Fabrication and Characterization of ZnO-based nanostructures and their applications on ultra violet humidity and pH sensors

  • 蔡 傅守

Student thesis: Doctoral Thesis


The dissertation aims at the preparation of ZnO-based nanostructure and theirs applications on UV humidity and pH sensing fields Comparison with traditional metal oxide-based film type sensing element nanostructures could provide larger surface to volume ratio quantum size effect and higher adsorption/desorption area In addition hydrothermal growth of one-dimensional (1-D) ZnO nanowire-based device possesses a much higher surface to volume ratio than thin film-based device It is very beneficial to the adsorption of sensing goal The related electric characteristics sensing performance and dynamic behaviors of these sensors measured under different UV light power of 0~6 mW/cm2 various relative humidity environments of 12~96% and pH value within range of 2~12 are presented and discussed At first in the second chapter of the present desseration through the use of n-ZnO NWs and oblique deposition p-SnO-film nano hetero junction for UV detection are studied and demonstrated Nano-heterojunction arrays (NHAs) were formed via the oblique-angle sputtering deposition of p-type tin monoxide onto vertically aligned ZnO NWs grown by hydrothermal growth (HTG) The prepared SnO/ZnO NW NHAs with different SnO thicknesses (50~1000 nm) have rectifying current-voltage characteristics and superior response to UV (254 nm) light illumination with UV sensitivity (IUV/Idark) as high as 8 5 and rise/fall time of ~5/20 s were obtained In the third chapter the use of a series connection of lateral ZnO NWs for nano optoelectronic sensors prepared by HTG is proposed A surface roughening using dilute HCl solution to promote UV response and antireflection properties is investigated and experimental results are demonstrated An UV (366 nm) sensing response (IUV/Idark) as high as 2 4 and response/recovery time as low as 10/32 s were obtained In the fourth chapter the use of laterally oriented ZnO NWs grown by a HTG method for relative-humidity (RH) sensing devices at room temperature (RT) is demonstrated A humidity sensor based on laterally oriented ZnO NWs with a sensing response (R12%/R96%) as high as 2 2 was obtained at RT The comparably good sensitivity of the proposed humidity sensors is attributed to the full utilization of the entire NW surface because the current path is aligned with the orientation of the bridged lateral ZnO NWs during the humidity sensing application Response/recovery time was estimated to be approximately 33 min/31 s and 27 min/56 s for the cases of 96-12-96% RH and 96-33-96% RH respectively In the fifth chapter through the use of prepared ZnO NSs for RH sensing devices at RT are proposed and investigated A sensing response (R12%/R96%) at RT of as high as 220 good sensing linearity in the range of 12~96% RH a fast sensing response time of as low as 600 s and a recovery time of 3 s are achieved Compared to ZnO NW-based humidity sensors a 100-fold improvement in the sensing response at RT was obtained which is mainly attributed to the ZnO NSs having a much higher surface-to-volume ratio and a porous-like surface Furthermore in the sixth chapter ZnO nanosheets (NSs) were prepared to serve as sensing element for extended-gate (EG) field effect transistor (EG-FET) pH Sensors A closed linear relationshipd between IDS (@ VDS= 5 V and Vref= 3 V) and pH levels in the range of 2-12 with a slope of about 0 026 mA/pH is obtained The sensitivity of the EG-FET pH sensors with ZnO NSs is as high as 25 mV/pH (@ IDS= 0 1 mA and VDS= 0 2 V) which is attributed to the sheet morphology providing a larger sensing area and increased oxygen-related binding sites during pH sensing There are still many drawbacks of the ZnO-based reference electrode such as its worse acid alkali tolerant low sensing response and low stability Metal oxide sensing nanostructure with higher acid alkali tolerant is still needed and further researched In the seventh chapter we used the facile HTG method for the growth of vertical TiO2 NWs TiO2 is much suitable than ZnO for pH sensing application due to its higher acid alkali tolerant The prepared TiO2 NWs as the EG electrode in EG-FET is proposed for pH sensing A closed linear relationshipd between IDS (@ VDS= 5 V and Vref= 3 V) and pH levels in the range of 2-12 with a slope of about 0 063 mA/pH is obtained The sensitivity of the EG-FET pH sensors with TiO2 NWs is as high as 62 mV/pH (@ IDS= 0 1 mA and VDS= 0 2 V) The sensing response has been improved about 2 times of the ZnO NSs-based EGFET pH sensors (25 mV/pH) which are attributed to the TiO2 with higher acid alkali tolerant than ZnO material Finally in the eighth chapter conclusions of a series of high-performance metal oxide semiconductor based UV humidity and pH sensing properties are made The improved performance of several types of sensing element including the ZnO NWs/SnO diode the serious connected roughed laterally grown ZnO NWs the laterally and vertically grown ZnO NSs and the vertically grown TiO2 NWs are summarized Improvement in sensing ability through the use of appropriate nanostructure and device structure is also highlighted Suggestions for further enhancing the sensing performance and improving the long-term operating stability continuously are proposed It is expected that the proposed metal oxide nanostructure based sensors with a suitable fabrication processes tuning could be an effective mean for future UV humidity pH and further bio sensing applications
Date of Award2014 Jan 6
Original languageEnglish
SupervisorShui-Jinn Wang (Supervisor)

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