Despite their importance, chemical reactions confined in a low dimensional space are elusive and experimentally intractable. In this work, we report doubly anisotropic, in-plane and out-of-plane, oxidation reactions of two-dimensional crystals, by resolving interface-confined thermal oxidation of a single and multilayer MoS₂ supported on silica substrates from their conventional surface reaction. Using optical second-harmonic generation spectroscopy of artificially stacked multilayers, we directly proved that crystallographically oriented triangular oxides (TOs) were formed in the bottommost layer while triangular etch pits (TEs) were generated in the topmost layer and that both structures were terminated with zigzag edges. The formation of the Mo oxide layer at the interface demonstrates that O₂ diffuses efficiently through the van der Waals (vdW) gap but not MoO₃, which would otherwise sublime. The fact that TOs are several times larger than TEs indicates that oxidation is greatly enhanced when MoS₂ is in direct contact with silica substrates, which suggests a catalytic effect. This study indicates that the vdW-bonded interfaces are essentially open to mass transport and can serve as a model system for investigating chemistry in low dimensional spaces.

中文翻译：

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MoS _2的界面约束双各向异性氧化

尽管它们具有重要意义，但限制在低维空间中的化学反应仍然难以捉摸，并且在实验上难以解决。在这项工作中，我们报告了二维晶体的各向异性各向异性，平面内和平面外的氧化反应，这是通过解决常规表面反应对二氧化硅衬底上负载的单层和多层MoS _2的界面约束热氧化而解决的。使用人工堆叠的多层光学二次谐波生成光谱，我们直接证明了晶体取向的三角形氧化物（TOs）在最底层形成，而三角形蚀刻坑（TEs）在最顶层形成，并且两种结构均以锯齿形终止边缘。在界面处形成的Mo氧化物层表明O ₂通过范德华（vdW）间隙有效扩散，但不会通过MoO ₃扩散，否则MoO ₃会升华。TOs大于TEs几倍的事实表明，当MoS ₂与二氧化硅基质直接接触时，氧化作用大大增强，这表明具有催化作用。这项研究表明，vdW键合的界面本质上对质量传输开放，并且可以用作研究低维空间中化学反应的模型系统。