Resonant Raman spectroscopy as tool to investigate colloidal semiconductor nanocrystals

胶体半导体纳米晶体或量子点（QDs）由于其作为光发射和激光，红外光电探测或太阳能转换的尺寸可调光电子材料的潜在用途而受到广泛关注。为了优化设计这种应用的量子点，重要的是要认识到，相当多的量子点特性是外在的材料性质，在很大程度上取决于量子点的表面终止。值得注意的例子包括光致发光量子产率、光致发光寿命和光稳定性。为了确保量子点原子的完全协调，并改善量子点的光学特性，一种既定的方法是将它们嵌入另一种半导体材料中，形成所谓的核/壳量子点。界面的确切性质很难调查，因为电子显微镜对比度通常不允许区分核和壳材料。关于光学性质，晶格振动声子与激发态载流子的相互作用也很重要，因为它控制着载流子的弛豫和输运。最近，在PbS量子点中观察到一个强的电子-声子耦合，解释了快速的多声子跃迁速率。然而，没有方便的实验方法来获得电子-声子耦合强度。在提出的项目中，我们计划通过共振拉曼光谱来解决这两个问题，结构特性以及电子-声子耦合。拉曼光谱研究了晶格的振动性质，并可以得出结构性质的结论。通过筛选激发波长和共振散射来执行拉曼光谱，可以将这些与电子结构联系起来。这种方法还没有建立在量子点上。最严重的问题是缺乏对量子点共振过程的理论认识。我们提出了一种实验-理论联合方法，以建立一个一致的理论，并对电子-声子耦合有一个很好的总体理解，并对核-壳和合金量子点的结构有新的认识。

Colloidal semiconductor nanocrystals or quantum dots (QDs) attract widespread attention due to their potential use as a size-tunable optoelectronic material for light emission and lasing, infrared photodetection or solar energy conversion. To optimally design QDs for such applications, it is important to realize that quite a few characteristics of QDs are extrinsic material properties that depend to a large extend on the QD surface termination. Notable examples include the photoluminescence quantum yield, the photoluminescence lifetime, and the photostability. An established approach to ensure full coordination of the quantum dot atoms, and to improve the optical properties of the QDs, is to embed them in another semiconductor material to form so-called core/shell QDs. The exact nature of the interface is difficult to investigate as electron microscopy contrast does often not allow to differentiate between core and shell materials. Regarding the optical properties, the interaction of lattice vibrations, phonons, with the excited carriers is also important as it governs the carrier relaxation and transport. Recently, a strong electron-phonon coupling in PbS QDs was observed, explaining fast multi-phonon transition rates. However, there is no convenient experimental method to access the electron-phonon couping strength. Within the proposed project, we plan addressing both, the structural properties as well as the electron-phonon coupling by resonant Raman spectroscopy. Raman spectroscopy investigates the vibrational properties of the lattice and allows to draw conclusions on the structural properties. Performing Raman spectroscopy with a screening of the excitation wavelengths, resonant scattering, allows to correlate these with the electronic structure. This method is not established for QDs yet. The most severe problem is the lack of a theoretical understanding of the resonant process in QDs. We propose a joint experiment-theory approach in order to establish a consistent theory and gain a good general understanding of the electron-phonon coupling and new insights into the structure of core-shell and alloyed QDs.