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Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 2797−2802

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Photoexcitation Dynamics in Janus-MoSSe/WSe2 Heterobilayers: Ab Initio Time-Domain Study Yan Liang,† Jianwei Li,‡ Hao Jin,*,‡ Baibiao Huang,† and Ying Dai*,† †

School of Physics, State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, People’s Republic of China College of Physics and Energy, Shenzhen Key Laboratory of Advanced Thin Films and Applications, Shenzhen University, 518060 Shenzhen, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: The photoexcitation dynamics plays a key role in determining the properties of van der Waals heterostructures (vdWHs). Based on the time-dependent density functional theory combined with nonadiabatic molecular dynamics, we investigate the charge transfer in JanusMoSSe/WS2 vdWHs. Ultrafast charge separation is observed, arising from the large overlapping between the donor and acceptor states. While the electron−hole recombination is 2 orders of magnitude slower than the charge separation, this can be understood by the fact that the initial and final states are strictly confined to different materials. Additionally, photoresponsivity performance of the vdWHs is also evaluated using density functional theory combined with the nonequilibrium Green’s functions. Simulated results of high photoresponsivity in a broad range of the spectrum endows proposed systems powerful potential in optoelectronic and photovoltaic applications. The atomistic picture revealed in our work provides chemical guidelines and facilitates the design of next-generation devices for light detecting and harvesting.

T

photoinduced charge separation and recombination dynamics in layered regular TMD vdWHs, the charge transfer dynamics have not been studied yet in janus-based vdWHs, especially for the influence of intrinsic out-of-plane dipole on the nonradiative relaxation. In this sense, a comprehensive understanding of the charge transfer process in janus-based vdWH is necessary. Inspired by the aforementioned remarkable work on MoS2/ WSe2 by Peng et al.,10 together with the small lattice mismatch between janus-MoSSe and WSe2, in this work, we focus on the janus-MoSSe/WS2 vdW heterostructure. We investigate the electronic structure and interlayer coupling between janusMoSSe and WSe2 systematically in the framework of timedependent density functional theory (TD-DFT). For the first time, the photoinduced charge separation and recombination dynamics at the janus-MoSSe/WSe2 vdWH are described explicitly in the time-domain. Ultrafast carrier transfer is observed within the heterointerface, ranging from 286 fs to 1.03 ps depending on the type of the vdWHs. However, the time scale for electron−hole recombination is, on the other hand, much longer than that of the charge separation, with the value up to 85 ps. The charge transfer mechanisms are analyzed by taking into account the effect of electronic states, electron− phonon coupling, and quantum coherence.14 Furthermore, the performance of janus-MoSSe/WSe2 vdWHs is evaluated using density functional theory combining with the nonequilibrium Green’s functions (NEGF-DFT). We find that the studied

wo-dimensional (2D) transition metal dichalcogenides (TMDs) with the general formula MX2 (M = Mo, W; X= S, Se) have drawn considerable attention due to their promising electronic and optical properties, which hold potential applications in fields such as optoelectronics, field-effect transistors, and spintronics.1−6 Very recently, the crystal configuration of sandwiched S−Mo−Se (Janus-MoSSe) has been successfully fabricated by replacing the top layer of Se (S) atoms with S (Se) atoms, while the bottom Se (S) layer remaining intact in the MoSe2 (MoS2) monolayer.7,8 In comparison with regular MX2, the janus-MoSSe monolayer is elucidated to possess intrinsic out-of-plane dipole due to the lack of reflection symmetry with respect to the central Mo atoms.8,9 To improve the electronic and optical properties of monolayer TMDs, van der Waals heterostructures (vdWHs) are developed by coupling different TMDs together. These 2D vdWHs usually exhibit type-II band alignments, where a staggered bandgap occurs at the interface. It is known that charge transfer is a fundamental process that controls electron− hole recombination and hence plays a significant role in determining the properties of vdWHs. In recent experiments, charge transfer dynamics in TMD vdWHs is investigated using femto-second pump−probe spectroscopy. For example, Peng et al. reported that electron transfer from WSe2 to MoS2 took place within 470 fs.10 Rigosi and co-workers found that hole transfer from the MoS2 layer to the WS2 layer took less than 50 fs after photoexcitation.11 On the other hand, an extremely long-time scale for electron−hole recombination are reported in TMD vdWHs, ranging from 250 ps to 1.8 ns.12,13 Although numerous works have been carried out to investigate the © XXXX American Chemical Society

