The precise precursor supply is a precondition for controllable growth of two-dimensional (2D) transition metal dichalcogenides (TMDs). Although great efforts have been devoted to modulating the transition metal supply, few effective methods of chalcogen feeding control were developed. Here we report a strategy of using active chalcogen monomer supply to grow high-quality TMDs in a robust and controllable manner, e.g., MoS2 monolayers perform representative photoluminescent circular helicity of ~92% and electronic mobility of ~42 cm2V−1s−1. Meanwhile, a uniform quaternary TMD alloy with three different anions, i.e., MoS2(1-x-y)Se2xTe2y, was accomplished. Our mechanism study revealed that the active chalcogen monomers can bind and diffuse freely on a TMD surface, which enables the effective nucleation, reaction, vacancy healing and alloy formation during the growth. Our work offers a degree of freedom for the controllable synthesis of 2D compounds and their alloys, benefiting the development of high-end devices with desired 2D materials.

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The accurate characterization of the chiral indices (n,m) of carbon nanotubes has greatly facilitated fundamental investigations and practical applications ranging from electronic circuits and quantum emission to biological detection 1-4 . To meet the growing miniaturizationdriven demand, handedness, the key structural quantity associated with mirror symmetry breaking, needs to be identified both accurately and efficiently 5-7 . To date, optical spectroscopic techniques with unprecedented high-throughput and noninvasive characteristics have achieved great success in identifying chiral indices even at the single-tube level 8-13 . However, none of these optical methods are capable of handedness characterization for single nanotubes due to their extremely weak chiroptical signals (~10 -7 ) compared to the excitation light 14,15 . Here, we demonstrate the complete structure identification of single nanotubes in terms of both chiral indices and handedness by Rayleigh scattering circular dichroism. The success originates from the background-free feature of Rayleigh scattering collected at an oblique angle, which enhances the chiroptical signal from nanotubes by three to four orders of magnitude compared to that using conventional absorption circular dichroism. We measured a total of 30 single-walled carbon nanotubes (including both semiconducting and metallic nanotubes) and found that their absolute chiroptical signals show an obvious structure dependence, which can be qualitatively understood through tightbinding calculations. Our strategy not only opens up exciting opportunities for unlocking the sophisticated functionality of nanotubes but also provides a new platform for chiral discrimination and chiral device exploration at the level of individual nanomaterials.

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Two-dimensional (2D) transition metal dichalcogenides (TMDs), with atomic thickness, strong spin–orbit coupling, enhanced light-matter interactions. and facile quantum control ability, have demonstrated great potential in the applications of nanoelectronics and optoelectronics. The realization of these high-performance applications strongly relies on the production of large-scale TMD films with high quality. Therefore, facile and accurate quality monitoring of TMDs is essential for their future applications. In this Review, we summarized the main defect types in TMD crystals obtained by different synthesis methods, and we discussed recent cutting-edge characterization techniques, including scanning transmission electron microscopy, etching or adsorption, optical spectroscopy, and field-effect transistors. Finally, we provide a short perspective on the future development of quality monitoring techniques for broad 2D materials.

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We report topologically-protected entanglement emitters, that emit topological Einstein-Podolsky-Rosen state and multiphoton entangled state from a plug-and-play silicon-photonic chip in ambient conditions. The device emulating a photonic anomalous Floquet insulator allows the generation of four-photon topological entangled states at nontrivial edge modes.