Controlled Growth And Performance Studies Of Graphene-based Vanadium Dioxide Materials

Phase change materials exhibit broad promising applications in photoelectric switch,intelligent glass,phase change storage and memristor fields.Vanadium dioxide(VO₂),as a typical phase change material,had attracted extensive attention because of insulation-metal phase transition at 68?.Unique thermal phase transition process was often accompanied by huge changes in physical properties such as electricity,optics and magnetism,which realized the application of various functional devices.The phase transformation behavior of VO₂ film was closely relatived to crystal quality,so the selection of substrate for growth was very important in the film preparation process.Using substrate with good lattice matching and photoelectric characteristics was helpful to prepare high-quality VO₂ film,thereby improved the performance of phase change device.Graphene demonstrated excellent optical and electrical properties.Combining with VO₂,graphene had a high application prospect in photoelectric phase change devices and transparent conductive films.VO₂ thin film prepared based on graphene substrate effectively played the advantages of graphene's layered structure and strong supporting role.After preparing high-quality VO₂ thin film with stoichiometric ratio,self-supporting stripping and transfer of the film be easily realized,thus further broadening the application range of devices.The common preparation methods of VO₂ thin films include pulsed laser deposition(PLD),molecular beam epitaxy(MBE),magnetron sputtering and other physical deposition techniques,but these methods often suffered from complex parameter control process and expensive equipment,which limited their practical applications.Thermal oxidation of vanadium metal films in oxygen atmosphere was a simple and feasible method.However,due to the existence of critical growth window,the films were sensitive to gas pressure,annealing parameters and time,and were easy to form peroxidation for vanadium atoms in polyvalent state.Different from the usual thermal oxidation,the wet oxidation technology used high temperature water vapor as a mild oxidant was a relatively mild oxidation method,argon gas pressure and temperature range were effectively controlled,and effectively avoided the peroxidation of the film.In addition,it was easy to prepare large-scale VO₂ thin films by using this method,therefore meet the requirements of practical application.Based on the above mentioned researches,high-quality VO₂films on graphene substrate surface were prepared by water oxidation method through optimization of preparation parameters.The main research contents were as follows:1.Fabrication and Performance Studies of VO₂/Graphene/6H-SiC films1)Assisted by copper atoms,the high quality graphene was prepared using molecular beam epitaxy(MBE)by annealing 6H-SiC(0001)substrate.Different amounts of copper atoms were pre-deposited on 6H-SiC substrates at different deposition times(5 mins,10 mins,20 mins,30 mins,40 mins)and then the 6H-SiC substrates were annealed.The results showed that pre-deposition of Cu atoms played a certain role in improving the quality of graphene.The quality of graphene prepared under the condition of Cu atom predeposition time of 10 mins was higher.The preparation of high-quality graphene laied a foundation for the subsequent VO₂ films with excellent growth performance.2)The vanadium films with appropriate thickness were deposited on the surface of Graphene/6H-SiC by magnetron sputtering technology.Subsequently,high quality VO₂ films were achieved after water oxidation treatment.The surface morphology and crystal structure of VO₂/Graphene/6H-SiC films were characterized by field emission scanning electron microscopy(FE-SEM),Raman spectroscopy,X-ray photoelectron spectroscopy(XPS)and so on.The effects of the Ar carrier gas flow and the annealing time on the quality of VO₂ have been systematically studied.The results showed that the Ar carrier gas flow rate of 0.6 L/min and the annealing time of60 mins were the optimal parameters for preparing VO₂/Graphene/6H-SiC films.Under the condition of the gas flow rate of 0.6 L/min,high temperature water vapor fully combined with vanadium metal film,which was conducive to the preparation of high quality VO₂ film.Appropriate annealing time optimized the integrity and continuity of VO₂ film surface and improved the phase transformation properties of the film.After optimization,the sample had certain phase transformation characteristics and the resistivity change close to 4 orders of magnitude.Compared with VO₂/6H-SiC films,the presence of graphene improved the phase change properties of VO₂/Graphene/6H-SiC films.2.Fabrication and Performance Studies of VO₂/Graphene/Al₂O₃ filmsThe vanadium films were deposited on the prepared Graphene/Al₂O₃ surface by magnetron sputtering,and then the VO₂ films were finally prepared by water oxidation method.The surface morphology,crystal structure and resistance changes of VO₂/Graphene/Al₂O₃ films before and after phase transformation were characterized and tested by a variety of characterization methods.The results showed that the heating temperature of 550?,the flow of Ar carrier gas of 0.7 L/min,and the oxidation time of 2 hours were favorable for the growth of VO₂ thin films.The samples had a narrow thermal width and a resistivity change of 3-4 orders of magnitude.Compared with the growth of VO₂ on sapphire,graphene as a buffer layer optimized the growth of VO₂film.In addition,the effect of different graphene thickness on the growth and phase transformation properties of VO₂ films was systematically studied.The results showed that with the increase of thickness,the thermal hysteresis width decreased and the resistivity increased.