Browsing by Subject "Plasmon"

Now showing 1 - 6 of 6

Interaction between structural and electronic phase changes of metal oxide semiconductor nanocrystals
(2017-08-09) Dahlman, Clayton John; Milliron, Delia (Delia Jane); Korgel, Brian A; Mullins, Charles B; Willson, Carlton G; Henkelman, Graeme
Semiconducting metal oxides have emerged as a core class of materials in functional electronic devices because of their versatile compositions and tunable electronic and optical properties. Applying a charge to metal oxides can modulate carrier properties and induce structural changes from charge-compensating defects. However, charge-mediated transformations are contingent upon efficient transport of carriers, compensating species, or field biases into the bulk. Nanostructured materials, including colloidal metal oxide nanocrystals, can accommodate efficient charge transport across the semiconductor interface, and exhibit sensitive optical and electronic properties that arise from their nanoscale geometry. This dissertation studies the relationship between charge-mediated electronic and structural phase changes in metal oxide nanocrystals, and correlates these transformations with their nanoscale geometry and interfacial environment. The first investigation studies anatase TiO₂ nanocrystals during electrochemical charging. TiO₂ nanocrystal films can undergo two independent charging processes within a Li-ion electrolyte: surface capacitance, which raises the Fermi level upon reduction and induces Drude-like infrared localized surface plasmon resonance without affecting structure, and intercalative charging caused by the insertion of Li⁺ into the nanocrystal lattice. These two charging processes create independent dual-spectrum visible (Li-ion intercalation) and infrared (plasmon resonance) optical responses to applied bias, with applications for versatile electrochromic smart windows. The optical and electrochemical properties of both charging mechanisms are isolated and studied independently to examine the role of structure and interfacial environments on these transformations. The second part of this dissertation explores charge-mediated transformations in nanocrystalline VO₂, which has a highly non-ideal, charge-correlated electronic structure. A charge-mediated electrochemical insulator to metal transformation in VO₂ is found to be highly sensitive to nanoscale grain size, leading to a secondary metal-insulator transformation for sufficiently confined particles. The results of these studies establish general principles to control the interplay between defect-mediated structural transformations, ideal semiconductor gating behavior and interfacial environments in metal oxide nanocrystals.
Loss compensation in a plasmonic nanoparticle array
(2013-05) Miller, Shannon Marie; Alú, Andrea
The problem of heavy material and radiative losses in plasmonic devices has held back their implementation for compact and high-speed data storage and interconnects. One of the most interesting solutions to this problem currently under exploration is the addition of a gain material in close proximity to the metallic nanostructures for loss compensation. Here the physics of light transport in a nanoparticle array, and the operation of gain media in contact with the structure, are described and analytically modeled. A two-dimensional array of closely spaced gold nanoparticles has been fabricated by focused ion beam milling, and its electromagnetic response in the presence or absence of a dye coating has been simulated in preparation for pump-probe optical testing. The compensation of losses via a fluorophore coating has been proven for the first time in this geometry, for a physically realized sample.
Novel tools for ultrafast spectroscopy
(2011-12) Jarvis, Thomas William; Li, Elaine; Fink, Manfred; Keto, John; Lim, Sang-Hyun; Shih, Chih-Kang; Sitz, Greg
Exciton dynamics in semiconductor nanostructures are dominated by the effects of many-body physics. The application of coherent spectroscopic tools, such as two-dimensional Fourier transform spectroscopy (2dFTS), to the study of these systems can reveal signatures of these effects, and in combination with sophisticated theoretical modeling, can lead to more complete understanding of the behaviour of these systems. 2dFTS has previously been applied to the study of GaAs quantum well samples. In this thesis, we outline a precis of the technique before describing our own experiments using 2dFTS in a partially collinear geometry. This geometry has previously been used to study chemical systems, but we believe these experiments to be the first such performed on semiconductor samples. We extend this technique to a reflection mode 2dFTS experiment, which we believe to be the first such measurement. In order to extend the techniques of coherent spectroscopy to structured systems, we construct an experimental apparatus that permits us to control the beam geometry used to perform four-wave mixing reflection measurements. To isolate extremely weak signals from intense background fields, we extend a conventional lock-in detection scheme to one that treats the optical fields exciting the sample on an unequal footing. To the best of our knowledge, these measurements represent a novel spectroscopic tool that has not previously been described.
Simulation of a plasmonic nanowire waveguide
(2009-05) Malcolm, Nathan Patrick; Howell, John R.; Shi, Li
In this work a Finite Difference Time Domain (FDTD) simulation is employed to explore local field enhancement, plasmonic coupling, and charge distribution patterns. This 3D simulation calculates the magnetic and electric field components in a large matrix of Yee cells using Maxwell’s equations. An absorbing boundary condition is included to eliminate reflection back into the simulation chamber, and a sample system of cells is checked for convergence. In the specific simulations considered here, a laser pulse of single wavelength is incident on a silicon substrate, travels through an embedded ZnO nanowire (NW) waveguide only (due to an Ag filter), then incites plasmonic coupling at the gap between an Au nanoparticle tip and an Au substrate, an Au nanoparticle (NP), or a trio of Au nanoparticles incident on an angled Si substrate. The angle between the axis of the NW and the normal of the substrate is varied from 0-60°. The NP perpendicular deflection with respect to the NW axis is also varied from -115 - 75 nm. The enhancement patterns reveal superior signal to noise ratio compared to Near Field Scanning Optical Microscopy (NSOM), three times smaller than the NP diameter 100 nm, as well as resolution and spot size of less than 50 nm. This method of Apertureless NSOM (ANSOM) using a NW waveguide grown on a transparent microcantilever therefore shows promise for imaging of single molecules incident on a substrate and NP-labeled cell membrane.
