Proceedings of Online nanoGe Fall Meeting 20 (OnlineNFM20)
Publication date: 4th October 2020
Layered perovskites belong to the family of two-dimensional (2D) Van der Waals materials with outstanding optoelectronic properties attracting intensive attention of the scientific community for their wide range of applications. Quality of 2D perovskites plays an important role on purity of the excitonic emission. Particularly presence of defects such as lattice distortion, vacancies or impurities may lead to exciton trap states. In this work, we propose a study of temperature dependent light emission properties of this unique 2D material with the lowest quantum well thickness of n=1,2. Ultra-thin flakes were mechanically exfoliated from solution-based synthetized bulk crystals PEA2PbI4 (n=1) and PEA2MAPb2I8 (n=2) onto standard SiO2/Si substrates and characterized by Raman and photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) methods. These flakes exhibit of a very bright room temperature PL with average linewidth of ≈50 meV for both 2D perovskite types n=1,2. Their PL is characteristic by double emission, where a higher energy peak is associated to free exciton emission (2.36 and 2.15 eV for n=1 and n=2 respectively) and lower energy peak could be related to trapped exciton emission [1][2] (2.32 and 2.1 eV for n=1 and n=2 respectively). By the lowering of temperature to 20 K, these peaks then become well-defined and sharper (linewidth ≈10 meV). Contrary to n=1 type, perovskites with n=2 phase show more dramatic behavior at low temperature with more dominant trapped exciton emission. The lower energy peak is at low temperature divided into 3 peaks separated by ≈8-20 meV. Further, with increasing of temperature from 20 up to 200 K, position of the higher energy excitonic peak shows slight blue shift (≈12 meV) for n=1 and slight red shift (≈7 meV) for n=2 related to the temperature induced change in their energy band gap. Free exciton PL lifetime of n=2 phase increases with temperature (20-200 K) from 0.8 to 3.5 ns, whereas for n=1 phase it sustains almost unchanged around value of ≈0.9 ns across the temperature range. Above that n=2 shows thermally induced significant fluctuations of PL lifetime and excitonic emission intensity, which are not recorded for n=1 phase. Finally, from the analysis follows that n=1 quantum well thickness exhibits a higher free excitonic transitions and a better temperature dependent emission stability, associated probably due to a lower presence of defects and a higher crystalline quality as supported by Raman spectroscopy.
This work was made possible by the Horizon 2020 research and innovation program through the S2QUIP (grant agreement No. 8204023) and DROP-IT (grant agreement No. 862656) projects, and by the Spanish MINECO through project No. TEC2017-86102-C2-1-R. M. K. and R.C.-A. acknowledge fellowships no. CPI-18-404 and CPI-18-418 on the S2QUIP project, respectively. Financial support from Spanish MINECO through Project No. TEC2017-86102-C2-1-R and the European Research Council (ERC) via Consolidator Grant (724424|No-LIMIT) are gratefully acknowledged by H.P.-A. and S.G.. S.G. acknowledges her \Grisolia" grant from Generalitat Valenciana.