Morphology-dependent electronic properties in cross-linked (P3HT-b-P3MT) block copolymer nanostructures

Disentangling "Bright" and "Dark" Interactions in Ordered Assemblies of Organic Semiconductors

We report on spatially correlated wavelength-resolved photoluminescence and Kelvin probe force microscopy to probe ground state charge-transfer coupling and its correlation with pi-stacking order in nanoscale assemblies of a small molecule n-type organic semiconductor, tetraazaterrylene (TAT). We find a distinct upshift in surface potential contrast (SPC) corresponding to a decrease in work function in TAT in the transition from disordered spun-cast films to ordered crystalline nanowire assemblies, accompanied by a nanowire size dependence in the SPC shift suggesting that the shift depends on both ground state charge transfer interaction and a size (volume)-dependent intrinsic doping associated with the nitrogen substitutions. For the smallest nanowires studied (surface height ≈ 10-15 nm), the SPC shift with respect to disordered films is +110 meV, in close agreement with recent theoretical calculations. These results illustrate how "dark" (ground-state) interactions in organic semiconductors can be distinguished from "bright" (excited-state) exciton coupling typically assessed by spectral measurements alone.

Uniform electroactive fibre-like micelle nanowires for organic electronics

Micelles formed by the self-assembly of block copolymers in selective solvents have attracted widespread attention and have uses in a wide variety of fields, whereas applications based on their electronic properties are virtually unexplored. Herein we describe studies of solution-processable, low-dispersity, electroactive fibre-like micelles of controlled length from π-conjugated diblock copolymers containing a crystalline regioregular poly(3-hexylthiophene) core and a solubilizing, amorphous regiosymmetric poly(3-hexylthiophene) or polystyrene corona. Tunnelling atomic force microscopy measurements demonstrate that the individual fibres exhibit appreciable conductivity. The fibres were subsequently incorporated as the active layer in field-effect transistors. The resulting charge carrier mobility strongly depends on both the degree of polymerization of the core-forming block and the fibre length, and is independent of corona composition. The use of uniform, colloidally stable electroactive fibre-like micelles based on common π-conjugated block copolymers highlights their significant potential to provide fundamental insight into charge carrier processes in devices, and to enable future electronic applications.

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.

Reverse Anti-solvent Crystallization Process for the Facile Synthesis of Zinc Tetra(4-pyridyl)porphyrin Single Crystalline Cubes

Synthesis of morphologically well-defined crystals of metalloporphyrin by direct crystallization based on conventional anti-solvent crystallization method without using any additives has been rarely reported. Herein, we demonstrate an unconventional and additive-free synthetic method named reverse anti-solvent crystallization method to achieve well-defined zinc-porphyrin cube crystals by reversing the order of the addition of solvents. The extended first solvation shell effect mechanism is therefore suggested to support the synthetic process by providing a novel kinetic route for reaching the local supersaturation environment depending on the order of addition of solvents, which turned out to be critical to achieve clean cube morphology of the crystal. We believe that our work not only extends fundamental knowledge about the kinetic process in binary solvent systems, but also enables great opportunities for shape-directing crystallization of various organic and organometallic compounds.

Investigation of the Effect of Thermal Annealing on Poly(3-hexylthiophene) Nanofibers by Scanning Probe Microscopy: From Single-Chain Conformation and Assembly Behavior to the Interfacial Interactions with Graphene Oxide

Poly(3-hexylthiophene) (P3HT) has been widely used in devices owing to its excellent properties and structural features. However, devices based on pure P3HT have not exhibited high performance. Strategies, such as thermal annealing and surface doping, have been used to improve the electrical properties of P3HT. In this work, different from previous studies, the effect of thermal annealing on P3HT nanofibers are examined, ranging from the single polymer chain conformation to chain packing, and the interfacial interactions with graphene oxide (GO) at nanoscale dimensions, by using scanning tunneling microscopy (STM), atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). High-resolution STM images directly show the conformational changes of single polymer chains after thermal annealing. The morphology of P3HT nanofibers and the surface potential changes of the P3HT nanofibers and GO is further investigated by AFM and KPFM at the nanoscale, which demonstrate that the surface potentials of P3HT decrease, whereas that of GO increases after thermal annealing. All of the results demonstrate the stronger interfacial interactions between P3HT and GO occur after thermal treatments due to the changes in P3HT chain conformation and packing order.

Ligand Controlled Morphology Evolution of Active Intermediates for the Syntheses of Gold Nanostars

Gold nanostars have unique plasmonic properties that are related to the highly branched nanostructures. However, it is challenging to precisely control these branches. Here we studied the reaction kinetics on the seed-mediated growth process of gold nanostars using in situ UV-vis spectroscopy. The impact of hydroquinone ligands on the formation and evolution of active intermediates was systematically explored. In addition, we improved the classical seed-mediated method to achieve a much better control on the final morphology of gold nanostars by a sudden addition of a high concentration ligand solution. Our method can significantly advance the syntheses of gold nanostars and provide numerous opportunities to prepare nanomaterials with unique morphology and plasmonic properties.

Formation and decay of charge carriers in aggregate nanofibers consisting of poly(3-hexylthiophene)-coated gold nanoparticles

Hybrid nanofibers consisting of poly(3-hexylthiophene)-coated gold nanoparticles have been facilely fabricated and comprehensively investigated by time-resolved emission and transient-absorption spectroscopy.

Charges on nano-islands and fibrils of poly(3-hexylthiophene-2,5-diyl) – light-modulation, injection and transportation

The impact of the nanostructures of conjugated polymers on their electronic properties is significant.

Work Function Modification in P3HT H/J Aggregate Nanostructures Revealed by Kelvin Probe Force Microscopy and Photoluminescence Imaging

We show that surface electronic properties of poly-3-hexylthiophene (P3HT) crystalline nanofibers as probed by Kelvin probe force microscopy (KPFM) depends sensitively on the degree of polymer packing order and dominant coupling type (e.g., H- or J-aggregate) as signaled by absorption or photoluminescence spectroscopy. Nominal HOMO energies between high molecular weight (J-aggregate) nanofibers and low-molecular weight (H-aggregate) nanofibers differ by ≈160 meV. This is consistent with shifts expected from H-type charge-transfer (CT) interactions that lower HOMO energies according to registration between thiophene moieties on adjacent polymer chains. These results show how KPFM combined with wavelength-resolved photoluminescence imaging can be used to extract information on "dark" (CT) interactions in polymer assemblies.

Single Step Process for Self-Assembled Block Copolymer Patterns via in Situ Annealing during Spin-Casting

We demonstrated a simple and time-efficient processing method for facilitating a microphase separation of block copolymers (BCPs) based on a single step of spin-casting with low volatile solvent and in situ annealing. Well-ordered lamellar patterns of poly(styrene-b-methyl methacrylate) BCP films having wide range of molecular weights (51-235 kg/mol) were fabricated by a single 3 min process of spin-casting, even without the conventional pretreatment of substrate neutralization. The formation of this well-ordered lamellar structure is attributed to a synergetic effect between slow solvent evaporation and thermal energy that may provide an efficient cooling profile for the BCP film during the spin-casting process.