From Self‐Assembly Hierarchical h‐BN Patterns to Centimeter‐Scale Uniform Monolayer h‐BN Film

Abstract

DOI: 10.1002/admi.201801493 Self-assembly into ordered patterns is ubiquitous in nature and involves a transition from disordered building blocks to ordered/crystalline assemblies.[1] The process allows the integration of individual components and contributes to pattern formation, ordered objects, and functional systems with efficiency and simplicity.[2,3] The ability to control pattern formation and understand the mechanism is of technological and scientific importance. For example, controlling the process of pattern formation has been extensively studied in several model systems, including snowflake formation,[4] metal aggregation on a substrate,[5] and colloidal nanocrystal growth.[6] However, owing to the many atomic-level complex processes involved, the formation of self-organized patterns at nanometer/micrometer length scales is very sensitive to various growth conditions, thus posing a great challenge for controllably tailoring the structures of patterned materials. In recent years, the fast development of 2D materials has extended their applications into numerous fields.[7–9] In the scale of 2D atomic layer, the assembly behavior is considered to be more obvious and further offers a direct way in probing properties and applications.[10,11] As a promising 2D candidate, hexagonal boron nitride (h-BN) has been gaining great interests in the past decade.[12–15] Being electrically insulating as well as chemically and thermally stable, atomically thin h-BN can be served as dielectric or substrate layers for other 2D materials such as graphene[8,14] and transition metal dichalcogenides (TMDs)[16] to improve the electrical performance. Further, by combining with other 2D materials, e.g., stacking or epitaxial growth, van der Waals’ heterostructure can be constructed for exploration of novel physical properties.[17–19] Given that h-BN crystals can be grown on a metal surface via a surface nucleation growth mechanism, a chemical vapor deposition (CVD) approach thus provides a platform to manipulate the shape and arrangement of these BN crystals.[20,21] It has shown that h-BN crystals exhibiting a variety of shapes, including triangle, hexagons, and six-sided polygons, can be formed by CVD under appropriate conditions.[22] However, the effort in selfassembly of individual h-BN crystals into hierarchical organized patterns with tunable periodicity (including the crystal size, spacing, orientation, and composition) and realization of a functional integrated system has met with very limited success. The self-assembly into highly ordered pattern is a universal phenomenon with unprecedented natural properties. However, the nonequilibrium processes in complex systems demand rigorous molecular formation mechanism, which are highly important for fundamental research. Herein, a large-scale formation of highly self-assembly hierarchical hexagonal boron nitride (h-BN) superordered structures on a liquid Cu surface by the chemical vapor deposition (CVD) method is demonstrated. The hierarchical h-BN superordered structure is found to be composed of two vertical stacking parts: one part is underneath h-BN film and the other part is top orientated branched h-BN patterns. In addition, the size, orientation, and morphology of the h-BN superordered structures can be precisely tuned by varying the gas flow rate and growth time. A kinetics-limited growth mechanism is proposed to elucidate the formation process, owing a well consistency with experimental results. Further, by mechanically peeling off the top few-layer h-BN, centimeter-scale uniform monolayer h-BN film is produced, demonstrating a direct and facile top-down fabrication method. This synthesis of highly ordered hierarchical h-BN patterns and following uniform monolayer h-BN film can be applied to other 2D materials, paving way for great potential in the investigation of growth mechanism and construction of homoand heterostructures.