未重构的α-Al$_{2}$O$_{3}$(0001)表面是不均匀且粗糙的
The Unreconstructed α-Al$_{2}$O$_{3}$(0001) Surface is Inhomogeneous and Rough
AI总结：
通过非接触原子力显微镜和密度泛函理论计算，发现未重构的α-Al$_{2}$O$_{3}$(0001)表面并非原子级平坦且均匀Al终止，而是本质亚稳态、粗糙且不均匀，挑战了常用原子模型的有效性。
摘要：
氧化铝（Al$_{2}$O$_{3}$）是薄膜生长和多相催化的关键材料，其原子表面结构对性能有重要影响。通过非接触原子力显微镜（nc-AFM）结合密度泛函理论（DFT）计算，我们挑战了未重构的$α$-Al$_{2}$O$_{3}$(0001)表面是原子级平坦且均匀Al终止的普遍假设。这种广泛接受的体相终止满足极性补偿要求，但导致表面Al阳离子高度欠配位。尽管这些Al阳离子有显著的内向弛豫，我们发现（1 ${\times}$ 1）表面相对于在1000°C以上高温形成的热力学稳定的$(\sqrt{31} \times \sqrt{31})R\pm9°$表面重构，本质上是亚稳态的。未重构表面的nc-AFM成像显示粗糙且无序的形貌，仅有纳米尺度的区域表现出有序的Al终止（1 $\times$ 1）结构。我们的结果表明，未重构的Al$_{2}$O$_{3}$(0001)表面本质上是非均匀的，调和了相互矛盾的实验观察，并挑战了常用原子模型的有效性。
英文摘要：
Alumina (Al$_{2}$O$_{3}$) is a key material for thin-film growth and heterogeneous catalysis, where the atomic surface structure critically impacts performance. Using noncontact atomic force microscopy (nc-AFM) combined with density functional theory (DFT) calculations, we challenge the common assumption that the unreconstructed $α$-Al$_{2}$O$_{3}$(0001) surface is atomically flat and uniformly Al-terminated. This widely accepted bulk termination satisfies polarity compensation requirements but results in highly undercoordinated surface Al cations at the surface. Despite substantial inward relaxation of these Al cations, we find that the (1 ${\times}$ 1) surface remains inherently metastable, relative to the thermodynamically stable $(\sqrt{31} \times \sqrt{31})R\pm9°$ surface reconstruction that forms at high temperatures above 1000 °C. Nc-AFM imaging of the unreconstructed surface reveals a rough and disordered morphology, with only nanometer-scale regions exhibiting the ordered Al-terminated (1 $\times$ 1) structure. Our results show that the unreconstructed Al$_{2}$O$_{3}$(0001) surface is intrinsically inhomogeneous, reconciling conflicting experimental observations and challenging the validity of commonly used atomistic models.
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