Greatly Enhanced Optical Absorption of a Defective MoS2 Monolayer

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Greatly Enhanced Optical Absorption of Defective MoS2 Monolayer through Oxygen passivation Huabing Shu, Yunhai Li, Xianghong Niu, and Jinlan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03242 • Publication Date (Web): 04 May 2016 Downloaded from http://pubs.acs.org on May 10, 2016

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ACS Applied Materials & Interfaces

Greatly Enhanced Optical Absorption of Defective MoS2 Monolayer through Oxygen Passivation Huabing Shu1, Yunhai Li1, Xianghong Niu1 and Jinlan Wang*1,2 1

2

Department of Physics, Southeast University, Nanjing, 211189, China

Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China

Abstract Structural defects in molybdenum disulphide (MoS2) monolayer are widely reported and greatly degrade the transport and photoluminescence. However, how they influence the optical absorption properties remains unclear. In this work, by employing many-body perturbation theory calculations, we investigate the influence of sulfur vacancies (SVs), the main type of intrinsic defects in MoS2 monolayer, on the optical absorption and exciton effect. Our calculations reveal that the presence of SVs creates localized midgap states in the bandgap, which results in a dramatic red-shift of the absorption peak and stronger absorbance in the visible light and near infrared region. Nevertheless, the SVs can be fine repaired by oxygen passivation and are beneficial to the formation of the stable localized excitons, which greatly enhance the optical absorption in the spectral range. The defect mediated/engineered absorption mechanism is well understood, which offers insightful guides for improving the performance of two-dimensional dichalcogenide based optoelectronic devices. Keywords: Molybdenum disulphide, sulfur vacancies, oxygen passivation, optical absorption, exciton

*

E-mail: [email protected]. 1

ACS Paragon Plus Environment


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1. Introduction Monolayer molybdenum disulfide (MoS2) is a quasi two-dimensional (2D) crystal, consisting of a monatomic Mo-layer and two monatomic S-layers. MoS2 monolayer owns a sizable direct band gap (~1.9 eV1,2), which makes it a potential material for field-effect transistors,3 solar cells,4 photodetectors,5,6 and flexible optoelectronic devices.7 However, due to its compound nature and high volatility of the chalcogenide, MoS2 monolayer produced by exfoliation1,3,8–10 and growth methods,11,12 contains abundant structural defects.13–16 Among them, sulfur vacancies (SVs) are the main type of intrinsic defects in MoS2 monolayer and the vacancy density can be up to ~1.3×1013 cm-2.15,17 The abundant SVs create localized states that can act as efficient traps for electrons, holes and excitons, which have been successfully characterized theoretically18–20 and experimentally.14,21 Also, SVs are believed to be the dominant factors which greatly degrade the performance of MoS2 based electronic and optoelectronic devices.6,15,17,22,23 For example, the experimentally attainable mobility24,25 is still one or two order of magnitude lower than the theoretical limitation of 410 cm2/V·s,26 which is attributed to the charge impurities, localized states or scattering centers caused by SVs.17,23 SVs also introduce a new photoemission peak and greatly limit photoluminescence (PL) intensity.27 Neutralizing unwanted carrier traps is one of the most important issues in semiconductor technology. A number of strategies have been developed to neutralize the harmful defects in 2D materials.13,28 For example, a low-temperature thiol chemistry route has been used to repair SVs, resulting in significant reduction of the charged impurities and traps and achieving a high mobility (>80 cm2/V·s) in backgated MoS2 field-effect transistors at room temperature.13 Another example is oxygen passivation of Se vacancies in WSe2 monolayer by 2


