Passive films overlayers

Summary. Scanning tunneling microscopy (STM) provides new possibilities to explore the link between the structure and the properties of thin oxide overlayers (passive films) formed electrochemically on well-defined metal surfaces. Passive oxide films protect many metals and alloys against corrosion. A better understanding of the growth mechanisms, the stability, and the degradation of passive films requires precise structural data. Recently, new results on the atomic structure of passive films have been obtained by STM. The important questions of crystallinity, epitaxy and the nature of defects have been addressed. Data on the structure of passive films on Ni, Cr, Fe, Al, and Fe-Cr alloys are reviewed with enq>hasis on atomically resolved structures. Ihe perspectives of future developments are discussed. 

Interesting Raman studies of the corrosion process, have utilized 7 or Au overlayers or sublayers to investigate the passive film [94]. It was found that the SERS of the passive film on iron has two characteristic, high intensity peaks at approximately 550 and 670 cm . SERS spectra were acquired during reduction of preformed passive films at different potentials, see Fig. 48. In order to obtain an observable signal, Au particles were deposited onto the surface to produce a surface-enhanced... 

Recent advances in the chemistry and structure of thin oxide overlayers (passive films) on metal and alloy surfaces have been achieved by using XPS and STM. XPS data have shovm that the passive film formed onNi in acid solution consists of a thin oxide layer (NiO) covered by hydroxyls. The total thickness ofthe passive film is about 1 nm. OnNi(l 1 STM imaging... 

Thin anodic oxide overlayers (passive films) can be highly resistant against corrosion. They also play a major role in adhesion, tribology, catalysis and micro-electronics. A better understanding of the relationship of the chemistry and structure to the properties of metal surfaces covered with thin oxide overlayers is needed. 

AR-XPS has been extensively used to investigate the chemical composition, the chemical states and the thickness of thin anodic oxide overlayers (passive films) formed on well-defined metal and alloy single crystal surfaces. More recently direct imaging of the surface structure by STM with atomic resolution provided new data on the crystallinity, the epitaxy and the nature of the structural defects existing in the thin oxide layers. Such data are useful... 

Thin films (qv) of vitreous silica have been used extensively in semiconductor technology. These serve as insulating layers between conductor stripes and a semiconductor surface in integrated circuits, and as a surface passivation material in planar diodes, transistors, and injection lasers. They are also used for diffusion masking, as etchant surfaces, and for encapsulation and protection of completed electronic devices. Thin films serve an important function in multilayer conductor insulation technology where a variety of conducting paths are deposited in overlay patterns and insulating layers are required for separation. 

Figure 14 Overlay of Pourbaix diagrams for Al, Fe, and Zn, showing that there exists a region of potential and pH within which Al and Zn would be expected to form solid films (i.e., passivate) and lose their ability to corrode sacrificially and protect the steel. (From Ref. 22.)...

Plasma-polymerised thiophene for passivating the surface defects on GaAs has been employed [70]. The paper showed the passivation of GaAs surface was made possible by sulfur present in an overlayer, provided by the thin film of plasma-polymerised thiophene. The deposition of polythiophene lowered the barrier height, reduced the surface recombination velocity and increased diffusion length. 

Generally, passive layers are not simple homogeneous oxide or hydroxide films. Usually, they have at least a bilayer or a multilayer structure, even for pure metal substrates. As already mentioned in Sec. 1.4.2 on surface analytical methods, oxides are located at the metal surface followed by an overlayer of hydroxide. Lower valency species are located inside, whereas higher valency cations are found in the outer parts of the films. In the case of alloys, very specific depth profiles are found which are related to the specific passive properties of these metals (Strehblow, 1997). The accumulation of one component relative to the other is a consequence of the thermodynamics of its anodic oxidation or its dissolution characteristics. As a consequence, one metal component may be accumulated at the metal surface, e.g., copper for AI/Cu (Strehblow et al., 1978) (Fig. 1-28) and copper for Cu/Ni alloys (Druska et al., 1996 Druska and Strehblow, 1996). The preferential dissolution of iron and the accumulation of Cr(III) within the film is another example (Fig. 1-27). 

Thin oxide films formed on metal eleetrodes are of widespread technical importance for passivation and/or catalysis of certain electrode reactions. For example, the stability of most engineered metallic structures towards environmental degradation, i.e., eorrosion, is largely dependent upon the formation of thin proteetive oxide overlayers. Alternatively, electrosynthe-... 

M. Danek, K.F. Jensen, C.B. Murray, M.G. Bawendi, Synthesis of luminescent thin film CdSe/ZnSe quantum dot composites using CdSe quantum dots passivated with an overlayer of ZnSe, Chem. Mater. 8 (1996)173-180. 

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