Studies Of Tungsten Oxide And Zinc Oxide Functional Thin Films And Their Applications For The Devices

Tungsten oxide and Zinc oxide used as important functional materials have beenwidely applied into chromic devices, microsensors, surface acoustic wave （SAW）devices and microfluidic devices. Based on the nanotechnology and advanceddeposition techniques, highly functionalized films with good crystallinity could begrown under the optimized conditions, which have a vital significance on theimprovement of the performance of the devices. The tungsten oxide nanocrystals withnovel microstructures have been controllably synthesized assisted by the dual-functionalreagents. The sputtered ZnO films have been used to fabricate the SAW devices usingthe microelectromechanical systems technology, and the high-performance SAWdevices have been successfully applied into the sensors and microfluidics.（1） Hexagonal tungsten oxide nanocrystals with microplate, urchin-like, andmicrospherical morphologies have been hydrothermally synthesized using ammoniumtartrate （AT）. The spherical microstructure changed to a microplate one and the crystalphase was partially transferred from hexagonal WO₃to orthorhombic WO₃when the pHvalue was reduced from1.08to0.6. Besides the adsorption action of the NH4+and Na+ions, the capping effect was reinforced by the hydrogen bonding from the tartrategroups with the increase in the concentration of AT. The optical analysis indicated thatthe bandgap increased with the decrease in the diameter of the hex-WO₃nanocrystals.The enhancement of the blue emission of the nanocrystals originates from the localintercalation of the Na+ions and the oxygen defects.（2） Hierarchical structured tungsten oxide （hydrate） nanocrystals have beensynthesized via hydrothermal route assisted by ammonium benzoate （AB）, whichshowed the products with various morphologies, including the flower-shape, sphericaland star-shape. Based on the analysis of the Fourier transform infrared spectra andobservations of the time-dependent growth, a new self-assembly growth mechanism hasbeen proposed for the formation of the different microstructures. The star-netted WO₃nanocrystals were applied as a sensitive layer for the relative humidity performed usinga ZnO/LiTaO₃Love mode SAW device. The frequency shift has been observed with a sensitive response to the change of the relative humidity from30%to90%, and thefrequency shift is linearly related to the relative humidity.（3） A new one-step method using the special holder has been developed tosputter-deposit the ZnO films. The typical columnar structure is inclined and the columninclination angle has been changed from0oto34o. X-ray diffraction analysis shows thatthe strains in the ZnO films decreased with substrate tilt angle. The opticalcharacterization indicates tha the band-gap energies decreased from3.19eV to3.07eV,which has a linear relation to the strain in the ZnO films.（4） The deposition process of ZnO/ultra-nanocrystal diamond （UNCD） film issimplified using the high target ultilisation sputtering technology and hot-filamentchemical vapor deposition technique, which allow the growth of the high-qualitymulti-layer films with strong （0002） texture and fine grains. The ZnO/UNCD films havebeen successfully applied to fabricate SAW devices and the transmission signalobtained for the Rayleigh mode SAW is better than those using the conventionalmagnetron sputtering technique. The phase velocities of the Rayleigh mode and Sezawamode SAWs show a significant thickness effect. The temperature coefficients of thefrequency （TCF） for the SAW devices have been measured and a value of less than-30ppm/oC for the TCF could be obtained when the thickness of the UNCD layer rangesfrom1.14to1.79μm. For the SAW device with ZnO thickness of7.68μm and UNCDthickness of1.06mm, the Rayleigh mode SAW achieves a coupling coefficient of5.2%.The low TCF of-23.4ppm/oC was obtained for the SAW devices with the2.72μm-thick ZnO and1.1μm-thick UNCD film.（5） Love mode surface acoustic wave ultraviolet sensors have been fabricatedusing sputtered ZnO films on36Y-cut LiTaO₃substrate. Results from the UV sensingmeasurements show that the amplitude response of the Love mode SAW UV sensoroperated at41.5MHz changed up to6.4dB and the fre-quency shift approached150kHz under a254nm illumination at the power density of350μW/cm2. Whereas under a365nm illumination at a power density of570μW/cm2the amplitude of thetransmission signal decreased only2.5dB with no significant frequency shift. Thefrequency hopping effect during the downshift and recovery periods was identified dueto the simultaneous interplays between the variations of the acoustic velocity andattenuation during the acoustic-electric interaction. （6） Surface acoustic wave has been excited from a SAW device with5.96μm-thickZnO and1.15μm-thick UNCD films, which is successfully used for the streaming,pumping, and jetting of microdroplets of different sizes. The critical behaviors ofdifferent microfluidic phenomena were analyzed at different powers applied to the IDTs.The microfluidic efficiencies of the different ZnO-based SAW devices wereinvestigated. Droplet pumping and jetting with different volumes were compared for theSAW devices with and without UNCD inter layers. Results show that the pumpingvelocities increase exponentially with the input power and those of the ZnO/UNCDdevices are much larger than those of the ZnO/Si SAW devices at the same power.Based on the accurate analysis of the jetting dynamics, the maximum jetting angle isobserved which depends on the droplet volume in the range1.5–8μLduring the jettingduration. The maximum jetting angle oscillates around69°for droplets larger than8μL.（7） Droplet streaming and nebulization on a36oY-cut LiTaO₃SAW device havebeen realized using the SH-SAW. The streaming inside the droplet is sensitive to theposition of the droplet, the coupling angle and droplet shape, which results in differentflow patterns. Droplet nebulization was enhanced with the increased aperture size of thedriven IDTs and mainly occurred in horizontal direction along two lateral sides of thedroplet, which is perpendicular to the designed SAW propagation direction. Theatomization duration increases with the droplet size, while it decreases with the drivenpower at a given drop size.