As materials shrink to the nanometer scale, interesting quantum phenomena emerge, which may lead to novel applications . Here, we describe a molecular detection method based on the conductance quantization in metallic nanowires. The phenomenom of conductance quantization occurs when the diameter of the wire is comparable to the electron Fermi wavelength . At the lowest conductance step (G=2e2/h), the narrowest portion of the wire has been shown to be as small as a few atoms . Because the few atoms dictate the conductance of the entire wire, adsorption of even a single molecule onto them may drastically change the conductance of the wire thereby making them useful as chemical sensors.
For a practical device, the nanowires must have long term stability and be producible in large quantities. The widely used mechanical method  that involve bringing two metallic wires in and out of contact are not suitable. In the present work, we fabricate highly stable nanowires by electrodeposition of Cu from electrolyte [4,5] onto Au microelectrodes separated by a 100nm gap. The electrodes were fabricated at Cornell Nanofabrication Facility by combining photolithography and focused ion beam techniques and were bridged by electrochemical deposition of Cu into the gap. The technique is reversible allowing etching of Cu if the wire is overgrown so that a desired conductance step can be stabilized. We note that a similar method was applied to fabricate facing electrodes with atomic scale gaps .
We studied the adsorption of three molecules 2,2' -bipyridine (22BPy), adenine, and mercaptopropionic acid (MPA). We used these 3 molecules because their adsorption strengths vary from very weak (22BPy) to very strong chemical adsorption (MPA). We have also recently done experiments with the neurotransmitter dopamine. We have been able to reproducibly stabilize the conductance of the nanowires at integer steps for several hours. After stablizing the wires at a desired quantum step, we added a drop of solution containing a sample molecule into the solution cell and monitored the subsequent change in the conductance continuously. Upon adsorption of the molecule to the wire, the quantized conductance decreases, typically to a fractional value, which may be attributed to the scattering of the conduction electrons by the adsorbates as predicted by a recent simulation . A further conductance change occurs when the nanowire is exposed to another molecule that has stronger adsorption.
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Department of Physics, Florida International University,
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