Scanning Tunneling Microscopy and Spectroscopy Studies of Single Wall Carbon Nanotubes
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Scanning tunneling microscopy and spectroscopy studies of single wall carbon nanotubes Teri Wang Odom, Jin-Lin Huang, Philip Kim, Min Ouyang, and Charles M. Liebera) Harvard University, Cambridge, Massachusetts 02138 (Received 6 February 1998; accepted 3 May 1998)
Scanning tunneling microscopy and spectroscopy have been used to characterize the atomic structure and tunneling density of states of individual single wall carbon nanotubes (SWNT’s) and ropes containing many SWNT’s. Analysis of atomically resolved SWNT images shows that the nanotubes consist of a wide range of diameters and helicities with no one structure clearly dominant. Tunneling spectroscopy measurements made simultaneously on atomically resolved SWNT’s exhibit semiconducting and metallic behavior that depend predictably on helicity and diameter. In addition the band gaps of the semiconducting tubes were also found to depend inversely on diameter. These results are compared to theoretical predictions, and the implications of these studies as well as important future directions are discussed.
I. INTRODUCTION
Currently, carbon nanotubes are the focus of intense interest worldwide. This attention to carbon nanotubes is not surprising in light of their promise to exhibit unique physical properties that could impact broad areas of science and technology, ranging from superstrong composites to nanoelectronics.1–3 Critical to realizing the potential of carbon nanotubes will be work that defines their intrinsic mechanical and electrical properties. There has been considerable experimental and theoretical progress to this end. For example, mechanical measurements have recently demonstrated that carbon nanotubes have the largest Young’s modulus of any known materials.4,5 Direct measurements of the full stress-strain behavior of individual nanotubes have also shown that nanotubes undergo a striking elastic buckling deformation, and are exceedingly tough materials.5 It is, however, the remarkable electronic properties of carbon nanotubes that have elicited the greatest interest.1,6–17 For single wall carbon nanotubes (SWNT’s), which consist of a single graphene sheet rolled into a seamless tube, theoretical calculations predict that both metallic and semiconducting nanotubes are possible depending only on the diameter and the helicity of the nanotube.6–10 The ability to display fundamentally distinct electronic properties without changing the local bonding sets nanotubes apart from other nanowire materials. The diameter and helicity of a defect-free SWNT are uniquely characterized by the roll-up vector ch na1 1 ma2 ; (n, m) that connects crystallographically equivalent sites on a two-dimensional (2D) graphene a)
Correspondence should be addressed to C.M.L. e-mail: [email protected]
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J. Mater. Res., Vol. 13, No. 9, Sep 1998
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sheet, where a1 and a2 are the graphene lattice vectors and n and m are integers (Fig. 1). Electronic band structure calcu
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