Physicochemical Characterization of Passive Films and Corrosion Layers by Differential Admittance and Photocurrent Spectroscopy

Two different electrochemical techniques, differential admittance and photocurrent spectroscopy, for the characterization of electronic and solid state properties of passive films and corrosion layers are described and critically evaluated. In order to get information on the electronic properties of passive film and corrosion layers as well as the necessary information to locate the characteristic energy levels of the passive film/electrolyte junction like: flat band potential (Ufb), conduction band edge (EC) or valence band edge (EV), a wide use of Mott-Schottky plots is usually reported in corrosion science and passivity studies. It has been shown, in several papers, that the use of simple M-S theory to get information on the electronic properties and energy levels location at the film/electrolyte interface can be seriously misleading and/or conflicting with the physical basis underlying the M-S theory. A critical appraisal of this approach to the study of very thin and thick anodic passive film grown on base-metals (Cr, Ni, Fe, SS etc..) or on valve metals (Ta, Nb, W etc..) is reported in this work, together with possible alternative approach to overcome some of the mentioned inconsistencies. At this aim the theory of amorphous semiconductor Schottky barrier, introduced several years ago in the study of passive film/electrolyte junction, is reviewed by taking into account some of the more recent results obtained by the present authors. Future developments of the theory appears necessary to get more exact quantitative information on the electronic properties of passive films, specially in the case of very thin film like those formed on base metals and their alloys. The second technique described in this chapter, devoted to the physico-chemical characterization of passive film and corrosion layers, is a more recent technique based on the analysis of the photo-electrochemical answer of passive film/electrolyte junction under illumination with photons having suitable energy. Such a technique usually referred to as Photocurrent Spectroscopy (PCS) has been developed on the basis of the large research effort carried out by several groups in the 1970’s and aimed to investigate the possible conversion of solar energy by means of electrochemical cells. In this work the fundamentals of semiconductor/electrolyte junctions under illumination will be highlighted both for crystalline and amorphous materials. The role of amorphous nature and film thickness on the photo-electrochemical answer of passive film/solution interface is reviewed as well the use of PCS for quantitative analysis of the film composition based on a semi-empirical correlation between optical band gap and difference of electronegativity of film constituents previously suggested by the present authors. In this frame the results of PCS studies on valve metal oxides and valve metal mixed oxides will be discussed in order to show the validity of the proposed method. The results of PCS studies aimed to get information on passive film composition and carried out by different authors on base metals (Fe, Cr, Ni) and their alloys, including stainless steel, will be also compared with compositional analysis carried out by well-established surface analysis techniques

Surface and Interface Effects in VLSI

ABSTRACT The effect of cation on the CdSe/polysulfide photoelectrochemical (PEC) solar cell has been investigated. The currentpotential response of the polysulfide electrolyte on a platinum electrode was measured as a function of the alkali metal cation in solution. There was a -33 mV shift in the redox potential and an increase in the current when the cation was changed from Na § to Cs § The performance of the CdSe/polysulfide PEC cell with both polycrystalline and single-crystal semiconductor electrodes increased in the open-circuit potential, the short-circuit current, the fill factor, and the energy conversion efficiency when Cs polysulfide was used as compared with K or Na polysulfide. Impedance measurements were made on the electrodes in polysulfide solutions and hydroxide solutions with the different cations. The results for both electrolytes showed a negative shift in the flatband potential, an increase in the apparent charge-carrier concentration, and an increase in the frequency dispersion of the measurements with the addition of Cs +. The impedance measurements also showed a dependence on the orientation of the CdSe crystal. The effects above are related to a combination of a change in the solution and the surface of the semiconductor. The stoichiometric distribution of the polysulfide species changes with the addition of Cs + and the surface of the semiconductor changes, facilitating charge transfer across the semiconductor/ electrolyte interface. Initial work on the cadmium chalcogenide/polysulfide PEC solar cell was published by Gerischer ~ and Ellis et aI.2 A great deal of work has been performed since those original studies on improvements to the semiconductor, the surface condition of the semiconductor electrode, and the electrolyte. CdS, CdSe, CdTe and mixed CdSe~T%x 3-~ have been investigated to increase the energy-conversion efficiency and stability of the PEC cells. Several researchers have investigated cadmium chalcogenide PEC solar cells using polycrystalline electrodes. Long-term stabilities have been demonstrated with conversion efficiencies up to 5% for polycrystalline CdSe. 7-1~ The semiconductor/liquid junction solar cell is particularly well suited to low cost polycrystalline electrodes. Changes in the make-up of the polysulfide electrolyte have a large effect on the performance of the solar cell; the effect of hydroxide ion concentration and the sulfideto-sulfur ratio have been investigated by some researchers.~l~ Both the hydroxide ion and the sulfur-to-sulfide ratio affect the equilibrium concentration of the electroactive species, the stability of the solution, and the light absorption by the solution. Changing the cation in the solution leads to changes in the performance of the cell. ~6-~9 The cation is not involved directly in the redox reactions at the semiconductor electrode but the performance of the cell increases significantly when the cation in solution is changed from Na § or K § to Cs § The improvement is in the stability of the semiconductor, the open-circuit photovoltage (Voc) and the photocurrent response. The flatband potential (V~) also shifts negative with the addition of Cs*; yet the only difference between the cations is the ionic radius and thus the charge density. The change in cation can affect the surface of the semiconductor by altering the degree of adsorption, or by incorporation into the semicon-* Electrochemical Society Active Member. a Present address: IBM Corporation, Hopewell Junction, New York 12533-6531. ductor itself; or by changing the activity of the species in solution, or the distribution of the polysulfide species. The CdSe/polysulfide system is examined here. First, the current-potential response of polysulfide on a platinum electrode was measured. Impedance measurements then were made to determine how the change in cation affects the band energies and charge-carrier concentration of the semiconductor. The measurements were made in a variety of systems to separate the different effects: CdSe singlecrystal electrodes in hydroxide electrolytes with the different cations, CdSe single-crystal electrodes in polysulfide with the different cations, and with CdSe polycrystalline electrodes in polysulfide. The impedance measurements indicate changes in the performance of both single-crystal and polycrystalline CdSe/polysulfide PEC cells with the different cations. Polysulfide Electrolyte Polysulfide solutions have been studied extensively in the pulp and paper industry. The equilibrium relationships of the solution species are presented in The electroactive species and rate-limiting step at the photoanode have been the subject of several papers. 14' 24-2~ I

Strictly positive-definite spike train kernels for point-process divergences

Exploratory tools that are sensitive to arbitrary statistical variations in spike train observations open up the possibility of novel neuroscientific discoveries. Developing such tools, however, is difficult due to the lack of Euclidean structure of the spike train space, and an experimenter usually prefers simpler tools that capture only limited statistical features of the spike train, such as mean spike count or mean firing rate. We explore strictly positive-definite kernels on the space of spike trains to offer both a structural representation of this space and a platform for developing statistical measures that explore features beyond count or rate. We apply these kernels to construct measures of divergence between two point processes and use them for hypothesis testing, that is, to observe if two sets of spike trains originate from the same underlying probability law. Although there exist positive-definite spike train kernels in the literature, we establish that these kernels are not strictly definite and thus do not induce measures of divergence. We discuss the properties of both of these existing nonstrict kernels and the novel strict kernels in terms of their computational complexity, choice of free parameters, and performance on both synthetic and real data through kernel principal component analysis and hypothesis testing