2, University of Electronic Science and Technology of China, Chengdu, , China
3, Yunnan University, Kunming, , China
4, Changan University, Xian, , China
CsPb2Br5 is a 2D perovskite where single layer Cs acts as a spacer, monolayer PbBr6 octahedrons are edge shared. Although CsPb2Br5 was synthesized more than 10 years ago[1, 2], only recently it began to draw a lot of attention. Despite extensive research over past two years, its fundamental bandgap and basic electronic and optical properties still remain controversial: is this a wide bandgap indirect semiconductor or a direct bandgap material with strong green luminescence at ~520 nm? Zhang et al. reported that the attachment of CsPb2Br5 dots to CsPbBr3 nanocrystals can enhance external quantum efficiency of CsPbBr3 light-emitting diode (LED) by 50%. Wang et al. later demonstrated a nearly 90% quantum efficiency with CsPb2Br5 nanocrystals. Using ion exchange with I and Cl, they further expended emission wavelength from UV to red. But these claims of high luminescence have met with skeptics. Li et al. observed no photoluminescence, in agreement with their calculation result of indirect wide bandgap semiconductor. Since then, the controversy remains, some group still report strong visible bandgap and strong photoluminescence and even demonstrate lasing action in CsPb2Br5 microplates[6, 7], while others reported the opposite. In this talk, I will discuss the reason for different observations and present our result of the bandgap investigation of CsPb2Br5.
 I. Y. Kuznetsova, I. S. Kovaleva, V. A. Fedorov, Russ. J. Inorg. Chem. 2001, 46, 1730.
 M. Rodova, J. Brozek, K. Knizek, K. Nitsch, J. Therm. Anal. Calorim. 2003, 71, 667.
 X. L. Zhang, B. Xu, J. B. Zhang, Y. Gao, Y. J. Zheng, K. Wang, X. W. Sun, Adv. Funct. Mater. 2016, 26, 4595.
 K. H. Wang, L. Wu, L. Li, H. B. Yao, H. S. Qian, S. H. Yu, Angew. Chem.-Int. Edit. 2016, 55, 8328.
 G. P. Li, H. Wang, Z. F. Zhu, Y. J. Chang, T. Zhang, Z. H. Song, Y. Jiang, Chem. Commun. 2016, 52, 11296.
 X. S. Tang, Z. P. Hu, W. Yuan, W. Hu, H. B. Shao, D. J. Han, J. F. Zheng, J. Y. Hao, Z. G. Zang, J. Du, Y. X. Leng, L. Fang, M. Zhou, Adv. Opt. Mater. 2017, 5.
 W. S. Longfei Ruan, Aifei Wang, Aishuang Xiang, Zhengtao Deng, The Journal of Physical Chemistry Letters 2017, DOI: 10.1021/acs.jpclett.7b01657.
 M. D. B. Ibrahim Dursun, Bekir Turedi, Badriah, A. S. Alamer, Jun Yin, Issam Gereige,, O. F. M. Ahmed Alsaggaf, Mohamed Eddaoudi,, a. O. M. Bakr, ChemSusChem 2017, ChemSusChem 10.1002/cssc.201701131.