Resonance Raman spectroscopy characterization of single-wall carbon nanotube separation by their metallicity and diameter

Evaluation of metallic and semiconducting single-walled carbon nanotube characteristics

The nature of the mixed electronic type metallic (M-) and semiconducting (S-) single-walled carbon nanotubes (SWNTs) synthesized by current methods has posed a key challenge for the development of high performance SWNT-based electronic devices. The precise measurements of M- to S-SWNT ratio in as-grown or separated samples are of paramount importance for the controlled synthesis, separation and the realization of various applications. The objective of this review is to provide comprehensive overview of the progress achieved so far for measuring the M/S ratio both on individual and collective levels of SWNT states. We begin with a brief introduction of SWNT structures/properties and discussion of the problems and difficulties associated with precise measurement of the M/S ratio, and then introduce the principles for obtaining distinguished signals from M-and S-SWNTs. These techniques are classified into different groups based either on the single/ensemble detection of SWNT samples or on the principles of techniques themselves. We then present the M/S ratio evaluation results of these methods, with emphasis on scanning probe microscopy (SPM)-based detection techniques. Finally, the prospects of precise and large-scale measurement of M/S ratio in achieving controlled synthesis and understanding growth mechanism of SWNTs are discussed.

A comparative study of single-walled carbon nanotube purification techniques using Raman spectroscopy

Raman spectroscopy has been utilized to show the increase of single-walled carbon nanotubes (SWCNTs) content in commercial grade samples synthesized by the chemical vapour deposition (CVD) technique with a minimization of impurities using both hydrochloric acid treatment and surfactant purification. Surfactant purification methods proved to be the most effective, resulting in a three-fold increase in the percentage of SWCNTs present in the purified product as determined by Raman spectroscopy.

Elucidation of the electronic structure of semiconducting single-walled carbon nanotubes by electroabsorption spectroscopy

We report benchmark calculations of electroabsorption in semiconducting single-walled carbon nanotubes that provide motivation to perform electroabsorption measurement on these systems. We show that electroabsorption can detect the continuum bands in different energy manifolds. Direct determination of the binding energies of excitons in higher manifolds thereby becomes possible. We also find that electroabsorption can provide evidence for Fano-type coupling between the second exciton and the lowest continuum band states.

Effects of Surfactant and Boron Doping on the BWF Feature in the Raman Spectrum of Single-Wall Carbon Nanotube Aqueous Dispersions

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.