Growing large-area, high-quality monolayers is the holy grail of graphene synthesis.In this work, the efficiency of graphene growth and the quality of their continuous films are explored through the time evolution of individual domains and their surface coverage on the substrate. Our phase-field modeling results and experimental characterization clearly demonstrate the critical roles of the deposition flux, edgereaction kinetics and the surface diffusion of active carbon sources in modulating the pattern evolution and rate of growth. We find that the contrast between the edgekinetics-limited and surface-diffusion-limited regimes is remarkable, which can be characterized by the evolution of domain patterns and considered as an indicator of the growth regime. However, common features exist in these two regimes, showing that the growth rate scales with time as t 2 in the early stage of growth and is regimeindependent, which is explained by the coarsen profiles of carbon concentration for both the compact and dendritic domains. The rate decays rapidly in the final stage of growth due to the competition between neighboring domains on the limited carbon sources diffusing on the substrate, which is highly regime-sensitive and extremely low in the surface-diffusion-limited regime with narrow gaps between the domains to be filled. Based on these findings, synthesis strategies to improve the growth efficiency and film quality are discussed.

The etching process can serve as an effective top-down approach that facilitates direct construction of tuned patterns, reversely studying growth mechanisms and further inducing unexpected physical properties. Currently, etching behaviors of monolayer transition-metal dichalcogenides (TMDs) have been rarely explored and the intrinsic etching mechanism still needs to be fully elucidated. Here, we demonstrate a facile and controllable oxygen-assisted anisotropic chemical etching of two-dimensional (2D) materials. Taking MoSe2 as an example, a series of well-defined etched patterns can be fabricated by precisely modulating the pretreatment time of oxygen plasma and the etching time. Atomic characterization exhibits that the edges of etched patterns are mainly zigzag-terminated. Density functional theory calculations highly agree with the experimental results, indicating an energy preference for zigzag edges. The as-produced MoSe2 flakes can be used as part of heterojunctions to display intriguing optoelectronic properties. The field-effect transistor based on as-etched MoSe2 flakes and the 2D poly­(3-hexylthiophene-2,5-diyl) crystal shows a distinct enhanced ambipolar photoresponse. Our strategy offers a new direction for preparing patterned 2D TMDs with desirable characteristics, opening the door for facilitating the development of optoelectronics.

Single-particle irradiation is a typical condition in space applications, which could be detrimental for electronic devices through processes such as single-event upset or latch-up. For functional devices made of few-atom-thick monolayers that are entirely exposed to the environment, the irradiation effects could be manifested through localized or delocalized electronic excitation, in addition to lattice defect creation. In this work, we explore the single-H irradiation effects on bare or coated graphene monolayers. Real-time time-dependent density functional theory-based first-principles calculation results elucidate the evolution of charge densities in the composite system, showing notable charge excitation but negligible charge deposition. A hexagonal boron nitride coating layer does not protect graphene from these processes. Principal component analysis demonstrates the dominance of localized excitation accompanied by nuclear motion, bond distortion and vibration, as well as a minor contribution from delocalized plasmonic excitation. The significance of coupled electron–ion dynamics in modulating the irradiation processes is identified from comparative studies on the spatial and temporal patterns of excitation for unconstrained and constrained lattices. The stopping power or energy deposition is also calculated, quantifying the dissipative nature of charge density excitation. This study offers fundamental understandings of the single-particle irradiation effects on optoelectronic devices constructed from low-dimensional materials, and inspires unconventional techniques to excite the electrons and ions in a controllable way.

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We demonstrate the structure evolution of hexagonal boron nitride (hBN) flakes grown on molten Cu in atmospheric pressure chemical vapor deposition by regulating the flux of precursor. We found that under lower precursor flux, tuned by temperature that controls the sublimation rates, the hBN grains change from triangle to truncated triangle shape with additional B-terminated edges, which could be understood through kinetic Wulff construction, while under higher flux, they form circular shape following deposition-controlled growth and predicted by a phase field modeling. In addition to the monolayer morphology from a single nucleation, adlayer patterns with centered aggregation and diffusive features at high precursor flux are observed and simulated by a two-dimensional (2D) diffusion-reaction model, where the random diffusion and deposition are revealed to be the dominating kinetics. The nucleation density and growth velocity could also be modulated by the ammonia borane heating temperature, where 80 °C is found to be optimal for the largest hBN grain size. Our transmission electron microscopy study shows that a misalignment of coalescing grains occurs on such molten Cu substrate, deviated from those observed on molten Au. Our results provide a new tool for the shape and grain size control of 2D materials and the understanding of their growth kinetics for large scale production.