Fabrication of stable metallic nanowires with quantized conductance

A metallic nanowire with quantized conductance was fabricated by electrochemically etching a narrow portion of a metallic wire supported on a solid substrate down to the atomic scale. The width of the nanowire was controlled flexibly by etching atoms away or depositing atoms back onto the wire with the electrochemical potential. Using a feedback loop this method can, at will, fabricate a single or an array of long-term stable nanowires with a pre-selected quantized conductance. These stable nanowires may be used in devices as digitized conductors and as sensors that detect chemicals in the air or in solutions. Using the conductance quantization as a feedback, this method may be used to fabricate nanoelectrodes by etching off the last few atoms in the thinnest portion of each nanowire. These nanoelectrodes may be connected to single molecules in molecular devices. The growing activity in the interdisciplinary area of nanometre-scale science and technology can be traced to the ever-increasing quest for the miniaturization of electronic devices, driven by the need for faster and more powerful electronics. In addition to the importance of developing new generation electronics, materials on the nanoscale often exhibit interesting quantum phenomena. An important example is the electrical conductance of a metallic wire with a diameter of a few atoms [1, 2]. Because the size of the wire is comparable to the Fermi wavelength of the conducting electrons in metal, the electrons transport ballistically along the wire and form well-defined quantum modes in the transverse direction. Each mode contributes equally to the conductance, thus, the conductance becomes quantized and is given by NG0, where G0 = 2e2/h is the conductance quantum andN is the number of the modes [2]. It has been proposed that such nanowires may be used as conductors [3, 4] and as single-atom digital switches [5] in nanoelectronic circuits. We have recently observed that the conductance quantization is sensitive to the adsorption of a molecule onto the nanowire which may lead to applications in chemical sensors [6]. For practical applications, however, a suitable method that can mass-produce stable nanowires must be found which is the subject of this paper. To date, the most widely used method for creating a nanowire that exhibits the conductance quantization phenomenon is based on mechanically breaking a fine metal wire [7–9] or separating two metal electrodes in contact [10– 12]. The breaking and separating are usually controlled by a break junction or a scanning tunnelling microscope (STM) that involves a stepping motor, or a piezoelectric transducer. A nanowire fabricated by the mechanical method cannot be † Author to whom correspondence should be addressed. E-mail address: [email protected] removed from the apparatus and is, therefore, unsuitable for many applications. Two non-mechanical methods have been recently demonstrated. One is to anodize an Al wire locally with an atomic force microscope (AFM) [13]. The use of the AFM makes it impracticable for mass-producing the nanowires. Furthermore, the method is not reversible which makes the control of the fabrication process difficult. Another non-mechanical method is to deposit a metal onto the STM tip held a few nanometres from a metal surface [14]. Although the method is reversible, the use of STM in the set-up once again makes it difficult for mass production. The lifetime of the nanowire is typically less than a few seconds because the gap between the STM tip and the metal surface drifts due to thermal expansion, acoustic noise and mechanical vibrations, which is also undesirable for practical applications. This paper describes a simple electrochemical etching/deposition method that can fabricate a single or an array of stable nanowires. The method can be automated such that a nanowire with a preset quantized conductance can be produced at will. The nanowires supported on a solid substrate may be removed from the fabrication set-up and used as a stand-alone device or for further investigation using various experimental probes. We have demonstrated the method using two different set-ups. In the first set-up, we started with a thin (5– 25 μm) Cu wire attached to a glass substrate (figure 1). The wire was coated with an insulation layer (wax or 5minutes-epoxy) except a region smaller than 1 μm near the centre. The central region was then exposed to 1 mM CuSO4 + 100 mM H2SO4 for electrochemical etching which was controlled by adjusting the electrochemical potential of the wire. For controlling the potentials, we used a homemade bipotentiostat with a Cu quasi-reference electrode and a Pt counter electrode [14]. The use of bipotentiostat 0957-4484/99/020221+03$19.50 © 1999 IOP Publishing Ltd 221