Li Da-Hai, Zheng Hua, Wang Zi-Yi, Zhang Rong-Jun, Zhang Hao, Zheng Yu-Xiang, Wang Song-You, Zhang David Wei, Chen Liang-Yao
Key Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China.
Phys Chem Chem Phys. 2017 May 17;19(19):12022-12031. doi: 10.1039/c7cp00660h.
Centimeter-scale WS ultrathin films were synthesized on sapphire substrates, and they showed highly oriented crystallographic growth along the c axis. Afterwards, the as-grown samples were systematically characterized using various detection methods. Reliable values of the roughness layer thickness and the film thickness were extracted using both atomic force microscopy (AFM) and spectroscopic ellipsometry (SE), and identified using Raman spectroscopy as well. The expansion and tensile strain along the [001] direction were discovered using X-ray diffraction (XRD) measurements. Accurate dielectric functions of WS films were derived from the point-by-point fitting results. The critical points (CPs) of WS, which have not been reported so far, are precisely extracted from the standard critical point (SCP) model. Their origins are uniquely assigned to different interband electronic transitions in the Brillouin zone, including some novel optical structures above 3 eV, which were not investigated in earlier studies. In this work, it is found that dielectric functions are thickness-dependent, while CPs have an opposite nature, and their intrinsic mechanisms are revealed. The as-obtained results can be expected to help people develop more extensive applications of WS.
在蓝宝石衬底上合成了厘米级的WS超薄膜，它们沿c轴呈现出高度取向的晶体生长。之后，使用各种检测方法对生长后的样品进行了系统表征。利用原子力显微镜（AFM）和光谱椭偏仪（SE）提取了粗糙度层厚度和薄膜厚度的可靠值，并通过拉曼光谱进行了确认。利用X射线衍射（XRD）测量发现了沿[001]方向的膨胀和拉伸应变。WS薄膜的精确介电函数是从逐点拟合结果中得出的。从标准临界点（SCP）模型中精确提取了迄今为止尚未报道的WS的临界点（CPs）。它们的起源被唯一地归因于布里渊区中不同的带间电子跃迁，包括一些高于3 eV的新型光学结构，这些在早期研究中并未被研究。在这项工作中，发现介电函数与厚度有关，而CPs具有相反的性质，并揭示了它们的内在机制。所获得的结果有望帮助人们开发WS更广泛的应用。
