Atomic steps, a defect common to all crystal surfaces, can play an important role in many physical and chemical processes. However, attempts to predict surface dynamics under nonequilibrium conditions are usually frustrated by poor knowledge of the atomic processes of surface motion arising from mass transport from/to surface steps. Using low-energy electron microscopy that spatially and temporally resolves oxide film growth during the oxidation of NiAl(100) we demonstrate that surface steps are impermeable to oxide film growth. The advancement of the oxide occurs exclusively on the same terrace and requires the coordinated migration of surface steps. The resulting piling up of surface steps ahead of the oxide growth front progressively impedes the oxide growth. This process is reversed during oxide decomposition. The migration of the substrate steps is found to be a surface-step version of the well-known Hele-Shaw problem, governed by detachment (attachment) of Al atoms at step edges induced by the oxide growth (decomposition). By comparing with the oxidation of NiAl(110) that exhibits unimpeded oxide film growth over substrate steps we suggest that whenever steps are the source of atoms used for oxide growth they limit the oxidation process; when atoms are supplied from the bulk, the oxidation rate is not limited by the motion of surface steps.oxidation | NiAl | surface steps S urface growth processes are often treated with a simplifying assumption that the substrate is immobile. The rationale behind this general belief is that the role of the rigid substrate is to serve as a structural template, guiding the arrangement of impinging atoms. However, many surface growth processes involve reaction with the substrate (i.e., require sources or sinks of substrate atoms). In such cases, the role of the metallic substrate goes far beyond that of a passive support because of its active participation in the growth process. Such surface growth processes are typified by the oxidation of metals, during which the interaction between oxygen and a metallic substrate results in oxide growth by consuming the substrate atoms. Although extensive interest in understanding surface oxidation has existed for decades owing to its critical role in many technological processes including high-temperature corrosion, heterogeneous catalysis, and thin film processing, many issues are still unresolved, particularly those concerning the early stages of oxidation. A detailed understanding of the initial oxidation processes of surfaces has always been complicated by overwhelming inhomogeneities owing to high density of defects. Atomic steps are present on virtually any crystalline material in any environment and serve as natural sources and sinks of substrate atoms owing to the reduced coordination of atoms at step sites. In this work we address oxidation-induced surface dynamics as a result of mass transfer from and to steps. This is a critical issue because it influences both the mechanism and kinetics of the surface physical and chemical pro...

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The oxidation behavior of NiAl(100) by molecular oxygen and water vapor under a near-ambient pressure of 0.2 Torr is monitored using ambient-pressure X-ray photoelectron spectroscopy. O exposure leads to the selective oxidation of Al at temperatures ranging from 40 to 500 °C. By contrast, HO exposure results in the selective oxidation of Al at 40 and 200 °C, and increasing the oxidation temperature above 300 °C leads to simultaneous formation of both Al and Ni oxides. These results demonstrate that the O oxidation forms a nearly stoichiometric AlO structure that provides improved protection to the metallic substrate by barring the outward diffusion of metals. By contrast, the HO oxidation results in the formation of a defective oxide layer that allows outward diffusion of Ni at elevated temperatures for simultaneous NiO formation.

The initial growth of ultrathin aluminum oxide film during the oxidation of NiAl(100) was studied with scanning tunneling microscopy. Our observations reveal that the oxide film grows initially as pairs of a double-row stripe structure with a lateral size equal to the unit cell of θ-Al2O3. These double-row stripes serve as the very basic stable building units of the ordered oxide phase for growing thicker bulk-oxide-like thin films. It is shown that the electronic properties of these ultrathin double-row stripes do not differ significantly from that of the clean NiAl surface; however, the thicker oxide stripes show a decreased conductivity.

The crystallization of amorphous aluminum oxide thin films formed on NiAl(100) has been investigated using in‐situ low energy electron microscopy, low energy electron diffraction, and scanning tunneling microscopy. It is found that both the annealing temperature and annealing time play crucial roles in the crystallization process. A critical temperature range of 450°C–500°C exists for the crystallization to occur within a reasonably short annealing time. The initially uniform oxide film first becomes roughened, followed by coalescing into amorphous‐like oxide islands; further annealing results in the conversion of the amorphous oxide islands into crystalline oxide stripes. The density of the crystalline oxide stripes increases concomitantly with the decrease in the density of the amorphous oxide islands for annealing at a higher temperature or longer time.