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the focused laser annealing in air, enhancing its conductivity by ~400 times and its photoconductivity by ~150 times.28 Theoretically, air passivation of chalcogen vacancies in MX2 (M= Mo, W; X=S, Se, Te) also shows that the chemisorbed O2 changes the chalcogen vacancies from harmful carrier-traps to electronically benign sites.29 Additionally, strong PL of MoS2 monolayer has also been reported through defect engineering and oxygen bonding on SVs.30 However, there is still lack of deep theoretical understanding on the defect mediated/engineered electronic structure and PL mechanism, especially on the optical absorption. In this work, by employing density functional theory (DFT) combining with GW approximation and Bethe-Salpeter equation (BSE) calculations, we show that the presence of SVs creates deep and shallow states in the bandgap and leads to the significant red-shift of the absorption spectrum, whereas surface functionalization via oxygen passivation can repair SVs and form stable localized excitons, which greatly enhances the optical absorption in the spectral range. 2. Computational methods We employed a three-step procedure to determine the electronic and optical properties of MoS2, SV-MoS2 and SVO2-MoS2. First, the Kohn-Sham energies and wave functions were obtained by DFT within the general gradient approximation (GGA) along with the Perdew-Burke-Ernzerh (PBE) exchange-correlation functional31 as implemented in the QUANTUM-ESPRESSO code.32 A 3×3×1 supercell was used and the minimum spacing was about 10 Å between neighboring images. A vacuum region of about 18 Å was inserted between the layers. All the calculations were done in a plane-wave basis using norm-conserving pseudopotentials33 with a 80 Ry energy cutoff and a k-point grid of 6×6×1. 3



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The valence electron configurations were 4s24p64d55s1 for Mo and 3s23p4 for S atoms, respectively. Van der Waals correction was included for weak interaction via the vdW-DF approach.34 The total energy was converged within 10-4 eV and forces acting on each atom were smaller than 0.01eV/Å. Then, the G0W0 approximation35 was employed to obtain quasi-particle (QP) band structures. The convergence of quasi-particle band gap with respect to the number of empty bands, the size of dielectric matrix and the Monkhosrt-Pack grid was carefully examined and a convergence within 0.1 eV was assured. Finally, the coupled electron-hole excitation energies and exciton wave functions were obtained by solving the Bethe-Salpeter equation (BSE).36,37 The involved unoccupied band number (960) was used to obtain the converged dielectric function within random phase approximation (RPA). A fine k-grid (12×12×1), 8 valence bands and 16 conduction bands were included to obtain the converged optical spectra. The G0W0 and BSE calculations were performed with the YAMBO code.38 3. Results and discussions

Figure 1. Optimized structures: (a) perfect MoS2 monolayer (MoS2), (b) MoS2 monolayer with single SV (SV-MoS2) and (c) MoS2 monolayer with an oxygen molecule adsorbed at SV site (SVO2-MoS2). The dMo-Mo and lS-Mo are emphasized by blue and yellow lines, respectively. 4


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A 3×3×1 supercell is built to model perfect MoS2 monolayer (MoS2), MoS2 monolayer with single SV (SV-MoS2), and MoS2 monolayer with an oxygen molecule adsorbed at SV site (SVO2-MoS2), and the optimized structures are shown in Figure 1. The equilibrium lattice constant (3.18 Å) and S-Mo bond length (2.41 Å) of perfect MoS2 monolayer from DFT-PBE calculation match well with the experimental values of 3.16 and 2.41 Å, respectively.39 The oxygen molecule is found to physically adsorb on the perfect MoS2 surface with a very small adsorption energy of 0.049 eV, suggesting that the adsorbed oxygen molecule would easily desorb from the ideal MoS2 surface. The presence of SV makes the Mo-Mo distance (dMo-Mo) surrounding SV and the S-Mo bond length (lS-Mo) shortened by ~2.2% and ~1.2% (see Table 1), respectively. Very interestingly, the presence of SV strongly strengthens the adsorption energy, yielding more than 35 times larger adsorption energy of 1.65 eV between MoS2 and O2, and thus chemisorption takes place on the surface of SV-MoS2. In the relaxed SVO2-MoS2 configuration (Figure 1c), the lower O atom is interacting with two neighboring Mo atoms and the center of O-O bond is located on SV. Also, the O-O bond length is significantly elongated by ~17% from 1.21 to 1.42 Å due to the formation of three Mo-O bonds. In fact, X-ray photoelectron spectroscopy measurement have confirmed the formation of Mo-O bonding in oxygen plasma treated MoS2.30 Our charge analysis also shows that about 1.09 electrons transfer from SV-MoS2 to O2, leaving SV-MoS2 p-type doped. This indicates that the oxygen passivation depletes the excess charges in SV-MoS2 through the Mo-O bonds as the channel.

