The synthesis of high-quality ultrathin overlayers is critically dependent on the surface structure of substrates, especially involving the overlayer–substrate interaction. By using in situ surface measurements, we demonstrate that the overlayer–substrate interaction can be tuned by doping near-surface Ar nanobubbles. The interfacial coupling strength significantly decreases with near-surface Ar nanobubbles, accompanying by an “anisotropic to isotropic” growth transformation. On the substrate containing near-surface Ar, the growth front crosses entire surface atomic steps in both uphill and downhill directions with no difference, and thus, the morphology of the two-dimensional (2D) overlayer exhibits a round-shape. Especially, the round-shaped 2D overlayers coalesce seamlessly with a growth acceleration in the approaching direction, which is barely observed in the synthesis of 2D materials. This can be attributed to the immigration lifetime and diffusion rate of growth species, which depends on the overlayer–substrate interaction and the surface catalysis. Furthermore, the “round to hexagon” morphological transition is achieved by etching-regrowth, revealing the inherent growth kinetics under quasi-freestanding conditions. These findings provide a novel promising way to modulate the growth, coalescence, and etching dynamics of 2D materials on solid surfaces by adjusting the strength of overlayer–substrate interaction, which contributes to optimization of large-scale production of 2D material crystals.

The lateral incorporation of graphene and hexagonal boron nitride (h-BN) onto a substrate surface creates in-plane h-BN/graphene heterostructures, which have promising applications in novel two-dimensional electronic and photoelectronic devices. The quality of h-BN/graphene domain boundaries depends on their orientation, which is crucial for device performances. Here, the heteroepitaxial growth of graphene along the edges of h-BN domains on Ni(111) surfaces as well as the growth dynamics of h-BN using chemical vapor deposition (CVD) are in situ investigated by surface imaging measurements. The nucleating seed effect of h-BN has been revealed, which contributes to the single orientation of heterostructures with epitaxial stitching. Further, the growth of h-BN prior to that of graphene is essential to obtain high-quality in-plane h-BN/graphene heterostructures on Ni(111). The "compact to fractal" shape transition of h-BN domains appears with the increasing surface concentration of the growth blocks, suggesting that the dynamic growth mechanism follows diffusion-limited aggregation (DLA) but not reaction-limited aggregation (RLA). Our results provide insights into the synthesis of well-defined h-BN/graphene heterostructures and deep understanding of the growth dynamics of h-BN on metal surfaces.

Fundamental understanding of chemistry confined to nanospace remains a challenge since molecules encapsulated in confined microenvironments are difficult to be characterized. Here, we show that CO adsorption on Pt(111) confined under monolayer hexagonal boron nitride (h-BN) can be dynamically imaged using near ambient pressure scanning tunneling microscope (NAP-STM) and thanks to tunneling transparency of the top h-BN layer. The observed CO superstructures on Pt(111) in different CO atmospheres allow to derive surface coverages of CO adlayers, which are higher in the confined nanospace between h-BN and Pt(111) than those on the open Pt surface under the same conditions. Dynamic NAP-STM imaging data together with theoretical calculations confirm confinement-induced molecule enrichment effect within the 2D nanospace, which reveals new chemistry aroused by the confined nanoreactor.