Gallium Plasmonics: Deep Subwavelength Spectroscopic Imaging of Single and Interacting Gallium Nanoparticles

Abstract

Gallium has recently been demonstrated as a phasechange plasmonic material offering UV tunability, facile synthesis, and a remarkable stability due to its thin, self-terminating native oxide. However, the dense irregular nanoparticle (NP) ensembles fabricated by molecular-beam epitaxy make optical measurements of individual particles challenging. Here we employ hyperspectral cathodoluminescence (CL) microscopy to characterize the response of single Ga NPs of various sizes within an irregular ensemble by spatially and spectrally resolving both in-plane and out-of-plane plasmonic modes. These modes, which include hybridized dipolar and higher-order terms due to phase retardation and substrate interactions, are correlated with finite difference time domain (FDTD) electrodynamics calculations that consider the Ga NP contact angle, substrate, and native Ga/Si surface oxidation. This study experimentally confirms previous theoretical predictions of plasmonic size-tunability in single Ga NPs and demonstrates that the plasmonic modes of interacting Ga nanoparticles can hybridize to produce strong hot spots in the ultraviolet. The controlled, robust UV plasmonic resonances of gallium nanoparticles are applicable to energy-and phase-specific applications such as optical memory, environmental remediation, and simultaneous fluorescence and surface-enhanced Raman spectroscopies.

Plasmonics in the Ultraviolet with Aluminum, Gallium, Magnesium and Rhodium

“…Similar results were obtained by Albella et al [60] through a numerical study based on the Discrete Dipole Approximation (DDA) [61]. Other methods used to study this type of nanoparticles include variable-angle spectroscopic and Mueller matrix ellipsometry [62] or hyperspectral cathodoluminescence imaging [63].…”

Immunosensing platform based on gallium nanoparticle arrays on silicon substrates

“…1a), we observe that the NP contact angle on the substrate plane is close to 90º, giving rise to hemispherical shapes. This morphology results from the surface tension of liquid Ga on the silicon substrate, and it is preserved during the functionalization works due to the formation of a thin oxide film on the NP surface (Knight et al 2015;Wu et al 2009). …”

Characterization of the topography and thickness of gallium thin films by scanning tunneling microscopy

“…While the formation of Ga grains from thinly deposited Ga films may be explained through Ostwald ripening [31] and references within, the formation of Ga grains on films whose thickness is much larger than the actual height of the grains is not trivial. Our STM measurements revealed that the first deposited Ga film had an approximate thickness of 32 µm and its surface is densely populated with grains that have large aspect ratio values from 1-1.8.…”

“…The Ga deposition method resembles the procedure used by Knight et al; [31] the main difference involved a longer deposition time, which resulted in the larger thicknesses of our Ga films. The films were prepared by depositing Ga (99.99% purity, Gallium Source, USA) on p type (boron doped) Si(111) mono-wafer substrates (Institute of Electronic Materials Technology, Poland).…”

“…Hence, some Ga grains may have a base that is equal or smaller than the projection of the Ga grain on the substrate, implying a contact angle at the base larger than 90 o , as reported in the literature. [31] The aspect ratio of the Ga grains, defined as the ratio between the height of the Ga grain to the half the diameter of its substrate projection, was obtained from these images. The aspect ratio of a hemisphere shaped grain was equal to 1, a value which can conveniently serve as a reference.…”