We were able to synthesize rods with an average diameter of 85 nm and average lengths of 96 (), 186, 321, 465, 495, 578, 641 (), 735 (), and 1175 nm (). Scanning electron microscopy (SEM) images and histograms reveal that only rod shaped materials with a narrow size distribution are present in the solution, .
Figure 1 SEM images of rods A) 96 ± 18 nm, B) 641 ± 47 nm, C) 735 ± 48 nm, and D) 1175 ± 49 nm long with 85 ± 10 nm diameters. All of these images and the corresponding histograms found in the insets illustrate the homogeneous (more ...)
The optical properties of these rods were investigated using UV-vis-NIR spectroscopy. When the length is 96 nm, one prominent broad peak around 600 nm is observed, which is indicative of both the transverse and longitudinal dipole modes, . The longitudinal plasmon, labeled I, overlaps the transverse dipole mode because the aspect ratio (length over diameter) is close to 1, and the rod spectrum resembles that of a near isotropic sphere or disc. However, Gans theory predicts that when the aspect ratio increases, the longitudinal mode will red-shift to longer wavelengths and the transverse mode will blue-shift to slightly shorter wavelengths.19
This trend was observed in gold nanorods made via solution phase synthesis.15,17
The gold nanorods synthesized via the hard template-directed method exhibit a similar trend, and (Supporting Information Figure 2
). One particularly interesting feature is the appearance of both even (labeled II, IV, VI) and odd (labeled I, III, V, VII) higher order multipole resonances when the aspect ratio is greater than 4, . These multipole resonances have not been observed previously in a colloidal suspension of gold nanorods and are assigned based on theoretical calculations which are discussed in further detail below.
UV-vis-NIR spectra of the A) 96, B) 641, C) 735, and D) 1175 nm long gold rods in D2O. The Roman numeral labels the multipole order associated with each plasmon resonance. Orders were assigned based on theoretical calculations.
When increasing the rod aspect ratio, both the even and odd higher order longitudinal modes (II–VII) red-shift to longer wavelengths, . Another noticeable feature is the increase in the number of multipole resonances. When the length of the rod reaches 641 nm, the transverse dipole mode, labeled * in appears at 530 nm while two other higher order modes, II and III, are observed at 846 nm and 1197 nm respectively. In this size regime the longitudinal dipole mode, I, is red-shifted to longer wavelengths and not detectable in the spectral range studied. Also a small shoulder is visible around 660 nm, which corresponds to the onset of multipole IV. Again, as the aspect ratio increases, the transverse mode remains around 530 nm, while the higher order modes red-shift and increase in intensity and number, . When the length is 735 nm, the II, III, and IV multipoles are at 1397 nm, 966 nm, 775 nm, respectively, and multipole V appears at around 660 nm. This same trend is followed in the spectrum for the rods that are 1175 nm long and up to seven multipoles are detected, . In the spectra each higher order mode is resolved from the transverse mode at around 650 to 700 nm. The minimum at around 800 nm is due to detector and lamp changeover in the instrument. The assignments in are based upon the characterization of these rods by SEM and electrodynamics results which we discuss below.