UPD-OPD transitions

The Cu adsorbate structure was studied using STM and EXAFS (extended x-ray absorption fine structure) techniques, but it is not yet well understood. UPD-OPD transition is in the range —82 to —71 mV. Bulk fee Cu spacing is reached after deposition of about lOCu monolayers. Holzle et al. (72) have shown that UPD Cu deposition on Au(l 11) is a combined adsorption-nucleation and growth process. 

UPD-OPD transition is in the range between —82 and —71 mV. The bulk fee Cu spacing is reached after deposition of about 10 Cu monolayers. 

First theoretical interpretations of Me UPD by Rogers [3.7, 3.12], Nicholson [3.209, 3.210], and Schmidt [3.45] were based on an idealized adsorption model already developed by Herzfeld [3.211]. Later, Schmidt [3.54] used Guggenheim s interphase concept" [3.212, 3.213] to describe the thermodynamics of Me UPD processes. Schmidt, Lorenz, Staikov et al. [3.48, 3.57, 3.89-3.94, 3.100, 3.214, 3.215] and Schultze et al. [3.116-3.120, 3.216] used classical concepts to explain the kinetics of Me UPD and UPD-OPD transition processes including charge transfer, Meloiy bulk diffusion, and nucleation and growth phenomena. First and higher order phase transitions, which can participate in 2D Meads phase formation processes, were discussed controversially by various authors [3.36, 3.83, 3.84, 3.92-3.94, 3.98, 3.101, 3.110-3.114, 3.117-3.120, 3.217-3.225]. 

Information about the influence of 2D UPD phases on thermodynamics and kinetics of subsequent 3D Me nucleation and growth can be obtained by UPD-OPD transition experiments. In general, the experiment has two stages. In the initial stage i, a 2D Me UPD phase is formed and eventually equilibrated at a selected underpotential AE. The final stage f of the system is characterized by an external potentiostatic pulse to t]f into the OPD range. There are two possibilities for pulse excitation techniques potentiostatic or galvanostatic conditions. 

Potentiostatic pulse polarization is recommended for UPD-OPD transition experiments since both undersaturation and supersaturation are held constant in the initial and final states ... 

The resulting cathodic current density transients, Of), can be analyzed in terms of the kinetics of the following reaction steps occurring during such a UPD-OPD transition experiment ... 

Tlie steps i) and ii) usually produce falling Hf) transients, whereas nucleative steps iii) and iv) give non-monotonous falling or rising i(f) transients (cf. Sections 3.5 and 4.2). The analysis of i t) transients in the time domain is not trivial because of the superposition of different steps i) - iv) within a UPD-OPD transition experiment. A quantitative analysis of the various step kinetics is only possible if the corresponding relaxation time constants are significantly different. 

Potentiostatic UPD-OPD transition experiments can be carried out either varying A i at T/f = const or changing rg at A i = const In the first case, the influence of a 2D Meads phase and/or of a 2D Me-S alloy phase on the nucleation and growth kinetics of the 3D Me bulk phase can be analyzed. In the second case, the nucleation and/or growth kinetics as a function of the supersaturation at constant initial state in the UPD range can be studied. 

Galvanostatic pulse polarization is less recommended for UPD-OPD transition experiments since only the undersaturation of the initial state can be held constant whereas the supersaturation of the final state becomes undefined ... 

The number of nucleation sites, Zo, in eq. (4.56) can be influenced by the initial underpotential A i. Therefore, UPD-OPD transition experiments can give information on the influence of the bare substrate structure on the 3D Me nucleation and growth kinetics in the OPD range. 

On the other hand, a linear relationship between In and rh is derived from eqs. (4.56) and (4.59) for potentiostatic UPD-OPD transition experiments with AE = const. The number of Me atoms within the cluster of critical size is obtained from the slope of a In /( /f) vs. Ir/f 1 plot at AE = const ... 

UPD-OPD transition experiments were carried out under potentiostatic conditions in different model systems in order to study the nucleation and growth kinetics of 3D Me bulk phase formation and the epitaxy between Me and S. 

This system was chosen as a typical example of a weak Me-S interaction. Potentiostatic UPD-OPD transition experiments in the system HOPG(0001)/Ag, CIO4 showed that the 3D Ag phase formation follows a Volmer-Weber island growth mode [4.43, 4.60-4.63]. Decoration of steps and other surface imperfections by a 3D Ag phase was observed at relatively low r f, whereas 3D Me crystallites are also formed on atomically flat terraces at relatively high 7f. The initial deposition kinetics... 

UPD-OPD transition experiments applying long-time polarization in the UPD range would reflect the influence of the slow 2D Me-S surface alloy formation in this system on the mechanism of 3D Ag bulk deposition in the OPD range. However, such measurements have not yet been published. 

This indicates that the UPD-OPD transition obviously proceeds via the Stranski-Krastanov mechanism (cf. Fig. Ic) involving the formation and growth of 3D Pb crystallites on top of the 2D internally strained Pb UPD adlayers which act as precursors for the nucleation and growth process in the OPD range. The unstrained 2D hep surface structure of a 3D Pb(lll) crystal face is reached after deposition of about 10 Pb monolayers as shown in Fig. 4.18. The interatomic distance corresponds to [Pg.194]

This system is also characterized by a strong Me-S interaction and a significant positive Me-S lattice misfit (do.Ti > do.Ag)- At low AiS Ag(lOO) and (111) substrates are modified by compressed 2D hep Tlads overlayers, which are higher order commensurate and incommensurate, respectively (cf. Section 3.4). UPD-OPD transition experiments and morphological investigations in the system Ag(M0/Tl, ... 

Au(lOO) substrate showed a commensurate 2D Cuads overlayer at low A . UPD-OPD transition experiments yielded epitaxial growing 3D Cu islands. The fee Cu bulk spacing was reached after deposition of about 10 Cu monolayers. Different Cu spacing in between was explained by the formation of thermodynamically unstable bcc Cu within the first monolayers to adjust the crystallographic misfit [4.82]. 

UPD-OPD transition phenomena can be used for the deposition of heterostructured ultrathin metal films from multicomponent Mef systems using, for example, the polarization routine for i = 2 shown in Fig. 6.16. 

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