Aspect-Ratio-Controlled Excitonic Zeeman Effects in Alkali Tungsten Bronze Nanocrystals Revealed by Magnetic Circular Dichroism Spectroscopy

Plasmonic nanostructures are characterized by a quantized collective oscillation of free charge carriers induced by resonant photon absorption. Charge carriers in plasmonic nanostructures can interact with other quasiparticles, including excitons, phonons, or magnons, under specific conditions, resulting in the emergence of unique quantum phenomena. Here we investigated the effect of geometry of substoichiometric plasmonic cesium-doped tungsten oxide (Cs:WO3–x) nanocrystals on the interactions between free electrons and excitons. By adjusting the colloidal synthesis conditions, we controlled the nanocrystal aspect ratio (height/width) and thereby the location, concentration, and spatial distribution of charge carriers and their interaction with the exciton in prism-shaped Cs:WO3–x nanocrystals. Using magnetic circular dichroism spectroscopy, we demonstrate the modulation of the magnetic-field-induced excitonic Zeeman effects in these nanocrystals as a function of their aspect ratio, enabled by the cooperative effect of strong electron localization on oxygen vacancies at the basal plane and the delocalization of Cs-derived electrons along the prismatic plane. The results of this work show that modulation of the carrier–exciton interactions in degenerately doped semiconductor nanocrystals can be achieved by manipulation of their aspect ratio, as a simple geometric parameter, providing a potential for applications of these materials for research in nanophotonics, optoelectronics, and correlated quantum materials.