AVS2012 Session NS-WeA: Nanophotonics and Plasmonics

We apply time dependent density functional theory to study the absorption spectrum of arrays of nano-scale pure Au chains and those doped with transition metal (TM) atoms. We find that as the number of chains in the array increases the plasmon peak shifts to higher energies and appears in the visible range for an array of three gold chains, each consisting of 10 atoms. Doping with TM atoms also leads to the formation of additional plasmon peaks close in energy to the main one for the undoped case and is especially pronounced for Ni-doped chains. However, the response is very different when we have two chains in the array each doped with one TM atom in the middle. We trace the origin of the additional modes to the interplay between the collective and local plasmon oscillations of the chains. We compare the calculated optical absorption spectrum of the doped chains for several different types of TM atoms at different positions in the chains, and provide rationale for the trends. We also analyze the case of arrays consisting of chains of two different noble metal atoms (Au-Ag) and of arrays in which one chain is of noble metal atoms and the other of TM atoms. We find that the plasmon mode is suppressed when the second chain is composed of TM atoms. In addition we study the effect of plasmon –exciton interaction in arrays of infinite Au chains.

Work supported in part by DOE Grant DE-FG02-07ER46354

One dimensional metal nanostructures such as nanorods and nanowires exhibit interesting linear and non-linear optical properties and find various applications as electronic, photonic and sensing devices. The optical properties of these composites are dominated by localized surface Plasmon resonance (LSPR) which results from the oscillations of conduction electrons in response to an external field. The resonant frequency of the electrons depends strongly on size, shape, distribution and the surrounding dielectric medium . By controlling the physical and chemical properties of the nanorods and nanowires the electronic and optical properties of the material can be tuned for appropriate applications. Nanorods and wires with well controlled aspect ratios can be grown by Electrodeposition and Polyol synthesis methods.

Localized surface plasmon resonances (LSPRs) in semiconductors offer new opportunities to engineer the interaction of electromagnetic radiation with solid-state materials. Importantly, the carrier density of semiconductors, and thus LSPR frequency, can be modulated via doping and/or electric field. In addition to realizing novel plasmonic devices, the direct integration of plasmonic and excitonic behavior also promises fundamentally distinct functionality. Here, we demonstrate and systematically control LSPRs in nanoscale Si for the first time. More specifically, Si nanowires are synthesized via the vapor-liquid-solid (VLS) technique with a combination of Si₂H₆ and PCl₃ precursors. PCl₃ simultaneously introduces P atoms to the nanowire core and delivers Cl atoms to the sidewall so as to minimize radial dopant incorporation. This chemistry enables growth sufficiently far from equilibrium such that dopant concentrations can exceed thermodynamic limits. Electron microscopy reveals that these nanowires are single crystalline and <111> oriented with very few lattice defects. Polarization dependent in-situ infrared spectroscopy measurements show intense mid-IR absorption bands only for the P-doped nanowires, which we assign to longitudinal LSPRs. A significantly weaker transverse mode is occasionally observed as well. The LSPR frequency can be readily adjusted by varying nanowire length. Mie-Gans theory supports our experimental results and indicates that electrically active dopant concentrations exceed 10²⁰ cm^-3.