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

The synthetic strategies, structural identification and electrocatalytic applications of single atom catalysts supported on graphene or graphene-like carbons are reviewed.