Engineering Plasmonic Nanocrystal Coupling Through Template-Assisted Self-Assembly

The construction of materials from nanocrystal building blocks represents a powerful new paradigm for materials design. Just as nature’s materials orchestrate intricate combinations of atoms from the library of the periodic table, nanocrystal “metamaterials” integrate individual nanocrystals into larger architectures with emergent collective properties. The individual nanocrystal “meta-atoms” that make up these materials are themselves each a nanoscale atomic system with tailorable size, shape, and elemental composition, enabling the creation of hierarchical materials with predesigned structure at multiple length scales. However, an improved fundamental understanding of the interactions among individual nanocrystals is needed in order to translate this structural control into enhanced functionality. The ability to form precise arrangements of nanocrystals and measure their collective properties is therefore essential for the continued development of nanocrystal metamaterials. In this dissertation, we utilize template-assisted self-assembly and spatially-resolved spectroscopy to form and characterize individual nanocrystal oligomers. At the intersection of “top-down” and “bottom-up” nanoscale patterning schemes, template-assisted self-assembly combines the design freedom of lithography with the chemical control of colloidal synthesis to achieve unique nanocrystal configurations. Here, we employ shape-selective templates to assemble new plasmonic structures, including heterodimers of Au nanorods and upconversion phosphors, a series of hexagonally-packed Au nanocrystal oligomers, and triangular formations of Au nanorods. Through experimental analysis and numerical simulation, we elucidate the means through which inter-nanocrystal coupling imparts collective optical properties to the plasmonic assemblies. Our self-assembly and measurement strategy offers a versatile platform for exploring optical interactions in a wide range of material systems and application areas

Active aerosols

We unite the research fields of active plasmonics and aerosols to create an active aerosol. We demonstrate control of the orientational and positional order of ensembles of gold nanorods suspended in air at standard temperature and pressure using externally applied electric fields. Light filter, valve and gradient responses are shown, establishing active aerosols as a unique type of optical element we term component-less optic

Smectic Nanorod Superlattices Assembled on Liquid Subphases: Structure, Orientation, Defects, and Optical Polarization

Directing the orientation of anisotropic nanocrystal assemblies is important for harnessing the shape-dependent properties of nanocrystal solids in devices. We control the orientation of smectic B superlattices of CdSe/CdS dot-in-rod nanocrystals through assembly on different polar interfaces and quantify the superlattice orientation through correlated small- and wide-angle grazing-incidence diffraction. Small-angle scattering is used to determine the phase of the nanorod superlattices and their preferential growth directions from the subphase. Wide-angle diffraction is used to quantify the orientations of nanorods within the superlattices and with respect to the substrate. Not only are the nanorod long axes aligned within the structures, but truncation of the short axes also coaligns the crystal axes of the nanorods with the zone axes in assembled smectic B crystals. Three dimensional orientational alignment of nanocrystals in superlattices is highly desirable in device applications. Depending on the subphase used for self-assembly, the films range from nearly quantitative vertical to horizontal alignment. Controlling for other variables, we find that the surface tension of the subphase is strongly correlated with the orientational ordering of the nanorod superlattices. The microstructure of nanorod superlattices shows many classic defects of atomic and liquid crystalline systems. The nature of defect structures supports a mechanism of nuclei formation on the subphase–solvent interface rather than in solution. Last, we demonstrate the relationship between optical absorption polarization and superlattice structure using correlated optical spectroscopy and electron microscopy

Plasmon Resonances in Self-Assembled Two-Dimensional Au Nanocrystal Metamolecules

We explore the evolution of plasmonic modes in two-dimensional nanocrystal oligomer “metamolecules” as the number of nanocrystals is systematically varied. Precise, hexagonally ordered Au nanocrystal oligomers with 1–31 members are assembled <i>via</i> capillary forces into polygonal topographic templates defined using electron-beam lithography. The visible and near-infrared scattering response of individual oligomers is measured by spatially resolved, polarized darkfield scattering spectroscopy. The response is highly sensitive to in-plane <i>versus</i> out-of-plane incident polarization, and we observe an exponentially saturating red shift in plasmon resonance wavelength as the number of nanocrystals per oligomer increases, in agreement with theoretical predictions. Simulations further elucidate the modes supported by the oligomers, including electric dipole and magnetic dipole resonances and their Fano interference. The single-oligomer sensitivity of our measurements also reveals the role of positional disorder in determining the wavelength and character of the plasmonic response. The progression of oligomer metamolecule structures studied here advances our understanding of fundamental plasmonic interactions in the transition regime between few-member plasmonic clusters and extended two-dimensional arrays

Plasmon-Enhanced Upconversion Luminescence in Single Nanophosphor–Nanorod Heterodimers Formed through Template-Assisted Self-Assembly

We demonstrate plasmonic enhancement of upconversion luminescence in individual nanocrystal heterodimers formed by template-assisted self-assembly. Lithographically defined, shape-selective templates were used to deterministically coassemble single Au nanorods in proximity to single hexagonal (β-phase) NaYF₄:Yb³⁺,Er³⁺ upconversion nanophosphors. By tailoring the dimensions of the rods to spectrally tune their longitudinal surface plasmon resonance to match the 977 nm excitation wavelength of the phosphors and by spatially localizing the phosphors in the intense near-fields surrounding the rod tips, several-fold luminescence enhancements were achieved. The enhancement effects exhibited a strong dependence on the excitation light’s polarization relative to the rod axis. In addition, greater enhancement was observed at lower excitation power densities due to the nonlinear behavior of the upconversion process. The template-based coassembly scheme utilized here for plasmonic coupling offers a versatile platform for improving our understanding of optical interactions among individual chemically prepared nanocrystal components

Angle-Independent Optical Moisture Sensors Based on Hydrogel-Coated Plasmonic Lattice Arrays

Plasmonic nanostructures provide excellent platforms for colorimetric sensors in chemical, biological, and environmental applications. In contrast to the existing library of plasmonic sensors, we report an angle-independent optical sensor that is designed for monitoring soil moisture and operating on rough surfaces. The optical moisture sensor is constructed by coating hydrogel on top of an ultrathin, plasmonic Au nanorod lattice array, where the refractive index changes of the hydrogel upon exposure to moisture are transduced into spectral shifts of the resonances of the array. A modified Langmuir adsorption isotherm model is used to capture the dynamics of water adsorption and desorption at the interface between the sensor and the ambient environment. The nanorod length and the nanorod array pitch are systematically tuned to decouple the localized surface plasmon resonance of the nanorods and the Rayleigh anomalies of the nanorod array, creating sensors with angle-independent resonances (∼0.2 nm/deg). As a proof of concept, we place the sensor on uneven soil surfaces and demonstrate the consistent sensor resonance shift that only depends on the soil wetness. Robust, eco-friendly optical moisture sensors with angle-independent resonances provide a promising sensing platform for smart soil moisture monitoring important to tackle the challenge of water scarcity in agriculture

The Case and Approach for Continuous, Simultaneous, Global Mars Weather Monitoring from Orbit

International audienc

The Case and Approach for Continuous, Simultaneous, Global Mars Weather Monitoring from Orbit

International audienc