Nonadiabatic Dynamics Simulations Reveal Distinct Effects of the

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Nonadiabatic Dynamics Simulations Reveal Distinct Effects of the Thickness of PTB7 on Interfacial Electron and Hole Transfer Dynamics in PTB7@MoS Heterostructures 2

Xiangyang Liu, Wen-Kai Chen, Weihai Fang, and Ganglong Cui J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01066 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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The Journal of Physical Chemistry Letters

Nonadiabatic Dynamics Simulations Reveal Distinct Effects of the Thickness of PTB7 on Interfacial Electron and Hole Transfer Dynamics in PTB7@MoS2 Heterostructures Xiang-Yang Liu, Wen-Kai Chen, Wei-Hai Fang, and Ganglong Cui*

Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China

Email: [email protected]

Abstract Mixed-dimensional hybrid heterostructures have attracted a lot of experimental attention because they can provide an ideal charge-separated interface for optoelectronic and photonic applications. In this contribution we have employed first-principles DFT calculations and nonadiabatic dynamics simulations to explore photoinduced interfacial electron and hole transfer processes in two PTB7-nL@MoS2 models (n=1 and 5). The interfacial electron transfer is found to be ultrafast and completes within ca. 10 fs in both PTB7-1L@MoS2 and PTB75L@MoS2 models, which demonstrates that the electron transfer is not sensitive to the thickness of the PTB7 polymer. Differently, the interfacial hole transfer is sensitive to the thickness of the PTB7 polymer. The transfer time is estimated to be ca. 70 ps in PTB71L@MoS2 while it is significantly accelerated to ca. 1 ps in PTB7-5L@MoS2. Finally, we have found that the electron transfer is mainly controlled by adiabatic electron evolution; whereas, 1 ACS Paragon Plus Environment

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in the hole transfer, nonadiabatic hoppings play a dominant role. These findings are useful for the design of excellent charge-separated interfaces of mixed-dimensional TMD-based heterojunctions for a variety of optoelectronic applications.

Electronic and optical properties of two-dimensional transition metal dichalcogenides (TMDs) have received much attention due to their potential applications in field-effect transistors, solar cells, and related optoelectronic devices.1-24 In particular, mixed-dimensional hybrid heterostructures combining TMDs with 0- and 1-dimensional materials through weak van der Waals interaction, for example with conjugated polymers, have attracted a lot of experimental attention because they could provide an ideal charge-separated interface for optoelectronic and photonic applications.15,25-38 Recently, Shastry et al. have presented an experimental realization of a large-area type-II photovoltaic heterojunction in which the MoS2 monolayer runs as the primary adsorber and the PTB7 polymer does as the donor.35 This heterostructure exhibits a tunable photoluminescence intensity as a function of the thickness of the PTB7 layer, which finally enables the quenching of the MoS2 photoluminescence.35 But, 2 ACS Paragon Plus Environment

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the underlying photoinduced carrier dynamics is not clear, which motivates further experimental studies. Zhong et al. have employed time-resolved transient absorption spectroscopies to investigate the ultrafast electron and hole transfer dynamics of the mixeddimensional PTB7@MoS2 heterojunction and concluded that the photoinduced electron transfer from PTB7 to electronically hot states of MoS2 occurs less than 250 fs.27 Followed are an exciton diffusion-limited electron transfer still from PTB7 to MoS2 and a hole transfer from MoS2 to PTB7 of less than 3 ps.27 Although these experimental studies provide preliminary spectroscopic and dynamic information on the carrier dynamics in PTB7@MoS 2, the photoinduced interfacial electron and hole transfer dynamics at the atomistic level is far from complete understanding and still requires theoretical studies.39-47

In this contribution we have constructed two heterostructure models to explore the photoinduced electron and hole transfer dynamics at the interface between PTB7 and MoS 2 by using both electronic structure calculations and nonadiabatic dynamics simulations at the DFT level.48-52 The first model is the simplest one in that it just includes a PTB7 layer, referred to as PTB7-1L@MoS2 hereinafter, which is used to explore the intrinsic interface properties. In order to explore the effects of the thickness of the PTB7 layers on the interfacial electron and hole transfer dynamics, we have constructed the second PTB7-5L@MoS2 heterojunction model, which includes five PTB7 layers, referred to as PTB7-5L@MoS2.

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Figure 1.

