Zelewski Szymon J, Kudrawiec Robert
Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370, Wrocław, Poland.
Sci Rep. 2017 Nov 13;7(1):15365. doi: 10.1038/s41598-017-15763-1.
Photoacoustic (PA) and modulated reflectance (MR) spectroscopy have been applied to study the indirect and direct band gap for van der Waals (vdW) crystals: dichalcogenides (MoS, MoSe, MoTe, HfS, HfSe, WS, WSe, ReS, ReSe, SnS and SnSe) and monochalcogenides (GaS, GaSe, InSe, GeS, and GeSe). It is shown that the indirect band gap can be determined by PA technique while the direct band gap can be probed by MR spectroscopy which is not sensitive to indirect optical transitions. By measuring PA and MR spectra for a given compound and comparing them with each other it is easy to conclude about the band gap character in the investigated compound and the energy difference between indirect and direct band gap. In this work such measurements, comparisons, and analyses have been performed and chemical trends in variation of indirect and direct band gap with the change in atom sizes have been discussed for proper sets of vdW crystals. It is shown that both indirect and direct band gap in vdW crystals follow the well-known chemical trends in semiconductor compounds.
光声（PA）和调制反射率（MR）光谱已被用于研究范德华（vdW）晶体的间接和直接带隙：二硫族化物（MoS、MoSe、MoTe、HfS、HfSe、WS、WSe、ReS、ReSe、SnS和SnSe）以及单硫族化物（GaS、GaSe、InSe、GeS和GeSe）。结果表明，间接带隙可以通过PA技术测定，而直接带隙可以通过对间接光学跃迁不敏感的MR光谱探测。通过测量给定化合物的PA和MR光谱并相互比较，很容易得出所研究化合物的带隙特性以及间接和直接带隙之间的能量差。在这项工作中，已经进行了这样的测量、比较和分析，并针对适当的vdW晶体组讨论了间接和直接带隙随原子尺寸变化的化学趋势。结果表明，vdW晶体中的间接和直接带隙都遵循半导体化合物中众所周知的化学趋势。
