Tuning the electronic properties of bondings in monolayer MoS₂ through (Au, O) co-doping

Abstract

Improving the electronic properties of Au–S bonding is the key to tuning the carrier transport of monolayer MoS₂-based nanodevices. Herein, we systematically investigate the electronic properties for Au-doped, O-doped, and (Au, O) co-doped monolayer MoS₂ to analysis the electronic properties of Au–S bondings using first-principles density functional calculations. Three gap states induced by Au–S bondings are observed at the band gap in Au-doped and (Au, O) co-doped monolayer MoS₂, which are n-type semiconductors. Moreover, the n-type barriers between the Fermi level of Au-doped and (Au, O) co-doped systems and the CBM of un-doped monolayer MoS₂ are 0.84 and 0.65 eV, respectively. In addition, low electron density and electron density difference are observed for Au–S bondings in Au-doped monolayer MoS₂, suggesting weak covalent Au–S bondings with high resistance; this explains the observed low carrier mobility of monolayer MoS₂ devices with an Au electrode. Upon introducing elemental O into Au-doped monolayer MoS₂, electron density and electron density difference of Au–S bondings in (Au, O) co-doped monolayer MoS₂ are increased to 0.58 and 0.15 eV Å⁻³, respectively, showing that the covalent Au–S bondings are strengthened, and their resistance and electron injection efficiency are further improved by the elemental O dopant. Our findings may provide an effective way to improve the electronic properties of Au–S bondings in monolayer MoS₂-based nanodevices with an Au electrode.

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1. Introduction

Monolayer transition metal dichalcogenides (mTMD) semiconductors are regarded as promising candidates for channel materials in next-generation nanoelectronic devices. Monolayer molybdenum disulfide (MoS₂), an important member of the two-dimensional mTMD semiconducting crystal, has attracted extensive attention due to its atomic thickness of ∼7 Å per layer, considerable band gap of ∼1.8 eV, planar nature, and distinctive electronic and optical properties.^1–4 Recently, monolayer MoS₂ was used to fabricate field-effect transistors (FETs), which show higher current ON/OFF ratios and lower power consumption than classical transistors.^5,6 Moreover, the monolayer MoS₂-based phototransistor has been demonstrated to have a better photoresponsivity compared with the graphene-based device.⁷

However, the observed carrier mobility in monolayer MoS₂-based FETs with an Au electrode is lower than expected,^8,9 owing to the weak Au–S bonding with low electron density, high contact resistance and large Schottky barrier at the Au–MoS₂ contacts in FETs.^5,10 A similar phenomenon has been observed in other mTMD FETs with common Au electrode.^11,12 To overcome this problem, Fang et al.¹¹ investigated the monolayer WSe₂-based FETs under the atmosphere of an oxidizing gas NO₂, and they found that the carrier mobility of monolayer WSe₂-based FETs significantly increased. The main reason may be that Au–Se bondings at the Au/WSe₂ interface region were improved by the adsorption of oxidizing gas.¹¹ A similar method might be used to improve the electronic properties of Au–S bonding and the carrier mobility of monolayer MoS₂-based FETs, because monolayer MoS₂ and WSe₂ are members of mTMD, and have similar geometric and electronic structures. Nevertheless, it is well known that the gas adsorption is reversible once exposed to ambient air. Permanent doping method should be explored to substitute the gas adsorption in the future. Previous studies^13,14 found that the elemental oxygen can stably exist in mTMD-based FETs. Thus, elemental oxygen, rather than oxidizing gas, doped monolayer MoS₂ was investigated in this study. In addition, to study the effect of elemental oxygen on the electronic properties of Au–S bondings in monolayer MoS₂, Au-doped and (Au, O) co-doped monolayer MoS₂ were comparatively studied.

To the best of our knowledge, the electronic structures and electronic properties of (Au, O) co-doped monolayer MoS₂ have not been well analyzed to date. Understanding the effect of elemental O on the electronic properties of Au–S bonding in Au-doped monolayer MoS₂ is vital for the possibility of tuning and designing monolayer MoS₂-based FETs with high carrier mobility. In this study, we comparatively investigate the electronic structures and electronic properties for un-doped, Au-doped, O-doped, and (Au, O) co-doped monolayer MoS₂ using first-principles density functional calculations.

2. Methods

In the present calculations, the exchange correlation of the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional as implemented in CASTEP code¹⁵ was employed. The electron–ion interactions were described by ultrasoft pseudo-potentials¹⁶ for Mo, S, Au and O atoms. It was recently reported that the GGA scheme is insufficient to correctly describe the electronic structure of transition metal oxides due to their localized d-states.¹⁷ To reliably describe the exchange-correlation effects and electron–ion interactions among the localized transition metal 4d electrons, the GGA+U (where U is the Hubbard term) method was also applied. Monolayer MoS₂ was modeled by a 4 × 4 supercell slab, which is periodic in the x and y directions, whereas separated by a 15 Å vacuum region in the z direction to minimize the interactions between adjacent image cells. In this study, all the dopants were doped into the monolayer MoS₂ in the form of substitutions. The structural schema of un-doped, Au-doped, O-doped and (Au, O) co-doped monolayer MoS₂ are presented in Fig. 1(a–d), respectively.

