How Depletion Layers Govern the Dynamic Plasmonic Response of In-doped CdO Nanocrystals

Doped metal oxide nanocrystals exhibit localized surface plasmon resonance that is widely tunable across the mid- to near-infrared region, making them useful for applications in optoelectronics, sensing, and photocatalysis. Surface states pin the Fermi level and induce a surface depletion layer that hinders conductivity and refractive index sensing, but can be advantageous for optical modulation. Several strategies have been developed to both synthetically and post-synthetically tailor the depletion layer towards particular applications, however, this understanding has primarily been advanced in Sn-doped In2O3 (ITO) nanocrystals, leaving open questions about generalizing to other doped metal oxides. Here, we quantitatively analyze the depletion layer in In-doped CdO (ICO) nanocrystals, which we show have an intrinsically wide depletion layer that leads to broad plasmonic modulation via post-synthetic chemical reduction and ligand exchange. Leveraging these insights, we apply depletion layer tuning to enhance the inherently weak plasmonic coupling in ICO nanocrystal superlattices. Our results demonstrate how electronic band structure dictates the radial distribution of electrons and governs the response to post-synthetic modulation, enabling the design of tunable and responsive plasmonic materials.