(28f) Dynamic-Template-Directed Assembly of 2D Monolayer Films of Conjugated Polymers and Their Distinct Electronic Properties

2D materials have been under vigorous investigation and indisputably represent the frontier of current research, owing to their unique properties compared to their bulk counterparts. The discovery of graphene and its astonishing properties led to rapid emergence of other 2D inorganic materials and has recently encouraged 2-dimensionalization of organic functional materials. Following the same footsteps, 2D monolayer films of conjugated polymers are gradually gaining an increasing attention, with their potential in modulation of optoelectronic and mechanical properties when low-dimensionalized to molecular layer thickness. Despite numerous efforts from the community, crystallization of highly ordered 2D monolayer films of conjugated polymers is still a challenge. This is partially due to high degree of conformational freedom of polymers and one-dimensional nature of their crystallization with fastest growth axis along the polymer backbone, which results in the conjugated polymers to mostly assemble as 1D nanofibers. Besides the fabrication challenge, fundamental questions on how film thickness effects the electronic structure and charge transport properties of the conjugated polymers had also not been explored before.

In this talk, we present dynamic-template-directed meniscus-guided coating as a new technique to fabricate 2D monolayer films of conjugated polymers over large area using a wide-processing window. By utilizing two donor-acceptor polymers, DPP2T-TT and PII-2T, we demonstrate that the dynamic-template enhances polymer 2D crystallization resulting in 2D monolayer films with highly ordered and aligned polymer backbone with edge-on 𝜋-𝜋 stacking. In contrast, the monolayer films on solid static substrates resulted in discontinuous morphology exhibiting 1D fiber-network with strong alkyl-chain stacking peaks at the expense of backbone stacking. As a result, charge transport was only observed on monolayer films printed on dynamic-template but not on those printed on solid substrates. As thickness was increased from monolayers to multi-layers, the polymer 𝜋-stacks changed from edge-on to bimodal orientation when the film reached a critical thickness of ~20 nm. UV-Vis spectroscopy and cyclic voltammetry analysis disclosed abrupt rise in J-aggregation, absorption coefficient and decrease in band gap and HOMO level until the film reached a critical thickness, possibly arising from straightened polymer backbone. Furthermore, increase in film thickness resulted in drastic increase of hole mobility in the polymer films, peaking at 0.7 cm²V^-1s^-1 near the critical thickness, which further corroborated with drastic planarity of polymer backbone with increase in film thickness. Finally, we also fabricated of organic field-effect transistor based chemical sensors incorporating DPP2T-TT films of varying thicknesses and demonstrated ultrahigh sensitivity (~83%) of ultrathin films of ~7 nm thickness (bilayers) to ammonia vapors of 1 ppb concentration.