Formation of atomic point contacts and molecular junctions with a ...

can flow between the electrodes. The current was typicallywell below 0.2 nA in the electrolyte shown as the grey solidline in Fig. 5b, much smaller than the conduction currentthrough the molecular junction. The electrochemical gateeffect observed here is rather weak, which could be partiallydue to the screening of the gate field by the proximity of thetwo electrodes (serving as source and drain).3 Ni atomic point contactThe same setup can create atomic point contacts by eithermechanically controlling the elongation of the Au electrodesor by electrodeposition/etching process (Fig. 4a). Theuse of electrodeposition makes it easy to fabricate atomicpoint contacts of different metals, and even a pointcontact between two different metals. To demonstrate thisapplication, we have fabricated Ni atomic point contactsusing this combined approach. The first step is to electrochemicallydeposit Ni onto Au electrodes by holding thepotentials of the two electrodes at 21.1 V against Ag/AgCl with 0.1 M nickel sulphamate in 0.5 M boric acid(pH 3.5). We can also choose to deposit Ni onto only oneof the two electrodes to fabricate an Au–Ni contact. Aftera point contact is formed, we can further control the sizeof the atomic point contact mechanically. Since Ni is aferromagnetic material with partial occupied d-orbital, ithas been argued that the strong exchange splitting of theelectron bands may lift the spin degeneracy. The up- anddown-spin electrons contribute independently to the electrictransport, which results in the conductance of atomic pointcontact located at integer multiples of e 2 /h. In the preliminaryexperiments, we have performed the measurementwith a magnetic field (0.4 T). A typical conductancetrace is shown in Fig. 6a. Unlike the results without a magneticfield (result not shown), steps near or at integer multiplesof e 2 /h have rather frequently been observed in Nipoint contacts. The conductance histogram (Fig. 6b) constructedfrom 18 individual curves from two differentdevices shows some evidence of conductance peakslocated at integer multiples of e 2 /h [30–32]. Furtherstudies are in progress to verify and understand the preliminaryfindings.4 SummaryWe have described a combined mechanical and electrochemicaldeposition method to fabricate atomic point contactsand molecular junctions. Using the method, we have createdatomic point contacts and studied electron transport through4,4 0 -bipyridine. The setup is particularly suitable for studyingatomic point contacts and molecular junctions inaqueous solutions, which is required for biologically relevantmolecules, molecular binding events and electrochemicalgate-control of single molecule conductance.5 AcknowledgmentsWe would like to thank Bingqian Xu and Joshua Hihath forthe help and acknowledge the financial support fromNSF-DMR-03-05242.6 ReferencesFig. 6 Conductance quantisation of Nia Conductance quantisation steps of a Ni atomic point contact. Thequantised conductance steps are located at integer multiples of e 2 /hb Conductance histogram constructed from 18 individual curves asshown in a1 Li, C.Z., Bogozi, A., Huang, W., and Tao, N.J.: ‘Fabrication of stablemetallic nanowires with quantized conductance’, Nanotechnology,1999, 10, pp. 221–2232 Morpurgo, A.F., Marcus, C.M., and Robinson, D.B.: ‘Controlledfabrication of metallic electrodes with atomic separation’, Appl.Phys. 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