Graciela B. Blanchet

Large area, high resolution, dry printing of conducting polymers for organic electronics

We show here that thermal imaging, a nonlithographic technique which enables printing multiple, successive layers via a dry additive process can be used in combination with tailored printable conductors in the fabrication of organic electronic devices. This method is capable of patterning a range of organic materials at high speed over large areas with micron size resolution and excellent electrical performance avoiding the solvent compatibility issues currently faced by alternative techniques. Such a dry, potentially reel-to-reel printing method may provide a practical route to realizing the expected benefits of plastics for electronics. We illustrate the viability of thermal imaging and imageable organics conductors by printing a functioning, large area (4000 cm2) active matrix backplane display circuit containing several thousand transistors.

Polyaniline nanotube composites: A high-resolution printable conductor

Polyaniline/single wall carbon nanotube composites were prepared to be used as printable conductors for organic electronics devices. We show here that the high aspect ratio of single wall carbon nanotubes enables percolation into a conducting network at extremely low nanotube concentration. The nature of the transport mechanism is revealed by the temperature dependence of the conductivity of these percolating composites. We demonstrate here that these thin composite films are printable via laser ablation with high resolution while retaining appropriate conductivity. The utility of these findings is illustrated by printing structures, which could serve as a source and drain with 7 μm channel and 2 S/cm conductivity for use in plastic transistors.

Laser ablation and the production of polymer films

The formation of high-quality thin films of polytetrafluoroethylene (PTFE) is important in many applications ranging from material reinforcement to molecular electronics. Laser ablation, a technique widely used to deposit a variety of inorganic materials, can also be used as a simple and highly versatile method for forming thin polymer films. The data presented show that PTFE films can be produced on various supports by the evaporation of a solid PTFE target with a pulsed ultraviolet laser. The composition of the ablation plume suggests that PTFE ablation and subsequent film formation occur by way of a laser-induced pyrolitic decomposition with subsequent repolymerization. The polymer films produced by this method are composed of amorphous and highly crystalline regions, the latter being predominantly in a chain-folded configuration with the molecular axis aligned parallel to the substrate surface.

Contact resistance in organic thin film transistors

This letter reports on the unexpected dependence of contact resistance on the dielectric layer for pentacene thin film transistors with printed organic conducting electrodes. While the intrinsic mobility is weakly reliant on the dielectric, the contact resistance does vary considerably with dielectric layer. We show that while morphological changes are not apparent, contact resistances vary by an order of magnitude. This result suggests that the barrier to charge injection may depend not only on interactions at the complex triple interface but also on the details of the electronic structure at the semiconductor/dielectric interface.

Suppression of Metallic Conductivity of Single-Walled Carbon Nanotubes by Cycloaddition Reactions

The high carrier mobility of films of semiconducting single-walled carbon nanotubes (SWNTs) is attractive for electronics applications, but the presence of metallic SWNTs leads to high off-currents in transistor applications. The method presented here, cycloaddition of fluorinated olefins, represents an effective approach toward converting the “as grown” commercial SWNT mats into high-mobility semiconducting tubes with high yield and without further need for carbon nanotube separation. Thin-film transistors, fabricated from percolating arrays of functionalized carbon nanotubes, exhibit mobilities >100 square centimeters per volt-second and on-off ratios of 100,000. This method should allow for the use of semiconducting carbon nanotubes in commercial electronic devices and provide a low-cost route to the fabrication of electronic inks.

Deposition of polytetrafluoroethylene films by laser ablation

Films of polytetrafluoroethylene (PTFE) were deposited by laser ablation using the fourth harmonic, at 266 nm, of a Nd‐YAG laser. The films are found to be stoichiometric with the correct optical properties. We suggest that UV absorption onsets the pyrolitic decomposition of PTFE leading to a monomer that subsequently repolymerizes onto a substrate.

Carbon nanotubes-semiconductor networks for organic electronics: The pickup stick transistor

We demonstrate an alternative path for achieving high transconductance organic transistors in spite of relatively large source to drain distances. The improvement of the electronic characteristic of such a scheme is equivalent to a 60-fold increase in mobility of the underlying organic semiconductor. The method is based on percolating networks, which we create from a dispersion of individual single-wall carbon nanotubes and narrow ropes within an organic semiconducting host. The majority of current paths between source and drain follow the metallic nanotubes but require a short, switchable semiconducting link to complete the circuit. With these nanotube-semiconducting composites we achieve effectively a 60× reduction in source to drain distance, which is equivalent to a 60-fold increase of the “effective” mobility of the starting semiconducting material with a minor decrease of the on/off current ratio. These field-induced percolating networks allow for the fabrication of high-transconductance transistors...

Deposition of amorphous fluoropolymers thin films by laser ablation

Thin films of the amorphous fluoropolymer, Teflon AF(r), were deposited by laser ablation using the 4th harmonic, at 266 nm, of a Nd‐YAG laser. Infrared spectroscopy indicated that the composition of the ablated films were that of the starting materials and x‐ray diffraction spectra corroborated the lack of crystallinity. The morphology of the films was controlled by the temperature of the substrate during film formation. We suggest pyrolitic decomposition and subsequent repolymerization as a possible mechanism to the formation of Teflon‐AF(r) films by laser ablation.

Direct patterning of conductive water-soluble polyaniline for thin-film organic electronics

We report a simple and low-cost technique for directly patterning conductive water-soluble polyaniline. This technique exploits the hydrophilic nature of polyaniline that was synthesized on a polymer acid template. To create conductive features, aqueous polyaniline solutions are directly spun cast onto substrates previously patterned with hydrophilic and hydrophobic regions. Polyaniline selectively adsorbs in the hydrophilic regions thereby resulting in features as small as 5 μm with an average conductivity of 0.3 S/cm over large areas. Polyaniline contacts for thin-film transistors were fabricated; such devices show on characteristics that are quantitatively similar to those of reference devices with gold electrodes.

Self-assembled three-dimensional conducting network of single-wall carbon nanotubes

We described here the self-assembling of a three-dimensional array of single-wall nanotubes (SWNTs). The distinctive choice of materials allowed for the self-assembly of SWNTs with low resistance conducting polymer links into a conducting network that when embedded into an insulating host shows no disruption of the conduction path. The ability to control network formation independently of the electrical properties of the host drastically changes the design of these conducting organic networks. Thus, enabling the tailoring of their electrical properties while addressing issues of film processability relevant for their application as printable conductors in organic electronic applications. These networks provide opportunities for applications in micro- and nanoelectronics.