First-principles analysis of MoS${}_2$/Ti${}_2$C and MoS${}_2$/Ti${}_2$C$Y_2$ ($Y=\mathrm{F}$ and OH) all-2D semiconductor/metal contacts

First-principles calculations are used to explore the geometry, bonding, and electronic properties of MoS${}_2$/Ti${}_2$C and MoS${}_2$/Ti${}_2$C$Y_2$ ($Y$ $=$ F and OH) semiconductor/metal contacts. The structure of the interfaces is determined. Strong chemical bonds formed at the MoS${}_2$/Ti${}_2$C interface result in additional states next to the Fermi level, which extend over the three atomic layers of MoS${}_2$ and induce a metallic character. The interaction in MoS${}_2$/Ti${}_2$C$Y_2$, on the other hand, is weak and not sensitive to the specific geometry, and the semiconducting nature thus is preserved. The energy level alignment implies weak and strong $n$-type doping of MoS${}_2$ in MoS${}_2$/Ti${}_2$CF${}_2$ and MoS${}_2$/Ti${}_2$C(OH)${}_2$, respectively. The corresponding $n$-type Schottky barrier heights are 0.85 and 0.26 eV. We show that the MoS${}_2$/Ti${}_2$CF${}_2$ interface is close to the Schottky limit. At the MoS${}_2$/Ti${}_2$C(OH)${}_2$ interface, we find that a strong dipole due to charge rearrangement induces the Schottky barrier. The present interfaces are well suited for application in all-two-dimensional devices.

Figure 1

Spin-polarized band structures of (a) free-standing Ti${}_2$C, (b) Ti${}_2$CF${}_2$, and (c) Ti${}_2$C(OH)${}_2$. Side views of Ti${}_2$C and Ti${}_2$C$Y_2$ are given in panels (d) and (e), respectively.

Figure 2

Lattice parameter optimization of the MoS${}_2$/Ti${}_2$C interface.

Figure 3

Side views of the six nonequivalent stacking patterns of the MoS${}_2$/Ti${}_2$C(OH)${}_2$ interface. The green, blue, gray, yellow, red, and white balls represent Mo, S, Ti, C, O, and H atoms, respectively.

Figure 4

(a) Spin-polarized total and (b) partial DOSs of adsorbed MoS${}_2$ and (c) charge density difference for MoS${}_2$/Ti${}_2$C for pattern fcc-I [inset in panel (c)]. The atomic positions are marked by solid circles. Arrows in panel (a) indicate the two spin channels. The dashed arrow in panel (b) highlights the metal-induced states in MoS${}_2$.

Figure 5

Band structures of (a) MoS${}_2$/Ti${}_2$CF${}_2$ atop-II, (b) MoS${}_2$/Ti${}_2$C(OH)${}_2$ atop-I, and (c) compressed (d) pristine MoS${}_2$. The MoS${}_2$-derived conduction and valence bands in the two hybrid systems are depicted by black dotted curves. (e) The energy level alignment of adsorbed MoS${}_2$ compared with the pristine material, referring to the vacuum level ($V_{\infty }$). The $n$-type Schottky barrier heights ${\Phi }_{\mathrm{B},\mathrm{n}}$ are indicated by pink fonts.

Figure 6

Plane-averaged electron density difference, Δρ($z$), for the systems (a) MoS${}_2$/Ti${}_2$CF${}_2$ atop-II and (b) MoS${}_2$/Ti${}_2$C(OH)${}_2$ atop-I. The positions of the atoms are indicated by solid circles, and $q$ is the charge transfer calculated by integrating Δρ($z$) over the full $z$ range.

Figure 7

Plane-averaged electrostatic potential (dotted line) along the interface normal of MoS${}_2$/Ti${}_2$C(OH)${}_2$ atop-I. The potential drop Δ$V_{\infty }$ across the vacuum is shown. The interface position is indicated by the vertical dashed line.