Debnath Tushar, Sarker Debalaya, Huang He, Han Zhong-Kang, Dey Amrita, Polavarapu Lakshminarayana, Levchenko Sergey V, Feldmann Jochen
Chair for Photonics and Optoelectronics, Nano-Institute Munich, Physics Department, Ludwig Maximilians-Universität (LMU), Munich, Germany.
Center for Energy Science and Technology, Skolkovo Institute of Science and Technology, Moscow, Russia.
Nat Commun. 2021 May 11;12(1):2629. doi: 10.1038/s41467-021-22934-2.
The halide ions of organic-inorganic hybrid perovskites can strongly influence the interaction between the central organic moiety and the inorganic metal halide octahedral units and thus their lattice vibrations. Here, we report the halide-ion-dependent vibrational coherences in formamidinium lead halide (FAPbX, X = Br, I) perovskite nanocrystals (PNCs) via the combination of femtosecond pump-probe spectroscopy and density functional theory calculations. We find that the FAPbX PNCs generate halide-dependent coherent vibronic wave packets upon above-bandgap non-resonant excitation. More importantly, we observe several higher harmonics of the fundamental modes for FAPbI PNCs as compared to FAPbBr PNCs. This is likely due to the weaker interaction between the central FA moiety and the inorganic cage for FAPbI PNCs, and thus the PbI unit can vibrate more freely. This weakening reveals the intrinsic anharmonicity in the Pb-I framework, and thus facilitating the energy transfer into overtone and combination bands. These findings not only unveil the superior stability of Br-based PNCs over I-based PNCs but are also important for a better understanding of their electronic and polaronic properties.
有机-无机杂化钙钛矿中的卤离子会强烈影响中心有机部分与无机金属卤化物八面体单元之间的相互作用，进而影响它们的晶格振动。在此，我们通过飞秒泵浦-探测光谱与密度泛函理论计算相结合的方法，报道了甲脒铅卤化物（FAPbX，X = Br、I）钙钛矿纳米晶体（PNC）中依赖于卤离子的振动相干性。我们发现，FAPbX PNC在带隙以上非共振激发时会产生依赖于卤化物的相干振子波包。更重要的是，与FAPbBr PNC相比，我们观察到FAPbI PNC的基模有几个更高的谐波。这可能是由于FAPbI PNC的中心FA部分与无机笼之间的相互作用较弱，因此PbI单元可以更自由地振动。这种减弱揭示了Pb-I框架中的固有非谐性，从而促进了能量转移到泛音和组合带中。这些发现不仅揭示了基于Br的PNC相对于基于I的PNC具有更高的稳定性，而且对于更好地理解它们的电子和极化子性质也很重要。
