Sondra L. Hellstrom

Polymer-Assisted Direct Deposition of Uniform Carbon Nanotube Bundle Networks for High Performance Transparent Electrodes

Flexible transparent electrodes are crucial for touch screen, flat panel display, and solar cell technologies. While carbon nanotube network electrodes show promise, characteristically poor dispersion properties have limited their practicality. We report that addition of small amounts of conjugated polymer to nanotube dispersions enables straightforward fabrication of uniform network electrodes by spin-coating and simultaneous tuning of parameters such as bundle size and density. After treatment in thionyl chloride, electrodes have sheet resistances competitive with other reported carbon nanotube based transparent electrodes to date.

Strong and Stable Doping of Carbon Nanotubes and Graphene by MoOx for Transparent Electrodes

MoO(x) has been used for organic semiconductor doping, but it had been considered an inefficient and/or unstable dopant. We report that MoO(x) can strongly and stably dope carbon nanotubes and graphene. Thermally annealed MoO(x)-CNT composites can form durable thin film electrodes with sheet resistances of 100 Ω/sq at 85% transmittance plain and 85 Ω/sq at 83% transmittance with a PEDOT:PSS adlayer. Sheet resistances change less than 10% over 20 days in ambient and less than 2% with overnight heating to 300 °C in air. The MoO(x) can be easily deposited either by thermal evaporation or from solution-based precursors. Excellent stability coupled with high conductivity makes MoO(x)-CNT composites extremely attractive candidates for practical transparent electrodes.

Lyotropic Liquid-Crystalline Solutions of High-Concentration Dispersions of Single-Walled Carbon Nanotubes with Conjugated Polymers†

The 1D structure of single-walled carbon nanotubes (SWNTs) leads to unique physical properties, which have been investigated extensively. Numerous applications and device prototypes have been demonstrated; however, most have used SWNTs grown in situ by chemical vapor deposition. This limits throughput and choice of substrate owing to the high growth temperatures involved. Solution-based postsynthesis device fabrication, typically involving purification, solubilization, chemical functionalization, cutting, and/ or controlled assembly of SWNTs, is more desirable because of low cost, scalability to large areas, and compatibility with flexible plastic substrates. Unfortunately, SWNTs are not readily soluble, and chemical functionalization strategies for their solubilization usually alter their electronic properties. Furthermore, to take full advantage of the anisotropic charge-transport properties of SWNTs and to enhance their performance in high-strength composite materials, it is necessary to align them over a large area. Noncovalent functionalization of SWNTs is a particularly attractive avenue for dispersion because it enables modification of material properties without altering the chemical structure of the nanotubes. To date, most high-concentration dispersions (>1mg mL ) have been obtained in aqueous solutions by mixing SWNTs with surfactants, doubleor

Driving High-Performance n- and p-type Organic Transistors with Carbon Nanotube/Conjugated Polymer Composite Electrodes Patterned Directly from Solution

We report patterned deposition of carbon nanotube/conjugated polymer composites from solution with high nanotube densities and excellent feature resolution. Such composites are suited for use as electrodes in high-performance transistors of pentacene and C(60), with bottom-contact mobilities of > 0.5 and > 1 cm(2) V(−1) s(−1), respectively. This represents a clear step towards development of inexpensive, high-performance all-organic circuits.

Ferroelectric Gated Electrical Transport in CdS Nanotetrapods

Complex nanostructures such as branched semiconductor nanotetrapods are promising building blocks for next-generation nanoelectronics. Here we report on the electrical transport properties of individual CdS tetrapods in a field effect transistor (FET) configuration with a ferroelectric Ba(0.7)Sr(0.3)TiO(3) film as high-k, switchable gate dielectric. A cryogenic four-probe scanning tunneling microscopy (STM) is used to probe the electrical transport through individual nanotetrapods at different temperatures. A p-type field effect is observed at room temperature, owing to the enhanced gate capacitance coupling. And the reversible remnant polarization of the ferroelectric gate dielectric leads to a well-defined nonvolatile memory effect. The field effect is shown to originate from the channel tuning in the arm/core/arm junctions of nanotetrapods. At low temperature (8.5 K), the nanotetrapod devices exhibit a ferroelectric-modulated single-electron transistor (SET) behavior. The results illustrate how the characteristics of a ferroelectric such as switchable polarization and high dielectric constant can be exploited to control the functionality of individual three-dimensional nanoarchitectures.

Contacting nanowires and nanotubes with atomic precision for electronic transport

Making contacts to nanostructures with atomic precision is an important process in the bottom-up fabrication and characterization of electronic nanodevices. Existing contacting techniques use top-down lithography and chemical etching, but lack atomic precision and introduce the possibility of contamination. Here, we report that a field-induced emission process can be used to make local contacts onto individual nanowires and nanotubes with atomic spatial precision. The gold nano-islands are deposited onto nanostructures precisely by using a scanning tunneling microscope tip, which provides a clean and controllable method to ensure both electrically conductive and mechanically reliable contacts. To demonstrate the wide applicability of the technique, nano-contacts are fabricated on silicide atomic wires, carbon nanotubes, and copper nanowires. The electrical transport measurements are performed in situ by utilizing the nanocontacts to bridge the nanostructures to the transport probes.

Surface Sorted Metallic Nanotube Thin Films for Transparent Electrodes

This work shows how to absorb only MET-SWNT on surfaces, ideal for electrodes since a lower sheet resistance will result. Additionally, this work has important implications for solar cell technology that requires thin transparent electrodes..