Making Nanotube Devices

The ability to manipulate carbon nanotubes on surfaces is the first step to controllably assemble them into electrical circuits and nanoscale devices. This toolkit has currently been extended to encompass tube translation, rotation, cutting, and even putting nanotubes on top of each other. Recently, Roschier et al. [64] and Avouris et al. [65] exploited nanotube translation and rotation by an AFM probe to create nanoscale circuits where a MWNT served as the active element. Roschier et al. [64] reported the fabrication of a MWNT single electron device (Fig. 17) and characterized its electrical properties. In addition, Avouris and co-workers [65] have fabricated a variety of structures to investigate the maximum current that can be passed through individual MWNTs, and the possibility of room-temperature electronics. In this latter regard they were able to fabricate a working field-effect transistor from a semiconducting nanotube.

Lefebvre et al. [66] reported the first tapping mode AFM manipulation of SWNT ropes to create crossed nanotube junctions. Their device fabrication strategy was similar to the manipulation method of [64,65], where translation

Fig. 17. AFM manipulation of a MWNT over pre-defined electrodes. The 410 nm long MWNT, the side gate, and the electrode structure are marked in the first frame. The last frame represents the measured configuration [64]

and rotation of nanotubes on surfaces occurred in small increments, typically 10 nm and 5 degrees at a time; however, instead of moving the nanotube over already-deposited electrodes, electrical contact was made to the nano-tubes after the manipulation. The top nanotube and degenerate Si substrate can both serve as gate electrodes for the lower nanotube, and the current measured at 5K through the lower nanotube exhibited current peaks reminiscent of two quantum dots in series. The authors speculate that the origin of the junction responsible for the double-dot behavior may either be mechanical (a combination of tube-tube and lower tube-surface interactions) or electrostatic (potential applied between lower tube and upper tube). In any case, the resulting local perturbation changes the nanotube band structure to preclude electron propagation along the lower tube. Interestingly, the tunnel barrier formed by the top bundle on the lower one can be tuned by the substrate voltage, and in the limit of strong coupling, the lower nanotube exhibits charging behavior of a single quantum dot.

As discussed above, manipulation of nanotubes on surfaces allows control in device fabrication of nanotube circuits, although this approach can also damage the nanotubes. Recently, Cheung et al. [67] reported a novel method of AFM manipulation and controlled deposition to create nanotube nanostructures. Their technique eliminates the laborious steps of incremental pushes and rotations as well as the unknown tube damaged caused by AFM manipulation across surfaces. Individual SWNTs and ropes were grown from Si-AFM tapping mode tips by chemical vapor deposition [68,69]. The SWNTs are deposited from the AFM tip to a pre-defined position on the substrate by three simple steps: (i) biasing the tip against the surface, (ii) scanning the nanotube tip along a set path, and (iii) then applying a voltage pulse to disconnect the tip from the nanotube segment on the substrate. This method can produce straight structures since tube-surface forces do not need to be overcome, and in addition, complex junctions between nanotubes may be created since the nanotube may be deposited at specified angles (Fig. 18).

Fig. 18. AFM images of SWNT deposition onto a substrate. (a) A SWNT deposited along the direction of the arrow. (b) A cross SWNT structure made by a second nanotube lithography step. The images in (a) and (b) are 2|im x 1.3|i.m [67]

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