Suppressed Carrier Recombination in Janus MoSSe Bilayer

6 days ago - Janus transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional materials with a vertical dipole ...
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Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 5564−5570

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Suppressed Carrier Recombination in Janus MoSSe Bilayer Stacks: A Time-Domain Ab Initio Study Bing Song,† Limin Liu,*,†,‡ and ChiYung Yam*,† †

Beijing Computational Science Research Center, Haidian District, Beijing 100193, China School of Physics, Beihang University, Beijing 100083, China

‡

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S Supporting Information *

ABSTRACT: Janus transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional materials with a vertical dipole moment. Here, using timedomain ab initio simulations, we show that electron−hole recombination can be substantially suppressed via different stacking orientations of bilayer MoSSe. Despite having a larger net dipole moment, a S−Se/S−Se oriented MoSSe bilayer has a shorter carrier lifetime due to strong nonadiabatic coupling and a small band gap. The electron−hole recombination is coupled to the interlayer out-of-plane motion. In contrast, the opposite vertical dipoles weaken interlayer interactions in symmetric oriented MoSSe bilayers. Consequently, initial and final states are localized within different layers, and this significantly suppresses carrier recombination, resulting in an order of magnitude longer excited carrier lifetime in Se−S/S− Se oriented MoSSe bilayers. Our simulations provide theoretical insights into the carrier dynamics and suggest a way to enhance the carrier lifetime in Janus TMDs for efficient energy harvesting.

O

Waals couplings between these heterostructures.21 Other studies show that the electronic structures can be tuned by different stackings.22 These properties underscore the important role of their interlayer interactions. In this Letter, we employ time-domain ab initio simulation to study the excited carrier dynamics in bilayer MoSSe with different vertical orientations. Carrier dynamics is fundamentally significant to various processes in physical, chemical, and biological systems. For applications such as solar cells, lightemitting diodes, and transistors, electron−hole separation and recombination play crucial roles in device operation. Here, we investigate the effect of the vertical dipole in Janus MoSSe on the rate of electron−hole recombination. The result shows that the interlayer interactions are weakened in symmetric S−Se/ Se−S and Se−S/S−Se oriented MoSSe bilayers due to the opposite vertical dipoles, and charge recombination is significantly suppressed. On the other hand, the strong nonadiabatic (NA) coupling and small band gap in the S− Se/S−Se oriented MoSSe bilayer lead to a short carrier lifetime. Our results imply that these Janus TMD stacks can have great potential application for solar energy conversion and photocatalysis. The simulations were performed using density functional theory (DFT) as implemented in the Vienna ab initio simulation package.23,24 The nonlocal exchange and correlation effects were treated by the Perdew−Burke−Ernzerhof (PBE) functional.25 van der Waals corrections were taken into

ver the previous decades, low-dimensional materials have received considerable interest owing to their unique physical and chemical properties such as large surface areas and abundant reactive sites.1−5 In particular, transition metal dichalcogenides (TMDs) with the general formula MX2 (M = Mo, W; X = S, Se, or Te) have become a focus of both experimental and theoretical studies because of their extraordinary properties, including strong catalytic activity, high photoluminescence efficiency, etc.6−10 Recently, monolayer MoSSe has been successfully synthesized and characterized.11 While monolayer MX2 consists of two layers of chalcogen atoms and one layer of transition metal atoms with an out-of-plane mirror reflection symmetry, these Janus monolayers of TMD with general formula MoXY (M = Mo; X/Y = O, S, Se and Te; X ≠ Y) break that mirror symmetry and result in a vertical dipole. The electronic properties of MoSSe monolayer and multilayer structures have been studied using first-principles calculations.12,13 Compared to the pristine MoX2 materials, these Janus MoXY exhibit superior photocatalytic activities with suitable band alignment for water splitting due to their intrinsic dipoles.14−16 The potential of MoSSe as alternative material for various device applications has also been demonstrated.17,18 The electronic properties of TMDs dramatically change from bulk to monolayer samples; for instance, the isolation of single layers of van der Waals bonded TMDs leads to the wellknown indirect to direct band gap transition of MoS2,19 and the reduced dielectric screening in monolayer and few-layer MoS2 drastically increases exciton binding energies.20 Recent experimental studies demonstrate that TMD heterostructures display ultrafast charge transfer, despite the weak van der © XXXX American Chemical Society

