Plasmon damping depends on the chemical nature of the nanoparticle interface

Abstract

The chemical nature of surface adsorbates affects the localized surface plasmon resonance of metal nanoparticles. However, classical electromagnetic simulations are blind to this effect, whereas experiments are typically plagued by ensemble averaging that also includes size and shape variations. In this work, we are able to isolate the contribution of surface adsorbates to the plasmon resonance by carefully selecting adsorbate isomers, using single-particle spectroscopy to obtain homogeneous linewidths, and comparing experimental results to high-level quantum mechanical calculations based on embedded correlated wavefunction theory. Our approach allows us to indisputably show that nanoparticle plasmons are influenced by the chemical nature of the adsorbates 1,7-dicarbadodecaborane(12)-1-thiol (M1) and 1,7-dicarbadodecaborane(12)-9-thiol (M9). These surface adsorbates induce inside the metal electric dipoles that act as additional scattering centers for plasmon dephasing. In contrast, charge transfer from the plasmon to adsorbates-the most widely suggested mechanism to date-does not play a role here.

Fig. 1. Plasmon energy loss through adsorbed M1 and M9 carboranethiols.

(A) Representative normalized scattering spectrum of a 22 ± 2 nm by 66 ± 4 nm single gold nanorod. The plasmon linewidth Γ is determined from a fit to a Lorentzian function. (B) Ball-and-stick model of M1 and M9 carboranethiols with the hydrogen atoms omitted for clarity. The difference between the M1 and M9 isomers is the placement of the two carbon atoms in the boron cage. The thiol group (-SH) is more acidic when attached to a carbon atom (M1) than to a boron atom (M9). (C) Plasmon linewidth broadening over time ΔΓ(t) = Γ(t) − Γ(t₀) of 115 and 130 single gold nanorods (red and blue dots) in M1 or M9 solutions (t > t₀) compared to only solvent at the start of the measurement (t₀). Langmuir adsorption isotherms (black lines) were used to estimate the adsorption time constant k and plasmon broadening ∆Γ_∞ for a fully thiol-covered gold nanorod. Inset: Schematic illustration of an uncoated, partially, and fully carboranethiol-coated gold nanorod in a microfluidic cell. (D) Plasmon linewidth broadening ∆Γ_∞ for fully M1 and M9 carboranethiol–covered gold nanorods obtained from the Langmuirian kinetics in (C). The dashed lines in (C) and error bars in (D) and (E) indicate the 95% confidence bounds of the fitted kinetics. The difference in plasmon broadening between M1 and M9 is clearly outside the error. (E) Mean scattering of 115 and 130 single gold nanorods after t = 114 to 120 min and t = 116 to 123 min in M1 and M9 solutions relative to the scattering intensity in ethanol at t = 0 min.

Fig. 2. Complete coverage of carboranethiols on gold nanorods.

(A) Adsorption time constant k from three independent adsorption experiments for M1 (red) and M9 (blue) carboranethiols (#1, #2, and #3). The adsorption time constants are similar in each experiment and reproducible, meaning that both carboranethiols adsorbed quickly and in similar fashion on the gold nanorods. (B) Plasmon broadening at infinite time ∆Γ_∞ for fully M1 (red) and M9 (blue) carboranethiol–coated gold nanorods from three independent experiments (#1, #2, and #3) obtained from Langmuir adsorption kinetics. The plasmon linewidth broadening ∆Γ_∞ differs between M1 and M9 carboranethiols but is consistent among the three experiments carried out for each carboranethiol. Corresponding adsorption isotherms with Langmuirian kinetics are given in section S3.

Fig. 3. Magnitudes of induced surface dipoles in the metal determine plasmon damping of carboranethiols.

(A) Schematic representation of plasmon damping in a nanorod: Conduction band electrons lose their energy by scattering with phonons, defects, and other electrons (15, 18). Surface adsorbates create additional scattering centers in the form of induced dipoles (35). The scattering efficiency depends on the average distance an electron needs to travel to reach the induced dipoles, i.e., the nanorod surface (15, 18, 23, 51). (B to D) ECW theory yields the total dipole moment μ_total of the M1 and M9 carboranethiols adsorbed on gold (B; red arrows) and in the gas phase (C; yellow arrows). The difference between these dipole moments gives the induced dipole moment in gold (D; violet arrows) due to carboranethiol adsorption (section S5).