Eric W. Tsai

Electrochemistry of some β‐Substituted Polythiophenes Anodic Oxidation, Electrochromism, and Electrochemical Deactivation

The anodic electrochemistry of the soluble poly(3-ethylmercaptothiophene) (PEMT) and poly[3,4-bis(ethylmercapto)thiophene] (PBEMT), is compared with the behavior of polythiophene (PT), poly(3-methylthiophene) (PMT), and poly(3-hexylthiophene) (PHT). In all five cases, the first anodic oxidation was reversible and was accompanied by reversible electrochromic behavior. When the polymers were driven to more positive potentials, there was immediate loss of electroactivity and concomitant color change to black. An electrochemical cross-linking/chain extension technique is discussed for PEMT as a means of growing smooth polymer films on electrode substrates starting from soluble oligomers in solution

Improvement on Intrinsic Electrical Properties of Low‐k Hydrogen Silsesquioxane/Copper Interconnects Employing Deuterium Plasma Treatment

The interaction between copper interconnects and low-k hydrogen silsesquioxane (HSQ) film was investigated using a Cu/HSQ/Si metal insulation semiconductor capacitor and deuterium plasma post-treatment. Owing to serious diffusion of copper atoms in HSQ film, the degradations of dielectric properties are significant with the increase of thermal stress. The leakage current beh avior in high-field conduction was well explained by the Poole-Frenkel (P-F) mechanism. By applying deuterium plasma treatment to HSQ film, however, the leakage current was decreased and P-F conduction can be suppressed. In addition, the phenomena of serious Cu penetration were not observed by means of electrical characteristic measurements and secondary ion mass spectroscopy analysis, even in the absence of diffusion barrier layers. This indicates that copper diffusion in low- k HSQ film can be effectively blocked by deuterium plasma post-treatment. Therefore, further improvement in resistance-capacitance reduction can be obtained due to the minimized thickness requirement for conventional barriers such as inorganic Si 3N4 and metallic TaN layers.

Charge storage and transport in thermal ruthenium oxide thin films: In situ detection of local pH changes and asymmetries in charge-discharge cycles at the oxide/aqueous electrolyte interphase

Abstract The charge-discharge cycle in thermal RuOx electrodes was studied by a combination of voltammetry and a new thin-layer spectroelectrochemical technique developed in this laboratory. The RuOx was deposited pyrolytically on Ti and Au supports for these two techniques, respectively. The oxide anneal temperature and the oxide loading on the substrate were varied to investigate their influence on the local pH changes and the charge consumed in the anodic oxidation and subsequent reduction at the oxide/electrolyte interphase. The potential scan rate was further used as a variable parameter in the voltammetric experiments. The spectroelectrochemical experiments showed conclusively that the voltammetric waves superimposed on a pseudo-capacitive background, which is a signature of RuOx represented proton ejection and reinjection during the anodic and cathodic cycles, respectively. Concomitantly, the integrated current efficiency for proton “collection” in the spectroelectrochemical measurements was much less than unity, signifying the importance of the capacitive components. Finally, the implications of the voltammetric and spectroelectrochemical data for the optimization and use of RuOx in electrochemical capacitors are discussed.

Electrochemistry of thioxanthene, thioxanthone and related compounds in acetonitrile: Substituent effects and correlation of electrochemical behavior with molecular orbital calculations

Abstract This paper presents the synthesis, electrochemistry, and molecular orbital (MO) picture of a series of conformationally-restricted diaryl sulfur compounds. The primary electrooxidative and electroreductive pathways of these compounds are compared with model systems including dibenzothiophene, thiochroman-4-one, and benzothiophene. The oxidation of these compounds is invariably irreversible (with the exception of thianthrene) and involves rapid dismutation of the radical cation which is formed in the primary electron transfer step. In the presence of “electrophoric” groups such as CO (e.g., thioxanthone) and SO 2 (e.g., dibenzothiophene sulfone), characteristic reversible electrochemical reduction responses are observed, which involve the radical anion in each case. The combined use of cyclic voltammetry and chronoamperometry permitted computation of the number of electrons involved in the electrochemical reaction and the diffusion coefficient for each compound. A series of C2-substituted thioxanthones was examined to probe the electronic influence of the substituent on the electrooxidation and electroreduction sites (i.e., on the electron densities at the 10- and 9-positions) respectively. These substituent effects are presented in terms of correlations of oxidation (or reduction) potentials with the substituent (Hammett) constants, and the highest occupied molecular orbital (HOMO), or lowest unoccupied molecular orbital (LUMO) energies respectively. The orbital eigenvalues were computed using the RPDDO method. Finally, the influence of varying bridging atoms in the above structure is discussed in terms of the ease of oxidation and the disposition of the HOMO orbital.

