Resonant Raman spectroscopy as tool to investigate colloidal semiconductor nanocrystals

• Applicants: Professor Dr. Gabriel Bester; Professor Dr. Holger Lange; Professorin Dr. Janina Maultzsch; Professor Dr. Horst Weller
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 410910897

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.

• DFG Programme: Research Grants