Estudo de desempenho e mecanismos de operação em transistors de interface eletrolítica baseados em óxido de tungstênio
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Data
2019-04-15
Autores
Barbosa, Martin Schwellberger
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Universidade Estadual Paulista (Unesp)
Resumo
Neste trabalho estudou-se sistemas de materiais derivados de trióxido de tungstênio (WO3) com diferentes eletrólitos para aplicações em transistores de filme fino com interface eletrolítica (EG-TFTs). Os materiais apresentaram diversas fases cristalinas (hexagonal, monoclínica, amorfa) e diferentes morfologias (granular, nanofibras, nanoplacas, nanopartículas e filmes lisos). Os principais eletrólitos utilizados foram líquidos iônicos e géis iônicos, contendo o cátion imidazolium. A motivação deste trabalho foi contribuir para o desenvolvimento do campo da iontrônica, ou seja, dispositivos em que a propriedade de um semicondutor é controlada por meio de fenômenos eletroquímicos. O uso de diferentes morfologias foi uma estratégia para controlar o desempenho dos dispositivos por meio das suas propriedades intrínsecas. Os materiais foram sintetizados por rotas químicas úmidas (sol-gel e hidrotermal) e de evaporação (RF sputtering). Caracterizações incluíram microscopia eletrônica, difração de raios X, espectroscopia de fotoelétrons, espectrometria de retroespalhamento Rutherford e área superficial (BET). Um estudo sistemático de operação de dispositivos transistors foi realizado utilizando os materiais granulares, nanofibras e nanoplacas com dois géis iônicos ([EMIM][TFSI]SOS e [EMIM][TFSI]SMS). Resultados de voltametria cíclica indicaram mecanismos eletroquímicos de dopagem com redução das espécies de W6+ com intercalação de prótons gerados a partir de resquícios de água. Além disso foi atribuído um mecanismo de dopagem química direta, com evolução eletroquímica de H2. Esses processos tiveram influência na modulação da condutividade do canal durante a operação dos dispositivos observando-se correlação entre os potenciais em que ocorriam aumentos de condutividade e os potenciais dos picos associados à redução. As diferentes morfologias apresentaram diferentes padrões de oxidoredução eletroquímica, que foram atribuídos ao arranjo iônico, e diferentes níveis de modulação, correspondentes à diferença na condução elétrica entre as partículas. Materiais com morfologia de nanoplacas tiveram maior desempenho de modulação de corrente e mobilidade de portadores, atingindo valores típicos de dispositivos de arquitetura de contato eletrolítico de alta eficiência. Os transistors foram também caracterizados em atmosferas com composição variável contendo H2, O2 e N2. Os objetivos foram elucidar os mecanismos de dopagem química e propor uma aplicação desses sistemas como sensores de gases. Observou-se que, principalmente principalmente com eletrodos de Pt, ocorreu uma dopagem irreversível dos filmes de WO3 com H2 por intermédio da interface eletrolítica, resultando na modulação da condutividade do semicondutor. A reversibilidade dessa dopagem ocorreu na presença de O2. Por fim, os mecanismos eletrostáticos dos transistors foram estudados por meio de perfis força-distância por microscopia de força atômica inoperando em EG-TFTs utilizando o liquido iônico [EMIM][TFSI]. Esses resultados indicaram que a camada iônica era altamente organizada mesmo que a superfície do WO3 fosse composta por nanopartículas. A espessura da primeira camada correspondeu a aproximadamente a dimensão de um cátion [EMIM]+. Além disso, essa camada foi deslocada para uma região mais próxima da superfície com a aplicação de campo elétrico, principalmente nas regiões centrais dos grãos. Os resultados obtidos neste trabalho reforçam a potencialidade da interface WO3 com líquidos iônicos para aplicações iontrônicas destacando como os mecanismos de dopagem podem ser usados de maneira estratégica para obter dispositivos com melhor desempenho.
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.
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.
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Palavras-chave
Electrolyte-gated transistor, Óxido de tungstênio, Eletroquímica, Sensor de hidrogênio, Tungsten oxide, Electrochemistry, Hydrogen sensor, Transistor de contato eletrolítico