Electron transport measurements
on nanotubes and nanowires. 

        Carbon nanotubes are a novel material that has became an object of intensive research activity. With the diameter on the order of 1 nm and large aspect ratios nanotubes effectively are one-dimensional nanostructures, and that adds to their electron transport properties such effects as nonlinear conductance, Luttinger liquid behavior and Coulomb blockade. It has been shown that electronic devices made of individual carbon nanotubes can act as conducting wires, field-effect transistors and simple logic elements – all that contributing to the progress of single-molecule electronics.
        In our group we study electron transport in single-walled carbon nanotubes (SWNTs).  Through collaboration with Prof. Jie Liu’s group (Duke, Department of Chemistry) we are getting samples of SWNTs grown by a novel CVD method on an oxidized doped Si substrate.  We attach contacts to the ends of the SWNTs by electron beam lithography. Two-terminal conductivity of a number of samples with different quality of the contacts has been measured at room and liquid nitrogen temperatures. The latest experiments were carried out in liquid 4He cryostat system at ~1.3K and magnetic fields up to 8 Tesla.



Fig.1A. AFM image of a contacted SWNT

 Fig 1B. SEM image

         Some our preliminary results

        Experimental observations have been made that agree with a number of both theoretical predictions and published experimental results. Depending on the tube’s chirality (which is defined by the vector connecting two atoms of 2D-graphite sheet that coincide when the cylinder is formed) SWNTs can be metallic or semiconducting. Both the former and the later ones were studied: the response to the application of gate voltage Vg to the Si substrate (Fig.2A) made it possible to distinguish between these two kinds of SWNTs during the measurement. The gate voltage does not noticeably affect conducting properties of the metallic tubes. In case of the semiconducting samples conductivity decreases by many orders of magnitude when positive gate voltage is applied (Fig.2B), consistent with p-type doping of the tube by absorbed oxygen. For two large metal terminals on the Si substrate surface connected by multiple SWNTs the difference in gating behavior provides a way to  get rid of all    connections by metallic tubes.  Only  metallic nanotubes carry current when there is a high positive voltage on the gate, and thus only they are subjected to electrical breakdown when the bias voltage V exceeds some limit 
Fig. 2A. Nanotube device schematic

 Fig. 2B. dI/dV vs. gave voltage for a nanotube device at 77K
   
    Cooling the samples down to 1.3K allows access the regime when temperature is lower than single-electron charging energy of the nanotube. For the samples with not-so-good contact quality, where effectively tunneling barriers separate SWNT and metal leads, Coulomb blockade oscillations have been observed (Fig. 3A). The measurements show that the location of the centers of the peaks along Vg axis is close to periodic (Fig. 3B) as in the case of a single quantum dot. This result may mean that the tube does not contain strong structural imperfections, which would effectively divide it into a number of short sections each behaving as an individual quantum dot. From the charging energy value taken from Fig .2B the length of the section of the nanotube available for added electrons was estimated. The fact that the estimated length equals to the length of the SWNT between the metal contacts means that the tube is effectively cut by the contacts.

      


Fig. 3A. dI/dV vs. gave voltage at 1.3K. Coulomb blackede oscillations can be observed

 Fig. 3B. Data on Fig .(3A) suggests reasonable peak periodicity
               Publications                    
   
    B. Zheng, C. Lu, G. Gu, A. Makarovski, G. Finkelstein and J. Liu, Nano Lett. 2, 895-898 (2002).

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