De Apurba, Bhunia Soumyadip, Cai Yichao, Binyamin Tal, Etgar Lioz, Ruhman Sanford
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem-91904, Israel.
J Am Chem Soc. 2024 Jul 24;146(29):20241-20250. doi: 10.1021/jacs.4c05412. Epub 2024 Jul 15.
Quantifying stimulated emission in semiconductor nanocrystals (NCs) remains challenging due to masking of its effects on pump-probe spectra by excited state absorption and ground state bleaching signals. The absence of this defining photophysical parameter in turn impedes assignment of band edge electronic structure in many of these important fluorophores. Here we employ a generally applicable 3-pulse ultrafast spectroscopic method coined the "Spectator Exciton" (SX) approach to measure stimulated-emission efficiency in quantum confined inorganic perovskite CsPbBr NCs, the band edge electronic structure of which is the subject of lively ongoing debate. Our results show that in 5-6 nm CsPbBr NCs, a single exciton bleaches more than half of the intense band edge absorption band, while the cross section for stimulated emission from the same state is nearly 6 times weaker. Discussion of these findings in light of several recent electronic structure models for this material proves them unable to simultaneously explain both measures, proving the importance of this new input to resolving this debate. Along with femtosecond time-resolved photoluminescence measurements on the same sample, SX results also verify that biexciton interaction energy is intensely attractive with a magnitude of ∼80 meV. In light of this observation, our previous suggestion that biexciton interaction is repulsive is reassigned to hot phonon induced slowdown of carrier relaxation leading to direct Auger recombination from an excited state. The mechanism behind the extreme slowing of carrier cooling after several stages of exciton recombination remains to be determined.
由于激发态吸收和基态漂白信号对泵浦 - 探测光谱的影响掩盖了半导体纳米晶体（NCs）中的受激发射，对其进行量化仍然具有挑战性。在许多这些重要的荧光团中，缺乏这一决定性的光物理参数反过来又阻碍了对带边电子结构的归属。在这里，我们采用一种普遍适用的三脉冲超快光谱方法，即“旁观者激子”（SX）方法，来测量量子限域无机钙钛矿CsPbBr NCs中的受激发射效率，其带边电子结构是当前激烈争论的主题。我们的结果表明，在5 - 6纳米的CsPbBr NCs中，单个激子使强烈的带边吸收带的一半以上发生漂白，而同一状态下受激发射的截面则弱近6倍。根据该材料的几个近期电子结构模型对这些发现进行讨论，结果表明它们无法同时解释这两种测量结果，这证明了这一新输入对于解决这一争论的重要性。连同对同一样品的飞秒时间分辨光致发光测量，SX结果还证实双激子相互作用能具有强烈的吸引力，大小约为80毫电子伏特。鉴于这一观察结果，我们之前关于双激子相互作用是排斥性的建议被重新归因于热声子导致的载流子弛豫减慢，从而导致从激发态直接俄歇复合。激子复合几个阶段后载流子冷却极端减慢背后的机制仍有待确定。
