Facet-Enhanced Dielectric Sensitivity in Plasmonic Metal Oxide Nanocubes

The resonant frequency of plasmonic nanoparticles depends on the refractive index of the local environment, a property which is directly useful for sensing applications and is indicative of potential utility for other applications based on near-field enhancement of light intensity. While the morphology dependence of dielectric sensitivity has been well studied in noble metal nanoparticles, less investigated is the sensitivity of degenerately doped metal oxide nanocrystals, whose plasmon resonances lie in the near- to mid-infrared. Here, we report the dielectric sensitivity of fluorine and tin co-doped indium oxide nanocubes, its dependence on their sharp faceting that gives rise to multiple plasmonic modes, and on their tin-dopant concentration. We find that the plasmon mode associated with the nanocube corners is the most sensitive and that raising dopant concentration increases dielectric sensitivity. Comparing to finite element simulations that assume a spatially uniform free electron distribution in the nanocubes, we show that the plasmon modes associated with the edges and the faces of the nanocubes are less sensitive than expected, and that their reduced dielectric sensitivity can be rationalized by the presence of band bending and a resulting surface depletion layer. Interestingly, simulations suggest that Fermi level pinning occurs predominantly on the cube faces, reshaping the free electron volume so that the depletion layer effectively insulates the faces and edges from the surrounding environment, while the corner mode remains sensitive.