Andrew J. Stapleton

Accurate thickness measurement of graphene

Graphene has emerged as a material with a vast variety of applications. The electronic, optical and mechanical properties of graphene are strongly influenced by the number of layers present in a sample. As a result, the dimensional characterization of graphene films is crucial, especially with the continued development of new synthesis methods and applications. A number of techniques exist to determine the thickness of graphene films including optical contrast, Raman scattering and scanning probe microscopy techniques. Atomic force microscopy (AFM), in particular, is used extensively since it provides three-dimensional images that enable the measurement of the lateral dimensions of graphene films as well as the thickness, and by extension the number of layers present. However, in the literature AFM has proven to be inaccurate with a wide range of measured values for single layer graphene thickness reported (between 0.4 and 1.7 nm). This discrepancy has been attributed to tip-surface interactions, image feedback settings and surface chemistry. In this work, we use standard and carbon nanotube modified AFM probes and a relatively new AFM imaging mode known as PeakForce tapping mode to establish a protocol that will allow users to accurately determine the thickness of graphene films. In particular, the error in measuring the first layer is reduced from 0.1-1.3 nm to 0.1-0.3 nm. Furthermore, in the process we establish that the graphene-substrate adsorbate layer and imaging force, in particular the pressure the tip exerts on the surface, are crucial components in the accurate measurement of graphene using AFM. These findings can be applied to other 2D materials.

Highly conductive interwoven carbon nanotube and silver nanowire transparent electrodes

Abstract Electrodes fabricated using commercially available silver nanowires (AgNWs) and single walled carbon nanotubes (SWCNTs) produced sheet resistances in the range 4–24 Ω □−1 with specular transparencies up to 82 %. Increasing the aqueous dispersibility of SWCNTs decreased the bundle size present in the film resulting in improved SWCNT surface dispersion in the films without compromising transparency or sheet resistance. In addition to providing conduction pathways between the AgNW network, the SWCNTs also provide structural support, creating stable self-supporting films. Entanglement of the AgNWs and SWCNTs was demonstrated to occur in solution prior to deposition by monitoring the transverse plasmon resonance mode of the AgNWs during processing. The interwoven AgNW/SWCNT structures show potential for use in optoelectronic applications as transparent electrodes and as an ITO replacement.

Transition metal-substituted Dawson anions as chemo- and regio-selective oxygen transfer catalysts for H2O2 in the epoxidation of allylic alcohols

Organic-soluble transition metal-substituted Dawson compounds [(n-C(4)H(9))(4)N](9)[P(2)W(17)O(61)M(Br)] (M(n+) = Co(2+), Ni(2+), Cu(2+) and Zn(2+)), [(n-C(4)H(9))(4)N](7)[HP(2)W(17)O(61)M(Br)] (M(n+) = Cr(3+), Mn(3+) and Fe(3+)) and [K/(n-C(4)H(9))(4)N](10-n)[P(2)W(17)O(61)M(H(2)O)] (M(n+) = Ir(4+), Ru(3+) and Pd(2+)) have been investigated as oxygen transfer agents for H(2)O(2) to a series of primary allylic alcohols to generate epoxides under biphasic reaction conditions (1,2-dichloroethane/H(2)O) at 30 or 35 degrees C, such that the effect of variations in the substituted transition metals could be evaluated. The allylic alcohols involved the species R(1)R(2)C=C(R(3))CH(2)-OH (where R(1), R(2) and R(3) = H or Me), as well as cyclic (2-cyclohexen-1-ol), bicyclic [(R-)-(-)-myrtenol and (R-)-(-)-nopol] and species with two unsaturated sites (geraniol and nerol). The reactions are highly chemoselective and regioselective. The order of reactivity for the M(II)-substituted species is Pd(II) > Zn(II) > Co(II) > Ni(II), and for M(III) and M(IV) substitution is Mn(III) approximately Ir(IV) > Fe(III) > Cr(III). The observed orders are consistent with the formation of metal(n+)-alcohol species as part of the reaction mechanism. For the more polarizing Ir(IV), however, Ir(IV)-alcoholate species are likely involved in the mechanism. Formation constants for the Mn(III) and Co(II)-phosphopolyoxotungstate-alcohol species with all of the above alcohols have been evaluated in 1,2-dichloroethane at 25 degrees C and range from 19.0-3.5 M(-1). The most likely transition state involves coordination of the alcohol to the transition metal substituted at the lacunary site, or alkoxide in the case of Ir(IV), along with interaction of the double bond of the alcohol with a peroxo group located at a W(VI) site adjacent to the substituted transition metal.

