D. Martin Taylor

Proton transport at the monolayer-water interface

It is shown that when monolayers of stearic acid, palmitic acid, DPPC, or DPPS are compressed above some critical area Ac a lateral conduction mechanism is initiated at the monolayer/water interface. The interfacial conductance increases on further increasing the molecular packing density in the monolayer. All compounds also show major changes in surface potential at Ac the potential becoming more positive in all cases. It is argued that this is a consequence of structural reorganisation at the headgroup/water interface causing a significant reduction in the local permittivity. The critical area, Ac, is approximately double the molecular areas estimated from the pressure-area isotherm, and experiments with stearic acid monolayers show that Ac decreases significantly when the chaotropic ion SCN-, which is known to disrupt the molecular structure of water, is added to the subphase. It is likely, therefore, that the structural changes occurring at Ac involve the formation of a hydrogen bonded network between monolayer headgroups and adjacent water molecules at the monolayer/water interface. It is suggested that the conduction mechanism initiated at Ac arises from proton hopping along this hydrogen-bond network.

Organic Digital Logic and Analog Circuits Fabricated in a Roll-to-Roll Compatible Vacuum-Evaporation Process

We report the fabrication of a range of organic circuits produced by a high-yielding, vacuum-based process compatible with roll-to-roll production. The circuits include inverters, NAND and NOR logic gates, a simple memory element (set-reset latch), and a modified Wilson current mirror circuit. The measured circuit responses are presented together with simulated responses based on a previously reported transistor model of organic transistors produced using our fabrication process. Circuit simulations replicated all the key features of the experimentally observed circuit performance. The logic gates were capable of operating at frequencies in excess of 1 kHz while the current mirror circuit produced currents up to 18 μA.

Surface plasmon resonance microscopy: Reconstructing a three‐dimensional image

A method for reconstructing three‐dimensional images of thin films from surface plasmon resonance (SPR) microscopy images is described. As an example, monolayers of a lipid were deposited onto a gold substrate using Langmuir–Blodgett technology and an SPR microscope image obtained. Using data from reflectance‐angle (R‐θ) curves obtained in supplementary measurements, the contrast in the microscope image was used to provide thickness information. Converted to a z modulation this was used to produce a three‐dimensional image of the monolayer.

Organic thin-film transistors with electron-beam cured and flash vacuum deposited polymeric gate dielectric

The electrical characteristics of pentacene organic field effect transistors (OFETs) based on cross-linked acrylic insulator as the gate dielectric are reported. Vacuum deposited thin films of cross-linked tripropyleneglycol diacrylate could be obtained by ultrahigh flash evaporation rate and subsequent irradiation using an electron-beam source. The characteristics of common gate OFETs, on highly conductive Si substrate, were tuned through the ease of control of the acrylic dielectric thickness achieving, without surface modification of the dielectric layer, a field effect mobility value of 0.09 cm2 V−1 s−1, a threshold voltage of 10 V, and an on/off current ratio of 1.3 × 103. This work could provide an alternative route to low cost and large area organic electronics manufacturing.

Behavior of avidin and avidin/bisbiotin polymers at the air-water interface

Abstract The surface behavior of avidin and two polymers, formed by affinity polymerization from two different stoichiometric mixtures of avidin and the bifunctional ligand bisbiotin, has been investigated at the air/water interface. Strong hysteresis in the pressure-area isotherms is attributed to mechanical distortion of the molecules. When subjected to a step change in pressure, the monolayer area relaxes to a new equilibrium value following first-order dynamics, the time constant being of the order of minutes for a pressure change of 5 mN m−1. Equilibrium isotherms for the fully relaxed monolayers are linear over most of the pressure range, with both compression and expansion isotherms exactly coincident. Monolayers of the avidin/bisbiotin polymer were deposited onto electron microscope grids by the LB technique and imaged in a TEM. The area per repeat unit was estimated to be ca. 30 nm2 in good agreement with previous estimates, but much smaller than the area per molecule obtained by extrapolating the pressure-area isotherm to zero pressure. Finally it is shown that spreading solvents containing a high percentage of chloroform cause avidin to denature at the air/water interface, this manifesting itself as a large molecular area at low surface pressure. However, it appears that denaturation may be reversed almost completely when the monolayer is compressed. The investigation suggests that it may be feasible to fabricate molecular electronic networks based on affinity polymerization.

Influence of lateral currents on capacitance spectra of organic metal-insulator-semiconductor structures with a ferroelectric insulator

The capacitance spectra of a metal-insulator-semiconductor (MIS) structure are discussed in terms of parasitic effects, caused by lateral currents along the insulator/ semiconductor interface. The organic materials Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and poly(3-hexylthiophene) (P3HT) were used for insulator and semiconductor, respectively. Capacitance-frequency (C-f) measurements are compared to numerically obtained spectra based on an equivalent circuit. Furthermore, capacitance spectra of a semiconductor-metal-insulator-metal (SMIM) structure were measured in order to identify the origin of the low frequency relaxation, found in the MIS C-f measurements. The studies suggested an increase of the capacitance by 10% at 0.01 Hz due to lateral currents. It was concluded that for probing frequencies > 1 Hz, parasitic effects on the capacitance spectra could be neglected. When the device was biased into accumulation prior to the measurement, the ferroelectric nature of the insulator led to an increase in the lateral spreading of charges.