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The promise of multiwalled carbon nanotubes (MWNTs) for supercapacitor electrodes remains unfulfilled due to their poor energy density, which is limited by their redox inactivity. Here, we show a simple, alternative path to achieve Faradaic charge storage by harnessing intrinsic heterogeneity (e.g., Fe catalyst) of as-synthesized MWNTs, obviating the challenges of combining disparate materials in hybrid composite electrodes. In acidic solutions, MWNTs are ruptured by voltammetric cycling beyond the electrolysis limit, thereby exposing residual catalyst nanoparticles. The addition of Faradaic charge storage associated with the Fe2+/Fe3+ transition, results in a 4-fold increase in peak capacitance of MWNT electrodes (290 F/g) compared to purified MWNT electrodes (70 F/g), along with a 60% increase in charge capacity.

Growing demands for high energy/high power energy storage systems have driven research efforts toward devices such as supercapacitors that bridge the gap between high power capacitors and high energy batteries. Supercapacitors have experienced commercial success with high surface area activated carbon electrodes; however, these systems are limited by the surface area and physical charge storage through the electrical double layer mechanism. Redox materials, such as metal oxides and conductive polymers, have attracted interest because of the increase in energy density obtained through faradaic charge storage processes. One of the main drawbacks of conducting polymers, which are far more abundant and cost-effective relative to oxides, is their relatively low theoretical charge capacity. In this work, we report the use of a high-charge capacity redox molecule, 1,4-benzoquinone, to increase the energy density of polypyrrole electrodes. The electrode synthesis conditions are shown to have a significant influen...

Many reactions in both chemistry and biology rely on the ability to precisely control and fix the solution concentrations of either protons or hydroxide ions. In this report, we describe the behavior of thermally programmable pH buffer systems based on the copolymerization of varying amounts of acrylic acid (AA) groups into N-isopropylacrylamide polymers. Because the copolymers undergo phase transitions upon heating and cooling, the local environment around the AA groups can be reversibly switched between hydrophobic and hydrophilic states affecting the ionization behavior of the acids. Results show that moderate temperature variations can be used to change the solution pH by two units. However, results also indicate that the nature of the transition and its impact on the pH values are highly dependent on the AA content and the degree of neutralization.

Organic thin-film transistors show tremendous potential for versatile electronic sensors. Critical challenges facing the integration of organic thin-film transistors (OTFTs) as chemical and biological sensors include reproducibility, sensitivity, and particularly stability. Here, we describe a bilayer organic semiconductor structure consisting of a pentacene active layer with copper phthalocyanine as the top surface passivation layer. In this architecture, the copper phthalacyanine (CuPc) acts as a surface passivation layer with efficient charge injection into the pentacene layer, thus providing a versatile method for the incorporation of many semiconductor materials as the sensing element. OTFTs fabricated with the described structure exhibited a field-effect mobility of 1.15 ± 0.2 cm2 V−1 s−1 at −2 V in ambient conditions with stable performance in aqueous media (0.5 cm2 V−1 s−1 at −1 V). X-ray diffraction (XRD) confirms that the bilayer film achieves a high degree of molecular order as suggested by the...

Ultrathin, transparent electronic materials consisting of solution-assembled nanomaterials that are directly integrated as thin-film transistors or conductive sheets may enable many new device structures. Applications ranging from disposable autonomous sensors to flexible, large-area displays and solar cells can dramatically expand the electronics market. With a practical, reliable method for controlling their electronic properties through solution assembly, submonolayer films of aligned single-walled carbon nanotubes (SWNTs) may provide a promising alternative for large-area, flexible electronics. Here, we report SWNT network TFTs (SWNTntTFTs) deposited from solution with controllable topology, on/off ratios averaging greater than 10(5), and an apparent mobility averaging 2 cm(2)/V.s, without any pre- or postprocessing steps. We employ a spin-assembly technique that results in chirality enrichment along with tunable alignment and density of the SWNTs by balancing the hydrodynamic force (spin rate) with the surface interaction force controlled by a chemically functionalized interface. This directed nanoscale assembly results in enriched semiconducting nanotubes yielding excellent TFT characteristics, which is corroborated with mu-Raman spectroscopy. Importantly, insight into the electronic properties of these SWNT networks as a function of topology is obtained.

Detecting trace amounts of analytes in aqueous systems is important for health diagnostics, environmental monitoring, and national security applications. Single-walled carbon nanotubes (SWNTs) are ideal components for both the sensor material and active signal transduction layer because of their excellent electronic properties and high aspect ratio consisting of entirely surface atoms. Submonolayer arrays, or networks of SWNTs (SWNTnts) are advantageous, and we show that topology characteristics of the SWNT network, such as alignment, degree of bundling, and chirality enrichment strongly affect the sensor performance. To enable this, thin-film transistor (TFT) sensors with SWNTnts were deposited using a one-step, low-cost, solution- based method on a polymer dielectric, allowing us to achieve stable low-voltage operation under aqueous conditions. These SWNT-TFTs were used to detect trace concentrations, down to 2 ppb, of dimethyl methylphosphonate (DMMP) and trinitrotoluene (TNT) in aqueous solutions. Along with reliable cycling underwater, the TFT sensors fabricated with aligned, sorted nanotube networks (enriched with semiconductor SWNTs) showed a higher sensitivity to analytes than those fabricated with random, unsorted networks with predominantly metallic charge transport.

