M.L. den Boer

Highly ordered thin films of polyheterocycles: A synchrotron radiation study of polypyrrole and polythiophene Langmuir-Blodgett films

Abstract Langmuir-Blodgett films have been made with 3-n-hexadecylpyrrole and 3-n-octadecylpyrrole monomers and copolymers with unsubstituted pyrrole made by chemical polymerization at the air-water interface on a subphase containing FeCl 3 . Langmuir-Blodgett films consisting of mixtures of stearic acid and alkylsubstituted polythiophenes have also been made as bilayer films. The orientation of single and multilayer films on platinum substrates have been studied by Near Edge X-ray Absorption Fine Structure Spectroscopy which also gives information about charge transfer interactions between the aromatic groups and the metallic substrates. The alkylsubstituted pyrroles form highly ordered two-dimensional structures. FeCl 3 initiated copolymerization with unsubstituted pyrrole leads to a more disordered system. In the case of polythiophene-stearic acid bilayers, the stearic acid layers are highly ordered. The poly(alkyl thiophene) layers sandwiched between stearic acid layers, on the other hand, exhibit random orientation of the thiophene moieties.

Iodine L-Edge X-Ray Absorption Fine Structure Studies of Polymer-Iodide Salt Complexes

Abstract We report measurements of extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) of the L edge of iodine in the polymer electrolytes PPO8NaI, PPO8LiI, and PEI10NaI. The EXAFS indicates a disordered environment for the iodine, although the PEI complex does exhibit several weak oscillations indicating a significantly less disordered environment of the anion in this complex. The XANES results suggest that the iodine is becoming more covalent in the sequence PPO8LiI-PEI10NaI-PPO8NaI, implying there are substantial interactions between the anions and cations.

Structure and Phase Transitions in PEO-KI Complexes

Abstract In the 1970s Wrightl1-2 and later Armand et aL3 suggested that poly(ethy1ene oxide) (PEO) complexed with alkali metal salts could be used as thin film polymeric solid electrolytes. Since this original work we have witnessed a rapid expansion of research and development of this class of conducting polymers.

Defect Physics of CuInSe2 for Photovoltaic Applications using Extended X-ray Absorption Fine Structure (EXAFS)

Native point defects in undoped samples of CuInSe2 (CIS) have been identified by a multiple-edge refinement of the extended x-ray absorption fine structure of the copper, indium and selenium absorption edges. Ab initio theoretical models for the pure compound and for various defect structures were constructed and carrier-type statistics were predicted by simultaneously fitting multiple absorption sites to these models. As expected, a model of our measurements based on pure compounds with no defects does not yield a good fit to the data. We find that a best fit requires a significant population of defects. Preliminary quantitative analysis suggests a 15 % vacancy in Cu, a 2-12% population of Cu-Se anti-sites and 15% In-Se anti-sites.

A Comparative Study of LiMn 2 0 4 From Various Sources

A process recently developed at Arthur D. Little, Inc. has enabled the direct and rapid production of LiMO x intercalation cathode materials having specific and controlled geometric and topographic attributes. Results of a comparative study of the electrochemical and spectroscopic characteristics of LiMn 2 0 4 from several commercial sources and LiMn 2 0 4 prepared using our process are reported. Improved electrochemical performance observed for the LiMn 2 0 4 cathode material produced using ADLs process are tentatively related to the small and uniform grain size and to the possible additional influence of electronic defects associated with surface states.

Unstable valence rare earths ion implanted into transition metals: valence variation studies

The technique of ion implantation is applied to study the problem of valence instabilities of Ce and Eu in transition‐metal hosts. L3 x‐ray absorption spectroscopy measurements are used to probe the rare‐earth valence state in these materials. Typical extreme valence variations are illustrated with Y, Ti, and Pd hosts. The important question of identifying extrinsic oxidation of the rare‐earth implant is addressed in detail. The signatures of rare earth oxidation in the x‐ray spectra of both bulk compound and implanted materials are discussed using specific examples.

The Role of Compound Electronic Structure in Ce-Valence Instabilities

The transition metals are the largest and most chemically diverse group of the periodic table. This has naturally lead transition metal compounds to a prominent position in the study of rare earth valence instabilities—a field where chemical flexibility and diversity are crucial to exploring the instability.1–4 Indeed, experience has shown that differing host electronic structure can alter the often subtle balance between magnetism, spin fluctuations, charge fluctuations and superconductivity in systems near to a valence instability.1–4 In this paper we explore the Ce valence instability in several very broad classes of transition metal compounds. We have several objectives in doing so. First, we wish to illustrate how rare earth valence state determination via L3 x-ray absorption spectroscopy can help draw together materials “tricks of the trade” into a more coherent picture of the underlying valence instability—electronic structure coupling. Second, these L3-valence measurements and their electronic structure tends can be used to efficiently identify new materials in the most interesting portion of the instability6. Finally, we wish to advance the notion that the 4f levels of an unstable valence atom can serve as a marker in the electronic structure of its host material. Moreover, that reading of this marker (via L3-valence determination for example) can in turn be thought of as a probe of the host electronic structure.

Correlation between electronic states of O, Cu, and Ba in several high‐Tc superconductors

X‐ray photoemission measurements of Y‐, Sm‐, Eu‐, and Yb‐based high‐temperature superconductors show a clear correlation between Cu 2p satellite features, which previous work has shown are associated with materials which exhibit superconductivity, and a high‐binding‐energy peak in the O 1s spectrum. In samples with varying oxygen content the intensity of the Cu 2p satellites changes, reflecting the Cu valence which varies between 1+ and 2+. The high‐binding‐energy O peak appears to track the Cu valence. This high‐binding‐energy peak may be indicative of the presence of O 2p holes, suggested in some models as necessary for superconductivity. The Ba 3d peak position also changes systematically with the Cu valence.