SEM image of the ITO thin film. The film is granular with typical grain diameters of about .
(Color online) Relative resistance (resistance of thin film–gas system divided by bare film resistance) measured using the two-probe resistance technique (on the left) and the four-probe resistance technique (on the right). Evidently, the relative resistance is nearly the same in both cases (the difference being only about 2.4%).
The experimental setup is illustrated where (a) the thin film is a slab with dimensions (the film thickness). The current runs parallel to the length . (b) The thin film with a uniform layer of adsorbed gas on the top surface. The gas has dimensions, , where is the time-dependent thickness of the adsorbed gas. (c) Geometry of the composite model where the adsorbed gas layer is assumed to consist of gas islands residing on the top surface of the thin film.
(Color online) Experimental reduced resistance (i.e., resistance of system/isolated film resistance) vs time for the thin film–gas (ITO-acetylene, methane, and sulfur dioxide) system (heavier, curved lines). The dimensions of the film were with thickness . Shown also are the results of the resistor network model (the straight lines). The parameter with units of is the resistor network fitting parameter.
(Color online) Same as Fig. 3 but comparison is made to the DEMA/DMGT models, which gave nearly identical results. The heavier curved lines are the experimental data. The parameters and the latter has units of , are the fitting parameters in these models, as discussed in the text.
(Color online) Top: Reduced resistance vs time assuming various choices of the conductivity ratio (, 1, 1.5, and ). For comparison the experimental results (heavy curve) are shown for acetylene and this covers up the DMGT/DEMA curve where . The fitting parameter for acetylene was used for all the curves. Also shown are the DEMA results with both in two and three dimensions. Bottom: or reduced resistance vs time is graphed. Notice the presence of the percolation threshold for the DEMA models at values of the gas filling fraction of (two dimensions) and (three dimensions).
Reduced resistance for thin film–acetylene gas system measured ten times in succession, with a delay between measurements. For each curve, the gas source was turned on for only and then turned off. The vertical lines in the figure indicate when the gas source was turned on and off. The ten measurements have been placed on the same curve so that at they all begin with reduced resistance of one. The rise in resistance after its minimum is presumably due to evaporation of the gas off the film surface. This graph demonstrates the reproducibility of the resistance of the thin film–gas system.
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