Janus single-layer transition metal dichalcogenides, in which the two chalcogen layers have a different chemical nature, push chemical composition control beyond what is usually achievable with van der Waals heterostructures. Here, we report such a Janus compound, SPtSe, which is predicted to exhibit strong Rashba spin–orbit coupling. We synthetized it by conversion of a single-layer of PtSe₂ on Pt(111) via sulfurization under H₂S atmosphere. Our in situ and operando structural analysis with grazing incidence synchrotron X-ray diffraction reveals the process by which the Janus alloy forms. The crystalline long-range order of the as-grown PtSe₂ monolayer is first lost due to thermal annealing. A subsequent recrystallization in presence of a source of sulfur yields a highly ordered SPtSe alloy, which is isostructural to the pristine PtSe₂. The chemical composition is resolved, layer-by-layer, using angle-resolved X-ray photoelectron spectroscopy, demonstrating that Se-by-S substitution occurs selectively in the topmost chalcogen layer.

Janus单层过渡金属二硫属化合物（其中两个硫属元素层具有不同的化学性质）使化学成分控制超出了范德华异质结构通常可以实现的范围。在这里，我们报道了这种Janus化合物SPtSe，预计它会表现出强Rashba自旋-轨道耦合。我们通过在H ₂ S气氛下硫化将PtSe ₂在Pt（111）上转化成单层PtSe ₂来合成。我们用掠入射同步加速器X射线衍射进行原位和操作结构分析，揭示了Janus合金形成的过程。生长的PtSe ₂的晶体长程有序单层首先由于热退火而损失。在硫源存在下的随后重结晶产生高度有序的SPtSe合金，该合金与原始PtSe ₂具有同构结构。使用角度分辨X射线光电子能谱逐层解析化学成分，表明Se-by-S取代选择性地发生在最上层的硫属元素层中。