UPD film

These XPS results cause great uncertainty as to the chemical state of emersed Pb(UPD) films on Ag and validate questions raised about partial desorption of UPD films after potential control is lost (3,9). Nevertheless, XPS chemical shifts of the Pb(4f) peaks show a dependence on the emersion potential i.e. the Pb coverage before emersion. Some emersed films also displayed at least two different chemical states of the UPD Pb (Figures 4b an 5b). 

The structure of lead UPD films on silver (hkl) faces is a consequence of the significant lattice misfit. The atomic radius of Ag is 0.144 nm, the radius of lead 0.174. This prevents a complete epitaxial structure. One can expect that the forced layer structure has a high internal strain. The elastic strain in the film can be estimated from the deviation of the distance of the atoms in the film d from the distance dg in the bulk crystal. The elastic strain is given by the equation... 

The system Cu—Au(l 11) is an example of the formation of a compact and completely discharged UPD film in two steps. The first peak potential represents a phase structure a, which, at a second potential, is transformed into the phase structure fi. The relevant free energy from the formation of the compact layer of metal B is then obtained by the sum of the free energy of formation of phase a and the free energy of transformation of phase a into phase fi. 

A third approach is the layer-by-layer deposition of monolayers. t xhis process is a combination of two sequential underpotential deposition processes. In the first process a metal is deposited as UPD film. Then after rinsing, the electrolyte is changed, and by an anodic oxidation (a type of anodic underpotential deposition) the anionic component is deposited as a second layer on the primary UPD film. In both deposition processes epitaxial film growth is expected and has been partially confirmed. 

Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

Single-phase ZnS films of a fine grain size (no XRD shown) and a band gap of 3.7 eV were electrodeposited from aqueous alkaline (pH 8-10) solutions of zinc complexed with EDTA, and thiosulfate as a sulfur source [101]. The voltammet-ric data implied that deposition occurred either by S-induced UPD of Zn or by a pathway involving both Zn " and thiosulfate concurrently. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Recent considerations of metal UPD on semiconductor surfaces suggest that light-assisted processes gain much significance in the relevant technology. The use of photoinduced UPD as an approach for the preparation of compounds and composite semiconductors either in thin films (layered structures) or in particulate suspensions is a challenging issue that will be outlined promptly. 

The UPD and anodic oxidation of Pb monolayers on tellurium was investigated also in acidic aqueous solutions of Pb(II) cations and various concentrations of halides (iodide, bromide, and chloride) [103]. The Te substrate was a 0.5 xm film electrodeposited in a previous step on polycrystalline Au from an acidic Te02 solution. Particular information on the time-frequency-potential variance of the electrochemical process was obtained by potentiodynamic electrochemical impedance spectroscopy (PDEIS), as it was difficult to apply stationary techniques for accurate characterization, due to a tendency to chemical interaction between the Pb adatoms and the substrate on a time scale of minutes. The impedance... 

Figure 3.21 Hydrogen evolution after each stage of BiPt surface alloy synthesis, (a) Pt film after deposition and anneal (b) immediately after Bi-UPD (c) after second anneal to form the BiPt surface alloy. Adapted from [Greeley et al., 2006a] see this reference for more details.

A representative example of the upd process is copper on gold and an extremely illuminating study of this system using repulsive AFM was reported by Manne et al. (1991). The authors employed a commercially available AFM, the essentials of which are shown in Figure 2.33. The reference electrode was a copper wire in contact with the electrolyte at the outlet of the cell. The counter electrode was the stainless steel spring clip holding the AFM cantilever in place. The working electrode was a 100 nm thick evaporated Au film (which is known to expose mainly the Au(111) surface) mounted on an (x, v) translator. 

Oxidative UPD involves the oxidation of species to form an atomic layer where the precursor contains the element in a negative oxidation state. A classic example is the formation of oxide layers on Pt and Au, where water is oxidized to form atomic layers of oxygen. Halide adsorption can be thought of similarly, where a species such as I oxidatively adsorbs on a metal surface as the halide atom. In that case, a bulk film is not formed at more positive potentials, but the diatomic is generated and diffuses into solution. With respect to compound formation, oxidative UPD from a sulfide solution is a good example ... 

Fig. 23. (a) X-t scan of an STM for Au(100) in 0.05 M H2SO4 + 0.4 mM CuS04 after stepping the potential from +500 mV to -250 mV vs. SCE (see arrow). The picture is shown in the differential mode (shaded) for a better contrast, (b) Cross section along the white line to-ti. The first ten copper layers (including the upd) form a smooth film on the substrate. With the deposition of the eleventh layer a striped structure appears, which is maintained during further deposition [78],... 

The use of UPD layers can in principle generate deposits with composition modulated on the atomic scale, and Pauling et al. have produced what they call hetero-structured ultra-thin films containing Ag, Pd and T1 by this method [158], Stickney and coworkers have assembled multilayered deposits of CdTe and GaAs by addition of one atomic layer of the individual components at a time, a process they call electrochemical atomic-layer epitaxy [159 162], The essential controlling feature in the UPD mechanism is that the deposited layers are allowed to reach equilibrium. Hence, the process represents an extreme of local, reversible control. 

---