Antolinez Felipe V, Rabouw Freddy T, Rossinelli Aurelio A, Cui Jian, Norris David J
Optical Materials Engineering Laboratory, Department of Mechanical and Process Engineering , ETH Zurich , 8092 Zurich , Switzerland.
Nano Lett. 2019 Dec 11;19(12):8495-8502. doi: 10.1021/acs.nanolett.9b02856. Epub 2019 Nov 13.
While ensembles of CdSe nanoplatelets (NPLs) show remarkably narrow photoluminescence line widths at room temperature, adding a CdS shell to increase their fluorescence efficiency and photostability causes line width broadening. Moreover, ensemble emission spectra of CdSe/CdS core/shell NPLs become strongly asymmetric at cryogenic temperatures. If the origin of these effects were understood, this could potentially lead to stable core/shell NPLs with narrower emission, which would be advantageous for applications. To move in this direction, we report time-resolved emission spectra of individual CdSe/CdS core/shell NPLs at 4 K. We observe surprisingly complex emission spectra that contain multiple spectrally narrow emission features that change during the experiment. With machine-learning algorithms, we can extract characteristic peak energy differences in these spectra. We show that they are consistent with electron "shakeup lines" from negatively charged trions. In this process, an electron-hole pair recombines radiatively but gives part of its energy to the remaining electron by exciting it into a higher single-electron level. This "shakeup" mechanism is enabled in our NPLs due to strong exciton binding and weak lateral confinement of the charge carriers. Time-resolved single-photon-counting measurements and numerical calculations suggest that spectral jumps in the emission features originate from fluctuations in the confinement potential caused by microscopic structural changes on the NPL surface (e.g., due to mobile surface charges). Our results provide valuable insights into line width broadening mechanisms in colloidal NPLs.
虽然CdSe纳米片（NPLs）的集合在室温下显示出非常窄的光致发光线宽，但添加CdS壳层以提高其荧光效率和光稳定性会导致线宽变宽。此外，CdSe/CdS核壳NPLs的集合发射光谱在低温下会变得强烈不对称。如果能理解这些效应的起源，这可能会导致发射更窄的稳定核壳NPLs，这对应用将是有利的。为了朝着这个方向发展，我们报告了单个CdSe/CdS核壳NPLs在4K时的时间分辨发射光谱。我们观察到令人惊讶的复杂发射光谱，其中包含多个在实验过程中发生变化的光谱窄发射特征。通过机器学习算法，我们可以提取这些光谱中的特征峰值能量差。我们表明，它们与带负电的三重激子产生的电子“振激线”一致。在这个过程中，电子-空穴对通过辐射复合，但将其部分能量通过将剩余电子激发到更高的单电子能级而给予该电子。由于激子的强束缚和电荷载流子的弱横向限制，这种“振激”机制在我们的NPLs中得以实现。时间分辨单光子计数测量和数值计算表明，发射特征中的光谱跳跃源于NPL表面微观结构变化（例如，由于移动表面电荷）引起的限制势的波动。我们的结果为胶体NPLs中的线宽展宽机制提供了有价值的见解。
