Low-Temperature Synergistic Molten Salt-Assisted Growth of Ultrathin WO ₂ Nanoflakes with Anisotropic Optical and Transport Properties

Two-dimensional (2D) nonlayered transition metal oxides present unique electronic, magnetic, optical, and catalytic properties but face significant challenges in controllable synthesis due to their intrinsic 3D bonding networks and typically ultrahigh synthesis temperatures. Herein, we demonstrate the controlled synthesis of monoclinic WO 2 nanoflakes with high crystallinity via a molten-salt-assisted chemical vapor deposition method at a substantially reduced temperature (down to 780 °C), which is ∼400 °C lower than the lowest reported temperature for vapor-phase synthesis. This strategy not only lowers the kinetic barrier but also promotes a high nucleation density, yielding samples with exceptional signal intensities for structural characterization. The as-grown WO 2 nanoflakes exhibit well-defined parallelogram shapes with record-small thickness down to ∼5 nm and lateral dimensions exceeding 100 μm, showcasing excellent crystallinity, phase purity, and good ambient stability. Angle-resolved polarized Raman spectroscopy unequivocally confirms the monoclinic crystal structure, high crystallinity, and anisotropic phonon polarization of the WO 2 nanoflakes. Cryogenic magneto-transport measurements reveal a low-temperature negative magnetoresistance (MR), along with an anisotropic MR response, which is attributed to extrinsic magnetic scattering. Thickness-dependent transport measurements further show the sensitivity of the WO 2 transport to the flake thickness. This work not only establishes a robust pathway for synthesizing crystalline 2D WO 2 but also unveils its intriguing structural and magneto-transport properties, highlighting its potential for fundamental studies and applications in nanoelectronics and spintronics.