Michael Foltýn

The plasmonic lightning‐rod effect refers to the formation of a strong electric near field of localized surface plasmons at the sharp features of plasmonic antennas. Although this effect is intuitively utilized in the design and optimization of plasmonic antennas, the relation between the magnitude of the electric field and the local curvature of the plasmonic antenna has not yet been rigorously established. Here, we provide such a study. We design sets of plasmonic antennas that allow us to isolate the role of the local curvature from other effects influencing the field. The electric near field is inspected by electron energy‐loss spectroscopy and electrodynamic simulations. We confirm the existence of the plasmonic lightning‐rod effect, demonstrate its decoupling from concurrent effects influencing the field, and establish its quantitative description, showing that its strength can be interpreted as comparable to that of the electrostatic lightning‐rod effect. We also provide a simple empirical formula for the spatial dependence of the field, which includes the effective radius of curvature significantly smaller than the geometrical radius.

This review summarizes studies of localized surface plasmons in non-noble metal nanostructures. It provides an overview of their plasmonic properties, including dielectric functions, theoretical quality factors, and tunability of the dipole mode.

Bismuth nanoparticles are being investigated due to their reported photothermal and photocatalytic properties. In this study, we synthesized spherical bismuth nanoparticles (50-600 nm) and investigated their structural and optical properties at the single-particle level using analytical transmission electron microscopy. Our experimental results, supported by numerical simulations, demonstrate that bismuth nanoparticles support localized surface plasmon resonances, which can be tuned from the near-infrared to the near-ultraviolet spectral region by changing the nanoparticle size. Furthermore, plasmonic resonances demonstrate stability across the entire spectral bandwidth, enhancing the attractiveness of bismuth nanoparticles for applications over a wide spectral range. Bismuth's lower cost, biocompatibility, and oxidation resistance make bismuth nanoparticles a suitable candidate for utilization, particularly in large-scale and even industrial plasmonic applications.

Bismuth is a particularly promising alternative plasmonic metal because of its theoretically predicted wide spectral bandwidth. In this study, we experimentally demonstrate the correlation between the shape and size of individual bismuth plasmonic antennas and their optical properties. To this end, we employ a combination of scanning transmission electron microscopy and electron energy loss spectroscopy. Bar-shaped and bowtie bismuth plasmonic antennas of various sizes were fabricated by focused ion beam lithography of a polycrystalline bismuth thin film. Our experimental findings demonstrate that these antennas support localized surface plasmon resonances and their dipole modes can be tuned through their size from the near-infrared to the entire visible spectral region. Furthermore, our findings demonstrate that bismuth exhibits a plasmon dispersion relation that is nearly identical to that of gold while maintaining its plasmonic performance even at higher plasmon energies, thus rendering it a promising low-cost alternative to gold.

Gallium is a phase-changing plasmonic material offering ultraviolet-to-near-infrared tunability, facile and scalable preparation, and good stability of nanoparticles. In this work, we explore the impact of the liquid-to-solid phase transition on their plasmonic properties at the single-particle level by analytical transmission electron microscopy. We observed a phase transition from liquid to β-gallium with a freezing temperature around -135 °C and a melting temperature around -20 °C. We have shown that the dipole mode of localized surface plasmon resonances can be tuned through their size from the ultraviolet to visible spectral region, while the differences in localized surface plasmon energies between liquid gallium at 25 °C and β-gallium nanoparticles at -177 °C are minor. Our results show that the performance of gallium nanoparticles is, in the case of temperature-dependent experiments, unaffected by the liquid-to-solid phase change of gallium and paves the way for suppressing the nonradiative recombination in surface-enhanced Raman spectroscopy at cryogenic temperature.

The behavior of plasmonic antennas is influenced by a variety of factors, including their size, shape, and material. Even minor changes in the deposition parameters during the thin film preparation process may have a significant impact on the dielectric function of the film, and thus on the plasmonic properties of the resulting antenna. In this work, we deposited gold thin films with thicknesses of 20, 30, and 40 nm at various deposition rates using an ion-beam-assisted deposition. We evaluate their morphology and crystallography by atomic force microscopy, X-ray diffraction, and transmission electron microscopy. Next, we examined the ease of fabricating plasmonic antennas using focused-ion-beam lithography. Finally, we evaluate their plasmonic properties by electron energy loss spectroscopy measurements of individual antennas. Our results show that the optimal gold thin film for plasmonic antenna fabrication of a thickness of 20 and 30 nm should be deposited at the deposition rate of around 0.1 nm/s. The thicker 40 nm film should be deposited at a higher deposition rate like 0.3 nm/s.