Simon J. Higgins

Metal–Organic Conjugated Microporous Polymers

Zwei vielseitige Strategien fur die Herstellung von Metall-organischen konjugierten mikroporosen Polymeren (MO-CMPs) mit Metallen wie Rhenium, Rhodium und Iridium werden beschrieben (siehe Beispiel). Diese Materialien vereinen in sich zwei Merkmale: eine ausgedehnte, unterbrechungsfreie elektronische Konjugation und das Vorhandensein katalytisch aktiver Metallzentren

Conjugated polymers incorporating pendant functional groups—synthesis and characterisation

The subject of this review is the synthesis and electropolymerisation of pyrroles or thiophenes with pendant functional groups (metal complexes, ligands, biomolecules for protein binding etc.) to afford electrodes coated with a polymeric matrix containing the desired functionality. Such electrodes are being examined in electrocatalysis and in sensor fabrication. More recently, well-defined, soluble polythiophenes with receptor groups for alkali metal ions or for non-covalent binding of electron-deficient aromatics have been made chemically. These polymers can be spin-coated onto electrode surfaces. Their optical and redox properties are changed as a consequence of reversible binding in readily detectable ways. Chemical synthesis of polymers avoids the quite positive potentials usually required for electropolymerisation, which can be a problem for delicate or expensive functional groups. Another approach is to attach the functionality to a suitably reactive polymer after electropolymerisation. Electropolymerisation can also result in poorly adherent or inhomogeneous polymer coatings, and the review concludes with an approach to tackling this problem with another class of functionalised pyrroles—those designed for the fabrication of monolayers, self-assembled on gold electrodes.

Long-range electron tunnelling in oligo-porphyrin molecular wires

Short chains of porphyrin molecules can mediate electron transport over distances as long as 5–10 nm with low attenuation. This means that porphyrin-based molecular wires could be useful in nanoelectronic and photovoltaic devices, but the mechanisms responsible for charge transport in single oligo-porphyrin wires have not yet been established. Here, based on electrical measurements of single-molecule junctions, we show that the conductance of the oligo-porphyrin wires has a strong dependence on temperature, and a weak dependence on the length of the wire. Although it is widely accepted that such behaviour is a signature of a thermally assisted incoherent (hopping) mechanism, density functional theory calculations and an accompanying analytical model strongly suggest that the observed temperature and length dependence is consistent with phase-coherent tunnelling through the whole molecular junction. A combination of calculations and electrical measurements on oligo-porphyrin wires in single-molecule junctions strongly suggest that the mechanism of long-range charge transport is phase-coherent electron tunnelling.

Measurement of single molecule conductivity using the spontaneous formation of molecular wires

A technique to measure the electrical conductivity of single molecules has been demonstrated. The method is based on trapping molecules between an STM tip and a substrate. The spontaneous attachment and detachment of α,ω-alkanedithiol molecular wires was easily monitored in the time domain. Electrical contact between the target molecule and the gold probes was achieved by the use of thiol groups present at each end of the molecule. Characteristic jumps in the tunnelling current were observed when the tip was positioned at a constant height and the STM feedback loop was disabled. Histograms of the measured current jump values were used to calculate the molecular conductivity as a function of bias and chain length. In addition, it is demonstrated that these measurements can be carried out in a variety of environments, including aqueous electrolytes. The changes in conductivity with chain length obtained are in agreement with previous results obtained using a conducting AFM and the origin of some discrepancies in the literature is analysed.

Identifying Diversity in Nanoscale Electrical Break Junctions

The realization of molecular-scale electronic devices will require the development of novel strategies for controlling electrical properties of metal/molecule/metal junctions, down to the single molecule level. Here, we show that it is possible to exert chemical control over the formation of metal/molecule...molecule/metal junctions in which the molecules interact by pi-stacking. The tip of an STM is used to form one contact, and the substrate the other; the molecules are conjugated oligophenyleneethynylenes (OPEs). Supramolecular pi-pi interactions allow current to flow through the junction, but not if bulky tert-butyl substituents on the phenyl rings prevent such interactions. For the first time, we find evidence that pi-stacked junctions can form even for OPEs with two thiol contacts. Furthermore, we find evidence for metal|molecule|metal junctions involving oligophenyleneethynylene monothiols, in which the second contact must be formed by the interaction of the pi-electrons of the terminal phenyl ring with the metal surface.