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Table 1. Structural parameters, band gaps and binding energies of excitons for MoS2, SV-MoS2 and SVO2-MoS2. The average S-Mo distance (lS-Mo) and the average Mo-Mo distance (dMo-Mo) around SV site. Excitation energies Ee(D)/Ee(A) for the first dark/bright states and corresponding exciton binding energies Eb(D) and Eb(A). Distances are in Å, energies are in eV.

System MoS2 SV-MoS2 SVO2-MoS2

lS-Mo 2.41 2.38 2.40

dMo-Mo 3.18 3.11 3.26

Eg(PBE) 1.73 0.91 1.50

Eg(G0W0) 2.89 2.01 2.17

Ee(D) 1.97 1.31 1.09

Ee(A) 1.97 1.34 1.35

Eb(D) 0.92 1.00 1.08

Eb(A) 0.92 0.97 1.01

Figure 2. (a-c) Band structures and projected density of states (PDOS) for MoS2, SV-MoS2 and SVO2-MoS2; (d) Local density of state (LDOS) integrated in the energy range of -0.25~0 and 0.60~1.22 eV for SV-MoS2. The red (black) solid lines are the results from G0W0 (PBE) calculation. Note that three green lines correspond to three localized states from PBE result. The energy of VBM is shifted to 0 eV.

The computed DFT-PBE band structures and density of states (DOS) of MoS2, SV-MoS2 and SVO2-MoS2 are displayed in Figure 2a-c. Both the valence band maximum (VBM) and conduction band minimum (CBM) of MoS2 are located at Γ point, yielding a direct band gap 6


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of 1.73 eV, which is in good agreement with reported theoretical values (1.74 eV,40 1.80 eV41,42). Moreover, from the orbital- and atom- projected DOS, we can see that the VBM exhibits strong hybridization between the S 3p states and Mo 4d states, whereas the CBM are mainly contributed by the 4d states of Mo (Figure 2a), consistent with earlier calculations.18 The presence of SV creates three localized states inside the gap of the host MoS2 (Figure 2b). One shallow localized state occupied by excess electrons is located around VBM, while other two deep unoccupied states are localized around 0.63 eV below CBM, which possibly arises from the broken trigonal crystal field around Mo atoms, as suggested by previous study.20 Local density of states (LDOS) further shows that these localized states are mainly from Mo atoms surrounding the missing S atom, with some mixing with the adjacent S atoms (Figure 2d). Once, electrons become attractive for holes at SV site, these localized states will be served as scattering centers of excitons. Meanwhile, three localized states affect the fundamental gap, reducing the band gap to 0.91 eV. When the O2 chemically adsorbs on the defective MoS2 monolayer, three characteristic localized states in SV-MoS2 largely change (Figure 2c). Two deep localized states are eliminated by the trapping of O2 at SV site, while the shallow localized state shifts down and transforms into a flat “molecular state” located at 0.015 eV above the VBM of the host SV-MoS2. The flat “molecular state” induced by the strong interaction between O2 and SV, belongs to the shallow doping and may act as the shallow donor state. As a result, the band gap of SVO2-MoS2 is significantly increased to 1.50 eV. This value is smaller than that of perfect MoS2, suggesting that electrons are easy to become charge carriers by a lower excitation energy. 7



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As standard Kohn-Sham DFT often underestimates the band gap, we further made G0W0 calculations to obtain accurate band structures of above three systems and presented in Figure 2a-c. The self-energy correction opens a sizable band gap of 2.89, 2.01 and 2.17 eV in MoS2, SV-MoS2 and SVO2-MoS2, respectively. Note that the band gap of MoS2 monolayer (1.9 eV) has been measured by optical absorption1 and photoluminescence,2 which is clearly overestimated by our G0W0 method. However, when the electron-hole interaction is included, the perfect MoS2 monolayer exhibits a strong absorption peak A at 1.97 eV corresponding to the optical band gap, which well reproduces the measured band gap.