Received: July 15, 2019 Accepted: September 1, 2019 Published: September 1, 2019 5564

DOI: 10.1021/acs.jpclett.9b02048 J. Phys. Chem. Lett. 2019, 10, 5564−5570

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The Journal of Physical Chemistry Letters

Figure 1. Side and top views of the (a) MoS2 and (b) MoSSe atomistic models. PDOS of monolayer (c) MoS2 and (d) MoSSe. Band structure of monolayer (e) MoS2 and (f) MoSSe.

Figure 2. Time evolution of the populations of the initial states for the recombination in (a) MoS2 and (b) MoSSe monolayers. Black lines: NAMD simulation results; red lines: exponential fits.

account by the DFT-D3 method of Grimme in both geometry optimization and molecular dynamics (MD) simulations.26,27 The projector-augmented wave approach was used to describe the interactions between ionic cores and valence electrons.28 A plane wave basis with an energy cutoff of 500 eV was used. The convergence criteria for energy and force were set to 10−5 eV/ atom and 0.02 eV/Å. To model a two-dimensional slab, a 10 Å vacuum space was added between bilayers to avoid interactions between periodic images. The Brillouin zone integration was

performed using the Monkhorst−Pack sampling scheme with 3 × 3 × 1 k-points.29 After relaxing the geometry at 0 K, the system was heated up to 300 K by repeated velocity rescaling.30−32 Adiabatic MD simulation was then carried out in the microcanonical ensemble for 7.0 ps with a time step of 1.0 fs. The electron−hole recombination dynamics was simulated using PYXAID,33,34 and 2000 geometries from the first 2 ps of the adiabatic MD trajectory were used as initial conditions for the nonadiabatic molecular dynamics (NAMD) simulations. In our 5565

DOI: 10.1021/acs.jpclett.9b02048 J. Phys. Chem. Lett. 2019, 10, 5564−5570

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The Journal of Physical Chemistry Letters simulations, decoherence effects play an important role due to the slow electron−hole recombination across the band gap.33,35 We employed the decoherence-corrected fewest switches surface hopping (FSSH) technique,31,36−38 where the decoherence effect is added to the FSSH algorithm as a semiclassical correction.34,39 To avoid heavy computational cost, the classical path approximation was employed, where nuclear dynamics was assumed to be unaffected by the dynamics of the electron degree of freedom. The approximation is mainly based on the fact that the changes in the nuclear geometry upon photoexcitation are smaller than the amplitude of the thermally induced fluctuations in the nuclear coordinates. The current approach has been applied to investigate photoexcitation dynamics in a broad range of systems.40−47 Due to the asymmetry of the atomic structure perpendicular to the xy-plane, there exists a built-in electric field in MoSSe with the direction pointing from the Se atomic plane to the S atomic plane. As demonstrated in the previous work, the internal electric field in the Janus structures can serve as an effective way to separate excitons into free electrons and holes.14 Our DFT results show that the dipole moment in a monolayer MoSSe is 0.19 D. To investigate the influence of the built-in electric field on the electron−hole recombination in Janus MoSSe, we first compare the electronic structures and excited-state dynamics in MoSSe with those in MoS2. Figure 1 shows the atomistic models and electronic band structures of monolayer MoS2 and MoSSe employed in this work. The calculated band gap of a pristine MoS2 monolayer is 1.77 eV, and that of a monolayer MoSSe is 1.67 eV, which are consistent with previous works.11,19,48 From Figure 1c,d, it is shown that the valence band maximum (VBM) and conduction band minimum (CBM) are mainly contributed by the d-orbital of Mo atoms. Due to the intrinsic dipole in MoSSe, the charge densities of the VBM and CBM are slightly polarized as compared to those in MoS2, as shown in Figure S1. Charge recombination is a major energy loss mechanism in electronics and photovoltaic applications. To evaluate the rate