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(left) PBE+D3 optimized PTB7-1L@MoS2 structures and (right) PTB7-5L@MoS2

structures (0 and 300 K). Also shown are inter-distances of the PTB7 layer to the MoS2 surface.

These initially constructed models are first optimized with the PBE+D3 method implemented in the Quickstep/CP2K package (see supporting information for simulation details).53-61 The resultant stable structures are shown in Fig. 1. In the PTB7-1L@MoS2 heterojunction, the PTB7 layer is physically adsorbed on the monolayer MoS 2 surface in a parallel way without forming any chemical bonds at the interface. The adsorption energy is estimated to be 2.12 eV at the HSE06 level.62-66 At this adsorption configuration, the monolayer MoS2 is essentially planar while the PTB7 layer is a little twisted because the π conjugation is broken at the boundary of the two conjugated moieties. The inter-distance between PTB7 and MoS2 is calculated to be 3.531 Å at 0 K. Upon heated to 300 K, the PTB7-1L@MoS2


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heterostructure does not change visibly except a subtly larger inter-distance between PTB7 and MoS2, 3.591 Å . In the PTB7-5L@MoS2 heterostructure, the five PTB7 layers are arranged in a nearly parallel way through bilateral non-covalent interaction and they are also physically adsorbed on the monolayer MoS2 surface. At the HSE06 level the corresponding adsorption energy is calculated to be 2.41 eV. Similarly, there is a small twisting due to the breaking of the π conjugation. Compared with those in the PTB7-1L@MoS2 heterojunction, the interdistance between the lowest PTB7 layer and the MoS2 surface is calculated to be 3.483 Å at 0 K (3.623 Å at 300 K).

Figure 2. DOS and PDOS of (A) PTB7-1L@MoS2 and (B) PTB7-5L@MoS2 calculated by the HSE06 functional. Also shown are the components of each state with respect to PTB7 and MoS2 (C and D). See supporting information for those by the PBE+D3 functional. 5 ACS Paragon Plus Environment


The density of states (DOS) and projected DOS (PDOS) are very helpful for qualitatively understanding the interfacial electron and hole transfer dynamics and we have thus employed the HSE06 method to calculate the corresponding DOS and PDOS of both PTB7-1L@MoS2 and PTB7-5L@MoS2 heterostructures. The panel A of Fig. 2 shows the DOS and PDOS of PTB7-1L@MoS2 and the panel C depicts the components of each state with respect to PTB7 and MoS2. To discuss more conveniently CBM and VBM are used to represent the conduction band minimum and the valence band maximum of MoS 2 while HOMO and LUMO are used for the highest occupied molecular orbital and the lowest unoccupied molecular orbital of PTB7. Clearly, the two highest occupied states i.e. HOMO and HOMO-1 are all from the PTB7 layer and the lowest unoccupied state i.e. CBM is from the MoS2 monolayer; thus, PTB71L@MoS2 is a type-II heterojunction. At the HSE06 level, CBM and VBM of MoS2 in this heterojunction is calculated to be 0.72 and -1.76 eV, respectively, from which the energy gap is estimated to be 2.48 eV. LUMO, HOMO, HOMO-1, and HOMO-2 energies of PTB7 are calculated to be 1.15, -0.70, -1.32, and -2.30 eV at the same computational level, respectively. HOMO-LUMO energy gap is predicted to be 1.85 eV. Similarly, the panels B and D of Fig. 2 show the counterparts for PTB7-5L@MoS2. Obviously, PTB7-5L@MoS2 is also a type-II heterojunction. CBM and VBM of MoS2 are calculated to be 0.565 and -1.909 eV at the HSE06 level, from which CBM-VBM energy gap is 2.47 eV, close to 2.48 eV in the above PTB71L@MoS2. HOMO and LUMO are estimated to be -0.548 and 0.813 eV; thus, HOMO-LUMO energy gap of the PTB7 layers, 1.36 eV, becomes smaller than 1.85 eV in PTB7-1L@MoS2. 6 ACS Paragon Plus Environment

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Furthermore, different from PTB7-1L@MoS2, the DOS from the PTB7 layers is significantly increased in PTB7-5L@MoS2. In the former, there exist only two occupied states of PTB7 between CBM and VBM of MoS2; but, in the latter, there are much more states in-between CBM and VBM of MoS2. Of course, this can be easily understood because of more PTB7 layers involved. One can also find that the lowest occupied state of PTB7 between CBM and VBM of MoS2 is much closer to its VBM in PTB7-5L@MoS2 than that in PTB7-1L@MoS2, which implies that the interfacial hole transfer from MoS2 to PTB7 is much easier for PTB7-5L@MoS2 due to a very smaller energy gap.

