Ultrathin surface alloy film

In electrochemical multicomponent systems", i.e., using electrolytes containing different Me , mixed UPD and OPD deposition of different metals can be used for a sequential deposition of different Me, monolayers forming sandwich-structured ultrathin metal films, S/Mei/Me2/...Me . The formation of 2D Me-S and Me,-Mey surface alloys and 3D Me-S bulk alloys can be utilized to form ultrathin surface alloy films such as S/Me-S, S/Me,-Mey, and S/Me, -S/Me/-Mey. Ultrathin sandwich-structured films and surface alloys will be denoted as heterostructures. 

In presence of significant Me-S lattice misfit, the epitaxy of isolated 3D Me crystallites or compact 3D Me films is strongly determined by the structure of internally strained 2D Meads overlayer and/or 2D Me-S surface alloy formed in the UPD range at high F or low AEi. The misfit between the lattice parameters of the 2D Meads phase and/or 2D Me-S surface alloy phase and the 3D Me bulk phase is mainly removed by misfit dislocations. The initial strain disappears after depositing a certain thickness of the 3D Me bulk phase. Usually, a thickness of n Me monolayers where 2 < < 20 is necessary to adjust the 3D Me bulk lattice parameters [4.58, 4.59]. If an incommensurate structure of a 2D Meads overlayer is formed in the UPD range, this structure will also be reflected epitaxially in 3D Me crystallites and ultrathin 3D Me films. 

The formation of ultrathin Me films on foreign substrates S (metals, superconductors, and semiconductors), S/Me, plays an important role in modern fields of technology such as micro- and nano-electronics, sensorics, electrocatalysis, etc. The process is often carried out by physical or chemical vapor deposition (PVD or CVD) of metals [6.152]. However, the difficult adjustment and control of the supersaturation via the gas flux is a great disadvantage of vapor deposition techniques. The situation becomes even more complicated, if more than one metal is deposited to form metallic sandwich layers and/or surface alloys. Therefore, electrochemical processes for the formation of ultrathin metal films and heterostructures became of great interest in modern thin layer technology. 

The electrochemical formation of ultrathin metal films, sandwich-structured layers and surface alloys with a defined thickness ranging from one to several monatomic layers on S can be obtained by metal deposition in the UPD and OPD ranges using special polarization routines. 

This book shows that the initial stages of 2D and 3D metal phase formation under electrochemical conditions are, in general, well developed and understood on an atomic level. The knowledge of the substrate surface properties is necessary for a well-defined preparation of 2D and 3D metal phases, surface alloys, ultrathin films, and heterostructures. The structural and epitaxial behavior of metal deposits determines their physical and chemical properties. 

Studies of single-crystal surfaces under UHV conditions have allowed us to quantify fundamental interactions at surfaces, and the majority of surface-science studies have been conducted in this manner. Utilization of XPD and LEIS techniques require the studies to be conducted under high vacuum, and studies of clean surfaces or precisely controlled adsorbate layers require UHV conditions. Here we discuss a few examples of the use of these two techniques in studies of single-crystal surfaces, illustrating their power and limitations. The surfaces discussed are metal surfaces that contain controlled amounts of adsorbates, ultrathin metal films, two-component metal alloy surfaces, and oxide surfaces. 

The most commonly used methods for the preparation of ultrathin oxide films are (1) direct oxidation of the parent metal surface, (2) preferential oxidation of one metal of choice from a suitable binary alloy, and (3) simultaneous deposition and oxidation of a metal on a refractory metal substrate. The detailed procedures for (1) and (2) are discussed elsewhere [7,56,57] procedure (3) is discussed here in detail. Preparation of a model thin-film oxide on a refractory metal substrate (such as Mo, Re, or Ta) is usually carried out by vapor-depositing the parent metal in an oxygen environment. These substrate refractory metals are typically cleaned by repeated cycles of Ar sputtering followed by high-temperature annealing and oxygen treatment. The choice of substrate is critical because film stoichiometry and crystallinity depend on lattice mismatch and other interfacial properties. Thin films of several oxides have been prepared in our laboratories and are discussed below. 

The lowest potential is measured in the center, where corrosion (i.e. anodic dissolution of iron) attacks most aggressively. At the edges, the potential increases somewhat in this zone oxygen reduction proceeds. The potential changes around the drop imply the presence of an ultrathin electrolyte film because the potential reaches values of the bare iron surface only at a considerable distance from the edges of the macroscopically observed drop [213]. Filiform corrosion of automotive aluminium alloy AA6016 has been studied with SKP [221]. 

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