Gallium (Ga), a group III metal, is of fundamental interest due to its polymorphism and unusual phase transition behaviours. New solid phases have been observed when Ga is confined at the nanoscale. Herein, we demonstrate the stable coexistence, from 180 K to 800 K, of the unexpected solid γ-phase core and a liquid shell in substrate-supported Ga nanoparticles. We show that the support plays a fundamental role in determining Ga nanoparticle phases, with the driving forces for the nucleation of the γ-phase being the Laplace pressure in the nanoparticles and the epitaxial relationship of this phase to the substrate. We exploit the change in the amplitude of the evolving surface plasmon resonance of Ga nanoparticle ensembles during synthesis to reveal in real time the solid core formation in the liquid Ga nanoparticle. Finally, we provide a general framework for understanding how nanoscale confinement, interfacial and surface energies, and crystalline relationships to the substrate enable and stabilize the coexistence of unexpected phases.
Reconfigurable plasmonics constitutes an exciting and challenging new horizon in nanophotonics. This blooming field aims at providing plasmonic nanostructures that present a dynamic and active plasmonic response that can be switched or manipulated by external stimuli to induce a controllable change in the optical properties. Most common plasmonic materials, such as the noble metals gold and silver, cannot deliver this type of behavior. Therefore, significant effort is being invested in developing alternative materials whose optical properties can be controllably modified to provide a reconfigurable plasmonic response. In this perspective, several materials including non-noble metals, transition metal oxides and nitrides, and chalcogenide compounds will be analyzed. The selected materials share interesting characteristics like low cost, good chemical and thermal stabilities, and CMOS compatibility while presenting a reconfigurable plasmonic response governed by different phase-change mechanisms.
Localized surface plasmon resonances optically excited in metallic nanoparticles (NPs) produce beneficial thermal and nonthermal effects. Nonthermal effects, such as enhancing and localizing fields on subwavelength scales and photo-generating hot carriers, have been extensively exploited, while interest in highly localized photothermal heating is reviving. Both effects may work together synergistically, such as increasing the efficiency of a photocatalytic process, or they may work against each other, such as accelerating the desorption of analytes in surface-enhanced spectroscopy. To compare how these effects depend on the composition and size of the NP, we report a quantitative survey of thermal and nonthermal properties in the visible-solar (1.7−4.1 eV) and ultraviolet (3.1−6.2 eV) ranges for 19 metals, including conventional plasmonic materials (gold, silver, copper), an alkaline earth metal (magnesium), post-transition metals (aluminum, gallium, indium), and a wide variety of transition metals. Figures of merit that reflect the resistive losses and electric field enhancement factor of the NPs were used in this comparative analysis.
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