Advances in Carbon Nanotube Characterization

Carbon nanotubes are expected to comprise an important class of materials for novel applications in electronics, optics, and high-strength materials. For the development of these applications, researchers invariably must understand the surface chemistry of nanotubes in solution for subsequent processing. Solution-phase chemical processing enables manipulation, separation, purification and provides direct routes to more complex materials. Understanding the physical and chemical interactions of such 1-D electronic materials under these conditions has proven difficult, however. The problem is not straightforward: carbon nanotubes invariably exist after synthesis as aligned aggregates or bundles that are tightly bound by an estimated 500 eV/micron of tube length. The problem of poor dispersion at the single nanotube level has greatly inhibited their study in solution. Recent breakthroughs allowing single tube dispersion have changed this situation, and from these techniques a wealth of knowledge regarding the optical and chemical properties has been obtained. In particular, advances in spectroscopic characterization including band-gap fluorescence and the assignments of metallic and semi-conducting spectral features for single walled carbon nanotubes allow for detailed, chirality based characterization of samples with great efficiency. Raman spectroscopy has been used extensively to characterize solid and solution phase carbon nanotube systems because it has the ability to probe distinct populations that have inter-band transitions in resonance with the excitation laser. In this way, Raman excitation profiles can be used to probe the unique geometric dependence of these transitions.

Early work using Raman spectroscopy to probe inter-band transitions has established the technique as a valuable tool for characterizing carbon. Kukovecz and co-workers examined HiPco produced carbon nanotubes using a series of excitation wavelengths. The authors quantitatively modeled peak shapes of tangential and disorder modes with success and described the radial breathing mode diameter dependence using a semiempirical relation. Canonico et al. measured Raman excitation profiles on laser-oven prepared nanotubes and used the line-shape to characterize armchair species. These earlier studies involved aggregated nanotube ropes, however, and in this state nanotubes have been shown to experience strong perturbations to their electronic structure that complicate the interpretation of such profiles. Dresselhaus and co-workers have pioneered experimental techniques enabling single nanotube spectroscopy to circumvent this limitation while Yu and Brus profiled bundle ends using micro-Raman spectroscopy to assign features using the results of the tight binding description of graphene. This work uses a solution phase dispersion of carbon nanotube recently shown to yield individually isolated species that fluoresce in the near-infrared. This fluorescence is a key indicator of dispersion and isolation. Hence, this method provides an opportunity to extend the results of past researchers to examine the resonant phonon spectra of a large number of isolated nanotubes in response to changes to their chemical environment.

Carbon nanotubes have a unique electronic structure that follows from the quantization of the electronic wave vector through a conceptual rolling of a graphene plane into a cylinder forming the carbon nanotube. The vector in units of the hexagonal elements connecting a continuous path tracing the nanotube circumference defines the nanotube chirality in terms of two integers: n and m. When |n-m| = 3q where q is an integer, the nanotube is metallic or semi-metallic while remaining species are semi-conducting with a diameter and chirality dependent band-gap. In the Raman spectrum, phonon modes at low Raman shift identified as radial breathing modes (RBMs) correspond to a uniaxial expansion and contraction and have shifts strongly dependent on nanotube diameter. In this way, nanotubes of a distinct chiral vector can be identified in the Raman spectrum readily and tracked as the excitation wavelength is varied through the absorption maxima of the nanotubes.

In this work, we review the covalent and non-covalent interactions of individual nanotubes in solution. We limit the discussion to samples where the nanotubes have been dispersed and purified as to yield mostly individually dispersed species. Such samples demonstrate well-resolved absorption spectra and band-gap fluorescence. The subject is divided into covalent interactions and non-covalent. The latter is then divided into what we consider charge transfer chemistries, where the interaction involves a shifting of the Fermi level of the nanotubes, and solvatochromic interactions, where the presence of the interaction is detected as a characteristic shift in either the fluorescence or the absorption transition.