Abstract
Vanadium oxides (VxOy) are classic “smart functional materials” used in a wide array of thermochromic, electronic, and catalytic applications. Specifically, vanadium dioxide (VO2) class nanomaterials are of enormous interest due to their unique first order reversible metal-insulator phase transition (MIT) behavior accompanied by a structural phase transition, inducing dramatic changes in electrical and optical properties with large lattice deformation. To date, a plethora of reports exemplifying the MIT characteristics of VO2, synthetic methods of VO2, and modulating VO2 phase transition temperatures (Tc) have been published. In this Tutorial Review, we present an overview on the fundamentals of the VO2 band structure and principles of MIT and outline various reported synthetic approaches for VO2 thin films, including dimensionally oriented VO2 nanostructures. Discussion on recent trends in VO2 applications, challenges in VO2 synthesis, and future perspectives are also elaborated in detail.
Highlights
Vanadium dioxide (VO2) class nanomaterials are of enormous interest due to their unique first order reversible metal-insulator phase transition (MIT) behavior accompanied by a structural phase transition, inducing dramatic changes in electrical and optical properties with large lattice deformation
A plethora of reports exemplifying the MIT characteristics of VO2, synthetic methods of VO2, and modulating VO2 phase transition temperatures (Tc) have been published. In this Tutorial Review, we present an overview on the fundamentals of the VO2 band structure and principles of MIT and outline various reported synthetic approaches for VO2 thin films, including dimensionally oriented VO2 nanostructures
A sudden change in the intrinsic electrical conductivity of vanadium oxides is normally related to a reversible semiconductor-to-metal transition (SMT), which is more widely known as a metal-to-insulator transition (MIT)
Summary
Vanadium oxides are ideal prototypical materials with strong electron–electron correlations discovered to demonstrate remarkable changes in their electrical properties from the insulating to the metallic state by applying external stimuli. Vanadium dioxide (VO2), for example, undergoes fully reversible MIT between insulating monoclinic VO2(M) to metallic rutile VO2(R) phases or vice versa triggered by externally applied stimuli such as heat or stress. A Magneli-type VO2 is an ideal textbook example of a system that exhibits abrupt structural transition from the metallic to the insulator state with an electrical conductivity change up to five orders of magnitude. The phase transition from a high symmetry tetragonal structure to a low symmetry monoclinic deviates from the position of the V atoms. In this case, dimerization creates zigzag V-atom chains with two different V–V distances of 3.19 and 2.60 Å between the nearest adjacent V atoms [Fig. 2(b)]. Cr-doped Cr0.2V0.8O2 demonstrated the highest Tc of 100 °C.31 Metal doping of VO2 can work up to a maximum doping fraction, beyond which MIT might disappear or materials may have more complex phases than the M1 and R phases
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