Raman spectroscopy of graphene and graphite: Disorder, electron phonon coupling, doping and nonadiabatic effects

Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.

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Raman spectroscopy as a versatile tool for studying the properties of graphene

Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to approximately 0.01 mg ml(-1), produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of approximately 1 wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry.

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High-yield production of graphene by liquid-phase exfoliation of graphite

Raman spectroscopy of carbon nanotubes

Raman spectroscopy has historically played an important role in the structural characterization of graphitic materials, in particular providing valuable information about defects, stacking of the graphene layers and the finite sizes of the crystallites parallel and perpendicular to the hexagonal axis Here we review the defect-induced Raman spectra of graphitic materials from both experimental and theoretical standpoints and we present recent Raman results on nanographites and graphenes The disorder-induced D and D′ Raman features, as well as the G′-band (the overtone of the D-band which is always observed in defect-free samples), are discussed in terms of the double-resonance (DR) Raman process, involving phonons within the interior of the 1st Brillouin zone of graphite and defects In this review, experimental results for the D, D′ and G′ bands obtained with different laser lines, and in samples with different crystallite sizes and different types of defects are presented and discussed We also present recent advances that made possible the development of Raman scattering as a tool for very accurate structural analysis of nano-graphite, with the establishment of an empirical formula for the in- and out-of-plane crystalline size and even fancier Raman-based information, such as for the atomic structure at graphite edges, and the identification of single versus multi-graphene layers Once established, this knowledge provides a powerful machinery to understand newer forms of sp2 carbon materials, such as the recently developed pitch-based graphitic foams Results for the calculated Raman intensity of the disorder-induced D-band in graphitic materials as a function of both the excitation laser energy (Elaser) and the in-plane size (La) of nano-graphites are presented and compared with experimental results The status of this research area is assessed, and opportunities for future work are identified

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Studying Disorder in Graphite-Based Systems by Raman Spectroscopy

As the technology for diamond film preparation by plasma-assisted CVD and related procedures has advanced, Raman spectroscopy has emerged as one of the principal characterization tools for diamond materials. Cubic diamond has a single Raman-active first order phonon mode at the center of the Brillouin zone. The presence of sharp Raman lines allows cubic diamond to be recognized against a background of graphitic carbon and also to characterize the graphitic carbon. Small shifts in the band wavenumber have been related to the stress state of deposited films. The effect is most noticeable in diamond films deposited on hard substrates such as alumina or carbides. The Raman line width varies with mode of preparation of the diamond and has been related to degree of structural order. The Raman spectrum of hexagonal diamond (lonsdaleite) is distinct from that of the cubic diamond and allows it to be recognized.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400004441

Characterization of diamond films by Raman spectroscopy

High quality diamond thin films were deposited on different substrates at temperatures from 300 to 1000 °C by the microwave plasma enhanced chemical vapor deposition (MPCVD) system. The quality of deposited diamond films was improved by adding oxygen in the gas mixtures. Different ratios of methane to oxygen concentration in hydrogen at different temperatures have been studied. At high temperatures (800–1000 °C), the addition of oxygen will not only enhance the growth rate of deposited films but also extend the region of diamond formation. At low temperatures ( 900 °C) were either graphitic or diamond containing a large amount of graphitic or amorphous carbon and at low temperatures (<500 °C) were white, soot-like coatings which were easily scraped off. The quality of the deposited films was characterized by Raman spectroscopy and scanning electron microscopy.

The effect of oxygen in diamond deposition by microwave plasma enhanced chemical vapor deposition

Thin diamond films were deposited on different substrates at temperatures below 500 °C by a microwave plasma‐enhanced chemical vapor deposition system. The deposited films were amorphous carbon or diamond films depending on the different gas mixtures used. The addition of oxygen to the gas mixtures was found to be critical for diamond growth at low temperatures. Without oxygen, the deposited films were white soots and easily scratched off. Increasing the oxygen input improved the quality of the Raman peaks and increased the film transparency. The diamond films were also characterized by scanning electron microscopy.

Effect of oxygen in diamond deposition at low substrate temperatures

By using surface‐enhanced Raman spectroscopy it was possible to clearly identify very thin diamond and amorphous carbon coatings which were not detectable by normal Raman spectroscopy. A very small amount of silver was sputtered onto the surface of thin diamond depositions. Raman spectra measured through the silver layer exhibited the 1332 cm−1 diamond line and broadbands due to other forms of carbon. Raman scattering measured through silver coatings directly on the silicon substrate revealed extremely thin layers of amorphous carbon.

Diane S. Knight

Papers

Characterization of diamond films by Raman spectroscopy

The effect of oxygen in diamond deposition by microwave plasma enhanced chemical vapor deposition

Effect of oxygen in diamond deposition at low substrate temperatures

Surface-enhanced Raman spectroscopy of chemical vapor deposited diamond films

Ferroelectricity in the ReBa2Cu3O7−δ superconductors