Effective Optoelectrical Switching by Using Pseudo-Single Crystal of Monolayer Array of 2D Polymer–Plasmonic Nanoparticles System

Abstract

The Langmuir–Blodgett (LB) technique is used to assemble molecular compounds and colloidal nanoparticles into a monolayer on the surface of a substrate. Although the LB technique was used successfully in the assembly of the nanoparticles and molecules into monolayer, ordering of the nanoparticles or the molecules inside the LB film was not highly controlled. Functionalization of long chain polymers, which are able to semicrystallize into a monolayer, with the surface of colloidal nanoparticles can lead to highly ordered 2D arrays upon LB assembly. Poly(ethylene glycol) (PEG) with an average molecular weight of 30 000 g/mol is able to organize both isotropic 40 nm gold nanocubes (AuNCs) and anisotropic 40 nm gold nanorods into highly ordered 2D arrays. The separation distances between the nanoparticles in such LB assemblies are comparable, and the distances decreased upon increasing LB surface pressure. A strong sharp localized surface plasmon resonance (LSPR) spectrum of 2D AuNC arrays of full width at half-maximum (fwhm) of 46 nm is obtained when they are organized into highly ordered, well-separated 2D arrays on the surface of a substrate. The high efficiency of the plasmonic AuNC 2D arrays is exploited for optoelectrical switching applications. The LSPR peak of the AuNC 2D arrays coated with a thin film of poly(3-hexylthiophene), an electrochromic polymer that changes its optical properties upon electrochemical oxidation, reversibly blue-shifts by 9 nm upon the electrochemical oxidation of the polymer and shifts back after electrochemical reduction. The narrow LSPR spectrum of the fabricated 2D arrays decreases the value of the figure of merit and thus improves their sensing efficiency.