Biography:

One of their most recent publications is Determination of anisotropic electron transport properties of two Langmuir-Blodgett organic multiple quantum wells. Which was published in journal Thin Solid Films.

More information about K.J. Donovan research including statistics on their citations can be found on their Copernicus Academic profile page.

K.J. Donovan's Articles: (6)

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

AbstractThe 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-C6H13)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 cm2 s-1 V-1 respectively.The results justify regarding these systems as organic multiple quantum wells.

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

AbstractThe 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C6H13)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 cm2 s−1 V−1 respectively. The results justify regarding these systems as organic multiple-quantum wells.

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

AbstractLangmuir-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

AbstractResults 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⊥=v0(E)exp(−2√2mAbhb) 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√2mAbh 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.

Anomalous effective hydrodynamic radius of octadecylamine functionalised single walled carbon nanotubes

AbstractIn a uniform electric field, single walled carbon nanotubes (SWCNTs) in suspension become electrically polarised and are caused to rotate due to the resulting torque acting on the induced dipole moment. This rotation is followed in real time as an induced optical anisotropy, linear dichroism, which is produced as a result. These observations allow the polarisability of the nanotubes and the effective viscosity of the suspending medium to be determined. These techniques are used here to determine the effective radius of an octadecylamine (ODA)-functionalised SWCNT and its polarisability and to compare these values with those found for non-functionalised SWCNTs. The effective radius is found to be a factor 25 times larger than that of a non-functionalised SWCNT and far in excess of the value expected on steric grounds alone. Alternatively the local viscosity seen by the functionalised SWCNTs is 2.2 times greater than the macroscopic viscosity of the dichloroethane in which they are suspended. The polarisability of the metallic SWCNTs is substantially unchanged as a result of the functionalisation.

Carrier range on conjugated polymers as molecular wires. Example: polydiacetylene, 4BCMU

AbstractThe crystalline polydiacetylene 4BCMU is studied using a photocharge collection technique following a 6 ns pulse of UV irradiation. From these studies it is shown, by two separate routes, that with blocking electrodes the single crystal polydiacetylene is a unity gain photoconductor. This entails that the distance of travel of a photocarrier on the polymer backbone of 100% polydiacetylene single crystals is much greater than the 2.5 μm length of the individual polymer chains that form the crystal and equal to the crystal length of up to several millimetres. Furthermore this knowledge of the photocarrier range enables the absolute determination of the photocarrier generation efficiency. Such information is crucial to the interpretation of other photoconduction experiments.

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