B. McCarthy

Multilevel conductance switching in polymer films

Multilevel conductance switching in poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) films is demonstrated. A thin-film structure, ITO-coated glass/MEH-PPV/Al, has shown the ability to store a continuum of conductance states. These states are nonvolatile and can be switched reproducibly by applying appropriate programing biases above a certain threshold voltage. The electrical conductivity of the highest and lowest states can differ by five orders of magnitude. Furthermore, these devices exhibit good cyclic switching characteristics and retention times of several weeks.

Evolution and evaluation of the polymer/nanotube composite

Abstract Composite structures, using MWNT and SWNT and the polymer (PmPV) exhibit properties which enhance those of the individual components. The polymer PmPV can act as an organic filter for the multiwalled system where the MWNT are indefinitely suspended in the polymer solution while the carbonaceous material falls out of solution. Raman measurements of this show a complete reduction of the amorphous line at 1350 cm-1. We see that we can alter the luminescence quantum yield of the composite, where the effects are different depending on which nanotubes are used. When we examine the SWNT/PmPV the quantum yield is increased. The MWNT composite also shows strong non-linear optical signal. The pristine polymer has an χ (3) of 10 −11 esu whereas the composite χ (3) is -10 −10 esu.

Optimisation of the arc-discharge production of multi-walled carbon nanotubes

Abstract The effect of varying current density and pressure during arc generation on the yield and purity of multi-walled nanotube-containing carbon soot has been studied in this work. Various soots were produced and characterised qualitatively using transmission electron microscopy and quantitatively using electron paramagnetic resonance and thermogravimetric analysis. It was found that both yield and purity increase as current density and pressure are increased to the limit of our experimental investigations, i.e. 195 A/cm 2 and 500 Torr of helium. Under these conditions a yield of 24 mg/min soot containing 48% by mass nanotubes was obtained.

A functional conjugated polymer to process, purify and selectively interact with single wall carbon nanotubes

Solutions of a semi-conjugated polymer are capable of suspending single wall carbon nanotubes indefinitely whilst the accompanying amorphous graphite settles out. Moreover, Raman spectroscopy indicates that the polymer preferentially interacts with nanotubes of specific diameters or range of diameters.

Complex nano-assemblies of polymers and carbon nanotubes

Since the discovery of carbon nanotubes in 1991 [1], researchers have envisaged potential applications such as nanoscale electronic circuits and the construction of complex carbon-based nano-machines. Thus, the assembly of basic building blocks of complex nano-architectures, such as conjugated polymers and nanotubes, has been a driving goal of much of the nano-science community. A first step towards realizing this goal may be the attachment to, or modification by carbon nanotubes of structures such as polymers. This leads to the possibility of assembling individual polymer molecules onto carbon nanotubes with the net effect being the modification of the polymers electronic properties and structure in a predictable way. To accomplish this, clearly, a more detailed understanding of the interactions between conjugated polymers and carbon nanotubes must be sought. In this paper, we describe the assembly of the polymer, poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV), into a coating around single-walled carbon nanotubes. Using scanning tunnelling microscopy, and scanning tunnelling spectroscopy, we demonstrate that the low-energy electronic structure of the assembled material is dominated by the one-dimensional nature of the nanotube as reflected in van Hove singularities. Further, we examine the modifications to electronic structure at higher energies using spectroscopy, which suggests that the polymers electronic structure is altered by the introduction of nanotubes.

Nonvolatile multilevel conductance and memory effects in organic thin films

Organic thin-film structures, including organic light-emitting diodes, are demonstrated to contain multiple nonvolatile conductance states at low-read voltages. Retention time of states is more than several weeks, and more than 20 000 write-read-rewrite-read cycles have been performed with minimal degradation. The electrical characteristics of these devices are consistent with metal diffusion or filament phenomena found in metal-insulator-metal structures, suggesting a possible mechanism by which the states are stored.

New and old cardiovascular risk factors: C-reactive protein, homocysteine, cysteine and von Willebrand factor increase risk, especially in smokers.

