High Spatial Resolution Mapping of Localized Surface Plasmon Resonances in Single Gallium Nanoparticles

Abstract

materials, as certain semiconductor systems, [6] have been demonstrated to show plasmonic behavior, where the plasmon resonance is achieved by the excitation of interband transitions. [7] Among metals, noble metals as gold and silver have been the most extensively studied and exploited plasmonic systems, whose response lies on the VIS-IR spectral region. However, expanding the operability range up to the UV region is highly desirable to cover further applications, [8] which requires exploring alternative plasmonic materials.Within this context, few material systems optically active at the UV, as Mg, Al, or Ga are currently catching the attention of many research groups. [9,10] The ability to obtain these materials as colloidal suspensions offers a further advantage since it allows for their easy and cheap synthesis as nanostructures. Aluminum is an attractive choice as UV plasmonic material due to its bulk plasma frequency (15 eV) and earth abundance. [11] The strong interband transitions in Al, at about 1.5 eV, damp the possible resonances at that spectral range (NIR), while the material can support LSPR below and above that region. The short wavelengths of Al resonances (typically, 150-250 nm) become comparable to the nanoparticle (NP) size, leading to the emergence of higher order LSPRs. Regarding the spectral shape, the large linewidth of Al resonances as compared to noble metals can be understood through the modified long wavelength approximation, where the radiative damping scales with the cubed oscillation frequency (ω 3 ). [12] Although much less studied until date, Mg also shows UV plasmon resonances as consequence of its bulk plasma frequency, supporting IR resonances, too, in contrast to Al. Up to three different LSPR modes have been reported for Mg hexagonal nanoplates. [9] Another alternative UV plasmonic material is gallium (Ga), having a high bulk plasmon energy, at 14 eV; [13] exhibiting strong UV LSPR due to its negative real part of the electric function along with its low imaginary part of the dielectric function. Simple synthetic procedures allow obtaining gallium nanoparticles (Ga NPs) in a cost effective way such as thermal evaporation. The plasmonic response of the synthesized Ga NPs can be tuned, for instance, as function of their size and shape. [14][15][16] Thus, Ga NPs are among the ideal candidates to spread the application range of plasmonic systems suitable, for instance, for DNA sensing. [17] Plasmonics has emerged as an attractive field driving the development of optical systems in order to control and exploit light-matter interactions. The increasing interest around plasmonic systems is pushing the research of alternative plasmonic materials, spreading the operability range from IR to UV. Within this context, gallium appears as an ideal candidate, potentially active within a broad spectral range (UV-VIS-IR), whose optical properties are scarcely reported. Importantly, the smart design of active plasmonic materials requires their characterization at high spatial and spectral resolu...

Plasmonic coupling in closed-packed ordered gallium nanoparticles

“…This two cases lead to aspect ratios of 0.7 and 0.71. Those values are compatible to have two different separated resonances as previously confirmed with EELS measurements 59 and strengthen our premise that the LSPR band around 200-400 nm in Fig. 2(f) corresponds to an out-of-plane resonant mode.…”

“…Furthermore, the main band for each Ga mass presents at higher energies a shoulder that is likely due to higher order resonance modes such as quadrupoles based on the similarity of the NP sizes and the light wavelength in all the cases. In fact, similar Ga NP diameters have been demonstrated to support dipoles and quadrupoles modes simultaneously as evidenced by electron energy loss spectroscopy (EELS) measurements 58 . However, apart from the main band, a lower intensity bands around 200-400 nm also exist.…”

Plasmonic Characterization of 3D Printing Metal-Polymer Nanocomposites

“…Therefore, in contrast to UV-Vis spectroscopy, EELS provides near-field measurements upon electron excitation. Moreover, the extraordinary combination of spatial and spectral resolution attainable by STEM allows 2D 32,33 and 3D 22,34 single particle studies (maps and tomographic reconstructions), higher-order multipole excitation and the ability of probing dark resonant modes as well as bright modes 22,35 . Consequently, plasmonic NPs have been broadly studied by means of EELS over the last years 28 , including substrate supported systems 21 .…”

Plasmonic Properties of Individual Gallium Nanoparticles

“…Importantly, the low melting temperature of gallium allows low-temperature fabrication with low energy consumption. Prior studies reported plasmonic properties of gallium nanoparticles, , tuning of the plasmon resonance by oxidation creating Ga–Ga 2 O 3 core–shell structures that were further studied in detail, including their thermal stability, gallium–indium alloy nanoparticles, and silver–gallium alloy nanoparticles . There are numerous applications for such nanoparticles.…”

“…Only two experimental studies have focused on the optical properties of individual gallium nanoparticles. These studies utilized either electron energy loss spectroscopy or cathodoluminescence . Here, we present a study combining electron energy loss spectroscopy in a scanning transmission electron microscope (STEM-EELS) and numerical simulations to address the optical response of individual gallium nanoparticles.…”