Islands Size-Controlled Phase Transition in MBE-Grown Two-Dimensional NbSe₂

Single layer (SL) NbSe₂ has recently attracted intensive research interest because of its unique phase-dependent electronic properties, thus it is important to control the phases in the growth. In molecular beam epitaxy (MBE) growth of NbSe₂, 1T and 2H phases can be synthesized by controlling the substrate temperature. However, other approaches to achieve phase control and an in-depth understanding of the mechanism are still lacking. Here, we report the realization of the phase transition from 1T to 2H phase in SL NbSe₂ by controlling the island size in MBE. Scanning tunneling spectroscopy images exhibit that SL NbSe₂ islands experienced three growth stages: nucleation of small 1T islands, coalescence and phase transition, and all large 2H islands. Statistical analysis and theoretical calculation both demonstrate that the mechanism of the phase transition is edge energy-controlled size effect (size-effect for short). Our results provide a feasible method to get large and compact 2H SL NbSe₂ islands instead of the fractal ones, deepening the understanding of the phase transition mechanism of SL NbSe₂, and may extend to various transition metal dichalcogenides (TMD) by MBE.