Ultrathin oxide layers

As in other fields of nanosdence, the application of STM techniques to the study of ultrathin oxide layers has opened up a new era of oxide materials research. New emergent phenomena of structure, stoichiometry, and associated physical and chemical properties have been observed and new oxide phases, hitherto unknown in the form of bulk material, have been deteded in nanolayer form and have been elucidated with the help of the STM. Some of these oxide nanolayers are and will be of paramount interest to the field of advanced catalysis, as active and passive layers in catalytic model studies, on the one hand, and perhaps even as components in real nanocatalytic applications, on the other hand. We have illustrated with the help of prototypical examples the growth and the structural variety of oxide nanolayers on metal surfaces as seen from the perspective of the STM. The selection of the particular oxide systems presented here refleds in part their relevance in catalysis and is also related to our own scientific experience. 

ULTRATHIN OXIDE LAYERS IN SILICON SCHOTTKY-TYPE SOLAR CELLS... 

Using GIR to investigate the thickness of the ultrathin oxide layer in Al-SiO c-Si devices, the number of Al—O bonds (band of 849 cm" ) was found to increase with thermal annealing [13] (Fig. 6.3). The decrease in the SiO thickness is a temperatnre-activated process with an activation energy of 0.98 eV and can be well described within the fi amework of an Al-A10y-SiO t Si model. In addition, contamination by aluminum can considerably influence the characteristics of the SiOj -Si interface, causing an increase in the density of electronic states, the effect being more pronounced for thinner silicon oxide layers [22, 23]. 

All Fe oxide films on Pt have strongly relaxed, unreconstructed bulk-terminated surfaces, but while the Fe304 and Fe203 oxide layers are similar to their respective bulk compounds, the ultrathin FeO layers are true 2D oxide phases that are different from the FeO bulk and stabilized by the metal-oxide interface. 

Materials with better properties are also of interest as possible replacements for aSiO for gate oxide applications, since the ultrathin SiO layers required for low-voltage, high-drive logic is inherently too leaky to be employed. Instead, a thicker layer of a material with a higher dielectric constant is needed. As in capacitor appli-... 

On the other hand, the progress of wet-processes as preparative techniques of metal oxide films has been remarkable. The so-called soft solution process that provides oxide layers by means of electrochemical oxidation of a metal surface is expanding as a synthetic method of various mixed metal oxides with controlled thickness [2], The two-dimensional (2D) sol-gel process based on the hydrolysis of metal alkoxides at the air/water interface has been reported as a preparative technique of ultrathin oxide films (Fig. 6.1a) [3]. It is also known that LB films of metal complexes of long-chain alkyl carboxylic acid can be converted to metal oxide films after removal of organic component by oxygen plasma [4] and UV-ozone treatments (Fig. 6.1b) [5]. Preparation of metal oxide... 

The cathode photocurrent is in proportion to the number of TCPP layers at least up to 10 cycles. The efficiency is greatly dependent on the kinds of oxide gel. These experimental observations suggest that electron transfer from the electrode to the porphyrin via the oxide gel layer is an essential mechanism of the photocurrent generation. Oxygen molecules as an electron acceptor readily diffuse in the oxide gel films of about 20 nm thickness. The electron transfer from the electrode to the porphyrin is assisted by satisfactory conductivity of the gel layer. The overall photocurrent value is considerably smaller than the conventional wet solar cell [11]. However, modification of the electrode surface by ultrathin oxide gel films will facilitate the design of novel light harvesting devices. 

One of the remarkable demonstrations of the capabilities of ultraviolet Raman spectroscopy to probe extremely thin ferroelectric oxide layers reported so far has been its application for studies of ultrathin BaTi03 films [48]. In order to investigate the size effect on the ferroelectric phase transitions, variable temperature UV Raman spectroscopy was applied to studies of a series of BaTi03 films with layer thicknesses varied from 1.6 to 10 nm (4—25 unit cells). 

Atrei A, Bardi U, Tarducd C, Rovida G (2002) Composition and structure of ultrathin vanadium oxide layers deposited on SnO2(110). Surf Sci 513 149... 

The quenching recovery provided the motivation for fabrication of OLEDs with AI2O3/AI cathodes.48 The AI2O3 was obtained by the natural oxidation of a pre-deposited ultrathin layer of A1 on the organic surface. It led to improved EL efficiency as long as the thickness of the initially deposited A1 layer did not exceed the depth of the native oxide layer. Further improvement was achieved when the A1... 

The electrochemical oxidation of silicon is of interest for the formation of ultrathin dielectric layers due to its low temperatures and easy controllability of the process [3]. In this case, the efficiency of the electrochemical treatment of the materials when oxide layers of low thicknesses are synthesized on their surfaces depends on the uniformity of the charge distribution at the solid phase/electrolyte interface. A great impact on this factor is made by adsorption processes, particularly, chemisorption ones, but their role in the formation of nano-sized films is, not yet understood. 

The electrocatalytic behavior of the thin palladium layers deposited on Au(hkl) surfaces for hydrogen adsorption/absorp-tion [77, 80], oxygen reduction [79], oxide formation/reduction [80], copper UPD [33, 77], electrochemical oxidation of formic acid [84] as well as formaldehyde [80] has been investigated in detail. These electrocatalytic activities depended significantly on the surface structure and thickness of the ultrathin palladium layers [80, 84]. 

Besides the standard methods of SIMS, AES, and XPS for investigation of these ultrathin dielectric layers in Schottky-type solar cells, IRRAS has also been applied [5-8]. Ultrathin (2-10-nm) Si02 films in metal-oxide-silicon strnctnres have also been investigated by IRRAS [9-14]. 

As discussed extensively in Section 3.1, the key feature in interpretating spectra of ultrathin dielectric and oxide layers on the surface of metals is the strong... 

Localized corrosion of metals and alloys occurs in aggressive media (e.g., containing chloride) as a consequence of the passivity breakdown, with major impact in practical applications and on the economy. This form of corrosion is particularly insidious since a component, otherwise well protected by a well-adherent, ultrathin oxide or oxyhydroxide barrier layer (i.e., the passive film), can be perforated locally in a short time with no appreciable forewarning. Extensive studies have been conducted over the last five decades to understand localized corrosion by pitting [1-10], but the detailed mechanisms accounting for the local occurrence of passivity breakdown remain to be elucidated and combined with kinetics laws to allow reliable prediction. 

The quantification of matrix components is possible in ultrathin films, layers, and bulk materials. The Hf and Al composition in ultrathin mixed Hf02/Al203 oxides could be determined via ratios of oxidic ions of the same type (e.g., HfO A10, HfO" A10- or Hf02 A102") under 250 eV Xe or Cs bombardment [263]. In a... 

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