Eugenia Matveeva

OBSERVATION OF NEW OSCILLATORY PHENOMENA DURING THE ELECTROCHEMICAL ANODIZATION OF SILICON

This paper reports the observation of large undamped voltage oscillations during the anodic polarization of silicon in electrol yte containing a combination of acids. One of them stimulates oxide growth and the other its chemical dissolution (in the present c ase, (0.01-0.1 M H3PO4) + (0.001- 0.01 M HF). This temporal patterning of the anodization process is shown to be due to the formation of a thin (50-90 nm) oxide layer at the sample surface and its subsequent lifting-off. The mechanism of oxide detachment i s thought to be an isotropic growth of micropores at the oxide/silicon interface triggered on by changes of electrochemical condi tions there.

Electrochemistry of the Indium-Tin Oxide Electrode in 1 M NaOH Electrolyte

Electrochemical behavior of the bare indium-tin oxide (ITO) electrode in 1 M NaOH electrolyte has been studied in a wide range of current density and applied charge in view of its possible application as a counterelectrode in rare earth optical windows. Nature and kinetics of both cathodic and anodic processes developed on ITO depend on electrolyte composition as well as on transformations occurring with the electrode. Contribution of oxygen vacancies and ions O 2 - is crucial for the electrochemical behavior of ITO. At the cathodic polarization, the electrode components are deeply reduced so that ITO is gradually and irreversibly converted to a metallic mirror with a noticeable decrease of oxygen content. At high anodic current density, the ITO electrode undergoes modifications and its conductivity decreases probably also due to the change of oxygen content in the oxides lattice. A phenomenological description of all the processes involved in ITO electrochemical performance and their reversibility is proposed and discussed. A stable operation of ITO in electrochemical devices can be achieved only in the case of usage of external RedOx dissolved in the electrolyte or deposited on the ITO electrode. Such active components could be selected from the different systems studied for flat panel displays.

Kinetics of the Electrochemical Loading of Hydrogen into Thin Yttrium Films Covered by Palladium

The behavior of a thin-film yttrium electrode (150-200 nm) capped by a palladium layer (20 nm) toward electrochemical hydrogen uptake was studied in I M NaOH electrolyte under galvanostatic conditions. The system is of interest for smart optical windows/ mirrors in devices with changeable transparency. The electrochemical behavior of the Y/Pd electrode depended on many factors, e.g., whether the loading is performed on a fresh or used electrode, current density, presence of reducible components in the electrolyte, time of spontaneous open-circuit potential relaxation and/or anodic discharge. Residual hydrogen kept by the system and its interaction with some electrolyte components is a reason for this behavior. A decisive role of the palladium layer in electrochemical kinetics was elucidated for the first time. We assume that the kinetics of the hydrogen uptake by underlying yttrium as well as the corresponding hydrogen release process is controlled by diffusion from/through hydrogenated palladium. The constant potential hydrogenation process realized at 0.045 V vs. the reversible hydrogen electrode (RHE) was proposed to be the α-β Pd hydride transition. Formation of the β-Y dihydride phase is completed on the way to the -0.095 V potential level (vs. RHE) and is irreversible. In contrast, the β-γ Y hydride transition realized at this potential level is reversible and could be repeated many times. The last potential drop to -0.345 V (vs. RHE) is a hydrogen evolution process and, simultaneously, a metal-semiconductor (and optical) transition in the γ-YH 3 phase. In a discharge process, a β-Y dihydride of different stoichiometry and α-Pd hydride are probably the only species that could be obtained. The kinetics depends extremely on the discharge conditions (open-circuit relaxation or anodic polarization), and the process shows two potential levels. Under anodic hydrogen release, surface Pd oxidation could also take place. We have proposed and discussed a scheme for hydrogenation/release in the Y/Pd electrode under different experimental conditions.

Equivalent circuit analysis of the electrical properties of conducting polymers: electrical relaxation mechanisms in polyaniline under dry and wet conditions

Abstract Alternating current impedance measurements were used to characterize the electrical properties of the emeraldine base form of polyaniline. This conjugated polymer presently attracts much attention due to the possibilities of practical applications in sensors. The impedance data were acquired in the range of frequencies from 0.1 to 105 Hz at constant temperature of 30°C. The single surface electrodes covered by the polymer suspension in different solvents (water, N-methyl-pyrrolidinone and m-cresol) were used in the work. The substance was gradually dried during the analysis, being subjected to the continuous flow of dry nitrogen. The results of measurements were interpreted using an electrical equivalent circuit approach. Theoretical curves to fit the experimental data were obtained using ZVIEW program. Corresponding equivalent circuits were found to include a number of capacitors, resistors and constant phase elements with their parameters sensitive to the experimental conditions. Electrical behavior of the samples at different stages of their drying from different types of suspensions was monitored employing this method. The results were interpreted in terms of the involvement of solvent molecules into the mechanism of the electrical conduction of the polymer.

