We have been attempting to study the nature of electrical conduction and magnetoresistance in one-dimensional structures. In collaboration with Prof. Honjie Dai's research group in the Department of Chemistry, we have prepared nanowires by evaporating a thin layer of metal onto a single-walled carbon nanotube that is suspended over a deep trench.
The trench is prepared by ebeam lithography of PMMA, coating a silicon nitride substrate. The pattern defines a pair of large contact pads separated by an 800 nm gap and isolated from the surrounding chip. Nanotube are grown from a catalyst deposited on one of the contact pads. The nanotubes grow in random directions so that the trench is not always spanned by a single tube, if at all, but under good growth conditions, approximately 10 percent of the devices are found to have a single nanotube spanning the trench, thereby forming a one-dimensional wire. Thin films of metals such as nickel and niobium are deposited by ebeam evaporation onto the contact pads and the nanotube. The trench is purposely undercut by HF wet-etching to eliminate step-coverage: the evaporated metal that falls to the bottom of the trench does not short-circuit the device. The nanowires thus formed have approximate dimensions of 30 angstroms (width) by 50 angstroms (deposited thickness), 800-1000 nm in length.
An SEM picture of a single nanotube spanning an 800nm trench. The blocks on the sides are the deposited catalyst from which the nanotubes are grown. The light areas adjacent to the trench are regions where the substrate has been undercut to prevent step-coverage during evaporation. The trench extends in both directions and maps out a box on each side that isolates the device. This nanotube has been coated with titanium and nickel.
Contact is made to the device by two wirebonds to each contact pad, so that a quasi-four point resistance measurement is possible. The samples are heat-sunk to the mixing chamber of an Oxford Instruments dilution refrigerator, with a base temperature of 6 mT and an external magnetic field capability of 16 T.
Oxford dilution refrigerator. The stainless steel rod at the bottom extends into the bore of a 16 T superconducting magnet.
We have been studying the magnetoresistance of the nickel nanowires at temperatures between 20 and 500 mK in a perpendicular magnetic field, and we plan to study the nanowires in a parallel field in the near future. We have also made some preliminary measurements of the niobium wires, in an effort to find a superconducting transition. No results have been published to date.