Molecular Orientations at Buried Conducting Polymer/Graphene Interfaces

Abstract

Graphene and semiconducting polymers have been extensively researched for use in many microelectronic devices. For both devices with layered structures and graphene–semiconducting polymer composite materials, molecular interactions between graphene and semiconducting polymers play significant roles in determining the interfacial properties and impact the device performance. In this research, molecular orientations of polymer backbones of three polythiophenes (PTs) with different side-chain lengths at the buried PT/graphene interfaces were determined using sum frequency generation (SFG) vibrational spectroscopy. It was found that the longer the PT side chain, the larger the PT backbone tilt angle measured. Therefore, the PT backbone with a longer side-chain length adopts a more lying-down pose on graphene. The same trend for orientation tilt angles of PTs with different side-chain lengths at PT/CaF₂ interfaces was observed. When comparing the same PT backbone orientation, PT at the PT/graphene interface generally lies down more compared to that at the PT/CaF₂ interface, due to stronger π–π interactions between the PT backbone conjugated rings and graphene. The different PT backbone orientations at interfaces are likely caused by varied interactions between the PT backbone and the substrate and between side chains and the substrate, as well as intermolecular and intramolecular interactions between PT molecules (including side chains and backbones). The determination of PT backbone orientations at buried interfaces helps to understand molecular interactions at these interfaces, aiding in the design of polymer/graphene interfaces with optimal structure and improved properties.