Tuning of Localized Surface Plasmons in Vanadium Dioxide Nanoparticles via Size and Insulator-Metal Transition

Vanadium dioxide has been identified as a promising phase-changing material for use in tunable plasmonic devices. In this study, we present a comprehensive modal analysis of single-phase and multi-phase vanadium dioxide nanoparticles. In-situ high-resolution electron energy loss spectroscopy was utilized to experimentally resolve the dipole plasmon peak, higher-order and breathing plasmonic modes, and bulk losses as a function of nanoparticle size. Furthermore, the focus is directed toward capturing the dynamic nanoscale optical response throughout the metal-insulator transition in a vanadium dioxide nanoparticle. This system possesses the ability to be gradually switched on and off in terms of the emergence of near-infrared plasmonic absorption. The switching is accompanied by a gradual spectral shift of the absorption peak, amounting to 0.18 eV for a 120 nm nanoparticle. It is envisioned that this phenomenon can be generalized to larger nanostructures with a higher aspect ratio, thereby introducing a wider tunability of the system, which is essential for functional nanodevices based on vanadium dioxide.

💡 Research Summary

This paper presents a comprehensive study of localized surface plasmons (LSPs) in individual vanadium dioxide (VO₂) nanoparticles, focusing on how particle size and the insulator‑metal phase transition (IMT) can be used to tune plasmonic resonances. The authors fabricated hemispherical VO₂ nanoparticles ranging from 50 nm to 220 nm in diameter on silicon nitride membranes using a two‑step thin‑film dewetting process. High‑resolution scanning transmission electron microscopy combined with electron energy‑loss spectroscopy (STEM‑EELS) was employed in situ to acquire spatially resolved loss spectra at three beam positions: outside the particle (≈10 nm from the edge), near the edge, and at the particle centre.

At low temperature (25 °C, insulating phase) the spectra show a broad feature around 1.4 eV, attributed to interband transitions of VO₂, with no distinct plasmon peak. Upon heating to 180 °C (metallic phase) two clear peaks emerge: a low‑energy peak near 0.9 eV, identified as the dipole LSP mode, and a higher‑energy peak around 1.28 eV, corresponding to the bulk plasmon of metallic VO₂. Lorentzian fitting of the loss spectra yields peak positions, amplitudes, and damping parameters (γ), allowing quantitative comparison of mode strength and quality factor.

Size‑dependent measurements reveal that the dipole LSP peak red‑shifts by about 0.32 eV as the particle diameter increases from 50 nm to 220 nm, while the bulk plasmon energy remains essentially constant at ~1.31 eV. Simulations using the boundary‑element method reproduce these trends and additionally predict the emergence of higher‑order modes (HOMs) for particles larger than ~300 nm, appearing as shoulders near 1 eV. The experimental data for the largest particles indeed show faint features consistent with these HOMs.

Dynamic experiments tracking a single 120 nm particle through repeated heating‑cooling cycles demonstrate continuous tuning of the dipole LSP: the peak shifts by 0.18 eV across the IMT, reflecting the gradual change in carrier density and complex dielectric function during the transition. Hysteresis in the peak position mirrors the well‑known thermal hysteresis of VO₂, confirming that the plasmonic response directly follows the electronic phase change.

The authors discuss the implications of these findings for active plasmonic devices. Because the IMT in VO₂ can be triggered electrically or optically on sub‑picosecond timescales, the demonstrated size‑ and temperature‑controlled plasmon tuning offers a pathway to ultrafast, reconfigurable nanophotonic components such as switches, modulators, and tunable metasurfaces. The relatively large damping in VO₂ leads to broadened resonances, which can be mitigated by engineering particle geometry (e.g., rods, disks) or by coupling to low‑loss dielectric environments.

In summary, the study establishes VO₂ nanoparticles as a versatile platform where both geometric (size) and external (thermal, electrical, optical) stimuli can be harnessed to modulate localized plasmonic modes. This dual‑control strategy expands the design space for functional nanodevices that require rapid, reversible tuning of optical properties, and it opens avenues for integrating phase‑change materials into next‑generation plasmonic and metamaterial technologies.