Tunable multiscale infrared plasmonics with metal oxide nanocrystals
(2017-10-26) Agrawal, Ankit, Ph. D.; Milliron, Delia (Delia Jane); Goodenough, John B; Willson, Carlton G; Truskett, Thomas M; Krogel, Brian A
Degenerately doped semiconductor nanocrystals (NC) exhibit a localized surface plasmon resonance (LSPR) that falls in the near- to mid-IR range of the electromagnetic spectrum. Unlike metal, the metal oxide LSPR characteristics can be further tuned by doping, and structural control, or by in situ electrochemical or photochemical charging. Here, we illustrate how intrinsic NC attributes like its crystal structure, shape and size, along with band structure and surface properties affects the LSPR properties and its possible applications. First, the interplay of NC shape and the intrinsic crystal structure on the LSPR was studied using model systems of In:CdO and Cs:WO₃, the latter of which has an intrinsic anisotropic crystal structure. For both systems, a change of shape from spherical to faceted NCs led to as anticipated higher near field enhancements around the particle. However, with Cs:WO₃, presence of an anisotropic hexagonal crystal structure, leads to additional strong LSPR band-splitting into two distinct peaks with comparable intensities. Second, plasmon-molecular vibration coupling, as a proof of concept for sensing applications, was shown using newly developed F and Sn codoped In₂O₃ NCs to couple to the C-H vibration of surface-bound oleate ligands. A combined theoretical and experimental approach was employed to describe the observed plasmon-plasmon coupling, the influence of coupling strength and relative detuning between the molecular vibration and LSPR on the enhancement factor, and the observed Fano lineshape by deconvoluting the combined response of the LSPR and molecular vibration in transmission, absorption, and reflection. Third, plasmon modulation through dynamic carrier density tuning was investigated using thin films of monodisperse ITO NCs with various doping level and sizes along with an in situ electrochemical setup. From the combination of the in-situ spectroelectrochemical analysis and optical modeling, it was found that often-neglected semiconductor properties, such as band structure modification upon doping and surface chemistry, strongly affect the LSPR modulation behavior. The influence of band structure and effects like Fermi level pinning by surface defect states were shown to cause a surface depletion layer that alters the LSPR properties, namely the extent of LSPR modulation, near field enhancement, and sensitivity of the LSPR to the surrounding.
Understanding effects of disorder on the plasmonic response of nanoparticle assemblies
(2024-08) Green, Allison Marie ; Truskett, Thomas Michael,1973-; Milliron, Delia (Delia Jane); Guihua Yu; Nathaniel Lynd; Adrianne Rosales; Thomas Truskett
Rational design of macroscopic assemblies from nanoscale particles requires an understanding of how the individual particles or building blocks interact, and how their properties are modified by structural contributions across length scales. Advances in experimental strategies to assemble inorganic nanoparticles into spanning structures, such as uniform superlattice films or disordered gel networks, have led to increased interest in their use as functional materials. However, it is difficult to predict a priori how the characteristics of individual building blocks and the structure of the macroscopic assembly will impact a targeted property, limiting the ability to efficiently realize design goals. For example, the optical response of nanoparticle macro-assemblies is useful for applications in diagnostics and molecular detection, but designing these materials is challenging because how structure-dependent coupling impacts the plasmonic response of individual nanoparticles is not well understood. To understand and improve strategies for the design of functional nanoparticle assemblies, we employed both experimental and computational approaches. We began with the investigation of the influence of nanoparticle surface modifications on their interactions and assembly via depletions attractions. We systematically varied a polymer shell coating the nanocrystals and studied the resulting phase behavior in the presence of a polymer depletant, using X-ray scattering and coarse-grained molecular dynamics simulations. Different polymer shells result in different inter-nanocrystal interactions, microstructures, and, therefore, distinct optical and rheological responses. Next, to better understand how differences in disordered microstructures contribute to resulting optical responses, we developed a Brownian dynamics simulation model of two-dimensional nanoparticle monolayers spanning a range of structural defectivity. We used a mutual polarization method for the simulation of the far-field and near-field response of these heterogeneous assemblies of thousands of particles and capture how variations in local environments contribute to distributions of particle-level resonant frequencies in strong correlation with hot spot intensities. Understanding underlying nanoscale distributions help rationalize the emergent macroscale properties. Lastly, we extended this model to three-dimensional assemblies to study the importance of structural disorder in the coupling of plasmons and photons in a film, resulting in the emergence of polariton modes which can considerably increase near-field enhancement. While these modes become broadened and less well-defined with increasing disorder, the polariton dispersion relation is defect-tolerant. These findings highlight strategies for the design of disordered nanoparticle assemblies with targeted plasmonic responses, important for reducing experimental trial-and-error.