Cycling 2D crystal growth: The electronic and optical properties of two-dimensional (2D) transitional metal dichalcogenides such as MoS ₂ and WSe ₂ could be modulated by creating in-plane superlattices and heterostructures from these materials. However, single layers of these materials are fragile and often do not withstand the processing conditions needed during subsequent growth steps. Zhang et al. developed a reverse-flow reactor that avoids thermal degradation and unwanted crystal nucleation. They demonstrate several examples of block-by-block epitaxial growth, such as 2D heterostructures where MoS ₂ surrounds a WS ₂ core and superlattices where the composition alternates between WS ₂ and WSe ₂ . Science , this issue p. 788

We present an overview of the recent advances, challenges and future perspectives on the chemical synthesis of two-dimensional atomic crystals, heterostructures and superlattices.

The tunable properties of halide perovskite/two dimensional (2D) semiconductor mixed-dimensional van der Waals heterostructures offer high flexibility for innovating optoelectronic and photonic devices. However, the general and robust growth of high-quality monocrystalline halide perovskite/2D semiconductor heterostructures with attractive optical properties has remained challenging. Here, we demonstrate a universal van der Waals heteroepitaxy strategy to synthesize a library of facet-specific single-crystalline halide perovskite/2D semiconductor (multi)heterostructures. The obtained heterostructures can be broadly tailored by selecting the coupling layer of interest, and can include perovskites varying from all-inorganic to organic-inorganic hybrid counterparts, individual transition metal dichalcogenides or 2D heterojunctions. The CsPbI₂Br/WSe₂ heterostructures demonstrate ultrahigh optical gain coefficient, reduced gain threshold and prolonged gain lifetime, which are attributed to the reduced energetic disorder. Accordingly, the self-organized halide perovskite/2D semiconductor heterostructure lasers show highly reproducible single-mode lasing with largely reduced lasing threshold and improved stability. Our findings provide a high-quality and versatile material platform for probing unique optoelectronic and photonic physics and developing further electrically driven on-chip lasers, nanophotonic devices and electronic-photonic integrated systems.