Browsing by Author "Cao, Yang"

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    Electron Energy Loss Spectroscopy and Optical Properties of Plasmonic Nanostructure
    (2015-04-15) Cao, Yang; Nordlander, Peter J.; Geurts, Frank; Halas, Naomi
    Plasmon is considered to be the incompressible self-oscillation of conducting electrons in small nanoparticles. A classical spring model could be used to describe plasmon’s behavior. Many different techniques have been applied to understand nanostructure’s plasmonic properties. Electron energy loss spectroscopy (EELS) is one of these tools, which is helpful for us to understand the interaction between fast moving electrons and nanomaterials. It could achieve very high spatial and energy resolution. Here, we develop a new finite-difference time-domain method to calculate EELS spectra and maps, which is based on a commercial software package “Lumerical”. The calculated results for different cases are compared with the well-known boundary element method (BEM) and show an excellent agreement. Our finite-difference time-domain (FDTD) method to calculate EELS spectra has further been proven really helpful by high-density plasmonic dimers’ experimental results. There are basically two different numerical techniques. One is based on finite difference method (FEM) and another is according to finite-difference time-domain method (FDTD). Both of them are very important to perform optical calculations in nanophotonics and plasmonics area. In general, they will try to solve Maxwell equations with many different boundary conditions numerically. Optical properties of nanomaterials are also very tremendous for us to understand plasmonics behavior in the external electromagnetic fields. We systematically performed FEM simulations for different dimensions’ split ring structure and identified each plasmon mode via induced charge plot. Later we also studied hollow Au Nanoshells: hollow Au-Ag Nanoshell and hollow Au-Co Nanoshell. The former showed the surprising in vivo instability in the near infrared region while the later has potential application in hot electron generation.
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    Impurity-Induced Plasmon Damping in Individual Cobalt-Doped Hollow Au Nanoshells
    (American Chemical Society, 2014) Thibodeaux, Christyn A.; Kulkarni, Vikram; Chang, Wei-Shun; Neumann, Oara; Cao, Yang; Brinson, Bruce; Ayala-Orozco, Ciceron; Chen, Chih-Wei; Morosan, Emilia; Link, Stephan; Nordlander, Peter; Halas, Naomi J.; Laboratory for Nanophotonics; Rice Quantum Institute
    The optical properties of plasmonic nanoparticles in the size range corresponding to the electrostatic, or dipole, limit have the potential to reveal effects otherwise masked by phase retardation. Here we examine the optical properties of individual, sub-50 nm hollow Au nanoshells (Co-HGNS), where Co is the initial sacrificial core nanoparticle, using single particle total internal reflection scattering (TIRS) spectroscopy. The residual Co present in the metallic shell induces a substantial broadening of the homogeneous plasmon resonance line width of the Co-HGNS, where the full width at half-maximum (fwhm) broadens proportionately with increasing Co content. This doping-induced line broadening provides a strategy for controlling plasmon line width independent of nanoparticle size, and has the potential to substantially modify the relative decay channels for localized nanoparticle surface plasmons.
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    The Surprising in Vivo Instability of Near-IR-Absorbing Hollow Au-Ag Nanoshells
    (American Chemical Society, 2014) Goodman, Amanda M.; Cao, Yang; Urban, Cordula; Neumann, Oara; Ayala-Orozco, Ciceron; Knight, Mark W.; Joshi, Amit; Nordlander, Peter; Halas, Naomi J.
    Photothermal ablation based on resonant illumination of near-infrared-absorbing noble metal nanoparticles that have accumulated in tumors is a highly promising cancer therapy, currently in multiple clinical trials. A crucial aspect of this therapy is the nanoparticle size for optimal tumor uptake. A class of nanoparticles known as hollow Au (or Au–Ag) nanoshells (HGNS) is appealing because near-IR resonances are achievable in this system with diameters less than 100 nm. However, in this study, we report a surprising finding that in vivo HGNS are unstable, fragmenting with the Au and the remnants of the sacrificial Ag core accumulating differently in various organs. We synthesized 43, 62, and 82 nm diameter HGNS through a galvanic replacement reaction, with nanoparticles of all sizes showing virtually identical NIR resonances at ∼800 nm. A theoretical model indicated that alloying, residual Ag in the nanoparticle core, nanoparticle porosity, and surface defects all contribute to the presence of the plasmon resonance at the observed wavelength, with the major contributing factor being the residual Ag. While PEG functionalization resulted in stable nanoparticles under laser irradiation in solution, an anomalous, strongly element-specific biodistribution observed in tumor-bearing mice suggests that an avid fragmentation of all three sizes of nanoparticles occurred in vivo. Stability studies across a wide range of pH environments and in serum confirmed HGNS fragmentation. These results show that NIR resonant HGNS contain residual Ag, which does not stay contained within the HGNS in vivo. This demonstrates the importance of tracking both materials of a galvanic replacement nanoparticle in biodistribution studies and of performing thorough nanoparticle stability studies prior to any intended in vivo trial application.