pubs.acs.org/NanoLett Letter Reversible Electrical Control of Interfacial Charge Flow across van der Waals Interfaces Shuai Fu, Xiaoyu Jia, Aliaa S. Hassan, Heng Zhang, Wenhao Zheng, Lei Gao, Lucia Di Virgilio, Sven Krasel, David Beljonne, Klaas-Jan Tielrooij, Mischa Bonn, and Hai I. Wang* Cite This: Nano Lett. 2023, 23, 1850−1857 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Bond-free integration of two-dimensional (2D) materials yields van der Waals (vdW) heterostructures with exotic optical and electronic properties. Manipulating the splitting and recombination of photogenerated electron−hole pairs across the vdW interface is essential for optoelectronic applications. Previous studies have unveiled the critical role of defects in trapping photogenerated charge carriers to modulate the photoconductive gain for photodetection. However, the nature and role of defects in tuning interfacial charge carrier dynamics have remained elusive. Here, we investigate the nonequilibrium charge dynamics at the graphene−WS2 vdW interface under electrochemical gating by operando optical-pump terahertz-probe spectroscopy. We report full control over charge separation states and thus photogating field direction by electrically tuning the defect occupancy. Our results show that electron occupancy of the two in-gap states, presumably originating from sulfur vacancies, can account for the observed rich interfacial charge transfer dynamics and electrically tunable photogating fields, providing microscopic insights for optimizing optoelectronic devices. KEYWORDS: van der Waals heterostructures, charge transfer, photogating, electrochemical gating, operando terahertz spectroscopy V ertical stacking of atomically thin materials constitutes van der Waals (vdW) heterostructures with exotic physics and synergistic properties beyond lattice-matching constraints. mechanism following resonant or above-bandgap photo- excitation,10,12,13,17 photothermionic emission (PTE) governs the CT process upon sub-bandgap photoexcitation.18,19 In the Such artificial material manipulation provides fertile ground for PTE scenario, the absorbed low-energy photons efficiently heat exploring fundamental physics and enabling novel optoelec- the electron bath in graphene, resulting in a thermalized hot tronics. Among the vast gamut of vdW heterostructures, carrier distribution. The thermalized hot carriers with graphene−transition metal dichalcogenide (TMDC) hetero- sufficiently high energy can be emitted over the Schottky structures have received particular attention because they barrier at the graphene−TMDC interface.10,15,18 inherit strong light−matter interactions and high charge carrier Following optical excitations and CT, transferred electrons mobility from their constituent building blocks.1−5 At the and holes reside in different materials: transient charge bond-free interface, graphene and TMDC are coupled by vdW separation states emerge. The resulting charge separation forces.6 The resulting interlayer atomic interactions enable time is critical in determining the photoconductive gain and strong electronic coupling, leading to interfacial charge flow operation bandwidth for many graphene-based optoelectronic which underpins various optoelectronic applications,7,8 includ- devices such as photodetectors.20 In our previous work,10 we ing photodetection. revealed that the charge separation time is closely related to As the backbone of these applications, understanding and defect states in TMDCs: electrons populating the excited states tuning the spatial distribution of photogenerated electron− of WS2 are rapidly and efficiently trapped by empty defect hole pairs at the vdW interface is paramount for photocurrent states within ∼1 ps and subsequently stored there for more operation. For that, recent spectroscopic studies have than ∼1 ns. This leads to long-lived charge separation and investigated photoinduced interfacial processes in graphene− TMDC heterostructures using ultrafast pump−probe techni- Received: December 6, 2022 ques.9−17 It has been reported that photoinduced charge Revised: February 12, 2023 transfer (CT) in graphene−TMDC heterostructures occurs Published: February 17, 2023 within ∼100 fs, and the activated CT trajectories critically depend on the pump photon wavelength. While direct hole tunneling from TMDC to graphene dominates the CT © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.nanolett.2c04795 1850 Nano Lett. 2023, 23, 1850−1857 Nano Letters pubs.acs.org/NanoLett Letter Figure 1. Conceptual illustration of probing and controlling interfacial charge dynamics in electrochemically gated graphene−WS2 