Study on Growth Mechanism of Atomically Thin WS2 Crystals and their Characterization

Abstract

Recently, two dimensional atomic-layered transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have been intensively studied in the last decades due to their semiconducting electronic band structure unlike a metallic graphene. The single-layered TMDCs are very attractive materials for photonic and optoelectronic applications because it has outstanding optical properties including the high quantum yield and the large exciton binding energy reaching up to 1 eV, which are absent in the bulk TMDCs. However, the understanding of nucleation and growth mechanism to realize the large-scale singe-layered TMDCs is still lacking. In this paper, we have studied the two series of single-layered WS2 crystals which varies the growth temperature and time by using thermal chemical vapor deposition (CVD) method. The powder of sulfur (S) and WO3 was used as the source and PTAS (Perylen-3, 4, 9, 10-tetracaboxylic acid tetrapotassium salt) was introduced as the promoter for the CVD growth. The as-grown WS2 crystals show the single layer domain size of ~ 20 m and the good optical/structural properties. Using the transmission electron microscopy (TEM), atomic force microscopy (AFM), and Raman scattering analyses, we confirmed the nucleation site is formed by the thick single-crystal WS2 which is not an intermediate form. We demonstrate that growth mode is gradually changed from 3D to 2D as the growth temperature increase 650 ~ 750 ℃ to 800 ~ 850 ℃, indicating the 3D nucleation site is early formed at initial stage and sufficient heat energy is required to facilitate the well-expandable the 2D mode after nucleation. Meanwhile, the ratio of domain- to core-size decreases and the optical property degrades as the growth time increases. These results clearly show that bulk phase of WS2 has thermodynamical stability compared to 2D phase and source deficient condition is reached with increasing the growth time. Our results offer new insight into the formation of 3D nucleation site in initial growth step and the presence of limited length-scale for the domain size in the growth of single-layered TMDCs. ⓒ 2015 DGIST