Research On The Fabrication And Applications Of Flexible Devices Based On Vanadium Dioxide Thin Films

In recent years,with the development of advanced artificial intelligence and wearable equipments,flexible and stretchable electronic devices have been widely employed in information,energy,medical and military fields.However,most of the large-scale integrated flexible circuits and flexible electronic devices are based on organic materials.Compared to organic materials,two-dimensional inorganic materials present excellent electrical,optical,thermal,mechanical and magnetic properties.With the progress of transfer printing technology,two-dimensional inorganic materials,such as silicon?Si?,gallium arsenide?GaAs?and lead zirconate titanate?PZT?are exploited in flexible electronic devices.As a consequence,high-performance inorganic flexible electronics would facilitate the replacement of conventional rigid electronics in the future.Due to the excellent metal-insulating phase transition property and the high temperature coefficient of resistance than other materials,vanadium dioxide?VO₂?thin films have been widely used in switching devices and infrared imaging.However,the research of flexible electronic devices based on VO₂ thin films remained in early stages.Hence,the fabrication of flexible devices based on VO₂ thin films used in electrical,thermal and mechanical fields is of great importance.In this dissertation,high-quality VO₂ thin films were deposited on SiO₂/Si substrates by polymer-assisted deposition?PAD?.Flexible electronic devices such as breath sensor,infrared sensor and temperature-mechanical sensor were fabricated in sandwich-like multi-layer structure by transferred printing VO₂ thin films on flexible polydimethylsiloxane?PDMS?substrates and flexible polyimide?PI?or polyethylene terephthalate?PET?substrates.These flexible devices demonstrated fast response property,high sensitivity and high reliability in strain tuning,breath sensing,infrared detecting and temperature-mechanical sensing when in use.In the study of VO₂ thin films grown by PAD,the thermodynamic process of depositon was analyzed and the VO₂ thin films were annealed in the reducing atmosphere at 780 K on SiO₂/Si substrates.The metal-insulator transition?MIT?temperature and insulator-metal transition?IMT?temperature of VO₂ thin films on SiO₂/Si substrates are 328 K and 360 K,respectively.The mechanical process of transfer printing was analyzed and the VO₂ thin films with a thickness of 130 nm were successfully transferred on PDMS substrates.The strain tuning of VO₂ thin films through PDMS substrates were achived during the thermal cycles from 300 K to 370 K.By changing the Young's modulus of the PDMS substrates,different tensile strains were applied on VO₂ thin films.As a result,the IMT temperature and MIT temperature were tuned about 7 K and 5 K,respectively.The strain tuning mechanisim was analyzed using slipping model and thermal expansion theory.Attributed to the PI/VO₂/PDMS structure,the flexible breath sensor can avoid the interference of external strain by PI layer.The response time and recovery time for exhalation and inhalation were less than 0.5 seconds.The results of long-term monitoring of different breath depths and different breath rates at different environment temperatures indicate that the VO₂ based flexible breath sensor is feasible in biomedical applications.Futhermore,flexible infrared sensor based on VO₂/PET structure was fabricated to act as an infrared detector to detect damages in infrared vision.The response time is less than 0.5 seconds for the detection of 850 nm infrared with light intensity of 0.3 mW/mm².The temperature-mechanical dual-parameter sensor was based on PET/VO₂/PDMS structure.The ultrahigh mechanical sensitivity to strains in the range of 0⁰.1%with a gauge factor over 400 was attributed to the disconnection-reconnection process of the zip-like nanoscale cracks under strain or vibration.Conbining the micro-thermal sensitivity of VO₂ thin films,the temperature and the mechanical signals were recorded simultaneously.After the fast Fourier transform?FFT?,the coupled signals were separated and make the sensor a promising application in temperature and pulse detection.