Nonequilibrium self-assembly of metals on diblock copolymer templates.
Most studies of self-assembled systems reveal that the highest order is associated with equilibrium states of the system. By systematically studying metal decoration of diblock copolymer templates, I show that a high degree of order can arise under strongly nonequilibrium conditions. Under a wide range of conditions, thermally evaporated gold decorates ultrathin, asymmetric, polystyrene-b-polymethylmethacrylate diblock copolymer films with isolated nanoparticles. These particles aggregate into nanoparticle chains inside the polystyrene block with a selectivity approaching 100%. However, even at metal loading fractions of up to 30% by volume no coalescence into continuous nanowires is observed. This behavior is also shared by indium, tin, lead, bismuth, and silver at low coverage (<30 A nominal thickness). At high coverage (>100 A nominal thickness), however, silver self-assembles to form nanowires. One can understand the formation of the chains of nanoparticles by understanding the equilibrium state of the system (metal+polymer). The silver nanowires are highly nonequilibrium structures and, to the best of my knowledge, unexplained by existing theoretical models. Assuming an energy difference for metallic particles for either side of the diblock, a mobility difference, and an attractive interaction between metallic particles, I modeled the self-assembly of the nanowires with a Monte Carlo simulation. This Monte Carlo simulation qualitatively agrees with the formation of the silver nanowires and their relaxation to equilibrium upon moderate heating.
Lopes
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The James Franck Institute and Department of Physics, The University of Chicago, Chicago, Illinois 60637, USA. wlopes@arryx.com