Metal deposition reflection spectroscopy

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

Until quite recently the very initial stages of metal deposition were difficult to characterize in detail by structure- and morphology-sensitive techniques. As a consequence and for practical purposes - multilayers were more useful for applications than monolayers - the main interest was focussed onto thick deposits. Optical and electron microscopy, ellipsometry and specular or diffuse reflectance spectroscopy were the classic tools, by which the emerging shape of the deposit was monitored [4-7],... 

The initial stages, notably the formation of a monolayer on a foreign substrate at underpotentials, were mainly studied by classical electrochemical techniques, such as cyclic voltammetry [8, 9], potential-step experiments or impedance spectroscopy [10], and by optical spectroscopies, e.g., by differential reflectance [11-13] or electroreflectance [14] spectroscopy, in an attempt to evaluate the optical and electronic properties of thin metal overlayers as function of their thickness. Competently written reviews on the classic approach to metal deposition, which laid the basis of our present understanding and which still is indispensable for a thorough investigation of plating processes, are found in the literature [15-17]. 

Growth experiments were carried out in two ultra high vacuum (UHV) cambers with sublimation sources of Si, Fe and Cr and quartz sensors of film thickness. Optical properties of the samples were studied in UHV chamber VARIAN (210 10Torr) equipped with differential reflectance spectroscopy (DRS) facilities. The samples surface was studied in the second UHV chamber (1 -10 9 Torr) equipped with LEED optics. Si(100) and Si(l 11) wafers were used as substrates for different series of the growth experiments. For the growth of silicide islands, metal films of 0.01-1.0 nm were deposited onto silicon surface. Silicon overgrowth with the deposition rate of 3-4 nm/min was carried out by molecular beam epitaxy (MBE) at 600-800 °C for different substrates. The samples were then analyzed in situ by LEED and ex situ by HRTEM and by... 

Besides cell designs that employ metals or other electrode materials in disc shape for external reflection spectroscopy, Robinson and McCreery have successfully employed cylindrical carbon fibers of 12 pm diameter [58, 59]. The carbon fiber was illuminated with a tunable dye laser. Scattered light was collected with fiber optics and guided to a photomultiplier detector. Because no thin layer arrangement and consequently poor electrochemical cell response were involved, fast experiments on a microsecond time scale were possible. Studies of polyaniline films deposited on platinum discs have been described [60]. 

Reflection spectroscopy, Raman spectroscopy, and ellipsometry complement the various electrochemical methods to study metal deposition. The optical methods can be used for a direct monitoring of the deposition process. The great advantage of optical spectroscopy... 

When reflectance spectroscopy was first used for the in situ study of electrode surfaces, the main emphasis was placed on monitoring the formation of adsorbates and thin films, e.g., the underpotential deposition of metals or the formation of oxides, because a rather strict correlation between coverage and adsorbate- or film-induced reflectance change was generally observed/ In the beginning of the seventies, the evaluation of adsorbate optical constants from measurements of with the help of stratified multilayer models... 

The deposition of metals onto semiconductor electrodes is sensitively monitored by differential reflectance spectroscopy because of the vast differences in the optical constants of metals and semiconductors. " Figure 58 illustrates the reflectance changes, R/R, for a ZnO electrode (which is an... 

Stamenkovic V, Arenz M, Ross PN, Markovic NM. 2004. Temperature-induced deposition method for anchoring metallic nanoparticles onto reflective substrates for in situ electrochemical infrared spectroscopy. J Phys Chem B 108 17915-17920. 

In spite of the overall success of this particular scheme, there are some issues which need to be clarified especially when it comes to details of the structure of the electrode/monolayer/electrode crossings on an atomistic level [4]. In a recent review on the vapor deposition of metal atoms on organic monolayers [65], the complexity of the process and the subtle effects of the surface structure and composition on the outcome are illustrated. A reflection-absorption infrared spectroscopy study [66] of a system, which is similar to the actual cross-junction, suggests undamaged organic mono-layers with Ti coatings, but further research is needed for the complete characterization of these complex structures. 

The FTIR spectrum of the PTFE film deposited by laser ablation was identical to that of the target [54], but that of the film produced by SR etching showed some visible differences (see Fig. 29). Obviously, the C-F2 deformation bands at 640 and 513 cm-1 appear much smaller in the bottom trace. To understand why these 640 and 513 cm-1 bands were so small in the SR case, we measured both normal and oblique transmission of FTIR with an incident angle of 0 and 80° [58]. Two FTIR spectrometers (PERKIN-ELMER and JASCO) were used to measure spectra in the range 400-3000 cm-1. For a cross-check, the film was also deposited on a metallic surface and infrared reflection absorption spectroscopy [62] was carried out to confirm our oblique transmission measurements. Typical changes in the FTIR transmission... 

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