Xin Hang, Zhang Jingyun, Yang Cuihong, Chen Yunyun
School of Physics & Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China.
Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing 210044, China.
Nanomaterials (Basel). 2022 Jan 27;12(3):414. doi: 10.3390/nano12030414.
It is known that complex growth environments often induce inhomogeneity in two-dimensional (2D) materials and significantly restrict their applications. In this paper, we proposed an efficient method to analyze the inhomogeneity of 2D materials by combination of Raman spectroscopy and unsupervised k-means clustering analysis. Taking advantage of k-means analysis, it can provide not only the characteristic Raman spectrum for each cluster but also the cluster spatial maps. It has been demonstrated that inhomogeneities and their spatial distributions are simultaneously revealed in all CVD-grown MoS, WS and WSe samples. Uniform p-type doping and varied tensile strain were found in polycrystalline monolayer MoS from the grain boundary and edges to the grain center (single crystal). The bilayer MoS with AA and AB stacking are shown to have relatively uniform p-doping but a gradual increase of compressive strain from center to the periphery. Irregular distribution of 2LA(M)/E2g1 mode in WS and E2g1 mode in WSe is revealed due to defect and strain, respectively. All the inhomogeneity could be directly characterized in color-coded Raman imaging with correlated characteristic spectra. Moreover, the influence of strain and doping in the MoS can be well decoupled and be spatially verified by correlating with the clustered maps. Our k-means clustering Raman analysis can dramatically simplify the inhomogeneity analysis for large Raman data in 2D materials, paving the way towards direct evaluation for high quality 2D materials.
众所周知，复杂的生长环境常常会在二维（2D）材料中诱导出不均匀性，并显著限制其应用。在本文中，我们提出了一种将拉曼光谱与无监督k均值聚类分析相结合的有效方法来分析二维材料的不均匀性。利用k均值分析，它不仅可以为每个聚类提供特征拉曼光谱，还可以提供聚类空间图。结果表明，在所有化学气相沉积（CVD）生长的MoS、WS和WSe样品中，不均匀性及其空间分布都能同时被揭示出来。在多晶单层MoS中，从晶界和边缘到晶粒中心（单晶）发现了均匀的p型掺杂和变化的拉伸应变。具有AA和AB堆叠的双层MoS显示出相对均匀的p型掺杂，但从中心到周边的压缩应变逐渐增加。分别由于缺陷和应变，揭示了WS中2LA（M）/E2g1模式和WSe中E2g1模式的不规则分布。所有的不均匀性都可以在带有相关特征光谱的彩色编码拉曼成像中直接表征。此外，通过与聚类图相关联，可以很好地解耦MoS中应变和掺杂的影响，并在空间上进行验证。我们的k均值聚类拉曼分析可以极大地简化二维材料中大量拉曼数据的不均匀性分析，为高质量二维材料的直接评估铺平道路。
