Threefold Symmetrical Growth of MoS2 Homoepitaxy with Enhanced Valley Polarization

Tilo H. Yang^1*, Hsiang-Chi Hu¹, Ting-Hua Lu¹, Yann-Wen Lan¹

¹Department of Physics, National Taiwan Normal University, Taipei City, Taiwan

* Presenter:Tilo H. Yang, email:hwyang1105@gmail.com

Controllable growth of highly crystalline transition metal dichalcogenide (TMD) patterns with predictable morphology and stacking configuration is crucially important for future optovalleytronic devices. Especially, rhombohedral stacking, the so-called 3R stacking TMDs are inversion symmetry breaking and can be considered as stacked monolayers, holding great potential in beyond-monolayer materials. Here we report single-crystalline MoS₂ with regular threefold symmetric bilayer patterns, which can be homoepitaxially grown on MoS₂ monolayer via chemical vapor deposition. The 3-fold symmetrical billayer patterns start from the 6-fold growth of nanoribbon from the initial bilayer nucleus, to self-aligned unidirectional ribbons, and finally to coalescence into triangle, implying the growth is a kinetic-dominated process and the growth of bilayer nanoribbons follow the preferential diffusion directions for MoS₂ adatoms. The fan-like MoS₂ growth can be explained by the distinct diffusion barriers of surface diffusion and edge diffusion under high molybdenum chemical potential. As confirmed by STEM investigation, the fan-shaped MoS₂ patterns are 3R-stacked with the underlying MoS₂ monolayer, and their boundaries are predominantly terminated by zigzag Mo edge structure. The fan-shaped MoS₂ pattern demonstrates strong degree of circular polarization around 40% at room temperature and ~82% at 77K, which exceeds that of monolayer MoS₂. Our homoepitaxial growth approach for making 3-fold symmetrical TMD epi-patterns is also demonstrated either for hexagonal MoS₂ first layer or for growth on sapphire.

Keywords: epitaxial growth, nanoribbon, 2D materials, transition metal dichalcogenides, circular polarization