Anna Drury

Experimental observation of scaling laws for alternating current and direct current conductivity in polymer-carbon nanotube composite thin films

Alternating current (ac) and direct current (dc) conductivities have been measured in polymer-nanotube composite thin films. This was carried out for a range of concentrations of multiwall nanotubes in two polymer hosts, poly(m-phenylenevinylene-co-2,5-dioctyloxyp-phenylenevinylene) (PmPV) and polyvinylalcohol (PVA). In all cases the dc conductivity σDC was ohmic in the voltage range studied. In general the ac conductivity displayed two distinct regions, a frequency independent region of magnitude σ0 at low frequency and a frequency dependent region at higher frequency. Both σDC and σ0 followed a percolation scaling law of the form σ∝(p−pc)t with pc=0.055% by mass and t=1.36. This extrapolates to a conductivity of 1×10−3 S/m for 100% nanotube content. Such a low value reflects the presence of a thick polymer coating, resulting in poor electrical connection between tubes. This leads to the suggestion that charge transport is controlled by fluctuation induced tunneling. In the high frequency regime the cond...

Enhanced brightness in organic light-emitting diodes using a carbon nanotube composite as an electron-transport layer

We have studied the effects of using a composite fabricated from carbon nanotubes and a host polymer, poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV), as an electron-transport layer in organic light-emitting diodes. Double layer devices using this composite as an electron-transport layer, triple layer devices with a composite electron-transport layer, and poly(9-vinylcarbazole) as a hole-transport layer, as well as poly(2,5-dimethoxy-1,4phenylene-vinylene-2-methoxy- 5(2′-ethylhexyloxy)-1,4-phenylene-vinylene (M3EH-PPV) single layer devices were prepared. Current–voltage–luminance and electroluminescent spectral measurements were performed using six different nanotube powder to polymer mass ratios (0%, 2%, 4%, 8%, 16%, and 32%) for all device structures studied. dc transport and photoluminescence behavior of the polymer-nanotube composite were also investigated. Although a potential barrier is introduced at the M3EH-PPV/composite interface, a significant increase in efficiency was ...

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.

Nonlinear optical response of multiwalled carbon-nanotube dispersions

Experimental measurements of nonlinear optical extinction of nanosecond laser pulses by a set of conjugated copolymer/multiwalled carbon-nanotube composites dispersed in solution are reported here. The polymer poly(para-phenylenevinylene-co-2,5-dioctyloxy-meta-phenylenevinylene) and multiwalled carbon-nanotube composites were varied according to nanotube mass content. The experiments were performed with an open-aperture Z scan with 6-ns Gaussian pulses at 532 nm from a frequency-doubled, Q-switched Nd:YAG laser. The nonlinear optical extinction of the incident pulses displays enhanced dissipation of the incident light for lower incident intensities relative to increasing multiwalled carbon-nanotube content. Either the multiwalled carbon nanotubes or the polymer dominates the nonlinear response of the composite depending on the relative mass of polymer to nanotube. Effective optical coefficients with a nonlinear absorption based model are calculated, and their intensity dependence is investigated. Mechanistic implications of the optical dissipation are also discussed.

Investigation of different synthetic routes to and structure–property relationships of poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene)

The synthesis of poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) by Horner–Emmons and Wittig condensation polymerisation in three different solvents is described. The chemical and optical properties of the derivatives thus formed are analysed, especially in relation to the differences in the cis∶trans ratio of the vinylene bonds. Although they all have the same chemical composition, it is found that their chemical and optical properties vary. NMR studies show that the morphologies of the polymers are different, due to the differences in the cis∶trans ratio of the vinylene bonds. It is found that the derivatives produced by Horner condensation contain a majority of trans bonds, and these derivatives show different spectral characteristics to the Wittig derivatives. The ability of the polymers to disperse carbon nanotubes is also studied. Here, not only is the synthetic route important, but the solvent used also plays a role. The derivatives that are produced by the Horner condensation route in DMF or chlorobenzene are found to have the best binding capabilities with carbon nanotubes.

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.

Synthesis and optical properties of phenylene-vinylene copolymers

Abstract A series of soluble phenylene-vinylene copolymers have been prepared by four different methods. Two of these methods use the Wittig reaction in two different solvents, whereas the other two were prepared by the Horner reaction using the same two solvents. The method of preparation was found to affect the cis/trans vinylene bond ratio and also the molecular weight characteristics of the polymer. Some of the polymers form high molecular weight cyclic oligomers. The optical properties of the polymers were investigated and compared. Concentration dependant fluorescence studies were done. In particular one polymer (Poly(m-phenylene-co-2,5-dioctoxy-p-phenylene-vinylene) or WToIPmPV prepared by the Wittig route in Toluene exhibited behaviour characteristic of an aggregate luminescence.

Hybrid light-emitting polymer device fabricated on a metallic nanowire array

An electrode comprised of a copper nanowire array, fabricated by electrodeposition into a porous alumina membrane, is incorporated into a single-layer organic light-emitting device. The 48-nm-diameter copper nanowires form an array of electrical nanocontacts. Current–voltage and electroluminescence data for this device type are presented and compared with an equivalent planar copper electrode device. The nanowire array electrode device sustained a higher operating current density. The results show that such an array electrode can be used to inject charge in a polymer light-emitting diode, opening up the possibility of producing arrays of nanosized light sources using this approach.

Synthesis and optical properties of group 14 element-thienyl polymers

The syntheses of several types of polymers containing silicon, germanium and tin in the main chain are described. Optical studies show that they exhibit absorption in the far-ultraviolet region of the spectrum (200–400 nm). Electrical studies show that these polymers behave as insulators. Preliminary studies indicate, that depending on the chemical structure of the backbone, it is possible to dope some of the polymers via partial oxidation in solution.