Threefold Symmetrical Growth of MoS2 Homoepitaxy with Enhanced Valley Polarization
Tilo H. Yang1*, Hsiang-Chi Hu1, Ting-Hua Lu1, Yann-Wen Lan1
1Department of Physics, National Taiwan Normal University, Taipei City, Taiwan
* Presenter:Tilo H. Yang,
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