Electronic Density of States of Atomically Resolved Single-Walled Carbon Nanotubes: Van Hove Singularities and End States

Abstract

The electronic density of states of atomically resolved single-walled carbon nanotubes have been investigated using scanning tunneling microscopy. Peaks in the density of states due to the onedimensional nanotube band structure have been characterized and compared with the results of tight-binding calculations. In addition, tunneling spectroscopy measurements recorded along the axis of an atomically-resolved nanotube exhibit new, low-energy peaks in the density of states near the tube end. Calculations suggest that these features arise from the specific arrangement of carbon atoms that close the nanotube end.PACS numbers: 71.20.Tx;61.16.Ch;73.20.At The electronic properties of single-walled carbon nanotubes (SWNTs) are currently the focus of considerable interest [1]. According to theory [2][3][4], SWNTs can exhibit either metallic or semiconducting behavior depending on diameter and helicity. Recent scanning tunneling microscopy (STM) studies of SWNT [5,6] have confirmed this predicted behavior, and have reported peaks in the density of states (DOS), Van Hove singularities (VHS), that are believed to reflect the 1D band structure of the SWNTs. A detailed experimental comparison with theory has not been carried out, although such a comparison is critical for advancing our understaning of these fascinating materials. For example, chiral SWNTs have unit cells that can be significantly larger than the cells of achiral SWNTs of similar diameter, and thus chiral tubes may exhibit a larger number of VHS than achiral ones [7]. Recent theoretical work [8,9] suggested, however, that semiconducting (or metallic) SWNTs of similar diameters will have a similar number of VHS near the Fermi level, independent of chiral angle. In addition, the electronic properties of localized SWNT structures, including end caps, junctions and bends [10][11][12][13], which are essential to proposed device applications, have not been characterized experimentally in atomically resolved structures.In this Letter, we report STM investigations of the electronic structure of atomically resolved SWNTs and compare these results with tight-binding calculations. Significantly, we find that the VHS in the DOS calculated using a straight-forward zone-folding approach agree with the major features observed in our experiments. We have observed new peaks in the local DOS (LDOS) at an end of a metallic SWNT and compared these results to calculations. This analysis suggests that the new peaks can be associated with a specific topology required to cap the SWNT. The implications of these results and important unresolved issues are discussed. Experimental procedures and are described elsewhere in detail [6,14]. In brief, SWNT samples were prepared by laser vaporization [15], purified and then deposited onto a Au (111)/mica substrate. Immediately after deposition, the sample was loaded into a UHV STM that was stabilized at 77 K; all of the experimental data reported in this Letter was recorded at 77 K. Imaging and spectroscopy were measured using etched tungst...

Purification and Characterization of Single-Wall Carbon Nanotubes (SWNTs) Obtained from the Gas-Phase Decomposition of CO (HiPco Process)

“…Well-spaced and symmetric structures, called van Hove singularities, appear in the local density of states of nanotubes due to the one-dimensional nature of the conduction electron states in nanotubes. 30 The one-dimensional nature of the energy bands has been illustrated previously by observation of the van Hove singularities with STM studies. [31][32][33] UVvis-near-IR spectra provide additional evidence of this phenomenon.…”

“…A third set of peaks centered near 650 nm is assigned to the first van Hove transition of metallic SWNTs. 30 The van Hove peaks are superimposed on a background that decreases smoothly from the ultraviolet to the near-infrared.…”

Tunneling spectroscopy on carbon nanotubes using STM

“…We point out the possibility that these fine structures are regarded as the splitting of the first peak. The DOS peak for metallic nanotubes is predicted to split into two peaks due to the anisotropy of equienergy contours around the K points [3,4]. Fine structures in present spectra might be related to this DOS splitting.…”

“…STM and STS measurements on single-wall carbon nanotubes (SWNTs) were reported by Wildoer et al [1] and Odom et al [2] at 4.2 and 77 K, respectively. Recently, the peak splitting for the metallic SWNT is reported [3,4].…”

Van Hove singularities as a result of quantum confinement: The origin of intriguing physical properties in Pb thin films

“…This is the reason why the STS of Pb films display a series of peaks [10] like a 1-D system [29], which has not been observed in any other 2-D system. The QWSs around the Κ point do not show an evident relationship with the DOS because they always cross the Fermi level very steeply.…”