Bisschop Suzanne, Geiregat Pieter, Aubert Tangi, Hens Zeger
Physics and Chemistry of Nanostructures , Ghent University , 9000 Ghent , Belgium.
Center for Nano and Biophotonics (NB Photonics) , Ghent University , 9000 Ghent , Belgium.
ACS Nano. 2018 Sep 25;12(9):9011-9021. doi: 10.1021/acsnano.8b02493. Epub 2018 Sep 12.
Colloidal quantum dots (QDs) are highly attractive as the active material for optical amplifiers and lasers. Here, we address the relation between the structure of CdSe/CdS core/shell QDs, the material gain they can deliver, and the threshold needed to attain net stimulated emission by optical pumping. On the basis of an initial gain model, we predict that reducing the thickness of the CdS shell grown around a given CdSe core will increase the maximal material gain, while increasing the shell thickness will lower the gain threshold. We assess this trade-off by means of transient absorption spectroscopy. Our results confirm that thin-shell QDs exhibit the highest material gain. In quantitative agreement with the model, core and shell sizes hugely impact on the material gain, which ranges from 2800 cm for large core/thin shell QDs to less than 250 cm for small core/thick shell QDs. On the other hand, the significant threshold reduction expected for thick-shell QDs is absent. We relate this discrepancy between model and experiment to a transition from attractive to repulsive exciton-exciton interactions with increasing shell thickness. The spectral blue-shift that comes with exciton-exciton repulsion leads to competition between stimulated emission and higher energy absorbing transitions, which raises the gain threshold. As a result, small-core/thick-shell QDs need up to 3.7 excitations per QD to reach transparency, whereas large-core/thin shell QDs only need 1.0, a number often seen as a hard limit for biexciton-mediated optical gain. This makes large-core/thin-shell QDs that feature attractive exciton-exciton interactions the overall champion core/shell configuration in view of highest material gain, lowest threshold exciton occupation, and longest gain lifetime.
胶体量子点（QDs）作为光放大器和激光器的活性材料极具吸引力。在此，我们探讨了CdSe/CdS核壳量子点的结构、它们所能提供的材料增益以及通过光泵浦实现净受激发射所需阈值之间的关系。基于初始增益模型，我们预测，减小围绕给定CdSe核生长的CdS壳层厚度将增加最大材料增益，而增加壳层厚度将降低增益阈值。我们通过瞬态吸收光谱法评估这种权衡。我们的结果证实，薄壳量子点表现出最高的材料增益。与模型定量一致的是，核和壳的尺寸对材料增益有巨大影响，对于大核/薄壳量子点，材料增益范围为2800 cm，对于小核/厚壳量子点则小于250 cm。另一方面，厚壳量子点预期的显著阈值降低并未出现。我们将模型与实验之间的这种差异归因于随着壳层厚度增加，激子 - 激子相互作用从吸引到排斥的转变。激子 - 激子排斥带来的光谱蓝移导致受激发射与更高能量吸收跃迁之间的竞争，从而提高了增益阈值。结果，小核/厚壳量子点每个量子点需要高达3.7次激发才能达到透明，而大核/薄壳量子点仅需要1.0次，这个数字通常被视为双激子介导的光学增益的硬极限。鉴于最高的材料增益、最低的阈值激子占据和最长的增益寿命，具有吸引性激子 - 激子相互作用的大核/薄壳量子点成为整体最佳的核壳配置。
