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| Author(s): |
Martin Schwellberger Barbosa
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | Araraquara. 2019-04-29. |
| Institution: | Universidade Estadual Paulista (Unesp). Instituto de Química. Araraquara |
| Defense date: | 2019-04-15 |
| Advisor: | Marcelo Ornaghi Orlandi |
| Abstract | |
This work was focused in tungsten oxide materials interfaced with multiple electrolytes for applications in electrolyte gated thin-film transistors (EG-TFTs). The materials presented different crystalline phases (hexagonal, monoclinic, amorphous) and different morphologies (granular, nanofibers, nanoplates and smooth films). The electrolytes considered included ion gels and ionic liquids using the imidazolium cation.The main motivation for this study was the interest for the developments in the field of iontronics, with devices that control the proprieties of semiconductors through electrochemical phenomena. The approach of using different materials was aimed to improve the devices using the materials intrinsic proprieties. The materials were synthetized using wet-chemical routes (sol-gel, hydrothermal) and evaporation (RF sputtering). Multiple characterizations techniques were used including scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry and surface area analysis (BET). A systematic study was performed using the granular, nanofiber and nanoplate materials interfaced with [EMIM][TFSI]SOS and [EMIM][TFSI]SMS ion gels. Cyclic voltammetry characterization indicated electrochemical doping mechanisms with the reduction of W6+ species with intercalation of protons from water in the electrolyte phase. A secondary direct chemical reduction mechanism intermediated by H2 electrochemical evolution was also observed. Both phenomena were attributed to the WO3 conductivity modulation, as increases in the value of the conductivity were correlated with the voltages where reduction peaks were observed. The materials of different morphologies presented different electrochemical peaks patterns that were attributed to the ionic arrangements and different modulation levels that were attributed to the different current conduction between particles. The materials with nanoplate morphology exhibited the best on/off ratio and electronic mobility, with figures-ofmerit reaching values typical of efficient electrolyte gated devices. The transistors were characterized under different atmospheric conditions containing H2, N2 and O2. This was performed in order to understand the direct chemical reduction mechanism and to test the devices for potential hydrogen sensor applications. It was observed that the films went through irreversible doping processes in the presence of H2 that were facilitated by the electrochemical bias and Pt electrodes. The reversibility of the doping processes was observed in the presence of O2. Lastly, the electrostatic mechanisms were studied by atomic force microscopy in-operando, using force distance profiling, in EG-TFT devices using the ionic liquid [EMIM][TFSI]. Results indicated that the ion layering is well organized even on the relatively rough film composed of WO3 nanoparticles. The thickness of the first ion layer was attributed to the distance of a single [EMIM]+ cation. It was observed that this layer could be shifted to a position closer to the surface with electrochemical bias, especially for measurements in the top of the grains. Overall the results in this work reinforce the potential applications of WO3 materials interfaced with ionic liquids and ion gels for iontronic applications and highlights how doping mechanisms can be used strategically for the optimization of the devices. (AU) | |
| FAPESP's process: | 14/27079-9 - Electrolyte-Gated transistors based on WO3 thin films: influence of the morphology and structure of the films on the device performance |
| Grantee: | Martin Schwellberger Barbosa |
| Support Opportunities: | Scholarships in Brazil - Doctorate |