Principle Scientist: Chengxiang Xiang
C. Xiang, A.G. Güell, M.A. Brown, J.Y. Kim, J.C. Hemminger, R.M. Penner*, Coupled Electrooxidation and Electrical Conduction in a Single Gold Nanowire, Nano Letters 8 (2008) 3017.pdf.
We are interested in using metal nanowires prepared using LPNE as transducers in chemical sensors. Can metal nanowires detect, as a resistance change, the chemical state of the wire surface? As an example, is any perturbation in the wire resistance seen when one molecular layer of oxide is grown on a gold nanowire? We sought to address this question using an experiment in which the resistance of a gold nanowire prepared using LPNE was measured in an acidic solution that permitted the growth on this nanowire of an oxide monolayer.
Figure 1. a) Photograph of the four-point probe used to measure the resistance of a potentiostatically- controlled gold nanowire in sulfuric acid, b) Optical micrograph showing edges of inner two gold electrodes, separated by 150 microns, and a gold nanowire spanning these electrodes. The entire region shown is covered with a poly-methylmethacrylate (PMMA) layer of thickness 400 nm, except for a rectangular window (width 46 microns) formed by electron beam writing at the center of the image. A section of this nanowire is exposed to the solution through this window. c) Scanning electron micrograph (SEM) showing the gold nanowire through the window in the PMMA resist. d) Higher magnification SEM image of the gold nanowire exposed within this window.
This experiment was carried out using the rig shown above. Briefly, a four point electrical contact was made to a single gold nanowire. The contacts and the nanowire were both encased in a layer of PMMA through which a rectangular window was cut, exposing the nanowire to the environment. We then immersed it in 0.1M sulfuric acid and adjusted its potential to grow an oxide layer using a potentiostat.
Figure 2. a) Array of gold nanowires (400 nm (w) x 60 nm (h)), deposited at 2 micron pitch on glass, investigated by x-ray photoelectron spectroscopy (XPS). b) Narrow scan XP spectrum of the gold 4f region for freshly-prepared gold nanowires that were equilibrated at +0.0 V vs. MSE in 0.10 M H2SO4 prior to emersion. This spectrum shows a binding energy of 84.0 eV for the 4f7/2 peak, characteristic of clean, elemental gold. c) XP spectrum of the gold 4f region for nanowires that were electrooxidized at +1.1 V vs. MSE. Deconvolution peak fitting of the shoulder on the high energy side of the elemental gold peaks (red cur ve) reveals the formation of an oxidized gold species with a binding energy of 85.6 eV assigned to Au(OH)3 . d) Plot of the gold intensity ratio between the peak assigned to Au(OH)3 peak and that of elemental gold for nanowires equilibrated at five potentials: 0.0 V, 0.7 V, 0.8 V, 1.0 V and 1.1 V vs MSE. The solid line provides a guide to the eye only.
The forward or oxidation scan is shown in Fig 3 for five nanowires with heights (the smallest dimension) that were both larger and smaller than the wire probed in Fig 3a, ranging from 63 nm to 18 nm. It is apparent that the increase in resistance seen in Fig 3a is even more pronounced for nanowires with a smaller height dimension. In fact, the relative increase in the resistance, dR/Rred , increases linearly with diminishing wire height for all of these six nanowires (Fig 3c) with the 18 nm sample producing a dR/Rred of 57% at +1.1V and 70% at +1.2V.
Figure 3. a) dR/R0.1V versus Eapp for the oxidation of five gold nanowires with dimensions as indicated. b) Plot of dR/R0.1V evaluated at +1.1 V vs. MSE versus height for the six nanowires in (a) and (b) (red trace). Also plotted are the calculated resistance change caused by the constriction in the wire diameter ((dR/Rred ), green trace) and the change in p from 0.38 to 0.0, blue trace) as well as the sum of these two contributions ((dR/Rred), black trace). c,d) schemtic diagrams depicting two mechanisms for the oxidation of a gold nanowire, the first (top) involving the formation of a superficial oxide layer and the second (bottom) an oxide layer that infiltrates grain boundaries.
In summary, the electrical resistance of gold nanowires measured in-situ in dilute sulfuric acid increases in concer t with the formation of a oxide monolayer by electrooxidation. The magnitude of the resistance increase is as large as +70% in gold nanowires with lateral dimensions of 18 x 95 nm; much larger than can be explained by the known mechanisms of dissipation in metals. The infiltration of oxide into the grain boundaries at the surface of these nanowires provides a physically reasonable explanation for this anomolous resistance increase but this mechanism is not confirmed by any direct experimental evidence.
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