Blades William H, Reinke Petra
Department of Materials Science and Engineering , University of Virginia , Charlottesville , Virginia 22904 , United States.
ACS Appl Mater Interfaces. 2018 Dec 12;10(49):43219-43229. doi: 10.1021/acsami.8b15210. Epub 2018 Dec 3.
The interaction of oxygen with Ni-Cr(100) alloy surfaces is studied using scanning tunneling microscopy (STM) and spectroscopy (STS) to observe the initial steps of oxidation and formation of the alloy-oxide interface. The progression of oxidation was observed for Ni(100) and Ni-Cr(100) thin films including Ni-8 wt % Cr(100) and Ni-12 wt % Cr(100), which were grown on MgO(100) in situ. These surfaces were exposed to between 1 and 150 L O at 500 °C, and additional annealing steps were performed at 500 and 600 °C. Each oxidation and annealing step was studied with STM and STS, and differential conductance maps delivered spatially resolved information on doping and band gap distributions. Initial NiO nucleation and growth begins along the step edges of the Ni-Cr alloy accompanied by the formation of small oxide particles on the terraces. The incubation period known in oxidation of Ni(100) is absent on Ni-Cr alloy surfaces illustrating the significant changes in surface chemistry triggered by Cr-alloying. Step edge faceting is initiated by oxide decoration along the step edges and is expressed as moiré patterns in the STM images. The surface oxide can be characterized by NiONi(6 × 7) and NiO-Ni(7 × 8) coincidence lattices, which have a cube-on-cube epitaxial relationship. Small patches of NiO are susceptible to reduction during annealing; however, additional oxide coverage stabilizes the NiO. NiO regions are interspersed with areas covered predominantly with a novel cross-type reconstruction, which is interpreted tentatively as a Cr-rich, phase-separated region. Statistical analysis of the geometric features of the surface oxide including step edge heights, and NiO wedge angles illustrates the layer-by-layer growth mode of NiO in this pre-Cabrera-Mott regime, and the restructuring of the alloy-oxide interface during the oxidation process. This experimental approach has offered greater insight into the progression of oxide growth in Ni-Cr thin films and underscores the dramatic impact of alloying on oxidation process in the pre-Cabrera-Mott regime.
利用扫描隧道显微镜（STM）和光谱学（STS）研究了氧气与Ni-Cr(100)合金表面的相互作用，以观察氧化的初始步骤以及合金-氧化物界面的形成。观察了原位生长在MgO(100)上的Ni(100)和Ni-Cr(100)薄膜（包括Ni-8 wt% Cr(100)和Ni-12 wt% Cr(100)）的氧化过程。这些表面在500℃下暴露于1至150 L的氧气中，并在500℃和600℃下进行了额外的退火步骤。每个氧化和退火步骤都通过STM和STS进行研究，微分电导图提供了关于掺杂和带隙分布的空间分辨信息。初始的NiO成核和生长沿着Ni-Cr合金的台阶边缘开始，同时在台面上形成小的氧化物颗粒。Ni-Cr合金表面不存在Ni(100)氧化过程中已知的孕育期，这说明了Cr合金化引发的表面化学的显著变化。台阶边缘刻面由沿着台阶边缘的氧化物修饰引发，并在STM图像中表现为莫尔条纹图案。表面氧化物可以由具有立方-立方外延关系的NiONi(6×7)和NiO-Ni(7×8)共格晶格来表征。小的NiO斑块在退火过程中容易被还原；然而，额外的氧化物覆盖使NiO稳定。NiO区域与主要覆盖着一种新型十字型重构的区域相互穿插，这种重构暂时被解释为富Cr的相分离区域。对表面氧化物的几何特征（包括台阶边缘高度和NiO楔角）的统计分析表明，在这个前卡布雷拉-莫特区域中NiO的逐层生长模式，以及氧化过程中合金-氧化物界面的重构。这种实验方法为Ni-Cr薄膜中氧化物生长的过程提供了更深入的了解，并强调了在这个前卡布雷拉-莫特区域中合金化对氧化过程的巨大影响。
