A Micellar Route to Ordered Arrays of Magnetic Nanoparticles: From Size‐Selected Pure Cobalt Dots to Cobalt–Cobalt Oxide Core–Shell Systems

Abstract

Starting with Co‐salt‐loaded inverse micelles, which form if the diblock copolymer polystyrene‐block‐poly(2‐vinylpyridine) is dissolved in a selective solvent like toluene and CoCl2 is added to the solution, monomicellar arrays of such micelles exhibiting a significant hexagonal order can be prepared on top of various substrates with tailored intermicellar distances and structure heights. In order to remove the polymer matrix and to finally obtain arrays of pure Co nanoparticles, the micelles are first exposed to an oxygen plasma, followed by a treatment in a hydrogen plasma. Applying in‐situ X‐ray photoelectron spectroscopy, it is demonstrated that: 1) The oxygen plasma completely removes the polymer, though conserving the original order of the micellar array. Furthermore, the resulting nanoparticles are entirely oxidized with a chemical shift of the Co 2p3/2 line pointing to the formation of Co3O4. 2) By the subsequent hydrogen plasma treatment the nanoparticles are fully reduced to metallic Co. 3) By exposing the pure Co nanoparticles for 100 s to various oxygen partial pressures p, a stepwise oxidation is observed with a still metallic Co core surrounded by an oxide shell. The data allow the extraction of the thickness of the oxide shell as a function of the total exposure to oxygen (p × time), thus giving the opportunity to control the ferromagnetic–antiferromagnetic composition of an exchange‐biased magnetic system.

Self-assembled arrays of zinc oxide nanoparticles from monolayer films of diblock copolymer micelles

“…As an active nanopattern, an array of cobalt nanoparticles was recently synthesized from diblock copolymer micelles although magnetic properties of the array were not reported. 12 In this communication, we demonstrate instant synthesis of optically active ZnO nanoparticles in a hexagonal order from a single-layered film of diblock copolymer micelles with oxygen plasma treatment. The near-band-edge emission was observed for the array of ZnO nanoparticles.…”

“…[8][9][10][11][12][13][14] In the case of self-assembled templates, a periodic nanostructure of diblock copolymers or their micelles was successfully employed to arrange nanoparticles with a considerable stability. [10][11][12][13][14] For example, the nanometre-sized micelles of diblock copolymers in a selective solvent for one of the blocks were transferred onto silicon wafers to form a hexagonally ordered monolayer film, which served as a structured template to direct the spatial location of nanoparticles. [10][11][12][13][14] In addition, an ordered array of metal or oxide nanoparticles was directly synthesized by employing plasma treatments as a successful method of oxidation or reduction of precursors of nanoparticles as well as removal of copolymer templates.…”

“…To place nanoparticles on a substrate in a specific arrangement, templates have been utilized, which can be lithographically fabricated or self-assembled. [8][9][10][11][12][13][14] In the case of self-assembled templates, a periodic nanostructure of diblock copolymers or their micelles was successfully employed to arrange nanoparticles with a considerable stability. [10][11][12][13][14] For example, the nanometre-sized micelles of diblock copolymers in a selective solvent for one of the blocks were transferred onto silicon wafers to form a hexagonally ordered monolayer film, which served as a structured template to direct the spatial location of nanoparticles.…”

“…[10][11][12][13][14] For example, the nanometre-sized micelles of diblock copolymers in a selective solvent for one of the blocks were transferred onto silicon wafers to form a hexagonally ordered monolayer film, which served as a structured template to direct the spatial location of nanoparticles. [10][11][12][13][14] In addition, an ordered array of metal or oxide nanoparticles was directly synthesized by employing plasma treatments as a successful method of oxidation or reduction of precursors of nanoparticles as well as removal of copolymer templates. Möller and coworkers demonstrated arrays of gold nanoparticles on various substrates with a combination of diblock copolymer micelles and plasma treatments.…”

“…Because toluene is a selective solvent for the PS block, spherical micelles of a PS corona and a PVP core were formed at this concentration. [10][11][12][13] A variety of metal salts, as a precursor of nanoparticles, can be selectively loaded into the PVP core. 10 Thus, ZnCl 2 was added to the PS-PVP micellar solution (molar ratio of ZnCl 2 : VP~0.5), then the mixed solution was stirred for at least 24 h for complete loading of ZnCl 2 into the PVP core.…”