Ultraviolet–Visible Plasmonic Properties of Gallium Nanoparticles Investigated by Variable-Angle Spectroscopic and Mueller Matrix Ellipsometry

Abstract

Self-assembled, irregular ensembles of hemispherical Ga nanoparticles (NPs) were deposited on sapphire by molecular beam epitaxy. These samples, whose constituent unimodal or bimodal distribution of NP sizes was controlled by deposition time, exhibited localized surface plasmon resonances tunable from the ultraviolet to the visible (UV/ vis) spectral range. The optical response of each sample was characterized using a variable-angle spectroscopic ellipsometer, and the dielectric response of the ensemble of NPs on each sample was parametrized using Lorentz oscillators. From this, a relationship was found between NP size and the deduced Lorentzian parameters (resonant frequency, damping, oscillator strength) for most unimodal and bimodal samples at most frequencies and angles of incidence. However, for samples with a bimodal size distribution, Mueller matrix ellipsometry revealed nonspecular scattering at particular frequencies and angles, suggesting a resonant interparticle coupling effect consistent with recently observed strong local field enhancements in the ultraviolet. KEYWORDS: plasmonics, ultraviolet, gallium, nanoparticles, variable-angle spectroscopic ellipsometry, variable-angle Mueller matrix ellipsometry R esearch into the optical response of confined electronic plasma oscillations in metallic nanostructures has become the foundation of modern plasmonics. Metallic nanostructures are routinely fabricated with desired spatial geometries for specific applications including single-molecule detection, enhanced fluorescence, toxic remediation, and catalysis. 1−6 Because the optical response depends sensitively on nanostructure composition, size, and shape, increasingly sophisticated characterization tools are required: aggregate behavior may be characterized by absorption spectroscopy or dark-field microscopy, while single-particle behavior may be characterized by confocal microscopy, near-field scanning optical microscopy, and cathodoluminescence. 7−12 These tools provide incredibly detailed understanding of the absorption and scattering properties of plasmonic metal nanoparticles, especially when complemented by sophisticated electromagnetic modeling techniques such as finite difference time domain, finite element codes, and the discrete dipole approximation. 13−19 Of growing interest is the need to monitor the plasmonic properties of substrate-supported nanoparticle (NP) ensembles during fabrication. Because it is impractical to use single-particle imaging tools for this application, advances in the more traditional tools are needed to understand single-particle behaviors from aggregate measurements. Spectroscopic ellipsometry (SE) has been used to monitor how NP ensembles alter the polarization state of an incident light beam as a function of wavelength and incidence angle. 20−24 Recently, a fixed incidence angle spectroscopic ellipsometer mounted on a molecular beam epitaxy (MBE) chamber has been used to monitor in real time the deposition of nanometer-scale metallic films and NPs through...

How an oxide shell affects the ultraviolet plasmonic behavior of Ga, Mg, and Al nanostructures

“…Comparative study in the far-field: influence of NP shapes Ellipsometry has been used to measured the pseudo-dielectric function of particle-on-substrate nanostructures in order to determine their plasmonic response [12,13]. The measured imaginary part of this pseudo-dielectric function correlates directly with the absorption efficiency that is easier and more accurate to determine numerically.…”

“…In this way, with the former we mimic experimental conditions for colloidal chemistry and with the latter, the typical experimental conditions for metallic nanostructures on substrates made of either Ga or Mg [11][12][13][14]. Although the typical Al NP geometry is usually not hemispherical, we use this geometry to facilitate comparison.…”

“…|E| 2 gives an estimate of the local field enhancement or, more precisely, the local enhancement of the local energy density, close to the particle surface in the near-field regime, whose spectral distribution is generally red-shifted with respect to Q abs due to the dominant evanescent character of the near electromagnetic field [24,25]. |E| 2 is useful for predicting the plasmonic performance in surface-enhanced applications such as Raman (SERS) or fluorescence (SEF) spectroscopy [26], while Q abs helps to describe the ellipsometric response of surfaces with metallic nanostructures, especially that related to the imaginary part of the measured pseudodielectric function [12].…”

“…Consequently, both humidity and Joule heating can produce efficiency losses in both metals and blue-shifts of the resonances in Al after several uses. Although gallium shows the weakest resonance and strongest oxide sensitivity, it is of interest because its oxide is thin and stable, its resonances in the UV [11,12] are broad, and its low melting point makes it suitable for eutectic alloys that may improve its plasmonic performance [37].…”

“…In previous studies, the plasmonic performance of various metals in the UV has been analyzed. Among them, aluminum (Al) [3,[6][7][8][9][10], gallium (Ga) [7,[10][11][12][13], magnesium (Mg) [7,14], and rhodium (Rh) [15][16][17] were identified as particularly promising candidates for UV plasmonics, with properties approaching those of silver (Ag) and gold (Au) widely used for plasmonic applications in the VIS-NIR [10,18]. However, the efficiencies of these metals depends sensitively on the surrounding dielectric environment, and all of them, except Au and Rh, form native oxides that wrap the NP with a shell, whose thickness depends on both the metal and the environmental exposure conditions (temperature and pressure).…”