Surface Plasmon Dynamics of High-Aspect-Ratio Gold Nanorods

Abstract

Ultrafast transient absorption studies are reported for high-aspect-ratio gold nanorods that were fabricated by electrochemical deposition in polycarbonate templates. The nanorods are 60 nm in diameter with distribution of lengths of up to 6 μm. The average aspect ratio was ∼50, resulting in a longitudinal surface plasmon resonance (SPR L ) band in the mid-IR, as well as a transverse (SPR T ) band in the visible. The rods were excited at 400 nm and probed at a range of wavelengths from the visible to the mid-IR to interrogate both SPR bands. The dynamics observed, including the electron-phonon coupling time and coherent acoustic breathing mode oscillations, closely resemble those previously reported for gold spherical nanoparticles and smaller-aspect-ratio nanorods. The electron-phonon coupling time was similarly determined to be 3.3±0.2 ps for both of the SPR bands. Also, oscillations with a 32-ps period were observed for probing near the SPR T band in the visible region due to impulsive coherent excitation of the acoustic breathing mode, which are consistent with the 60-nm diameter of the nanorods determined by scanning electron microscopy. The results demonstrate that the dynamics for long gold nanorods are similar to those for smaller nanoparticles.

Plasmonic Optical Properties and Applications of Metal Nanostructures

“…It has been found that the relaxation time is similar to that in spherical nanoparticles and bulk. [180][181][182][183][184] This indicates that the electron-phonon coupling is not affected by the shape of the nanostructure in any significant manner.…”

Physical properties of elongated inorganic nanoparticles

“…This is an empirical formula, which was derived by fitting data extracted from dark field optical spectroscopy of individual nanorods of various sizes and aspect rations. In a recent work, Au nanorods with very large aspect ratios (around 50), which were synthesized via an electrochemical approach, exhibited a shift in the longitudinal plasmon resonance that was far bigger than that predicted by the Gans' theory [261]. The maximum of the band was at 12 µm, while Gans' theory predicts only 4 µm.…”

“…Therefore the electron-phonon coupling constant, at least for Au, remains practically unchanged down to 2 nm. The ratio is instead larger in other materials, like for example gallium, silver and tin (see [261] and references cited therein). As an example, for tin nanoparticles (tin has 4 valence electrons and much lighter mass) decreasing relaxation times with decreasing particle size (from 6 down to 2 nm) were measured [288].…”

“…Other experiments have confirmed these results. In a recent study [261] the surface plasmon behavior of nanorods with very large aspect ratios (around 50) was studied, for which the longitudinal plasmon resonance was found to fall in the mid-infrared (12 µm, as discussed in Section 4.5). Also in that case, electron-phonon coupling constants similar to that of the spheres and of shorter rods were measured.…”

Application of Gold Nanorods for Plasmonic and Magnetic Imaging of Cancer Cells

“…Among the various nanoparticles being actively investigated for biomedical applications, gold nanorods (GNRs) have attracted increasing attention as efficient tags of various diagnostic and therapeutic agents, primarily owing to their distinct optical and plasmonic properties, nontoxicity, and biocompatibility [24][25][26][27][28][29][30][31][32][33][34][35]. GNR was selected as a nanovector for a variety of bio-applications because of the distinct features of GNR among the other nanoparticles.…”