Figure 3. Spatial distributions of selected states involved in the interfacial electron and hole transfer dynamics in (left) PTB7-1L@MoS2 and (right) PTB7-5L@MoS2 models. 7 ACS Paragon Plus Environment


Next we are dedicated to exploring the initially populated electron and hole states in PTB7-1L@MoS2 and PTB7-5L@MoS2, which are useful for studying the following interfacial electron and hole transfer dynamics. These electron and hole states are generated due to the electron transition; thus, we first explore the most possible electron transitions in the FranckCondon region. First, we focus on the electron transition of PTB7-1L@MoS2. At the HSE06 level, it is found that the electron transitions from state-385 (HOMO) to state-400 (LUMO), state-382 to state-386 (CBM), and state-381 to state-387 have the largest oscillator strengths, whose excitation energies [oscillator strengths] are 1.86 eV [0.56], 2.62 eV [0.53], and 2.62 eV [0.53]. The first electron transition of 1.86 eV corresponds to the excitation within the PTB7 layer, close to the experimental value of 1.77 eV [700 nm];27 whereas, the latter two of 2.62 eV are related to the excitation within MoS2 (exp. 2.34 eV, 530 nm).27 As a result, when an electron is promoted from HOMO to LUMO of PTB7 upon photoexcitation, a hole will be generated in HOMO. In this situation, the interfacial hole transfer is not allowed because HOMO is much higher than VBM of MoS2 in energy. On contrary, the interfacial electron transfer is possible in that LUMO is much higher than CBM of MoS2 in energy (see Fig. 2). In parallel, if an electron is excited into either state-386 or state-387 within MoS2, a hole will occupy either state-381 or state-382. In this case, the interfacial electron transfer is not favorable because both state-386 and state-387 are much lower than LUMO of PTB7; instead, the interfacial hole transfer becomes possible owing to the fact that the hole states 381 and 382 are lower than the two occupied states of PTB7, i.e. states 384 and 385 (see Fig. 2). The left panel of Fig. 3 shows the spatial distribution of selected states involved in the interfacial electron and hole transfer 8 ACS Paragon Plus Environment

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of PTB7-1L@MoS2. It is clear that both HOMO and LUMO are mainly located on PTB7 and the others are on MoS2.

Second, we look at the electronic transition of PTB7-5L@MoS2. As with the situation of PTB7-1L@MoS2, there are two kinds of excitations, which are separately located on PTB7 and MoS2. In the former the electronic transition from state-677 (HOMO) to state-713 (LUMO+4) has the largest oscillator strength with an excitation energy of 1.92 eV, also close to the experimental value of 1.77 eV [700 nm].27 In the latter the electronic transitions from state-664 to state-678 (CBM) and state-665 to state-679 have the largest oscillator strengths with same 2.63 eV excitation energies [exp. 2.34 eV, 530 nm].27 Clearly, the electronic transition of state-677 (HOMO) to state-713 (LUMO+4) within PTB7 excites an electron into LUMO+4 and leaves a hole in HOMO. Since HOMO of PTB7 is much higher than VBM of MoS2, the interfacial hole transfer is not allowed thermodynamically; instead, an interfacial electron transfer is possible in energy because LUMO+4 is much higher than CBM of MoS2. Alternatively, when the electronic transition takes place within MoS2, e.g. from state-664 to state-678 (CBM), the interfacial electron transfer is unfavorable because all unoccupied states of PTB7 are higher than CBM of MoS2 in energy. On contrary, the interfacial hole transfer from MoS2 to PTB7 is allowed thermodynamically. The right panel of Fig. 3 shows the spatial distribution of selected states involved in the interfacial electron and hole transfer dynamics in PTB7-5L@MoS2. Similarly, both HOMO and LUMO+4 are mainly located on PTB7 in spite of that the latter has some tail on MoS2, while the others are essentially on MoS2 except 9 ACS Paragon Plus Environment


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state-664 having some tail on PTB7. These tails enhance nonadiabatic couplings benefitting the interfacial hole transfer in PTB7-5L@MoS2 as will be seen in the following nonadiabatic dynamics simulations.

