Substrate-Dependent Properties of Polydiacetylene Nanowires on Graphite and MoS₂

Scanning tunneling microscopy (STM) has been used to study polydiacetylene (PDA) nanowires and their electronic coupling with the surface. PDA nanowires exhibit intriguing substrate-dependent electronic effects when probed at varying sample bias voltage conditions on different substrate electrode materials, in this case, highly ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS(2)). An analysis of nanowire heights over a wide range of bias voltages shows strong polymer-substrate contact effects, the strength of which is reflected in the asymmetry of the height-voltage data on each substrate. On HOPG, PDA nanowires exhibit a decrease in height as the bias voltage magnitude is reduced, and the height is substantially greater at negative voltages than at positive voltages. On MoS(2), PDA nanowires appear with much higher contrast than on HOPG when imaged at the same negative bias conditions. At positive bias voltages on MoS(2), the nanowires are invisible in all STM images, yet the unpolymerized molecules can still be imaged. These effects are necessarily electronic in origin. Surprisingly, only the polymer nanowires exhibit any bias-dependent change; the unpolymerized molecules are imaged at all bias voltages on both substrates. Additionally, the substrate affects how the unpolymerized molecules are ordered. In some areas, the molecules are arranged such that part of the monolayer is offset from the correct threefold symmetry direction by a slight misfit angle. On HOPG, this misfit is approximately 6 degrees, while on MoS(2), it is approximately 11 degrees. Interactions with the substrate thus play a role both in electronic structure and in molecular alignment.