Quantum Nature of THz Conductivity: Excitons, Charges, and Trions in 2D Semiconductor Nanoplatelets and Implications for THz Imaging and Solar Hydrogen Generation

As for many technological applications, excitation transport determines their performance, we investigate the THz conductivity of electrons, holes, excitons, and trions in 2D semiconductor nanoparticles. While the non-Drude-like frequency response of charge carriers in these systems has been established recently, the responses of excitons and trions remain not fully understood. We show that the exciton polarizability is related to intraexcitonic transitions between different states of relative motion and independent of the center-of-mass motion of an exciton. In contrast to simplifying models, a thermal distribution among those states leads to a considerable alteration of the resultant polarizability. To understand experimental data, we develop a quantum mechanical model for the mobility of trions and describe a linear-response-based formalism for the polarizability of excitons with a thermal distribution. Discussing the size- and aspect ratio dependent mobility of these species, we show that the particle manifold can be tuned. While for small nanoplatelets and a high number of background electrons signatures of negative trions dominate the THz response, in contrast, for extended 2D systems, excitons prevail. Like the conductance for charge carriers, the polarizability of excitons as well as mobility of trions is altered by quantization effects. Our results give basic insights to the understanding of the THz spectra of colloidal, epitaxial, and free-standing 2D semiconductors, for instance, monolayer perovskites and TMDCs, materials of current interest for solar energy harvesting, photocatalysis, or high bandwidth and low-noise nanoelectronics or THz detection in imaging systems for security applications. We provide a toolbox for the analysis of experiments and improved microscopic understanding, which in reverse allows optimization of technological applications.