A. Jorio

Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy

Raman spectroscopy is here shown to provide a powerful tool to differentiate between two different sp(2) carbon nanostructures (carbon nanotubes and graphene) which have many properties in common and others that differ. Emphasis is given to the richness of both carbon nanostructures as prototype examples of nanostructured materials. A glimpse toward future developments in this field is presented.

Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an interdefect distance ∼3 nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity.

General equation for the determination of the crystallite size La of nanographite by Raman spectroscopy

This work presents a systematic study of the ratio between the integrated intensities of the disorder-induced D and G Raman bands (ID∕IG) in nanographite samples with different crystallite sizes (La) and using different excitation laser energies. The crystallite size La of the nanographite samples was obtained both by x-ray diffraction using synchrotron radiation and directly from scanning tunneling microscopy images. A general equation for the determination of La using any laser energy in the visible range is obtained. Moreover, it is shown that ID∕IG is inversely proportional to the fourth power of the laser energy used in the experiment.

RAMAN SPECTROSCOPY ON ISOLATED SINGLE WALL CARBON NANOTUBES

A review is presented on the resonance Raman spectra from one isolated single wall carbon nanotube. The reasons why it is possible to observe the spectrum from only one nanotube are given and the important structural information that is provided by single nanotube spectroscopy is discussed. Emphasis is given to the new physics revealed by the various phonon features found in the single nanotube spectra and their connection to spectra observed for single wall nanotube bundles. The implications of this work on single wall carbon nanotube research generally are also indicated.

Characterizing carbon nanotube samples with resonance Raman scattering

The basic concepts and characteristics of Raman spectra from carbon nanotubes (both isolated and bundled) are presented. The general characteristics of the radial breathing mode, tangential mode (G band), disorder-induced mode (D-band) and other Raman features are presented, with the focus directed toward their use for carbon nanotube characterization. Polarization analysis, surface enhanced Raman spectroscopy and complementary optical techniques are also discussed in terms of their advantages and limitations.

Defect characterization in graphene and carbon nanotubes using Raman spectroscopy

This review discusses advances that have been made in the study of defect-induced double-resonance processes in nanographite, graphene and carbon nanotubes, mostly coming from combining Raman spectroscopic experiments with microscopy studies and from the development of new theoretical models. The disorder-induced peak frequencies and intensities are discussed, with particular emphasis given to how the disorder-induced features evolve with increasing amounts of disorder. We address here two systems, ion-bombarded graphene and nanographite, where disorder is represented by point defects and boundaries, respectively. Raman spectroscopy is used to study the ‘atomic structure’ of the defect, making it possible, for example, to distinguish between zigzag and armchair edges, based on selection rules of phonon scattering. Finally, a different concept is discussed, involving the effect that defects have on the lineshape of Raman-allowed peaks, owing to local electron and phonon energy renormalization. Such effects can be observed by near-field optical measurements on the G′ feature for doped single-walled carbon nanotubes.

Electron and phonon renormalization near charged defects in carbon nanotubes

Owing to their influence on electrons and phonons, defects can significantly alter electrical conductance, and optical, mechanical and thermal properties of a material. Thus, understanding and control of defects, including dopants in low-dimensional systems, hold great promise for engineered materials and nanoscale devices. Here, we characterize experimentally the effects of a single defect on electrons and phonons in single-wall carbon nanotubes. The effects demonstrated here are unusual in that they are not caused by defect-induced symmetry breaking. Electrons and phonons are strongly coupled in sp(2) carbon systems, and a defect causes renormalization of electron and phonon energies. We find that near a negatively charged defect, the electron velocity is increased, which in turn influences lattice vibrations locally. Combining measurements on nanotube ensembles and on single nanotubes, we capture the relation between atomic response and the readily accessible macroscopic behaviour.

Double resonance Raman spectroscopy of single-wall carbon nanotubes

A review of double resonance Raman spectroscopy is presented. Non-zone centre phonon modes in solids can be observed in the double resonance Raman spectra, in which weak Raman signals appear in a wide frequency region and their combination or overtone modes can be assigned. By changing the excitation laser energy, we can derive the phonon dispersion relations of a single nanotube.

Nanowires and nanotubes

Nanowires and nanotubes are now at the forefront of materials science at the nanoscale. This article starts with introductory comments about nanowires and nanotubes and then addresses in more detail the special structure and properties of bismuth nanowires and carbon nanotubes, which are considered as prototype examples of nanowires and nanotubes. Both nano-materials are important for the new nanoscience concepts that they introduce and for their promise for practical applications. Both provide a system that is simple enough so that detailed calculations of their properties can be carried out, and predictions about their physical behavior can be made. The occurrence and control of unusual and unique properties of specific nanostructures are the drivers for the exploitation of nanoscience in nanotechnology applications.

Raman spectroscopy for probing chemically/physically induced phenomena in carbon nanotubes

The use of recent advances in resonance Raman spectroscopy studies on isolated carbon nanotubes and the scientific knowledge achieved so far from these studies is discussed in the context of advancing carbon nanotube-based technology. Changes in the Raman spectra can be used to probe and monitor structural modifications of the nanotube sidewalls that come from the introduction of defects and the attachment of different chemical species. The former effect can be probed through the analysis of the disorder-induced Raman modes and the latter through the upshifts/downshifts observed in the various Raman modes due to charge transfer effects.