(Invited) Plasmonic Fabrication of Chiral and Magneto-Chiral Nanostructures
Authors: Tatsuma Tetsu , Ishida Takuya 
Published: December 2024

Chiral and magneto-chiral plasmonic nanostructures attract attention because they have various potential applications including catalysts, chemical sensors, and optical and optoelectronic materials and devices. In many cases, those nanostructures are fabricated by lithographic techniques. However, those top down methods are generally time-consuming and expensive. Therefore, we have developed photoelectrochemical methods in which site-selective deposition or dissolution reactions are driven by optical near field generated around anisotropic metal nanoparticles under right- or left-circularly polarized light (CPL).
We have used gold or silver nanocuboids,1 nanorods,2 nanocubes,3 triangular nanoplates,4 and intricate nanoporous films5 on semiconducting or dielectric substrates as anisotropic metal precursors for preparation of chiral plasmonic nanostructures. If an anisotropic metal nanoparticle is irradiated with CPL, chiral electric field is generated around the nanoparticle, and energetic electron-hole pairs generate at the resonance sites, where electric field is localized. As a result, reductive deposition of silver or oxidative deposition of lead oxide proceeds preferentially at the resonance sites, resulting in formation of chiral plasmonic nanostructures, which exhibit circular dichroism (CD).
Chiral metal nanoparticles were also prepared from less anisotropic, circular metal nanodisks. In this case, initial nucleation of metal at an arbitrary site of the nanodisk edge breaks its symmetry and gives rise to chiral electric field distributions under CPL. As a result, the nanodisk is grown into chiral plasmonic nanostructure.
We also prepared magneto-chiral nanostructures by employing superparamagnetic magnetite nanocubes as precursors. The magnetite nanocubes on a glass substrate exhibit magnetic circular dichroism (MCD) but not CD. Magnetite is also reported to be conducting enough for its nanoparticles to show LSPR. Therefore, chiral electric field generates at around a magnetite nanocube under CPL and plasmonic photoelectrochemical reactions can proceed at those localized resonance sites. In addition, magnetite also has valence band and conduction band, and an electron in the valence band can be excited to the conduction band by a photon with energy higher than the band-gap energy. This photoexcitation can also occur at the localized resonance sites.6 As a result of those localized photoelectrochemical reactions induced by CPL, chiral magnetite-silver nanocomposite structures were obtained, and those nanocomposites not only MCD but also CD based on the chiral morphologies. Thus, the nanocomposites break both inversion and time-reversal symmetries. As a result, they exhibit nonreciprocal transmission of visible light, which is recognized also as magneto-chiral dichroism (MChD).

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