Fig. 1 (a–d) Atomic configurations of un-doped, Au-doped, O-doped, and (Au, O) co-doped monolayer MoS₂, respectively.

During calculations, the plane-wave cutoff energy was set to 500 eV after extensive convergence analysis. The Brillouin-zone of the monolayer MoS₂ was performed over the 7 × 7 × 1 k-point grids using the Monkhorst–Pack method, where the self-consistent convergence of the total energy is 1.0 × 10⁻⁵ eV per atom. A conjugate gradient scheme was used to relax the supercells until the component of the forces on each atom was less than 0.02 eV Å⁻¹.

3. Results and discussions

3.1 Electronic structures

For understanding the effect of elemental O on the electronic properties of Au–S bondings in Au-doped monolayer MoS₂, the band structures and density of states of the un-doped, Au-doped, O-doped, and (Au, O) co-doped monolayer MoS₂ models were calculated in comparison to the optimized structures. Fig. 2(a) comparatively presents the electronic structures of un-doped monolayer calculated by GGA and GGA+U methods in the first Brillouin zone along the high symmetry directions. It can be found that the band structures that are adjacent to the band gap of un-doped monolayer MoS₂ calculated by GGA and GGA+U methods are mainly dominated by the hybridization of Mo 4d and S 3p orbitals, which is consistent with previous calculated results.^18,19 However, some deviations between the band gaps calculated by GGA and GGA+U methods are observed in the un-doped monolayer MoS₂. In the case of GGA method, the band gap of the un-doped monolayer MoS₂ is about 1.74 eV, which is in good agreement with previous reports.^18,19 However, the calculated band gap is underestimated compared to the experimental band gap of 1.98 eV,² due to the self-interaction error (SIE) of the exchange functional of the generalized gradient approximation.²⁰ Upon taking the effective Hubbard parameter (U_eff) for Mo 4d states (about 7.5 eV (ref. 21)) into consideration, band gap of the un-doped monolayer MoS₂ is effectively improved to 1.97 eV, as presented by the red curve shown in Fig. 2(a); this is consistent with the experimental value. Therefore, the electronic properties of the abovementioned doped monolayer MoS₂ systems were calculated by the GGA+U method to accurately describe the d-states interactions.

Fig. 2 (a–b) Electronic band structures and corresponding partial density of states of un-doped and Au-doped monolayer MoS₂, respectively. In figure (a), the blue and red curves present the electronic structure of un-doped monolayer MoS₂ calculated by GGA and GGA+U, respectively. (c) Band structures of Au-doped monolayer MoS₂ system. The original band structures of un-doped monolayer MoS₂ are also plotted for reference (black curves), which are superimposed on the new band structure (red curves) so that the old and new sub-bands can be compared.

When one Au atom substitutes a Mo atom in monolayer MoS₂, the conduction bands still mainly consist the hybridization from Mo 4d and S 3p orbitals, although the conduction bands slightly shift down, as exhibited in Fig. 2(b). In addition, it can be found that three partially filled gap states, which are marked by ellipse, prism, and hexagon, respectively, are observed around the Fermi level. The reason may be that the Au–S bonding affects the Mo–S bonding in Au-doped monolayer MoS₂, and results in the spreading of the orbitals at the band edges into the band gap.¹² In regards to the first gap states marked by ellipse, they originate from the coupling of the hybridization states between Au 5d and S 3p orbitals with Mo 4d and S 3p orbitals. For the second gap states, they almost overlap with the Fermi level and are mainly composed of Au 5d orbitals. In the case of the third gap states, although they consist of two band curves, both are dominated by the hybridization between Au 5d and S 3p orbitals. Moreover, as the doping concentration increases, more and more gap states occupy the band gap; thus, the band gap of the Au-doped monolayer MoS₂ disappears. Therefore, the Au/MoS₂ interface shows a metallic character.⁵ However, these gap states are not effective for either electron or hole, and thus degrade the mobility of the charge carriers.²² Therefore, the mobility of MoS₂-based nanodevices with Au electrodes is lower than expected.^7,8 Fig. 2(c) compares the band structures of the un-doped and Au-doped monolayer MoS₂. It can be found that the position of E_F of the Au-doped monolayer MoS₂ shifts toward the original conduction band minimum (CBM) of the un-doped monolayer MoS₂, indicating that the monolayer MoS₂ is doped n-type by Au, which is consistent with previous results.¹⁰ However, a large n-type barrier (Φ_n) of about 0.84 eV between the E_F of the Au-doped monolayer MoS₂ and the original CBM of the un-doped monolayer MoS₂ is found. This is similar to the observed n-type wide Schottky barrier (Φ_SB,n) at the Au/MoS₂ interface in the experimental and theoretical results.^10,23

