Gold Nanowire Thermophones
Ultra-long (mm scale) polycrystalline gold nanowires were investigated for their ability to perform as thermophones, or thermoacoustic sound emitters. Arrays of ~4000 linear gold nanowires are fabricated at 5 um pitch on glass surfaces using lithographically patterned nanowire electrodeposition (LPNE). The properties of nanowire arrays for generating sound are evaluated as a function of frequency (from 5 - 120 kHz), angle from the plane of the nanowires, input power (from 0.30 - 2.5 W) and the width of the nanowires in the array (from 270 to 500 nm.) Classical theory for thermophones based upon metal films accurately predicts the measured properties of these gold nanowire arrays. Angular "nodes" for the off-axis sound pressure level (SPL) versus frequency data, predicted by the directivity factor, are faithfully reproduced by these nanowire arrays. The maximum efficiency of these arrays (~10^-10 at 25 kHz), the power dependence, and the frequency dependence is independent of the lateral dimensions of these wires over the range from 270 to 500 nm.