Silver-Overgrowth-Induced Changes in Intrinsic Optical Properties of Gold Nanorods: From Noninvasive Monitoring of Growth Kinetics to Tailoring Internal Mirror Charges

Abstract

We investigate the effect of surfactant-mediated, asymmetric silver overgrowth of gold nanorods on their intrinsic optical properties. From concentration-dependent experiments, we established a close correlation of the extinction in the UV/vis/NIR frequency range and the morphological transition from gold nanorods to Au@Ag cuboids. Based on this correlation, a generic methodology for in situ monitoring of the evolution of the cuboid morphology was developed and applied in time-dependent experiments. We find that growth rates are sensitive to the substitution of the surfactant headgroup by comparison of benzylhexadecyldimethylammonium chloride (BDAC) with hexadecyltrimethylammonium chloride (CTAC). The time-dependent overgrowth in BDAC proceeds about 1 order of magnitude slower than in CTAC, which allows for higher control during silver overgrowth. Furthermore, silver overgrowth results in a qualitatively novel optical feature: Upon excitation inside the overlap region of the interband transition of gold and intraband of silver, the gold core acts as a retarding element. The much higher damping of the gold core compared to the silver shell in Au@Ag cuboids induces mirror charges at the core/shell interface as shown by electromagnetic simulations. Full control over the kinetic growth process consequently allows for precise tailoring of the resonance wavelengths of both modes. Tailored and asymmetric silver-overgrown gold nanorods are of particular interest for large-scale fabrication of nanoparticles with intrinsic metamaterial properties. These building blocks could furthermore find application in optical sensor technology, light harvesting, and information technology.

Figure 1

Schematic depiction of the synthetic pathway to Au@Ag cuboids: CTAB-stabilized spherical single-crystalline seed nanoparticles are grown into rods using CTAB and AgNO₃ as directing agents and HQ as reducing agent. In a second, step the as-prepared gold nanorods are transferred to desired surfactants (BDAC, CTAC) and subsequently overgrown with silver into Au@Ag cuboids.^,,,

Figure 2

Normalized UV/vis/NIR extinction of (a) gold nanorod cores and Au@Ag cuboids prepared with (b) CTAC or (c) BDAC as surfactant. Simulated extinction spectra are included in red color. Insets show TEM images of the final nanoparticle morphology.

Figure 3

Au@Ag cuboid growth in static experiments: heat maps of the concentration-dependent evolution of plasmonic modes complied from normalized UV/vis/NIR extinction spectra in (a) CTAC and (g) BDAC. Longitudinal modes (L-LSPR), length, width, and corresponding aspect ratio of Au@Ag cuboids synthesized in (b) CTAC and (h) BDAC plotted versus Ag to Au ratio. TEM micrographs of Au@Ag cuboids prepared with Ag to Au ratios of 0.2, 1.2, 2, and 6.4 for (c–f) CTAC and (i–l) BDAC.

Figure 4

Au@Ag cuboid growth in dynamic experiments and compared to simulation: heat maps of time-dependent UV/vis/NIR extinction spectra of syntheses performed with (a) CTAC and (b) BDAC at Ag to Au ratio of 8 (also compare Figure S6). Representative TEM insets show the final Au@Ag cuboids. The plasmonic mode (1) features an initial blue-shift followed by a slight red-shift as highlighted by the white dashed line. (c) Modeled evolution of the spectral signature considering both changes in dimension (size) and edge rounding (edge) (for simulation parameters see Table S1).

Figure 5

Surfactant-controlled overgrowth kinetics in (a) CTAC and (b) BDAC: volume change ΔV/V_Ag⁰ (black), i.e., the number of reduced ions N_Ag⁰, over time. The corresponding growth rates (red), given by the first derivative dN_Ag0/dt, exhibit three growth regimes: increasing growth rate, constant growth rate, and the cease of growth. Please note changes in scaling.

Figure 6

Schematic representation of the three-step silver overgrowth mechanism.

Figure 7

(a) Plasmonic modes evaluated from simulated extinction spectra for increasing aspect ratios for gold nanorod overgrowth with silver (solid lines) and pure silver cuboid (intersected lines). (b) Plasmonic modes (surface charge distribution) of the gold nanorod, Au@Ag cuboid, and pure silver cuboid relative to their excitation wavelength. (c) Electric field distribution of transversal modes of the Au@Ag cuboid. The edge rounding was fixed to 8%.

Figure 8

Evolution of plasmonic mode (2) during the tailored overgrowth process to emphasize the nature of in-phase and mirror charges. The mirror charges occur exclusively in the overlap region of interband and intraband of gold and silver, respectively. Surface charge distribution plots of (a) the gold nanorod core and (b–e) Au@Ag cuboids of different dimensions [width/length] and shell thicknesses (side/end). All units are in nanometers.