Upon introducing elemental O into the Au-doped monolayer MoS₂, three partially occupied gap states show significant change in the band gap, as shown in Fig. 3(b). In the case of the first gap states, they do not only consist of the coupling of the hybridization states between Au 5d and S 3p orbitals with Mo 4d and S 3p orbitals, but also originate from the coupling of the hybridization states between Au 5d and O 2p orbitals with Mo 5d and O 2p orbitals. For the second gap states, which are similar to the Au-doped monolayer MoS₂, they almost overlap with the Fermi level and are composed of Au 5d orbitals. The third gap states only contain one band curve and consist of the hybridization states between Mo 4d and S 3p orbitals. Moreover, the third gap states gradually integrate into a valence band with increasing concentrations of O dopant in (Au, O) co-doped monolayer MoS₂, indicating additional scattering centers, which degrade the mobility of the charge carriers, are decreased. In addition, the position of E_F moves further toward the original conduction band compared to that of the Au-doped monolayer MoS₂, as shown in Fig. 3(c), although the (Au, O) co-doped monolayer MoS₂ shows an n-type character. Moreover, the Φ_n of (Au, O) co-doped monolayer MoS₂ is about 0.65 eV, which is lower than that of the Au-doped monolayer MoS₂ because the elemental O decreased the work function of monolayer MoS₂.¹¹ These factors mean that the carriers will be more easily transported in the band of (Au, O) co-doped monolayer MoS₂. Therefore, this may be the reason for the mTMD-based FETs to exhibit higher electronic properties under the oxidization conditions.^11,24 In addition, it can be found from Fig. 3(a) that the band gap of monolayer MoS₂ reduces to 1.58 eV when the monolayer MoS₂ is doped with O. Moreover, the band gap of the O-doped monolayer MoS₂ declines as the doping concentration increases. Because the hybridization of the anti-bonding between O 2p and Mo 4d orbitals in the conduction bands of O-doped monolayer MoS₂ leads to the shift of the conduction band toward low energy. Therefore, the O-doping method may be an effective approach to improve the electronic properties of Au/MoS₂ interface and Au-doped monolayer MoS₂.

Fig. 3 Electronic band structures and corresponding partial density of states of O-doped monolayer MoS₂ (a) and (Au, O) co-doped monolayer MoS₂ (b). (c) Band structures of (Au, O) co-doped monolayer MoS₂ system. The original band structure of undoped monolayer MoS₂ is also plotted for reference (black curves), which is superimposed on the new band structure (red curves) so that the old and new sub-bands can be compared.

3.2 Charge density

The charge densities of Mo–S and Au–S bondings are calculated and plotted in Fig. 4. The minimum charge densities of Mo–S and Au–S bondings are presented as ρ_Mo and ρ_Au, respectively. In Fig. 4(a), the ρ_Mo value of un-doped monolayer MoS₂ is about 0.60 eV Å⁻³, which is in good agreement with previous reports.²⁵ This means that the Mo–S bondings in un-doped monolayer MoS₂ show strong covalent characters, which is consistent with other theoretical results.^26,27 In the case of the Au-doped monolayer MoS₂, the ρ_Mo value of about 0.65 eV Å⁻³ exhibits a small increment compared to that of the un-doped monolayer MoS₂, as shown in Fig. 4(b). Nevertheless, it can be observed from Fig. 4(d) that the ρ_Au value of the Au-doped monolayer is about 0.30 eV Å⁻³, which is lower than that of ρ_Mo. This indicates the higher resistance and lower electron injection efficiency of Au–S bondings than that of Mo–S bondings. A similar phenomenon is also observed at the Au–MoS₂ interface region, which leads to the lower carrier mobility of monolayer MoS₂-based FETs than expected.^8,9

Fig. 4 Charge density contour profile of bondings. (a–c) The Mo–S bondings in un-doped, Au-doped, and (Au, O) co-doped monolayer MoS₂, respectively. (b–d) The Au–S bondings in Au-doped and (Au, O) co-doped monolayer MoS₂, respectively. The values in each panel indicate the minimum charge density of Mo–S (ρ_Mo) and Au–S (ρ_Au) bonding (in units of eV Å⁻³), respectively.