IMPEDANCE STUDY OF AGING POROUS SILICON FILMS

Abstract-The electrical properties of luminescent porous silicon layers (PS) were studied using a method of nc impedance spectroscopy in a range of electric field frequencies 0.175-lo5 Hz (amplitude 1 V) and temperatures 20-15D”C. It was found that two processes are responsible for the impedance properties of PS: the relaxation process with characteristic time of about 3 x 10-4s and a low-frequency conductivity of Warburg type. An electric equivalent circuit was designed ensuring good fit of the experimental data acquired at differently treated PS samples. The components of the equivalent circuit were assigned to physical elements of PS (an amorphous phase at the top of PS and a layer of underlying bulk PS material). Different post-growth treatments (training by ac electric field and increasing temperature, pore filling by inert electrolyte, evacuation at increased temperatures) result in changing the properties of the PS samples. This is interpreted in terms of an existence of residuals of the electrolyte inside the pores and their influence onto the aging stability of PS. Key words: porous silicon, electrical impedance

Optical and Electrical Properties of Mg ∕ Ni Alloy Subjected to Electrochemical Hydrogenation

The work is aimed at studying properties of the Mg–Ni alloy thin film during electrochemical hydrogenation in an alkalineelectrolyte.Wehavesimultaneouslymeasuredthekineticsofthecathodicprocessesundergalvanostaticconditionsandchangesin optical transmission. We have performed in situ ac impedance measurements of gradually hydrogenated films to monitorchangesintheirelectricalproperties.Mg–Nialloysdemonstratehighopticalcontrastduringelectrochemicalhydrogenationduetoswitchingbetweenreflectiveandtransparentstates,excellentreversibilityofthehydrogenation–dehydrogenationreaction,andfairservicetime.Thevalueoftheacimpedancetechniqueforstudyingthecomplexelectricalprocessesaccompanyingtheincorpo-rationofhydrogenintoactivemetalsandalloysisdemonstrated.©2005TheElectrochemicalSociety. @DOI:10.1149/1.2083107 # Allrightsreserved.ManuscriptsubmittedMarch29,2005;revisedmanuscriptreceivedJuly21,2005.AvailableelectronicallyOctober17,2005.

Hydrogenation Behavior of Yttrium Thin-Film Electrode in 1 M NAOH Electrolyte after Removal of Palladium Cap

Electrochemical loading/release of hydrogen into a thin-film yttrium ~Y! electrode ~200 nm! capped with a Pd protective layer ~20 nm! is reported and compared with the behavior of the same electrode after removal of the protection. The system is of interest for the development of rare-earth based optical switchable devices. The stripping procedure was done on the previously cycled electrode when the uppermost protective Pd layer had been detached from the yttrium film. The Pd detachment as shown by scanning electron microscopy and chemical analysis of the different areas of the surface made it evident that a Pd remnant was still present on the electrode ~5-7 nm!. The electrochemical behavior of such a stripped Y electrode was stable but different from the former protected one and optical switching was observable as well. We discuss the role of the surface protection in optical devices based on hydrides, materials and morphology, and oxidation of the active metal ~yttrium in our case! after the protection failure. It is proposed to use the Pd-containing alloys developed for hydrogen permeation systems in the form of surface patches/gates for hydrogen uptake into the active material and oxidize the rest of the surface to prevent it from further alteration.

Oscillatory electrochemical reactions at corroding silicon surface

The paper analyses the nature of chaotic and well-ordered oscillations of the anodic potential and open circuit potential of silicon immersed in aqueous electrolytes. These oscillations are observed when experimental conditions are fine tuned in what corresponds to the current flowing through the system, composition of electrolyte, its viscosity, etc. It is assumed that the oscillations are due to the accumulation of mechanical stress in the thin (50-80 nm) oxide film formed at the surface of silicon as a result of electrochemical anodic reaction. The stress is released by local etching of the oxide and its lifting-on from the Si surface. The process repeats again and again yielding long-lasting oscillations of the anodic potential value (amplitude around 1-15 V, period 20-150 s) or of the open circuit potential (several hundreds milli-volts). Along with temporal ordering of the process (oscillations of potential) there occurs a spatial ordering in the system - the surface of corroding Si sample is covered with hexagonally ordered semi-spherical cells (diameter about 700 nm). The effect is well-fit by the general phenomenology of chaos-order transitions in che4mical systems (bifurcations), strange attractors are the intrinsic features of these oscillations) and its kinetics is very similar to that of the Belousov-Zabotinsky reaction. However, oscillatory processes on the corroding Si surface are caused by quite specific physical and chemical mechanisms, which are not well understood presently. We present the microscopic model for the oscillatory behavior which involves, generation of local mechanical stress at the Si/electrolyte interface, non-linear electrochemical etching of Si, localization of the electric field at the etched surface, etc.