K. Scott

Enhanced electronic transport properties in complementary binary discotic liquid crystal systems

Abstract The electronic transport properties of the molecular stacks in discotic liquid crystals (DLCs), have generated much recent interest. In particular, among a certain class of these DLCs, the triphenylenes, some derivatives exhibit high hole mobilities as demonstrated by the time of flight transit signals obtained in transient photoconduction experiments. The stacks have also been shown to exhibit one-dimensional transport behaviour. If the DLCs are to find application as new electronic materials it is desirable to improve their electronic transport properties. In this paper we demonstrate that the hole mobility and range are greatly increased as a result of the ordering imposed in complementary binary mixtures formed by addition of a large core discogen to the triphenylene molecules.

Determination of the parallel and perpendicular intermolecular tunnelling rates in two Langmuir-Blodgett quantum well systems

Abstract The first experimental determination of the anisotropy of the electron transfer rate in two Langmuir-Blodgett insulator multilayer systems is presented. A key feature of these two multilayer systems is the presence of the planar conjugated phthalocyanine ring. The first system is composed of tetra-tertiary-butyl phthalocyanine monosulphonic acid. In it the ring planes are approximately normal to the layer plane. The second system is composed of the two-ring phthalocyanine HOSiPcOSiPcOSI( n C 6 H 13 ) 3 . In it the rings lie in the layer plane. The anisotropy of the systems is determined from a model of the recombination rate, at high density, of photoelectrons and photoholes on the same layer. The ratio of the intralayer to interlayer transfer rates between adjacent rings is found to be 1500 and 570 respectively. Coupled with a previous determination of the interlayer transfer time of 1.6 and 0.30 ns, the intralayer transfer times are then determined as 1.07 and 0.56 ps respectively. The in-plane mobilities can then be determined as 0.34 and 1.2 cm 2 s −1 V −1 respectively. The results justify regarding these systems as organic multiple-quantum wells.

Electron transport across organic quantum wells : new results on amphiphilic phthalocyanines

Electron transfer rates through Langmuir-Blodgett multilayers of two new phthalocyanine molecules have been measured and are presented. Photocreated electrons are shown to travel through up to forty layers with interlayer tunnelling times of 1.6 ns and 150 ps for the two molecules. These times are resolved in the experiments described.

Molecular Engineering the Phototransport Properties of Discotic Liquid Crystals

This paper explores how charge carrier mobilities in columnar discotic liquid crystals may be enhanced. The hole mobility and its temperature dependence has been measured in a variety of triphenylene based DLCs using the time of flight method in the mesophase and where possible in the glassy (or crystalline) phase. A model using the Holstein small polaron is fitted to the results and the resulting polaron self energy and bandwidth are found. Where trapping is observed it shows a tendency to trap at a faster rate as electric field is increased. This is discussed in terms of dimensionality of the carrier transport. Some transient data is fitted to the Noolandi multiple trapping model and trapping rates and detrapping rates using that model are recovered. Their variation with electric field is discussed.

Control of electron transfer in a nanostructure assembled from organic molecules

Abstract Langmuir-Blodgett multilayers of a photoactive two-ring phthalocyanine have been assembled and excitation with a fast laser pulse used to create photocarriers on the conjugated rings. Applying an electric field perpendicular to the plane of the multilayer, in the z direction, interlayer electron tunnelling is measured giving a range of carrier motion that indicates that all the phthalocyanine layers are crossed. In a novel experiment, molecular bilayers and saturated organic molecules have been inserted in designed z locations in the multilayer structure. Electron transfer has been demonstrated to be blocked by the bilayers. That is, the electron transfer has been controlled at a nanometre level by molecular electronic design. The technique can in principle be extended to create electronic blocks which can be turned on and off by electric fields, or by light pulses, thus opening the way to molecular electronic devices.

Determination of anisotropic electron transport properties of two Langmuir-Blodgett organic multiple quantum wells

Abstract The first experimental determination of the anisotropy of the electron transfer rate in two- Langmuir-Blodgett insulator multilayer systems is presented. A key feature of these two multilayer systems is the presence of the planar-conjugated phthalocyanine ring. The first system is composed of tetra-tertiary-butyl phthalocyanine monosulphonic acid. In it the ring planes are approximately normal to the layer plane. The second system is composed of the two-ring phthalocyanine, HOSiPcOSiPcOSi(n-C 6 H 13 ) 3 . In it the rings lie in the layer plane. The anisotropy of the systems is determined from a model of the recombination rate, at high density, of photoelectrons and photoholes on the same layer. The ratio of the intralayer to interlayer transfer rate between adjacent rings is found to be 1500 and 570 respectively. Coupled with a previous determination of the interlayer transfer time of 1.6 and 0.15 ns, the intralayer transfer times are then determined as 1.07 and 0.28 ps respectively. The in-plane mobilities can then be determined as 0.34 and 2.4 cm 2 s -1 V -1 respectively. The results justify regarding these systems as organic multiple quantum wells.

