Opportunities and Challenges for Alternative Nanoplasmonic Metals: Magnesium and Beyond

Abstract

Materials that sustain localized surface plasmon resonances have a broad technology potential as attractive platforms for surface-enhanced spectroscopies, chemical and biological sensing, light-driven catalysis, hyperthermal cancer therapy, waveguides, and so on. Most plasmonic nanoparticles studied to date are composed of either Ag or Au, for which a vast array of synthetic approaches are available, leading to controllable size and shape. However, recently, alternative materials capable of generating plasmonically enhanced light-matter interactions have gained prominence, notably Cu, Al, In, and Mg. In this Perspective, we give an overview of the attributes of plasmonic nanostructures that lead to their potential use and how their performance is dictated by the choice of plasmonic material, emphasizing the similarities and differences between traditional and emerging plasmonic compositions. First, we discuss the materials limitation encapsulated by the dielectric function. Then, we evaluate how size and shape maneuver localized surface plasmon resonance (LSPR) energy and field distribution and address how this impacts applications. Next, biocompatibility, reactivity, and cost, all key differences underlying the potential of non-noble metals, are highlighted. We find that metals beyond Ag and Au are of competitive plasmonic quality. We argue that by thinking outside of the box, i.e., by looking at nonconventional materials such as Mg, one can broaden the frequency range and, more importantly, combine the plasmonic response with other properties essential for the implementation of plasmonic technologies.

Figure 1

(a) Real and (b) imaginary part of the dielectric function of various plasmonic metals with the corresponding merit indices (c) Q1 and (d) Q2. Values were obtained from Johnson and Christy (Au, Ag, Cu), Palik (Mg), Rakić (Al), Smith (Na, K), and Mathewson and Myers (In).

Figure 2

LSPR size and shape control. (a) Plasmonic spectral range of Ag, Au, Cu, Al, Mg, and In NPs over the UV–vis–NIR spectrum for various NP shapes. Paler colors indicate intrinsically low plasmonic performance regions due to interband transitions, and dashed lines indicate a potential extension of LSPR frequency in the NIR. Adapted from ref (80) with permission from the Royal Society of Chemistry. (b) Dipolar plasmon energy dependence on the plasmon length of Au NPs with different geometries. Reprinted from ref (58). Copyright 2012 American Chemical Society. (c) Experimental (open markers, dashed lines) and calculated (filled markers, solid lines) size dependence of the three lowest energy LSPRs observed in hexagonal Mg NPs. Reprinted from ref (79). Copyright 2018 American Chemical Society.

Figure 3

Single crystal and twinned shapes of FCC and HCP NPs. Adapted with permission from ref (94). Copyright 2021 Boukouvala et al.

Figure 4

Factors affecting NP toxicity.

Figure 5

Cost comparison of metal precursors for (a) high-purity bulk metals and (b) different metal salts used in colloidal synthesis in the literature for Au (HAuCl₄·3H₂O, AuBr₃, AuCl₃, KAuCl₄), Ag (AgNO₃, CH₃CO₂Ag, Ag₂SO₄), Cu (CuSO₄·5H₂O, CuSO₄, (CH₃CO₂)₂Cu, CuCl₂, CuCl₂·5H₂O), In (InCl₃, InCl₃·H₂O, (CH₃CO₂)₃In), Al (C₂H₅N(CH₃)₂·AlH₃, Al(CH₂CH(CH₃)₂)₃, ((CH₃)₂CHCH₂)₂AlH, AlCl₃), and Mg ((CH₃(CH₂)₃)₂Mg, MgCl₂); costs are from Sigma-Aldrich. Error bars represent the cost spread across the various salts.