Department of Mechanical Engineering & Multidisciplinary Program in Materials Science and Engineering, State University of New York, Binghamton, NY 13902;
Department of Chemistry, Physics and Engineering, Biola University, La Mirada, CA 90639; and.
Proc Natl Acad Sci U S A. 2015 Jan 13;112(2):E103-9. doi: 10.1073/pnas.1420690112. Epub 2014 Dec 29.
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.
原子台阶是所有晶体表面共有的缺陷，在许多物理和化学过程中都起着重要作用。然而，由于对源自表面台阶处物质输运的表面运动原子过程缺乏了解，尝试预测非平衡条件下的表面动力学通常会受阻。通过使用时空分辨的低能电子显微镜研究 NiAl(100) 的氧化过程中的氧化膜生长，我们证明了表面台阶对于氧化膜生长是不可渗透的。氧化的推进仅在同一梯台上发生，并且需要表面台阶的协调迁移。氧化生长前沿前表面台阶的堆积逐渐阻碍了氧化的进行。在氧化分解过程中，这个过程会被逆转。研究发现，基体台阶的迁移是众所周知的赫勒-肖沃问题的表面台阶版本，由氧化物生长（分解）引起的台阶边缘处的 Al 原子的脱离（附着）来控制。通过与 NiAl(110) 的氧化相比，后者在基体台阶上表现出无阻的氧化膜生长，我们提出，只要台阶是用于氧化生长的原子源，它们就会限制氧化过程；当原子由体相提供时，氧化速率不受表面台阶运动的限制。
