Theoretical and Experimental Insight into the Mechanism for Spontaneous Vertical Growth of ReS₂ Nanosheets

Rhenium disulfide (ReS₂) differs fundamentally from other group-VI transition metal dichalcogenides (TMDs) due to its low structural symmetry, which results in its optical and electrical anisotropy. Although vertical growth is observed in some TMDs under special growth conditions, vertical growth in ReS₂ is very different in that it is highly spontaneous and substrate-independent. In this study, the mechanism that underpins the thermodynamically favorable vertical growth mode of ReS₂ is uncovered. It is found that the governing mechanism for ReS₂ growth involves two distinct stages. In the first stage, ReS₂ grows parallel to the growth substrate, consistent with conventional TMD growth. However, subsequent vertical growth is nucleated at points on the lattice where Re atoms are “pinched” together. At such sites, an additional Re atom binds with the cluster of pinched Re atoms, leaving an under-coordinated S atom protruding out of the ReS₂ plane. This under-coordinated S is “reactive” and binds to free Re and S atoms, initiating growth in a direction perpendicular to the ReS₂ surface. The utility of such vertical ReS₂ arrays in applications where high surface-to-volume ratio and electric-field enhancement are essential, such as surface enhanced Raman spectroscopy, field emission, and solar-based disinfection of bacteria, is demonstrated.