N. Stein

In situ spectroscopic ellipsometric study of porous alumina film dissolution

Porous alumina films have been synthesised by anodisation of high purity aluminium in 0.4 M phosphoric acid at constant current density. The dissolution process of those coatings in 2 M phosphoric acid has been investigated by in situ spectroscopic ellipsometry (SE) combined with electrochemical measurements. A sharp drop of the open current potential is observed due to film removal. The dissolution time depends on total barrier layer thickness. Those results are confirmed by SE measurements. A three layer optical model has been used to interpret SE spectra. During the dissolution process, the porous top layer has a quasi-constant thickness, whereas porosity increases significantly. The dissolution proceeds within the pores. Only the thickness of pure alumina barrier layer (upper part) decreases during the dissolution process whereas the interface barrier layer (lower part) does not change.

Real time in situ ellipsometric and gravimetric monitoring for electrochemistry experiments

This work describes a new system using real time spectroscopic ellipsometer with simultaneous electrochemical and electrochemical quartz crystal microbalance (EQCM) measurements. This method is particularly adapted to characterize electrolyte/electrode interfaces during electrochemical and chemical processes in liquid medium. The ellipsometer, based on a rotating compensator Horiba Jobin-Yvon ellipsometer, has been adapted to acquire Psi-Delta spectra every 25 ms on a spectral range fixed from 400 to 800 nm. Measurements with short sampling times are only achievable with a fixed analyzer position (A=45 degrees ). Therefore the ellipsometer calibration is extremely important for high precision measurements and we propose a spectroscopic calibration (i.e., determination of the azimuth of elements according to the wavelength) on the whole spectral range. A homemade EQCM was developed to detect mass variations attached to the electrode. This additional instrument provides further information useful for ellipsometric data modeling of complex electrochemical systems. The EQCM measures frequency variations of piezoelectric quartz crystal oscillator working at 5 MHz. These frequency variations are linked to mass variations of electrode surface with a precision of 20 ng cm(-2) every 160 ms. Data acquisition has been developed in order to simultaneously record spectroscopic ellipsometry, EQCM, and electrochemical measurements by a single computer. Finally the electrodeposition of bismuth telluride film was monitored by this new in situ experimental setup and the density of electroplated layers was extracted from the optical thickness and EQCM mass.

In-situ ellipsometric study of copper passivation by copper heptanoate through electrochemical oxidation

Inhibition of copper in a desearated sodium heptanoate solution (0.08 M, pH 8.0) by electro-oxidation has been investigated. Rp measurements were carried out to compare inhibition efficiency using this original method with that based on other usual chemical oxidation methods. In-situ spectroscopic and kinetic ellipsometric measurements have been performed to study the passivation mechanism of copper. The inhibition of copper is attributed to the formation of a protective layer composed of a mixture of copper II heptanoate and copper hydroxide. Two growth rates are observed during the passivation process. A model of a duplex layer is proposed. The thickness of the passive film is evaluated at 14±1.5 nm.

Thermal conductivity of Bi2Te3 nanowires and nanotubes

The thermal conductivity and the structural stability of Bi2Te3 nanowire and nanotubes are investigated by molecular dynamics simulations with growth orientations [0 0 1] [0 1 0]. The thermal conductivity of nanowires is lower than the bulk thermal conductivity by a factor of ten, while for the nanotubes by a factor of twenty. The striking result is that while the nanowires in the direction [001] are dynamically unstable, nanotubes in this direction remain stable. The stability and the ultralow thermal conductivity of Bi2Te3 nanotubes suggest improvement for next thermoelectric generation devices.