Figure 3. (a) Atomic structure of the supercell of S−Se/S−Se oriented bilayer MoSSe. (b) Band structure and (c) PDOS of the bilayer MoSSe.

overlap between the states at the VBM and CBM, and a larger overlap of the states typically leads to stronger coupling and a faster NA transition between them. Overall, the NA coupling for the recombination in Janus MoSSe is weakened, and the carrier lifetime in MoSSe is found to be slightly longer than that in MoS2. Recent work shows that a nanosecond time scale MD trajectory can be constructed to obtain a more accurate fit to the carrier lifetime.46 Along this line, it is anticipated that a stronger built-in electric field might facilitate charge carrier separation. This can be readily achieved by stacking a multilayer Janus TMD in the same orientation. Figure 3a depicts the atomic structure of an optimized bilayer MoSSe with a S−Se/S−Se stacking orientation. The distance between adjacent Mo atomic planes (Δz) is 6.77 Å, and the calculated dipole moment in bilayer MoSSe is 0.34 D, which is about 2 times larger than that in its monolayer counterpart. The band gap of S−Se/S−Se oriented bilayer MoSSe is significantly reduced to 1.21 eV, as shown in its band structure in Figure 3b. Further analysis of the projected density of states (PDOS) reveals that both the VBM and CBM are mainly contributed by the d-orbital of Mo atoms, similar to monolayer MoSSe. Figure 4a shows the evolution of the population of the CBM for the S−Se/S−Se oriented bilayer MoSSe. A plateau is observed in the evolution of the excited-state population from 1.8 to 2.7 ps. To understand the excited carrier dynamics, we plot the time evolutions of NA electron−phonon coupling for the recombination and the interlayer distance in Figure 4b,c, respectively. Interestingly, it is found that the electron−hole recombination is coupled to the out-of-plane motion of the two layers. A clear correlation

Table 1. Band Gap, Root-Mean-Square NA Coupling, Dephasing Time, and Nonradiative Electron−Hole Recombination Time for MoS2 and MoSSe

MoS2 MoSSe

band gap (eV)

NA coupling (meV)

dephasing time (fs)

carrier lifetime (ps)

1.77 1.67

1.40 1.28

21.01 25.38

1548.54 1869.27

of electron−hole recombination, the NAMD of excited charge carriers in MoS2 and MoSSe is simulated. It is assumed that electron−hole pairs are generated upon photoexcitation in the beginning of the NAMD simulations. Because the intraband relaxation of hot carriers occurs on a much faster time scale, we therefore assume that they have relaxed to band edges prior to the recombination. Figure 2 shows the evolution of excitedstate population in the monolayer MoS2 and MoSSe. The rates of charge recombination are obtained by a linear fit, exp(−t/τ) ≈ 1 − t/τ. The results show that the intrinsic dipole in MoSSe has only a minor effect on the separation of charge carriers. The band gap, root-mean-square of NA couplings, dephasing time, and carrier lifetime for MoS2 and MoSSe are summarized in Table 1. The NA coupling depends significantly on the 5566

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Figure 4. Time evolution of (a) the excited carrier population in S− Se/S−Se oriented bilayer MoSSe. Black line: NAMD simulation result; red line: exponential fit. (b) NA electron−phonon coupling and (c) interlayer distance Δz.

Figure 6. (a) Comparison of the excited carrier population in S−Se/ S−Se, S−Se/Se−S, and Se−S/S−Se oriented bilayer MoSSe. Time evolution of (b) NA electron−phonon coupling and (c) interlayer distance in Se−S/S−Se oriented bilayer MoSSe.

between the interlayer distance and the NA coupling is observed. Due to the different electronegativities of Se and S atoms, the charge distribution within each MoSSe layer is polarized. This gives rise to an electrostatic attractive force between the bilayer, and hence, a perpendicular interlayer oscillation results. When the two layers are close, the NA electron−phonon coupling for electrons and holes is strong, which facilitates electron−hole recombination. On the other hand, when the two layers are far apart, the NA coupling between the excited carriers vanishes. The electronic states are essentially decoupled, and transitions between them are prohibited, which explains the plateau in the time evolution of excited-state population. This observation is further confirmed by the charge distributions of the VBM and CBM at different time shots. Figure 5a,b shows the charge distributions of the VBM and CBM in S−Se/S−Se ordered bilayer MoSSe at 5 and 1648 fs, respectively. At 5 fs, the two MoSSe layers are far apart, and clearly, the states at the VBM

