Preparation And Basic Application Of High Performance VO₂ Nanostructure Thin Films

As a transition metal oxide with unique metal to insulator transition（MIT）,VO₂ has been received increasing attention for decades.VO₂ has promising applications in electrical switch,electrical oscillator,smart windows,laser weapon protection films,memory devices and so on due to dramatic changes of its electrical resistivity and infrared transmission before and after MIT.In this dissertation,we successfully prepare nanostructural VO₂ thin films by solution method,systematically investigate the growth process,microstructure,electrical and optical properties and their relations,and reveal the growth mechanism.The main content and important results are presented as follows:Part 1.Self-Assembling VO₂ Nanonet with High Switching Performance at Wafer-ScaleTechnologically controlling over nanostructures is essential to tailor the functionalities and properties of nanomaterials.Various methods free from lithography-based techniques have been employed to fabricate 2D nanostructures,however it is still hard to achieve a well interconnected 2D regular nanostructure.Here,we demonstrate a facile chemical solution method to self-assemble a regular and interconnected VO₂ nanonet at wafer-scale.The nanonet shows a well-defined2D truss network constructed by VO₂ nanorods with twinning relationships.The growth direction and crystallographic orientation of nanorods are synchronously controlled,leading to horizontally epitaxial growth of nanorods along three symmetric directions of the（001）single-crystal sapphire substrate.The unique nanonets enable the acquisition of excellent resistance switching properties and dramatic fatigue endurance.A large resistance change of near 5 orders with 1.7°C width of hysteresis loop is characterized comparable to the properties of single crystals without detectable degradation after 500 cycles over the metal-to-insulator transition.It indicates the nanonet can serve as an exceptional candidate for practical application in switching functional devices.Our findings offer a novel pathway for self-assembly of 2D ordered nanostructures,which would provide new opportunities for the bottom-up integration of nanodevices.Part 2.Evolution of Structural and Electrical Properties of Oxygen-Deficient VO₂ under Low Temperature Heating ProcessStructural stability and functional performances of Vanadium dioxide（VO ₂）are strongly influenced by oxygen vacancies.However,the mechanism of metal-insulator transition（MIT）influenced by defects is still under debate.Here,we study the evolution of structure and electrical property of oxygen-deficient VO₂by a low temperature annealing process（LTP）based on a truss-structured VO₂nanonet.A continuous lattice reduction is observed during LTP.No recrystallization and structural collapse of the VO₂ nanonet are found after LTP.The amount of oxygen vacancies in VO₂ is easily controlled by the LTP process.The valence-band XPS measurements indicate that the oxygen deficiency strongly affects the energy level of valence band edge.Correspondingly,the resistance changes of the VO ₂films from 1 to 4.5 orders of magnitude are achieved b y LTP.This work demonstrates a novel and effective way to control the amount of oxygen vacancies in VO₂,facilitating the further researches on the role of oxygen vacancy in structure and MIT of VO₂,which is important for the deep understanding of MIT of VO₂.Part 3.Hydrothermal Growth of VO₂ Nanoplate Thermochromic Films on Glass with High Visible TransmittanceThe preparation of thermochromic vanadium dioxide（VO₂）films in an enconmical way is of interest to realizing the application of smart windows.Here,we reported a successful self-assembly of VO₂ nanoplate films on TiO₂-buffered glass by a facile hydrothermal process.The VO₂ films composed of triangle-shaped plates standing on substrates exhibit a self-generated porous structure,which favors the transmission of solar light.The porosity of films is easily controlled by changing the concentration of precursor solutions.Excellent thermochromic properties are observed with visible light transmittance as high as 70.3%and solar modulating efficiency up to 9.3%in a VO₂ film with porosity of³5.9%.This work demonstrates a promising technique to promote the commercial utilization of VO ₂in smart windows.Part 4.Preparation of xCr-VO₂ by Simple Hydrothermal Method for Uncooled Infrared DetectorVO_x have attracted many attentions as thermistor materials for uncooled infrared detector.Here,we demonstrate a facile hydrothermal method to prepare xCr-VO₂ thermistor thin films by doping Cr.The phase transitions（MIT）of VO₂thin films are suppressed by Cr doping and appropriate thermal treatment process.The Cr doped VO₂ thin films show a large TCR value（temperature coefficient of resistance）of-4.029%in the temperature change from 20 to 100 ^oC.In addition,the TCR values of xCr-VO₂ thin films are successfully tuned though varing thermal treatment process.