A.B. Dalton

A Composite from Poly(m‐phenylenevinylene‐co‐2,5‐dioctoxy‐p‐phenylenevinylene) and Carbon Nanotubes: A Novel Material for Molecular Optoelectronics

As research progresses towards smaller and more efficient devices, the need to develop alternative molecular scale electronic materials becomes apparent. Integrated electronic component fabrication from organics has been recognized theoretically as the ultimate goal. In order to gain a comprehensive insight into these materials, extensive research has been carried out on conjugated carbon systems over the last few decades to optimize their optical and electrical properties. For example, doping polyacetylene with I2 has been shown to result in a large increase in conductivity compared to the pristine material. However, doping polymers tends to retard their optical properties as regards luminescence by reducing their bandgaps and introducing trapping sites such as solitons, polarons, or bipolarons. The simple lesson over the years is that if materials are to be considered for luminescence, doping should not be carried out despite the desire to improve charge transport properties. We report here the first physical adopingo, to use the traditional term, using small concentrations of multiwalled nanotubes in a conjugated luminescent polymer, poly(m-phenylenevinylene-co-2,5-dioctoxyp-phenylenevinylene) (PmPV), in a polymer/nanotube composite. This can increase electrical conductivity of the polymer by up to eight orders of magnitude. The nanotubes appear to act as nanometric heat sinks, preventing the buildup of large thermal effects, caused either optically (photobleaching) or electrically, which degrade these conjugated systems. We also report that electroluminescence was achieved from an organic light-emitting diode (LED) using the composite as the emissive layer in the device. Since initial work on conjugated systems, attempts have been made to find an area where polymers and/or fullerenes could be used as active semiconductor components. Although many new and interesting materials have been synthesized to this end, very few have found a practical application. One exception is polyphenylenevinylene (PPV), first reported by Burroughes et al. as being the light-emitting semiconductor in a Schottky diode. This encouraged scientists to study a wide variety of conjugated systems, including derivatives of this polymer, in order to optimize the efficiency of light emission from such devices. Polymers for use in LEDs must possess a number of important qualities. A high quantum yield of photoluminescence is necessary and the material must remain undoped, as dopants act as trapping sites, quenching the radiative decay of excitons. It is essential therefore to find a polymer that is reasonably conductive while maintaining its luminescent properties. Most undoped polymers possess a very low conductivity and so require high aturn-ono fields to generate sufficient carriers in order to produce the excitons, which decay radiatively. This is, in practical terms, very inefficient as fields generally induce large thermal effects, consequently causing device breakdown. There are other problems that must be addressed, but elimination of these very basic ones should substantially improve efficiencies and soon lead to applications for these polymers. The polymer used in our studies is PmPV, whose structure is a variation of the more common PPV. In this case the substitution pattern leads to dihedral angles in the chain and, according to molecular mechanics energy minimization calculations, the polymer chain tends to coil, forming a helical structure. The calculated diameter of this helix in vacuum is ca. 20 x8a, whilst the pitch is ca. 6 x8a. Multiwalled nanotubes were produced by the arc discharge method, resulting in multiwalled nanotubes of 20 nm average diameter and lengths between 500 nm and 1.5 mm. The nanotube powder and PPV were mixed together in toluene and sonicated briefly. It is probable that the coiled polymer conformation allows it to surround layers of nanotubes, permitting sufficiently close intermolecular proximity for p±p interaction to occur. The color change was dramatic in that the polymer has a bright yellow color while the composite, at high nanotube concentrations, possesses a deep green color. Photoluminescence studies were carried out using an Ar laser at the pump wavelength of 457 nm. Electrical conductivity was measured using a twopoint probe sandwich geometry and Pt electrodes. The LED was fabricated by casting the composite onto indium tin oxide (ITO) then sputtering an aluminum electrode on top. As the polymer structure possesses helicity, it is not surprising that it is able to wrap itself around the nanotubes and keep them suspended in solution indefinitely. The actual texture of the composite can be observed in Figure 1,

Phase Separation of Carbon Nanotubes and Turbostratic Graphite Using a Functional Organic Polymer

sorbed in the vesicle bilayer. Photoinitiated polymerizations were performed in a thermostatted quartz reactor using either an UV-lamp (HPR 125 W, Philips) or a pulsed excimer laser (Lambda Physics XeF, 351 nm, 2 Hz pulse frequency, 30 mJ energy per pulse) as irradiation source. Conversions were determined by HPLC analysis of the residual monomers. Details concerning the use of cryo-electron microscopy have been described earlier [19].

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.

Physical doping of a conjugated polymer with carbon nanotubes

A semi-conjugated, organic polymer was mixed with carbon nanotubes to form a wholly organic composite. Composite formation from low to high nanotube concentration increases the conductivity dramatically by ten orders of magnitude, indicative of percolative behaviour. Effective mobilitys and carrier densities were calculated from the space charge regions of the current voltage characteristics for the 0% to 8% mass fractions.

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.

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.

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.

Electron paramagnetic resonance as a quantitative tool for the study of multiwalled carbon nanotubes

We have described a method that maximizes the phase separation of graphitic particles (GP) and multiwalled carbon nanotubes (MWNT) in solutions of various organic polymeric hosts. This involves the formation of sediment and a solute. These components were characterized for MWNT and GP content using electron paramagnetic resonance (EPR) measurements. All EPR signals could be deconvoluted into nanotube and GP components. When normalized, these components are representative of the mass of MWNT and GP present. This allows us to make quantitative measurements of nanotube and GP content in different environments. The most successful polymer host was poly (m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV). In this case the solute contained 63% of the added nanotubes with only 2% of the added graphite remaining.

Spectroscopic investigation of conjugated polymer/single-walled carbon nanotube interactions

Abstract Composite materials, based on single-walled carbon nanotubes and a poly( p -phenylene vinylene) derivative, show an interaction between the components capable of solubilising the nanotubes, which has not been otherwise achieved. Here these materials are characterised by electron microscopy, and optical and vibrational spectroscopy. The spectroscopic behaviour of the polymer is seen to be dramatically affected, which is attributed to conformational changes due to the effect of the nanotubes.

Measurement of nanotube content in pyrolytically generated carbon soot

Carbon nanotubes can be efficiently separated from impurity nmaterial in carbon soot using a conjugated polymer filtration system as nmonitored by EPR, allowing the calculation of purity of the crude carbon nsoot.