Vanadium Dioxide Nanowires Fabrication And Research On M Phase

Vanadium dioxide （VO₂） is one of the most interesting materials due to its phasetransition at a temperature slightly above room temperature （Tc=343K）. The first-ordermetal-insulator transition （MIT） is from the low-temperature, insulating, monoclinicphase to the high-temperature, metallic, tetragonal phase. The transition features drasticchanges in the lattice structure, electrical conductivity, and optical reflectivity,especially the4-5order of magnitude abruptly change of electrical conductivity.Additionally, people start to study one-dimensional nanowires due to the M-R domainequal to the size of nanowires.There are many different kinds of phase of vanadium dioxide. M phase is the mostpopular structure because of its metal-insulator transition, and it contains M1and M2.All the time, M2is thought to be caused by small perturbation, such as temperature,stress and doping. However, recently research found that it also shows M1when dopingW, and then what will happen when doping with low valency elements, like Al and Cr,whether resulting in M2structure. Therefore, in this article we will fabricate vanadiumdioxide nanowires doped with low valency elements and verify the structure ofnanowires doped with Al and Cr.Firstly, pure vanadium dioxide nanowires was fabricated by CVD. V2O₅powderwas placed in a quartz boat in the center of a horizontal tube furnace. The reactionproduct is collected on unpolished quartz substrates downstream from the source quartzboat. The growth was carried out in the following condition: evaporation temperature850°C, Ar carrier gas flow rate30sccm, pressure2000Pa, evaporation time2h, andfinaly down to room temperature naturally.Secondly, vanadium dioxide nanowires doped with Al or Cr is achieved byhydrothermal method. H2C2O4is used for reducing agent, and reactants contain V2O₅,Al₂O₃, CrO3. Ultrapure water （ρ=18.2MΩ cm） is used to prepare all aqueous solutions.After mixing the reaction product for30min, the resulting mixture was transferred to a25mL PPL cup and then heated in a sealed autoclave. The hydrothermal reaction wasperformed at250°C for150h.Finaly, MIT temperature is tested by DSC, and the detail structure is achieved byRaman spectrum. The experiment shows that pure vanadium dioxide nanowires’ phasetransition temperature is68°C, while VO₂doped with Al or Cr is higher,91.5°C with Crdoping. Additionaly, it is M2structure when doping with Al or Cr achieved by Ramantest. Therefore we can draw a conclusion that vanadium dioxide doped with low valencyelements is M2structure at room temperature.