Tunable Size Dependence of Quantum Plasmon of Charged Gold Nanoparticles

The quantum behavior of surface plasmons has received extensive attention, benefiting from the development of exquisite nanotechnology and the diverse applications. Blueshift, redshift, and nonshift of localized surface plasmon resonances (LSPRs) have all been reported as the particle size decreases and enters the quantum size regime, but the underlying physical mechanism to induce these controversial size dependences is not clear. Herein, we propose an improved semiclassical model for modifying the dielectric function of metal nanospheres by combining the intrinsic quantized electron transitions and surface electron injection or extraction to investigate the plasmon shift and LSPR size dependence of the charged Au nanoparticles. We experimentally observe that the nonmonotonic blueshift of LSPRs with size for Au nanoparticles is turned into an approximately monotonic blueshift by increasing the electron donor concentration in the reduction solution, and it can also be transformed to an approximately monotonic redshift after surface passivation by ligand molecules. Moreover, we demonstrate controlled blueshift and redshift for the electron and hole plasmons in ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ core-shell nanoparticles by injecting electrons. The experimental observations and the theoretical calculations clarify the controversial size dependences of LSPR reported in the literature, reveal the critical role of surface electron injection or extraction in the transformation between the different size dependences of LSPRs, and are helpful for understanding the nature of surface plasmons in the quantum size regime.

Figure 1

Hard boundary box model of charged AuNPs and calculated extinction spectra. (a) Core-shell model of AuNP. (b) Electron is injected into the surface layer of Au ($\mathrm{\Delta }{\rho }_{\mathrm{CT}}>0$). (c) Electron is extracted from the surface layer of Au ($\mathrm{\Delta }{\rho }_{\mathrm{CT}}<0$). (d) Calculated LSPRs of AuNPs ($D=6\mathrm{nm}$) with different $\mathrm{\Delta }{\rho }_{\mathrm{CT}}$. (e) Calculated LSPRs of AuNPs with different diameters ($\mathrm{\Delta }{\rho }_{\mathrm{CT}}=0$). (f) LSPR wavelength of AuNPs ($D=3.5$, 6.0, and 12.0 nm) with varied $\mathrm{\Delta }{\rho }_{\mathrm{CT}}$. (g) LSPR size dependences of AuNPs with $\mathrm{\Delta }{\rho }_{\mathrm{CT}}=-0.3$, 0, 0.4, and 1.0.

Figure 2

Nonmonotonic-shifting LSPR size dependence of AuNPs synthesized by a reduction reaction. (a) Measured and nominal sizes ($D$ and $D_{\text{Nom}}$) of AuNPs prepared with various volumes of ${\mathrm{HAuCl}}_4$ ($V_{\mathrm{Au}+}$). (b) Size-dependent extinction spectra of AuNPs with $m_{\mathrm{AA}}/m_{\mathrm{Au}+}=1.5$ ($D_{\text{Nom}}=2.1–8.0\mathrm{nm}$). The spectra are vertically shifted for clarity. (c) Size dependence of the plasmon resonance wavelength of the AuNPs synthesized with $m_{\mathrm{AA}}/m_{\mathrm{Au}+}=1.5$.

Figure 3

LSPR size dependences of AuNPs with electron injection and electron extraction. (a) Schematic illustration of electron injection from electron donors (AA) to AuNPs. (b) Schematic illustration of electron extraction from AuNPs to ligand molecules. (c) The LSPR size dependence is gradually transformed from the nonmonotonic shift to an approximately monotonic blueshift when the value of $m_{\mathrm{AA}}/m_{\mathrm{Au}+}$ increases from 5.0 to 450 (0 day). (d) The LSPR size dependence is gradually transformed from the nonmonotonic shift to an approximately monotonic redshift when the passivation time increases from 1 to 6 days ($m_{\mathrm{AA}}/m_{\mathrm{Au}+}=5.0$).

Figure 4

LSPR blueshift of ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ heteronanostructures with electron injection. (a),(b) Schematic illustration of the ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ core-shell heteronanostructure and band alignment. (c) TEM images of ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ with $D_{\mathrm{Au}}=7.5\pm 0.4\mathrm{nm}$ and average ${\mathrm{Cu}}_{2-x}\mathrm{S}$ shell thickness of 5 nm. (d) Extinction spectra of ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ ($D_{\mathrm{Au}}=2.1\mathrm{nm}$) prepared with various volumes of ${\mathrm{NaBH}}_4$ as the electron donor. The volume of 1 M ${\mathrm{NaBH}}_4$ is gradually increased from $2.5\mu \mathrm{L}$ to $50\mu \mathrm{L}$. The gray dotted line refers to the original extinction spectrum of AuNPs with diameter of 2.1 nm. The spectra are vertically shifted for clarity. (e) The h-SPR intensity at 1100 nm and e-SPR wavelength as a function of the ${\mathrm{NaBH}}_4$ volume of ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ with $D_{\mathrm{Au}}=2.1\mathrm{nm}$ and $D_{\mathrm{Au}}=6.0\mathrm{nm}$.

Figure 5

LSPR size dependences of ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ and ${\mathrm{Cu}}_2\mathrm{S}@\mathrm{Au}$. (a) Size-dependent extinction spectra of ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ with $D_{\mathrm{Au}}$ varying from 8.0 to 2.1 nm (without adding ${\mathrm{NaBH}}_4$). The spectra are vertical shifted for clarity. (b),(c) LSPR size dependences of ${\mathrm{Cu}}_{2-x}\mathrm{S}@\mathrm{Au}$ and ${\mathrm{Cu}}_2\mathrm{S}@\mathrm{Au}$ (reduced by $50\mu \mathrm{L}$ ${\mathrm{NaBH}}_4$).