Study On The Preparation,Phase Control And Application Of Flexible VO₂ Thin Films

With the rapid development of electronics industry,wearable,low energy consumption and environmentally friendly flexible electronic materials have attracted more and more attention.Vanadium dioxide(VO₂)has a wide application prospect in intelligent windows,photoelectric switches,optical storage,infrared sensors and infrared laser protection because of its rapid,reversible and multi-stimulus response phase transition behavior.The development of flexible VO₂ films and devices can further promote the development of flexible electronic materials and may play an important role in the next generation of flexible electronics.However,high quality and high crystallinity VO₂films require high growth temperature,which makes it impossible to deposit them on the flexible substrate of common organic polymers,thus hindering their application in flexible electronic materials.Focus on the issue,this study aims to prepare flexible epitaxial VO₂ thin films with high crystal quality on flexible substrates by conventional high-temperature deposition method.And its application in low energy consumption,flexible and multi-functional optoelectronic devices is explored and discussed though characterization and effective regulation of its performance.The main research contents and results are as follows:1.By pulsed laser deposition,M-VO₂ and B-VO₂ films with good crystallization are prepared on flexible mica substrate by van der Waals(vdW)epitaxy growth at550 and 450?,respectively.M-VO₂ film exhibits excellent phase transition properties with resistance mutation of?3 orders of magnitude.The metal-insulator(MI)transition in bending states with bending radius of 8mm and 6mm are almost same as that in the flat state.In addition,because of the weak vdW bond between VO₂ and mica pieces,mica pieces can be removed by wet method,and a free-standing VO₂ film with excellent properties can be obtained and transferred to other substrates.The preparation of flexible VO₂ films with high quality has laid a solid foundation for the application of VO₂ films in flexible electronic devices.2.Based on the above high quality epitaxial M-VO₂ thin film on mica substrate,a transparent flexible double-layer thin film transistor was further prepared by using solid flexible ionic gel as gate dielectric layer.Highly reversible and continuable modulation of MI transition is realized by applying small gate voltages(±4 V).X-ray diffraction(XRD)and secondary-ion mass spectroscopy analyses,together with theoretical calculation,confirm that electrically controlled phase transformation is mainly caused by reversible and nonvolatile H⁺doping into VO₂ lattices.With this regulation,the optical properties of the VO₂ thin film can be further regulated effectively(the near-infrared transmittance regulation range can be up to 40%).The non-volatile,reversible and continuous modulation of the VO₂ phase transition shows a promising application for smart window,near-infrared optoelectronic switches and infrared laser protection devices with high efficiency.3.The electric field induced conductance modulation of VO₂ thin film in the above flexible transistor exists two mechanisms:volatile electrostatic and non-volatile electrochemical regulation,which well correspond to the short-and long-term plasticity(STP/LTP)of the synaptic weight in the neural synapse.Various functions of biological synapses,such as pair-pulse facilitation(PPF),long-term potentiation/depression(LTP/LTD),and spike frequency/time dependent plasticity(SRDP/STDP),are all successfully simulated through the design of input gate voltage pulses in our synaptic transistor.On this basis,an artificial nociceptor with characteristics of threshold,relaxation and sensitization can be perfectly realized in our synaptic Mott transistors.Moreover,the simulation results of a convolution neural network(CNN)built from these synaptic Mott transistors show that the recognition accuracy of handwritten digits can be achieved as high as 95%.This study provides a new approach for the application of VO₂ thin film and the flexible VO₂ thin film transistor is expected to be used in the future wearable neuromorphic systems with high performance.4.Based on the electrochemical reaction of ion doping,the B-VO₂ film epitaxial on mica sheet is directly used as the electrode material of supercapacitors and its charge-discharge properties are characterized in detail.The electrochemical reaction test results show that the B-VO₂ film possess highly reversible charge and discharge performance,with a specific capacitance of up to 275 F/g,and a retention rate of up to 78%after1000 charge and discharge cycles.At the same time,XRD results show that the crystal structure of B-VO₂ film changes significantly during charging and discharging process(insertion and removal of Li ions),even if after 1000 cycles.In addition,B-VO₂ films exhibit excellent bending stability and their electrochemical properties remain unchanged after 500 bending cycles.The preparation of thin film electrodes with excellent electrochemical activity is undoubtedly conducive to the realization of miniaturization and integration of energy storage devices.Therefore,this study provides new idea and option for the design and preparation of flexible and high-performance energy storage devices.