Electrochemical ellipsometry

Amongst other spectroscopic teclmiques which have successfiilly been employed in situ in electrochemical investigations are ESR, which is used to investigate electrochemical processes involving paramagnetic molecules, Raman spectroscopy and ellipsometry. 

Electrochemical techniques have been developed into very powerful tools for research and technology. However, decades ago, researchers started to understand that even more insight could be obtained if electrochemical techniques were combined with additional spectroscopic tools. Among these it is sufficient to mention infrared spectroscopy, Raman spectroscopy, luminescence techniques, electroreflection or ellipsometry. 

Ellipsometry is used to study film growth on electrode surfaces. It is possible to study films at the partial monolayer level and all the way up to coverage of thicknesses of thousands of angstroms while doing electrochemical measnrements. To get nseful data it is important to determine A and j/ for the bare electrode snrface and the surface with a film. These data are processed to derive the film thickness, d, and the refractive index, h, which consists of a real (n) and imaginary part (k), h = n- ik. So ellipsometry gives information on the thickness and refractive index of snrface hlms. 

Electrochemical Quartz Crystal Microbalance Fntnre advances will require the coupling of EQCM with spectroscopic techniqnes that yield chemical information. EQCM has been conpled with ellipsometry (Gottesfeld et al., 1995). However, ellip-sometry does not yield chemical information. 

Gottesfeld, S., Y.-T. Kim, and A. Redondo, Recent applications of ellipsometry and spec-troellipsometry in electrochemical systems, in Physical Electrochemistry, I. Rnbenstein, Ed., Marcel Dekker, New York, 1995, p. 393. 

During the anodic polarization of platinum to potentials of about 3.0 V (RHE), one or several layers (but no more than three) of chemisorbed oxygen are formed, which sometimes are called the a-oxide of platinum. The limiting thickness of these layers is about 1.3 nm. They can be studied both by electrochemical methods and by ellipsometry. At more positive potentials phase-oxide surface layers, the p-oxides are formed. The quantitative composition and structure of these layers and the exact limits of potential for their formation depend on many factors composition of the electrolyte solution, time of polarization, surface history, and often remain unknown. 

The evidence for two reasonably well defined layers comes primarily from electrochemical studies, ellipsometry and Raman spectroscopy, The ellipsometric studies of Ord and DeSmet (1976) show good evidence of an abrupt... 

Spectroscopic ellipsometry is a non-destructive, interface sensitive, in situ technique for interface characterization. Time resolved ellipsometric spectroscopy was used to determine the mechanism of electrochemical deposition of photoresists on copper electrodes under potentiostatic, anodic conditions. Nucleation of photoresist deposition occurs randomly. During the early stages of nucleation the semi-spherical particles are separated by about 100 A. The deposits tend to grow like "pillars" up to 50 A. Further growth of the "pillars" lead to coalescence of the photopolymer deposits. 

Time resolved ellipsometry is an ideal technique to investigate the mechanism of nucleation and film growth of electrochemically deposited films. 

Time resolved ellipsometry is a surface sensitive technique that can be used to study the kinetics and mechanism of electrochemical deposition of photopolymer films. 

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

The monotonic increase of immobilized material vith the number of deposition cycles in the LbL technique is vhat allo vs control over film thickness on the nanometric scale. Eilm growth in LbL has been very well characterized by several complementary experimental techniques such as UV-visible spectroscopy [66, 67], quartz crystal microbalance (QCM) [68-70], X-ray [63] and neutron reflectometry [3], Fourier transform infrared spectroscopy (ETIR) [71], ellipsometry [68-70], cyclic voltammetry (CV) [67, 72], electrochemical impedance spectroscopy (EIS) [73], -potential [74] and so on. The complement of these techniques can be appreciated, for example, in the integrated charge in cyclic voltammetry experiments or the redox capacitance in EIS for redox PEMs The charge or redox capacitance is not necessarily that expected for the complete oxidation/reduction of all the redox-active groups that can be estimated by other techniques because of the experimental timescale and charge-transport limitations. 

A number of methods are available for the characterization and examination of SAMs as well as for the observation of the reactions with the immobilized biomolecules. Only some of these methods are mentioned briefly here. These include surface plasmon resonance (SPR) [46], quartz crystal microbalance (QCM) [47,48], ellipsometry [12,49], contact angle measurement [50], infrared spectroscopy (FT-IR) [51,52], Raman spectroscopy [53], scanning tunneling microscopy (STM) [54], atomic force microscopy (AFM) [55,56], sum frequency spectroscopy. X-ray photoelectron spectroscopy (XPS) [57, 58], surface acoustic wave and acoustic plate mode devices, confocal imaging and optical microscopy, low-angle X-ray reflectometry, electrochemical methods [59] and Raster electron microscopy [60]. 

