When aliovalent dopants are sufficiently segregated to the core or near the surface of semiconductor nanocrystals, charge carriers donated by the dopants are also segregated to the core or near the surface, respectively. In Sn-doped indium oxide nanocrystals, we find that this contrast in free charge carrier concentration creates a core and shell with differing dielectric properties and results in two distinctly observable plasmonic extinction peaks. The trends in this dual-mode optical response with shell growth differ from core/shell nanoparticles composed of traditional plasmonic metals such as Au and Ag. We developed a model employing a core/shell effective medium approximation that can fit the dual-mode spectra and explain the trends in the extinction response. Lastly, we show that dopant segregation can improve sensitivity of plasmon spectra to changes in refractive index of the surrounding environment.
Electron transfer to and from metal oxide nanocrystals (NCs) modulates their infrared localized surface plasmon resonance (LSPR), revealing fundamental aspects of their photophysics and enabling dynamic optical applications. We synthesized and chemically reduced dopantsegregated Sn-doped In 2 O 3 NCs, investigating the influence of radial dopant segregation on LSPR modulation and near-field enhancement (NFE). We found that core-doped NCs show large LSPR shifts and NFE change 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 Fermi level and increasing the donor defect density in the shell region. These results establish dopant segregation as a useful strategy to engineer the dynamic optical modulation in plasmonic semiconductor NC heterostructures going beyond what is possible with conventional plasmonic metals.
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.
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