Quantum Efficiency of Photogeneration in Discotic Liquid Crystals: Part 1: Temperature and Wavelength Dependence

Photocarrier generation quantum efficiencies of columnar discotic liquid crystals are determined using DC photoconduction. Photoaction spectra, of single triphenylene and binary systems comprising 50:50 stoichmetric mixtures of triphenylene with a larger triphenylene based macrocycle, demonstrate that the main generation process is due to the excitation of the smaller triphenylene molecule. Other transient photoconduction experiments have shown that carriers trap in the crystalline phase of the single systems while time of flight signals are obtained in the mesophase. Temperature dependence experiments allow carrier range to be determined in the crystalline phase for the single material and demonstrate that charge transits the binary system in both the mesophase and glassy phase.

Comparison of tunnelling rates in two Langmuir-Blodgett quantum well structures differing only in barrier width

Abstract Langmuir-Blodgett multilayer structures were fabricated from two homologous, amphiphilic phthalocyanine molecules and differed only in interlayer separation. Sandwiching such multilayers between electrodes and using a pulsed photoconduction technique it proved possible to excite mobile carriers and to observe their motion directly as an accumulation of charge at the electrodes. From such observations the interlayer tunnelling time was measured for each structure and the differences in this parameter correlated with the change in tunnelling barrier width, which is known for each multilayer structure. Such correlation leads to a successful prediction of the barrier height and the attempt rate for tunneling in these molecules.

Electron tunnelling rates as a function of intermolecular distance, measured in a Langmuir-Blodgett assembly

Abstract Results are presented that demonstrate an exponential dependence of the electron tunnelling rate on interlayer separation. Direct measurements of transient photocurrents with sub-nanosecond resolution have been made on a series of Langmuir-Blodgett multilayer structures assembled from two amphiphilic bis-phthalocyanine molecules. Using either homostructures of the two molecules or a heterodimer structure incorporating both molecules, three tunnelling gaps were attainable. The attempt rate, v0, as determined by Franck-Condon factors, and the well depth in the three structures was identical for each, being determined by the phthalocyanine ring structure and electron affinity. It is thus possible to relate any change in measured tunnelling rate with the change in barrier width, b. The tunnelling rate, k⊥, is related to the tunnelling barrier width by: k ⊥ =v 0 (E)exp( −2√2mA bh b) where A is the well depth (electron affinity of the conjugated rings) and m is an effective mass for the tunnelling carrier. A plot of ln(k⊥) vs. b reveals a straight line whose gradient is −2 √2mA bh which intercepts the k⊥ axis at k⊥ = v0(E). From the data a value of A = 2.4 eV and v0 = 7 × 1016 Hz at an electric field of 2 × 108 V m−1 are found. The well depth is in good agreement with expectations. The attempt rate is high but there is a large uncertainty in this value and it is in overall agreement with expectations. v0 is expected to depend on electric field and this is the subject of a further study.

Manipulation and immobilization of alkane-coated gold nanocrystals using scanning tunneling microscopy

Colloidal nanoparticles, comprised of gold nanocrystals, of mean diameter 2.8 nm, coated with an insulating chemically stable self-assembled monolayer of dodecanethiol, have been prepared. Monolayers of nanoparticles have been physisorbed on highly oriented pyrolitic graphite, first by self-assembly, and second by assembly as Langmuir films and subsequent deposition. Nanoparticles have been self-assembled on gold, and immobilized by chemisorption, using decanedithiol during assembly as a linking molecule. Scanning tunneling microscope images of the monolayers are obtained. At high substrate–tip voltages, >0.6 V, the tip is able to climb above the nanoparticles. The tunneling is then a two-step event, tunneling from the substrate to the gold nanocrystal, and subsequently from the gold nanocrystal to the tip. At low voltage, 0.25 V, the Coulomb blockade prevents one extra electron occupying the gold nanocrystal. The tip cannot then climb above the nanoparticles. The theoretical threshold of the blockade is ...