STM atomic resolution images of single-wall carbon nanotubes

We have obtained atomically resolved STM images of individual single-wall carbon nanotubes. The interpretation of the apparent lattice is nontrivial. In most cases we observe a triangular arrangement instead of the expected hexagonal carbon lattice. However, the chirality of the nanotubes can be unambiguously determined from the images, which is an important result because the electronic properties are predicted to be strongly dependent on the atomic structure. Since the discovery of carbon nanotubes [1, 2] much attention has been directed to the electronic properties of these fullerene molecules. A remarkable result of theoretical calculations [2, 3] is the strong dependence of the electronic bandstructure of a nanotube on its chiral structure and diameter. Nanotubes can be either metallic or semiconducting, with energy gaps that depend on the tube diameter. The predictions are based on single-wall nanotubes, which consist of single graphene layers wrapped into cylinders. Only recently has it been possible to synthesize single-wall carbon nanotubes with high yield and structural uniformity [4]. The relation between the chirality of a nanotube and its electrical properties can be explored by scanning tunneling microscopy (STM), since it allows both topographic imaging and scanning tunneling spectroscopy (STS). STS is done by positioning the STM tip above a nanotube, switching off feedback and measuring the tunnel current as a function of the bias voltage. From these measurements, the local density of states can be obtained. To be able to correlate the electronic structure of a tube to its chirality, it is essential to obtain atomically resolved images, from which the chiral structure can be determined. There have been several reports of topographic imaging of bundles and individual carbon nanotubes by STM [5–10], as well as some preliminary STS data on nanotubes [5, 8, 10, 11]. These measurements concerned mostly multi-wall nanotubes. So far, there has been no report of high-quality spectroscopic data in combination with atomically resolved images. Atomic resolution on carbon nanotubes was achieved by Ge and Sattler [7]. In their work a mixture of multi-wall and single-wall nanotubes was produced by vapor condensation of carbon on highly oriented pyrolytic graphite (HOPG) substrates in high vacuum. In this paper we present atomically resolved STM images of single-wall carbon nanotubes that were deposited on atomically flat gold surfaces. These samples were very suitable for the STM investigation of nanotubes for the following two reasons. First, gold has little structure in the density of states itself, in contrast to HOPG which simplifies interpretation of the STM spectroscopy measurements. Second, the nanotubes were well characterized before the deposition. They were synthesized by a laser vaporization technique [4] and examined by X-ray diffraction, transmission electron microscopy (TEM) and Raman scattering spectroscopy measurements [4, 12]. These measurements showed the high structural uniformity of these tubes. The material consists mainly of single-wall nanotubes of ∼1.4 nm in diameter. The small diameter of the tubes confirm that they have to be single-walled. Measurements on these samples demonstrated the possibility to observe with STM both the atomic and electronic structure of carbon nanotubes, which are predicted to be related. From the STS results we can distinguish the two predicted classes of carbon nanotubes: The semiconducting tubes, with energy gaps inversely proportional to the diameters and the metallic tubes. These results will be discussed elsewhere [13]. In this paper we discuss the atomically resolved topographic images of the nanotubes. These atomically resolved images clearly reveal the chiralities of the nanotubes. In most cases the atomic lattice appears to be triangular. The nanotubes were deposited from a dispersion in 1, 2 dichloroethane on single-crystalline Au(111) facets. The samples were dried within a few seconds by applying a weak nitrogen flow. A home-built STM [14] operated at 4 K has been used for all measurements. We used Pt/Ir tips, cut in ambient by scissors. Atomic resolution was readily achieved on most nanotubes. Typical bias parameters were those of Fig. 1, viz., a tunnel current of 60 pA and a bias voltage of 0.5 V. About 30 tubes were investigated. Figure 1 shows an image of an individual carbon nanotube. The carbon lattice can be clearly observed from which