Lateral Assembly of Metal Nanoparticles Directed by Nanodomain Control in Block Copolymer Thin Films

We have produced and assembled palladium (Pd) nanoparticles in two-dimensional (2-D) periodic arrays in nanoscales by using block copolymer (BC) thin films as templates. A symmetric diblock copolymer of poly(methyl methacrylate)-block-poly(2-hydroxyethyl methacrylate) (PMMA-b-PHEMA) was coated on a Si-wafer by dip-coating from three different solvents having different solubility against each component. Two of them are selective solvents for PMMA and PHEMA, respectively, and the third one is a common solvent for the two components. A monolayer of the micelles was formed from the solutions of the selective solvents and the micelles are arranged in a hexagonal 2-D lattice, whereas a lamellar pattern was obtained from a homogeneous solution of the common solvent. The monolayer films thus-prepared were exposed to the vapor of palladium (II) bis(acetylacetonato), and Pd nanoparticles were selectively produced in the PHEMA phase due to its stronger reducing power than the PMMA phase. The patterns thus-obtained were successfully transferred to a Si-wafer by reactive dry etching technique because of the high etching resistivity of the metal nanoparticles against gas plasmas. By using this process, three lateral periodic arrays of dots, holes, and lines were created with 20-nm repeating distances.