Cross-linked polymer films were investigated as new gate dielectric materials for low-voltage thin-film transistors. Poly(4-vinylphenol) (PVP) was cross-linked through esterification reactions with commercially available bifunctional anhydrides, acyl chlorides, and carboxylic acids. The polymer dielectric films were evaluated based on surface morphology, capacitance, leakage current, and their compatibility with organic semiconductors. Thin insulating PVP films cross-linked with dianhydrides yielded a capacitance as high as 400 nF/cm2 with leakage currents below 10−8 A/cm2. Organic thin-film transistors (OTFTs) fabricated on these gate dielectric layers exhibited charge carrier mobilities as high as 3 cm2/(V s) for p-channel pentacene on octadecyltriethoxylsilane (OTS)-modified PVP and 0.045 cm2/(V s) for n-channel perfluorinated copper phthalocyanine (FCuPc).

Low-voltage organic thin-film transistors were fabricated to investigate the stability of organic semiconductors in water and determine the critical parameters for aqueous phase chemical detection. We showed that the molecular structure, alkyl-chain length, and thin-film morphology strongly influence the electrical characteristics in water. Highly two-dimensional films of alkyl-substituted semiconductors greatly reduced the influence of water on the electrical characteristics. The sensitivity and response times of OTFT sensors were also impacted by the thin-film structure of the organic semiconductor. OTFT sensors were used to detect 10 parts per billion cysteine and 40 ppm TNT in aqueous solutions.

In organic thin film transistors (OTFT), modifying the molecular chemical structure affects the molecular packing and thin film morphology, which both sensitively influence the charge carrier mobility. A detailed understanding of the interplay of molecular chemical structure, molecular packing, and thin film morphology is therefore necessary to improve OTFT performance. Fluorene-bithiophene-fluorene (FTTF) derivatives have demonstrated great potential for use as the active layer for OTFTs. A series of FTTF asymmetrically substituted derivatives were synthesized to fine-tune the film properties. In this study, the thin film microstructure details of FTTF and several FTTF derivatives with asymmetrically substituted alkyl-chains of different lengths are studied and compared with their electrical performance in thin film transistor devices. The respective unit cells were characterized using grazing incidence X-ray diffraction (GIXD), and for FTTF the detailed molecular packing was determined from the GIXD int...

Air-stable p-type semiconducting oligofluorene–thiophene derivatives were vacuum-deposited on octadecyltriethoxysilane-treated SiO2/Si substrates. Effects of end-substituents and substrate deposition temperature (TD) on molecular orientation, crystalline morphologies, and structures in these thin films were investigated by two-dimensional grazing incidence X-ray diffraction and atomic force microscopy, and those results were correlated with charge mobility in top-contacted devices. Crystalline morphologies of the first monolayer thin film in direct contact with the dielectric surface, influenced by TDs (25, 90, and 140 °C) and end-substituted groups (hydrogen, hexyl, and dodecyl), could be categorized as dendrite, compact disk, and single-crystal-like layered grains. The results of grazing incidence X-ray diffraction strongly support that molecular orientation in the films can be finely tuned through controlling substrate, TD, and molecular architecture, resulting in high air stability and field-effect mo...

The measurement and optimization of microstructure development in organic semiconductor films is valuable because microstructure in many cases critically impacts electronic performance. We demonstrate a general method to measure microstructure thickness dependence in thin films using surface-sensitive near edge X-ray absorbance fine structure (NEXAFS) spectroscopy. The method is applied to an oligofluorene derivative DDFTTF, which consists of a fluorene-bithiophene-fluorene core that is end-substituted with linear dodecyl groups. The substrate-relative orientations of the aromatic core and the aliphatic end chains are independently determined, and comparing these orientations to terrace heights from atomic force micrographs proves that the end chains are interdigitated or folded. By measuring microstructure development from 6 to 150 nm, we find that DDFTTF exhibits two different preferential microstructures: one with large terraces within which molecules exhibit a strongly vertical orientation, and one with much smaller domains within which molecules exhibit a mildly horizontal orientation. The relative distribution of these two preferential microstructures depends on the distance of the domains from the substrate and the substrate temperature during deposition. The utility of this method is tested using a lamination technique to measure the saturation hole mobility at the top and bottom interface of DDFTTF films. We find that local microstructures with greater pi orbital alignment in the source-drain plane correlate directly to better local saturation hole mobilities.

Fluorene−thiophene oligomers show great promise as the active material in p-type organic thin film transistors (TFTs) because of their good performance and good stability under ambient conditions. In this study, a series of fluorene−thiophene co-oligomers were asymmetrically substituted with an alkyl group to probe the effect chemical structure has on their thin film properties. The alkyl groups are n-hexyl, n-octyl, and n-dodecyl, respectively. These oligomers were characterized by elemental analysis, mass spectrometry, differential scanning calorimetry, and thermogravimetric analysis, whereas the thin films of these oligomers were characterized by X-ray diffraction, atomic force microscopy, and field-effect transistor measurements. We found that the performance of these asymmetric molecules is similar to their symmetric counterparts. Mobilities as high as 0.16 cm2 V-1 s-1 were found for the n-dodecyl-substituted oligomer.