Figure 3. (a-c): Optical absorption spectra of MoS2, SV-MoS2 and SVO2-MoS2 with and without electron-hole interaction included, i.e. G0W0+BSE and G0W0+RPA, respectively. A denotes the first absorption peak and two short lines show the positions of the first dark exciton (cyan) and the first bright exciton (dull-red), respectively; (d): Optical absorption spectra of MoS2, SV-MoS2 and SVO2-MoS2 with electron-hole interaction. A 0.15 eV Lorentzian broadening is employed in these plots. 8


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Now, let’s focus on the optical absorption of MoS2, SV-MoS2 and SVO2-MoS2. Figure 3a-c depicts their optical absorption spectra with and without electron-hole interaction. Comparing to the independent-particle picture, the optical absorption spectra with electron-hole interaction included witness a weight redistribution of the oscillator strength, and thus a global red-shift of the whole spectrum is seen. Moreover, the excitonic effects are significant in these materials and play a decisive role on the optical absorption spectra, due to the reduced dimensionality and weakened electronic screening. The perfect MoS2 has a strong absorption peak A at 1.97 eV in which the first bright exciton (the first bright state) and the first dark exciton (the lowest excited state) are located (Figure 3a). The presence of SV dramatically red-shifts the absorption edge of the optical spectrum (Figure 3d), which is attributed to the large density of vacancy (~1.3×1014 cm-2) in our model. At present, such large red-shift of absorption edge in monolayer MoS2 is difficultly observed experimentally, because the small SV density of ~1.3×1013 cm-2 obtained from experiment.15,17 The peak A is located at 1.34 eV with slightly attenuated intensity. More interestingly, although the optical absorbance is overall weakened in the spectral range of 0~5 eV, the optical absorption between 1 and 2.5 eV is greatly strengthened. The enhanced optical absorption is attributed to the dramatically increased number of bright excitons in the SV-MoS2 which is twenty times more than that in perfect MoS2 monolayer in the energy range of 1~2.5 eV. When passivating SV via O2, the position of peak A almost does not change in comparison with SV-MoS2, while the intensity of peak A and the optical absorbance in the energy range of 0~5 eV are enhanced remarkably. Experimentally, the optical absorption was strengthened with decreasing electron doping in the exfoliated MoS2 monolayer.43 The enhancement in optical 9



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absorbance mainly arises from the increased number of bright excitons in the spectral range. As discussed above, the presence of SV leaves excess electrons at a vacancy site, which mainly come from the three unsaturated Mo atoms, leading to the charged exciton (trion), while the oxygen molecule depletes excess electrons in SV-MoS2 via the formation of the Mo-O bonds. As a result, the number of neutral excitons, especially bright excitons in SVO2-MoS2 is twice more than that in SV-MoS2. The previous experiment43,44 has also demonstrated that the depletion of charge carriers in the exfoliated MoS2/MoSe2 monolayer would stabilize (destabilize) neutral (charged) excitons, leading to a transition from the charged exciton (eeh) to neutral exciton (eh).

Figure 4. Diagram of the transition processes of the first dark exciton (a, c, e) and the first bright exciton (b, d, f) corresponding to the first absorption peak A in MoS2, SV-MoS2 and SVO2-MoS2.

To get profound insight into the absorption mechanism, we plot the transition processes of the first dark exciton and the first bright exciton in Figure 4a-f. The bright exciton in the SV-MoS2 mainly originate from the optical transitions between VBM of the hosting MoS2 and the unoccupied localized states at Γ-point (Figure 4d), suggesting that the exciton is bound to SV. Clearly, the presence of the unoccupied localized states decreases transition

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matrix element, leading to the descended intensity of peak A. When an O2 is adsorbed on SV, the first bright exciton is largely contributed by the optical transition between the VBM of hosting SV-MoS2 and the CBM at the Γ-point (Figure 4e-f), indicating that the molecular state has negligible influence on the formation of the first bright exciton and the transition process is similar to perfect MoS2, directly giving rise to the increase of transition matrix element. Therefore, the intensity of peak A is remarkably enhanced.

