Tunable type-II band alignment and electronic structure of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ heterostructure: Interlayer coupling and electric field

In this work, we perform a first-principle study to investigate the atomic and electronic structures of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ van der Waals heterostructure (vdWH) as well as its tunable electronic structure via interlayer coupling and an external perpendicular electric field. The ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH is structurally and thermodynamically stable at room temperature. Our results demonstrate that the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH exhibits a semiconducting characteristic with a direct band gap of 1.86/2.66 eV as given by the PBE/HSE06 calculation. This value of band gap conveniently lies in the visible light energy range, thus unraveling the strong optical absorption of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH in the technologically important visible light regime. The band edges of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH separately from the ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ layers, thus resulting in a type-II band alignment, which is highly desirable for achieving efficient electron-hole separation. Remarkably, the electronic structure and the band alignment types can be flexibly tuned between type-I and type-II by applying an external electric field, by changing the interlayer distance and by applying the in-plane strain. Our findings reveal the potential of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH as a tunable hybrid material with strong potential in optoelectronic applications.

Figure 1

[(a) and (e)] Atomic structure, [(b) and (f)] PBE and HSE06 band structures, [(c) and (g)] weighted band structure, and [(d) and (h)] phonon dispersion curves of ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ monolayers.

Figure 2

(a) Top view and (b) side view of the optimized atomic structure of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ heterostructure. Red, cyan, green, and orange balls represent the carbon, nitrogen, molybdenum, and silicon atoms, respectively.

Figure 3

Band structures of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH and (c) its weighted band structure obtained by PBE calculation.

Figure 4

(a) Electrostatic potential and (b) electron density difference of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH. Yellow and cyan regions represent the charge accumulation and depletion, respectively. The isosurface is set to be 2.5 $\times {10}^{-3}\mathrm{e}$/ Å${}^3$.

Figure 5

The change in the band gap of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH as a function of interlayer coupling by changing the interlayer distances $D$. The inset represents the schematic model of applied strain along the $z$ direction.

Figure 6

Band structures of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH under different interlayer couplings of (a) Red and black lines represent the contributions of ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ layers, respectively.

Figure 7

Projected band structures of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH with different pressures where the topmost and bottom-most layers are fixed, (a) $P$ = 3.01, (b) 1.24, (c) 0.71, (d) 0.65, (e) 0.67, and (f) 0.69 GPa. Red and green circles represent the contributions of ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ layers, respectively.

Figure 8

The variation of the band gap of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH as a function of in-plane strain. The inset represents the schematic model of applied in-plane strain.

Figure 9

Projected band structures of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH under the in-plane strain of (a) $\varepsilon =-6\%$, (b) $-4\%$, (c) $-2\%$, (d) $+2\%$, (e) $+4\%$, and (f) $+6\%$. Red and green circles represent the contributions of ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ layers, respectively. The dashed black line is the Fermi level, which is set to be zero.

Figure 10

The band gap of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH as a function of electric field. The inset represents the schematic of applying electric field along the $z$ direction of vdWH.

Figure 11

Projected band structures of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH under different strength of electric field: (a) $-0.9$, (b) $-0.6$, (c) $-0.3$, (d) $+0.3$, (e) $+0.6$, and (f) $+0.9\mathrm{V}$/Å. The Fermi level is set to be zero. Red and black lines represent the contributions of ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ layers, respectively.