Planar silver nanowire, carbon nanotube and PEDOT:PSS nanocomposite transparent electrodes

Abstract Highly conductive, transparent and flexible planar electrodes were fabricated using interwoven silver nanowires and single-walled carbon nanotubes (AgNW:SWCNT) in a PEDOT:PSS matrix via an epoxy transfer method from a silicon template. The planar electrodes achieved a sheet resistance of 6.6 ± 0.0 Ω/□ and an average transmission of 86% between 400 and 800 nm. A high figure of merit of 367 Ω−1 is reported for the electrodes, which is much higher than that measured for indium tin oxide and reported for other AgNW composites. The AgNW:SWCNT:PEDOT:PSS electrode was used to fabricate low temperature (annealing free) devices demonstrating their potential to function with a range of organic semiconducting polymer:fullerene bulk heterojunction blend systems.

Pathway to high throughput, low cost indium-free transparent electrodes

A roll-to-roll compatible, high throughput process is reported for the production of highly conductive, transparent planar electrode comprising an interwoven network of silver nanowires and single walled carbon nanotubes imbedded into poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The planar electrode has a sheet resistance of between 4 and 7 Ω □−1 and a transmission of >86% between 800 and 400 nm with a figure of merit of between 344 and 400 Ω−1. The nanocomposite electrode is highly flexible and retains a low sheet resistance after bending at a radius of 5 mm for up to 500 times without loss. Organic photovoltaic devices containing the planar nanocomposite electrodes had efficiencies of ∼90% of control devices that used indium tin oxide as the transparent conducting electrode.

Effect of a calcium cathode on water-based nanoparticulate solar cells

Water-based nanoparticulate (NP) and bulk heterojunction (BHJ) organic photovoltaic (OPV) devices based on blends of poly(9,9-dioctylfluorene-co-N,N-bis(4-butylphenyl)-N,Ndiphenyl-1,4-phenylenediamine) (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT) have been fabricated with aluminium and calcium/aluminium cathodes. The NP devices exhibit power conversion efficiencies (PCEs) that are double that of the corresponding BHJ device. Moreover, the addition of calcium into the cathode structure results in a dramatic increase in open circuit voltage and PCEs approaching 1% for water-based polyfluorene OPV devices.

Carbon nanotube modified probes for stable and high sensitivity conductive atomic force microscopy

Conductive atomic force microscopy (C-AFM) is used to characterise the nanoscale electrical properties of many conducting and semiconducting materials. We investigate the effect of single walled carbon nanotube (SWCNT) modification of commercial Pt/Ir cantilevers on the sensitivity and image stability during C-AFM imaging. Pt/Ir cantilevers were modified with small bundles of SWCNTs via a manual attachment procedure and secured with a conductive platinum pad. AFM images of topography and current were collected from heterogeneous polymer and nanomaterial samples using both standard and SWCNT modified cantilevers. Typically, achieving a good current image comes at the cost of reduced feedback stability. In part, this is due to electrostatic interaction and increased tip wear upon applying a bias between the tip and the sample. The SWCNT modified tips displayed superior current sensitivity and feedback stability which, combined with superior wear resistance of SWCNTs, is a significant advancement for C-AFM.

High‐Performance Thin Film Transistor from Solution‐Processed P3HT Polymer Semiconductor Nanoparticles

Nanoparticulate suspensions of semiconducting polymer poly‐3‐hexylthiophene (P3HT) have been prepared in water through a mini‐emulsion process using sodium dodecyl sulphate (SDS) as the surfactant. Using these suspensions, we have fabricated organic thin film transistors (OTFTs) in a top gate configuration. These devices operate at a low voltage and show output characteristics similar to those achieved when the P3HT film is spun from chloroform. To characterize the properties of the film made from the nanoparticle suspension, differential thermal analysis (TGA), differential scanning calorimetry (DSC), atomic force microscopy (AFM), fluorescence spectra analysis, ultraviolet/visible (UV/VIS) spectrophotometry and X‐ray photoelectron spectroscopy (XPS) have been used.

Low Vπ modulators containing InGaAsP∕InP microdisk phase modulators

The authors have demonstrated a low switching voltage modulator comprised of a 12μm radius indium phosphide (InP)-based optical microdisk phase modulator in an external Mach-Zehnder interferometer. The chip-scale phase modulator operated with a measured Vπ of 0.6mV.