Research On The Preparation,Characterization And Photoelectric Properties Of Vanadium Oxide-Carbon Nanotube Thin Films

The structures,optical and electrical properties of vanadium oxide(VO_x)prepared by Sol-Gel have been widely investigated.However,the overall performances of such materials need to be improved to meet the rapid device development.In order to achieve this goal,composite materials based on VO_x have been developed,among which,vanadium oxide-carbon nanotube(VO_x-CNT)composites draw considerable attention.Most researches and applications on VO_x-CNT composite materials are involved in supercapacitors and sensors,the applications of such materials on infrared detectors have never been reported.In this dissertation,VO_x films and VO_x-CNT composite films were fabricated by Sol-Gel and systematically investigated.The performances of vanadium oxide-single walled carbon nanotube(VO_x-SWCNT)composite films as thermistor materials for bolometers were evaluated.VO_x films were prepared by Sol-Gel through different annealing processes.Scanning electronic microscope(SEM),X-ray diffraction(XRD),high resistance meter,UV-Vis spectroscopy and Fourier transform infrared spectroscopy(FTIR)were employed to investigate the structures,morphologies,crystallinity,electrical and optical properties of the resulting films.The results indicate that the organic vanadium alkoxide can not be fully decomposed at low annealing temperature or short annealing time,while high annealing temperature and long annealing time will result in the destruction of the V=O bonds and formation of too much VO_x with higher valance state V ions,respectively,both of which will decrease the overall properties of the VO_x films.In contrast,VO_x films annealed at 310 ? for 1 h consist of V2O5 and a small quantity of low valance state vanadium oxide,exhibiting the best optical and electrical properties,and thus holding great potential for the applications as thermistor materials in infrared detectors.Vanadium oxide-multi walled carbon nanotube(VO_x-MWCNT)and VO_x-SWCNT composite films were prepared by spin-coating and spray-coating method.Results reveal that the structures,electrical and optical properties of the composite films are influenced by different methods and different kinds of carbon nanotube.Specifically,the electrical resistances and TCR of the composite films prepared by mixed spin-coating and layered spin-coating are slightly decreased compared with those of the pristine VO_x films,which is attributed to the weak interactions between VO_x and CNTs.Differently,for the spray-coating method,the electrical resistances and TCR of the VO_x-MWCNTs composite films are decreased sharply.In contrast,the VO_x-SWCNT composite films exhibit good uniformity and moderate electrical properties because of the moderate interactions,and thus such composite films are promising in the applications as thermistor materials in bolomters.The influences of the SWCNT concentration on the structures,electrical and optical properties of the VO_x-SWCNT composite films were systematically investigated.Results show that VO_x can chemically interact with SWCNTs in the composite films,and particularly,such interactions reach saturated at about 6 wt% SWCNT.Accordingly,the content of V=O bonds,optical transmittance,optical band gap,sheet resistance and TCR vary with the SWCNT concentration,and they similarly reach saturated if the SWCNT concentration is about 6 wt%.However,the VO_x-SWCNT composite film containing 4 wt% SWCNTs exhibits optimal comprehensive properties of high film uniformity,high optical absorptivity,desirable sheet resistance(141 kW(14)?)and TCR(-1.73 %K-1),favorable for applications in uncooled infrared detectors.Spectroscopic ellipsometry(SE)was utilized to characterize the VO_x–SWCNT composite films prepared by sol-gel.Five Tauc-Lorentz oscillators model was employed to describe the dispersions in the optical responses of VO_x and VO_x-SWCNT thin films.Remarkably,the optical and electrical properties deduced from the SE results are similar with those of our previous work.Particularly,the peak transition energies of the 5 Tauc-Lorentz oscillators were assigned to the specific transitions according to the band structures of VO_x.These experimental results will be helpful for better understanding the VO_x-SWCNT composite films,and promoting future characterizations of other SWCNT-based composites by SE.Electrical conductivities of VO_x films and VO_x-SWCNT composite films were systematically investigated.Results show that the electrical conductivity of VO_x film obeys Meyer-Neldel Rule(MNR)with a positive Meyer-Neldel activation energy(EMN)of +16 meV.After incorporation of VO_x with SWCNTs,a normal MNR with a positive EMN of +13 meV is similarly measured at low annealing temperatures(?310 oC).But at high temperatures(?310 oC),a different,inverse MNR with a negative EMN of-43 me V is observed.In addition,electrical conductivities of VO_x-SWCNT composite film with different SWCNTs concentrations follow MNR with EMN of +46 meV.The MNR behaviors of the VO_x film and VO_x-SWCNT composite film were explained.These results will be helpful not only for better understanding the electrical transports of VO_x and VO_x-SWCNT films,but also for gaining insights into the origins of and modifying the MNR behaviors.