Unresolved problems associated with the production of graphene materials include the need for greater control over layer number, crystallinity, size, edge structure and spatial orientation, and a better understanding of the underlying mechanisms. Here we report a chemical vapor deposition approach that allows the direct synthesis of uniform single-layered, large-size (up to 10,000 μm 2 ), spatially self-aligned, and single-crystalline hexagonal graphene flakes (HGFs) and their continuous films on liquid Cu surfaces. Employing a liquid Cu surface completely eliminates the grain boundaries in solid polycrystalline Cu, resulting in a uniform nucleation distribution and low graphene nucleation density, but also enables self-assembly of HGFs into compact and ordered structures. These HGFs show an average two-dimensional resistivity of 609 AE 200 Ω and saturation current density of 0.96 AE 0.15 mA∕μm, demonstrating their good conductivity and capability for carrying high current density.atomic crystal | electronic materials G raphene has attracted considerable attention because of its extraordinary physical properties and potential electronic and spintronic applications (1-3). It is critical to find ways of precisely controlling the graphene layer number (4-6), crystallinity, size, edge structure, and even spatial orientation. The chemical vapor deposition (CVD) approach is a powerful and cost-effective technique for the production of high-quality and large-scale graphene films. In spite of the complexity of CVD procedures involving different catalysts, carbon sources, and other variables, the physical principles underlying this method are relatively simple. It is widely accepted that CVD mainly involves either surface catalytic reaction (7,8) or bulk carbon precipitation onto the surface during cooling (9, 10) for catalysts with low-carbon and high-carbon solubility, respectively. In both cases, graphene nucleation on a catalyst surface is one of the critical steps in the growth process. Various factors affect the initiation of the graphene nucleation process, including the type (11, 12) or surface microstructure of the catalyst, carbon source (13), carbon segregation from metal-carbon melts (14), processing history, and parameters in CVD growth (15)(16)(17). In general, nucleation densities on substrates such as Cu or Ni are nonuniform. This nonuniformity causes a large dispersion of both nucleus density and size distribution of graphene, representing a general problem in graphene CVD growth systems.It has been found that low-pressure CVD synthesis of graphene on Cu foil provides a good way of fabricating uniform single-layer graphene films (7). Studies have shown that the continuous films were formed by connecting randomly oriented, irregular-shaped, and micrometer-sized graphene flakes, resulting in the presence of a large amount of both low-and high-angle grain boundaries composed of pentagons and heptagons, which leads to a dramatic degradation in electronic properties compared with those of pristine graphene (7...

Hybrid organic-inorganic perovskite materials have received substantial research attention due to their impressively high performance in photovoltaic devices. As one of the oldest functional materials, it is intriguing to explore the optoelectronic properties in perovskite after reducing it into a few atomic layers in which two-dimensional (2D) confinement may get involved. In this work, we report a combined solution process and vapor-phase conversion method to synthesize 2D hybrid organic-inorganic perovskite (i.e., CH3NH3PbI3) nanocrystals as thin as a single unit cell (∼1.3 nm). High-quality 2D perovskite crystals have triangle and hexagonal shapes, exhibiting tunable photoluminescence while the thickness or composition is changed. Due to the high quantum efficiency and excellent photoelectric properties in 2D perovskites, a high-performance photodetector was demonstrated, in which the current can be enhanced significantly by shining 405 and 532 nm lasers, showing photoresponsivities of 22 and 12 AW(-1) with a voltage bias of 1 V, respectively. The excellent optoelectronic properties make 2D perovskites building blocks to construct 2D heterostructures for wider optoelectronic applications.

Vertical heterojunctions of two two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention recently. A variety of heterojunctions can be constructed by stacking different TMDs to form fundamental building blocks in different optoelectronic devices such as photodetectors, solar cells, and light-emitting diodes. However, these applications are significantly hampered by the challenges of large-scale production of van der Waals stacks of atomically thin materials. Here, we demonstrate scalable production of periodic patterns of few-layer WS2, MoS2, and their vertical heterojunction arrays by a thermal reduction sulfurization process. In this method, a two-step chemical vapor deposition approach was developed to effectively prevent the phase mixing of TMDs in an unpredicted manner, thus affording a well-defined interface between WS2 and MoS2 in the vertical dimension. As a result, large-scale, periodic arrays of few-layer WS2, MoS2, and their vertical heterojunctions can be produced with desired size and density. Photodetectors based on the as-produced MoS2/WS2 vertical heterojunction arrays were fabricated, and a high photoresponsivity of 2.3 A·W(-1) at an excitation wavelength of 450 nm was demonstrated. Flexible photodetector devices using MoS2/WS2 heterojunction arrays were also demonstrated with reasonable signal/noise ratio. The approach in this work is also applicable to other TMD materials and can open up the possibilities of producing a variety of vertical van der Waals heterojunctions in a large scale toward optoelectronic applications.