Monochromatic double coloring effect on V2O5 thin films deposited by electron beam

ABSTRACT Electrochromic material is a thin film technology that has a close relationship with environmental sustainability since they can control the light and heat that enter and leave a building. Vanadium oxide films are electrochemically active for lithium and electron co-intercalation, presenting low optical variation in the low energy visible light, which is suitable for application as a passive electrode in the electrochromic devices. The spectral double coloration of vanadium oxide thin film has been investigated but the monochromatic, in particular, for high photons energy, remains poorly explored. To understand the optical behavior of the vanadium oxide films, in high photons energy (E > 2.0 eV), films were opto-electrochemically investigated. The films were electron beam deposited and modified by heat treatment in an inert atmosphere. All samples show crystallographic orientation in a [110] direction, and the charge capacity showed to be dependent on the film conditioning. The charge capacity of the as-grown film is about 6 mC while for the film heat-treated at 100 ºC it is five times higher. The vanadium oxide films showed reversible Li+ intercalation, presenting a double spectral and monochromatic coloration. It was concluded that for low energy photons the small polaron model fully explains the phenomenon of double monochromatic coloration, and, for high-energy, the monochromatic double coloration is due to the crystallographic phase transition.

摘要：电致变色材料（Electrochromic material）是一类薄膜技术，可通过调控建筑内外进出的光与热，与环境可持续性紧密相关。氧化钒薄膜（Vanadium oxide films）对锂与电子共插层具有电化学活性，在低能可见光区间展现出微弱的光学变化，适用于作为电致变色器件（electrochromic devices）中的无源电极（passive electrode）。目前学界已对氧化钒薄膜的光谱双色效应开展研究，但针对单色变色（尤其是高光光子能量区间）的探索仍相对匮乏。为阐明氧化钒薄膜在高光光子能量（E>2.0 eV）下的光学行为，本研究通过光电化学方法对其进行了系统表征。实验所用薄膜通过电子束沉积制备，并经惰性气氛热处理改性。所有样品均呈现[110]方向的晶体取向，且电荷容量受薄膜后处理条件影响显著：原生薄膜的电荷容量约为6 mC，而经100℃热处理后的薄膜电荷容量提升至前者的五倍。实验结果表明，氧化钒薄膜具备可逆锂离子插层能力，可实现光谱与单色双重变色效应。本研究最终得出结论：对于低能光子，小极化子模型（small polaron model）可完全解释单色双重变色现象；而对于高能光子，该效应则源于晶体相变（crystallographic phase transition）。