Atomic-Scale Visualization of Spontaneous Bilayer MoS

Transition metal dichalcogenide (TMD) nanotubes, particularly those of molybdenum disulfide (MoS2), exhibit exceptional optoelectronic, superconducting, and mechanical properties, rendering them highly promising for diverse advanced applications. Despite this potential, the controlled synthesis of small-diameter, single-walled TMD nanotubes, especially MoS2, remains a significant challenge, primarily due to their high bending rigidity. Here, we provide direct atomic-scale evidence for the spontaneous transformation of bilayer MoS2 nanoribbons into fully closed single-walled nanotubes within the confined environment of carbon nanotubes (CNTs). By employing atomic-scale high-resolution transmission electron microscopy (HRTEM), nanobeam electron diffraction (NBED), annular dark-field scanning transmission electron microscopy (ADF-STEM), and electron energy-loss spectroscopy (EELS), we identify a cooperative mechanism involving simultaneous axial rotation and out-of-plane bending of the bilayer nanoribbons. This coupled motion facilitates seamless edge reconstruction, ultimately leading to the formation of defect-free, fully closed single-walled MoS2 nanotubes. Quantitative geometric reconstruction and strain analyses, supported by density functional theory (DFT) calculations, reveal that although the transformation induces radial, axial, and shear strains (dominated by radial bending strain), spontaneous interlayer Mo-S edge bonding provides crucial energetic stabilization for the resulting tubular structure. Statistical analysis of multiple transformation events confirms the necessity of this coupled mechanism, which inherently generates interlayer twist and results in exclusively chiral nanotubes, predominantly with diameters between 4.1 and 4.9 nm. Our findings elucidate a deterministic, self-limiting transformation pathway driven by coupled axial rotation and bending, providing a strategy for potentially controlling the structure (chirality and diameter) of quasi-one-dimensional TMD nanotubes for future nanoelectronics, photonics, and energy applications.