ZnO is a well-known Metal Oxide Semiconductor (MOS) material used in gas sensor applications due to its good electrical property, wide band gape of 3.37 eV, ~60 meV exciton binding energy, low cost, and high mechanical stability. Especially, ZnO nanorod structure is widely investigated in gas detection due to its high surface-to-volume ratio, nontoxicity, and suitable doping. Numerous methods have been applied to enhance the sensing performance of ZnO nanostructure gas sensors. In this research, a coaxial nanorod/nanotube structure was fabricated to improve the sensing response of ZnO nanorods to ethanol vapor, as shown in Figure 1. Also, due to additional oxygen vacancies, Al doped ZnO (AZO) thin films were deposited on the surface of ZnO nanorod/nanotube to further enhance the sensitivity of ZnO nanorods.

In this study, the ZnO seed layers were deposited on Si wafer by Atomic Layer Deposition (ALD) with diethylzinc ((C₂H₅)₂Zn) and DI water as precursors of zinc and oxygen. Physical vapor deposition (PVD) was carried out to synthesize aligned ZnO nanorods shown in Figure 1(a). There was a layer of Al₂O₃ as sacrificial spacer deposited on the surface of ZnO nanorods by ALD with TMA (Al₂(CH₃)₆) and DI water as precursors shown in Figure 1(b). An additional layer of ZnO thin film was synthesized by ALD shown in Figure 1(c) and Figure 2(a). To expose the Al₂O₃ sacrificial spacer layer, Precision Ion Polishing System (PIPS) was introduced to remove the top cover of synthesized ZnO nanorods shown in Figure 1(d) and Figure 2(b). Then Sodium hydroxide (NaOH) was applied to remove the Al₂O₃ layer to form ZnO nanorod/nanotube structure as shown in Figure 1(e) and Figure 2(c). As shown in Figure 1(f) and Figure 2(d), AZO thin films were deposited on the surface of ZnO nanorod/nanotube by ALD to further enhance its sensing performance to ethanol vapor. Field emission scanning electron microscopy (FE-SEM) to investigate the morphology of ZnO nanorod/nanotube before and after coated with AZO thin films by ALD. The crystal structures of ZnO nanorod/nanotube before and after coated with AZO thin films were analyzed by X-ray diffraction (XRD).

To investigate the sensing performance of ZnO nanorod/nanotube to ethanol vapor, a gas sensor testing system was designed and built with a sealed reaction chamber, ethanol vapor generator, testing circuit, and controlled heating system. The physical change of ZnO nanorods to ethanol vapor is resistance. Therefore, the resistance of ZnO nanorod/nanotube was recorded by the home-built sensor testing system based on Labview and CompactRio system from National Instruments. ZnO nanorod/nanotube samples before and after coated with AZO thin films were tested under various volumes of ethanol vapor at different temperatures to analyze their sensing responses. A sensing response comparison was made between ZnO nanorods and ZnO nanorod/nanotube.