Zinc Oxide Nanostructure-Based Langasite Crystal Microbalance Ultraviolet Sensor

An ultraviolet (UV) sensor consisting of a langasite (La
₃
Ga
₅
SiO
₁₄
) thickness shear mode resonator and ZnO nanostructure sensing medium was fabricated. For the first time, ZnO nanostructures, including nanorods, polycrystalline film, and well-aligned nanowires, were grown directly on the active area of langasite crystal microbalance (LCM) by the thermal evaporation method without the aid of any transfer technique. As a key part of the growth process, zinc vapor trapping and two-stage temperature ramping were employed to achieve catalyst-free, self-seeding growth of ZnO nanowires at growth temperatures below 600°C. Under illumination of 365-nm UV light, a downshift in resonant frequency was observed for the LCMs due to acousto-electric interaction between photoinduced carriers and the shear waves. ZnO nanowires showed enhanced UV photosensitivity compared with that of nanorods or polycrystalline film. LCM with nanowires exhibited maximum shift of 157.5 Hz for several ON-OFF UV cycles and generated photocurrent of 235 μA for 5 V bias. Nanowire-based sensor also demonstrated excellent response and recovery times of 11 and 43 s, respectively. All the ZnO nanostructure-coated LCMs showed good stability and repeatability in UV sensing.