Plasmonic Metal Oxide Nanocrystals as Building Blocks for Infrared Metasurfaces

Abstract

Metamaterials operating at infrared (IR) frequencies have garnered significant attention due to the opportunities for resonant interactions with vibrational modes of molecules and materials and manipulation of thermal emission. These metamaterials usually consist of periodic arrangements of subwavelength scale metallic or dielectric elements, patterned either top-down by nanolithographic methods or bottom-up by nanocrystal (NC) assembly. However, conventional metals are inherently constrained by their fixed electron concentrations, which limits the degrees of freedom in the design of the meta-atom unit cells to achieve the desired optical response. In this context, doped metal oxide NCs, with the prototypical case being tin-doped indium oxide (ITO) NCs, are exceptional candidates for self-assembled IR metamaterials, owing to their relatively low and synthetically tunable electron concentrations that govern the frequencies of their IR plasmon resonances. Focusing on ITO NCs as building blocks, this Account describes recent progress in the synthetic tuning of NC optical properties, NC superlattice monolayer preparation methods for fabricating IR resonant metamaterials, and the emerging understanding of the optical response, facilitated by recently developed simulation methods.

Based on experimental and simulation methods we helped develop, we are advancing a mechanistic understanding of how self-assembled NC metamaterials can produce distinctive near- and far-field optical properties not readily achievable in lithographically patterned structures. First, the impacts of the inevitable defects and disorder associated with self-assembly can be rationalized and, in some cases, recognized as advantageous. Second, self-assembly enables intimate nanoscale intermixing of different NC and molecular components. By incorporating probe molecules within the gaps between NCs where the electric field enhancement is the strongest, we show enhanced detection of molecular vibrations that can be optimized by tuning the size and resonance frequency of the NCs. We show how metasurfaces incorporating mixtures of NCs with different doping concentrations can achieve an epsilon-near-zero dielectric response over a broad frequency range. Finally, considering the NC metasurface itself as a building block, we show how photonic structures incorporating these assemblies can harness and amplify their distinctive properties. Through modeling the NC monolayer as a slab with an effective permittivity response, we designed a frequency-tunable IR perfect absorber by layering the NCs on a simple open cavity structure. Since the perfect absorption architecture further enhances the IR electric field localization strength, we expect that this integration strategy can enhance molecular vibration coupling or nonlinear optical response. The versatility of the NC assembly and integration approach suggests opportunities for various metal oxide NC superstructures, including mixing and stacking of NCs beyond a single monolayer, representing a vast parameter space for the design of linear and nonlinear IR optical components.