Electroreflectance method

The ideal systems to test this conjecture are single-crystalline poly-diacetylenes (PDAs), notably perfect PDA chains embedded in a crystalline lattice [15]. Employing the electroreflection method, Weiser and Mbller [136] analyzed the... 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

The adsorption of Cu2+ ions on the Ti02 forms electroactive surface states within the band gap of the oxide, whose energy position was determined by the electrolyte electroreflection method [285, 286]. These copper-induced surface states were established to be located ca. 1.1 eV below the conduction band edge. Information concerning the subbands of the surface states in the Ti02 electrodes modified with Ag, Pd, Pt and Au one can find in [286, 310, 311], as well as in Chapter 6 of this book. 

In the electroreflectance method a sinusoidal wave alternating potential (about 10—100 Hz) is superimposed on a conventional linear potential generally used in specular reflection measurements. A lock-in amplifier is used to detect resulting changes in which reflected light intensities are proportional to dRIdE. The response of l(l// o)(df /3 ) is recorded as a function of the linear potential on an XY recorder. 

Commercial versions of PR are available. Other contactless methods of electro-modulation are Electron-Beam Electro-reflectance (EBER) and Contacdess Electroreflectance (CER). In EBER the pump beam of Figure 2 is replaced by a modulated low-energy electron beam (- 200 eV) chopped at about 1 kHz. However, the sample and electron gun must be placed in an ultrahigh vacuum chamber. Contactless electroreflectance uses a capacitor-like arrangement. 

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. 

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)... 

As is widely known, bulk species which have chromophores that absorb in the UV-Vis can be analyzed quantitatively and qualitatively by this spectroscopy. The study of electrodes or species adsorbed as thin layers by UV-Vis is more difficult, due to sensitivity problems and the availability of the appropriate chromophores [50], Another use of this type of analysis is electroreflectance [51,52], Adsorption of species on reflective electrode surfaces changes their reflectivity. Thus, this method can indicate electroadsorption processes very sensitively, in situ, although it does not provide specific information on the structure and composition of surface layers. 

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. 

In-situ spectroelectrochemical techniques are covered by chapters on infrared, Raman EPR, ellipsometry, electroreflectance, and photocurrent spectroscopy. Ex-situ UHV experiments are treated in a separate chapter. New electrochemical directions are described in chapters on hydrodynamic methods, channel electrodes, and microelectrodes. A final chapter covers computing strategies for the on-line accumulation and processing of elec-trochemial data. 

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]. 

This method is sometimes called reflectance difference spectroscopy (RDS) and, because of considerable overlap, this method is sometimes also considered to be a variation of electroreflectance spectroscopy (see p. 50 for further details). 

The electrolyte electroreflectance (EER) method is successfully used [36, 37] to determine the flat band potential (t/fb). In this method, the optical reflectance at the semiconductor electrode surface is measured under modulation of the electrode potential by superimposition of a small AC voltage. The modulation of the electrode potential causes modulation of the density of majority carriers near the semiconductor sxrrface, which in turn, causes modulation in light reflectance. 

Several techniques can be used to determine the flatband potential of a semiconductor. The most straightforward method is to measure the photocurrent onset potential, ( onset- At potentials positive of (/>fb a depletion layer forms that enables the separation of photogenerated electrons and holes, so one would expect a photocurrent. However, the actual potential that needs to be applied before a photocurrent is observed is often several tenths of a volt more positive than ( fb- This can be due to recombination in the space charge layer [45], hole trapping at surface defects [46], or hole accumulation at the surface due to poor charge transfer kinetics [43]. A more reliable method for determining ( fb is electrolyte electroreflectance (EER), with which changes in the surface free electron concentration can be accurately detected [47]. The most often used method, however, is Mott- chottky analysis. Here, the 1/ Csc is plotted as a function of the applied potential and the value of the flatband... 

For SAMs with attached redox molecules, k (units of s ) can be measured by cychc voltammetry, chronoamperome-try (CA), alternating current impedance spectroscopy (ACIS), alternating current voltammetry (AGV), AG electroreflectance spectroscopy, and an indirect laser-induced temperature (ILIT) jump method. 

Amongst the optical techniques there are also the more traditional methods such as the ellipsometry, electroreflectance and particularly, surface plasmons, where experimental and theoretical advances have made it possible to offer a picture of the surface electronic states of the metal in some selected cases, such as the silver (111) phase. We should mention here the measurement of image potential induced surface states by electroreflectance spectroscopy. In this case, besides the normal surface... 

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