Growth of Atomically Thin Metastable β-Tungsten in Single-Walled Carbon Nanotubes for Stable One-Dimensional Ferromagnets
Xin Zhao, Kun Wang, Bowen Li, Quan Xiao, Meihui Song, Wu Wang, Luyao Zhang, Fenfa Yao, Boyuan Yu, Yingbo Li, Xiao Wang, Shu Guo, Chuanhong Jin, Jiaqing He, Feng Yang
Abstract
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 W 2 C, with 85% of β-W nanowires along the anisotropic ⟨010⟩ direction. Atomically resolved electron microscopy directly unveils the dynamic evolutions of W 2 C-to-β-W and further β-to-α - W within SWCNTs, depending on the H 2 -annealing time. Detailed mechanistic studies by theoretical calculations and experiments reveal that oxygen diffused within the W 2 C 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.