Mechanistic studies of surface polymerization by ion-assisted deposition

Polythiophene is a conductive polymer that has attracted much interest in recent years because its properties are desirable for applications that include light-emitting diodes, field-effect transistors, and photovoltaics. Optimization of the performance of polythiophene in these devices requires the development of processing methods that can simultaneously control its chemistry and morphology on the nanometer scale. One such method is surface polymerization by ion-assisted deposition (SPIAD) in which conducting polymer thin films are grown on substrates by the simultaneous deposition of hyperthermal polyatomic ions and thermal neutrals in vacuum. Here, mass-selected beams of thiophene ions are deposited on α-terthiophene oligomers in experiments, and density functional theory-molecular dynamics (DFT-MD) simulations are carried out to determine the dominant mechanisms responsible for the SPIAD process. Both neutral and positively charged systems are considered in the simulations in order to assess the effect of charge on the results. The experimental results show that polymerization occurs preferentially under a narrow set of ion energy and ion/neutral ratio conditions. The DFT-MD simulations illustrate the manner in which ion energies affect polymerization and reveal how secondary chemical reactions can substantially modify both the thin film and the substrate.