Ultrafast Transient Absorption and Terahertz Spectroscopy as Tools to Probe Photoexcited States and Dynamics in Colloidal 2D Nanostructures

Abstract

Two-dimensional (2D) semiconductors hold high potential for the implementation of efficient ultrathin electronics (e.g. field-effect transistors, light emitting diodes and solar cell devices). In recent years, colloidal methods to synthesize ultrathin 2D materials have been developed that offer alternatives (like the production of non-layered 2D materials and upscaling) to mechanical exfoliation methods. By focusing on optoelectronic applications, it is important to characterize the nature and dynamics of photoexcited states in these materials. In this paper, we use ultrafast transient absorption (TA) and terahertz (THz) spectroscopy as optimal tools for such a characterization. We choose recently synthesized ultrathin colloidal 2D InSe nanosheets (inorganic layer thickness 0.8-1.7 nm; ≤5 nm including ligands) for discussing TA and THz spectroscopic studies and elucidate their charge carrier dynamics under photoexcitation with TA. THz spectroscopy is then used to extract contactless AC mobilities as high as 20±2 cm²/Vs in single InSe layers. The obtained results underpin the general applicability of TA and THz spectroscopy for characterizing photoexcited states in 2D semiconductors.