Interface electroreflection

The reflectivity of bulk materials can be expressed through their complex dielectric functions e(w) (i.e., the dielectric constant as a function of frequency), the imaginary part of which signifies absorption. In the early days of electroreflectance spectroscopy the spectra were often interpreted in terms of the dielectric functions of the participating media. However, dielectric functions are macroscopic concepts, ill suited to the description of surfaces, interfaces, or thin layers. It is therefore preferable to interpret the data in terms of the electronic transitions involved wherever possible. 

The theoretical developments in the above areas were influenced, to a considerable extent, by concepts borrowed from semiconductor physics and the physics of surfaces. Other fields of photoelectrochemistry of semiconductors were affected to a greater degree by progress achieved in the study of metal electrodes. Here we mean photoemission of electrons from semiconductors into solutions and electroreflection at a semiconductor-electrolyte interface. 

Photoelectrochemistry (PEC) is emerging from the research laboratories with the promise of significant practical applications. One application of PEC systems is the conversion and storage of solar energy. Chapter 4 reviews the main principles of the theory of PEC processes at semiconductor electrodes and discusses the most important experimental results of interactions at an illuminated semiconductor-electrolyte interface. In addition to the fundamentals of electrochemistry and photoexcitation of semiconductors, the phenomena of photocorrosion and photoetching are discussed. Other PEC phenomena treated are photoelectron emission, electrogenerated luminescence, and electroreflection. Relationships among the various PEC effects are established. 

Electrolyte Electroreflectance (EER) is a sensitive optical technique in which an applied electric field at the surface of a semiconductor modulates the reflectivity, and the detected signals are analyzed using a lock-in amplifier. EER is a powerful method for studying the optical properties of semiconductors, and considerable experimental detail is available in the literature. ( H, J 2, H, 14 JL5) The EER spectrum is automatically normalized with respect to field-independent optical properties of surface films (for example, sulfides), electrolytes, and other experimental particulars. Significantly, the EER spectrum may contain features which are sensitive to both the AC and the DC applied electric fields, and can be used to monitor in situ the potential distribution at the liquid junction interface. (14, 15, 16, 17, 18)... 

Electroreflectance has been studied at the interface BuS2 (Tributsch, 1997). It passes through a maximum at about 0.3 V SCE. 

Abrantes L. M., Peat R., Peter L. M. and Hamnett A. (1987), Electroreflectance at the semiconductor-electrolyte interface—a comparison of theory and experiment for n-GaAs , Ber. Bunsenges. Phys. Chem. 91, 369-374. 

Gilman J. M. A., Batchelor R. A. and Hamnett A. (1993), Surface processes at electrolyte highly-doped semiconductor interfaces analysed by electroreflectance modelling , J. Chem. Soc. Earaday Trans. 89, 1717-1722. 

Salvador P., Tafalla D., Tributsch H. and Wetzel H. (1991), Reaction mechanisms at the n-FeSi/I interface—an electrolyte electroreflectance study , J. Electrochem. Soc. 138, 3361-3369. 

Tafalla D., Pujadas M. and Salvador P. (1989), Direct measurements of flat-band potential shifts under illumination of the semiconductor electrolyte interface by electrolyte electroreflectance , Surface Sci. 215, 190-200. 

It is important to note in this context that electroreflectance measurements on silver electrodes (in the absence of pyridine) have found structures attributed to surface states, which were unusually and strongly dependent on the electric potential. Normally, we would expect the energies of species in the interface to change not more than the change in the electric potential. When these findings are further verified, they may turn out to be very important for understanding SERS and interface problems in general. 

Otto suggested that, as in electroreflectance, where changes in the structure of the interface cause changes in the reflectance, modulations of the interface by the molecular vibrations can change it as well. Thus, the reflectivity R is given by... 

Both the intensity and the frequency of the -OH stretch in water are critically dependent upon the extent of hydrogen bonding to the immediate environment [40] and thus is a sensitive probe of the various types of water existing at the electrode-electrolyte interface. However, there are additional physical effects that may be detected by the phase-sensitive IR-ATR technique such as those derived from the variation of the electric field at the surface these electroreflectance effects are primarily due to changes in the surface electron density. 

The detailed interpretation of electroreflectance spectra is still in its infancy, but enough has already been learnt to indicate that the technique will form a most valuable adjunct to other methods that have recently been developed to study the semiconductor/electrolyte interface. The next few years should see this technique become a standard weapon in the armoury of the semiconductor electrochemist. 

The steady-state and dynamic properties of the Au(lll)/aqueous electrolyte interface were investigated experimentally with a variety of electrochemical and structure sensitive methods. Examples are impedance measurements [13], electroreflectance [14], LEED [14,15] SHG [16], in-situ surface X-ray scattering [17] and STM [18-20] experiments. In the following paragraph we will only focus on some essential results of our in-situ STM and SEIRAS studies [21,22]. 

The combination of classical electrochemical measurements with ex situ transfer experiments into UHV [242], and in situ structure-sensitive studies such as electroreflectance [25], Raman and infrared (IR)-spectroscopies [29, 243], and more recently STM and SXS [39] provided detailed knowledge on energetic, electronic and structural aspects of (ordered) anion adsorption and phase formation. These experimental studies have been complemented by various theoretical approaches (1) quantum model calculations to explore substrate-adsorbate interactions [244-246] (2) computer simulation techniques to analyze the ion and solvent distribution near the interface [247] (3) statistical models [67] and (4) MC simulations [38] to describe phase transitions in anionic adlayers. 

In this chapter, electrochemical properties of ET proteins at electrode interfaces studied by spectroelectrochem-ical techniques are described. In situ spectroelectrochemical techniques at well-defined electrode surfaces are sufficiently selective and sensitive to distinguish not only steady state structures and oxidation states of adsorbed species but also dynamics of reactants, products, and intermediates at electrode surfaces on a monolayer level. The spectroelectrochemical techniques used in studies of ET proteins include IR reflection-absorption, potential-modulated UV-vis reflectance (electroreflectance), surface-enhanced Raman scattering (SERS) and surface plasmon resonance, total internal reflection fluorescence, (TIRE) and absorbance linear dichroism spectroscopies. 

Electroreflectance, fluorescence and elastic light-scattering experiments used to study the mechanism by which surfactants spread at the MS interface are described in [25]. Monochromatic linearly polarized light was focused onto the electrode surface at a 45° angle. The specularly reflected light was collected in electroreflectance experiments. Fluorescence and elastically scattered light were collected at an angle of 90° with respect to the plane of incidence and 45° with respect to the electrode surface. 

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