Thermodynamics and Kinetics Synergy for Controlled Synthesis of 2D van der Waals Single-Crystal NbSe₂ via Modified Chemical Vapor Transport

Two-dimensional van der Waals (vdW) materials possess novel physical properties and promising applications. Despite a wide range of 2D vdW materials having been obtained via the chemical vapor transport (CVT) method, the understanding of the growth mechanism has not grown at the same pace as the lack of investigation into the quality control. In this work, we develop the controllable growth of 2H-NbSe₂ single crystals via the temperature-oscillating CVT method. A thermodynamic calculation was done to investigate the chemical equilibrium of the NbSe₂ + iodine mixture. A kinetic process, operated by temperature oscillation in a two-zone furnace, was used to establish the supersaturation-dependent growth. Screw-dislocated crystals as well as the homogeneous hexagon crystals were obtained under different cooling rates during oscillation. The quality of fabricated crystals was characterized by X-ray diffraction, high-resolution transmission electron microscopy, and superconductivity measurements. Remarkably, crystals of the best quality were successfully grown under high supersaturation and low growth temperature (600/570 °C). This work provides a generally applicable method for optimizing CVT growth and offers high-quality crystals for studying intrinsic physical properties and fabricating 2D electronic devices.