Shinde Aparna, Rajput Parikshit Kumar, Makhija Urmila, Tanwar Riteeka, Mandal Pankaj, Nag Angshuman
Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune 411008, India.
Nano Lett. 2023 Aug 9;23(15):6985-6993. doi: 10.1021/acs.nanolett.3c01627. Epub 2023 Jul 24.
Typically, bright excitons (X) emit light in two-dimensional (2D) layered hybrid perovskites. There are also dark excitons (X), for which radiative recombination is spin-forbidden. Application of a magnetic field can somewhat relax the spin-rule, yielding X emission. Can we obtain X light emission in the absence of a magnetic field? Indeed, we observe unusually intense X emission at ∼7 K for (Rac-MBA)PbI, (Rac-4-Br-MBA)PbI, and (R-4-Br-MBA)PbI (Rac-MBA: racemic methylbenzylammonium), which crystallize in a lower symmetry monoclinic phase. For comparison, orthorhombic (R-MBA)PbI does not exhibit X emission. X has a lower energy than X, with energy difference Δ. In monoclinic samples, Δ ∼ 20 meV is large enough to suppress the thermal excitation of X to X, at temperatures <30 K. Consequently, X recombines by emitting light with a long lifetime (∼205 ns). At higher temperatures, the emission switches to the spin-allowed X (lifetime < 1 ns).
通常情况下，明亮激子（X）在二维（2D）层状杂化钙钛矿中发光。也存在暗激子（X），其辐射复合是自旋禁戒的。施加磁场可在一定程度上放宽自旋规则，产生X发射。在没有磁场的情况下我们能否获得X发光呢？确实，我们观察到对于（外消旋-MBA）PbI、（外消旋-4-溴-MBA）PbI和（R-4-溴-MBA）PbI（外消旋-MBA：外消旋甲基苄基铵），在约7 K时出现异常强烈的X发射，它们结晶为较低对称性的单斜相。相比之下，正交晶系的（R-MBA）PbI不表现出X发射。X的能量低于X，能量差为Δ。在单斜样品中，Δ约为20 meV，大到足以在温度<30 K时抑制X到X的热激发。因此，X通过发射长寿命（约205 ns）的光进行复合。在较高温度下，发射转变为自旋允许的X（寿命<1 ns）。
