Thermochromism is a color change phenomenon for a compound when it undergoes a heating/cooling process. The observation of this phenomenon mostly relies on the naked eye, rather than an exact optical technique, which retards better new thermochromic material design and is detrimental to the understanding of its fundamental mechanism. In this paper, we present a facile and noninvasive in situ ellipsometric method to monitor the optical properties evolution of CsPbI₃ film in its thermochromic process with three notable phase transitions from room temperature to 340 °C. We monitored the in situ structure evolution of CsPbI₃ film by three techniques, namely, temperature-dependent X-ray diffraction, Raman spectroscopy, and spectroscopic ellipsometry. Four phases of CsPbI₃, that is, α, β, γ, and δ, could be detected in temperature-dependent spectroscopic ellipsometry measurement. Especially, we found the most expected “stable” α and γ phases show temperature-dependent bandgap evolution even in their phase-stable temperature region. The in situ spectroscopic ellipsometry therefore could function as a facile and noninvasive technique for detection of phase and band evolution with potential applications in smart windows, semiconductor, and optical devices.

中文翻译：

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原位光谱椭偏仪用于热致变色CsPbI ₃相演化产品组合

热变色是化合物经过加热/冷却过程时的变色现象。对这种现象的观察主要依靠肉眼，而不是精确的光学技术，这会延迟更好的新型热致变色材料设计，并且不利于其基本机理的理解。在本文中，我们提出了一种简便且无创的原位椭圆偏振法，以监测CsPbI ₃膜在热致变色过程中的光学性质演变，该过程具有从室温到340°C的三个明显的相变。我们通过三种技术监测了CsPbI ₃薄膜的原位结构演变，这三种技术分别是温度依赖性X射线衍射，拉曼光谱和椭圆偏振光谱。CsPbI _3的四个阶段，即α，β，γ和δ，可以在依赖温度的椭圆偏振光谱测量中检测到。尤其是，我们发现最期望的“稳定”α和γ相即使在其相稳定温度范围内也显示出与温度相关的带隙演化。因此，原位光谱椭圆偏振法可作为一种简便且无创的技术，用于检测相位和谱带演化，并在智能窗，半导体和光学设备中具有潜在的应用。