Maskless fabrication of polythiophene patterns by photochemical conversion of regioselectively condensed 2,5-diiodothiophene

Patterning of conjugated polymers for organic optoelectronic devices

Conjugated polymers have been attracting more and more attention because they possess various novel electrical, magnetical, and optical properties, which render them useful in modern organic optoelectronic devices. Due to their organic nature, conjugated polymers are light-weight and can be fabricated into flexible appliances. Significant research efforts have been devoted to developing new organic materials to make them competitive with their conventional inorganic counterparts. It is foreseeable that when large-scale industrial manufacture of the devices made from organic conjugated polymers is feasible, they would be much cheaper and have more functions. On one hand, in order to improve the performance of organic optoelectronic devices, it is essential to tune their surface morphologies by techniques such as patterning. On the other hand, patterning is the routine requirement for device processing. In this review, the recent progress in the patterning of conjugated polymers for high-performance optoelectronic devices is summarized. Patterning based on the bottom-up and top-down methods are introduced. Emerging new patterning strategies and future trends for conventional patterning techniques are discussed.

Chemical template directed iodine patterns on the octadecyltrichlorosilane surface

A carboxylic-terminated nanometer-scale chemical pattern on an octadecyltrichlorosilane (OTS) surface can guide the deposition and crystallization of iodine, forming an iodine pattern on the chemical pattern. The iodine in the pattern is gel-like when fabricated by the solution-deposit method. In contrast, a dendritic, snowflake-shaped polycrystalline iodine sheet is formed by the vapor-phase condensation method. The data demonstrate that iodine is a good tracing and visualizing agent for studying liquid behavior at the nano scale. The topography of the iodine stain reveals that the "coffee ring" effect can be suppressed by reducing the pattern size and increasing the evaporation rate. The chemical template-bound iodine pattern has an unusually low vapor pressure and it can withstand prolonged baking at elevated temperature, which differs significantly from bulk iodine crystals.

Microwave-Assisted Coating of PMMA beads by silver nanoparticles

Microwave (MW) irradiation was found to be a new technique for coating silver nanoparticles with an average size of approximately 31 nm onto the surface of poly(methyl methacrylate) PMMA beads (3 mm diameter). The microwave polyol reduction was carried out under an argon atmosphere. Silver nanoparticles were obtained by the MW irradiation of a solution mixture containing silver nitrate (or silver acetate), poly(ethylene glycol), ethanol, water, and 24 wt % aqueous ammonia for 5 min in the presence of PMMA beads, yielding a PMMA-nanosilver composite. By controlling the atmosphere and reaction conditions, we could achieve the deposition of silver nanoparticles onto the surface of poly(methyl methacrylate) and vary the amount of the silver anchored to the surface. The resulting silver-deposited PMMA samples were characterized using X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, high-resolution scanning electron microscopy, X-ray photoelectron spectroscopy, and volumetric titration with potassium thiocyanate (KSCN) according to the Folgard method.

Functional nanostructures from surface chemistry patterning

Nanoscale devices are expected to provide important advances for a number of applications. While many methods to generate nanoscale patterns exist, their use is confined to a relatively narrow range of materials. To fabricate nanoscale structures of a material with useful properties, the most convenient route is to transfer the geometry of an existing pattern into another material. Methods to achieve this pattern transfer are summarized and organized in this review. Methods to generate the original patterns, as well as applications of the final structure are also described.

Dendritic aggregation of oligothiophene during desorption of 2,5-diiodothiophene multilayer and topography-induced alignment of oligothiophene nanofibers

The multilayer desorption behavior of 2,5-diidothiophene and the dendritic aggregation of photochemical reaction products during the desorption of 2,5-diiodothiophene multilayers have been studied. Like many other aromatic compounds, 2,5-diiodothiophene shows a multilayer desorption behavior different from the typical zeroth-order kinetics, a metastable desorption peak growth at approximately 220 K followed by a thick multilayer peak growth at approximately 235 K. Traditionally, these desorption behaviors have been attributed to the formation of three-dimensional clusters. This paper provides the direct evidence of this clustering process by producing nondesorbing photoreaction products in the multilayer and by imaging their clusters after the multilayer desorption. Oligothiophene species are produced via photochemical reactions of 2,5-diiodothiophene during the multilayer deposition at approximately 180 K in ultrahigh vacuum (UHV). Upon heating the multilayer to room temperature, the oligothiophene species forms into fibrous aggregates with a fractal dimension varying from 1.37 to 1.81 depending on their surface concentration. Using a topographical alteration of the substrate with a repeating pattern, these oligothiophene fibers can be aligned to a certain direction. This may allow in-situ fabrication of aligned conjugated polymer fibers directly on a target substrate.

Physical insights into the photoactivated Ullmann coupling process producing highly conjugated oligothiophene films on a copper substrate

This paper describes the details of surface reactions producing >100-nm-thick conjugated polymer films. When 2,5-diiodothiophene films deposited on copper are irradiated with UV at room temperature in Ar environments, oligothiophene films are synthesized. The average conjugation length of the produced film varies from approximately 7 to 3-4 as the film thickness increases from approximately 100 to approximately 500 nm. The X-ray photoelectron spectroscopy analysis of the produced films reveals evidence for the formation of organo-copper intermediate species at the copper-monomer film interface and their diffusion from the copper surface into the monomer film during the photochemical process. A one-dimensional diffusion-reaction model is presented to explain the formation, diffusion, and reaction of organo-copper intermediates in the multilayer film during the photochemical reaction. The model simulation results qualitatively explain the decrease of the Ullmann coupling contribution in the photochemical reaction with the film thickness.

Guided Growth of Nanoscale Conducting Polymer Structures on Surface-Functionalized Nanopatterns

We describe an electrochemical method of directly growing conducting polymer nanostructures between metal electrodes with the geometry controlled by hydrophilic/hydrophobic patterns. The surface patterning can be achieved by a large number of lithographic methods such as AFM, electron-beam, elastomeric microprinting, and photolithography and is compatible with industrial semiconductor fabrication processes. Conducting polymer structures so formed have good alignment compared to bulk synthesis and are grown in place between electrodes. Polypyrrole field effect transistors have been produced using this method. Electrical measurements show conductivity strongly dependent on the presence of anionic dopant species during growth. Devices grown with a high concentration of dopant show metallic behavior, while those with less doping behave as p-type semiconductors.