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

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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 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.
Funding Information
  • National Key Research and Development Program of China (2021YFA0717400)
  • Shenzhen Municipal Fundamental Research Program (JCYJ20220530115215035)
  • Guangdong Provincial Pearl River Talents Program (2021QN02C104)
  • National Natural Science Foundation of China (22205091, 22222504, 22475093, 92161124, 92461307)
  • State Key Laboratory for Modification of Chemical Fibers and Polymer Materials (KF2504)

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