With all its complications and uncertainties, impedance spectroscopy, as seen at the end of the twentieth century, is a growing technique in fundamental electrodic analysis [cf. the seminal contributions of (independently) D. D. and J. R. MacDonald]. Among its advantages is that the necessary equipment is less expensive than that of competing spectroscopic equipment and that it can provide information on any electrochemical situation (e.g., it is not limited by, say, the need for specular reflectance, as in ellipsometry). 

However, while the evidence for the existence of polarons was extremely convincing, that for bipolarons was rather more problematical in that it was largely effectively negative in nature the absence of an absorption peak in the optical spectrum, the absence of a signal in epr studies on the decline of the observed signal. In essence, bipolarons had not been actually observed. This fact was remedied by the work of Christensen and Hamnett (1991) who employed ellipsometry and FTIR to study the growth and electrochemical cycling of polypyrrole in situ in aqueous solution. 

Several factors have contributed to this goal in the recent past development of electrochemical techniques for the study of complex reactions at solid electrodes, use of physical methods such as ESCA, Auger, LEED, etc. for the study of surfaces in the ultrahigh vacuum (UHV) environment and in situ techniques under the same conditions as the electrode reaction. Ellipsometry, electroreflectance, Mossbauer, enhanced Raman, infrared, electron spin resonance (ESR) spectroscopies and measurement of surface resistance and local changes of pH at surfaces were incorporated to the study of electrode kinetics. 

Optical techniques - including ellipsometry, Raman spectroscopy, potential modulation reflectance, and photo-electrochemical technique - and an understanding of what really happens during the passivation process... 

Our approach to this problem involves a detailed mechanistic study of model systems, in order to identify the (electro)chemical parameters and the physicochemical processes of importance. This approach takes advantage of one of the major developments in electrochemical science over the last two decades, namely the simultaneous application of /ton-electrochemical techniques to study interfaces maintained under electrochemical control [3-5]. In general terms, spectroscopic methods have provided insight into the detailed structure at a variety of levels, from atomic to morphological, of surface-bound films. Other in situ methods, such as ellipsometry [6], neutron reflectivity [7] and the electrochemical quartz crystal microbalance (EQCM) [8-10], have provided insight into the overall penetration of mobile species (ions, solvent and other small molecules) into polymer films, along with spatial distributions of these mobile species and of the polymer itself. Of these techniques, the one upon which we rely directly here is the EQCM, whose operation and capability we now briefly review. 

Among the large variety of in situ experiments that have been described one can distinguish (1) those whose purpose is investigation of the electrochemical doping process itself cyclic voltammetry, quartz balance [17], mirage effect [18], and ellipsometry [19], and (2) those developed for studies of the properties of the CP UV-near-IR spectroscopy [20], IR [21], ESR [22], conductivity [23], impedence [24], and so on. 

Cohen SL, Brusic VA, Kaufman FB, Frankel GS, Motakef S, Rush B. X-ray photoelectron spectroscopy and ellipsometry studies of the electrochemically controlled adsorption of benzotriazole on copper surfaces. J Vac Sci Technology 1990 A8(3) 2417. 

Ellipsometry can measure films from subnanometer to a few micrometers, depending on material properties and wavelength of the light source. It has been widely used for thin film measurement in various applications, from biology to semiconductor, and from solid/solid to solid/liquid interfaces [24,25]. Ellipsometer with electrochemical cell for in situ thin film analysis is available from J.A. Woollam Co., Inc. and has been used in the research on electrochemical deposition [26]. However, in situ measurement of anodic films is more challenging because the films are usually metal complexes with unknown optical properties and difficult to verify with other ex situ techniques. 

Techniques for Characterization of Electrodes and electrochemical Processes, R. Varma, J.R. Selman, Eds., Wiley (1991). (Various spectroscopies, ellipsometry. Impedance spectroscopy). 

Shortly after Chidsey and co-workers initial papers. Miller et al. reported full characterization of Au-S(CH2) OH monolayers (System 5, = 6-12, 14, 16) by ellipsometry, XPS and electrochemical methods [44]. The nearly defect-free nature of the monolayers was attributed to hydrogen-bonding interactions between neighboring adsorbate chains at the film-electrolyte interface. The level of defects was probed by varying bridging halides, which should change electron-transfer processes at pinholes from outer to inner sphere. Electrochemical annealing was found to improve the EBE [44]. Later, they showed that defects in the SAMs are on the... 

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