vdW heterostructures by operando optical-pump THz-probe (OPTP) spectroscopy. (a) Schematic representation of a graphene−WS2 vdW heterostructure supported by a sapphire substrate (side view). A polymer electrolyte (LiClO4 dissolved in poly(ethylene oxide)) is deposited on top of the graphene layer for electrochemical gating and is transparent to light at both UV−vis and THz frequencies. Note that the gate electrodes are not in contact with the heterostructure. (b) Schematically illustration of the operando OPTP and four-point probe measurements (top view). A 1.55 eV femtosecond laser pulse selectively excites the graphene layer, and a collinearly propagating, single-cycle THz pulse probes the photoconductivity of the heterostructure as a function of pump−probe delay. The Fermi level (EF) of the heterostructure is controlled by the voltage applied between the gate and the electrode deposited on graphene. Meanwhile, the resistance of the graphene layer is measured by the four- point probe method. The measurements were performed at room temperature in a dry N2 environment. (c) Simplified band diagram of the heterostructure. ED1 and ED2 represent the energies of the two in-gap defect states involved in the photogating effect, as described in the main text. efficient photogating in graphene, in line with the ultrahigh heterostructures. By simulating the hot carrier distribution in photoresponsivity observed in high-performance graphene− the heterostructure following photoexcitation, we further TMDC-based photodetectors.1 Rational defect engineering provide compelling experimental evidence that the observed and manipulation represents one of the key strategies to rich interfacial CT dynamics and electrically tunable photo- further optimize the performance of optoelectronic devices. gating fields at graphene−WS2 vdW interfaces can be Nonetheless, realizing the full potential of defect engineering rationalized by electron occupancy of two distinct in-gap for optoelectronics is nontrivial, as it requires concerted efforts states, presumably originating from sulfur vacancies. Our to understand the origin of defects, develop effective methods results not only reveal fundamental interfacial photophysics to control their nature/density, and elucidate their impact on across the vdW interface but also provide insights into the microscopic interfacial charge carrier dynamics. So far, limited design of advanced (opto)electronic applications through efforts have been devoted to tackling these issues. defect engineering. Here, we track and manipulate nonequilibrium charge As schematically shown in Figure 1a, we fabricated a carrier dynamics in electrochemically gated graphene−WS2 vertically stacked graphene−WS2 heterostructure (see details heterostructures by operando optical-pump terahertz (THz)- in the Supporting Information). The monolayer nature of the probe (OPTP) spectroscopy. Electrochemical gating effectively bare WS2 monolayer is manifested by two pronounced optical controls the chemical potential in graphene and, thus, the resonances at 2.04 and 2.43 eV (Figure S1a), corresponding to interfacial energetics for CT. More importantly, it allows the well-established A- and B-exciton transitions in WS2.21 In modulation of the filling of electronic defect states at the addition, the Raman spectrum of the bare WS2 monolayer interface, which strongly affects the photogating mechanism. shows resonances at ∼355 and ∼419 cm−1 (Figure S2a), We report the first experimental implementations of electrical corresponding to its in-plane (E12g) and out-of-plane (A1g) control of photogating field direction (i.e., the direction of the vibration modes, in line with previous reports.22,23 When built-in interfacial electric field induced by the CT and thus brought in contact with graphene, WS2 exhibits red-shifted photogating effect) and efficiency in vdW heterostructures: for exciton resonances (Figure S1b) due to the dielectric screening p-doped heterostructures with empty in-gap defect states, effect of the graphene layer.24 Meanwhile, the line width of the photoinduced electron injection or trapping into defects in A-exciton resonance of WS2 in the heterostructure broadens WS2 governs the photogating mechanism, leading to an (Figure S1b), which we attribute to the static interlayer CT “electron photogating” effect; for n-doped heterostructures process.25 These optical signatures indicate good electronic with fully filled in-gap defect states, photogenerated holes are coupling at the vdW interface. For the heterostructure, along injected into defects, resulting in a “hole photogating” scheme. with the