D.A. Batzel

Phthalocyanine Langmuir-Blodgett film microsensors for halogen gases

Abstract Microsensors for halogen gases fabricated by Langmuir-Blodgett films of phthalocyanine [(C 6 H 13 ) 3 SiOSiPcOGePcOH] (where Pc is the phthalocyanato dianion) and planar microelectronic technologies are evaluated. The sensor incorporates a micro heater and a diode to control the temperature of the film. The gas concentration is measured by the conductance change. The film temperature effects on sensitivity, rise and decay times, as well as pulsed operation have been studied experimentally. The Cl 2 sensor reaches a sensitivity peak at 130 °C and the conductivity of the film changes from 10 -10 to 10 -6 mho cm -1 in 111 ppm pre-mixed Cl 2 /N 2 mixture. The response and recovery times are about 1 min at 130 °C. For the I 2 sensor, the peak response is at 20 °C and the conductivity changes from 10 -9 to 10 -8 mho cm -1 for 40 ppm I 2 in air. The response and recovery times are about 20 s.

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.

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.

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.