title: Gold nanorod length controls dispersion, local ordering, and optical absorption in polymer nanocomposite films.
abstract: The dispersion, local orientation and optical absorption of polystyrene (PS, degree of polymerization P) nanocomposites containing PS-grafted gold nanorods (Au NRs, PS degree of polymerization N), with aspect ratios (ν = length/diameter) ranging from 2.5 to 6.3, are studied using quantitative scanning electron microscopy (SEM) and optical spectroscopy. The experimentally observed nanorod assemblies and optical absorptions are compared with predictions from self-consistent field theory (SCFT) and finite difference time domain (FDTD) calculations, respectively. A pair correlation function for Au NRs is calculated from SEM images, and contains no correlation peaks for P/N = 0.9, indicating nanorods are dispersed within the nanocomposite. Large correlation peaks are observed for P/N = 7.6, representative of interparticle separation distances within nanorod aggregates, which do not vary with ν. On the basis of SCFT calculations, aggregation is attributed to significant depletion-attraction forces in the composite for P/N > 1. When Au NRs disperse, the longitudinal surface plasmon resonance (LSPR) peak red shifts from the visible into the near-IR as ν increases. No shift in the dispersed LSPR position is observed for v = 2.5 and 3.3 upon aggregation because the ratio of the interparticle distance to the nanorod length is too large for surface plasmon coupling. However, for v = 6.3, significant coupling between surface plasmons leads to a blue shift of the LSPR by approximately 140 nm, in agreement with FDTD calculations.
authors.affiliation: Department of Materials Science & Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104, USA. composto@seas.upenn.edu.
authors.forename: Dongliang
authors.initials: D
authors.lastname: Wang
authors.forename: Michael J A
authors.initials: MJ
authors.lastname: Hore
authors.forename: Xingchen
authors.initials: X
authors.lastname: Ye
authors.forename: Chen
authors.initials: C
authors.lastname: Zheng
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Murray
authors.forename: Russell J
authors.initials: RJ
authors.lastname: Composto