D.M. Taylor

Interface states in polymer metal-insulator-semiconductor devices

The admittance of polymer metal-insulator-semiconductor (MIS) capacitors has been measured as a function of frequency and applied voltage. The results reveal the presence of hole trapping states at the interface of the polysilsesquioxane insulator and the poly(3-hexylthiophene) semiconductor. The states appear to be distributed in two bands: one close to the equilibrium Fermi level at the semiconductor surface, the other ∼0.5eV above. Annealing the devices under vacuum for several hours at 90°C increases the concentration of the shallower traps to ∼3×1012cm−2eV−1, while decreasing the concentration of deep traps to ∼1×1010cm−2eV−1. Annealing improves the bulk hole mobility to ∼1×10−4cm2V−1s−1 while reducing the field-effect mobility in MIS field-effect transistors (FETs) slightly to ∼7×10−3cm2V−1s−1. Although the concentration of interface states is sufficiently great to account for gate-bias-induced threshold voltage instability in MISFETs, their associated time constants are much too short to explain th...

Modelling the surface potential-area dependence of a stearic acid monolayer

Abstract Based on a 3-layer capacitor approach used previously to describe the surface potential of condensed phase monolayers we have developed a model which appears to explain the main features of the surface potential-area (ΔVA) isotherm of stearic acid. The model suggests that a large reduction in the local permittivity in the neighbourhood of the headgroup dipoles from 78.4 to 6.5 is the cause of the rapid increase in ΔV when the monolayer is compressed below a critical area per molecule ( A c ≌ 30 A 2 molecule ). We associate this with the growth of domains in the monolayer.

Surface plasmon resonance studies of chemisorbed biotin-streptavidin multilayers

Abstract Surface plasmon resonance spectroscopy has been used to confirm that a monolayer of biotin may be immobilized by chemisorption onto evaporated gold films and subsequently used to bind a monolayer of streptavidin. As expected, the strong affinity of the protein for the biotin ligand ensures that the binding of the protein layer is highly specific. No non-specific binding occurs to the biotinylated gold surface; the protein layer is resistant to washing in 1 M NaCl. It is suggested that multilayer structures may be assembled by chemisorption of successive protein monolayers using a bifunctional bisbiotin ligand for connecting individual layers.

The surface potential of monolayers formed on weak acidic electrolytes: Implications for lateral conduction

Abstract Surface potential measurements on stearic acid monolayers formed on dilute NaCl solutions and ultrapure water show that the double-layer potential, ψ 0 , obeys the simple Gouy-Chapman theory only if account is taken of the effect of subphase ion concentration on monolayer dissociation. It is concluded that the enhanced lateral conductance observed during compression of stearic acid monolayers cannot arise from the higher proton concentration in the diffuse double-layer but rather from a H-bonded network at the monolayer/water interface as described previously for phospholipid monolayers.

Molecular nanostructures and their electrical probing

Abstract A brief review is provided of a self-assembly technique based on the specific interaction between the protein streptavidin and its complementary ligand, biotin. This methodology is capable of being developed for fabricating the 2D molecular networks that will be essential to the realisation of a molecular electronic system. Such a system is taken as an example of a prototypical memory array in which data is stored as an electrostatic charge or molecular dipole and the information ‘read’ using the electric force mode of an atomic force microscope (AFM). Based on a simple, spherical model of the AFM tip, equations are derived for the electrostatic forces and force gradients experienced by the tip as it scans over (a) a point charge and (b) a point dipole. It is shown that the spatial resolution of the probe is governed largely by the tip-to-surface height with the tip radius being less important. It is shown that for operation in the non-contact mode in the thermal vibration limit, the probe should be capable of detecting a single electronic charge at a tip-to-surface distance of ∽15 nm and a dipole of moment 1 Debye at ∽1.5 nm. For a typical tip-to-surface distance (∽50 nm) which allows electrostatic effects to be decoupled from Van der Waals interactions, the sensitivity of the probe decreases to ∽10 electronic charges and ∽10 4 dipoles of moment 1 Debye. Finally, it is shown that application of an external potential to the tip while significantly improving sensitivity leads to a loss of spatial resolution.

