Contact transfer curves

Figure 3.25. The dependence of the quantum yield cp(ro) on the contact quantum yield

Information on charge carrier injection was obtained from analysing contact resistances in OFETs. In contrast to Section 17.5, we now use transfer curves... 

The transfer curve of an ideal device that would exactly follow these equations is drawn in Fig. 5, together with its first and second derivative. The first derivative is a step function, while the second derivative reduces to a sharp peak at Vg = Vx, thus allowing a precise and reliable estimation of the threshold voltage [22-24]. A major interest of the method is that it is not sensitive to both gate voltage dependence and contact resistance. 

Before considering the role of the electrode material in detail, there is one further factor which should be pointed out. The product of an electrode process may be dependent on the timescale of the contact between the electroactive species and the electrode surface, particularly when a chemical reaction is sandwiched between two electron transfers in the overall process. This was first realized when it was found that ir E curves and reaction products at a dropping mercury electrode were not always the same as those at a mercury pool electrode (Zuman, 1967a). For example, the reduction of p-diacetylbenzene at a mercury pool was found to be a four-electron process, giving rise to the dialcohol, while at a dropping mercury electrode the product was formed by a two-electron process where only one keto group was reduced (Kargin et al., 1966). These facts were interpreted in terms of the mechanism... 

OS 94][R 13][P 74]ForadmixtureofsampleswifhvaryingconcentrationsofCo(ll) and Cu(II), the respective changes in the Co(ll) chelate complex concentration as a function of contact time were optically derived [28]. Analysis was performed in the reaction/extraction area and also in the decomposition/removal area (Figure 4.102). As expected, more complex is formed in the reaction/extraction area with increasing contact time. Also, more complex results when increasing the Co(ll) concentration at constant Cu(ll) concentration. This proves that mass transfer is efficient (as high concentrations can also be handled) and that no interference from other analytes falsifies the measurement. As a result, calibration curves were derived. 

A Ru(0001) sample, with vacuum deposited Cu, has been characterized by cyclic voltammetry by transferring to an electrochemical cell (16). Figures 4e-4h shows the anodic stripping curves for four different coverages of Cu. A single stripping peak was observed at +110 mV for 0.6 ML Cu and shifted to +145 mV for 5.2 ML Cu. This peak represents the removal of the first monolayer of Cu or Cu in direct contact with the Ru surface. The curve for 5.2 ML Cu shows an additional peak at -20 mV for the stripping of multilayer Cu. 

Table II presents the vadues of v, the rate constant for the electron transfer reaction with the donor and acceptor in contact, calculated by deconvolution of the fluorescence decay curves for a number of excited porphyrin-cOkyl halide systems. It appears that the rate parauneter depends strongly on the calculated exothermicity for these reactions. Parauneter i/ contadns information about the Framck-Condon factor of the electron-tramsfer reaction, which is in itself dependent on the reaction exothermicity and reorgauiization energy (22.23). Whether the rate constauit for the electron-transfer reactions depends on the exothermicity in the manner predicted by theory, that is with a simple Gaussian dependence (22), cannot be ainswered at present because of the uncertainties in the energetics of the particular reactions studied here.

Figure 10.16 Potential energy curves for the transformation of dihydrogen-bonded complex 2t-2TFA to contact ion pair 3t-2TFA. The O-H distance is taken as a reaction coordinate of the transferring proton. Energies are given in kcal/mol. (Reproduced with permission from ref. 6.)...

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