Research On Fabrication, Conductivity Characteristic And Phase Transition Controllability In Vanadium Dioxide Nanowires

In the condensed matter physics and material science,vanadium dioxide?VO₂?is a typical kind of metal-insulator transition?MIT?material.VO₂ exhibits many interesting physical and chemical properties due to the special d electron orbit.Recent studies show that in all the polymorph structrues of VO₂,there are only two structures showing the distinct MIT properties,VO₂?A?and VO₂?M?.Additionally,low-dimensional vanadium dioxides also show an obvious metal-insulator phase transition,which can be used as sensors,optical switching,resistance switching,Mott FET and so on.Now,studies of low-dimentional VO₂?A?and VO₂?M?nanostructures are very few,especially one-dimentional vanadium dioxides whose conductivity properties,M-R domain structures and tuning of the phase transition are still under debate.For these reasons,VO₂?A?and VO₂?M?nanowires were successed fabricated by a facile hydrothermal method and a chemical vapor deposition in this thesis.Using the self-designed shielded box,the conductivity properties and phase transition were studied in detail.In addition,we investigated the evolution of M-R domain structures and the relationship between lattice mismatch and MIT phase transition.These researches provide theoretical and experimental bases for the smart nano applications of vanadium dioxide nanowires.And the main research contents are as follows:Studies of synthesis of high quality single crystalline VO₂?A?and VO₂?M?nanowires.VO₂?A?nanowires were synthesized by hydrothermal method using oxalic acid as reducing agent and polyethylene glycol 6000 as surfactant.It was found that polyethylene glycol 6000 can effectively accelerate the transformation from V₂O₅ precursor to VO₂?B?,and there is no intermediate phases(V₃O₇?H₂O and V₆O₁₃)formed during fabrication process.Thus less time and lower temperature were needed in the hydrothermal method.The investigation of growth mechanism indicates an oriented attachment growth process in the hydrothermal system.VO₂?M?nanowires were synthesized by chemical vapor deposition?CVD?.V₂O₅ powder and rough quartz plate were used during synthesis process.Due to the low melting point of V2O5 and rough surfaces of quartz plate,much more VO₂?M?nanowires were succeesed synthesized.Less time?²h?and lower synthesis temperature?¹133K?were needed during the fabrication process.More importantly,there are lots of free-standing nanowires when using rough quartz plates as substrates.The structure transition and temperature-driven conductivity switching of the VO₂?A?nanowires were investigated.The experimental results show that VO₂?A?nanowire exhibits a distinct structural transition accompanied with an order of magnitude change in resistance,and a clear temperature-dependent current switching hysteresis.Theoretically,the electrical conductivity behavior was found to be consistent with Mott's small polaron model.In addition,due to the size and surface effects,the nanowires with different widths show great disparities in the hysteresis loop,phase transition temperature and electrical conductivity.The phase transition temperature is linearly dependent on the inverse nanowire widths.More interestingly,the first-order phase transition of the nanowire even coverts into high-order continuous phase transition when the width is below a critical size.To explore the intrinsic influence of the size and surface effects,the analysis of TEM measurements showed that the rough surface structure of the nanowire is very different to the internal structure.And the surface structure has a remarkable effect on the phase transition characteristics with the decrease of the nanowire width,and the suitable heterogeneous nucleation originating from the rough surface structure should play a crucial role in properties of the VO₂?A?nanowires.Size-dependent phase transition features of the VO₂?A?nanowires also suggest that the size and surface effects must be taken into consideration when designing the VO₂ nanodevices.The effects of lattice mismatch to the M-R domain structures,conductivity properties and tuning of phase transition of VO₂?M?nanowires have been studied systematically.The results show that M-R domain structures periodically change as the increase of temperatures.Due to the lattice mismatch,VO₂?M?nanowires show great disparities in the hysteresis loop,phase transition temperature and electrical conductivity compared with those free-standing VO₂?M?nanowires.For instance,width of hysteresis becomed wider and phase transition temperature rised.Interestingly,it exhibited a special transformation between M1 and M2.VO₂?M?nanowires also display multi phase transition owing to the lattice mismatch which can be used in multi-bit cell field.Tuning of VO₂?M?phase transition has been explored for advanced storage appliances.The experimental results show that the transformation between M and R keep continuous circulation when appling a proper voltage,which can be potentially used as flash memory.Futhermore,this demonstrates that control of morphology,structure and gowth direction plays a crucial role in tuning the phase transition of VO₂?M?nanowires.This thesis comprehensively investigated synthesis methods,conductivity propertities,size and surface effects and tuning of phase transition of VO₂?A?and VO₂?M?nanowires.These studies are important for understanding the mechanism of phase transition and lay the foundation for applications of micro-nano devices based on vanadium dioxide nanowires.