Howard A. Mizes

Small particle adhesion: measurement and control

Abstract A number of technologies, including electrophotographic printing, require an understanding of particle–surface adhesion forces. At Xerox, we have developed and applied new techniques in order to understand the dependence of particle adhesion on physical properties, such as particle charge, shape and surface roughness. Atomic force microscopy and computer modeling were used to investigate the effects of surface roughness, external additives and applied electric fields on the adhesion of single particles. Centrifugal detachment was used to measure the adhesion force distribution of several hundred particles simultaneously and to determine its sensitivity to particle charge and size. Electric field detachment has enabled rapid characterizations of the adhesion of particle layers, providing insight into the roles of particle–surface contact area and nonuniform particle charging on the adhesion of ensembles of particles. The addition of digital photography to the detachment techniques has allowed in-situ visualization of particle detachment, enabling us to probe interparticle effects on adhesion.

Polarons and their stability in poly(phenylenevinylene)

Abstract To study properties of polarons in poly(phenylenevinylene) (PPV) we set up a tight-binding Hamiltonian along the lines of the SSH Hamiltonian for polyacetylene. The parameters were determined by fitting the bandgap, valence bandwidth and vinylene geometry to better than 1%. We used the resulting Hamiltonian to calculate the geometry of the polaron and the distance of the polaron level from the closest band edge as a function of chain length. To describe interchain coupling we took into account the actual separation of each atom from the closest atom on the neighboring chain which, unlike polyacetylene, is different for each atom in the PPV monomer. It was found that interchain coupling does not destabilize the polaron in PPV.

Atomic force microscopy adhesion measurements of surface-modified toners for xerographic applications

Abstract We have used an atomic force microscope to measure the adhesion of surface-modified toner particles to various surfaces of relevance to xerography. We will present adhesion data on several toner and toner surface additive surface modification techniques and use supporting machine performance data to discuss the advantages and disadvantages of the techniques.

Does photogeneration produce bipolarons in poly (para-phenylene vinylene)?

Abstract Extensive photoinduced absorption (PA) and photoconductivity (PC) measurements have been carried out on poly ( para -phenylene vinylene) (PPV) and its derivatives to determine which excitations are present in different time frames after exposure to light. Picosecond PA experiments have led to the conclusion that the large majority of photons above the absorption edge create long-lived (ns) polaron pairs, positive and negative polarons bound on adjacent chains by Coulomb attraction. We suggest that the steady-state PA that has been attributed to bipolarons in undoped samples is also due to polaron pairs. The main evidence for this is the coincidence of the observed steady-state visible and near-infrared PA with that of the corresponding ps PA due to polaron pairs, and the fact that the frequencies of the observed infrared-active vibrational modes (IRAV) require strong pinning. The long tail of the PC that has been attributed to the slow motion of bipolarons in the bulk is more reasonably explained by the effect of traps, for example, thermally and photochemically created carbonyls. The increase in PA and PC observed for photon frequencies higher than about 1/2 eV above the absorption edge, that has been variously attributed to generation of bipolarons and of free electrons in a continuum of energy levels, we assert is due to free polarons. At such photon energies some electrons and holes may be able to escape their Coulomb attraction and avoid being trapped into either polaron pair or exciton states. We conclude that there is no evidence that photogeneration results in a significant number of bipolarons in PPV and its derivatives, apart from samples with particular impurities or defects that foster bipolaron creation.

Polarons and Their Stability in Poly (P-Phenylene Vinylene)

Abstract We have parametrized a tight-binding description of the n electrons, with [sgrave] bond compressibility, on a poly (p-phenylene vinylene), PPV, chain by fitting the valence band width, the band gap and the chain geometry to better than 1%. With the resulting Hamiltonian we found that for long chains the polaron forms a localized state 0.18 eV from the band edge. Interchain coupling was described by using the results of ab initio electronic structure calculations and the actual separations of the atoms with the closest interchain contacts. It was found that interchain coupling does not destabilize the polaron in PPV.

Nonlinear digital filtering of scanning‐probe‐microscopy images by morphological pseudoconvolutions

A recently developed class of digital filters known as morphological pseudoconvolutions is applied to scanning tunneling microscopy (STM) images. These filters use a nonlinear branch of image processing known as morphology to improve the characteristics of both moving mean and moving median filters. They filter equally in both the x and y directions, so as not to introduce artifacts, and they have an adjustable parameter that allows the user to restore the observed image completely as the parameter tends to infinity. Very few assumptions are made concerning image and noise content; only the shape of typical data is taken into account. These filters are shown to outperform, both visually and in the mean‐square‐error sense, previously introduced Wiener filtering techniques. The filters are compared on typical STM‐type images, using both modeled and actual data. The technique is general, and has been shown to perform very well on all types of STM and atomic force microscopy images.

Effect of interchain coupling on polaron stability and other properties

Abstract Recent three-dimensional band structure calculations on trans-polyacetylene and poly(phenylene vinylene) have corroborated the suggestion that interchain coupling is too strong in these materials for polarons to exist. Nevertheless there is evidence that polarons do exist in these materials. We show that conjugation breaks, by decreasing the effective interchain coupling, allow the existence of polarons. The effect of such breaks on optical absorption and hopping transport of polarons is discussed.

Soliton model of metallic state in polyacetylene

Abstract Doping of polyacetylene by more than 4–7% causes an insulator-metal transition, as evidenced by the appearance of sizable Pauli susceptibility and large conductivity. The nature of the metallic state has been the subject of wide speculation. We show that there is strong evidence, mainly from optical absorption measurements, that the soliton lattice characteristics of doped polyacetylene below the transition persists in the metallic state. Another strong piece of evidence for the soliton lattice model is the existence of samples in which free carrier absorption is present at 300 K, but disappears at low temperatures. We show that other proposed models of the metallic state (polaron lattice, uniform chain and polson model) cannot explain the data.

Photogeneration of Polaron Pairs in Poly (Phenylene Vinylene)

Abstract Photoinduced absorption measurements of poly (phenylene vinylene), PPV, made picoseconds after above-gap photon excitation, have been interpreted as showing that a majority of the photons create polaron pairs bound by Coulomb attraction. We have calculated the peaks and approximate widths of the absorption bands due to a polaron pair on adjacent chains in PPV, and find good agreement with the observed absorption.

Random solitons in polyacetylene and the insulator-metal transition

Abstract A chain with a random arrangement of dimers, i.e. pairs of sites with energy ϵ b while all other sites have energy ϵ a , has been shown to have a set of delocalized states. The number of delocalized states is N 1 2 , where N is the number of sites on the chain. It has been proposed that the solitons or bipolarons produced by doping conducting polymers are equivalent to the dimers defined above and the resulting delocalized states are responsible for the insulator-metal transition found in these polymers. We point out that polyacetylene chains and solitons have quite different properties from the chains and dimers specified above. In the metallic state the solitons are reflectionless for all electron energies, not just a finite range of energies. Although a random soliton distribution has been shown to lead to an insulator-metal transition at a dopant concentration even smaller than that found experimentally, allowing the chain to relax in order to minimize the energy removes the randomness and reopens the gap. Using the observed infrared absorption due to solitons, we show that the soliton distribution cannot depart much from uniformity at room temperature and below for doping concentration less than about 6%.