Two-dimensional superconducting MoSi₂N₄(MoN)_4n homologous compounds

The number and stacking order of layers are two important degrees of freedom that can modulate the properties of 2D van der Waals (vdW) materials. However, the layers' structures are essentially limited to the known layered 3D vdW materials. Recently, a new 2D vdW material, MoSi₂N₄, without known 3D counterparts, was synthesized by passivating the surface dangling bonds of non-layered 2D molybdenum nitride with elemental silicon, whose monolayer can be viewed as a monolayer MoN (-N-Mo-N-) sandwiched between two Si-N layers. This unique sandwich structure endows the MoSi₂N₄ monolayer with many fascinating properties and intriguing applications, and the surface-passivating growth method creates the possibility of tuning the layer's structure of 2D vdW materials. Here we synthesized a series of MoSi₂N₄(MoN)_4n structures confined in the matrix of multilayer MoSi₂N₄. These super-thick monolayers are the homologous compounds of MoSi₂N₄, which can be viewed as multilayer MoN (Mo_4n+1N_4n+2) sandwiched between two Si-N layers. First-principles calculations show that MoSi₂N₄(MoN)₄ monolayers have much higher Young's modulus than MoN, which is attributed to the strong Si-N bonds on the surface. Importantly, different from the semiconducting nature of the MoSi₂N₄ monolayer, the MoSi₂N₄(MoN)₄ monolayer is identified as a superconductor with a transition temperature of 9.02 K. The discovery of MoSi₂N₄(MoN)_4n structures not only expands the family of 2D materials but also brings a new degree of freedom to tailor the structure of 2D vdW materials, which may lead to unexpected novel properties and applications.