Received: March 24, 2018 Accepted: May 10, 2018 Published: May 10, 2018 2797

DOI: 10.1021/acs.jpclett.8b00903 J. Phys. Chem. Lett. 2018, 9, 2797−2802

Letter

The Journal of Physical Chemistry Letters

or bonds broken after heating the system at 300 K for 6 ps. These results further demonstrate that the studied systems are stable even at room temperature. The charge transfer processes under investigation are schematically shown in Figure 3a. Upon illumination, the photoinduced holes transfer from MoSSe layer with lower energy level to WSe2 layer with higher energy level, while electrons transfer in the opposite way. It is believed that the electron−hole recombination rate is low if the electrons and holes are confined to different materials. Before describing the nonadiabatic molecular dynamic (NAMD) results, it is informative to study their electronic properties. The calculated band structures using PBE functional for SSeIH and SeSeIH are plotted in Figure 3b. Different colors are employed to label the contributions of each single layer, i.e., blue for WSe2 and red for MoSSe monolayers. In both cases, direct band gaps are observed, with the values of 0.79 and 1.32 eV for SSeIH and SeSeIH, respectively. It is worth noting that these 2D vdWHs show type-II band alignment, in which the conduction band minimum (CBM) and valence band maximum (VBM) are staggered between two monolayers. The band offsets of SSeIH are 0.75 eV for VBM and 0.59 eV for CBM. Such large energy difference leads to the band bending at the heterointerface, which provides driving force and pushes electrons (holes) to move from WSe2 (MoSSe) to MoSSe (WSe2) sides. While for SeSeIH, the band offsets are relatively smaller, indicating less energy is lost to vibrational motions during the charge transfer process. It should be emphasized that, although the rigorous HSE06 functional can give a more accurate description of the electronic properties, they are significantly computationally expensive and can not be combined with NAMD, which requires thousands of electronic structure calculations.16 Although, on the other hand, PBE functional usually underestimates the band gap, it can reproduce the experimental charge transfer dynamics correctly. This is because it is accurate enough to describe the spatial distributions of electronic states and the states coupling, which is crucial for dynamic simulations.17 In Figure S3, we plot the band structures of SSeIH and SeSeIH using the HSE06 functional. The results show that, despite the band gaps, the dispersion of the band structures are similar as compared with the results obtained by

vdWHs show high photoresponsivity in a broad range of spectrum, indicating great potential in optoelectronic and photovoltaic applications. As shown in Figure 1a, monolayer janus-MoSSe consists of S, Mo, and Se three atomic planes. The top and bottom planes are

Figure 1. (a) Trigonal prismatic coordination geometry of janusMoSSe. Side views of janus-MoSSe/WSe2 vdWH with (b) S and (c) Se side of MoSSe is interfaced with WSe2, which are denoted as SSeIH and SeSeIH, respectively.

chalcogen atoms in a triangular lattice structure, while the middle sublayer is another triangular lattice of metal atoms. In the present work, two types of janus-MoSSe/WSe2 vdWHs are considered. The first one is called SSeIH, in which the top layer of S atoms in janus-MoSSe touches with the bottom layer of Se atoms in WSe2 monolayer (see Figure 1b), whereas the other one is named SeSeIH, where the Se layer in janus-MoSSe connects with the WSe2 monolayer (see Figure 1c). Following the previous study,15 we adopt the most stable AB stacking arrangement (see Figure S1b of the Supporting Information (SI)), which has the lowest formation energy among the six staking configurations (see Table S1 in SI). To examine the stabilities of the proposed SSeIH and SeSeIH, vibration spectrum as well as the molecular dynamics calculations are carried out. Dynamically stabile structures are heralded when calculated dispersions of phonon modes have positive values of frequency throughout the Brillouin zone. As shown in Figure 2a and Figure S2a, there are no imaginary frequencies in the phonon dispersion curves for both SSeIH and SeSeIH. In addition, ab initio adiabatic molecular dynamics calculations are also carried out in this work. The temperature and total energy fluctuation are presented in Figure 2b,c and Figure S2b,c, which indicate that no geometric reconstructions

Figure 2. Phonon dispersion (a). Fluctuation of total energy (b) and temperature (c) with time obtained from adiabatic molecular dynamic simulation of SSeIH at 300 K. 2798

DOI: 10.1021/acs.jpclett.8b00903 J. Phys. Chem. Lett. 2018, 9, 2797−2802

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The Journal of Physical Chemistry Letters

