Table S1. Electrical properties obtained from ZTO-based TFTs treated at 500 °C and prepared from 0.3 M and 0.15 M precursor solutions. Table S2. Electrical properties obtained from ZTO-ZTO TFTs treated at temperatures from 300 °C to 500 °C. Table S3. Summary of the fitting parameters namely, layer thickness, SLD and interface width used to fit the experimental XRR spectra for each system studied. The metal oxide systems were simulated assuming a smooth and continuous surface. Table S4. Percentage contents and position of O 1s components for bilayer and trilayer films. Table S5. Atomic concentration (%) and calculated composition ratios at etch cycle five following XPS depth profiling of bilayer and trilayer films. Table S6. Electrical properties for L = 80 µm, 100 µm & 120 µm obtained before and after the ZTO-ZTO TFTs treated at 500 °C were exposed to IPA in the air. Table S7. Electrical properties for L = 80 μm, 100 μm & 120 μm obtained before and after the ZTO-ZTO TFTs treated at 500 °C were exposed to acetone in the air. Table S8. Electrical properties for L = 80 μm, 100 μm & 120 μm obtained before and after the ZTO-ZTO TFTs treated at 500 °C were exposed to toluene in the air. Table S9. Comparison of the TFT-based gas sensors’ performance. Figure S1. (a) Output characteristics at Vds = 0 to 40 V and Vgs = 0 to 40 V for ZTO-ZTO TFTs treated at 500 °C and (b) 450 °C. Transfer characteristics in the saturation regime at Vds = 40 V and Vgs = −20 to 40 V for the ZTO-ZTO TFTs treated at temperatures from 300 °C to 500 °C. Figure S2. Individual elemental mapping of the (a)-(e) ZTO-ZTO-ZnO and (f)-(j) ZTO-ZnO-ZTO films. Figure S3. AFM image (1 µm2 scan area) of (a) ZTO (b) ZTO-ZTO and (c) ZTO-ZnO films with calculated rms surface roughness. Figure S4. SEM images of the (a) ZTO-ZnO-ZTO and (b) ZTO-ZTO-ZnO thin films. (c) High magnification images of the ZTO-ZTO thin film and (d) calculated thickness from a cross-section. Figure S5. XPS wide scan of (a)-(b) ZTO-ZTO and (c)-(d) ZTO-ZnO films at etch cycles zero and twenty. Figure S6. XPS wide scan of (a)-(b) ZTO-ZTO-ZnO and (c)-(d) ZTO-ZnO-ZTO films at etch cycles zero and twenty. Figure S7. XPS depth profile of (a) ZTO-ZTO (b) ZTO-ZnO (c) ZTO-ZTO-ZnO and ZTO-ZnO-ZTO films. Figure S8. Transfer characteristics in the saturation regime at Vds = 40 V, Vgs = −40 to 40 V and L = 80 μm, 100 μm & 120 μm, obtained before and after the ZTO-ZTO TFTs treated at 500 °C were exposed separately to (a) acetone, (b) IPA and (c) toluene in the air. Figure S9. Transfer characteristics for the ZTO-ZTO TFTs treated at 500 °C in the saturation regime at Vds = 40 V, Vgs = −40 to 40 V and L = 80 μm following overnight exposure to (a) IPA, (b) acetone and (c) toluene vapors. Recovery with heat treatment is shown by the solid cyan lines. Figure S10. Transfer characteristics in the saturation regime at Vds = 40 V, Vgs = −40 to 40 V and L = 80 μm for the ZTO-ZnO TFTs treated at 500 °C following overnight exposure to (a) IPA and (b) acetone vapors. (c) Calculated responsivity. (d) Transient sensor response at fixed Vgs = 15 V, 25 V and 35 V in the linear and (e) saturation regimes. Figure S11. Bespoke testing chamber with device stage illustrations corresponding to source-drain electrode shadow mask.

M.U.C. thanks the Engineering and Physical Sciences Research Council (EPSRC) (New Investigator Award # EP/V037862/1 and Capital Equipment Grant EC/RF080422) for financial support. A.G.G. acknowledges funding from Spanish MCIN (MCIN/AEI/10.13039/501100011033) and “ERDF A way of making Europe” under project grant PID2022-139671OB-I00, as well from the Gobierno de Aragón (DGA) under projects T03_23R (Grupos de Investigación Reconocidos). G.M. gratefully acknowledges financial support from the project SASS AV0027152 from FEDER-FSE-2014-2020 and the project 16-IDEX-0001 CAP 20–25.

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