Representation of Vivid Colors of Cyan, Magenta, Yellow, and Green in Ag Deposition-Based Plasmonic Electrochromic Device By Precise Control of Shape and Density of Deposited Ag Nanoparticles

Abstract

Ag nanoparticles exhibit various colors depending on their localized surface plasmon resonance (LSPR). Based on this phenomenon, Ag deposition-based electrochromic devices can represent various optical states in a single device such as the three primary colors (cyan, magenta, and yellow), silver mirror, black and transparent.1 A control of the morphology of Ag nanoparticles can lead to dramatic changes in color, as their size and shape influence the LSPR band. For this purpose, the “voltage-step method” was applied to a Ag deposition-based EC device to obtain multiple colors by shifting the LSPR band. In this method, two different voltages are successively applied : the first voltage V 1 is applied for a very short time t 1 to initiate Ag nucleation, and the second voltage V 2 is applied for a time t 2 to promote growth of the Ag nuclei. As V 2 is more positive than the nucleation voltage, further nucleation is no longer possible during t 2. Therefore, the Ag nanoparticle growth and the resultant device color can be controlled by changing t 2.In this study2, we aimed to improve the color quality of Ag deposition-based multicolor EC devices by precisely controlling the supply of Ag+ ions to the electrode surface. Initially, the effect of coloration by decreasing the concentration of Ag+ ions in the Ag deposition-based EC device was investigated. As a result of optimizing the driving voltage, a lower concentration device (10 mM Ag+ ion) showed vivid and bright colors of cyan and magenta. This improvement was due to the generation of fine Ag nanoparticles and the prevention of the connection of Ag particles. From the electrochemical investigation, it was suggested that the slow supply rate of Ag+ ions in the lower concentration device induced electrochemical deposition of fine and isolated Ag nanoparticles.Consequently, to obtain more uniformly deposited Ag nanoparticles, the nucleation voltage of V 1 was set to a lower voltage in the 10 mM device. As a result, spherical Ag nanoparticles were successfully obtained, and the device showed yellow and green colors. By comparing the LSPR behavior of the device with that of Ag nanoparticles dispersed in solution and using FDTD calculations, the mechanism of the yellow and green colors was investigated in detail. Thus, it indicated that the yellow coloration with an absorption band in the 400–500 nm region was caused by LSPR absorption of isolated spherical Ag nanoparticles. Furthermore, it was also observed that the coexistence of isolated spherical particles and aggregations of Ag nanoparticles induced two types of LSPR bands, leading to a good green color. References A.Tsuboi, K.Nakamura and N.Kobayashi, Chem. Mater., 2014, 26, 6477-6485S.Kimura, T.Sugita, K.Nakamura and N. Kobayashi, Nanoscale, 2020 (in press)