Figure 3. (a) Schematic of the photoexcitation and charge transfer dynamics at the janus-MoSSe/WSe2 interface due to ① electron transfer,② hole transfer and ③ recombination of electron and hole. (b) Band structures of SSeIH and SeSeIH.

the PBE functional, suggesting that PBE functional is sufficient to ensure the validity of the obtained results. To obtain a deep insight into the origin of the charge transfer dynamics, we analyze the charge distributions of the donor and acceptor states in the vdWHs. The results are shown in Figure 4. In the charge separation processes, excitation of MoSSe

are characterized in Figure 5a−d. The time constants are obtained by the exponential fitting, i.e., f(t) = a + b exp (−t/τ).

Figure 4. Charge densities of the key states at (a) SSeIH and (b) SeSeIH: (1) Valence band maximum (VBM) of MoSSe. (2) VBM of WSe2. (3) Conduction band minimum (CBM) of MoSSe. (4) CBM of WSe2. Green, gray, yellow, and purple spheres denote the Se, W, S, and Mo atoms, respectively.

causes hole transfer from MoSSe to WSe2. Thus, the VBMs of MoSSe (states 1) and VBMs of WSe2 (states 2) are denoted as donor and acceptor states of holes in the following analysis. Likewise, absorption of a photon by WSe2 leads to electron transfer from WSe2 to MoSSe. Therefore, the CBMs of WSe2 (states 4) and CBMs of MoSSe (states 3) are named as donor and acceptor states of electrons. In addition, states 2 and 3 are also the initial states of the recombining hole and electron, in which charge recombination occurs at these two states. As has been discussed above, the CBM (state 3) and VBM (state 2) of the vdWHs, are localized strictly within the single monolayer. Consequently, it is expected that interlayer interactions are weak, leading to low electron−hole recombination rate. On the other hand, the situations for donor orbitals involved in the separation dynamics between MoSSe and WSe2 monolayers are different. As shown in Figure 4a, the donor states of electrons (state 4) in SSeIH contain charge densities of both materials. These delocalized states indicate strong interlayer couplings between MoSSe and WSe2 for electrons, while the donor states of holes (state 1) are localized and confined to the WSe2 side. As a result, the interlayer coupling is weaker for holes than that of electrons in SSeIH. By contrast, the interlayer coupling becomes much stronger in SeSeIH. As plotted in Figure 4b, large overlapping states within the interlayer can be observed for donor orbitals of electrons (state 4) and holes (state 1), which facilitate the nonadiabatic electron−phonon coupling. The dynamics of photoinduced charge separation processes resulting from photoexcitation of either janus-MoSSe or WSe2

Figure 5. (a) Electron and (c) hole separation dynamics in SSeIH, as well as (b) electron and (d) hole separation dynamics in SeSeIH. The separation time is fitted by an exponential relation, f(t) = a + b exp (−t/τ).

In all studied systems, ultrafast charge transfer is observed, with the time scale ranging from 286 fs to 1.028 ps. In particular, the electron transfer in SeSeIH is only 286 fs, which is much shorter as compared with the values in SeSeIH (773 fs) and MoS2/WSe2 vdWH (470 fs).10 The ultrafast charge transfer can be understood in the framework of the interlayer coupling. In general, the wave function mixing between the initial and final states reflects the nonadiabatic charge dynamics, which is related to the electron−phonon coupling. More delocalized donor states favor the NA coupling. As indicated in Figure 4, the donor orbitals in SeSeIH are more delocalized, especially for photogenerated electrons, which have densities extending into the interlayer space (state 4 in Figure 4b). Due to the large overlapping states within the heterointerlayer, strong donor− acceptor interactions are expected, which provide driving force for fast charge separation. These features make SeSeIH a promising candidate for ultrafast optoelectronic applications. By contrast, in SSeIH, the donor states of holes (state 1 in Figure 4b) are localized within the janus-MoSSe monolayer, which results in the weaker interaction between donor and acceptor orbitals. As a result, the hole separation process in 2799

DOI: 10.1021/acs.jpclett.8b00903 J. Phys. Chem. Lett. 2018, 9, 2797−2802

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The Journal of Physical Chemistry Letters SSeIH is relatively slow, i.e., 1028 fs, which is about 4 times longer than that of the electron separation process in SeSeIH. In addition to the charge separation dynamics, electron−hole recombination plays an important role in determining the performance of the optoelectronic and photovoltaic devices. The charge recombination across the SSeIH and SeSeIH are characterized in Figure 6a. The electron−hole recombination in

