Principle Scientists: Ben Murray
B.J. Murray, E.C. Walter, and R.M. Penner*, Amine Vapor Sensing With Silver Mesowires, NanoLetters 4 (2004) 665.
B.J. Murray, J.T. Newberg, E.C. Walter, Q. Li, J.C. Hemminger, and R.M. Penner*, Fast, Reversible Resistance Modulation in Mesoscopic Silver Wires Induced by Exposure to Amine Vapor. Analytical Chemistry 77 (2005) 5205.
Gold, silver, platinum and palladium, are ductile and resistant towards oxidation in aqueous solutions over a broad range of pH. These two attributes recommend noble metal nanowires for applications in chemical sensors. However in contrast to semiconductor nanowires, the resistance of metal nanowires is not affected by the capacitive coupling of charge carriers to immobilized charge (e.g., ions) at the surface of the wire. Instead, the electric field produced by immobilized ions is screened by the high carrier density within a few Angstroms of the surface resulting in no significant change in carrier density or resistance. So you might not expect metal nanowire to be sensitive to the adsorption of molecules, but we have found that for several noble metals (silver, copper, and platinum), this is simply not the case. We focus attention, in these two papers, on the surprising behavior of silver nanowires upon exposure to ammonia.
Figure 1. (A) Schematic diagram of electrodeposition pulse method used to prepare silver nanowires having a width of one grain. (B) Scanning electron microscope images of silver nanowires as a function of the growth time. Shown at top are the silver nuclei formed by the nucleation pulse. These nuclei are gown to coalescence to form a continuous silver nanowire.
Silver nanowire ensembles (AgNEs) with diameters ranging from 200 nm to 1.0 micron were prepared by electrochemical step edge decoration (ESED). These AgNEs showed a rapid (< 5s), reversible increase in resistance upon exposure to the vapor of ammonia, trimethylamine, and pyridine. The amplitude of the resistance change was up to +3000%, more than two orders of magnitude larger than can be explained based on boundary layer scattering effects. We experimentally probe the mechanism for this resistance modulation in the case of ammonia, and we propose a model to describe it. Conductive-tip atomic force microscopy (AFM) was used to probe individual sections of nanowires in AgNEs; these data revealed that the resistance change caused by NH3 exposure was concentrated within a minority (about 10%) of the 5 micron wire segments that were probed - not uniformly distributed along each nanowire. All AgNEs showed a temperature dependence of their resistance that was smaller than expected for silver metal. Highly sensitive AgNEs sometimes showed a negative temperature coefficient of resistance characteristic of semiconductors, but negative values were never observed for AgNEs with a low sensitivity to NH3. AgNEs did not respond to hydrocarbons, O2, H2O, N2, CO, or Ar, but a large (R/Ro> |-50%|) irreversible decrease in resistance was seen upon exposures to acids including HCl, HNO3, and H2SO4. Based on these and other data, we propose a model in which oxidized constrictions in silver nanowires limit the conductivity of the wire, and provide a means for "gating" conduction based on the protonation state of the oxide surface.
Figure 2. (A) Current-voltage plots for a silver nanowire array before (dark blue) and after (red) exposure to 100% NH3 in air. A relative increase in the resistance of this array of +390% was observed. Light blue I-V curves were recorded immediately after returning the array to a pure air ambient. (B) Resistance versus time for an array of silver wires during exposure to 5 s pulses of ammonia vapor in N2 at the concentrations indicted. The initial resistance of this array, Ro, was 3500 ohms. R vs. time traces at different NH3 concentrations are displaced along the resistance axis for purposes of clarity.
The data of Figure 2 shows that ensembles of silver nanowires prepared by ESED show a resistance that is modulated, reversibly and rapidly, by ammonia and other amines. The amplitude of this resistance change for wires as large as 300 nm in diameter argues against a boundary layer scattering mechanism. Instead, we advance a model involving the presence of Ag2O bridges along these nanowires. Since Ag2O is an n-type semiconductor (EBG = 1.2 eV), the resistance of these bridges is sensitive to the ionization state of the hydroxyls at the oxide surface: A negatively charged, deprotonated surface is associated with a high resistance state for the bridge where as a neutral, protonated surface is associated with a low resistance state. We term these Ag2O bridges "chemically responsive interparticle boundaries" or CRIBs.
The existence of CRIBs is inferred from two experiments: 1) Conductive tip AFM investigations that show the resistance modulation is confined to isolated sections of nanowires that possess a dramatically elevated electrical resistance, and, 2) Measurements of the temperature coefficient of resistance (alpha) for AgNEs. Measured alpha values for AgNEs are always lower than the alpha for bulk silver. AgNEs that respond strongly to ammonia deviate most strongly from bulk with some AgNEs even exhibiting thermally activated conduction (negative alpha values). In addition, CRIBS also provide an immediate explanation for the variability in terms of the amplitude of the resistance response since wires without CRIBs can be expected to dramatically attenuate the resistance modulation caused by amines by shunting the current around more resistance wires in an ensemble that contain one or more CRIBs.
We further demonstrate that CRIBs can be electrochemically synthesized. Electrochemical oxidation of silver nanowires in alkaline electrolytes induces the removal of silver (etching) at interparticle boundaries followed by Ag2O electrodeposition. This treatment substantially reduces the variability in sensitivity to NH3 with all oxidized AgNEs showing a resistance response to NH3 of more than 25%. This result shows that charge-gating can be artificially enhanced in metal nanowires.