Influence of temperature on the voltammetric response of thermal ruthenium oxide electrodes

Thermal ruthenium oxide (RuOx) electrodes were voltammetrically cycled in 0.1 M H2SO4 at varying solution temparatures from 22 to 65°C. A systematic increase in the voltammetric charge (q) with temperature was noted the magnitude of which was dependent on the oxide loading of the Ti substrate. This increase obeyed an Arrhenius temperature dependence and activation enthalpies of ∼4–5 kJ mol−1 were deduced from plots on ln qvsT−1. The magnitude of the activation enthalpy was insensitive to oxide loading in the range 0.7–4.0 mg cm−2 and the switching potential at the upper limit from 0.40 to 0.90V (vs sce). In stark contrast to the behavior in acidic electrolytes, cyclic voltammograms in 0.1 M K2SO4 revealed negligible temperature dependence at potentials below the O2 evolution reaction. These results are interpreted in terms of two time-constant model for charge storage/transport in RuOx involving the external and interior surfaces of the oxide micropore structure. The temperature dependence in tentatively assigned to a diffusion or hopping mechanism involving protons at the external surfaces of the oxide micropore structure.

Electrochemical modulation of luminescence from an interfacial probe during redox switching of polypyrrole

Steady-state measurements were used to demonstrate how luminescence from an interfacial probe such as pyrene or [Ru(bpy)3Cl2](bpy = 2,2′-bipyridine) may be modulated by redox switching of a polypyrrole electrode between the insulating and electronically conductive states.

Proton transport accompanies redox switching of polypyrrole: a spectroelectrochemical study

Spectroelectrochemical detection of local pH changes has shown that proton expulsion accompanies redox switching of polypyrrole perchlorate thin films from the insulating to the conductive state.

Electrochemistry of some N-substituted phospha-λ5 azenes in acetonitrile: Mechanistic aspects, electrophoric group behavior, and substituent effects

Abstract Cyclic voltammetry (CV) data are presented for the oxidation and reduction of a series of N-aryl-P,P,P-triphenylphospha-λ5-azenes. The electrochemical behavior of these compounds is compared with the corresponding N-sulphonyl- and N-acyl-compound series. In all three cases, the electronic influence of the substituent. R, in the aryl ring system is quantified in terms of Zuman plots of the CV oxidation or reduction potential vs. the Hammett substituent constant. It is shown that insertion of the SO2 and CO spacer groups between the aryl ring system and the N reaction site attenuates the electronic influence of R on the electrochemical oxidation response. The latter, therefore, is largely centered at the N reaction site. By way of contrast, the influence of R on the reduction response is minimal. These observations lead to the categorization of two types of electrochemical responses. One is intrinsic to the molecule as a whole, and the other is caused by the presence of an ‘electrophoric” group. We show herein how these N-substituted phospha-λ5-azenes represent good model systems for defining and illustrating the concept of an electrophoric group; i.e. a portion of a molecule giving rise to an electrochemical response that is characteristic (fingerprint) of that moiety itself rather than of the molecule as a whole. Finally, the oxidation mechanism involving dimerization of the parent compound in the N-aryl series is elucidated via the combined use of CV, constant potential coulometry, 31P NMR spectroscopy, and HPLC. Similarities in the oxidation behavior of this compound with anilines are pointed out.

Luminescent probes of ion transport in polypyrrole: new strategies for luminescence modulation and assay of ion content in conductive polymers

Polypyrrole membranes containing luminescent counterions and probes enable the electrochemical modulation of luminescence, detection of ion transport, and assay of doping levels during switching of the polymer between the electronically conductive and insulating redox states to be carried out.

Optical probes of ion transport in electrochemical processes based on in situ derivatization; application to polypyrrole and polythiophene thin films

Dopant ion fluxes during the electrochemical reduction of polypyrrole perchlorate and polythiophene tetrafluoroborate thin films were measured at parts per billion senstivity, and the polymer doping levels assayed via derivatization with 2,6-di-p-tolyl-4-phenylpyrylium reagent followed by extraction of the ion pair complex into chlorobenzene, and its detection by spectrofluorimetry.