Electroreflectance measurements

Electrolyte contacts have been used to characterize as-deposited and annealed CdS/CdTe solar cell structures by photocurrent spectroscopy and electrolyte elec-troabsorbance/electroreflectance measurements (EEA/EER) [267-269]. 

The XPS results obtained by Kolb and Hansen are reproduced in Fig. 6 and they clearly demonstrate not only that cations as well as anions stay on the surface but also that the amount of ions exhibits the expected potential dependence even in the case of specific adsorption. The preservation of the double layer charge after emersion was also shown by other techniques like charge monitoring [28] and electroreflectance measurements [29],... 

In order to explain the changing optical properties of AIROFs several models were proposed. The UPS investigations of the valence band of the emersed film support band theory models by Gottesfeld [94] and by Mozota and Conway [79, 88]. The assumption of nonstoichiometry and electron hopping in the model proposed by Burke et al. [87] is not necessary. Recent electroreflectance measurements on anodic iridium oxide films performed by Gutierrez et al. [95] showed a shift of optical absorption bands to lower photon energies with increasing anodic electrode potentials, which is probably due to a shift of the Fermi level with respect to the t2g band [67]. 

Photoluminescence excitation spectroscopy (PLE) is generally used to identify the excited-state structure in quantum wells. For GalnN/GaN quantum wells, Im et al [14] used PLE to study single wells of various widths. Similarly to the results from absorption, electro-absorption, and electroreflectance measurements, a large Stokes shift of the onset of the PLE spectrum with respect to the dominating photoluminescence peak was observed at low temperature [14]. 

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. 

The spectrum of an ER signal, i.e. the plot of (AR/R)er as a function of k, is called the electroreflectance (ER) spectrum. Electroreflectance measurement can also be carried out during linear potential scan or potential step of Ejc- Under linear potential scan, one obtains the voltammogram of the ER signal, the so-called ER voltammogram. 

Surface CTC of chlorpromazine (CPZ) has been shown to be involved in hypothermia and sedation of mice in vitro surface adducts between chlorpromazine and collagen have been demonstrated, as well as with lignocain and with phenytoin. The existence of such CPZ surface adducts has been directly confirmed by electroreflectance measurements. 

The reflectance of an electrode is modulated when an alternating potential is applied to an electrode. This technique has been applied to the study of the anodic layer on Pt by Conway and Gottesfeld. These ac electromodulation reflectance measurements yield results analogous to ac admittance measurements whereas the change of reflectance and ellipsometric parameters with and without the film present are analogous to the charge vs. potential curves. The ac electroreflectance measurements confirm the reversible adsorption at low coverage ( h < 0.9 V) and the irreversible character of film formation at Eu > 1.0 V. 

Irrespective of some still unresolved details of the primary process that leads to generation of electron-hole pairs one can determine the effective absorption coefficient, called a], that determines the photoelectric geminate pair yield in the low field limit, i.e. at fields where (j)o is constant. The result is shown in fig.7 for DCH. has a singularity close to the energy where electroreflectance measurements of Sebastian and Weiser (24) locate the onset of the transition between delocalized valence and conduction band states of the chain. The feature near 3.5 eV is due to charge transfer from the carbazole substituent to the polymer chain (49). 

The most successful technique of modulation spectroscopy has been the electroreflectance measurements on PEA-OCH and -TS single crystals (9, 33, 34). In the thermo- and plexoreflectance measurements the maximum value of was close to the location of pecUc... 

Frequently, electrochemical information can be interpreted better in the presence of additional nonelectrochemical information. Typically, however, there is one significant restriction electrochemical and spectroscopic techniques often do not detect exactly the same mechanisms. With spectroscopic measurements (e.g., infrared spectroscopy), products that are formed by electrochemical processes may be detected. In other cases (luminescence techniques) mechanisms may be found by which charge carriers are trapped and recombine. Other techniques (electroreflection studies) allow the nature of electronic transitions to be determined and provide information on the presence or absence of an electric field in the surface of an electrode. With no traditional technique, however, is it... 

A relatively new arrangement for the study of the interfacial region is achieved by so-called emersed electrodes. This experimental technique developed by Hansen et al. consists of fully or partially removing the electrode from the solution at a constant electrical potential. This ex situ experiment (Fig. 9), usually called an emersion process, makes possible an analysis of an electrode in an ambient atmosphere or an ultrahigh vacuum (UHV). Research using modem surface analysis such as electron spectroscopy for chemical analysis (ESCA), electroreflectance, as well as surface resistance, electrical current, and in particular Volta potential measurements, have shown that the essential features (e.g., the charge on... 

The above considerations are illustrated by Fig. 38, which shows the dependences of electroreflection of a silicon electrode on its potential. In fact, the signal changes the sign at the flat band potential (the latter was measured independently by the differential capacity technique). 

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. 

We will illustrate the difficulties and the opportunities which are associated with two complementary measuring techniques Relaxation Spectrum Analysis and Electrolyte Electroreflectance. Both techniques provide information on the potential distribution at the junction of a "real" semiconductor. Due to the individual characteristics of each system, care must be taken before directly applying the results which were obtained on our samples to other, similarly prepared crystals. 

Electroreflectance spectra were measured for n-CdSe in the liquid junction configuration, and variations of the lineshape as a function of potential were observed. As the potential was reduced below the flatband potential, the electroreflectance signals changed sign. The potential at which this change occurs correlates well with the turn-on potential for light-induced photocurrent and with the intercept of the Mott-Schottky plot. 

In this section, we consider some techniques, other than d.c. photocurrent measurements, that use illumination as a probe. Most directly related to the previous section are techniques employing sub-bandgap radiation and we shall consider these first. We shall then treat those techniques involving the emission of radiation and then photovoltage and a.c. photocurrent measurements. Finally, we shall treat electroreflectance and thermoreflectance measurements. 

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. 

For instance, at the present, by optical measurements (by in situ electroreflectance) it was shown that the symmetry properties of the crystal faces were not perturbed by contact with aqueous solutions. For copper, all faces studied present their azimuthal anisotropy with respect to the plane of polarization of the incident light, except for (100), (111), (211), and some neighboring faces of (111) for which no azimuthal anisotropy is observed. " For silver, the twofold symmetry of the (110) face was observed the electroreflectance spectra at normal incidence differ markedly when the electric field vector of light is parallel or perpendicular to the surface atomic rails. This is not the case for the (111) and (100) faces which have higher symmetry. For gold, no azimuthal anisotropy was observed for the (111) and (100) faces with respect to the plane of polarization of the incident light, while the (110) and (311) faces present an azimuthal anisotropy. It does not necessarily mean that the outermost layer of surface atoms is 1 x 1, because a surface reconstruction such as 1 x2 for Au (110) would have the same symmetry order as 1x1. These are already studies of the metal surface in the presence of the electrochemical dl. 

This approach has been used by Tomkiewicz et al. [17] to rationalise the electroreflectance spectrum of CdIn2Se4 unfortunately, the crystal studied in this report disintegrated before measurement of the donor intensity could be carried out to verify that the experimental conditions were such that the low-field theories could reasonably be expected to hold. Nevertheless, the intensity of the electroreflectance peak showed a marked dependence on potential, decreasing by a factor of ten over a 1V range as shown in Fig. 10(a). 

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