well-preserved vibration modes of WS2, we obtain the Such reversal of the photogating mechanism is of relevance expected G- and 2D-bands originating from graphene (Figure and importance for optoelectronic applications based on vdW S2b). The single symmetric 2D-band confirms the monolayer 1851 https://doi.org/10.1021/acs.nanolett.2c04795 Nano Lett. 2023, 23, 1850−1857 Nano Letters pubs.acs.org/NanoLett Letter Figure 2. Electrically tunable interfacial charge dynamics at the graphene−WS2 vdW interface. (a) Electrical resistance of the graphene layer as a function of gate voltage. (b) Photoconductivity dynamics at the graphene−WS2 vdW interface operated at different gate voltages. The sample was photoexcited by a 1.55 eV ultrashort laser pulse with a pump fluence of 60 μJ/cm2. The photoconductivity dynamics are divided into three representative time windows according to their characteristics. Note that the data in the second and third time windows are multiplied by a factor of 10 for better visualization. Here Vg and VD are the applied gate voltage and the voltage corresponding to the Dirac point of graphene, respectively. (c) Peak THz photoconductivity as a function of gate voltage. Figure 3. Defect-mediated CT and photogating at vdW interfaces. (a, b) Photoconductivity (a) at ∼20 ps and (b) averaged from 100 to 700 ps as a function of gate voltage under different pump fluences. (c, d) Simulated EF-dependent majority conducting charge density change in graphene based on the defect-mediated photogating scenario (c) before (at ∼20 ps) and (d) after (>100 ps) the back-transfer of charge carriers from the conduction and valence bands of WS2 under different pump fluences. 1852 https://doi.org/10.1021/acs.nanolett.2c04795 Nano Lett. 2023, 23, 1850−1857 Nano Letters pubs.acs.org/NanoLett Letter nature of graphene, and the G-band position at ∼1586 cm−1 We observe a clear sign transition of Δσ20ps upon suggests an initial EF of ∼0.15 eV in the graphene layer electrochemical gating, from positive for the p-doped (referring (equivalent to a free carrier density N of ∼1.4 × 1012 cm−2; to graphene) regime to negative for the n-doped regime. We Supplementary Section 1). understand the observed gating-modulated photoconductivity To investigate the interfacial charge dynamics at the as CT-induced transient electron loss (i.e., the decrease of graphene−WS2 vdW interface while tuning the filling of defect electron density) in graphene, which downshifts EF and thus states, we perform OPTP measurements on electrochemically increases (decreases) the conductivity of p-doped (n-doped) gated graphene−WS2 vdW heterostructures (see details in the graphene at all gate voltages employed. We also observe that Supporting Information). As depicted in Figure 1b, in the Δσ>100ps is positive regardless of the gate voltage, indicating a OPTP measurement, a 1.55 eV ultrashort laser pulse (∼50 fs transition from CT-induced electron loss to gain while tuning duration) selectively excites charge carriers in graphene. graphene from the p- to n-doped regime. This result further Subsequently, a freely propagating single-cycle THz pulse indicates a reversal of the interfacial photogating field, from (∼1 ps duration) passes through the heterostructure. The pointing away from graphene in the p-doped regime to pump-induced relative attenuation of the transmitted THz pointing toward graphene in the n-doped regime. Our previous electric field (−ΔE/E) scales linearly with the photo- study10 reported that electrons trapped in empty defects in conductivity26 (Δσ) and is recorded as a function of pump− WS2 are responsible for photogating in p-doped hetero- probe delay time. The heterostructure used in this study is structures. In the following, we show that holes trapped in the made of polycrystalline graphene and WS2 layers grown by electrochemically filled in-gap defects in WS2 are responsible chemical vapor deposition (CVD) with typical domain sizes of for photogating in n-doped heterostructures. Furthermore, by a few micrometers. As the diameter of the probe beam used for comparing Δσ20ps (Figure 3a) and Δσ>100ps (Figure 3b), we OPTP measurements is ∼1 mm, the measured photo- find that the CT-induced photoconductivity undergoes a conductivity dynamics represent the average response of negative-to-positive transition in the n-doped regime. This heterostructures with mixed twist angles. indicates that the interfacial charge equilibrium of n-doped Figure 2a shows the resistance of the graphene layer as a heterostructures following CT is time-dependent, leading to a function of gate voltage (Vg − VD). The resistance reaches a dynamic