Spectroscopic Routes to Structural and Electrical Characterization of 2D Colloidal Semiconductor Nanocrystals

A promising alternative to gas-phase deposition of 2D semiconductors is solution-based synthetic strategies. These low-temperature methods yield colloidal “inks” of liquid-dispersed 2D nanostructures that allow for the straightforward fabrication of flexible devices. However, the colloidal nature of these 2D semiconductors introduces new challenges to the characterization of their materials properties. Here, we employ two distinct optical spectroscopic techniques to extract structural and electrical information from these 2D nanocrystals. First, bottom-up solution routes were applied to produce nearly monodisperse colloidal 2D SnS semiconductors. Next, their crystallographic phase and structure was investigated using Raman spectroscopy, where we probed the anisotropic, black phosphorus-like bonding arrangement found within the layers of SnS. Finally, we interrogate their electrical transport properties using a novel time-resolved terahertz spectroscopy strategy that allows for the non-contact determination of photoconductivity and carrier mobility within individual 2D colloidal nanocrystals. We believe these metrological innovations will prove applicable and of paramount importance to the characterization of other solution-synthesized semiconductor nanocrystal systems.