Background The relative importance of new risk factors for heart disease singly or in combination is uncertain. We assessed relationships between C-reactive protein, homocysteine, cysteine, von Willebrand factor, activated factor XII and stable heart disease, as well as interaction with established risk factors. Methods A case-control study of 260 cases of stable heart disease from the Irish component of the European Action on Secondary Prevention through Intervention to Reduce Events (EUROASPIRE) II cohort and 260 age, sex-matched controls. C-reactive protein, homocysteine, cysteine, von Willebrand factor, activated factor XII and conventional risk factors were assayed or recorded. Interaction effects between new and conventional factors were assessed using additive and multiplicative models. Results C-reactive protein, homocysteine, cysteine and von Willebrand factor were significantly higher in cases than controls. Comparing the top fifth with the bottom four-fifths showed independent associations between heart disease and C-reactive protein [odds ratio (OR) 1.79; 95% confidence interval (CI) 1.12–2.86; P = 0.01], cysteine (OR 2.00; 95% CI 1.25–3.20; P = 0.004), von Willebrand factor (OR, 3.0; 95% CI 1.9–4.8; P < 0.0001). For homocysteine, the association was independent comparing the top tenth to the bottom nine-tenths (OR 1.95; 95% CI 1.02–3.41; P = 0.04). Activated factor XII was not associated with risk. The association between C-reactive protein and disease was U-shaped and a graded association existed between homocysteine, cysteine, von Willebrand factor and disease. C-reactive protein, homocysteine, cysteine and von Willebrand factor considerably increased risk associated with other factors, particularly smoking. Conclusions Independent associations exist between stable heart disease and C-reactive protein, homocysteine, cysteine and von Willebrand factor. Strong combined effects were observed between these and conventional risk factors, particularly smoking. Smoking cessation may profoundly reduce risk associated with other risk factors. We found no evidence of a relationship between activated factor XII and disease.

Controlling the Optical Properties of a Conjugated Co-polymer through Variation of Backbone Isomerism and the Introduction of Carbon Nanotubes

Abstract The need to control the formation of weakly emitting species in polymers such as aggregates and excimers, which are normally detrimental to device performance, is illustrated for the example of the polymer poly( m -phenylenevinylene-co-2,5-dioctyloxy- p -phenylenevinylene), using the model compound, 2,5-dioctyloxy- p -distyrylbenzene as a comparison. Two different methods, namely a Horner–Emmons polycondensation in dimethylformamide (DMF) and a Wittig polycondensation in dry toluene, have been used during synthesis resulting in a polymer with a predominantly trans -vinylene backbone and a polymer with a predominantly cis -vinylene backbone, respectively. Photoluminescence and absorption spectroscopy indicate that the polymer forms aggregate species in solution with spectra that are distinctly red-shifted from those associated with the intra-chain exciton. Concentration dependent optical studies were used to probe the evolution of aggregation in solution for both polymers. The results indicate that inter-chain coupling in the predominantly cis -polymer is prominent at lower concentrations than in the case of the trans -counterpart. These results are supported by pico-second pump and probe transient absorption measurements where, in dilute solutions, the polymer in a cis -configuration exhibits highly complex excited state dynamics, whereas the polymer in a trans -configuration behaves similarly to the model compound. It is proposed therefore that the degree of backbone isomerism has a profound impact on the morphology of the polymeric solid and control over it is a route towards optimising the performance of the material in thin film form. Another method to inhibit inter-chain effects using multi walled carbon nanotubes (MWNT) as nano-spacers in the polymer solutions is proposed. By comparison to spectroscopic analysis, aggregation effects are shown to be reduced by the introduction of nanotubes. Electron microscopy and computer simulation suggest a well-defined interaction between the polymer backbone and the lattice of the nanotube.

Enhanced Raman scattering for temperature measurement of a laser-heated atomic force microscope tip

Illuminating a silicon cantilever of an atomic force microscope with a focused laser beam creates heat that can be funneled into a nanoscale area at the apex of its tip. To characterize the heating dynamics and measure the temperature of the tip, a Raman scattering pump-and-probe method is used. It is found that at the apex of the tip the intensity of the Raman Stokes and anti-Stokes components are significantly enhanced relative to those obtained on a bulk silicon sample. Modeling the temperature rise at the tip of the cantilever by a closed-form analytical expression gives good agreement with the Raman measurements. This model can be used to design the structure of the cantilever so that the heat delivery to its tip is optimized. Such an optimized cantilever can potentially be used in high-density, heat-assisted magnetic recording, optical data storage using phase-change media and thermomechanical recording systems, for example, where nanoscale heated regions are of importance.

Optical absorption and fluorescence of a multi-walled nanotube-polymer composite

We report on the optical studies of an organic composite containing an unusual phenylene vinylene copolymer and multi-walled carbon nanotubes in solution. The nanotubes appear to be held in the polymer matrix through a weak interaction between the backbone of the polymer and the lattice of the nanotube. This incorporation greatly effects the absorption and emission properties of the composite. We have also shown that it is possible to control the quantum efficiency of the system by varying the mass fraction of the nanotubes present.