Binary Nanoparticles Coassembly in Bioinspired Block Copolymer Films: A Stepwise Synthesis Approach Using Multifunctional Catechol Groups and Magneto-Optical Properties

The development of hybrid films containing binary nanoparticles is one of the current key objectives in nanotechnology. Different nanoparticles dispersed in a polymer matrix exhibit diverse functions, and their properties are influenced by the interplay between the nanoparticles. The poly­(vinyl catechol)-<i>block</i>-polystyrene (PVCa-<i>b</i>-PSt) block copolymer containing a catechol group, which is inspired by the adhesive protein found in the mussel foot, is an attractive platform for the coassembly of binary nanoparticles because the catechol group has various desirable properties, including the ability to coordinate metal oxides and reduce metal ions. In the fabrication of hybrid materials based on PVCa-<i>b</i>-PSt thin films, it is challenging to control the microdomain morphologies and the nanoparticle assembly process. In this work, we investigate the impact of solvent vapor annealing on microphase-separated nanostructures in lamellae-forming PVCa-<i>b</i>-PSt. We show that a fast solvent vapor annealing in a tetrahydrofuran atmosphere for 10 min induces the formation of perpendicularly oriented lamellar structures within PVCa-<i>b</i>-PSt thin films. We also propose a stepwise approach to create binary nanoparticles coassemblies in PVCa-<i>b</i>-PSt thin films. In the first step of this process, we exploit the metal–coordination properties of PVCa-<i>b</i>-PSt catechol groups to drive the directed self-assembly of magnetite (Fe₃O₄) nanoparticles for the preparation of magnetic hybrid thin films. The Fe₃O₄ nanoparticles are dispersed and localized mainly within PVCa microdomains of the lamellar PVCa-<i>b</i>-PSt thin films. In the second step, the catechol group reduces the Ag⁺ ion in the magnetic hybrid thin films, which leads to the formation of hybrid thin films containing Ag nanoparticles. Plasmonic Ag nanoparticles and magnetic Fe₃O₄ nanoparticles coassemble in the PVCa microdomains. The resulting plasmonic/magnetic hybrid thin films exhibit an enhanced magneto-optical Kerr effect because of the localized surface plasmon resonance of the Ag nanoparticles near Fe₃O₄ nanoparticles within the PVCa microdomains. Such magneto-optical (MO) properties make the hybrid thin films interesting for imaging of magnetic fields and MO devices. Our results indicate that PVCa-<i>b</i>-PSt is a promising platform for developing well-ordered hybrid thin films containing different nanoparticles

Magnetoresistance Effect in Rubrene-Based Spin Valves at Room Temperature

We fabricate spin-valve devices with an Fe₃O₄/AlO/rubrene/Co stacking structure. Their magnetoresistance (MR) effects at room temperature and low temperatures are systemically investigated based on the measurement of MR curves, current–voltage response, etc. A large MR ratio of approximately 6% is achieved at room temperature, which is one of the highest MR ratios reported to date in organic spin valves. With decreasing measurement temperatures, we observe that the MR ratios increase because of decrease in spin scattering, and the width of the MR curves becomes larger owing to increase in the coercivity of the electrodes at low temperature. A nonlinear current–voltage dependence is clearly observed in these organic spin valves. From the measurement of MR curve for the spin valves with different rubrene layer thickness, we observe that the MR ratios monotonously decrease with increasing rubrene-layer thickness. We discuss the spin-dependent transport mechanisms in these devices based on our experimental results and the present theoretical analysis. Moreover, we note that the devices exhibit smaller MR ratios after annealing compared to their counterparts without annealing. On the basis of atomic force microscopy analysis of the organic films and device resistances, we deduce that the increase of interface spin scattering induced by large surface roughness after annealing most probably leads to reduction in the MR ratios