Blueshift of the surface plasmon resonance in silver nanoparticles studied with EELS

Abstract: We study the surface plasmon (SP) resonance energy of isolated spherical Ag nanoparticles dispersed on a silicon nitride substrate in the diameter range 3.5-26 nm with monochromated electron energy-loss spectroscopy. A significant blueshift of the SP resonance energy of 0.5 eV is measured when the particle size decreases from 26 down to 3.5 nm. We interpret the observed blueshift using three models for a metallic sphere embedded in homogeneous background material: a classical Drude model with a homogeneous electron density profile in the metal, a semiclassical model corrected for an inhomogeneous electron density associated with quantum confinement, and a semiclassical nonlocal hydrodynamic description of the electron density. We find that the latter two models provide a qualitative explanation for the observed blueshift, but the theoretical predictions show smaller blueshifts than observed experimentally.in a homogeneous medium.

Blueshift of the Surface Plasmon Resonance in Silver Nanoparticles Studied with EELS

The document in focus details an experimental study on the surface plasmon (SP) resonance behavior in silver (Ag) nanoparticles, conducted via electron energy-loss spectroscopy (EELS). The research targets Ag nanoparticles dispersed on silicon nitride substrates and explores their diameters spanning from 3.5 to 26 nanometers. Crucially, it observes a pronounced blueshift in SP resonance energy, amounting to 0.5 eV as the nanoparticle size dwindles from 26 to 3.5 nm. This is a significant finding as it diverges from the classical predictions, which suggest size-independent resonance energies for subwavelength particles.

Study Methodology

The nanoparticles were synthesized chemically and stabilized in aqueous solutions, followed by deposition onto silicon nitride membranes. The experiment utilized STEM equipped with EELS to scrutinize the SP resonances demarcated from both structural and chemical particulars on the nanoscale. This technique pinpointed resonance energies with precision, facilitating the characterization of blueshift phenomena.

Theoretical Models

Three models were examined to elucidate the experimental findings:

1. Classical Local-Response Drude Model: This model presumes a homogenous electron density within the Ag nanoparticles and predicts a uniform SP resonance energy. However, it fails to account for the observed blueshift, rendering it inadequate in this context.

2. Semiclassical Local-Response Model with Quantum Confinement Correction: This model adopts an inhomogeneous electron density profile, considering the quantum confinement effects. It offers qualitative insight into the blueshift but remains quantitatively dissonant when juxtaposed with the experimental data.

3. Semiclassical Nonlocal Hydrodynamic Model: Incorporating nonlocality, this model considers variations from the classical density profile due to quantum kinetic effects within the electron gas. It aligns closely yet not entirely with experimental results, indicating the necessity for further refinement in theoretical descriptions.

Analyses and Implications

The observed blueshift is considerably robust against variations in experimental conditions, hinting at intrinsic quantum properties of the electron gas in Ag nanoparticles, rather than being induced by extrinsic factors. The implications are profound, suggesting that quantum mechanics plays an increasingly pivotal role in plasmonic behavior at diminutive scales.

Speculation about the apparent quantitative discrepancy between theory and experiment suggests that additional factors such as substrate interactions and multipole contributions may play a significant part. Furthermore, combining both the inhomogeneous density profile approach and nonlocal hydrodynamics could provide a pathway to resolving observed discrepancies.

Future Directions

For deeper insight, future research might pursue:

• Advanced Theoretical Models: Integrating substrate effects and engaging thorough multipole calculations to better accommodate the nonlocal responses and inhomogeneity in electron densities.
• Cross-Material Studies: Conducting similar EELS studies on other metals and configurations, including varied substrates to further elucidate the quantum effects governing plasmon resonance blueshift at the nanoscale.
• Substrate and Environmental Variability: Exploring different substrate materials and environmental conditions to encompass a broader spectrum of electron plasmonic interactions.

This paper fortifies the emerging narrative of quantum-driven modifications in plasmonic responses and sets a precedent for both experimental and theoretical advancements in nanoscale optics and material sciences.