講演情報
[22p-A304-3]Synthesis and optical properties of bismuth-based mixed-anion compounds epitaxial thin films
〇Zhengkang PENG1,2, Daichi Oka2, Yasushi Hirose2, Tomoteru Fukumura1 (1.Tohoku Univ., 2.Tokyo Metro. Univ.)
キーワード：
epitaxial thin film、mixed-anion compounds、optical property
Bismuth-based mixed-anion compounds (BMCs) are attracting much attention as photo-energy conversion materials because of their flexible bandgap and anisotropic layered structure. The conduction band bottom and the valence band top are mainly formed by Bi 6p orbital and the p orbitals of the constituent anions, respectively, which are sensitive to the local structure [1]. Therefore, it is essential to understand the relationship between the crystal structure and band gap to design BMCs with suitable optical properties for applications. We recently succeeded in synthesizing Bi2OS2 and Bi3O2S2Cl (Fig. 1) in the form of epitaxial thin film by mist chemical vapor deposition (CVD) [2]. In this study, we investigated the crystal structure and the optical properties for the Bi2OS2 and Bi3O2S2Cl thin films.
The Bi2OS2 and Bi3O2S2Cl thin films were selectively synthesized from N,N-dimethylformamide solutions of BiCl3 and thiourea (CH4N2S) on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (100) single crystal substrates by mist CVD with different setups. X-ray diffraction, energy dispersive X-ray spectroscopy, and X-ray photoemission spectroscopy confirmed epitaxial growth of these compounds without any impurity.
Figure 2 shows the Tauc plots of optical absorption spectra for (001)-oriented Bi2OS2 and Bi3O2S2Cl epitaxial thin films. Bi3O2S2Cl showed a significantly smaller bandgap of 0.6 eV than that of 1.0 eV for Bi2OS2 despite the formation of S 2p orbital at the valence band top in both the compounds. First-principles calculations indicated that the different local structures surrounding the Bi cations caused the reduction of bandgap in Bi3O2S2Cl [3].
References
[1] T. Li et al., Adv. Mater. 33, 2008574 (2021).
[2] Z. Peng et al., 69th JSAP spring meeting 2022, 25a-F307-5.
[3] B. Ruan et al., J. Am. Chem. Soc. 141, 3404 (2019).
The Bi2OS2 and Bi3O2S2Cl thin films were selectively synthesized from N,N-dimethylformamide solutions of BiCl3 and thiourea (CH4N2S) on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (100) single crystal substrates by mist CVD with different setups. X-ray diffraction, energy dispersive X-ray spectroscopy, and X-ray photoemission spectroscopy confirmed epitaxial growth of these compounds without any impurity.
Figure 2 shows the Tauc plots of optical absorption spectra for (001)-oriented Bi2OS2 and Bi3O2S2Cl epitaxial thin films. Bi3O2S2Cl showed a significantly smaller bandgap of 0.6 eV than that of 1.0 eV for Bi2OS2 despite the formation of S 2p orbital at the valence band top in both the compounds. First-principles calculations indicated that the different local structures surrounding the Bi cations caused the reduction of bandgap in Bi3O2S2Cl [3].
References
[1] T. Li et al., Adv. Mater. 33, 2008574 (2021).
[2] Z. Peng et al., 69th JSAP spring meeting 2022, 25a-F307-5.
[3] B. Ruan et al., J. Am. Chem. Soc. 141, 3404 (2019).