Highly Responsive Plasmon Modulation in Dopant-Segregated Nanocrystals

Electron transfer to and from metal oxide nanocrystals (NCs) modulates their infrared localized surface plasmon resonance (LSPR) absorption, revealing fundamental aspects of their photophysics and providing opportunities for sensing and dynamic window technologies. However, the achievable shift of the LSPR is diminished by the presence of a near-surface depletion layer with low electron concentration. The depletion layer also separates the plasmonic NC core from the surrounding environment with a deleterious effect on the near-field enhancement (NFE) of the electromagnetic field. Here, we synthesized a series of Sn-doped In2O3 NCs with dopants segregated either in the core or the shell region to tune the depletion layer and introduce a second region of band bending at the interface between doped and undoped regions. By chemically reducing these NCs, we investigated the influence of radial dopant segregation on LSPR modulation and NFE. We found that core-doped NCs show large LSPR shifts during chemical titration, enabling broadband modulation in LSPR energy of over 1000 cm−1 and of peak extinction over 300%. Simulations reveal that the evolution of the LSPR spectra during chemical reduction results from raising the surface potential and increasing the donor defect density in the shell region. Although the computationally predicted NFE is greater for shell-doped NCs as synthesized, the change in NFE when adding or removing electrons is larger for core-doped NCs. These results establish dopant segregation as a useful strategy for engineering the responsive properties of metal oxide NCs, highlighting opportunities for dynamic optical modulation in plasmonic semiconductor NC heterostructures that go beyond those accessible with conventional plasmonic metals.