Self-feeding formation of atomically thin molybdenum nanoflakes on MoS₂ monolayer

The decoration of atomically thin metal flakes on two-dimensional (2D) material membranes to impart intriguing properties for applications such as sensors and catalysis has attracted tremendous interest. Here, we report the formation of atomically thin Mo nanoflakes on a molybdenum disulfide monolayer (ML-MoS₂) via a 'self-feeding' process using in situ transmission electron microscopy. Driven by energetic e-beam irradiation and thermal excitation, metallic Mo atoms preferentially segregate out and aggregate around mirror twin boundaries in the host MoS₂ ML, which then assemble into metallic nanoflakes: the associated dynamic process captured at the atomic scale. The Mo atoms constituting the nanoflakes tend to sit on the Mo-top and S-top sites if they are viewed as absorbed atoms with respect to the ML-MoS₂ substrate, which is further confirmed by theoretical calculations. Density functional theory calculations reveal that the entire system, i.e. metallic Mo nanoflakes sitting on the ML-MoS₂ structure, exhibits overall metallic behavior, thus offering an efficient way to tune the electronic properties of the host MoS₂. Such controlled fabrication can also enrich the 2D material family and provide new opportunities for exploiting their applications in catalytic and sensing devices. (paper)