The shape of nanoparticles (NP) is critical for nanoscale phenomena including plasmonics, self-assembly, and catalysis. The common plasmonic metals Au, Ag, Cu and Al all form face centered cubic (FCC) structures with variants of cuboctahedral, singly twinned, pentatwinned, and icosahedral shapes. In contrast, Earth-abundant, plasmonic Mg crystallizes in a hexagonal close packed (HCP) configuration, giving rise to single crystal hexagonal plates as well as a potentially large array of singly twinned NP shapes. We have previously shown uncommon shapes such as folded structures that are twinned along one of the many low-energy twin boundaries, where the twin plane determines the folding angle, forming e.g., tents, chairs, tacos, and kites. Here the Wulff construction is applied to predict the full range of possible crystal shapes based on the known low energy HCP twin planes, i.e., (10-11x), x= 1, 2, 3 and (11-2y), y= 1, 2, 3, 4, yielding two groups of shapes which we refer to as folded hexagons and kite shapes, respectively. Twin and re-entrant enhancement parameters in the Wulff model allow us to simulate kinetic growth conditions, leading to various aspect ratios as well as “filled” convex shapes that resemble rods. These can explain well the singly-twinned shapes that have been observed experimentally in NP syntheses. Results from systematic optical and EELS simulations reveal the shape dependent plasmonic behaviour of these Mg NPs, with resonance energies spanning the ultraviolet, visible, and near-infrared ranges, and show the influence of the twin plane angle and filling on both the far-field and near-field response. For example, filled shapes exhibit broader scattering peaks while a stronger field localization occurs in the concave corner of shapes with a smaller folding angle. By shedding light on the properties of these Mg NPs, this theoretical study helps guide application tailored Mg NP syntheses which are currently gaining increasing popularity in the field of nanoplasmonics.

Shape and plasmonic response of twinned magnesium nanoparticles: A theoretical study

ACS Spring Meeting 2021

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