Wu Yu-Ning, Wuenschell Jeffrey K, Fryer Robert, Saidi Wissam A, Ohodnicki Paul, Chorpening Benjamin, Duan Yuhua
National Energy Technology Laboratory, United States Department of Energy, Pittsburgh, PA 15236, United States of America.
Leidos Research Support Team, Pittsburgh, PA 15236, United States of America.
J Phys Condens Matter. 2020 Jul 8;32(40). doi: 10.1088/1361-648X/ab9d4f.
To gain fundamental understanding of the high-temperature optical gas-sensing and light-energy conversion materials, we comparatively investigate the temperature effects on the band gap and optical properties of rutile and anatase TiOexperimentally and theoretically. Given that the electronic structures of rutile and anatase are fundamentally different, i.e. direct band gap in rutile and indirect gap in anatase, it is not clear whether these materials exhibit different electronic structure renormalizations with temperature. Usingmethods, we show that the electron-phonon interaction is the dominant factor for temperature band gap renormalization compared to the thermal expansion. As a result of different contributions from the acoustic and optical phonons, the band gap is found to widen with temperature up to 300 K, and to narrow at higher temperatures. Our calculations suggest that the band gap is narrowed by about 147 meV and 128 meV at 1000 K for rutile and anatase, respectively. Experimentally, for rutile and anatase TiOthin films we conducted UV-Vis transmission measurements at different temperatures, and analyzed band gaps from the Tauc plots. For both TiOphases, the band gap is found to decrease for temperature above 300 K quantitatively, agreeing with our theoretical results. The temperature effects on the dielectric functions, the refractive index, the extinction coefficient as well as the optical conductivity are also investigated. Rutile and anatase show generally similar optical properties, but differences exist in the long wavelength regime above 600 nm, where we found that the dielectric function of rutile decreases while that of anatase increases with temperature increase.
为了深入了解高温光学气敏和光能转换材料，我们通过实验和理论方法对比研究了温度对金红石型和锐钛矿型TiO₂的带隙和光学性质的影响。鉴于金红石型和锐钛矿型的电子结构存在根本差异，即金红石型为直接带隙，锐钛矿型为间接带隙，目前尚不清楚这些材料的电子结构重整化是否会随温度变化。通过计算方法，我们发现与热膨胀相比，电子 - 声子相互作用是导致带隙随温度重整化的主要因素。由于声学声子和光学声子的贡献不同，发现带隙在温度升至300 K时会变宽，而在更高温度下会变窄。我们的计算表明，在1000 K时，金红石型和锐钛矿型TiO₂的带隙分别缩小约147 meV和128 meV。在实验方面，我们对金红石型和锐钛矿型TiO₂薄膜在不同温度下进行了紫外 - 可见透射测量，并从Tauc图分析了带隙。对于两种TiO₂相，发现带隙在温度高于300 K时会定量减小，这与我们的理论结果一致。我们还研究了温度对介电函数、折射率、消光系数以及光导率的影响。金红石型和锐钛矿型通常表现出相似的光学性质，但在波长大于600 nm的长波长区域存在差异，我们发现金红石型的介电函数随温度升高而减小，而锐钛矿型的介电函数随温度升高而增加。
