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0915-R04-07

Gas Sensing with Gold Nanoparticle-Decorated GaN Nanowire Mats Vladimir Dobrokhotov, David McIlroy, and Chris Berven Department of Physics, University of Idaho, Moscow, ID, 83844-0903

ABSTRACT The electrical properties of chemical sensors constructed from mats of bare GaN nanowires and GaN nanowires decorated with gold nanoparticles are presented. The sensors were tested in vacuum and at atmospheric pressures of Ar, N2 and methane. The current-voltage (I-V) curves of the sensor constructed with bare GaN nanowires were Ohmic and the device was insensitive to all gases tested. The I-V curves of the sensor constructed from GaN decorated with Au nanoparticles were non-linear and exhibited a drop in conductivity of five orders of magnitude relative to bare GaN nanowire sensors. The Au nanoparticle decorated nanowires also exhibited electrical responses that were chemically selective. The sensor exhibited a nominal response to Ar and a slightly greater response to N2 relative to vacuum. A suppression of the conductivity of the Au-GaN device of 50% was observed upon exposure to methane. Both the drop in conductivity of the Au-GaN nanowire-based sensor, relative to bare GaN nanowires, and the response to methane are explained in terms of the formation of a depletion layer and an increase in the depletion layer width due to physisorption induced surface potentials. INTRODUCTION When constituted in the form of nanoparticles, gold displays remarkable reactivities providing researchers and engineers with new opportunities for chemical sensing applications. When exposed to gases, the bonds between the molecules and the Au nanoparticles can have purely physisorptive character making sensors based on gold nanoparticles highly reversible. Because the amount that has been adsorbed will vary with changing temperature or pressure, extremely useful gas sensors based on this reactivity can be constructed. In the majority of the sensors that use Au nanoparticles, the chemical response was measured by monitoring the intensity of the Au surface plasmons. This is not a very convenient mechanism by which to construct a sensor. In contrast, sensors based purely on electrical current-voltage (I-V) measurements can be much more compact, can be easily integrated with other electrical circuits, as well as, provide much more accurate quantitative data. The goals of this work have been to design sensitive and reversible gas-detecting devices, which at the same time are compact, inexpensive to construct and readily commercialized, as well as to develop a theoretical model of their electrical transport properties as a function of gas adsorption. EXPERIMENT The GaN nanowires were grown in a standard tubular flow furnace at atmospheric pressure. The substrates were 10 mm diameter c-axis oriented sapphire coated with a Ni seedlayer approximately 50 nm thick. The nitrogen source was ammonia and the Ga source consisted of a pellet of Ga. The growth temperature was 950o C. The sapphire substrate was placed approximately 2.5 cm from the Ga sou