Advanced Nested Coaxial Thin Film ZnO Nanostructures Synthesized by Atomic Layer Deposition for Improved Sensing Performance

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Abstract
We report a new synthesis method for multiple walled nested thin film nanostructures by combining hydrothermal growth methods with Atomic Layer Deposition (ALD) thin film technology and sacrificial films thereby increasing the surface-to-volume ratio to improve the sensing performance of novel ZnO gas sensors. Single crystal ZnO nanorods serve as the core of the nanostructure assembly and were synthesized hydrothermally on fine-grained ALD ZnO seed films. Subsequently the ZnO core nanotubes were coated with alternating sacrificial co-axial 3-D wraparound ALD Al2O3 films and ALD ZnO films. Basically, an ALD 3-D wrap-around Al2O3 sacrificial layer was coating the center nanorod to realize a nested coaxial ZnO thin film nanotube. To increase the surface-to-volume ratio of the nested multiple film nanostructure both front and backside of the nested coaxial ZnO films must be exposed by selectively removing the intermittent Al2O3 sacrificial films. The selective removal of the sacrificial films exposes the front and backside of the free-standing ZnO films for interacting with target gases during sensing operation while steadily increasing the surface-to-volume ratio. The sensing response of the novel ZnO gas sensor architecture with nested nanotubes achieved a maximum 150% enhancement at low temperature compared to a conventional ZnO nanorod sensor.

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