Confinement-Controlled Morphology and Stability of One-Dimensional CrI(3) Nanotubes.

Integrating monolayers derived from 2D van der Waals (vdW) magnetic materials into next-generation technological applications remains a significant challenge due to their structural and magnetic instability issues. Template-assisted encapsulation is a potential route for the growth of stable 2D monolayers aimed at designing novel 1D heterostructures, opening new avenues for studying low-dimensional quantum effects and spin-related phenomena. In this study, we explored the diameter-dependent encapsulation of 2D CrI crystals using multiwalled carbon nanotubes (MWCNTs) as nanoscale host templates. Advanced microscopic analysis revealed distinct structural transitions, ranging from internal nanorod encapsulation to external shell formation, directly influenced by the host nanotube diameter (2-20 nm). Furthermore, statistical analysis of structural morphologies indicates that CrI nanorods preferentially form within MWCNTs with inner diameters from 2 to 7 nm, while single-walled CrI nanotubes are stabilized in CNTs with diameters from 3 to 10 nm. For host CNTs exceeding ∼10 nm in diameter, CrI predominantly forms surface coatings rather than confined one-dimensional structures. In situ electron beam irradiation demonstrates the superior structural stability of single-walled CrI confined within MWCNTs, while externally coated CrI undergoes decomposition into metallic Cr clusters. Prolonged irradiation induces a morphological transformation of CrI nanotubes into nanorods. Encapsulated structures also remain stable under long-term ambient conditions, highlighting the dual protective role of CNT confinement. These insights lay the groundwork for engineering robust, tunable 1D magnetic heterostructures of CrI for spintronic and data storage applications.