State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China.
Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences, Shenyang 110016, P. R. China.
Nanoscale. 2019 Dec 21;11(47):22762-22771. doi: 10.1039/c9nr04270a. Epub 2019 Sep 4.
Two-dimensional (2D) materials usually exhibit interesting layer-dependent dielectric and optical properties, which play important roles in structure optimization and performance improvement of related devices. Recently, 2D WSe has attracted considerable attention in atomically thin electronics and optoelectronics, due to its exotic photoelectric properties. In this paper, high-quality, continuous, and centimeter-scale 2D WSe with different layers on a sapphire substrate are prepared by an ultrafast ambient-pressure chemical vapor deposition method. We comprehensively investigate the evolution of the layer-dependent dielectric and optical properties of 2D WSe from a single layer to five layers by spectroscopic ellipsometry over an ultra-broad energy range (0.73-6.42 eV). We identify the critical points (CPs) in the dielectric function spectra of 2D WSe with different layers, and reveal physical origins of the corresponding optical transitions at these CPs by the CP analysis method and first-principles calculations. Results demonstrate that the center energies of these CPs exhibit intriguing layer-dependencies, which can be interpreted as the alternative domination of the decreasing exciton binding energy and the striking band renormalization. For the first time, we found that the imaginary part of the dielectric function of WSe at these CPs exhibits a valley-like shape versus the layer number, and the bottom appears at 3-layers. This non-monotonic evolution is explained as a competition between the layer-dependent decrease of the exciton effect and the layer-dependent increase of the joint density of states.
二维（2D）材料通常表现出有趣的层依赖介电和光学性质，这些性质在相关器件的结构优化和性能提升中起着重要作用。最近，二维 WSe 因其奇特的光电性质，在原子薄电子学和光电子学中引起了相当大的关注。在本文中，我们采用超快常压化学气相沉积方法在蓝宝石衬底上制备了高质量、连续、厘米级的不同层数的二维 WSe。我们通过光谱椭圆偏振术在超宽能量范围（0.73-6.42 eV）内全面研究了从单层到五层的二维 WSe 层依赖介电和光学性质的演变。我们确定了不同层数的二维 WSe 介电函数谱中的临界点（CPs），并通过 CP 分析方法和第一性原理计算揭示了相应光学跃迁的物理起源。结果表明，这些 CPs 的中心能量表现出有趣的层依赖性，可以解释为激子结合能的减小和能带重整化的显著影响的交替主导。我们首次发现，这些 CPs 处 WSe 的介电函数虚部的层数依赖性呈现出类似于山谷的形状，谷底出现在 3 层。这种非单调演化可以解释为激子效应的层依赖性减小和联合态密度的层依赖性增加之间的竞争。
