Observation of Field‐Effect Transistor Behavior at Self‐Organized Interfaces

Abstract

sion-like behavior behind the leading edge of the phase front was not captured. The assumption of a fixed electron density did not take into account the important fact that additional electrons are transferred to the polymer once charge neutrality is re-established locally by the arrival of the cation front. Also, the assumption that the diffusion constant is fixed, rather than a function of the oxidation level, is not valid, as shown by the striking differences in shape and velocity during the first-ever reduction. The next stage in our modeling will take these effects into account.While the experimental and modeling curves in Figure 4 are strikingly similar, it is important to keep in mind that the experimental curves are color intensity profiles. In order to convert the intensities in the images into ion concentrations (the quantity in our model), the Faradaic charge consumed by the PPy needs to be determined, and these measurements are under way. Once we knowẼ in the film, we will extract the mobility from the data in Figure 3. Future studies will also more closely examine the diffusion terms in the model.In conclusion, we have introduced a simple experimental tool that directly measures ion transport in electrochromic materials and have presented preliminary results on the reduction process in PPy(DBS). We have shown that there are different rate-limiting processes at different potentials, that migration plays a critical role in ion transport, and that a simple model can account for much of the behavior. Initial physics-based modeling has correctly predicted the existence of the phase fronts and captured much of their shapes. Future experimental work will include quantification of the coefficients in the equations as well as studies of the oxidation process and correlations between ion transport and volume change. The next stages in modeling will take additional physics into account. ExperimentalThe fabrication process for the devices was as follows. Over an oxidized silicon wafer, an adhesion layer of Cr (100 thick) was deposited by either electron-beam evaporation or sputtering, followed immediately by a layer of Au (3000 thick) that served as the working electrode. A layer of PPy(DBS) was electrochemically deposited from an aqueous solution of 0.1 M NaDBS and 0.1 M pyrrole (pH 7) at a constant potential of 0.47 V (versus Ag/AgCl) with an EcoChemie pgstat30 to a thickness of 0.3 lm, at which the color changes are very apparent. All thicknesses were measured by profilometery (Tencor Alphastep 500). A 2 lm thick layer of a photosensitive polymer (SU8-2000, MicroChem Corp.) was photolithographically patterned over the PPy into the shape of a rectangular stripe following the manufacturer's directions. The PPy that was not covered by SU8 was removed by dry etching in an oxygen plasma [22], leaving a stripe of PPy covered by SU8 on the electrode surface.All cycling experiments were performed under a nitrogen atmosphere to prevent oxygen doping. The electrochromic response was recorded with a digital camera (Niko...

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