## 3 Results and Discussion  
### Physical Characteristics of the ZnO Nanords and Nested Nanotubes  
The FE-SEM images of Figure 6a-e highlight the resulting morphology at the various stages of the entire sequence of the nested ZnO thin-film nanorod/nanotube gas sensor synthesis process. Figure 6a shows the ZnO nanorods grown by the hydrothermal method, clearly revealing single crystalline ZnO nanorods with a crystallographic hexagonal cross-section structure. Following 3D wrap-around coatings with an ALD Al\({}_{2}\)O\({}_{3}\) sacrificial film and on top of it another ZnO ALD film coating, the surface of the center ZnO nanorods was covered and sealed with a top ZnO covering, as shown in Figure 6b. The PIPS process was utilized to remove the top cover and to expose the Al\({}_{2}\)O\({}_{3}\) sacrificial layer from the top as shown in Figure 6c,d. Afterwards, the sacrificial alumina layer was preferentially removed by wet chemical dissolution in an NaOH alkali solution. This constitutes a key process step in our novel sensor design and isolated the center hydrothermal ZnO nanotubes. The creation of an empty annular gap remaining after the removal of the sacrificial layer exposed two additional reaction-sensing surfaces, as shown in Figure 6e. The higher magnification micrograph of Figure 6e highlights the nanotube surfaces exhibiting a granular structure which is indicative of polycrystalline material. Figure 6f shows a cross-sectional view of the ZnO nanorod/nanotube gas sensors, revealing an average nanorod height of \(\sim\)750 nm. Figure 5: Schematic diagram of the improved 2 nd-generation gas sensor testing system. For further physical characterization of the ALD-synthesized ZnO sensing films, we have conducted X-ray diffraction (XRD) analysis, energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). Figure 7a,b provides X-ray diffraction (XRD) results showing that ZnO films synthesized by ALD technology are polycrystalline with a preferred growth along the (002) plane or c-axis of the hexagonal wurtzite structure. Figure 7b demonstrates the progressive improvement of the full width at half maximum (FWHM), with increasing film thickness indicated by the number of ALD growth cycles. To investigate potential sample contamination, we ran energy-dispersive X-ray spectroscopy (EDS) for elemental chemical analysis using a scanning electron microscope (SEM) for our ALD ZnO film on Si substrate. The EDS of Figure 8 demonstrates that only Zn, O and Si signals were obtained, with no other detectable contaminants. This confirms the well-documented fact that atomic layer deposition (ALD) technology always provides undoped intrinsic semiconducting ZnO films without any detectable contamination based on the semiconductor-grade ALD precursors used to synthesize the ZnO films. The transmission electron microscopy (TEM) analysis corroborates the random polycrystalline nature of our ALD ZnO nanotubes. Figure 9a shows TEM micrographs of several detached ALD ZnO nanotubes harvested for TEM investigation. The grain structure of the polycrystalline ZnO nanotube walls can be clearly seen. Figure 9b zooms in for a high-resolution HR-TEM section of a ZnO nanotube wall to provide a detailed view of individual ZnO grains and the accompanying grain boundaries. The TEM micrographs provide a good summary of the physical properties of our ALD-synthesized nested nanotube films used for ethanol vapor sensing. Figure 6: FE-SEM images of the novel nested ZnO thin-film nanorod/nanotube coaxial structure synthesis process. (**a**) Intrinsic hydrothermal ZnO nanorods with single-crystal hexagonal wurtzite structure. (**b**) Polycrystalline wrap-around ALD coating on ZnO nanorods after coating with an Al\({}_{2}\)O\({}_{3}\) sacrificial film followed by ZnO thin films. (**c**,**d**) Reveals top cover removed by PIPS ion milling to expose the sacrificial layer. (**e**) Finally, the sacrificial layer has been preferentially etched away by a pH 11 NaOH solution.