单纳米晶和系综水平上的钙钛矿纳晶和频振动光谱

Sum Frequency Generation Vibrational Spectra of Perovskite Nanocrystals at the Single-Nanocrystal and Ensemble Levels

在单纳米晶和系综水平上探测有机-无机杂化钙钛矿纳米晶的分子结构对于理解其与尺寸相关的光电特性以及纳米晶体组装机制至关重要,但目前实现该探测依然存在很大的挑战性.本文首次证明飞秒和频振动光谱可以提供一种高灵敏的探测工具用于探测单纳米晶上与玻尔直径(约10 nm)相当的纳米晶分子结构.利用(R-MBA)PbBr_(3)和(R-MBA)_(2)PbI纳米晶(MBA:甲基苄胺)中苯基的光谱特征来监测和频信号,发现在系综水平上,和频谱图在3067(±3)cm(v模)处有一个很强的共振峰,在3045(±4)cm^(−1)(v模)处有一个弱的肩膀峰;而在单纳米晶水平上,主要由v模和在3025(±3)cm^(-1)处的v模组成.单纳米晶水平的纳米晶颗粒倾向于平躺在表面.而随系综数目和平均尺寸的增加,纳米晶逐渐竖立起来.这一发现可能为依赖于纳米晶尺寸的光物理性质和光致发光闪烁动力学的结构起源提供有价值的信息.

Determination of molecular structures of organicinorganic hybrid perovskite(OIHP)nanocrystals at the single-nanocrystal and ensemble levels is essential to understanding the mechanisms responsible for their size-dependent optoelectronic properties and the nanocrystal assembling process,but its detection is still a bit challenging.In this study,we demonstrate that femtosecond sum frequency generation(SFG)vibrational spectroscopy can provide a highly sensitive tool for probing the molecular structures of nanocrystals with a size comparable to the Bohr diameter(∼10 nm)at the single-nanocrystal level.The SFG signals are monitored using the spectral features of the phenyl group in(RMBA)PbBr_(3) and(R-MBA)_(2)PbI_(4) nanocrystals(MBA:methyl-benzyl-ammonium).It is found that the SFG spectra exhibit a strong resonant peak at 3067±3 cm^(−1)(ν2 mode)and a weak shoulder peak at 3045±4 cm^(−1)(ν_(7a) mode)at the ensemble level,whereas a peak of theν2 mode and a peak at 3025±3 cm^(−1)(ν20b mode)at the single-nanocrystal level.The nanocrystals at the single-nanocrystal level tend to lie down on the surface,but stand up as the ensemble number and the averaged sizes increase.This finding may provide valuable information on the structural origins for size-dependent photo-physical properties and photoluminescence blinking dynamics in nanocrystals.