Self-seeded ZnO nanowires on SAW sensors: on-chip growth and methods for sensitivity enhancement

Abstract

SAW devices can be realized as sensors by coating a suitable sensing medium on the acoustic path of the device to detect various physical and chemical measurands. ZnO is a compound widely employed as sensing and wave propagation medium in SAW devices. In particular, high aspect ratio ZnO nanowires are a promising candidate as sensing medium for SAW gas sensors and UV detectors because of its favorable sensing properties, high surface-to-volume ratio and variety of growth techniques. In this thesis, high aspect ratio catalyst-free ZnO nanowires were directly grown as sensing medium on the surface of SAW devices for the first time through the self-seeding thermal evaporation method. Initially, growth of ZnO nanowires were conducted on blank 128°-YX LiNbO3 substrates and the alignment nature of the as-grown nanowires was studied. As-grown ZnO nanowires exhibited a crisscross aligned growth pattern due to step bunching of the polished LiNbO3 surface during the nanowire growth process. However, it was experimentally observed that nanoscale etching of the LiNbO3 surface could realize growth of well-aligned ZnO nanowires because of the added surface roughness. The thermal evaporation method was then adopted for direct growth of ZnO nanowires on SAW devices. High temperature stable Rayleigh mode SAW resonators were designed, and fabricated using Pt/Ti electrodes on 128°-YX LiNbO3 substrates to facilitate the on-chip nanowire growth conducted at 620°C. As a key part of the on-chip growth process, a mask and sleeve setup was used to protect one-half of the IDT aperture during the nanowire growth process to enhance post-growth device functionality. It was observed that the proposed device had approximately 3 times higher sensitivity to UV light of 365 nm wavelength as compared to the only previous report where 1-D ZnO nanostructures were directly grown on a LiNbO3 based SAW device using solution grown method for UV detection purposes. Further, FEM simulations were conducted to study the prospects of coupled resonance phenomenon towards enhancing the sensitivity of the ZnO nanowire integrated SAW device to conductivity changes in the sensing medium. It was observed that sensitivity increases by more than 200 times when the ZnO nanowire heights are adjusted such that the resonance frequency of the nanowires approaches the original resonance frequency of the SAW device. FEM simulations were also used to investigate the possibility of switching from Rayleigh waves to higher order Sezawa waves propagating on layered SAW devices to enhance the mass loading sensitivity of SAW sensors. As an example, it was shown that Sezawa waves propagating on a ZnO/Si layered SAW device has 2 to 8 times higher mass loading sensitivity as compared to Rayleigh waves propagating on the same device. The use of the self-seeding thermal evaporation method instead of the conventionally used solution grown method opens up possibilities towards scalable growth of catalyst-free ZnO nanowires without fusion among their roots which increases the surface area of the sensing medium. Moreover, on-chip growth of ZnO nanowires in the absence of a liquid medium has future prospects for in situ measurement of the SAW device response.