Modulating the electronic structure and interface contact of $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ van der Waals heterostructures by strain engineering: Insights from first-principles calculations

As a recent member of the two-dimensional (2D) van der Waals (vdW) heterostructures, the $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure has received considerable attention due to its fascinating characteristics compared with the constituent 2D materials. In this paper, we performed first-principles calculations to investigate its structural, electronic, and magnetic properties and explored the effects of interlayer and in-plane strains on these properties. Our results reveal that the antiferromagnetic (AFM) ground state in the $\mathrm{Cr}{\mathrm{Se}}_2$ layer of the heterostructure is maintained owing to weak vdW interactions between the $\mathrm{Cr}{\mathrm{Se}}_2$ and $\mathrm{W}{\mathrm{Se}}_2$ layers. However, the AFM state can be transformed into the ferromagnetic state at interlayer compressive strain of −19% or in-plane tensile strain of 2.5%. Moreover, the $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterointerface belongs to the metal-semiconductor interface and exhibits $p$-type ohmic contact and low contact resistance. The transition from $p$-type ohmic contact to $p$-type Schottky contact or $n$-type Schottky contact can be achieved by interlayer or in-plane strain engineering, which is associated with the strain-induced energy shifts of the valence band maximum and conduction band minimum of $\mathrm{W}{\mathrm{Se}}_2$. Additionally, the tunneling probability of the heterostructure rises dramatically (up to 100%) with interlayer coupling, which is favorable for carrier transport at the heterointerface. Our findings demonstrate that strain engineering is an effective way of modulating metal-semiconductor interfaces and provide theoretical guidance for designing electronic and magnetic devices based on the $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ vdW heterostructure as well as broadening its applications in future functional devices.

Figure 1

Top and side views of (a) $\mathrm{W}{\mathrm{Se}}_2$ and (d) $\mathrm{Cr}{\mathrm{Se}}_2$ monolayers. Black rhombuses in (a) and (d) refer to the unit cells of $\mathrm{W}{\mathrm{Se}}_2$ and $\mathrm{Cr}{\mathrm{Se}}_2$. Band structures of (b) $\mathrm{W}{\mathrm{Se}}_2$ and (e) $\mathrm{Cr}{\mathrm{Se}}_2$ monolayers. Projected density of states (PDOS) of (c) $\mathrm{W}{\mathrm{Se}}_2$ and (f) $\mathrm{Cr}{\mathrm{Se}}_2$ monolayers.

Figure 2

$\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure: (a) Relaxed structure (Configuration I), (b) spin densities in antiferromagnetic (AFM) and ferromagnetic (FM) states. The yellow and cyan isosurfaces refer to spin-up and spin-down densities, respectively, (c) projected band structure, (d) planar electronic density difference $\mathrm{\Delta }\rho$, and (e) electrostatic potential along the $Z$ direction.

Figure 3

(a) Total energy of $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure in antiferromagnetic (AFM) and ferromagnetic (FM) states at different interlayer strains and the energy difference between the two states. (b) Total energy of $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure in the AFM and FM states and the energy difference between the two states as a function of biaxial strain.

Figure 4

(a) Projected band structures of $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure under different interlayer strains. The red part represents the contribution of $\mathrm{W}{\mathrm{Se}}_2$ supercell, and the blue part represents the contribution of $\mathrm{Cr}{\mathrm{Se}}_2$ supercell. (b) Band edge position and (c) Schottky barrier of $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure as a function of interlayer strain.

Figure 5

(a) Schottky barrier of the heterostructure subjected to biaxial strain. (b) Projected band structures of $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure under different biaxial strains. The red and blue parts, respectively, represent the contributions from $\mathrm{W}{\mathrm{Se}}_2$ and $\mathrm{Cr}{\mathrm{Se}}_2$ supercells. (c) Band edge position of the heterostructure subjected to biaxial strain.

Figure 6

The electrostatic potential of $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure along the $Z$ direction at (a) interlayer and (b) biaxial strains. Barrier height, barrier width, and tunneling probability of the heterostructure at (c) interlayer and (d) biaxial strains.