transition from electron loss to electron gain in maximum value of ∼1 kΩ in the vicinity of the Dirac point and graphene (see detailed simulations below). decreases in both the p-doped and n-doped regimes. Figure 2b To describe the electrically tunable CT-induced photo- shows the exemplary photoconductivity dynamics of the conductivity and its evolution over time, we propose a defect- graphene−WS2 vdW heterostructure at different gate voltages. mediated CT and photogating mechanism. In this mechanism, electrochemical gating modulates the defect occupancy in WS2, Notably, the photoconductivity dynamics in the first 10 ps thereby regulating interfacial CT channels and long-lived (gray area, Figure 2b) switches its sign from positive for the charge separation states across the interface. Here we assume low-doped regime to negative for the highly doped regimes, as that (i) CT occurs at the graphene−WS2 vdW interface only also demonstrated by the evolution of the peak photo- via PTE following sub-bandgap excitation in graphene, based conductivity (Δσpeak) in Figure 2c. The sign transition of on previous reports;10,15,18,34 (ii) the conduction band of WS2 Δσpeak while tuning EF has been well reported in graphene27−29 and empty in-gap defect states serve as electron acceptors, and can be simply understood as follows: in intrinsic or low- while the valence bands of WS2 and electrochemically filled in- doped graphene, photoexcitation leads to a substantial increase gap defect states are hole acceptors, whenever the interfacial in the charge carrier density, thereby increasing the energetics is favorable; and (iii) following CT, the back- conductivity (i.e., positive Δσpeak). In contrast, in highly transfer makes the system relax back to initial conditions and doped graphene, photoexcitation elevates the electron temper- dictates the charge separation lifetime. We assume that the ature, leading to increased momentum scattering of charge lifetime of charge carriers trapped in the defect states is carriers and, thus, a transient reduction in conductivity (i.e., substantially longer than that of charge carriers populated in negative Δσpeak). Altogether, the evolution of resistance the conduction and valence bands of WS2 because of their (Figure 2a) and Δσpeak (Figure 2c) with gate voltage localized nature.10 Within this framework, we simulate demonstrates the successful tuning of EF in the heterostructure thermalized hot carrier distributions in graphene at different by electrochemical gating. initial EF following photoexcitation and thermalization by We now focus on photoconductivity dynamics on longer taking into account EF-dependent heat capacity, energy time scales beyond 20 ps. Because the intrinsic hot carrier conservation, and particle number conservation (Supplemen- response of bare graphene is negligible within 10 ps,27,30 the tary Section 2). A fixed portion of the thermalized hot remaining photoconductivity is directly related to the CT- electrons (holes) energetically distributed above (below) the induced conductivity change in graphene.10,31−33 As shown in corresponding acceptor states are assumed to be able to Figure 2b, around 20 ps (marked in light pink), the CT- transfer across the vdW interface. induced photoconductivity demonstrates a substantial varia- Based on the model, a critical and open question concerns tion with gate voltage, indicating that EF-dependent interfacial the density, filling state, and quantity of defect states involved CT processes take place at the vdW interface. On the other in CT (on short time scales) and photogating processes (on hand, beyond 100 ps, the photoconductivity is always positive long, approximately nanosecond time scales). Recent scanning and shows a weak dependence on gate voltage (blue area, tunneling microscope (STM) studies35 and ab initio theoretical Figure 2b). For better comparison, we summarize the gating- calculations36 revealed that sulfur vacancies (VS) in WS2 forms dependent photoconductivity at around 20 ps (defined as two energetically narrow unoccupied defect states inside the Δσ20ps) and beyond 100 ps (average of data between 100 and bandgap because of an exceptionally strong spin−orbit 700 ps, defined as Δσ>100ps) under three different pump coupling in WS2. Furthermore, the presence of defect states fluences, as shown in Figures 3a,b. in WS2 has been supported by ultrafast spectroscopic studies, 1853 https://doi.org/10.1021/acs.nanolett.2c04795 Nano Lett. 2023, 23, 1850−1857 Nano Letters pubs.acs.org/NanoLett Letter Figure 4. Snapshots illustrating the nonequilibrium interfacial processes that take place in p-doped, undoped, and