Infrared Electromagnetic Field Redistribution on Graphene by Silver Nanoparticle Dimers

Background

Graphene, the thinnest conductive material known, offers exceptional electronic, thermal, and mechanical properties that have propelled it into diverse applications—from optoelectronics [1] to energy storage [8]. Its ability to confine electromagnetic energy below the diffraction limit via surface plasmons has spurred intense research in plasmonics, underpinning advances in SERS [10], sensing, and catalysis.

Hybrid structures that combine graphene with metal nanoparticles have attracted particular interest. Typically, the electromagnetic field is trapped on the metal particles, limiting the interaction with graphene. Recent work on metal film–nanoparticle systems, however, demonstrates that induced image charges can redirect the field onto the film surface—a process known as electromagnetic field redistribution [22]. While nanoparticle dimers enhance confinement relative to monomers, the effect of this redistribution on graphene had not been explored until now.

In this study we investigate the distribution of electromagnetic fields in silver nanoparticle monomers and dimers on monolayer graphene, using both numerical simulations and experimental SERS measurements. Our results show that, at infrared wavelengths, the field concentrates on the graphene surface, a behavior attributed to the graphene plasmon’s metallic response in this spectral range.

Methods/Experimental

Material and Sample Preparation

Silver nanoparticles were synthesized by reducing AgNO₃ with sodium borohydride in the presence of polyvinylpyrrolidone (PVP). Monolayer graphene was grown on copper foils by atmospheric‑pressure chemical vapor deposition (CVD) using a 25 % CH₄/H₂ flow at 1000 °C for 10–30 min, then transferred onto Si substrates with PMMA support [28]. The nanoparticles were subsequently deposited onto the graphene to form the hybrid structures.

Simulation Parameters

All electromagnetic analyses were performed with the FDTD method. The silver dimer was placed 1 nm above the graphene, with a 1 nm inter‑particle gap and 0.5 nm PVP coating. Plane waves at 633, 2000, and 3000 nm were normally incident from the nanoparticle side, with an electric field amplitude of 1 V m^–1. Perfectly matched layer (PML) boundaries absorbed outgoing waves, and frequency‑domain monitors captured the field distribution. The incident polarization was aligned along the dimer axis to maximize plasmon coupling.

Results and Discussion

Figure 1 illustrates the simulated electric‑field maps for monomer and dimer configurations at 633, 2000, and 3000 nm. At 633 nm the field localizes in the inter‑particle gap of the dimer, while the monomer shows field confined to its sides. In contrast, at 2000 nm the dimer still concentrates field at both the particle gap and the particle–graphene gap, and at 3000 nm the strongest enhancement appears in the particle–graphene gap, indicating that the graphene surface dominates the field distribution in the infrared.

Surface‑charge simulations (Figure 2) confirm this trend: at 633 nm charges cluster on the silver particles, whereas at 3000 nm they accumulate on the graphene sheet. The permittivity of monolayer graphene changes from dielectric (ε ≈ 1.5) at 633 nm to metallic (ε ≈ –19) at 3000 nm, enabling strong coupling with image charges on the nanoparticle surface. This coupling drives the field redistribution and concentrates energy on the graphene surface in the infrared.

Experimental SERS measurements (Figure 3) validate the simulations. Monolayer graphene deposited on silver monomers and dimers exhibits markedly enhanced Raman signals compared to graphene alone, demonstrating that the nanoparticles effectively amplify the local field. While the simulated enhancement factor for the dimer (~2.3 × 10² at 633 nm) exceeds the measured value, the discrepancy is attributed to experimental variations in particle geometry and gap size.

Further simulations (Figure 4) compare dielectric nanoparticle dimers with varying permittivities on graphene. Only when the nanoparticle possesses a metallic response does the field concentrate on the graphene surface at 3000 nm, reinforcing that graphene plasmons drive the observed redistribution.

Conclusion

We have demonstrated that silver nanoparticle dimers can redirect infrared light onto monolayer graphene, achieving electromagnetic field redistribution that is absent at visible wavelengths. The effect originates from the strong coupling between graphene plasmons and image charges on the nanoparticle surface. This insight broadens the potential of graphene‑based plasmonic devices for infrared sensing, photodetection, and energy conversion.

Abbreviations

CVD

Chemical vapor deposition

EM

Electromagnetic field

FDTD

Finite‑difference time‑domain

PML

Perfectly matched layer

PMMA

Poly(methyl-methacrylate)

SEM

Scanning electron microscope

SERS

Surface‑enhanced Raman spectroscopy