Abstract
The present investigation reported on a novel oxygen-assisted etching growth method that can directly transform wafer-scale plain VO2 thin films into pyramidal-like VO2 nanostructures with highly improved field-emission properties. The oxygen applied during annealing played a key role in the formation of the special pyramidal-like structures by introducing thin oxygen-rich transition layers on the top surfaces of the VO2 crystals. An etching related growth and transformation mechanism for the synthesis of nanopyramidal films was proposed. Structural characterizations confirmed the formation of a composite VO2 structure of monoclinic M1 (P21/c) and Mott insulating M2 (C2/m) phases for the films at room temperature. Moreover, by varying the oxygen concentration, the nanocrystal morphology of the VO2 films could be tuned, ranging over pyramidal, dot, and/or twin structures. These nanopyramidal VO2 films showed potential benefits for application such as temperature−regulated field emission devices. For one typical sample deposited on a 3-inch silicon substrate, its emission current (measured at 6 V/μm) increased by about 1000 times after the oxygen-etching treatment, and the field enhancement factor β reached as high as 3810 and 1620 for the M and R states, respectively. The simple method reported in the present study may provide a protocol for building a variety of large interesting surfaces for VO2-based device applications.
Highlights
Structural characteristics including dimensionality, morphology, and crystal structure can have significant influences on the properties and applications of functional materials
Upon the metal-to-insulator phase transition (MIPT), the crystalline structure changes from a high-symmetry tetragonal rutile phase (P42/mnm, R phase) to a lower symmetry monoclinic phase (P21/c, M1 phase) with dimerized V–V pairs exhibiting alternating zig-zag like chains [4,5]
Among the numerous reports on the synthesis of VO2 films, only a few distinct surface structures, e.g., rough pores [41], subwavelength nanoholes [42], nanotetrapods [43], nanobeams [19], have been developed. This may derive from the complexity of the vanadium oxide system [44] which has multiple oxidation states, different elemental compositions [45,46], and various coordination polyhedra [47]; these severely restrict their applications as high performance devices
Summary
Structural characteristics including dimensionality, morphology, and crystal structure can have significant influences on the properties and applications of functional materials. Among the numerous reports on the synthesis of VO2 films, only a few distinct surface structures, e.g., rough pores [41], subwavelength nanoholes [42], nanotetrapods [43], nanobeams [19], have been developed This may derive from the complexity of the vanadium oxide system [44] which has multiple oxidation states (from +2, as in VO, to +5, as in V2O5), different elemental compositions (such as VnO2n−1 and VnO2n+1) [45,46], and various coordination polyhedra (including the tetrahedron, trigonal bipyramid, square pyramid, regular octahedron, and distorted octahedron) [47]; these severely restrict their applications as high performance devices. The simple and large-area available processing reported in this work may provide a practical strategy for the production of VO2-based integration devices
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