Non-equilibrium self-assembly of metals on diblock copolymer templates

Typically, the most perfectly ordered, self-assembled structures correspond to equilibrium states of the system. Here, I show that a high degree of order can arise out of strongly non-equilibrium conditions. I report on a systematic study of non-equilibrium aspects of the decoration of diblock copolymer ultrathin films by evaporated metals. I observe two distinct behaviors for selectively decorating the diblock copolymer: either the metal decorates the diblock copolymer template with nanoparticles or the metal decorates the template with nanowires. Remarkably, these nanowires remain stable under non-equilibrium conditions. I focus on results obtained with evaporated gold and silver on asymmetric polystyrene-b-polymethylmethacrylate (PS-b-PMMA). Gold and a number of other metals (indium, tin, lead, bismuth, aluminum) decorate the diblock copolymer with chains of nanoparticles and don't form wires. Silver forms chains of nanoparticles at low coverage (<30 A), but at high coverage (>100 A), silver forms nanowires. One can understand the formation of the chains of nanoparticles by understanding the equilibrium state of the system (metal + polymer). The silver nanowires, however, are highly non-equilibrium structures. To understand their formation, 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.