Charge transfer metal deposition

The same effects occur with most other semiconductors but were not further analyzed. As already discussed in Chapter 5, the proof of the existence of surface states is rather difficult because spectroscopic methods are not sufficiently sensitive. Another way of reducing the influence of surface states is to catalyze a charge transfer by depositing a metal or metal oxide on the semiconductor surface. This problem will be treated separately in Section 7.9. 

The unhindered ionic charge transfer requires many open pores of the smallest possible diameter to prevent electronic bridging by deposition of metallic particles floating in the electrolyte. Thus the large number of microscopic pores form immense internal surfaces, which inevitably are increasingly subject to chemical attack. 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

Reactions involving charge transfer through the interface, and hence the flow of a current, are called electrochemical reactions. Two types of such reactions are indicated in Fig. 1.1. The upper one is an instance of metal deposition. It involves the transfer of a metal ion from the solution onto the metal surface, where it is discharged by taking up electrons. Metal deposition takes place at specific sites in the case shown it is a hollow site between the atoms of the metal electrode. The deposited metal ion may belong to the same species as those on the metal electrode, as in the deposition of a Ag+ ion on a silver electrode, or it can be different as in the deposition of a Ag+ ion on platinum. In any case the reaction is formally written as ... 

The film has noticeable planar conductivity, which depends on the number of monolayers as shown in Figure 7.2. The conductivity ofthe film is detectable for two monolayers, but the value is small for very thin films (two to six monolayers). From six monolayers the conductivity begins to increase linearly with the number of monolayers, a feature that is also found in LB films made of charge-transfer salts, and is perhaps a function of the imperfection in continuity of the first monolayers on the metal electrode-quartz substratum boundary. This imperfection came about during the deposition process as a result of different hydrophilic properties of metal and quartz surfaces. 

In this chapter, we shall use DFT to investigate the extent to which the oxide support alters the electronic structure of the deposited metal as a result of charge transfer at the metal-oxide interface. We will use CO chemisorption as a function of Pt film thickness to demonstrate how changes in the metal electronic structure can lead to chemisorption trends that deviate from expectations based on the current theory of molecular adsorption. 

The possibilities afforded by SAM-controlled electrochemical metal deposition were already demonstrated some time ago by Sondag-Huethorst et al. [36] who used patterned SAMs as templates to deposit metal structures with line widths below 100 nm. While this initial work illustrated the potential of SAM-controlled deposition on the nanometer scale further activities towards technological exploitation have been surprisingly moderate and mostly concerned with basic studies on metal deposition on uniform, alkane thiol-based SAMs [37-40] that have been extended in more recent years to aromatic thiols [41-43]. A major reason for the slow development of this area is that electrochemical metal deposition with, in principle, the advantage of better control via the electrochemical potential compared to none-lectrochemical methods such as electroless metal deposition or evaporation, is quite critical in conjunction with SAMs. Relying on their ability to act as barriers for charge transfer and particle diffusion, the minimization of defects in and control of the structural quality of SAMs are key to their performance and set the limits for their nanotechnological applications. 

Kinetic Scheme. Generally, metal ions in a solution for electroless metal deposition have to be complexed with a ligand. Complexing is necessary to prevent formation of metal hydroxide, such as Cu(OH)2, in electroless copper deposition. One of the fundamental problems in electrochemical deposition of metals from complexed ions is the presence of electroactive (charged) species. The electroactive species may be complexed or noncomplexed metal ion. In the first case, the kinetic scheme for the process of metal deposition is one of simple charge transfer. In the second case the kinetic scheme is that of charge transfer preceded by dissociation of the complex. The mechanism of the second case involves a sequence of at least two basic elementary steps ... 

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