Three-dimensional nanoscale alignment of metal nanoparticles using block copolymer films as nanoreactors

We present a simple dry process to create nanoscale arrangements of metal nanoparticles within block copolymer films. This process involves only one step such that the vapor of palladium(II) acetylacetonato (Pd(acac)(2)) is exposed to a polymer film in a nitrogen atmosphere at 180 degreesC for periods up to 2 h. The Pd(acac)2 vapor penetrates into the film and then is decomposed and reduced to a metallic state to form nanoparticles therein. When block copolymer films having nanoscale microdomain structures are used, the metal complex is selectively reduced in one of the phases to give nanoscale arrangements of the metal particles that reflect the microdomain structure of the used block copolymer. In symmetric diblock copolymers of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA), Pd(acac)(2) is selectively reduced in the PS lamellae to yield Pd nanoparticles with the average diameter ranging from 3 to 4 nm with a narrow size distribution, depending on the molecular weight. As a result, the Pd nanoparticles are arranged in a periodic lamellar manner within the film. In an asymmetric diblock copolymer having an alcohol group in the side chain of one of the blocks, poly(styrene)-block-poly(hydroxylated polyisoprene) (PS-b-PIOH), where the PIOH block forms spherical domains in the PS matrix, the Pd particles are selectively assembled in the PIOH domains, not in the PS phase. This approach would be of general applicability to any block copolymers, and the particles should be assembled in the phase having relatively stronger reducing power. The vapor of the metal complex can penetrate into the film deeply, and thus the arrangements of the particles are in a three-dimensional array within a film. Co(acac)(2) is also evaluated and gives Co nanoparticles within the PIOH domains in the PS-b-PIOH film as does asPd(acac)(2). Electron spectroscopic imaging and electron energy loss spectroscopy on an energy-filtering transmission electron microscope were performed for nanoscale chemical analysis to investigate the reaction products of Co(acac)(2) within the polymer films in detail.