Resonant Raman spectroscopy as tool to investigate colloidal semiconductor nanocrystals

胶体半导体纳米晶或量子点(QD)由于在发光和激光、红外光电探测或太阳能转换等方面具有可调尺寸的光电材料的潜在用途而受到广泛关注。要为这种应用优化设计量子点，重要的是要认识到，量子点的相当多的特性是在很大程度上依赖于量子点表面端接的非本征材料特性。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配位，并改善量子点的光学性质，一种公认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研究，因为电子显微镜对比通常不能区分核和壳材料。在光学性质方面，晶格振动、声子与激发载流子的相互作用也很重要，因为它控制着载流子的驰豫和输运。最近，在PbS量子点中观察到了强烈的电子-声子耦合，这解释了快速的多声子跃迁速率。然而，目前还没有一种方便的实验方法来获得电子-声子耦合强度。在拟议的项目中，我们计划通过共振拉曼光谱来研究结构性质和电子-声子耦合。拉曼光谱研究晶格的振动性质，并允许得出关于结构性质的结论。通过筛选激发波长，即共振散射进行拉曼光谱，可以将这些波长与电子结构相关联。这种方法还没有为量子点建立起来。最严重的问题是对量子点中的共振过程缺乏理论上的理解。我们提出了一种实验-理论相结合的方法，以建立一致的理论，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.