Elsevier Ltd. All rights reserved.