Comparison of the conductance of three types of porphyrin-based molecular wires: β,meso,β-fused tapes, meso-Butadiyne-linked and twisted meso-meso linked oligomers.

The length dependence of charge transport is evaluated in three families of porphyrin-based wires. Planar edge-fused tapes and alkyne-linked oligomers mediate efficient charge transport with exceptionally shallow distance dependence, whereas the conductances of the twisted singly linked chains decrease steeply with increasing oligomer length. The planar tapes are more conjugated than the alkyne-linked oligomers, but these two types of wires have similar conductance attenuation factors.

Simplifying the conductance profiles of molecular junctions: the use of the trimethylsilylethynyl moiety as a molecule–gold contact

Conductance across a metal|molecule|metal junction is strongly influenced by the molecule-substrate contacts, and for a given molecular structure, multiple conductance values are frequently observed and ascribed to distinct binding modes of the contact at each of the molecular termini. Conjugated molecules containing a trimethylsilylethynyl terminus, -C≡CSiMe(3) give exclusively a single conductance value in I(s) measurements on gold substrates, the value of which is similar to that observed for the same molecular backbone with thiol and amine based contacting groups when bound to under-coordinated surface sites.

Variable contact gap single-molecule conductance determination for a series of conjugated molecular bridges

It is now becoming clear that the characteristics of the whole junction are important in determining the conductance of single molecules bound between two metal contacts. This paper shows through measurements on a series of seven conjugated molecular bridges that contact separation is an important factor in determining the electrical response of the molecular junction. These data are obtained using the I(t) method developed by Haiss et al since the scanning tunnelling microscope tip to substrate separation can be controlled through choice of the set-point (I(0)) current and calibrated with current-distance curves and knowledge of the terminal to terminal length of the molecular wire. The contact gap separation dependence is interpreted as arising from tilting of these molecules in the junction and this model is underpinned by ab initio transport computations. In this respect we make the general observation that conductance increases rather dramatically at higher tilt angle away from the normal for conformationally rigid molecular wires and that this increase in conductance arises from increased electronic coupling between the molecular bridge and the gold contacts.

Grafting and electrochemical characterisation of poly-(3,4-ethylenedioxythiophene) films, on Nafion and on radiation-grafted polystyrenesulfonate–polyvinylidene fluoride composite surfaces

The preparation and characterisation of poly-(3,4-ethylenedioxy)thiophene (PEDOT) films coated onto fluorocarbon membranes containing sulfonate groups, such as Nafion-117 and poly(styrene-4-sulfonate)-grafted poly(vinylidene fluoride) (PVDF-g-PSSA) membranes is described. The PEDOT layer is found to adhere very strongly on to both membranes, and is highly conductive. The organic polymer membranes can be used directly as working electrodes in electrochemical experiments, without any inert metal or semiconductor support. The composite materials are characterised by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

Conduction processes in conjugated, highly regio-regular, high molecular mass, poly(3-hexylthiophene) thin-film transistors

Highly regio-regular poly(3-hexylthiophenes) (P3HT) thin films have been produced using the Tzrnadel method. They have head to tail counts approaching very close to 100% and high molecular mass. Thin-film transistors and Schottky diodes have been used to study the effects of counter ion and carrier density on field-effect and bulk mobility, respectively. The density of counter ions was increased using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and had a profound effect on both the field effect and bulk mobility with a ratio between the two of 102, in a similar way to DDQ in poly(β′-dodecyloxy-α,α′,-α′,α″terthienyl) (polyDOT3), but with substantially higher drift mobilities. A method is described, using Schottky barriers, of simply and accurately determining carrier drift mobility. The effect of carrier density on hole mobility is believed to be indirect, filling traps in the regions separating the highly ordered domains until trap free conduction and hence high mobility is reached.