14 research outputs found
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 natureâs materials orchestrate intricate combinations of atoms from the library of the periodic table, nanocrystal âmetamaterialsâ integrate individual nanocrystals into larger architectures with emergent collective properties. The individual nanocrystal âmeta-atomsâ 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 âtop-downâ and âbottom-upâ 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 subphaseâsolvent 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 âmetamoleculesâ as the number of
nanocrystals is systematically varied. Precise, hexagonally ordered
Au nanocrystal oligomers with 1â31 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 NanophosphorâNanorod 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 (ÎČ-phase) NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> 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 lightâs 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 (âŒ0.2 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