## 4 Conclusions  
A novel nested coaxial ZnO thin-film nanorod/nanotube gas sensor architecture was fabricated by an integrated process combining hydrothermal growth of single-crystal nanorod center anchors, followed by 3D wrap-around coatings of sacrificial ALD alumina films alternating with ALD ZnO polycrystalline films. This novel nested ZnO thin-film gas sensor architecture was examined by a newly upgraded gas sensor testing system connected with digital mass flow controllers for precise target vapor concentration. At the start, ZnO nanorods were synthesized by the hydrothermal method on a fine-grain polycrystalline ZnO seed layer deposited by ALD to anchor the final complex nested sensor nanostructure. An ALD Al\({}_{2}\)O\({}_{3}\) sacrificial layer was a key concept to create nested coaxial ZnO thin-film nanotubes. After removing accumulated ALD films from the top surface by PIPS, the Al\({}_{2}\)O\({}_{3}\) sacrificial layer was preferentially removed by wet chemical etching in a NaOH solution with pH 11. Consequently, a nested ZnO nanorod/nanotube structure was created with two additional reaction surfaces, which provide a significant increase in surface-to-volume ratio. In principle, our approach is amenable to adding more than one sacrificial layer and additional ZnO sensing films. Theoretically, every additional annular sacrificial layer will add two new ZnO reaction surfaces available for sensing after wet chemical etch removal. With increasing reaction surfaces and sites, the sensing response of nested ZnO thin-film nanorod gas sensors achieved a significant response enhancement, employing nested coaxial ZnO nanotubes with empty annular sacrificial layers. In particular, the saturation level was improved from 800 ppm to 1000 ppm. Compared to conventional ZnO nanorod gas sensor benchmarks, our novel nested ZnO nanotube gas sensors achieved a sensing response improvement of roughly 150% at a low temperature of 25 \({}^{\circ}\)C. In summary, this work demonstrates that our nanotechnology process design of creating empty annular gaps surrounding free-standing nested ZnO nanotubes is effective in increasing the surface-to-volume ratio and significantly improves the sensing response of ZnO solid-state gas sensors. Since the hydrothermal growth direction of the ZnO nanorods did not result in all nanorods aligned vertically along the [001] direction, PIPS could only remove the top cover of the highest nanorods, missing the inclined rods falling short on height. The formation ratio of successfully opened nested nanotube structures was around 70%, while 30% of the nanorods failed to have the ALD capping layer removed by ion milling because they did not reach the required rod height of 750 nm. In the future, the growth method will be optimized to achieve all vertically aligned ZnO nanorods, in which case the formation ratio will be improved to 100%. In addition, production reliability and repeatability will be optimized in the future for commercialization. This study is focused on the investigation of nested ZnO nanotube sensor designs based on DC resistance measurements with ohmic contacts to the ZnO samples. In general, this nested nanotube technology with sacrificial layers represents a pathway for further improvement of the sensitivity response and a higher saturation level, thereby enhancing the application potential of ZnO gas sensors for volatile organic compounds. Data curation; Formal analysis; experimental Investigation; methodology; writing-original draft, P.L. ; assisting experimental Investigation, L.S.Z. ; conceptualization, investigation, data curation, formal analysis and writing-original draft, K.Z. ; supervision, funding acquisition, project administration; resources; conceptualization, validation, writing & editing original draft, H.B. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.