Deposition of metal atoms

In contrast to the older techniques, a newer method is to use a scanning tuimeling electron microscope to deposit metal coatings in microscopic images as small as 0.001 pm. The ultimate surface metallization techniques allow deposition of metals atom by atom in controlled three-dimensional arrays. 

Underpotential Deposition of Metal Atoms Because of the energy of interaction between a foreign substrate and the adsorbed metal atoms formed by discharge, cathodic discharge of a limited amount of metal ions producing adatoms is possible at potentials more positive than the equilibrium potential of the particular system, and also more positive than the potential of steady metal deposition. 

Observing the variation of electroreflection spectra in the course of electrochemical treatment of electrodes, one may determine qualitatively reversible and irreversible changes in the near-the-surface layer, for instance, partial decomposition of ZnSe under anodic polarization (Lemasson et al, 1980) and deposition of metal atoms on the surface of ZnO under cathodic polarization (Kolb, 1973). 

Unlike anions that specifically adsorb at electrodes, cations normally do not lose their solvation shell due to their smaller size and are electrostatically adsorbed at electrodes at potentials negative to the pzc. However, depending on the affinity with the foreign substrate, cations can be reduced to a lower oxidation state or even discharged completely to the corresponding metal atom at the sub-monolayer or monolayer level at potentials positive to the equilibrium Nernst potential for bulk deposition. This deposition of metal atoms on foreign metal electrodes at potential positive to that predicted by the Nernst equation for bulk deposition has been called underpotential deposition and has been extensively investigated in recent years. Detailed discussion of the... 

In spite of the overall success of this particular scheme, there are some issues which need to be clarified especially when it comes to details of the structure of the electrode/monolayer/electrode crossings on an atomistic level [4]. In a recent review on the vapor deposition of metal atoms on organic monolayers [65], the complexity of the process and the subtle effects of the surface structure and composition on the outcome are illustrated. A reflection-absorption infrared spectroscopy study [66] of a system, which is similar to the actual cross-junction, suggests undamaged organic mono-layers with Ti coatings, but further research is needed for the complete characterization of these complex structures. 

The high mass sensitivity of ETSM sensors renders them particularly suited for the analysis of monolayer and submonolayer films. In fact, the earliest applications of the ETSM involved studying the electrochemical deposition of monolayers, including the formation of metal oxides [207], electrosorption of halides [208], and the underpotential deposition of metal atoms [209-213]. In some cases, the electrovalency (i.e., the ratio of moles of electrons transferred at the electrode to moles of adsorbate deposited) was found to vary with adsorbing species the adsorption of iodide onto gold, for example, occurs with complete charge transfer from the halide to the electrode, whereas the adsorption of bro-... 

The experimental results and the tentative explanations clearly indicate that the deposition of metal atoms on the MgO films is followed by complex diffusion mechanisms, which depend quite strongly on the nature of the metal-MgO bond. The use of CO as a probe molecule provides invaluable information to identify the metal adsorption sites but also contributes to complicate the diffusion mechanisms as new M-CO species, quite strongly bound in the case of Rh and Pd, can become mobile on the surface and compete with the mobility of the individual metal atoms. 

Synergism via deposition of metal atoms on the surface has been postulated for bimetallic clusters of Pt or Rh with Pb, Tl, Bi, and Cd for HCOOH electrooxidation (278). The catalytic enhancement by the nonnoble surface... 

This process describes the reaction of metal atoms with ligands L forming metal ions (dissolution, corrosion) or the deposition of metal atoms by reduction of metal ions. The solvent molecules are a special example of ligands that reacts with the metal ions. A partial step of the total process, ad-atom deposition and dissolution, was aheady discussed in Chapter 4. The rate equations are similar to Eqs. (6.2) and (6.3). 

The formation of clusters may be regarded as a continuous deposition of metal atoms on a polymeric film over the time t at the rate W then the corresponding rate equations are... 

Underpotential deposition (UPD) is one useful way to the fabrication of metal mono-layer on metal nanoparticles [57]. UPD has been widely used in the deposition of metal atoms electrochemically onto another metal substrate at a potential slightly lower than the equilibrium potential for its reduction to metal [58]. This deposition is self-limiting, and atoms can only deposit up to single monolayer. When Pt is used as the core metal, several metals including Ru, Pd, and Ag can be directly deposited as monolayer via UPD [59]. 

Figure 2. Voltammogram changing with the deposition of metal atoms, (a) In situ cyclic voltammogram for Rh depositing (100) in a 0.05 M H2SO4 +...

Amount of deposition of metal atoms was controlled by monitoring the in-situ voltammogram. After then the single crystal surface was washed with ultra-pure water replaced in the quartz cell, and then it was transferred into the main chamber for the LEED and XPS or AES analysis. The Rh/Pt(100) and PtZRh(lOO) surfaces... 

The polymer-metal interface shown in Fig. was derived from an electron micrograph obtained by Mazur and Reich.They electrodeposited silver from a silver ion solution diffusing through a polyimide film. Particles not connected to the diffusion source were removed by computer analysis. The deposited silver particles essentially "decorate the concentration profile and permit the diffusion front to be observed. A 1000-A thin slice was used to aproximate two dimensional diffusion. The fractal dimension of this interface was determined by computer analysis to be approximately 1.7. Similar ramified interface fronts are created by vapor deposition of metal atoms on polymers and by certain ion bombardment treatments of polymer surfaces. The fractal front is fairly insensitive to the details of the concentration profile. However, strong chemical potential gradients in asymmetric interfaces may promote a more planar, less ramified structure. The fractal characteristics of polymer interfaces... 

These considerations suggest that very thin oxide films may present specific electronic properties, which might find interesting applications, for example in the field of micro-electronics or opto-electronics, and an enhanced reactivity similar to that of oxide powders. This point is more and more recognized in the literature. Thin oxide films may be grown on metallic substrates by deposition of metallic atoms and subsequent oxidation. By this method, many crystalline films have been obtained, for example BaO on W(llO) (Shih et al, 1988 Mueller et al, 1990) - a... 

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