Figure 5. (a-c) Top views of the electron wave functions of first dark excitons in MoS2, SV-MoS2 and SVO2-MoS2. The black dots indicate the hole position.

To further explicitly understand the correlation between excited electrons and holes, we depict the wave function of the first dark exciton of MoS2, SV-MoS2 and SVO2-MoS2 by diagonalizing the Bethe-Salpeter two-particle’s Hamiltonion. The excitonic wave function can be written as

| ΨS (re , rh )〉 = ∑AScvk ψck (re )ψ*vk (rh ) ,

(1)

cvk

where re and rh are the electrons and holes coordinates in real space, respectively, and Ψ 2

is the quasi-particle wave function. The electron probability distribution ψS (re , rh ) , as a function of the electron position re with the fixed hole position rh for the first dark exciton, describes how the excited electrons and holes are correlated in real-space. Real-space electron distribution presented in Figure 5 clearly shows that the first dark exciton in perfect 11



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MoS2 is largely spread, with an exciton diameter over 46 Å, in concordance with the small binding energy of 0.92 eV (Table 1); while that of SV-MoS2 is confined to a smaller range of 38 Å, accompanied by a larger binding energy of 1 eV. In contrast, the first dark exciton of SVO2-MoS2 is greatly localized with a spatial extension of only ~20 Å and its binding energy is as large as 1.08 eV. The formation of highly stable and localized dark excitons around SVs in SVO2-MoS2 is expected to avoid the nonradiative recombination. Additionally, it is found that the number of excited states between the first dark state and the first bright state significantly increases in SVO2-MoS2, as shown in Figure 6c. According to Kasha’s rule, system at high excited states must relax to the first dark state first and then emits light through radiative decay process to the ground state. The excessive intermediate states between the first dark state and the first bright state of SVO2-MoS2 will provide efficient paths for relaxation, leading to its high PL efficiency. This can well explain the huge photoluminescence enhancement at defects/cracks sites of MoS2 monolayer through defect engineering and oxygen bonding observed experimentally.30

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Figure 6. (a-c) Oscillator strength of the excited states of MoS2, SV-MoS2 and SVO2-MoS2 in the energy range of 1~2.5 eV. The green and red short dot lines refer to the positions of the first dark state and first bright state, respectively.

4. Conclusion In summary, we performed DFT combined G0W0 and BSE calculations to explore how the electronic property and optical absorption of MoS2 monolayer are affected by SVs, and how they are further engineered through O2 passivation. It is found that SVs induce localized midgap states in the bandgap, leading to the red-shift of the absorption peak and the stronger absorbance in the visible light and near infrared region. However, the O2 passivation at SV sites can greatly enhance the optical absorption of SV-MoS2 in the spectral region. This is due to the formation of stable localized neutral exciton and the transition from charged excitons to neutral free excitons. Our results provide a clear picture on the SV mediated/engineered optical properties of MoS2 monolayer and well explain available experimental observations that a strong photoluminescence enhancement of monolayer MoS2 through defect engineering and oxygen bonding, which offers insightful guides for improving the performance of 2D dichalcogenide based optoelectronic devices.

Notes The authors declare no competing financial interest. Acknowledgments This work is supported by the NSFC (21525311, 21373045, 11404056) and NSF of Jiangsu (BK20130016) and SRFDP (20130092110029) in China and the Scientific Research 13



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Foundation of Graduate School of Southeast University (YBJJ1521) in China. The authors appreciate the computational resources provided by Southeast University.

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Toc Graphic

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Greatly Enhanced Optical Absorption of a Defective MoS2 Monolayer

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