Layered Poly(3-hexylthiophene) Nanowhiskers Studied by Atomic Force Microscopy and Kelvin Probe Force Microscopy

Kelvin Probe Microscopy Investigation of Poly-Octylthiophene Aggregates

Conductive polymers have fundamental relevance as well as novel technological applications in the organic optoelectronics field. Their photophysical and transport properties strongly depend on the molecular arrangement, and nanoscale characterization is needed to fully understand the optoelectronic processes taking place in organic devices. In this work, we study the electrostatic properties of poly-3-octylthiophene isolated structures: disordered low-packed polymer chains and crystalline layered lamellar assemblies. We characterize the electronic ground state using Kelvin probe microscopy. This allows us to resolve a rich variety of surface potential regions that emerge over the different polymer structures. These SP regions are correlated with different molecular aggregates.

Conformation modification of terthiophene during the on-surface synthesis of pure polythiophene

On-surface coupling under ultra-high vacuum is employed as a versatile approach to synthesize pure polythiophene from a 5,5''-dibromo-2,2':5',2''-terthiophene (DBTT) precursor and the corresponding temperature-dependent stepwise reaction mechanism is systematically studied by scanning tunneling microscopy (STM). After thermal deposition of the precursor onto a Au(111) surface that is kept at room temperature, a triangle-like pattern and a linear self-assembled pattern are formed with different molecular coverages through BrBrS halogen bonds and BrBr type-I contact bonds, respectively. In the self-assembled nanostructures, the thiophene units adopt trans-conformation. Mild annealing promotes the structural transition of both nanostructures into ordered zigzag organometallic linear chains with all-cis configured thiophene units connected through coordination bonds to the Au adatoms. Such conformational variety is easily recognized by STM, particularly in the case of DBTT-CH3 with the extra -CH3 signals. The covalently coupled products from the DBTT precursor are obtained by further annealing the organometallic intermediate at higher temperatures, which leads to the removal of Au atoms and the formation of ordered polymer chains and disordered polythiophene networks. Further characterization suggests that the reaction mechanism is associated with Ullmann-type coupling to form the ordered chains as well as Ullmann-type and dehydrogenative C-C coupling to fabricate cross-linked polymer networks. Compared with the on-surface synthesis process of DBTT on the Cu(111) surface, it can be confirmed that the Au adatoms are vital to synthesize polythiophene. These findings provide important insight into the reaction mechanism of on-surface synthesized pure polythiophene and on-surface coupling can potentially be applied to synthesize other functional conjugated polymers.

Quantifying the Mechanical Anisotropy in Poly(3-hexylthiophene) Nanofibers

Correlating the structure with nanomechanical property of semicrystalline conjugated-polymer crystal is of essential importance for the performance improvement and design of flexible electronic devices. Although it is well-known that the semicrystalline conjugated-polymer crystal exhibits anisotropic structure owing to the π-π and layer stacking of highly coplanar conjugated backbones, the structure-nanomechanical property relationship is missing. Here, we investigated the axial mechanical anisotropy of the P3HT nanofiber by using thermal shape-fluctuation analysis and a three-point bending test based on atomic force microscopy. Our results show that Young's modulus in the layer-stacking direction (E_L) is 1-2 orders of magnitude greater than that in the π-conjugated backbone direction (E_B). We attribute this mechanical anisotropy to the π-stacking of the P3HT backbone, but the layer stacking will decrease E_L, which weakens the mechanical anisotropy. Moreover, we demonstrated that the P3HT nanofiber shows a loading-rate-independent Young's modulus and deformation-dependent resilience in the layer-stacking direction.

Aggregation of P3HT as a preferred pathway for its chemical doping with F4-TCNQ

In the chemical doping reaction of P3HT with F₄-TCNQ, the aggregation of P3HT occurs before the charge transfer step.

The Impact of Aggregation on the p-Doping Kinetics of Poly(3-hexylthiophene)

The morphological effects of regioregular poly(3-hexylthiophene) (P3HT) on its p-doping kinetics with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) in solution are studied using optical absorption spectroscopy and stopped-flow technique. Two morphological forms, solubilized (s-P3HT) and nanowhiskers (nw-P3HT), are investigated. Both P3HT solubilized and aggregated solutions show similar characteristic near-IR absorption bands for integer charge transfer products with F₄-TCNQ. Kinetic analysis on p-doping of s-P3HT with F₄-TCNQ indicates that the doping reaction proceeds with a single reaction mechanism that is first order in s-P3HT. The doping kinetics of P3HT aggregate solution shows two distinctive reaction mechanisms. The slow mechanism has a reaction rate constant similar to that of solubilized P3HT solution, so it likely results from s-P3HT components that are present in the aggregate solution. The fast one is assigned to the nw-P3HT component, probably due to more efficient charge delocalization in the aggregated P3HT nanostructures. Additionally, the kinetic trends of the p-doping reactions are better fitted with the consideration of a Gaussian-like distribution of reactivities from P3HT, matching the complexity of polymeric systems originating from molecular weight and morphology variations. This study highlights the importance of considering different morphological forms of conjugated polymers on their charge-transfer reaction kinetics. The knowledge gained here should be fundamentally and practically important for future chemical doping applications in organic electronic device fabrications.

Distinguishing between Mechanical and Electrostatic Interaction in Single Pass Multi Frequency Electrostatic Force Microscopy Measurements on a Molecular Material

Single-pass electrostatic force microscopy is postulated as one of the most advanced techniques in terms of spatial resolution and fastness in data acquisition for the study of electrostatic phenomena at the nanoscale. However, crosstalk anomalies, in which mechanical interactions combine with tip-sample electrostatic forces, are still a major issue to overcome, specifically in soft and biological samples. In this paper we propose a novel method based on bimodal-atomic force microscopy to distinguish mechanical crosstalk from electrostatic images. The method is based in the comparison of bimodal AFM images with electrostatic ones, where pure mechanical interaction can be discerned from a mixture of mechanical and electrostatic interactions. The proposed method is optimized and demonstrated using a supramolecular charge transfer material. Finally, the method is used as a tool to depict different crosstalk levels in tetrathiafulvalene-based (TTF) assemblies, discerning between electrical and mechanical interactions. This kind of observation is important for obtaining accurate descriptions of charge distribution in samples made from organic and molecular layers and materials.