In Situ Closing the van der Waals Gap of Two-Dimensional Materials

Self-intercalation in two-dimensional (2D) materials, converting 2D materials into ultrathin covalently bonded materials, presents great possibilities for studying a new family of quantum-confined materials with the potential to realize multifunctional behavior. However, understanding the mechanisms and associated in situ kinetics of synthesizing self-intercalated 2D (ic-2D) materials, particularly at the atomic scale, remains elusive, greatly hindering the practical applications of ic-2D crystals. Here, we successfully in situ synthesized ic-2D thin films via thermal annealing of their parental TMDCs inside an electron microscope. We atomically visualized the evolution from TaS₂ and NbS₂ into the corresponding ic-2D Ta₁₊S₂ and ic-2D Nb_1+xS₂, respectively, by in situ scanning transmission electron microscopy (STEM). The self-intercalation process in TaS₂ is atomically realized by metal adatom edge adsorption and subsequent diffusion in an atom-by-atom manner. On the other hand, MoS₂ and MoSe₂ tend to coalesce into metal crystals under the same annealing conditions, suggesting that the self-intercalation process is predominantly controlled by thermodynamic factors as further verified by density functional theory (DFT). By varying the ramping rate and annealing temperature, the coverage and spatial arrangement of the filling sites can be precisely tuned, ranging from 2a × $$a, $$a × $$a, or Ta trimers, as predominantly gauged by kinetic factors. Our work sheds light on the thermodynamics and growth kinetics involved in ic-2D formation and paves the way for growing highly crystalline ic-2D materials with intercalation concentration and topology-dependent properties.