n-doped graphene−WS2 vdW heterostructures following photoexcitation and thermalization. where defects lead to rapid trapping in WS246 and long-lived defect state also fails to reproduce our experimental interfacial charge separation at the graphene−WS2 interface.10 observations. This is because, following back-CT at ∼100 ps, The typical densities of these defects lie in the range of 1011− ΔNmajor immediately flips sign when EF crosses the Dirac point 1013 cm−2.35,36 Given the operating range of the electro- and the defect energy (Supplementary Figures S4 and S5). On chemical gating method, we electrically tune only the filling of the other hand, the two-defect model illustrated in Figure 4 (as the in-gap defect states, while that of the filled defect state in discussed below) captures all the essential gating- and time- the valence band of WS2 is beyond our reach. In our dependent photoconductivity characteristics in Figure 3a,b: simulation, we consider three scenarios: (i) none, (ii) one, and ΔNmajor is zero in the undoped regime and becomes positive in (iii) two in-gap defect states. The defect density is realistically the doped regimes, as shown in Figure 3c,d. varied between 1011 and 1013 cm−2, and its (or their) On this basis, we further discuss the PTE model involving energetics are fully relaxed within the bandgap of WS2 for two in-gap defect states following the schemes shown in Figure simulation. 4. For highly p-doped heterostructures, both in-gap defect As discussed in Supplementary Sections S3 and S4, we find states are empty and therefore serve as electron acceptors (left that models involving no or only one in-gap defect state are panel in Figure 4). In contrast, for the highly n-doped case, the unable to reproduce the main features shown in Figure 3a,b. in-gap defect states are fully occupied and can trap photo- Briefly, in the absence of defect states, the conduction and generated holes following optical excitations and CT (right valence bands of WS2 are the only CT channels to harvest panel in Figure 4). When EF lies between these two in-gap thermalized hot electrons and hot holes, respectively, from defect states, electron trapping to the empty in-gap defect state graphene. We calculate the majority conducting carrier density and hole trapping to the occupied in-gap defect state can take change (ΔNmajor; electrons for n-doped graphene and holes for place simultaneously, resulting in approximately null net gating p-doped graphene) induced by CT in graphene using the charges residing in WS2 and negligible photogating effects difference between the number of thermalized hot electrons (middle panel in Figure 4). For ease of description, we define distributed above the conduction band minimum of WS2 and the energy and density of the energetically higher defect state the number of thermalized hot holes distributed below the as ED1 and ND1 and those of the energetically lower defect state valence band maximum of WS2 (Supplementary Section 3). As as ED2 and ND2. By numerically calculating ΔNmajor at different shown in Figure S3, the pure PTE model, without considering EF (Supplementary Section 5) following the above-mentioned any defect states, fails to reproduce the main features of the schemes, we can reproduce all the main features shown in experimental results, in particular for Δσ>100ps: (i) in the n- Figure 3a,b under the three different pump fluences used in the doped regime, the absolute value of the CT-induced (negative) experiments, as shown in Figure 3c,d. From the simulation photoconductivity continuously increases with the upward results, we find that on early time scales (∼20 ps), the electron shift of EF because the increasing EF makes hot electron loss at all gate voltages can be rationalized by the fact that PTE transfer energetically more favorable than hot hole transfer; (ii) of thermalized hot electrons to the conduction band of WS2 is following the sub-100 ps back-CT process, the photo- energetically more favorable than that of thermalized hot holes conductivity in the hybrid system is expected to relax back to the valence band of WS2. On longer time scales (>100 ps), to the equilibrium state without exhibiting photogating effects in line with our assumption and previous reports, the nature (Figure S3b), which is in contradiction with the experimental and density of defects dictate the photogating mechanisms. As results. Similarly, the PTE model involving only one in-gap shown in Figure 3d, beyond 100 ps, following the back-transfer 1854 https://doi.org/10.1021/acs.nanolett.2c04795 Nano Lett. 2023, 23, 1850−1857 Nano Letters pubs.acs.org/NanoLett