Clayton Mauldin

Nanostructured Organic Semiconductors via Directed Supramolecular Assembly

Organic small molecule semiconductors have many advantages over their polymer analogues. However, to fabricate organic semiconductor-based devices using solution processing, it is requisite to eliminate dewetting to ensure film uniformity and desirable to assemble nanoscopic features with tailored macroscopic alignment without compromising their electronic properties. To this end, we present a modular supramolecular approach. A quaterthiophene organic semiconductor is attached to the side chains of poly(4-vinylpyridine) via noncovalent hydrogen bonds to form supramolecular assemblies that act as p-type semiconductors in field-effect transistors. In thin films, the quaterthiophenes can be readily assembled into microdomains, tens of nanometers in size, oriented normal to the surface. The supramolecules exhibited the same field-effect mobilities as that of the quaterthiophene alone (10(-4) cm(2)/(V.s)). Since the organic semiconductors can be readily substituted, this modular supramolecular approach is a viable method for the fabrication of functional, nanostructured organic semiconductor films using solution processing.

Bifunctional Patterning of Mixed Monolayer Surfaces Using Scanning Probe Lithography for Multiplexed Directed Assembly

The patterning of a surface with more than one type of functionality in spatially resolved fashion is described. A scanning probe was used to create patterns composed of two orthogonal types of functionality within a dense, electrochemically active mixed monolayer via a simple modulation of the applied surface bias. One reductive pathway produces surface-bound amine moieties while the other creates an oxidized surface. The two newly created surface functionalities can each be used independently to locally deposit complementary materials such as electron- or hole-transporting molecules via self-assembly.

Superhydrophobic SAM Modified Electrodes for Enhanced Current Limiting Properties in Intrinsic Conducting Polymer Surge Protection Devices

Surface interface engineering using superhydrophobic gold electrodes made with 1-dodecanethiol self-assembled monolayer (SAM) has been used to enhance the current limiting properties of novel surge protection devices based on the intrinsic conducting polymer, polyaniline doped with methanesulfonic acid. The resulting devices show significantly enhanced current limiting characteristics, including current saturation, foldback, and negative differential effects. We show how SAM modification changes the morphology of the polymer film directly adjacent to the electrodes, leading to the formation of an interfacial compact thin film that lowers the contact resistance at the Au-polymer interface. We attribute the enhanced current limiting properties of the devices to a combination of lower contact resistance and increased Joule heating within this interface region which during a current surge produces a current blocking resistive barrier due to a thermally induced dedoping effect caused by the rapid diffusion of moisture away from this region. The effect is exacerbated at higher applied voltages as the higher temperature leads to stronger depletion of charge carriers in this region, resulting in a negative differential resistance effect.

Sensitivity to molecular order of the electrical conductivity in oligothiophene monolayer films.

Using conducting probe atomic force microscopy (CAFM), we show that electrical conductivity in oligothiophene molecular films deposited on SiO(2)/Si wafers is extremely sensitive to degree of crystalline order in the film. By locally distorting the molecular order in the films through the controlled application of pressure with the AFM tip, the lateral charge transport was reduced by factors varying from 2 to 10, even when no changes in the height of the film could be observed.

Modular small-molecule directed nanoparticle assembly

We demonstrate a simple and versatile approach to produce hierarchical assemblies of nanoparticles by combining block copolymers and small molecules. Directed nanoparticle assemblies can be achieved with a variety of nanoparticles and small molecules without modification of the nanoparticle ligands. This novel approach opens up new routes toward the fabrication of nanoparticle-based functional devices using a “Bottom-Up” approach.