Advanced Nested Coaxial Thin-Film ZnO Nanostructures Synthesized by Atomic Layer Deposition for Improved Sensing Performance  
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2 DUD-Applied Research Center at Thomas Jefferson National Accelerator Laboratories,  
12050 Jefferson Avenue, Suite 721, Newport News, VA 23606, USA; [EMAIL_ADDRESS]  
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Herein, coaxial nested ALD thin-film nanotube structures were synthesized to improve the sensing performance of ZnO nanorods to ethanol vapor. The primary objective of the nested coaxial ZnO nanotube design is to provide two additional reaction surfaces compared to the conventional ZnO nanorod gas sensors for each added wrap-around sacrificial layer surrounding the anchoring center ZnO nanorod. Basically, the nested coaxial ZnO nanotube increases the surface-to-volume ratio further. An ALD 3D wrap-around Al2O3 sacrificial layer was coating each center nanorod to realize a nested coaxial nanotube consisting of free-standing ZnO thin films. The sacrificial layer was selectively removed by immersion and dissolution in a sodium hydroxide (NaOH) solution due to the different solubility ranges of Al2O3 and ZnO [10]. Figure 1 depicts a 3D schematic drawing of the concept of the coaxial nested tube-in-tube ZnO thin-film nanostructure device shown after selective removal of the sacrificial Al2O3 layer, which exposes two extra sensing surfaces separating the outer ZnO nanotube from the center ZnO nanorods. These two additional ZnO sensing surfaces fabricated by this novel process design method serve to increase the surface-to-volume ratio and to improve the sensing response of the gas sensor. Due to the average diameter size of ZnO nanorods and the available distance between nanorods synthesized by hydrothermal method, the thickness of the added sacrificial layer and the subsequent ZnO coating are restricted to approximately 20 nm. Therefore, ALD technology was applied to coat the core anchoring ZnO nanorods with 3D wrap-around Al2O3 sacrificial layers alternated with ALD ZnO layers with precisely controlled deposition thickness on the nanometer scale. To expose the sacrificial layer for wet etching, the precision ion polishing system (PIPS) was utilized to remove the accumulated top film covers composed of the ALD Al2O3 sacrificial layer and the ZnO thin films deposition.  
For the study of the sensing performance of the novel ZnO nanotube gas sensor design to ethanol vapor concentration detection, a newly upgraded gas sensor testing system was designed and custom-built with two mass-flow controllers (MFCs) to precisely control the concentration of ethanol vapor input. We have investigated the sensing performance of the novel ZnO nanorod gas sensor design and benchmarked our nested coaxial nanotube at different temperatures with saturation-level ethanol vapor and at optimum working temperature as a function of various concentrations of ethanol vapor.