Letter of electrons and holes from WS2, the photoinjected or trapped defect density and species in TMDCs,41−44 which is relevant electrons in the empty in-gap defect states in the p-doped for optimizing photogating and thus photodetection applica- heterostructure lead to an “electron photogating” scheme in tions of graphene−TMDC heterostructures. For instance, VS graphene, while trapped holes in the initially filled in-gap defect can be either generated in a controlled fashion by electron states cause a “hole photogating” mechanism. Furthermore, we beam irradiation45 or chemically repaired by rational use of notice that the effect of pump fluence on Δσ>100ps depends superacids and/or sulfur-containing agents.46,47 The ability to largely on EF. In the doped regimes, Δσ>100ps increases precisely control the initial defect type/density and implement sublinearly with pump fluence: it gets gradually saturated electrical control of EF and defect filling in graphene−TMDC with fluence (starting from the low-gating potential side), heterostructures provides a new degree of freedom for limited by the defect density (fixed for a given sample) used for manipulating interfacial dynamics at the vdW interface and the photogating. In the undoped regime, Δσ>100ps shows no developing high-performance optoelectronic devices. On the obvious pump fluence dependence and is close to zero. This is other hand, rational passivation of VS and fabrication of a result of the trade-off between electron loss and gain in undoped heterostructures can suppress defect-induced long- graphene, as EF lies between the two defect states. Our lived photogating at the interface, which is relevant for proposed model simplifies some detailed processes that may designing graphene−TMDC-based modulators with fast contribute to CT. For instance, we have assumed that the modulation speed and wide bandwidth. quantum yield of CT is EF-independent and linearly scales with In summary, we investigate in situ the interfacial charge ΔNmajor, which neglects the rate competition between CT and dynamics in graphene−WS2 vdW heterostructures under hot carrier relaxation in graphene (which is EF-dependent). electrochemical gating. The EF modulation reversibly controls The slightly asymmetric gating-dependent photoconductivity the defect occupancy in WS2 and the hot carrier distribution in shown in Figure 3b may be partially attributed to such an graphene, allowing precise tuning of the direction and assumption, which could be further augmented by the efficiency of the photogating field at the vdW interface. This electron−hole mobility asymmetry in graphene.37,38 work provides a fundamental understanding and control of the The results shown in Figure 3 clearly demonstrate the complex interfacial charge dynamics in vdW heterostructures, critical role of electrochemical gating on interfacial CT opening up new possibilities for developing optoelectronic dynamics. For forward CT, graphene loses electrons in both devices via defect and doping engineering. n- and p-doped regimes, i.e., the CT direction is not modulated. On the other hand, electrochemical gating substantially modifies the long-lived charge separation states ■ ASSOCIATED CONTENT * Supporting Information sı (electrons in empty in-gap defect states or holes in filled in-gap The Supporting Information is available free of charge at defect states) and, thus, the photogating field by controlling https://pubs.acs.org/doi/10.1021/acs.nanolett.2c04795. the defect occupancy. Although defects states can also be populated by other methods (e.g., electrical or electrochemical Additional information for sample fabrication; operando methods) to induce gating effects, the photogating effect optical-pump THz-probe spectroscopy; UV−vis and reported in this study is unique because conductance Raman characterizations, and simulation details of modulation is triggered by light absorption and the subsequent thermalized hot carrier distribution in graphene and CT process, which holds great promise for light sensing. photothermionic emission models involving no defect, Furthermore, in contrast to the conventional photogating one defect, and two defects (PDF) mechanism in which the interfacial photogating field direction is determined by the CT direction (e.g., electron injection from graphene to WS2 leads to electron gating to graphene10), our study provides a novel scheme to electrically control the ■ AUTHOR INFORMATION Corresponding Author photogating field direction by tuning defect occupancy without Hai I. Wang − Max Planck Institute for Polymer Research, D- changing the CT direction. 