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.
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