Figure 4. Time-dependent electron amount that is still located on the PTB7 moiety in (A) PTB7-1L@MoS2 and (B) PTB7-5L@MoS2 models; Time-dependent hole amount that is still located on the MoS2 moiety in (C) PTB7-1L@MoS2 and (D) PTB7-5L@MoS2 models.

To estimate the interfacial electron and hole transfer dynamics between PTB7 to MoS2 in PTB7-1L@MoS2 and PTB7-5L@MoS2, we have carried out nonadiabatic dynamics


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simulations under the classical path approximation, which has been widely used for simulating carrier dynamics in a series of organic and inorganic materials.39-42

The interfacial electron transfer in PTB7-1L@MoS2 is mainly from the electronic transition from state-385 (HOMO) to state-400 (LUMO) within PTB7 (see above). As a result, state-400 is chosen as initial electron state. Different from state-385 that is entirely on PTB7, state-400 has some tail on MoS2 (see Fig. 3). This will increase the electronic coupling between PTB7 and MoS2, which drives the interfacial electron transfer. The panel A of Fig. 4 shows the time-dependent population of the excited electron that is still localized on PTB7. Due to the tail of state-400, the initial electron population on PTB7 is about 0.5. After only about 10 fs, this amount of electron is transferred to MoS2. This ultrafast electron transfer is mainly ascribed to the fact that the initially populated electron state i.e. state-400 of PTB7 has very small energy gaps with accepting conduction band states and large density of accepting states of MoS2 (see Fig. 2). For PTB7-5L@MoS2, state-713 is chosen as the initial electron state. As with state-400 in the above PTB7-1L@MoS2, state-713 in PTB7-5L@MoS2 also has a tail on MoS2. The interfacial electron transfer is also ultrafast and completed within the first 10 fs as shown in the panel B of Fig. 4. Similarly, small energy gaps between state-713 of PTB7 with accepting conduction band states and large density of accepting states of MoS2 are responsible for this ultrafast interfacial electron transfer. Finally, it is noteworthy that adiabatic electron transfer is dominant and in charge of the interfacial electron transfer in either PTB7-1L@MoS2 or PTB7-5L@MoS2. 11 ACS Paragon Plus Environment


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The interfacial hole transfer dynamics from MoS2 to PTB7 is complicated. In PTB71L@MoS2, as discussed above, the two electronic transitions from state-382 to state-386 and from state-381 to state-387 have the largest oscillator strengths and are mainly responsible for generating hole states within MoS2. Therefore, state-382 and state-381 are respectively chosen as initial hole states in our simulations. As seen in Fig. 3, both state-381 and state-382 are completely located on MoS2 without visible tails on PTB7, which implies a small nonadiabatic coupling between MoS2 and PTB7. In addition, due to the existence of a comparable energy gap between states 381-382 of MoS2 and states 384-385 of PTB7, as shown in the panel C of Fig. 2, the interfacial hole transfer from MoS2 to PTB7 should be slow. The panel C of Fig. 4 shows the time-dependent hole population on MoS2, which is averaged over the two dynamics simulations in which state-381 and state-382 are chosen as initial hole states, respectively (see SI for individuals). It can be found that the interfacial hole transfer is indeed very slow. Through a simple exponential fitting on the time-dependent hole population in the panel C of Fig. 4, the time for the interfacial hole transfer is roughly estimated to be ca. 70 ps.

However, the situation is changed in PTB7-5L@MoS2. An ultrafast interfacial hole transfer is observed as shown in the panel D of Fig. 4. Its hole transfer time is estimated to be about 1 ps through fitting the time-dependent hole population with a simple exponential function. Interestingly, in contrast to the electron transfer, nonadiabatic hole hoppings play a major role in the interfacial hole transfer in PTB7-5L@MoS2. How to understand such ultrafast interfacial hole transfer in particular compared with that in PTB7-1L@MoS2? First, the initial 12 ACS Paragon Plus Environment

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hole states 664 and 665 have remarkable tails on PTB7, which thereby enhance the hole coupling between MoS2 and PTB7. This is unlike the situation of states 381 and 382 in PTB71L@MoS2 where there is no tail spreading to PTB7 (see Fig. 3). Second, the energy gap between the initial hole state of MoS2 and the energetically nearest accepting state of PTB7 is much smaller in PTB7-5L@MoS2 than that in PTB7-1L@MoS2. For example, it is 0.44 eV between VBM of MoS2 and HOMO-1 of PTB7; while, it is reduced to 0.24 eV between VBM of MoS2 and HOMO-9 of PTB7 as shown in Fig. 5.

