E.G. Wilson

Intrinsic photoconduction and photoemission in polyethylene

Intrinsic photoemission and photoconduction of polyethylene in the photon energy range 7<E<14 eV have been experimentally investigated. Both phenomena are parabolic in E with the same threshold energy EG=8.8 eV. The main features of a model that accounts for the observations are: (a) EG is the intrinsic electron-hole band gap. (b) Strong absorption for 7.6<E<8.8 eV is due to exciton creation. (c) The bottom of the conduction band lies an energy A above the vacuum level, where 0<A<1.2 eV. (d) The photocurrents are due to rapid motion of carriers through a short distance approximately=10 nm along individual polyethylene chains. (e) The carriers subsequently travel through the polyethylene with a very low mobility.

Demonstration of an ultra-high mobility organic polymer

Abstract Experimental evidence is given for a high mobility (μ ∼ 20 m2 s−1 V−1) and a low field-saturated drift velocity (v d ∼ 2·2 × 103 m s−1) for carrier motion along a polymer chain. Photoconduction experiments in time domains from 10−9 to 102 seconds and fields from 102 V m−1 to 106 V m−1 are described. The polymer, the bis(p-toluene sulphonate) ester of 2,4-hexadiyne-1,6-diol, is a single crystal in which the conjugated semiconducting carbon chains are all parallel to each other. Carriers travel a distance approaching 1 mm along a chain before trapping at a defect. It is possible to cut the chains and detect the resulting reduction of range of carrier motion.

Investigation of the surface morphology of thermally evaporated thin gold films on mica, glass, silicon and calcium fluoride substrates by scanning tunneling microscopy

The surface morphologies of thin gold films thermally evaporated on glass, mica, CaF2 and Si substrates were investigated by scanning tunneling microscopy (STM) and compared to each other. The surface roughness of the gold films and average area of the grains observed was investigated as a function of the temperature and length of time of prebake of the substrates. The dependence of the gold surface roughness on substrate temperature is discussed. For substrates held at room temperature the ionic interaction between gold particles and substrate surface determines the size of the grains. Large flat areas of dimensions of the order of 200×200 nm2 are obtained for 80 nm thick films grown on glass heated at 300°C for 6 h and areas flat over a macroscopic distance greater than 500 nm are obtained on mica heated at 400°C for the same period of time. For CaF2 gold epitaxial growth starts to occur above 200°C. Epitaxial growth of fcc metals on alkaline halides is discussed.

A new theory of acoustic solitary-wave polaron motion

The formation and break-up of the acoustic solitary-wave polaron from a free electron and acoustic phonons in one dimension is described. In an electric field the polaron moves as a massive rigid classical particle. Energy dissipation by acoustic phonons is weak, the mobility in the continuum limit is infinite, and the polaron drift velocity is the velocity of sound. The theory is applied to polydiacetylene and explains the experimental ultra-high mobility and saturated drift velocity found in that material by Donovan and Wilson (1981).

Electronic excitations of a conjugated polymer crystal

A highly crystalline polymer is described in which all the polymer chains are parallel and the backbone of the chains is conjugated. It is postulated that the low-lying (<5 eV) electronic excitations are electrons and holes in one-dimensional bands confined to individual chains. The band structure is deduced. The crystal is a semiconductor, EG approximately=2 eV, for charge transfer along the chain direction and an insulator in perpendicular directions. Singular Van Hove optical transitions characteristic of one dimension are discussed and shown to lead to peaks in the reflectivity. The optical and Raman properties of the model are compared with experimental results.

The ultraviolet spectra of polyethylene and long-chain paraffins

Ultraviolet absorption spectra of thin films of polyethylene and of the paraffins C28H58 and C42H86 in the range 6-12 eV have been measured at 300 K and 4 K. This range covers the onset of intrinsic electronic excitations. The significance of the results is discussed briefly.

Photocarrier creation in one dimension

Abstract Experimental results, and theoretical discussion, are presented on photocarrier creation in large single crystals of a conjugated polymer: the bis(p-toluene sulphonate) ester of 2,4-hexadiyne-1,6-diol. The quantum efficiency for carrier creation as a function of photon energy is found absolutely. The probability φ of carrier pairs avoiding geminate recombination is characteristic of a one-dimensional system, and well described by a theory due originally to Onsager. φ is linear in field at low fields, going over to Poole–Frenkel–Schottky variation at high fields. φ is exponentially dependent on temperature, with an activation energy depending on field, and on the mechanism of carrier creation.

Observation of Charge Motion along Molecular Wires Laid Down as Langmuir-Blodgett Multilayers

Abstract Fast photocurrents have been observed in Langmuir-Blodgett multilayers of polymerised 10, 12 penta cosa diynoic acid (PDA 12–8). They are linear in electric field and vary sublinearly with light intensity.

Electron motion perpendicular to Langmuir-Blodgett multilayers of conjugated macrocyclic compounds: The organic quantum well

Abstract The principle of a three-dimensional molecular memory is outlined. Initial experiments aimed at understanding the transfer of electrons between the layers of a Langmuir-Blodgett multilayer are described.

Anomalous electric field dependence of carrier drift in one-dimensional systems. Example: organic PDA-TS

Some years ago Donovan and Wilson (1981) concluded that carrier motion on a single chain of the quasi-one-dimensional polymer crystal PDA-TS exhibits highly unusual features. From the electric field dependence of the peak photocurrent and carrier generation rate they concluded that carriers had an ultra-high mobility and a low-field saturated drift velocity. The authors report new measurements in the t- alpha time-dependent domain and at the same time propose a transport model to explain the data. They show that both the t- alpha decay law and the sublinear field scaling of the effective drift velocity can be associated with one and the same phenomenon: one-dimensional carrier drift in the presence of strongly scattering defects.