In this work, we also evaluate the photoresponsivity performance of SSeIH and SeSeIH using a two-probe model. A detailed description of the constructed system is shown in Figure S4. Upon illumination, the photoinduced current is generated throughout the heterojunction. The photocurrent Jph is obtained as23,24 Jph =

e ℏ

∫ d2πE ∑ Tα(E)

(3)

α

where Tα(E) is the effective transmission coefficient: < > Tα(E) = Tr{i Γα[(1 − fα )Gph + fα Gph ]}

(4)

and Γα, fα, and stand for the line-width function, the Fermi function, and the greater/lesser Green’s function including electron-photon interactions, respectively. Once the photocurrent is obtained, the photoresponsivity (Rph) is calculated by Jph R ph = eFph (5) G>/< ph

Figure 6. (a) Electron−hole recombination dynamics and (b) puredephasing functions across SSeIH and SeSeIH. The decay time scale of dephasing represents the elastic electron−phonon scattering time.

where Fph is the photon flux defined as the number of photons per unit time per unit area.25,26 An external bias voltage Vds = 0.2 V is applied across the vdWHs. In our simulations, the incident light power density is l kW·m−2, i.e., AM1.5, which is the standard test condition. The predicted photocurrents for SSeIH and SeSeIH as a function of wavelength are shown in Figure 7a. We find that both heterobilayers exhibit high

SSeIH is fitted to be 85 ps, which is about 2 orders of magnitude longer than that of charge separation. Such low recombination rate benefit from their type-II feature akin to other TMD vdWHs, such as MoS2/MoSe2 heterostructure.13 One primary reason that accounts for the low electron−hole recombination rate can be rationalized based on the electronic interactions. As has been discussed above, the initial states of the recombining electrons (state 3 in Figure 4) and holes (state 2 in Figure 4) in SSeIH are confined to each single layer. No overlapping states are observed within the interlayer space. As a result, the interlayer interactions in recombination are weak, leading to low electron−hole recombination rate. Besides the electronic interactions, another important factor that influences the electron−hole recombination process is the electron− phonon interaction. The transfer process of photoinduced electron and hole gives rise to energy relaxation into phonons, which prevents back-transfer of the charges and enables a long lifetime charge separation. The elastic electron−phonon interactions result in the loss of quantum coherence, which can be estimated using optical response theory by calculating the pure-dephasing functions:18 D(t ) = e−g(t )

(1)

where g (t ) =

∫0

t

∫0

dτ1

τ1

C(τ2) dτ2

Figure 7. (a) Calculated photocurrents Jph as a function of wavelength for SSeIH and SeSeIH. (b) Simulated photocurrents Jph as a function of photon energy and polarizing angle (θ) for SSeIH.

(2)

In general, long-lived coherence leads to fast quantum transitions, while shorter coherence retards the dynamics. The dynamics stops if the coherence time becomes infinitely small, which is known as the quantum Zeno effect.19 The calculated D(t) of SSeIH and SeSeIH is shown in Figure 6b. The results demonstrate that quantum coherence loses faster in SSeIH than that in SeSeIH, confirming that the charge recombination rate is lower for SSeIH as compared with the SeSeIH. We should point out that due to the localized CBM and VBM in vdWHs, decoherence is essential for slow charge recombination that involves a single quantum transition across a large energy gap.16,20−22

photocurrents in a broad range of spectrum. Furthermore, SeSeIH shows better optoelectronic performance than that of SSeIH, with the photoresponsivity up to 0.017 A W−1 at 442 nm incident laser wavelength. This value is comparable with those of InSe/InTe in-plane heterostructure (0.030 A W−1)25 and MoS2/WSe2 heterobilayer (0.011 A W−1).27 Several factors rationalize why SeSeIH shows high performance of photoresponse. First, SeSeIH has a direct bandgap with the value of 1.32 eV, which facilitates light absorption in a broad range of the spectrum. Second, the donor−acceptor interaction is strong 2800

