Optical resonances of colloidal gold nanorods: From seeds to chemically thiolated long nanorods

Following an adapted three-step seed-mediated method, we synthesize colloidal Au thin and long (L > 100 nm) nanorods (NRs) and characterize the metallic nanostructures evolving from the initial Au nanoparticle (NPs) seeds to thiolate-functionalized NRs, using HRTEM and ultraviolet, visible, and near-infrared absorption spectroscopy (UV-vis-NIR). For the long NRs we analyze the role of several solvents and rod concentration on the spectral features of the assembled products, which are further studied with simulated spectra. Superimposed to a broad resonance in the range 700-900 nm, which corresponds to short (L < 100 nm) interacting nanorods, the growth of long nanorods is clearly identified with the emergence of a robust resonance at 960 nm, linked to the three half-wavelength antenna plasmon mode. This mode is enhanced when the nanorod concentration decreases and splits into two peaks when the thiolate coverage chemically modifies the rod surface by a thin layer of nanometer size. This behavior is explained with a dielectric model based on ab initio calculations of thiolate-gold cluster surfaces.
We gratefully acknowledge the support of the Basque Departamento de Educacion and the UPV/EHU (Grant No.IT-366-07) and Spanish Ministry of Economy and Competitiveness MINECO (FIS2013-48286-C2-1-P and FIS2013-41184-P) and the ETORTEK research program funded by the Basque Departamento de Industria and the Diputacion Foral de Guipuzcoa (nanoGUNE 2014). F.J. Recio thanks a Fondecyt 3130538 Postdoctoral Grant.