Electron energy-loss spectroscopy studies of single wall carbon nanotubes

doi:10.1016/S0008-6223(98)00263-2

Carbon

Volume 37, Issue 5, 9 April 1999, Pages 733-738

https://doi.org/10.1016/S0008-6223(98)00263-2 Get rights and content

Abstract

We have carried out momentum-dependent measurements of the density response function of bulk samples of purified single wall nanotubes using electron energy-loss spectroscopy. Carbon nanotubes support both excitations between delocalized and localized electronic states. The π-plasmon exhibits significant q-dependence, with a dispersion relation similar to that of the graphite plane, demonstrating the graphitic nature of the nanotube electron system along the tube axis. In contrast, the interband excitations observed at low energy have vanishingly small dispersion in q. These excitations between localized states are related to characteristic interband transitions between the singularities in the nanotube electronic density of states, and can thus be used to show that our samples contain significant quantities of both semiconducting and metallic single wall nanotubes.

Introduction

Carbon nanotubes are a new member of the growing family of novel fullerene-based materials, and represent a promising extension of this material class as they can be used as ideal building blocks for nanoengineering as a result of their special electronic 1, 2 and mechanical [3] properties. Regarding their molecular structure, nanotubes can be envisaged as rolled-up graphene sheets which are capped with fullerene-like structures. Interestingly, their electronic properties are predicted to vary depending upon the wrapping angle and diameter of the graphene sheet, thus offering the possibility to selectively form either metallic or semiconducting nanotubes 4, 5, 6.

The best system in which to investigate the intrinsic properties of this new material class are single wall nanotubes (SWNTs). Macroscopic nanotube samples generally contain a distribution of tubes with different diameters and chirality and thus present the experimentalist with an averaged picture of their properties. Therefore, many studies have been carried out on individual SWNTs. Transport measurements [7] and scanning tunnelling spectroscopic (STS) and topographic (STM) studies of single nanotubes [8] have done much in the recent months to advance our knowledge regarding the properties of SWNTs, for example by experimentally verifying the remarkable relationship between nanotube geometry and their electronic properties [8]. In addition, spatially-resolved electron energy-loss spectroscopy (EELS) has been performed on individual multi-wall nanotubes (MWNT) 9, 10, 11 or on a single bundle of SWNTs [12].

Much less has been done using methods that can be applied to macroscopic samples. Combined electron spin resonance, microwave and DC resistivity measurements [13] have led to the conclusion that bulk SWNT material is metallic. Resonant Raman measurements have been interpreted in terms of the vibrational modes of non-chiral `armchair' SWNTs [14], which is in contrast to the results from scanning tunnelling microscopy and EELS in transmission in which chiral as well as non-chiral (`armchair' and `zig-zag') nanotubes were positively identified 8, 15. Additionally, it has been shown that bundles of SWNTs can be intercalated with alkali metals or halogens in order to achieve n- or p-type doping, respectively 16, 17.

In this contribution we present high resolution momentum-dependent EELS in transmission measurements of purified SWNTs, in which we show that they support two types of electronic excitations. The first group of excitations are non-dispersive, whereby their energy position is characteristic of the separation of the van Hove singularities in the electronic density of states of the different types of nanotubes present in the sample. The second type of excitation shows considerable dispersion, which parallels that observed in the graphite plane, and is related to a collective excitation of the π-electron system polarized along the nanotube. In addition, the close similarity to graphite is also indicated by the C1s excitation edges.

Section snippets

Experimental

SWNTs were produced by a laser vaporization technique [18]. The material consists of up to 60% SWNTs with approximately 1.4 nm mean diameter and was purified as described in Ref. [19]. Free-standing films for EELS of effective thickness about 1000 Å were prepared on standard copper microscopy grids via vacuum filtration of a nanotube suspension in a 0.5% surfactant (Triton X100) solution in de-ionised water, with a SWNT concentration of ∼0.01 mg ml⁻¹. The surfactant was them rinsed off and the

Results and discussion

In Fig. 2 we present the EELS C1s excitation spectrum of purified SWNT compared to that of highly oriented pyrolytic graphite (HOPG, measured with the momentum transfer perpendicular to the graphite plane). In such a measurement, excitations of the core electron into unoccupied states with C2p character are probed. The spectrum of the SWNTs strongly resembles that of graphite. By comparison with directional-dependent C1s excitation measurements of HOPG 21, 22, we see that the SWNT spectrum most

Summary

In summary, we have demonstrated that momentum-dependent high-resolution EELS in transmission measurements of SWNTs show that they represent a text-book example of a system supporting both excitations between localized and delocalized electronic states. Along the nanotube axis SWNTs support a π-plasmon whose dispersion relation is very similar to that of the graphite plane, proving the graphitic nature of the electronic system in this direction. At low energies, non-dispersive features are

Acknowledgements

T.P. thanks the European Union for funding under the `Training and Mobility of Researchers' program. The work at Rice was supported by the National Science Foundation (DMR9522251), the Advanced Technology Program of Texas (003604-047) and the Welch Foundation (C-0689).

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Electron energy-loss spectroscopy studies of single wall carbon nanotubes

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Summary

Acknowledgements

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