Abstract
The standard D-G-2D pattern of Raman spectra of sp2 amorphous carbons is considered from the viewpoint of graphene domains presenting their basic structure units (BSUs) in terms of molecular spectroscopy. The molecular approximation allows connecting the characteristic D-G doublet spectra image of one-phonon spectra with a considerable dispersion of the C=C bond lengths within graphene domains, governed by size, heteroatom necklace of BSUs as well as BSUs packing. The interpretation of 2D two-phonon spectra reveals a particular role of electrical anharmonicity in the spectra formation and attributes this effect to a high degree of the electron density delocalization in graphene domains. A size-stimulated transition from molecular to quasi-particle phonon consideration of Raman spectra was experimentally traced, which allowed evaluation of a free path of optical phonons in graphene crystal.
Highlights
Raman scattering has become an overwhelming method of testing graphene-based solid carbons
The standard D-G-2D pattern of Raman spectra of sp2 amorphous carbons is considered from the viewpoint of graphene domains presenting their basic structure units (BSUs) in terms of molecular spectroscopy
We propose to look at the spectra in Figure 8 from the viewpoint of analytical triad: the size and heteroatom necklace of the relevant BSUs, as well as the BSUs package—as it was done with the one-phonon spectra in the previous section
Summary
Raman scattering has become an overwhelming method of testing graphene-based solid carbons. A deep similarity of the spectra for a large set of samples of defect and disordered crystals of graphite and graphene [4,5,6,7,8,9,10] has created a favorable basis for successful use and further improvement of the solid-state approach in its phonon-confinement format [11,12] This format has legitimated the use of RSs parameters, such as position, bandwidth and intensity of the D, G, and 2D bands Recent detailed investigations have clearly revealed a molecular nature of sp amorphous carbons (ACs) (amorphics for simplicity) [17,18], again raising the question about the choice between solid-state and molecular approach Meeting this request, in the current article we intend to inspect which new information about sp ACs can be obtained from their spectra analysis based on the molecular spectroscopy background.
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