Figure 5. Schematic mechanism for the interfacial electron and hole transfer dynamics in (left) PTB7-1L@MoS2 and (right) PTB7-5L@MoS2 models. Also shown are the estimated electron and hole transfer times and the corresponding energies of CBM and VBM and some molecular orbitals located on the PTB7 polymer. See text for discussion. 13 ACS Paragon Plus Environment


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Our above simulations not only help to rationalize experimental phenomena but also provide new mechanistic insights on the interfacial electron and hole transfer dynamics at the interface of PTB7 and MoS2. First, it is experimentally inferred that 700 nm photon mainly excites PTB7 while 530 nm photon does MoS2.27 This is also confirmed by our calculated electronic transitions in either PTB7-1L@MoS2 or PTB7-5L@MoS2 models. In addition, the interfacial electron transfer is experimentally assumed to start from delocalized electron state so that electron does not need diffuse to the interface before completely transferring to MoS2localized states.27 In our DFT calculations, these delocalized electron states have been observed, for example, state-400 in PTB7-1L@MoS2 and state-713 in PTB7-5L@MoS2 in Fig. 3.

Second, our estimated time of < 10 fs for the interfacial electron transfer in both PTB71L@MoS2 and PTB7-5L@MoS2 models is reasonably consistent with recent experiments in which the photoinduced electron transfer from PTB7 to MoS2 is found to occur in less than IRF-limited 250 fs.27 The discrepancy between experiments and simulations can be understood very well taking into account that our constructed models are far from realistic PTB7@MoS2 heterojunction in which a variety of defects and trapping states of MoS2 exist and the thickness of the PTB7 layer is at the nanometer scale, etc. Nevertheless, our simulations indeed provide important dynamical information and details about this interfacial electron transfer. In addition, it can be found that the thickness of the PTB7 polymer has negligible effects on the interfacial electron transfer dynamics because both PTB7-1L@MoS2 and PTB7-5L@MoS2 models exhibit similar dynamical feature as shown in the panels A and B of Fig. 4. 14 ACS Paragon Plus Environment

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Third, through comparing the results of PTB7-5L@MoS2 with those of PTB7-1L@MoS2, one can find that the thickness of the PTB7 polymer have remarkable influence on the interfacial hole transfer dynamics. The estimated time for the hole transfer in PTB7-1L@MoS2 is about 70 ps; while, in PTB7-5L@MoS2, it is decreased to ca. 1 ps, which is close to the recent experimental data, i.e. less than 3 ps.27 The different hole transfer time in two models also demonstrates that the thickness of the PTB7 polymer has remarkable influence on the interfacial hole transfer dynamics. Considering that it takes place from MoS 2 to PTB7, we can thus qualitatively understand why the quenching of the MoS2 photoluminescence varies as a function of the thickness of the PTB7 polymer in experiments.35

In summary we have employed first-principles DFT calculations and related nonadiabatic dynamics simulations to explore the photoinduced interfacial electron and hole transfer dynamics in the mixed-dimensional PTB7@MoS2 heterojunction. The interfacial electron transfer is found to be ultrafast and is in complete within about 10 fs in both PTB71L@MoS2 and PTB7-5L@MoS2 models, which is fully consistent with recent experiments.27 This electron transfer is also found to be not sensitive to the thickness of the PTB7 polymer. Different from the electron transfer, the hole transfer is sensitive to the thickness of the PTB7 polymer, the time is estimated to be about 70 ps in PTB7-1L@MoS2 but is decreased to be about 1 ps in PTB7-5L@MoS2, which is close to experimentally determined time of less than 3 ps. 27 Finally, we have found that the electron transfer is mainly controlled by the adiabatic electron evolution; whereas, in the hole transfer, nonadiabatic hoppings play a dominant role. These 15 ACS Paragon Plus Environment


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new insights could be useful for the design of excellent charge-separation interfaces of mixeddimensional TMD-based heterojunctions for optoelectronic applications.

Supporting Information. The following files are available free of charge. The theoretical backgrounds, simulation details, additional figures and tables. (PDF)

AUTHOR INFORMATION Corresponding Author Email: [email protected] (G.C.)

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work has been supported by the grants NSFC 21522302 (G.C.) and NSFC 21520102005 (G.C. and W.F.).

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