Table 2. Band Gap, Root-Mean-Square of NA Coupling, Dephasing Time, and Carrier Lifetime of S−Se/S−Se, S− Se/Se−S, and Se−S/S−Se Oriented Bilayer MoSSe

S−Se/S−Se S−Se/Se−S Se−S/S−Se

band gap (eV)

NA coupling (meV)

dephasing time (fs)

carrier lifetime (ps)

1.21 1.45 1.45

1.441 0.907 0.406

9.25 10.59 13.15

315.46 2176.18 11875.86

and CBM are well-separated, whereas significant overlap between states at the VBM and CBM is observed at 1648 fs when the two MoSSe layers are close. Taking into account the interlayer motion, a carrier lifetime of about 315.46 ps is obtained by an exponential fit. Details of the fitting is given in the Supporting Information. In our simulations, the interlayer motion is mainly governed by electrostatic interactions. The validity of the classical path approximation is based on the fact

Figure 5. Charge distributions of the VBM and CBM in S−Se/S−Se oriented bilayer MoSSe at (a) 5 and (b) 1648 fs. 5567

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Figure 7. Simple model for evaluation of the Coulombic force between the Janus MoSSe monolayers. The interplane distances r1, r2, and r3 are set as 1.53, 1.71, and 7.0 Å, respectively.

down the radiative recombination of the electron−hole pairs. For systems with a large interband dipole moment, a radiative channel should be considered for a more quantitative analysis. This can be taken into account by calculating the coupling matrix between exciton momenta and radiation.51,52 For comparison, the band gaps, root-mean-square of NA couplings, dephasing time, and electron−hole recombination time for S− Se/S−Se, S−Se/Se−S, and Se−S/S−Se ordered MoSSe bilayers are reported in Table 2. The different interlayer interactions can be explained by electrostatic interactions between the MoSSe layers. A simple model is constructed to evaluate the Coulombic forces between the MoSSe monolayers, as depicted in Figure 7. The model comprises six atomic planes. Using the parameters obtained from DFT simulations, it is found that the repulsive force is stronger in the Se−S/S−Se oriented MoSSe bilayer as compared to that in the S−Se/S−Se oriented bilayer. Details of the model are given in the Supporting Information. Due to the stronger interlayer repulsive force in the Se−S/S−Se oriented MoSSe bilayer, the interlayer distance in the Se−S/ S−Se oriented MoSSe bilayer is larger and a weaker interaction is expected. Therefore, it is found that the intrinsic dipoles of MoSSe have an indirect effect on the NA coupling in multilayer MoSSe, which in turn has a significant effect on the lifetimes of excited carriers in the materials. Experimentally, the MoSSe monolayer can be synthesized by first replacing the top layer sulfur atoms in MoS2 with hydrogen atoms using remote hydrogen plasma. This is followed by a subsequent thermal selenization, which substitutes the hydrogen atoms with selenium atoms, forming a structurally stable Janus MoSSe monolayer.11 In practice, large-area synthesis of bilayer MoS 2 films has been demonstrated using chemical vapor deposition.53 To synthesize symmetric S−Se/Se−S and Se−S/S−Se oriented MoSSe bilayers, similar techniques can be applied to bilayer MoS2 by replacing the sulfur atoms with selenium atoms in both the top and bottom layers. In summary, on the basis of first-principles calculations, we investigate the influence of a built-in electric field on the electron−hole recombination in Janus MoSSe materials. It is found that the intrinsic dipole in MoSSe has only a minor effect on the separation of charge carriers. Instead, the carrier

that the charge polarizations in the MoSSe bilayer are similar for the ground state and excited state. In addition, the induced charge changes in the ground state and excited state due to the thermal fluctuations should be qualitatively the same. Thus, the interlayer dynamics should be essentially unaffected. Surprisingly, the carrier lifetime in S−Se/S−Se ordered bilayer MoSSe is about an order of magnitude shorter than that in the monolayer despite the fact that there is a stronger built-in electric field. The reason for the short carrier lifetime in bilayer MoSSe is its reduced band gap because the probability of NA Eg

