Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan. Graduate Institute of Electro-Optical Engineering and Department of Electronic and Computer Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan.
Nanotechnology. 2017 Jun 9;28(23):235203. doi: 10.1088/1361-6528/aa6f51.
ReS and ReSe have recently been enthusiastically studied owing to the specific in-plane electrical, optical and structural anisotropy caused by their distorted one-layer trigonal (1 T) phase, whereas other traditional transition-metal dichalcogenides (TMDCs, e.g. MoS and WSe) have a hexagonal structure. Because of this special property, more and versatile nano-electronics and nano-optoelectronics devices can be developed. In this work, 2D materials in the series ReS Se (0 ≤ x ≤ 2) have been successfully grown by the method of chemical vapor transport. The direct and indirect resonant emissions of the complete series of layers can be simultaneously detected by polarized micro-photoluminescence (μPL) spectroscopy when the thickness of the ReS Se is greater than ∼70 nm. When it is less than 70 nm, only three direct excitonic emissions-E , E and E -are detected. For the thick (bulk) ReS Se , more stacking of the ReX monolayers even flattens and shifts the valence-band maximum from Γ to the other K- or M-related points, thus leading to the coexistence of direct and indirect resonant light emissions from the c-plane ReX. The transmittance absorption edge of each bulk ReX (a few microns thick) usually has a lower energy than those of the direct E and E excitonic emissions to form indirect absorption. The coexistence of direct and indirect emissions in ReX is a unique characteristic of a 2D layered semiconductor possessing triclinic low symmetry.
由于其扭曲的单层三角（1T）相引起的特殊面内电学、光学和结构各向异性，ReS 和 ReSe 最近受到了热烈研究，而其他传统的过渡金属二卤代物（TMDCs，例如 MoS 和 WSe）具有六方结构。由于这种特殊性质，可以开发更多种类的纳米电子学和纳米光电学器件。在这项工作中，通过化学气相输运法成功地生长了 ReSSe 系列的二维材料（0≤x≤2）。当 ReSSe 的厚度大于约 70nm 时，通过偏振微光电致发光（μPL）光谱可以同时检测到完整系列层的直接和间接共振发射。当厚度小于 70nm 时，仅检测到三个直接激子发射-E1、E2 和 E3。对于厚（体）ReSSe，ReX 单层的更多堆积甚至会使价带最大值从 Γ 平坦化并转移到其他 K 或 M 相关点，从而导致 c 平面 ReX 的直接和间接共振光发射共存。每个体 ReX（几微米厚）的透射吸收边缘通常具有比直接 E1 和 E2 激子发射更低的能量，从而形成间接吸收。在具有三斜低对称性的二维层状半导体中，直接和间接发射的共存是其独特特征。