DOI: 10.1021/acs.jpclett.8b00903 J. Phys. Chem. Lett. 2018, 9, 2797−2802

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The Journal of Physical Chemistry Letters for the charge transfer. The photogenerated electron has density extending into the interlayer space, which contributes to the fast charge separation process. Third, the electron−hole recombination time is extremely long, i.e., 2 orders of magnitude larger than that of the electron−hole separation process, suggesting the electron−hole recombination rate is relatively low. These results demonstrate that, upon illumination, the photogenerated electrons and holes in SeSeIH are easy to separate with low recombination rate, leading to large net photocurrent as well as high photoresponsivity. In addition, the influence of polarization angle (θ) on the photocurrents is also investigated. For the linearly polarized light vector, θ is defined as the included angle with respect to the y-axis (e1⃗ ) direction. The calculated photocurrents of SeSeIH at different θ with photon energies ranging from 0.5 to 3.5 eV are plotted in Figure 7b. The results indicate that this hexagonal heterostructure shows higher photoresponsivity when θ = 0° or 180°. In summary, by employing the TD-DFT combined with NAMD, we systematically studied the photoinduced charge transfer process in the 2D janus-MoSSe/WSe2 vdWHs with different arrangements. It is found that the ultrafast electron− hole separation occurs with the time-scale ranging from 286 fs to 1.028 ps. The ultrafast charge separation process can be understood based on the fact that the photoexcited charges delocalize between the donor and acceptor states. Meanwhile, a relatively low dynamic rate for electron−hole recombination is observed in our studied cases. The predicted time-scale of charge recombination is up to 85 ps, which is about 2 orders of magnitude longer than that of separation process. This can be rationalized based on the interlayer interactions. The VBM and CBM, which are the initial states of the recombining holes and electrons, strictly localize at each single layer respectively, leading to weak states coupling between the interlayer. As a result, low recombination rate of the electron−hole pair is expected in the vdWHs. In addition, high photoresponsivity in a broad range of the spectrum is found, especially for SeSeIH. The ultrafast photoinduced charge separation, low electron− hole recombination rate, and high photoresponsivity make janus-based TMD vdWHs promising candidates in photovoltaic and optoelectronic applications. We employ the Vienna ab initio simulation package (VASP)28 for geometry optimization and electronic structure calculation. Adiabatic molecular dynamic trajectories are computed with the Quantum Espresso program29 using a converged plane-wave basis set. The generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE)30 exchange-correlation functional, and the projected augment wave (PAW)31 approach with plane wave cutoff energy of 500 eV. The convergence criteria is set to be 10−5 eV in energy and 0.01 eV Å−1 in force. A vacuum spacing of ∼18 Å is used to avoid the interactions between repeated images. vdW interactions are included in the simulations using the D2 approach.32 The system is then heated to 300 K through repeated velocity rescaling. Photocurrent calculations are performed on the basis of NEGF-DFT method,33 as implemented in the Nanodcal code.34,35 Phonon spectra are calculated by using the PHONOPY code.36 NAMD calculations are performed using PYXAID code developed by Akimov and Prezhdo,37,38 which uses fewest switching surface hopping (FSSH)39,40 technique within TDDFT, and starting from the time-dependent Schrödinger equation and Kohn−Sham orbitals:

iℏ

∂ Ψn(r, t ) = H(r, R, t )Ψn(r, t ) ∂t

Ψn(r, t ) =

∑ Ckn(t )Φk(r, R, (t )) k

(6)

(7)

According to the solutions of these equations, the probability of transition between adiabatic states i and j can be obtained by the wave function expansion coefficients and coupling, defined as dij = −iℏ Φi

∂ ∂t

Φj . The approach has been applied in a

broad range of systems, including semiconductor quantum dot,41 monolayer42 and heterointerface,43 and the detailed description of the theory can be found in the previous studies.37,38,43 A total of 100 geometries were randomly selected from the first 1 fs of the 6 ps trajectory and used as initial conditions in the NAMD calculations.

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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.8b00903. Several additional details, including the stacking of heterostructures in six high-symmetry sequences and their energy differences, phonon dispersion and monocular dynamic simulation for SeSeIH, band structures using HSE06 method, and the schematic model used in NEGF-DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.D.). *E-mail: [email protected] (H.J.). ORCID

Hao Jin: 0000-0002-5085-6144 Ying Dai: 0000-0002-8587-6874 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science foundation of China (No. 11604213, No. 11374190, and No. 21333006), the Taishan Scholar Program of Shandong Provice, and the Shenzhen Key Lab Fund (Grant No. ZDSYS20170228105421966).

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DOI: 10.1021/acs.jpclett.8b00903 J. Phys. Chem. Lett. 2018, 9, 2797−2802