( )

transitions is proportional to exp − k T where Eg is the band B

gap, kB is the Boltzmann constant, and T is the temperature. The reduced band gap substantially increases the interband transition probability and thus the recombination rate. MoSSe bilayers can exist as two other configurations, the S− Se/Se−S and Se−S/S−Se stacking orientations. Here, we further study the excited-state dynamics in symmetric oriented bilayer MoSSe. Figure 6a plots the time evolution of the excited-state population for the Se−S/S−Se ordered bilayer. Compared to that in the S−Se/S−Se ordered bilayer, carriers reside much longer in the Se−S/S−Se ordered bilayer, with a fitted lifetime of 11.9 ns. The changes of the NA electron− phonon coupling and the interlayer distance along the MD trajectory are also plotted in Figure 6b,c, respectively. Clearly, the average interlayer distance is longer due to the opposite vertical dipoles, and this results in a weaker interaction between the two TMD layers. The spatial distributions of the states at the VBM and CBM at different time shots are analyzed in Figure S3. It can be seen that these states are mostly decoupled along the MD trajectory, which significantly suppresses charge recombination and results in an order of magnitude longer carrier lifetime. Similar results are obtained for the S−Se/Se−S oriented bilayer, and the calculated carrier lifetime is 2.2 ns. Detailed analysis of its excited-state dynamics is given in Figure S4. It has been reported that the radiative decay can be the major recombination channel in MoS2.49,50 In such a case, a large interband dipole moment is required for efficient radiative decay of electron−hole pairs. For bilayer MoSSe in this study, interlayer excitons are involved where the CBM and VBM are separated in the two layers. It is expected that the interband dipole moment is small and thus slows 5568

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lifetimes depend significantly on the NA coupling between occupied and unoccupied states. Due to strong NA coupling and a small band gap, the S−Se/S−Se oriented MoSSe bilayer has a short carrier lifetime. In contrast, the opposite vertical dipoles in symmetric S−Se/Se−S and Se−S/S−Se oriented MoSSe bilayers weaken interlayer interactions and result in weaker NA coupling. This leads to a significantly reduced carrier recombination and results in an order of magnitude longer excited carrier lifetime in Se−S/S−Se oriented MoSSe bilayers. We believe that the results provide theoretical insights into the excited carrier dynamics and suggest a way for enhancing the carrier lifetime in Janus TMDs for efficient energy-harvesting applications. We expect that the findings in this work would also apply to other Janus TMDs that possess an out-of-plane dipole moment. However, for different heterostructures, other recombination mechanisms may be involved. For instance, ultrafast charge transfer had been reported in MoS2/WS2 heterojunctions, despite the weak binding of these heterostructures. Wang et al. attributed the ultrafast charge transfer to the plasma oscillations that resulted from the collective motion of excitons at the interface.54 In such cases, carrier dynamics would also depend on excitation frequencies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b02048. Spatial distributions of the VBM and CBM of monolayer MoS2 and MoSSe at 0 K; time evolution of the excitedstate carrier population in S−Se/S−Se oriented bilayer MoSSe; analysis of Se−S/S−Se and S−Se/Se−S excited-state dynamics; and a simple physical model for evaluation of the Coulombic force between Janus MoSSe monolayers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bing Song: 0000-0002-4346-4425 Limin Liu: 0000-0003-3925-5310 ChiYung Yam: 0000-0002-3860-2934 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Science Challenge Project (TZ2018004) and the National Natural Science Foundation of China (21673017 and U1930402) is gratefully acknowledged. Computational support from the Special Program for Applied Research on Super Computation of the NSFCGuangdong Joint Fund (the third phase) (U1501501) and the Beijing Computational Science Research Center (CSRC) is also acknowledged.

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The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.9b02048 J. Phys. Chem. Lett. 2019, 10, 5564−5570