Phase breaking in three-terminal contacted single-walled carbon nanotube bundles

Abstract

The three-terminal electrical transport through single-walled carbon nanotube bundles with low resistive metal contacts is investigated at room temperature. After correcting for the lead resistance, two-probe resistances close to the value expected for a metallic single-walled carbon nanotube are found. Analysis of the experimental data in the frame of the Landauer-Büttiker formalism reveals the phase-and momentumrandomizing effect of the third electrode, which is at floating potential, on the quasiballistic transport. Within this model, the phase-coherence length of the charge carriers is estimated to be Ϸ300 nm at room temperature.The potential application of carbon nanotubes ͑CNT's͒ as molecular wires in submicron devices has initiated a variety of conductance experiments. Phenomena like Coulomb blockade 1,2 and signatures of Luttinger liquids 3 in singlewalled carbon nanotubes ͑SWCNT's͒, as well as AharanovBohm oscillations, 4 quantized conductance and quasiballistic transport at room temperature ͑RT͒ in multiwalled carbon nanotubes ͑MWCNT's͒ were reported.5 Except for the observations of Coulomb blockade, a low contact resistance between leads and the CNT is required in such experiments.In the present study, electron-beam lithography ͑EBL͒ was used to contact SWCNT's with electrodes on top, similar to Ref. 3. Electrode arrays consisting of three equidistant stripes were prepared from AuPd ͑40 wt %/60 wt %͒. The electrical transport investigations at room temperature presented here focus on samples with SWCNT bundles, which are connected by three low-Ohmic contacts, in contrast to recent measurements with only two contacts. 3,6 For sample preparation, 7 arc-discharge SWCNT raw material was dispersed by ultrasonic treatment in aqueous surfactant solution, and purified by centrifugation. As substrate, an As doped Si wafer with a thermally grown SiO 2 layer was used. Before adsorption of the SWCNT's, the substrate was treated with a 0.1 wt % aqueous solution of 3-͑aminopropyl͒triethoxysilane for 2 min. In order to perform EBL ͑EBL 100 system, LEICA͒ on top of the SWCNT's, a two-layer resist system was used.2 After EBL, AuPd ͑thickness Ϸ17 nm͒ or Au ͑thickness Ϸ24 nm͒ was thermally evaporated on the substrate at a base pressure of pϷ10 Ϫ7 mbar using a rate of 1 Å/s. Figure 1 shows a scanning force microscope ͑SFM͒ image ͑Tapping Mode, Digital Instruments, Nanoscope IIIa͒ of a typical sample we have investigated. A thin SWCNT bundle is connected to three AuPd electrodes. The electrode stripes are separated by 100 nm, and are approximately 100 nm in width. The electrical transport measurements were performed in vacuum (pр10 Ϫ3 mbar͒ at RT. In the upper inset of Fig. 1, the SFM height profile across electrode stripe 2 is shown. The contour of the bundle can be clearly detected in the profile, revealing a height that coincides with the height of the uncovered parts of the bundle ͑Ϸ3 nm͒. This result indicates that its structural integrity is preserved during metal evaporation due to the high mechanical stability of ...

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