2D transition-metal dichalcogenides (TMDs) are an emerging class of materials with superior properties that make them highly attractive for fundamental studies of novel physics and for applications ranging from nanoelectronics and nanophotonics to sensing and catalysis. [1][2][3][4][5] As the most extensively Research on transition metal dichalcogenides (TMDs) has been accelerated by the development of large-scale synthesis based on chemical vapor deposition (CVD) growth. However, in most cases, CVD-grown TMDs are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs), which seriously degrade their electrical and photoelectrical properties. Here, the epitaxial growth of highly aligned MoS 2 grains is reported on a twofold symmetry a-plane sapphire substrate. The obtained MoS 2 grains have an unusual rectangle shape with perfect orientation alignment along the [1-100] crystallographic direction of a-plane sapphire. It is found that the growth temperature plays a key role in its orientation alignment and morphology evolution, and high temperature is beneficial to the initial MoS 2 seeds rotate to the favorable orientation configurations. In addition, the photoluminescence quenching of the well-aligned MoS 2 grains indicates a strong MoS 2 −substrate interaction which induces the anisotropic growth of MoS 2 , and thus brings the formation of rectangle shape grains. Moreover, the well-aligned MoS 2 grains splice together without GB formation, and thus that has negligible effect on its electrical transport properties. The progress achieved in this work could promote the controlled synthesis of large-area TMDs single crystal film and the scalable fabrication of high-performance electronic devices.

Structure and energy band engineering of 2D materials via selective doping or phase modulation provide a significant opportunity to design them for optoelectronic devices. Here, the synthesis of high-quality Mo x Re 1-x S 2 alloys with tunable composition and phase structure via chemical vapor deposition growth is reported, and their novel energy band structures and optoelectronic properties are explored. The phase separation and structure reconstruction, which are found to be two serious problems in the synthesis of these alloys, are successfully suppressed through tuning their growth thermodynamics. As a result, the obtained Mo x Re 1-x S 2 alloys have uniform composition, phase structure, and crystal orientation. Together with X-ray photoelectron spectroscopy analysis and first-principle calculation, the Re/Mo doping-induced Fermi level up-shift/down-shift, new electronic states, and "sub-gap" formation in Mo x Re 1-x S 2 alloys are revealed. Especially, a strong band bowing effect is discovered in the Mo x Re 1-x S 2 alloys with structure transition between 1T′ and 2H phases. Furthermore, these alloys reveal tunable conduction behavior from n-type to bipolar and p-type in 1T′ phase, as well as novel "bipolar-like" electron conduction behavior in 2H alloys. The results highlight the unique alloying effects, which do not exist in the single-phase 2D alloys, and provide the feasibility for potential applications in building novel electronic and optoelectronic devices.

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