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.