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

TL;DR

This work addresses how localized surface plasmon resonances in VO_2 nanoparticles can be tuned by size and by partial insulator–metal transitions. It combines in-situ STEM-EELS with Lorentzian deconvolution and COMSOL-based simulations to resolve dipole, higher-order, and bulk plasmon modes in single VO_2 NPs across insulating and metallic phases, and tracks the dynamic optical response during the transition. The authors demonstrate size-dependent redshifts of dipole LSPs, a near-constant bulk plasmon energy, and low quality factors due to damping, as well as tunability of up to $0.18\ \mathrm{eV}$ in partially switched nanoparticles, offering a route to temperature-sensitive, active nanophotonic elements and generalizing to higher-aspect-ratio structures for broader tunability near telecom wavelengths. These findings provide design principles for phase-change plasmonic devices that leverage the MIT in VO_2 for fast, nanoscale optical control.

Abstract

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.

Figures (7)

• Figure 1: Vanadium dioxide nanoparticles. (a) Schematic of the hemispherical VO$_2$ NP on a SiN$_x$ membrane with the incident electron beam. (b) HAADF STEM image of one region of an ensemble of VO$_2$ NPs. (c) Size distribution of the whole ensemble of NPs. (d) Measured EEL spectra of 170nm NP (highlighted in (b) with a green circle) at low (25) and high (180) temperature. The particle switches from an insulating (blue) phase to a metallic (red) phase. In the metallic phase, a pronounced plasmon peak emerges. The EEL spectra are background-subtracted and integrated over the whole particle.
• Figure 2: Spatial modal analysis of 170nm VO$_2$ NP. (a-c) Insulating VO$_2$ NP. (a) Measured EEL spectra for the insulating VO$_2$ NP (at 25) at the three regions (center, edge, out; highlighted next to the HAADF STEM micrograph of the VO$_2$ NP). (b) Numerically simulated EEL spectra for the corresponding three electron beam positions. (c) Calculated loss function of insulating bulk VO$_2$. (d-k) Metallic VO$_2$ NP. (d) Measured EEL spectra for the metallic VO$_2$ NP (at 180) from the same three regions as in (a). (e,f) Determination of the dipole mode (e) and bulk plasmon (f) energy. (g) Processing of the EEL spectrum integrated over the edge region by two Lorentzians corresponding to the dipole and bulk plasmon. (h) Numerically simulated EEL spectra. (i) Normalized magnitude of the induced electric field with the field lines around the NP at the energy 0.9eV resembling the in-plane dipole field. Electron beam passing 70nm outside the particle is marked with the green dot. (j) Calculated loss function of metallic bulk VO$_2$. (k) Processing of the calculated EEL spectrum in the center of the NP by two Lorentzians, one with fixed peak energy and FWHM, corresponding to the bulk loss function, and one with free parameters, corresponding to the breathing mode (BM).
• Figure 3: Size dependence of EEL spectra for metallic VO$_2$ NPs. (a) HAADF STEM microcrographs of five representatives. The scalebars correspond to 100nm. (b) Measured EEL spectra taken from the region outside the NPs, where the LSP (dipole mode -- DM) is excited. (c) Measured EEL spectra taken from the center of the NPs, where the bulk plasmon (BP) is excited. (d) Simulated EEL spectra for the beam position 10nm from the outer edge of the NPs. We observe the emergence of higher-order modes (HOM) for the largest NPs. (e) Simulated EEL spectra for the electron beam positioned in the center. The sizes of the NPs are at the individual colorbar legends. (f) Peak energy, (g) peak loss probability, and (h) Q factor of the dipole (maroon) and bulk (orange) plasmon mode. The experimental energies in (f) are fitted with silver dashed lines as a guide for the eyes.
• Figure 4: Tuning of the dipole plasmon mode via partial insulator-metal transition. (a) HAADF STEM image of a partially switched NP and EEL maps (1.0--1.5eV) for the NP measured at different temperatures (top) and the schematic depiction of the thermally tunable NP. The green arrows indicate the approximate size of the metallic part (brighter area). (b) Measured EEL spectra for four transition states of the NP. (c) Simulated EEL spectra for 120nm hemisphere with a beam 10nm outside. We change the position of the boundary between the insulating and the metallic phase, increasing the size $L$ of the metallic part. (d,e) Experimental peak energy (d) and peak maxima (e) dependence on temperature. (f,g) Simulated peak energy (f) and peak maxima (g) as a function of the size $L$ of the metallic part.
• Figure S1: Processing of EEL spectra. (a) HAADF STEM micrograph with marked integration areas for center (orange), edge (red), and out (maroon) signal as well as for the background, i.e., the signal from a pure silicon nitride membrane (cyan). (b) Typical zero-loss peak proving the high energy resolution of our experiments. (c) Full EEL spectrum for the out position (dashed maroon) overlaid by the background (cyan) and background-subtracted EEL spectrum (solid maroon). (d) Full EEL spectrum for the edge position (dashed red) overlaid by the background (cyan) and background-subtracted EEL spectrum (solid red). (e) Full EEL spectrum for the center position (dashed orange) overlaid by the background (cyan) and background-subtracted EEL spectrum (solid orange). Within the manuscript, background-subtracted EEL spectra are presented.