Active Plasmonic Color Tuning of Self-Assembled Ag Nanocube Monolayer

The purpose of this study is to demonstrate plasmonic color modulation for dynamic full-color tuning devices. Active plasmonic color tuning is a technology for generating various colors continuously and instantaneously on a single plasmonic substrate and is expected to be applied to variable color filters, strain sensors, and anti-counterfeiting tags. In this study, we focused on the crystalline Ag nanocube monolayer for active color tuning because the resonance wavelength of hybridization plasmon mode shifts by controlling the inter-cube distance [1]. Moreover, the nanocube shape provides higher plasmon resonance due to the smaller dispersion of electron vibrations than the nanosphere shape, leading to a narrow-band spectrum and vivid color. Here, we proposed the active color tuning by continuously controlling the inter-cube distance of the monolayer using a PDMS stretchable and transparent substrate [2].<br/>Crystalline Ag nanocubes were chemically synthesized by the polyol process [3], and these nanocubes were assembled in monolayer and high density by the Langmuir-Blodgett (LB) method [4]. The average size of the Ag nanocubes was 78.3 ± 4.6 nm, and the average cube density of the monolayer was 57.9 cubes/μm². The monolayer was fabricated in a PDMS stretchable substrate, and the transmission spectra were measured as biaxial stretching the substrate under random polarization. The color of the transmitted light changed continuously from magenta, orange, to yellow with stretching from 0 % to 20 %. We experimentally demonstrated the active plasmonic color tuning of the Ag nanocube monolayer by stretching the substrate. Furthermore, FDTD (Finite-Difference-Time-Domain) simulations were performed to investigate the dependence of the plasmon resonance wavelength on the inter-cube distance of the monolayer. The inter-cube distance of the monolayer increased linearly from 5.8 ± 5.4 nm to 24.8 ± 16.1 nm with 20 % substrate stretching by comparing the experimental results of resonance wavelength shift with FDTD simulation results.<br/>We are currently considering an electrical stretching of the substrate based on MEMS (Micro Electro Mechanical Systems) technology for the practical use of active plasmonic color tuning. The technique that combines the PDMS stretchable substrate with MEMS has already been reported as polymer-silicon hybrid nanophotonic MEMS [5]. By applying this technique, the development of a device to fine control the stretching of the substrate in two dimensions is expected. In this presentation, we would like to show the results of active plasmonic color tuning and the recent progress in electrical stretching of the substrate based on MEMS.<br/>[1] A. Mizuno, <i>et al</i>., Appl. Surf. Sci. <b>480</b>, 846-850 (2019).<br/>[2] A. Mizuno, <i>et al</i>., ACS Appl. Nano Mater. <b>4</b>, 9, 9721-9728 (2021).<br/>[3] Y. Sun, <i>et al</i>., Science <b>298</b>, 5601, 2176-2179 (2002).<br/>[4] J. A. Zasadzinski, <i>et al</i>., Science <b>263</b>, 5154, 1726-1733 (1994).<br/>[5] Steven C. Truxal, <i>et al</i>., 31st Annual International Conference of the IEEE EMBS Minneapolis, pp. 6693-6695.