When elemental O is introduced into the Au-doped monolayer MoS₂, the ρ_Mo of about 0.63 eV Å⁻³ shows a slight decrease compared to that of the Au-doped monolayer MoS₂. Moreover, the decrease in the ρ_Mo is still inconspicuous, even though the concentration of elemental O increases. However, it can be found that the ρ_Au increases to 0.58 eV Å⁻³, which is not only higher than that of Au-doped monolayer MoS₂, but also closer to the value of ρ_Mo in the un-doped monolayer MoS₂. This suggests that the dopant elemental O can significantly improve the resistance and electron injection efficiency of Au–S bondings. In addition, it is also found that the ρ_Au exhibits a slight increase with the increasing concentration of O dopant in the (Au, O) co-doped monolayer MoS₂. Thus, the conductivity of Au–S bondings was improved by the elemental O dopant. This may be the true reason for the carrier mobility of mTMD FETs to be evidently improved under the condition of oxidizing gas.^11,24

3.3 Charge density difference

The charge density differences of Mo–S and Au–S bondings are plotted in Fig. 5. A positive value (red) of charge density difference indicates electron accumulation, while a negative value (green) denotes electron depletion. In Fig. 5(a), a certain amount of electrons, about 0.13 eV Å⁻³, accumulates between the Mo and S atoms in un-doped monolayer MoS₂, indicating the strength of the covalent Mo–S bondings; this is consistent with previous experimental and theoretical studies.^26,27 Moreover, the electron accumulation slightly increases to 0.16 eV Å⁻³ and 0.15 eV Å⁻³ in the Au-doped and (Au, O) co-doped monolayer MoS₂, respectively, as shown in Fig. 5(b and c). Thus, values of the ρ_Mo of Au-doped and (Au, O) co-doped monolayer MoS₂ are higher than that of the un-doped monolayer MoS₂. Moreover, this means that the covalent character and conductivity of Mo–S bondings are enhanced by introducing Au and O atoms. Nevertheless, as the concentration of elemental O increases in the (Au, O) co-doped monolayer MoS₂, the covalent character of Mo–S bondings slightly weakens due to the weakening of electron accumulation. In Fig. 5(d), a large amount of electron transfer from Au atoms to S atoms, and small electron accumulation between Au and S atoms, indicate that the Au–S bondings show high ionic character and low covalent character in Au-doped monolayer MoS₂. Thus, the ρ_Au is relatively low in the Au-doped monolayer MoS₂. When elemental O is introduced into the Au-doped monolayer MoS₂, the growing number of electrons localize between Au and S atoms, and the electron accumulation of Au–S bonding increases to 0.10 eV Å⁻³. As a result, the Au–S bondings are dominated by strong covalent character, rather than ionic character. Furthermore, the covalent character of the Au–S bondings becomes stronger with the increasing concentration of elemental O in (Au, O) co-doped monolayer MoS₂. Thus, the electron density of the Au–S bondings and the conductivity of Au–S bondings are improved after introducing elemental O into Au-doped monolayer MoS₂.

Fig. 5 Charge density different contour profile of bondings. (a–c) The Mo–S bondings in un-doped, Au-doped, and (Au, O) co-doped monolayer MoS₂, respectively. (b–d) The Au–S bondings in Au-doped and (Au, O) co-doped monolayer MoS₂, respectively. The values in each panel indicate the maximum values of charge density difference at Mo–S and Au–S bonding (in units of eV Å⁻³), respectively.

4. Conclusion

In this study, the electronic properties of Au-doped, O-doped and (Au, O) co-doped monolayer MoS₂ have been comparatively investigated using the first-principles plane-wave pseudopotential method based on density functional theory. Results show that the band gap of monolayer MoS₂ decreases after introducing the elemental O dopant, and three gap states that are induced by Au–S bonding are observed at the band gap in the Au-doped and (Au, O) co-doped monolayer MoS₂. Moreover, the large n-type barriers between the doped systems and the CBM of un-doped monolayer MoS₂ are observed. In addition, low electron density and electron density difference are observed between Au–S bondings in Au-doped monolayer MoS₂, which explains the phenomenon that the observed carrier mobility of a monolayer MoS₂ device with an Au electrode is lower than expected. Notably, the electron density and electron density difference of Au–S bondings in the (Au, O) co-doped monolayer MoS₂ are increased significantly compared with those in Au-doped monolayer MoS₂, showing that the Au–S bondings are dominated by covalent character, and their resistance and electron injection efficiency are further improved by elemental O dopant. Moreover, the effect of elemental O concentration on the electronic properties of Au–S bondings in co-doped system is weak. Our findings provide an effective way to improve the electronic properties of Au–S bondings in monolayer MoS₂-based nanodevices with Au electrodes.

Acknowledgements

We acknowledge the National Natural Science Foundation of China under grant No. 61376091, the Natural Science Foundation of Shaanxi Province under grant No. 2012JM6012, the Fundamental Research Funds for the Central Universities under grant No. 3102014JCQ01033.

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