Possible doping strategies for MoS${}_2$ monolayers: An ab initio study

Density functional theory is used to systematically study the electronic properties of doped MoS${}_2$ monolayers, where the dopants are incorporated both via S/Mo substitution or as adsorbates. Among the possible substitutional dopants at the Mo site, Nb is identified as suitable $p$-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS${}_2$ conduction band, making the system $n$ type. Finally, the adsorption of charged molecules is considered, mimicking an ionic liquid environment. We find that molecules adsorption can lead to both $n$- and $p$-type conductivity, depending on the charge polarity of the adsorbed species.

Figure 1

Structure of the supercell used for the calculations. In (a) we show a $5\times 5$ MoS${}_2$ monolayer supercell with the possible substitutional dopants. Mo is substituted by a transition metal (green dot): Y, Zr, Nb, Re, Rh, Ru, Pd, Ag, Cd; S is substituted by nonmetals and halogens (red dot): P, N, As; F, Cl, Br, I. In (b) we display the same supercell where we indicate the possible adsorption sites defined in the text. Light gray spheres indicate Mo atoms; light yellow spheres indicate S.

Figure 2

Band structure (left) and DOS (right) of a pristine (undoped) MoS${}_2$ monolayer. The red curve indicates the DOS projected on the Mo atoms.

Figure 3

DOSs for a $5\times 5$ MoS${}_2$ supercell in which one S atom is replaced by (a) F, (b) Cl, (c) Br, and (d) I. Negative DOS values refer to minority spins, while positive are for the majority. The blue dashed line indicates the Fermi energy, while the colored shaded areas indicate the DOS projected over the dopants. Note that all the DOSs are aligned to have a common Fermi level, $E_{\mathrm{F}}=0$.

Figure 4

DOSs for a $5\times 5$ MoS${}_2$ supercell in which one S atom is replaced by (a) N, (b) P, and (c) As. Negative values refer to minority spins and positive values to majority spins. The blue dashed line indicates the Fermi energy; the colored shaded areas indicate the DOS projected on the dopants. Note that all the DOSs are aligned to have a common Fermi level, $E_{\mathrm{F}}=0$.

Figure 5

DOSs for a $5\times 5$ MoS${}_2$ supercell in which one Mo atom is replaced by (a) Re, (b) Ru, (c) Rh, (d) Pd, (e) Ag, and (f) Cd. The blue dashed line marks the Fermi energy; the colored shaded areas indicate the DOS projected onto the dopants. Note that all the DOSs are aligned to have a common Fermi level, $E_{\mathrm{F}}=0$.

Figure 6

DOSs for a $5\times 5$ MoS${}_2$ supercell in which one Mo atom is replaced by (a) Nb, (b) Zr, and (c) Y. The blue dashed line indicates the Fermi energy, the colored shaded areas indicate the DOS projected on the dopants. Note that all the DOSs are aligned to have a common Fermi level, $E_{\mathrm{F}}=0$.

Figure 7

DOSs for a $5\times 5$ MoS${}_2$ supercell in which an atom chosen between (a) Cs, (b) K, (c) Li, and (d) H is adsorbed on the MoS${}_2$ surface. The blue dashed line indicates the Fermi energy; the colored shaded areas indicate the DOS projected on the adsorbates. Note that all the DOSs are aligned to have a common Fermi level, $E_{\mathrm{F}}=0$.

Figure 8

DOS for a MoS${}_2$ supercell, where a single Mo is replaced by Nb, and where a Cs atom is adsorbed. The blue dashed line indicates the Fermi energy, $E_{\mathrm{F}}=0$; the colored curves indicate the DOS projected on the dopants.

Figure 9

Schematic diagrams of the electric double layer transistor operation with an ionic liquid electrolyte. Here, “S” indicates the source, “D” indicates the drain, and “G” indicates the gate. The solid circles denote the ions in the liquid, where the different colors represent cations and anions. (a) When no gate voltage is applied (${\text{V}}_{\mathrm{G}}=0$), cations and anions are uniformly distributed and both adsorbed at the interface of the semiconductor with equal probability. (b) When a finite gate bias is present (${\text{V}}_{\mathrm{G}}=v$), cations or anions adsorb predominantly at the gate electrode, depending on the bias polarity, and the oppositely charged ions adsorb predominantly on the semiconductor. These adsorbed ions lead to an accumulation of a screening charge at the semiconductor surface, which implies a large surface carrier concentration.

Figure 10

Optimized geometries for [(a),(b)] NH${}_4^{+}$ and [(c),(d)] BF${}_4^{-}$ ions adsorbed on a MoS${}_2$ monolayer. In (a) and (c) the top view is shown, whereas the side view is in (b) and (d). Pink spheres indicate H, green spheres N, cyan spheres F, and red spheres B.

Figure 11

DOS for (a) NH${}_4^{+}$ and (b) BF${}_4^{-}$ molecule adsorbed on MoS${}_2$. The results reported here are for a $5\times 5$ supercell. The blue dashed line indicates the Fermi energy; the colored shaded areas indicate the DOS projected over the adsorbates. Note that the DOSs have the Fermi level at 0 ($E_{\mathrm{F}}=0$), but they are aligned according to their valence and conduction bands.

Figure 12

DOSs for the doped MoS${}_2$ monolayer, calculated with the HSE06 functional. We report results for (a) Nb substitutional at the Mo site, and for (b) Cs, (c) NH${}_4^{+}$, and (d) BF${}_4^{-}$ adsorbed on the MoS${}_2$ surface. The blue dashed line indicates the Fermi energy ($E_{\mathrm{F}}=0$); the colored shaded areas indicate the DOS projected on the adsorbates. In the plot we have aligned all the conduction and valence bands.