Two-dimensional superconducting MoSi2N4(MoN)4n homologous compounds

Abstract

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.

Keywords: 2D layered materials; MoSi2N4; homologous compounds; iDPC-STEM; superconductivity.

Figure 1.

The atomic structure of multilayer MoSi₂N₄. (a and b) iDPC (a) and the corresponding dDPC (b) images of bilayer MoSi₂N₄, clearly showing the atomic sites of Mo (blue balls), Si (green balls) and N (red balls). (c) iDPC image of a rhombohedral-structured multilayer MoSi₂N₄ with ABC-typed stacking order.

Figure 2.

The atomic structure of MoSi₂N₄(MoN)₄. (a and b) HAADF-STEM image of MoSi₂N₄(MoN)₄ confined in multilayer MoSi₂N₄ (a) and the zoomed-in view of the MoSi₂N₄(MoN)₄ (b). (c and d) iDPC (c) and dDPC (d) images of MoSi₂N₄(MoN)₄. The blue, green and red balls represent the Mo, Si and N atoms, respectively.

Figure 3.

The atomic structure of super-thick MoSi₂N₄(MoN)_4n. (a and b) HAADF-STEM image of MoSi₂N₄(MoN)₈ confined in multilayer MoSi₂N₄ (a) and iDPC image of MoSi₂N₄(MoN)₈ (b). The A, B and C lines indicate the Mo atomic layers aligned with the MoSi₂N₄ matrix. (c and d) HAADF-STEM image of one end of the MoSi₂N₄(MoN)_4n confined in 11-layer MoSi₂N₄ (c) and iDPC image of MoSi₂N₄(MoN)₂₈ (d). The blue, green and red balls represent the Mo, Si and N atoms, respectively.

Figure 4.

Calculated structures and properties of MoSi₂N₄(MoN)_n. (a) Diagram illustrating the process of structure screening of MoSi₂N₄(MoN)₄. (b and c) The atomic models (b) and electronic band structures (c) of the energetically favorable structure and experimental structure of MoSi₂N₄(MoN)₄. (d) Young's modulus of MoSi₂N₄(MoN)_n and Mo_n+1N_n+2 for n = 0−4. The Young's modulus of Mo₂N₃ was not given due to its mechanical instability. (e and f) Phonon dispersions, vibrational PHDOS, Eliashberg function and accumulated EPC constants of MoSi₂N₄(MoN)₄ (e) and Mo₅N₆ (f), where the circles in phonon dispersion plots represent the phonon linewidth γ_qν, related to the contribution to α²F(ω) at each q and each mode ν.