Two-dimensional(2D)materials have attracted increasing interest since the first isolation of graphene from graphite in 2004[1]and subsequent exfoliation of other 2D crystals from a variety of lay-ered,van derWaals(vdW),materials[2].For such crystals,atoms within each layer are connected by strong chemical bonds,while the adjacent layers are stacked with weak vdW forces.Therefore monolay-ers can be exfoliated from the three-dimensional(3D)layered crystals,also called the parent materials.So far,dozens of such 2D vdW materials have been synthesized,such as graphene,h-BN,transition metal dichalcogenides and phosphorene[2-4].The discovery of such 2D vdW materials has led to the ob-servation of numerous exciting physical phenomena and exotic properties and already resulted in a number of intriguing applications.Furthermore,such 2D materials enable creation of a very broad class of artificial materials,vdW het-erostructures,by layer-by-layer stacking in a designed sequence[3,4].However,the structure of 2D vdW materials is essentially limited for those exfoliated from the parent materials.Synthesizing 2D vdW materials without known 3D lay-ered parents would provide a huge oppor-tunity for engineering materials with new attributes and functionality.