Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS₂ and MoS₂ monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS₂ and MoS₂ monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS₂ and MoS₂ monolayers and to exploit their intrinsic properties in a multitude of applications.

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二维（2D）半导体，以2D过渡金属二金属卤化物（TMD）表示，由于强的自旋轨道/自旋谷耦合而表现出丰富的波谷物理特性。探测此类2D系统的最常见方法是利用光学方法，该方法可以监视各种激子态的光发射，并进一步帮助理解此类现象背后的物理原理。因此，对具有良好光学质量的二维TMDs有很大的需求。在这里，我们报告了一种以高产率和高光学质量在六方氮化硼（hBN）薄片上直接生长外延WS ₂和MoS ₂单层的方法。这些单层显示出比天然晶体剥离的单层更好的固有发光特性。第一次，WS ₂的山谷塞曼分裂并已对hBN上的MoS ₂单分子层进行了可视化和系统的研究。这项研究为生产高光学质量的WS ₂和MoS ₂单层膜并在众多应用中利用它们的内在特性开辟了一条新途径。

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