Michael C. Petty

Polyaniline thin films for gas sensing

Thin films of polyaniline have been deposited by spinning, evaporation and by the Langmuir-Blodgett technique. The films are shown to possess slightly different in-plane electrical conductivities, reflecting differences in their chemical structure and layer morphology. The conductivity is found to depend on the gas ambient. All types of polyaniline films are sensitive to H2S and NOx at concentrations down to 4 ppm. However, only spun and evaporated films are responsive to SO2.

The preparation and properties of stable metal-free phthalocyanine Langmuir-Blodgett films☆

Abstract In this paper we show for the first time that it is possible to prepare stable Langmuir-Blodgett films of metal-free phthalocyanine. They are of reproducible quality and adhere tenaciously to various substrates and to each other. Preparation conditions and isotherms are described for both a tetra-tert-butyl-substituted phthalocyanine and the unsubstituted molecule. Preliminary results describing the structural, electrical and optical properties of the films are also presented.

Langmuir-blodgett films

The Langmuir-Blodgett technique enables uniform ultra-thin organic films to be produced. These layers could find wide ranging uses in electronic devices and in artificial biological systems. This article describes the deposition of monomolecular and multilayer films and outlines a few of their possible applications.

New electroluminescent bipolar compounds for balanced charge-transport and tuneable colour in organic light emitting diodes: triphenylamine–oxadiazole–fluorene triad molecules

This work describes bipolar 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids which incorporate triphenylamine or carbazole units within the π-electron system, viz. compounds 7, 8, 14 and 16. A related bipolar bis(oxadiazolyl)pyridine system 20 is reported. The syntheses of these five new materials are discussed, along with their optoelectronic absorption and emission properties, and their solution electrochemical redox properties. Anodic electropolymerisation of 20 was observed. Calculations using DFT (density functional theory) establish that they all possess a significantly higher HOMO energy level (by 0.60–1.02 eV) than 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7) due to the presence of electron-rich amine moieties and increased conjugation lengths, thereby leading to more balanced charge-transport characteristics. Devices were fabricated by spin-coating techniques using the bipolar compounds as the emitters in the simple device architecture ITO:PEDOT-PSS:X:Ca/Al (X = 7, 8, 14, 16 or 20). The turn-on voltages were 2.9, 5.5, 3.6, 4.5 and 3.4 V for the devices incorporating 7, 8, 14, 16 and 20, respectively. The highest external quantum efficiency (EQE) was observed for compound 7: viz. EQE 0.36%; current efficiency 1.00 cd A−1; power efficiency 0.56 lm W−1 at 5.7 V. The EQE of the device fabricated from 8 was considerably lower than for devices using other materials due to low light emission. The EL emission peaked at λmax 430, 487, 487 and 521 nm for 8, 14 and 16, and 7, respectively. For the 20 device λmax = 521 nm and 564 nm. Thus the HOMO–LUMO gap has been modified, allowing the colour of the emitted light to vary from light blue through to green by the systematic chemical modification of the molecular subunits. The high chemical and thermal durability of these materials combined with the simplicity of the device structure and low turn-on voltages offers considerable potential for OLED applications.

Hybrid silicon-organic nanoparticle memory device

We demonstrate a nonvolatile electrically erasable programmable read-only memory device using gold nanoparticles as charge storage elements deposited at room temperature by chemical processing. The nanoparticles are deposited over a thermal silicon dioxide layer that insulates them from the device silicon channel. An organic insulator deposited by the Langmuir–Blodget technique at room temperature separates the aluminum gate electrode from the nanoparticles. The device exhibits significant threshold voltage shifts after application of low-voltage pulses (⩽±6 V) to the gate and has nonvolatile retention time characteristics.

Electronic devices incorporating stable phthalocyanine Langmuir-Blodgett films☆

Abstract This paper describes the critical role of the solvent in the preparation of monolayers of phthalocyanine. Pressure-area isotherms demonstrate that realistic monomolecular areas can be achieved for a number of compounds provided that the central hydrogen atoms in the ring system are replaced by appropriate metal ions. Particularly good results have been obtained with an asymmetrically substituted copper phthalocyanine. Transmission electron diffraction data for copper phthalocyanine tris(CH2NHC3H7-iso) show a preferred orientation over a sample region of 3 mm. The excellent stability of the Langmuir-Blodgett films has enabled us to incorporate them in a number of electronic devices. These include a gas-sensitive structure based on silicon, a bistable switch using GaAs and an electroluminescent metal-insulator-semiconductor ZnSeS diode.

Second-harmonic generation in mixed hemicyanine: fatty-acid Langmuir–Blodgett monolayers

The second-order nonlinear optical properties of a hemicyanine-dye-containing monolayer are measured by using second-harmonic generation. An unexpected large enhancement of the second-harmonic signal from the dye is observed on dilution by optically passive fatty acid, with the maximum enhancement occurring at around 50% dilution. Possible mechanisms are discussed.

Chemosensor devices: voltammetric molecular recognition at solid interfaces

This article reviews progress in modifying electrode surfaces with functional organic molecules which act as signalling units upon molecular recognition of a guest molecule or ion. This process triggers a change in the physicochemical properties of the host assembly which can be monitored by an electrochemical response. Topics covered include: molecular recognition in conjugated polymers; molecular recognition in monolayers formed by Langmuir-Blodgett and self assembly techniques; incorporation of recognition sites (‘molecular gates’) in otherwise passivating monolayers; ion channel sensors. Representative redox moieties include: polythiophene and polypyrrole derivatives, paraquat, ferrocene, tetrathiafulvalene, metalloporphyrins, C 60 , Ru(NH 3 ) 6 3+ and Fe(bpy) 2 (CN) 2 0 . Issues relevant to the development of practical, commercially-viable sensors are considered.

Effect of composition on the electrical conductance of milk

Abstract The contribution of the various components in cow’s milk to its electrical conductivity has been studied using the technique of admittance spectroscopy. Measurements at 100 kHz and 8 °C confirm previous observations that the milk conductance is predominantly determined by the salt fraction. Lactose showed very little effect on the conductivity, while the presence of fat resulted in a decrease in the milk conductance with increasing fat content. Sodium caseinate possessed a very low conductance; nevertheless, we suggest that the physical and chemical nature of the casein micelles can influence the overall milk conductivity.

Vapour recognition using organic films and artificial neural networks

Abstract Organic thin-film sensors bases on the thermal evaporation and dip-coating of polyaniline, and on the Langmuir-Blodgett deposition of a vanadium porphyrin, have been fabricated. The d.c. electrical resistance of the individual elements are found to exhibit different changes on exposure to simple vapours (water, proano, ethyl acetate and acetone). These data have been used successfully to train an artificial neural network, based on a back-propagation technique, to recognized two of the vapours.