Growth of Atomically Thin Metastable β-Tungsten in Single-Walled Carbon Nanotubes for Stable One-Dimensional Ferromagnets

Thin-film β tungsten (β-W), a metastable phase of tungsten, holds significant potential in the fabrication of superconducting and spin-memory devices. However, due to the rapid surface passivation of tungsten in oxygen and moisture, the synthesis of nanosized metastable β-W with the intrinsic atomic surface is still difficult, and their magnetic properties remain rather unexplored. Inspired by the strong host-guest interaction-induced stabilization, we reported the synthesis of atomically thin (1.0-1.3 nm) metastable β-W nanowires within single-walled carbon nanotubes (SWCNTs) through an oxygen-assisted transformation of starting W₂C, with 85% of β-W nanowires along the anisotropic ⟨010⟩ direction. Atomically resolved electron microscopy directly unveils the dynamic evolutions of W₂C-to-β-W and further β-to-α-W within SWCNTs, depending on the H₂-annealing time. Detailed mechanistic studies by theoretical calculations and experiments reveal that oxygen diffused within the W₂C lattice governs the formation and stabilization of ultrathin β-W nanowires within the SWCNTs. Additionally, the nanoconfinement of SWCNTs, restricting the thickness of W nanowires down to 2 nm, also benefits the thermodynamically favorable nucleation of β-W than α-W. With the protection of a single graphene layer against water erosion, β-W@SWCNTs exhibit a ferromagnetic response at ∼130 K, with higher chemical stability than fully exposed thin-film β-W. This work may provide a feasible way to design the ferromagnetic nanowire metamaterials based on aligned SWCNT arrays that have the potential to fabricate microwave and spin devices.