(Invited) New Class of Plasmonic Alloy Nanoparticles
Authors: Teranishi Toshiharu 
Published: September 2023

The past 150 years of research on inorganic nanomaterials that exhibit intrinsic localized surface plasmon resonance (LSPR) in the visible region has focused on well-disperse nanoparticles (NPs) composed of coinage metals (Au, Ag, and Cu). Recent theoretical studies on photo-excited electron dynamics in Au NPs have revealed that not only the interband transition but also the screening caused by the vibrations of bound electrons by visible light are important for visible LSPR [1]. However, the correlation between the electronic and crystal structure (as a means to control the screening effect) remains unclear because most alloys based on coinage metals have a face-centred cubic (fcc)-based structure. Here, we show that well-disperse pseudospherical intermetallic Pd-based alloy NPs with a B2 structure and PtIn2 NPs with a C1 (CaF2-type) structure intrinsically exhibit LSPR absorption in the visible region.
It is well known that chemical conversion of inorganic NPs via element substitution reactions, such as ion exchange reactions [2] and galvanic replacement reactions, can overcome the difficulties associated with controlling the size, shape, chemical composition, and crystal structure in conventional syntheses [3-5]. Especially, alloying enables us to drastically modulate the electronic properties of metallic NPs. The retained shape of the parent-NPs in element replacement reactions provides an opportunity to obtain non-equilibrium unique structures and even new structures of inorganic NPs. We demonstrate that various Pd-based alloy NPs with a B2 structure were synthesized by the element replacement reactions with Pd–P NPs and showed LSPR in the visible region [6]. This is an alchemy in nanoplasmonics in the sense that the plasmonic properties of group 11 elements are reproduced by using non-group 11 elements. On the other hand, we successfully synthesized pseudospherical PtIn2 intermetallic NPs with a C1 structure as uncommon types of visible plasmonic NPs [7].
Simulations of above alloy NPs and fcc-Au NPs indicate that both the reduction in the interband transition and the strong screening by a small number of bound d-electrons are the main origins of the visible plasmonic features of alloy NPs. Our results demonstrate that the composition and crystal structure of the intermetallic NPs of our work determine the interband transition and the screening effect. Our findings dramatically expand the plasmonic NP library and highlight remarkable NPs that are far superior, in terms of physical and chemical properties, to conventional plasmonic NPs.
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[6] Sato, R.; Takekuma, H.; Teranishi T. et al., to be submitted.
[7] Takekuma, H.; Sato, R.; Iida, K.; Teranishi, T. et al., submitted.