Evidence for π-Interactions in Stacked Polymers by STM Simulations

Abstract

The influence of π-interactions in thin layers of stacked polymers has been studied with first-principles DFT calculations and STM simulations. Improving π–π coupling between polymeric layers in the stack contributes to enlarging the size of STM contrasts and to attenuating the fine atomic structure features in the topmost superficial polymer layer. In addition, the tunneling transport within the stack is strongly influenced by the distance separating the polymer layers within the stack. Consequently, it appears very difficult to determine the structural properties (stack height, interlayer distance) of thin-layer stacks only on the basis of STM contrast measurements. For multiple-layer stacks, we have clearly identified two different tunneling regimes where one is assisted by π-coupling that enhances the tunneling current, whereas the other is a pure tunneling transport mechanism. Our results suggest that STM could constitute a powerful technique to probe the existence of π-electron interactions in π-conjugated stacks.