In Situ\nClosing the van der Waals Gap of Two-Dimensional\nMaterials

Self-intercalation\nin two-dimensional (2D) materials, converting\n2D materials into ultrathin covalently bonded materials, presents\ngreat possibilities for studying a new family of quantum-confined\nmaterials with the potential to realize multifunctional behavior.\nHowever, understanding the mechanisms and associated in situ kinetics\nof synthesizing self-intercalated 2D (ic-2D) materials, particularly\nat the atomic scale, remains elusive, greatly hindering the practical\napplications of ic-2D crystals. Here, we successfully in situ synthesized\nic-2D thin films via thermal annealing of their parental TMDCs inside\nan electron microscope. We atomically visualized the evolution from\nTaS<sub>2</sub> and NbS<sub>2</sub> into the corresponding ic-2D Ta<sub>1+<i>x</i></sub>S<sub>2</sub> and ic-2D Nb<sub>1+x</sub>S<sub>2</sub>, respectively, by in situ scanning transmission electron\nmicroscopy (STEM). The self-intercalation process in TaS<sub>2</sub> is atomically realized by metal adatom edge adsorption and subsequent\ndiffusion in an atom-by-atom manner. On the other hand, MoS<sub>2</sub> and MoSe<sub>2</sub> tend to coalesce into metal crystals under\nthe same annealing conditions, suggesting that the self-intercalation\nprocess is predominantly controlled by thermodynamic factors as further\nverified by density functional theory (DFT). By varying the ramping\nrate and annealing temperature, the coverage and spatial arrangement\nof the filling sites can be precisely tuned, ranging from 2<i>a</i> × 3<i>a</i>, 3<i>a</i> × 3<i>a</i>, or Ta trimers, as predominantly\ngauged by kinetic factors. Our work sheds light on the thermodynamics\nand growth kinetics involved in ic-2D formation and paves the way\nfor growing highly crystalline ic-2D materials with intercalation\nconcentration and topology-dependent properties.