Preparation Of WO₃ Films With Different Structures And Optical And Electrical Control Of Their Proerties

WO₃ has a variety of different crystal structures and non-stoichiometric properties,which enable the gate voltage of ionic liquids to effectively control its structure and optical and electrical properties,thus having great application potential in many fields such as smart glass,information storage and processing,and sensors.So far,there are few studies on the regulation of ionic liquid gate voltage on WO ₃ materials with different structures.In this paper,WO₃ is used as the research object,the pulsed laser deposition method is used to prepare WO₃ films with different structures,and the influence of ionic liquid gate voltage and ultraviolet light irradiation on the structure,electric al and optical properties of WO₃ films with different structures is studied,And explored its internal mechanism.（1）The preparation of WO₃ films with monoclinic phase,triclinic phase,and amorphous state.In the atmosphere of high oxygen pressure,the substrate temperature may be the key factor to decide the structure of WO₃ film.For the fixed substrate temperature,the oxygen pressure may influence the number of oxygen vacancies in WO ₃,and the corresponding the composition and microstructures of WO₃.The WO₃ films with monoclinic phase,triclinic phase,and amorphous state were obtained by optimizing the deposition temperature and oxygen pressure.（2）WO₃ fims modulated by the protic ionic liquid gating.The resistances of WO₃films with monoclinic phase,triclinic phase,and amorphous state can be modulated by ionic liquid gating in some extent.The resistance of the WO₃ film can be dramatically modulated as the larger gating voltage was applied for longer time.For the same gating conditions,the different WO₃ structures cause varying degrees of the modulation.Compared with the amorphous WO₃ fims,the WO₃ films with monoclinic phase,triclinic phase can be more dramatically modulated.And the gated state of the WO₃ films can sustain in a period of time.In our study,the optimized gating condition is gating voltage of+3.5 V for 20 min.The monoclinic WO₃ films gated at+3.5 V for 20 min was chosen as the research object,and further microstructures and properties were studied.After gating,the transparent WO₃ film turns dar blue,and its resistance is decreased from more than 10¹²Ωto 101Ω.And valence of W in the pristine film is mainly+6.After gating,the W⁵⁺and W⁶⁺ions coexist in the film.These phenomena may be related with H⁺ions intercalating into the WO₃ films driven by ionic-liquid gating and the corresponding formation of H_xWO₃.（3）Monoclinic WO₃ fims modulated by ultraviolet light stimulation.The transparent WO₃ film becomes light blue after ultraviolet light stimulation.And,its resistance is decreased from more than 10¹²Ω to 10⁴Ω.And W ions mainly exist as+6 in the pristine film.After ultraviolet light stimulation,the W⁵⁺and W⁶⁺ions coexist in the film.The results may be explained as follows:electrons and holes are formed when WO₃ fims irradiated with ultraviolet light.The necessary H⁺ions for the coloration can be generated from the reaction of the adsorbed water with holes.Subsequently the protons produced can diffuse into the Mo O₃ lattice under the electric field produced by the rest of the electrons in the film.The photogenerated electrons injected into the conduction band of WO₃ react with WO₃ and the H⁺to form H_xWO_3.In conclusion,the H⁺ ions are intercalated into WO₃ film and the corresponding H_xWO₃ is formed by the optical and electrical modulation such as ionic liquid gating and ultraviolet light irradiation.Correspondingly,the microstructures,color and resistance are modulated in a certain degree.The properties of WO₃ after modulating can be sustained in a period of time.The difficulty of the optical and electrical modulation is different.And the performance of the modulated state is also various.This study can provide novel clues for designing multifunctional devices,and further emulating the biological synapse behavior and neuromorphic visual sensors.