The interaction of rubidium atoms with a model molecule for polyacetylene: a photoelectron spectroscopy investigation

The interaction of rubidium, a first group metal, with (alpha,omega-diphenyltetradecaheptaene) (DP7), a diphenylpolyene having seven double bonds in the polyene chain, has been studied by means of X-ray and UV photoelectron spectroscopy. DP7 can be considered as a model molecule for polyacetylene since the frontier pi-orbitals are mainly localized on the polyene part of the molecule. The experimental data are interpreted with the help of related experimental data from sodium-doping of DP7 and quantum chemical calculations. The interaction with rubidium results in n-type doping of the DP7, i.e., an electron-transfer process from the metal to the pi-conjugated system occurs. The doping process affects mainly the polyene part of the molecule, and new electronic states are created in the previously forbidden energy gap. At low doping levels, it appears that one new broad peak is developed in the band gap and, most important, a finite density of states at the Fermi energy can be detected. Upon further doping, there is a slight shift of the single peak, and a second peak is formed. The evolution of the new doping-induced states can be explained in terms of a transition from a polaron-like situation to a soliton-antisoliton pair formation

Polaron to bipolaron transition in a conjugated polymer: Rubidum-doped poly(para-phenylenevinylene)

Photelectron spectroscopy has been used to study the evolution of the electronic structure induced by n-type doping of two pi-conjugated systems: poly(p-phenylenevinylene), or PPV, and trans,trans-1,4-distyrylbenzene, a model molecule for PPV. Doping was carried out in UHV by physical vapor deposition of rubidium atoms. In both systems, two different doping regimes are observed. At low doping levels, for both materials, one new electronic state appears in the originally forbidden energy gap, and a finite density of states is observed at the Fermi level. Upon increased doping, the observed state moves to lower binding energy and a second state appears within the original energy gap. The evolution of the gap states, together with the appearance of a finite density of states at Fermi level at low doping levels, indicates a transition from polaron to bipolaron charge storage states in these conjugated systems. It should be stressed that these results constitute the first direct measurements of a polaron to bipolaron transition in a conjugated polymer using ultraviolet photoelectron spectroscopy

Atomic force microscopy investigations of morphologies in ultrathin polyaniline films

Preliminary results of two studies of the morphology of polyaniline, studied by means of atomic force microscopy (AFM), are reported. (1) Emeraldine base (EB)/camphorsulfonic acid (CSA) highly oriented fibers were prepared by a 'hot-drawing' technique. In addition to imaging the drawn fibers themselves, within the fibers, an internal structure with a clear orientation in the drawing direction is observed. (2) Submonolayer coverages of EB/CSA spin-coated from a chloroform solution onto highly oriented graphite (HOPG) were studied, and geometrically shaped crystalline islands are observed. The internal structure of these islands is attributed to the presence of compact coils. Morphological changes upon 'secondary doping' using m-cresol vapor were studied. Upon secondary doping, the crystallinity is lost

The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films

Films of poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS), prepared by coating the aqueous PEDOT–PSS dispersion and by coating a mixture of the PEDOT–PSS dispersion and different solvents, have been studied using four-point probe conductivity measurements, atomic force microscopy and photoelectron spectroscopy. The electrical conductivity of thin films of the second type (further on called PEDOT–PSS–solvents) was increased by a factor of about 600 as compared to films of the first type (further on called PEDOT–PSS–pristine). Morphological and physical changes occur in the polymer film due to the presence of the solvent mixture, the most striking being that the ratio of PEDOT-to-PSS in the surface region of the films is increased by a factor of 2–3. This increase of PEDOT at the surface indicates that the thickness of the insulating PSS ‘shell’ that surrounds the conducting PEDOT–PSS grains is reduced. The (partial) reduction of the excess PSS layer that surrounds the conducting PEDOT–PSS grains is proposed to lead to an increased and improved connectivity between such grains in the film and hence a higher conductivity. When PEDOT–PSS–solvents receives a post-coating heat treatment, the increased PEDOT-to-PSS ratio at the surface is maintained or even slightly improved, as is the increase in electrical conductivity, even though spectroscopy show that the solvent molecules are removed. This suggests that screening or doping by the solvents throughout the film are not likely to be the key mechanisms for the improved conductivity and supports our proposed mechanism of improved conductivity through improved connectivity between the conducting grains

Electronic structure of conjugated polymers and interfaces in polymer-based electronics

Electron emission, in resonance with soft X-ray absorption, appears to provide a measure of the dispersion of the highest occupied and the lowest, core-hole distorted, unoccupied pi-bands in poly(p-phenylenevinylene)