On the preparation of thin metal oxides by Langmuir-Blodgett film deposition

Abstract An attempt at forming thin ZnO films by the thermal decomposition of Zn stearate Langmuir-Blodgett (LB) films is described. The stearate films were prepared by spreading stearic acid on a water subphase containing ZnCl 2 . To ensure maximum conversion of the acid to salt, a high pH is required, but owing to micellar formation the maximum practical pH for stable monolayer formation was found to be 7.2. The monolayer was transferred to a solid substrate by vertical dipping with dipping ratios close to unity. The uniformity of the deposited LB multilayers was investigated by scanning surface potential measurements, phase contrast microscopy and surface plasmon resonance spectroscopy. Conversion of the acid to the Zn salt was confirmed by electron beam microanalysis and Rutherford backscattering, the latter suggesting that a fraction of the Zn ions may be incorporated into the monolayer as lower valence complexes. Despite the initial uniformity of the films, upon heating, the multilayers broke up into a myriad of microscopic droplets, the size of which increased with increasing film thickness.

Characterization of monolayers and LB multilayers of dioctadecyldimethylammonium chloride

Abstract Pressure-area ( π - A ) isotherms were obtained from dioctadecyldimethylammonium chloride (DODAC) monolayers at the air-water interface for a range of subphase salt concentrations. Simultaneous surface potential measurements confirmed the important role of electrostatic repulsion between ionized headgroups in controlling the liquid-expanded to liquid-condensed phase transition. The extrapolated area per molecule measured in the condensed phase, 0.58 nm 2 , was in good agreement with previously reported X-ray diffraction measurements on hydrated crystals. The Langmuir-Blodgett (LB) deposition of uniform monolayers of DODAC onto aluminium and gold substrates was investigated by surface potential measurements, phase contrast microscopy and surface plasmon resonance microscopy. Atomic force microscopy confirmed the regular molecular order of the deposited monolayer, yielding a spacing of approximately 0.46 nm between rows of adjacent molecules.

Effects of space charge at the conjugated polymer/electrode interface

Films of the polymer poly(4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (PCDM) were electrodeposited onto indium tin oxide (ITO) coated glass substrates. The formation of a polymer was confirmed by the presence of an absorption band at ∼900 nm (∼1.4 eV) previously attributed to a π–π* transition. Diodes formed by evaporating aluminum counterelectrodes onto the films displayed nonlinear current–voltage characteristics that were independent of voltage polarity. Interestingly, at low applied voltages, the diodes exhibited two, stable, reversible conductance states differing by about an order of magnitude. The effect is similar to that reported recently in diodes formed from chromium-doped SrZrO3. ac admittance measurements carried out over a range of frequency coupled with the voltage dependence of the device capacitance at low frequency suggests that the two states arise from the presence of a field-dependent space charge at the ITO/PCDM interface.

Monolayer behaviour of monovalent and divalent salts of long-chain alkylammonium [Ni(dmit)2] and alkylammonium [Ni(mnt)2]

Abstract Monovalent and divalent long-chain alkylammonium salts of [Ni(dmit) 2 ] and [Ni(mnt) 2 ] are shown to form monolayers at the air-water interface albeit with difficulty. Reproducible pressure-area isotherms are only obtained after monolayers are left fully expanded at the air-water interface for long periods of time. Surface potential measurements and phase contrast microscopy have been used to show that the quality of LB films of these charge transfer salts are greatly improved by long spreading times on the water surface. Mixing the pure salt with a long-chain fatty acid is found to improve the spreading characteristics of the salt but the resulting mixed monolayer is likely to be composed of two separate phases.

The specific adsorption of streptavidin to a tetrabiotinylated porphyrin monolayer at the air-water interface

Abstract The specific interaction at the air-water interface between streptavidin and a monolayer of the tetrabiotinylated ligand 5,10,15,20-tetrakis {α-[4-(biotinylamidomethyl)pyridinium bromide]-p-tolyl porphyrin, stabilised by complexation with sodium octadecyl sulphate, is observed directly by surface pressure measurements and by fluorescence microscopy. Changes in the structure of the monolayer, especially the appearance of domains, after adding protein to the subphase confirm that the concomitant expansion of the pressure-area isotherm is caused by the specific adsorption of protein to biotin ligands in the monolayer.