Preparationand Thermochromic Properties of VO2 Micro-nano Thin Film

VO2微納米結構薄膜的製備及相變性能研究

Student thesis: Doctoral Thesis

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Award date7 Aug 2018

Abstract

Vanadium dioxide (VO2) is a correlated transition metal oxide. It has attracted much attention because of its reversible metal-insulator transition (MIT) at 341 K. During the phase transition, the structure transforms from a low-temperature monoclinic phase to a high-temperature rutile phase in nanosecond. Furthermore, its electronic and optical properties change tremendously. Just because of this, VO2 can be utilized as resistive switching elements, optical storage media, light modulators, and energy-saving smart windows. However, the phase transition temperature is still too high, which impedes its practical applications. In addition, the transformation stress and thermal heat stress produced in phase transition process may lead to the separation of the films. To cure the above problems, the influence of oxygen vacancies, nanoporous structure and composite films on the structural stability and phase transition performance of VO2 thin films was studied. The main contents and innovative results are as following:

(1) The influence of oxygen vacancy on phase transition performance was studied in this thesis. The vanadium oxide thin films are fabricated by magenetron sputtering method at room temperature, and then annealed in protective atmosphere of Argon (Ar) at 723 K for 2 h. The concerntration of oxygen vacancies increases with increasing Ar pressure. It is demonstrated that the phase transition temperature decreases to 319 K when Ar pressure is 100 Pa. According to the principle of electric neutrality, oxygen vacancies introduce electrons into films and the introduced electrons reduce the band gap and make the phase transition from insulator to metal phase more easily, resulting in the low phase transition temperature. In addition, oxygen vacancies weaken the hybridization between V3d and O. Hence, the bond breaking and reforming become easier, which also reduces the phase transition temperature.

(2) The influences of porous structure on stability and phase transition performance of VO2 thin films were studied. The oxygen flow rate in sputtering process plays a role in porosity and pore size. It is demonstrated that the hysteresis width decreases and the stress is released timely due to the porous morphology. To an extent, the nanoholes can enhance the structural stability and extend the service life of the thin films. In addition, the porous structure is also influenced by sputtering power. When the sputtering power is lower than 240 W, VO2 thin film with columnar particles is obtained and the nanopores are formed between the columnar particles. The compact thin film with spherical nanoparticles is obtained when the sputtering power is 280 W. The highest porosity of 10.2 % is obtained at the sputtering power of 220 W. The visible transmittance (Tvis = 36-62 %) is enhanced with increasing porosity (0-10.2 %). The phase transition properties of the porous VO2 thin film are substantially improved when compared to the dense one. The porous thin film will be the inevitable tendency in the futher research.

(3) To further improve the component constancy of VO2 film, we fabricated VO2 composite films. It is found that the precursor concentration plays a dominate role in phase evolution process. The phase evolution procrsses (VO2(B) → VO2(B) + VO2(A) → VO2(A) + VO2(M) → VO2(M);VO2(B) → VO2(B) + VO2(M) → VO2(M);VO2(B) → VO2(D) → VO2(M)) and mechanisms were studied. For different phase evolution processes, the interphases exhibited different influence on the hysteresis width (ΔH = 6.9-104.5 K). In addition, the results indicate that the near-infrared modulation (ΔT2500 = 16-40%) could be enhanced by decreasing the particle size. The nanoparticles with 30-40 nm in size were fabricated by optimizing the preparation technique, and the optical properties of VO2 film were further improved. However, the hysteresis width of nanoparticles is large (ΔH = 47.3 K), due to the elimination of the nucleation defects of phase transition in nanoparticles.

(4) VO2(M) nanosheets not only have smaller dimension, but also have narrower hysteresis width when compared with nanoparticles. The nanosheets were fabricated by the hydrothermal method and subsequently annealed in Ar. In order to improve the dispersity of the nanosheets, the process parameters are optimized to make the nanosheets arranged vertically on the surface of the hollow spheres. It is demonstrated that both the dispersion and heat treatment show enormous impact on the phase transition performance. Under maintaining the invariability of the nanosheets, the optical performance is enhanced by increasing the annealing temperature. With increasing dispersion of the nanosheets, the visible transmittance (Tvis) and the solar modulation (ΔTsol) are improved from 33% and 5.2% to 60% and 11.2%, respectively. Meanwhile, a large contrast transmittance of 60% at 2500 nm is obtained. In addition, the VO2 nanosheets exhibit narrow hysteresis width of under 10 K, which is much smaller than the nanoparticles. In a word, the nanosheets improve the phase transition performance to a large extent, and promote the practical applications of VO2(M) based smart windows.

    Research areas

  • Vanadium dioxide, Phase transition performance, Energy efficient window, Porous structure, Nanosheets