Plasmon Coupling in Nanorod Assemblies: Optical Absorption, Discrete Dipole Approximation Simulation, and Exciton-Coupling Model

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• 作者：Prashant K. Jain ; Susie Eustis ; Mostafa A. El-Sayed
• 刊名：Journal of Physical Chemistry B
• 出版年：2006
• 出版时间：September 21, 2006
• 年：2006
• 卷：110
• 期：37
• 页码：18243 - 18253
• 全文大小：272K
• 年卷期：v.110,no.37(September 21, 2006)
• ISSN：1520-5207

文摘

The shape anisotropy of nanorods gives rise to two distinct orientational modes by which nanorods can beassembled, i.e., end-to-end and side-by-side, analogous to the well-known H and J aggregation in organicchromophores. Optical absorption spectra of gold nanorods have earlier been observed to show a red-shift ofthe longitudinal plasmon band for the end-to-end linkage of nanorods, resulting from the plasmon couplingbetween neighboring nanoparticles, similar to the assembly of gold nanospheres. We observe, however, thatside-by-side linkage of nanorods in solution shows a blue-shift of the longitudinal plasmon band and a red-shift of the transverse plasmon band. Optical spectra calculated using the discrete dipole approximation methodwere used to simulate plasmon coupling in assembled nanorod dimers. The longitudinal plasmon band isfound to shift to lower energies for end-to-end assembly, but a shift to higher energies is found for the side-by-side orientation, in agreement with the optical absorption experiments. The strength of plasmon couplingwas seen to increase with decreasing internanorod distance and an increase in the number of interactingnanorods. For both side-by-side and end-to-end assemblies, the strength of the longitudinal plasmon couplingincreases with increasing nanorod aspect ratio as a result of the increasing dipole moment of the longitudinalplasmon. For both the side-by-side and end-to-end orientation, the simulation of a dimer of nanorods havingdissimilar aspect ratios showed a longitudinal plasmon resonance with both a blue-shifted and a red-shiftedcomponent, as a result of symmetry breaking. A similar result is observed for a pair of similar aspect rationanorods assembled in a nonparallel orientation. The internanorod plasmon coupling scheme concluded fromthe experimental results and simulations is found to be qualitatively consistent with the molecular excitoncoupling theory, which has been used to describe the optical spectra of H and J aggregates of organic molecules.The coupled nanorod plasmons are also suggested to be electromagnetic analogues of molecular orbitals.Investigation of the plasmon coupling in assembled nanorods is important for the characterization of opticalexcitations and plasmon propagation in these nanostructures. The surface plasmon resonance shift resultingfrom nanorod assembly also offers a promising alternative for analyte-sensing assays.