层状二硫化钨的可控制备和生长机制研究

  半导体性的二维过渡金属硫族化合物（Two-Dimensional Transition Metal Dichalcogenides, 2D-TMDCs），如二硫化钨（WS₂）、二硫化钼（MoS₂）等，具有较高的理论载流子迁移率（~1103 cm^-1 V^-1 s^-1）和较高的开关比（10⁶-10⁸），在新一代柔性电子中具有广阔的应用前景。然而，高质量2D-TMDCs薄膜的大面积可控制备依然存在较大困难，这严重制约了2D-TMDCs的工业化应用。化学气相沉积（Chemical Vapor Deposition, CVD）被认为是制备低缺陷、大面积2D-TMDCs薄膜的有效方法。然而，该方法制备的2D-TMDCs样品普遍存在单晶尺寸小、层数不可控、薄膜质量差和重复性不佳等问题。研究表明，这源于CVD反应难以精确调控前驱体中的元素浓度比，如硫：钨（S:W）比。

  据此，本论文以层状WS₂的生长为研究对象，提出了一种新型的旋涂-限域策略来提升CVD反应中前驱体浓度的调控，用于可控制备大尺寸、高质量的WS₂薄膜。主要研究内容和结果如下：用COMSOL Multiphysics软件模拟了限域空间中硫蒸汽随时间的动态分布。该模拟结果表明限域空间可以较长时间保持S:W ~ 2的优化比值，并促进WS₂的面内生长。我们研究了限域空间、金属前驱体浓度、硫源位置等参数对WS₂的形核密度、晶粒尺寸和层数的影响，进一步从实验上验证了旋涂-限域策略能够有效控制WS₂的生长动力学，并实现了大尺寸的单层、双层WS₂的可控制备。此外，我们成功制备了MoS₂-WS₂平面异质结并发现该材料相比单一组分的WS₂或MoS₂具有更高效的电催化析氢性能。本论文工作加深了对CVD反应过程中WS₂晶体生长动力学的理解，并为实现大单晶、层可控的WS₂薄膜以及相关异质结的制备提供了简单有效的策略。

   Semiconductive two-dimensional transition metal dichalcogenides (2D-TMDCs), such as tungsten disulfide (WS₂) and molybdenum disulfide (MoS₂), have high theoretical carrier mobility (~1103 cm^-1V^-1 s^-1) and high on/off ratio (10⁶-10⁸), which have broad application prospects in the next generation of flexible electrons. However, the controllable fabrication of high-quality and wafer-scale 2D-TMDCs films remain difficulties, which seriously restricts the industrial application of 2D-TMDCs. Chemical vapor deposition (CVD) is considered to be an effective method to prepare 2D-TMDCs thin films with low defect density and large-size. However, 2D-TMDCs samples prepared by CVD always suffer from small single crystal size, uncontrollable layer number, poor film quality and poor repeatability. Studies show that this is mainly due to the difficulty of CVD reaction in accurately regulating the element concentration ratio in the precursor (e.g. S:W ratio).

    In view of this, the growth of layered WS₂ is taken as the research object, and a spin-coating confined space strategy is innovatively proposed in this work to improve the modulation of precursor concentration in CVD, which is applied in the controllable preparation of large-size and high-quality WS₂ films. The main research content and conclusions are as follows. COMSOL Multiphysics is used to simulate the change of precursor vapor over time in spin-coating confined space CVD. The simulated results indicate that the optimal S:W ratio (~2) can be maintained in the confined space for a long time, which is beneficial to promote the in-plane growth of WS₂. We continue to study the effects of confined space, concentration of metal precursor, location of S source and other reaction parameter on the nucleation density, domain size and layer number of WS₂. We successfully achieve the controllable preparation of monolayer and bilayer WS₂ with large domain size. In addition, we successfully prepare in-plane heterojunction of MoS₂-WS₂ and find it exhibits more efficient hydrogen evolution reaction performance than WS₂ or MoS₂. This work provides an in-depth understanding of the growth kinetics of 2D WS₂ crystals during CVD reaction, and proposes a simple and effective strategy for the controllable growth of 2D-TMDCs with large single crystal, controllable layer number and in-plane heterostructure.

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