Origin of the $n$-type and $p$-type conductivity of MoS${}_2$ monolayers on a SiO${}_2$ substrate

Ab initio density functional theory calculations are performed to study the electronic properties of a MoS${}_2$ monolayer deposited over a SiO${}_2$ substrate in the presence of interface impurities and defects. When MoS${}_2$ is placed on a defect-free substrate, the oxide plays an insignificant role since the conduction band top and the valence band minimum of MoS${}_2$ are located approximately in the middle of the SiO${}_2$ band gap. However, if Na impurities and O dangling bonds are introduced at the SiO${}_2$ surface, these lead to localized states, which modulate the conductivity of the MoS${}_2$ monolayer from $n$- to $p$-type. Our results show that the conductive properties of MoS${}_2$ deposited on SiO${}_2$ are mainly determined by the detailed structure of the MoS${}_2$/SiO${}_2$ interface, and suggest that doping the substrate can represent a viable strategy for engineering MoS${}_2$-based devices.

Figure 1

Side and top views of reconstructed structures for the O-terminated SiO${}_2$ (0001) (siloxane) surface [(a) and (b)], and the fully hydroxylated SiO${}_2$ (0001) (silanol) one [(c) and (d)] (color code: cyan $\to$ Si, red $\to$ O, violet $\to$ H). For clarity, only the top layers of SiO${}_2$ (above the dashed line) are shown in the top views.

Figure 2

Top view of the optimized structure of MoS${}_2$ placed on a defect-free (a) siloxane and (b) silanol surface (color code: light gray $\to$ Mo, yellow $\to$ S, cyan $\to$ Si, red $\to$ O, violet $\to$ H). Only the top layers of SiO${}_2$ are shown (see Fig. 1).

Figure 3

Electronic structure of the SiO${}_2$/MoS${}_2$ hybrid system when various defects are present at the SiO${}_2$ siloxane surface. (a) The total DOS for the defect-free surface (black, dashed curves), and when either Na (green, solid curves), or H adsorbed (magenta, solid curves) are adsorbed. (b) The DOS of a pristine freestanding MoS${}_2$ monolayer. The total DOS and the PDOS for MoS${}_2$, when the MoS${}_2$ monolayer is placed on the defect-free siloxane surface (c), on a siloxane surface with one adsorbed Na (d), or with one adsorbed H (e). The blue dashed vertical line indicates the Fermi level, which has been set to zero in all the panels. The red shaded areas indicate the MoS${}_2$ PDOS. Positive and negative DOS are respectively for spin-up (majority spins) and spin-down (minority spins) electrons.

Figure 4

Schematic band diagram for MoS${}_2$ placed on the defect SiO${}_2$ substrate (a), and on a substrate including a defect-induced donor (b) or acceptor (c) level. This demonstrates the modulation of the conductivity from $n$- to $p$-type as the impurity state redefines the Fermi energy in the oxide. The energy levels $E_V$, $E{}_C$, $E_F$, and $E_{\mathrm{vac}}$ define the valence band maximum (VBM), the conduction band minimum (CBM), the Fermi energy, and the vacuum level, respectively. The subscripts 1 and 2 refer to SiO${}_2$ and MoS${}_2$, respectively. The blue dashed lines indicate the Fermi energy of the hybrid system (common to SiO${}_2$ and MoS${}_2$). The thick green line in (b) indicates the donor level and the thick red line in (c) represents the acceptor state in the oxide. Note that in general due to charge transfer from MoS${}_2$ to the gap states, and the related dipole formation, the level alignment between $E_{V1}$ and $E_{V2}$ will also change in the defective systems.

Figure 5

The optimized geometry for MoS${}_2$ placed on (a) the siloxane surface incorporating a Na impurity and (b) the dangling oxygen bond on the silanol surface, obtained by removing a H atom. The arrows indicate the positions of the defects on the surface (color code: green $\to$ Na, while the other colors are the same atoms as in Fig. 2). The arrows indicate the location of the impurities/defects. Only the top layers of SiO${}_2$ are shown (see Fig. 1).

Figure 6

Electronic structure of the SiO${}_2$/MoS${}_2$ hybrid system when various defects are present at the SiO${}_2$ silanol-reconstructed surface. (a) The DOS for the defect-free surface (black, dashed curve) and for the one where one O dangling bond is induced by a single H removal (green, solid curve labeled as SiO${}^{*}$). The total DOS and the MoS${}_2$ PDOS for the SiO${}_2$/MoS${}_2$ composite when the MoS${}_2$ monolayer is placed on (b) the defect-free surface, (c) on the surface with a single adsorbed Na atom, and (d) on the surface with an O dangling bond created by removing a single H atom. The blue dashed line indicates the Fermi energy, which is set to zero in all panels. The red shaded areas indicate the MoS${}_2$ PDOS. Positive and negative DOS are, respectively, for spin-up (majority spins) and spin-down (minority spins) electrons.

Figure 7

Density of states for the defective SiO${}_2$/MoS${}_2$ composite calculated with the ASIC XC functional. In panel (a), we report the DOS for the siloxane reconstruction with an intercalated Na atom [corresponding to Fig 3d], while in (b) that for the silanol reconstruction and an O dangling bond obtained by removing a surface H atom [corresponding to 6d].