Unique identification of single-walled carbon nanotubes in composites

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Abstract

The physical properties of single-walled carbon nanotubes (SWNTs) depend strongly upon their structure and so a reliable method to determine this structure is essential to their future applications. Resonant Raman spectroscopy has proven to be a powerful tool to characterize SWNTs, and also to monitor their deformation. The radial breathing modes (RBMs) are commonly used to identify SWNTs, since their position is inversely proportional to the nanotube diameter. In this present study, the RBM intensity variations in epoxy- and poly(vinyl alcohol) (PVA)-SWNTs composites were measured under uniaxial strain using lasers of three wavelengths 632, 785 and 830 nm. These structure-dependent intensity variations are attributed to the effect of strain upon the energy of the SWNT electronic transitions, moving SWNTs closer to or further away from the resonance conditions that enhance the Raman signal. Combining the RBM position, resonance theory and this additional information about the behaviour under strain, a unique nanotube structure can be assigned to each RBM observed in the composites.

Introduction

A single-wall carbon nanotube (SWNT) can be considered as a rolled graphene sheet and its structure is defined by two indexes (n,m) that indicate how the graphene sheet is rolled [1]. The electronic properties of carbon nanotubes are heavily dependent on their structure [1] and yet, despite the recent advances, synthesis products generally contain a large range of nanotube diameters and chiralities [2]. A reliable method to determine the (n,m) indexes is essential for future applications and academic studies. Several methods exist to determine the SWNT structure, such as scanning tunneling microscopy (STM) [3], electron diffraction [4], and more recently fluorescence spectroscopy [5]. STM and electron diffraction are not suited for the study of SWNTs on a large scale and the analysis of the data is a delicate task [6], [7]. Fluorescence spectroscopy cannot be used to characterize metallic nanotubes, solid particles, bulk samples or nanotubes dispersed in an organic solvent [8] or in composites. It does not provide a diameter value and the fluorescence peak positions are very sensitive to the SWNT environment [9].

Resonant Raman spectroscopy has proven to be a powerful tool for the determination of nanotube structure and to probe their electronic density of states. The position of the radial breathing modes (RBMs) ωRBM, which correspond to the collective radial vibrations of the carbon atoms, was found to be inversely proportional to the nanotube diameter d [10]. Due to the one-dimensionality of the nanotubes, their electronic density of states spectrum exhibits van Hove singularities in the conduction and valence bands. According to resonance theory, the intensity of these RBMs is predicted to reach its maximum when the laser excitation energy Elaser matches the energy separation Eii between the ith pair of van Hove singularities [11]. We will focus in this study on E11M for the metallic types and E22S for the semiconducting ones. Using only the RBM position and the theoretical Eii values from tight-binding calculations, a number of attempts to assign chiralities to the RBMs of isolated nanotubes or small bundles have been reported [11], [12], [13], [14], [15], [16]. Several nanotubes within a close diameter range and with similar Eii values can be assigned to a single RBM. Using the RBM position and resonance theory, Jorio et al. were able to reduce the number of possibilities [11]. The trigonal warping effect [17] helped reduce even further the possibilities for metallic nanotubes, but additional information was needed for the semiconducting ones to assign them a unique structure. Also, these methods rely heavily on the theoretical Eii values, which can be affected by curvature or the assembly of nanotubes into bundles [14]. Also, nanotubes within the same diameter range can have very close Eii values, so additional information is required to reduce the number of suitable candidates.

Here we report the intensity variations of RBMs as a function of uniaxial strain using lasers of three wavelengths 632, 785 and 830 nm. These structure-dependent variations are attributed to the modification of the resonance conditions under strain. The behaviour of the nanotubes under strain in composites was used as an additional factor to assign the (n,m) indexes to the RBMs observed.

Section snippets

Experimental

Two types of SWNTs were studied: purified HiPco and Elicarb nanotubes. The purified HiPco nanotubes were obtained from Carbon Nanotechnologies Inc. (USA) and produced by disproportionation of carbon monoxide. Their mean diameter is about 1 nm [18] and they contain up to 10% by weight of catalyst residue. The purified Elicarb SWNTs from Thomas Swan & Co. Ltd. (UK) were produced by chemical vapor deposition and contain very little catalyst residue. They have a mean diameter of less than 2 nm and

RBMs of SWNT composites

Typical Raman spectra collected from HiPco and Elicarb SWNTs embedded in a polymer matrix are displayed in Fig. 1. For HiPco SWNT composites, the Elaser = 1.49 eV (λ = 830 nm) spectrum is fitted with 5 Lorentzian lines centred at 207, 217, 227, 237, and 266 cm−1, the Elaser = 1.58 eV (λ = 785 nm) spectrum with 5 centred at 210, 232, 238, 268, and 272 cm−1, and the Elaser = 1.96 eV (λ = 632 nm) spectrum with 5 centred at 200, 221, 257, 284, and 299 cm−1. For Elicarb SWNT composites, the Elaser = 1.49 eV spectrum is

Strain effects

The intensity variations caused by deformation exceed the range of experimental error of about ±10%. Any orientation effects from the four-point bending tests can be ruled out. The nanotube bundles were dispersed isotropically in the epoxy/SWNT composite and in the PVA film. Low-frequency Raman spectra were collected at the same location, at 0% strain, with different orientation of the sample with respect to the incident laser polarization and the RBM positions and relative intensities were

Structure assignments using the tight binding model

Using tight-binding (TB) calculations, the RBM intensity variations under strain are an additional criterion for the assignment of SWNT structure. Table 1 is a list of RBMs observed in epoxy/SWNT composites or PVA/SWNT films with three different lasers and their intensity variation behaviour in four-point bending tests. Only the SWNTs with their axes parallel to the strain axis and the laser polarization are considered. Any strain along the circumferential direction was neglected, since the

Conclusions

The strain-induced intensity variations of the Raman RBMs of SWNTs in epoxy/SWNT composites and PVA/SWNT films were investigated using three different lasers of wavelengths 830, 785 and 632 nm. Variations of between 10% and 200% of the RBM intensities were observed over a range of strain between −0.6% and 0.7%. The trend (increase or decrease) as well as the magnitude of the intensity variations depends on the nanotube diameter and its chirality. RBMs separated by just 1–2 cm−1, which is close to

Acknowledgements

The project was supported through the European Union Thematic Network CNT-NET and one of the authors (M. L.) is grateful to the EPSRC for financial support under Grant No. GR/S61706/01.

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