MULTIPROBE TRANSPORT EXPERIMENTS ON INDIVIDUAL SINGLE-WALL CARBON NANOTUBES

Deposition of individual single-wall carbon nanotubes over multiple (up to seven) Pt nanoelectrodes is realized. Two-probe and four-probe transport measurements between adjacent pairs of electrodes show similar but not identical single-electron Coulomb charging signatures at low temperatures. The observations indicate that nanotubes can behave as a chain of quantum wires connected in series. We argue that the local barriers separating these islands may be caused by bending of the tube near the edges of electrodes. [S0031-9007(98)05997-3] PACS numbers: 73.61. Wp, 72.80. Rj Carbon nanotubes [1] are attracting much attention because of their unique electrical, mechanical, and capillary properties. The electrical properties of carbon nanotubes strongly depend on their diameter and the chiral angle of the atomic lattice: “Zigzag” or “chiral” nanotubes are predicted to be semiconductors with either a substantial (,1 eV) gap or a very low gap s,meV sd, whereas “armchair” tubes are expected to be truly one-dimensional (1D) metals [2,3]. Recently, a high-yield method for the synthesis of single-wall carbon nanotubes (SWCNTs) was discovered [4], which enabled first experimental studies on the electronic properties of this model variety of nanotubes [3,5,6]. In our many experiments with individual SWCNTs between metallic electrodes, three types of behavior can be distinguished: (i) The current-voltage ( I-V) characteristics are nonlinear already at room temperature, with a rather high s.10 MVd zero-bias resistance which increases upon cooling. These tubes are identified as large-gap semiconducting tubes [7]. (ii) I-V curves are linear at room temperature with a lower two-probe resistance (typically ,1 MV). Upon cooling, this resistance is almost constant down to ,100 K below which it rises due to Coulomb charging. A finite density of states is found even at mK temperatures [5], indicating that these tubes are metallic and presumably of the armchair variety. (iii) A third type of behavior is observed, where nanotubes display similar room-temperature characteristics and also a finite low-temperature density of states. However, the Coulomb charging signatures of these samples are quite different. In particular, a strong temperature dependence of the resistance is observed (increase of at least a factor of 20 from ,100 to 4 K) which is absent in the second class of nanotubes. It is tempting to associate this third class with the low-band-gap semiconducting “quasimetallic” nanotubes. In this Letter we report on the investigation of this third class of nanotubes. We succeeded in depositing individual SWCNT molecules on four or more metal leads [Fig. 1(a)], and investigating their two-probe and four-probe resistance versus length, temperature, and gate potential. The low-temperature properties appear to be determined by Coulomb blockade effects [8], where, in order to tunnel onto the nanotube, electrons should overcome the charging energy of the molecule with an integer number of electrons on it. The Coulomb charging signatures measured between adjacent pairs of electrodes are similar but not identical. A strong temperature dependence of the peak conductance is observed. Both features demonstrate that the nanotube internally consists of a small number of islands in series. Comparing with previous experiments [5] we thus find a different type of behavior, where a tube does not behave as one continuous quantum wire with extended quantum states. Instead it is electrically broken up into a chain of weakly coupled 1D quantum wires separated by local barriers. These barriers may FIG. 1. (a) AFM image of the sample showing a single nanotube over seven Pt electrodes. The spacing between the leads is about 400 nm. The Pt gate electrode (not visible) is positioned at ,10 mm from the tube. The apparent height of the SWCNT is ,1.2 nm on top of the leads as well as in between them. (b) Two-probe measurements sVbias › 1 mVd of the current versus gate voltage between various pairs of leads i-j at 4 K. Curves are vertically offset for clarity. The curve “1-3 model” is the current calculated for electrodes 1-3 from the currents 1-2 and 2-3 under the assumption of a series connection of resistors (see text).