In-plane x-ray diffraction for characterization of monolayer and few-layer transition metal dichalcogenide films
In-plane x-ray diffraction for characterization of monolayer and few-layer transition metal dichalcogenide films
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DOI：
10.1088/1361-6528/aaa1bd
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发表时间：
2018-02-02
期刊：
影响因子：
3.5
通讯作者：
Redwing, Joan M.
中科院分区：
文献类型：
--
作者：
Chubarov, Mikhail;Choudhury, Tanushree H.;Redwing, Joan M.
过渡金属二硫族化合物（TMD）的单晶单层和少层薄膜的生长及其在光学、电子、传感、催化等方面的潜在应用具有重要意义。这些材料的特性是决定其性能和应用的关键。二维薄膜的超薄性质对使用传统的Bragg-Brentano几何结构的x射线衍射（XRD）分析二维薄膜的结晶度和外延取向提出了挑战。为了避免这个问题，我们演示了使用实验室规模的设备，使用标准的Cu x射线管来分析TMD单层和少层薄膜的结晶度。在单晶衬底上化学气相沉积WSe2和WS2薄膜的几个例子中，证明了该技术的适用性。采用面内XRD测定了生长在c面蓝宝石上的WSe2与外延层为石墨烯的SiC上的WSe2的外延关系。研究了蓝宝石表面WS2薄膜晶体结构取向随生长温度的变化规律。最后，确定了蓝宝石衬底上WS2/WSe2垂直异质结构的外延关系。我们观察到，在所有研究条件下，WSe2在本研究中使用的两种衬底上都呈外延生长，而WS2在蓝宝石衬底上表现出不同的择优取向，这与温度有关。与蓝宝石衬底相比，沉积在WSe2上的WS2只表现出一种优先取向，这可能为更好地控制WS2的取向和晶体质量提供了一条途径。在SiC上外延石墨烯的情况下，在面内XRD中没有观察到石墨烯相关的峰，而在拉曼光谱中证实了石墨烯的存在。这表明了平面内XRD技术在表征低电子密度材料方面的局限性。
There is significant interest in the growth of single crystal monolayer and few-layer films of transition metal dichalcogenides (TMD) and other 2D materials for scientific exploration and potential applications in optics, electronics, sensing, catalysis and others. The characterization of these materials is crucial in determining the properties and hence the applications. The ultra-thin nature of 2D layers presents a challenge to the use of x-ray diffraction (XRD) analysis with conventional Bragg-Brentano geometry in analyzing the crystallinity and epitaxial orientation of 2D films. To circumvent this problem, we demonstrate the use of in-plane XRD employing lab scale equipment which uses a standard Cu x-ray tube for the analysis of the crystallinity of TMD monolayer and few-layer films. The applicability of this technique is demonstrated in several examples for WSe2 and WS2 films grown by chemical vapor deposition on single crystal substrates. In-plane XRD was used to determine the epitaxial relation of WSe2 grown on c-plane sapphire and on SiC with an epitaxial graphene interlayer. The evolution of the crystal structure orientation of WS2 films on sapphire as a function of growth temperature was also examined. Finally, the epitaxial relation of a WS2/WSe2 vertical heterostructure deposited on sapphire substrate was determined. We observed that WSe2 grows epitaxially on both substrates employed in this work under all conditions studied while WS2 exhibits various preferred orientations on sapphire substtate which are temperature dependent. In contrast to the sapphire substrate, WS2 deposited on WSe2 exhibits only one preferred orientation which may provide a route to better control the orientation and crystal quality of WS2. In the case of epitaxial graphene on SiC, no graphene-related peaks were observed in in-plane XRD while its presence was confirmed using Raman spectroscopy. This demonstrates the limitation of the in-plane XRD technique for characterizing low electron density materials.