55128 Mainz, Germany; orcid.org/0000-0003-0940- We now discuss the nature of the defects that govern 3984; Email: wanghai@mpip-mainz.mpg.de interfacial photophysics in our study. While other possibilities cannot be completely ruled out, we show compelling evidence Authors that considering only the energetics and density of in-gap Shuai Fu − Max Planck Institute for Polymer Research, D- defect states generated by sulfur vacancies (VS) can explain the 55128 Mainz, Germany; orcid.org/0000-0003-4038- electrically tunable photoconductivity dynamics shown in 2384 Figures 2 and 3. As experimentally revealed by STM and Xiaoyu Jia − Max Planck Institute for Polymer Research, D- theoretically calculated by ab initio theory, VS in WS2 has a 55128 Mainz, Germany typical density of 1011−1013 cm−2 and forms two low-lying in- Aliaa S. Hassan − Max Planck Institute for Polymer Research, gap defect states,35,36 which perfectly match the inferred in-gap D-55128 Mainz, Germany defect density of ∼1012 cm−2 and state quantity of 2. Heng Zhang − Max Planck Institute for Polymer Research, D- Furthermore, on the basis of simulations, we deduce that the 55128 Mainz, Germany; orcid.org/0000-0002-5175- energy positions of the two defect states should be relatively 7367 symmetric with respect to the Dirac point of graphene. The Wenhao Zheng − Max Planck Institute for Polymer Research, inferred energetics are in good agreement with recent STM D-55128 Mainz, Germany studies35 of graphene−WS2 vdW heterostructures. Lei Gao − Max Planck Institute for Polymer Research, D- Sulfur vacancies are known to be prevalent in both exfoliated 55128 Mainz, Germany; School of Physics and Key and CVD TMDCs due to their low formation energy.39,40 Laboratory of MEMS of the Ministry of Education, Southeast Recent advances in defect engineering enable fine-tuning of University, Nanjing 211189, China 1855 https://doi.org/10.1021/acs.nanolett.2c04795 Nano Lett. 2023, 23, 1850−1857 Nano Letters pubs.acs.org/NanoLett Letter Lucia Di Virgilio − Max Planck Institute for Polymer (5) Sierra, J. F.; Fabian, J.; Kawakami, R. K.; Roche, S.; Valenzuela, Research, D-55128 Mainz, Germany S. O. Van Der Waals Heterostructures for Spintronics and Opto- Sven Krasel − Max Planck Institute for Polymer Research, D- Spintronics. Nat. Nanotechnol. 2021, 16 (8), 856−868. 55128 Mainz, Germany (6) Liu, Y.; Huang, Y.; Duan, X. Van Der Waals Integration before David Beljonne − Laboratory for Chemistry of Novel and beyond Two-Dimensional Materials. Nature 2019, 567 (7748), 323−333. Materials, Université de Mons, 7000 Mons, Belgium; (7) Hu, Z.; Liu, X.; Hernández-Martínez, P. L.; Zhang, S.; Gu, P.; orcid.org/0000-0002-2989-3557 Du, W.; Xu, W.; Demir, H. V.; Liu, H.; Xiong, Q. Interfacial Charge Klaas-Jan Tielrooij − Catalan Institute of Nanoscience and and Energy Transfer in van Der Waals Heterojunctions. InfoMat Nanotechnology (ICN2), Barcelona 08193, Spain; 2022, 4 (3), No. e12290. orcid.org/0000-0002-0055-6231 (8) Jin, C.; Ma, E. Y.; Karni, O.; Regan, E. C.; Wang, F.; Heinz, T. F. Mischa Bonn − Max Planck Institute for Polymer Research, Ultrafast Dynamics in van Der Waals Heterostructures. Nat. D-55128 Mainz, Germany; orcid.org/0000-0001-6851- Nanotechnol. 2018, 13 (11), 994−1003. 8453 (9) Chen, Y.; Li, Y.; Zhao, Y.; Zhou, H.; Zhu, H. Highly Efficient Hot Electron Harvesting from Graphene before Electron-Hole Complete contact information is available at: Thermalization. Sci. Adv. 2019, 5 (11), No. eaax9958. https://pubs.acs.org/10.1021/acs.nanolett.2c04795 (10) Fu, S.; du Fossé, I.; Jia, X.; Xu, J.; Yu, X.; Zhang, H.; Zheng, W.; Krasel, S.; Chen, Z.; Wang, Z. M.; et al. Long-Lived Charge Author Contributions Separation Following Pump-Wavelength-Dependent Ultrafast Charge H.I.W. and M.B. conceived and supervised the project. S.F. Transfer in Graphene/WS2 Heterostructures. Sci. Adv. 2021, 7 (9), performed the measurements with help from J.X., A.S.H., H.Z., No. eabd9061. W.Z., L.G., L.D.V., and S.K..; S.F. developed the PTE models (11) Yuan, L.; Chung, T. F.; Kuc, A.; Wan, Y.; Xu, Y.; Chen, Y. P.; with help from J.X. and K.-J.T. All authors contributed to data Heine, T.; Huang, L. Photocarrier Generation from Interlayer Charge- interpretation; S.F. and H.I.W. wrote the paper with input Transfer Transitions in WS2-Graphene Heterostructures. Sci. Adv. from all authors. 2018, 4 (2), No. e1700324. Funding (12) Aeschlimann, S.; Rossi, A.; Chavez-Cervantes, M.; Krause, R.; Open access funded by Max Planck Society. Arnoldi, B.; Stadtmuller, B.; Aeschlimann, M.; Forti, S.; Fabbri, F.; Coletti, C.; Gierz, I. Direct Evidence for Efficient Ultrafast Charge Notes Separation in Epitaxial WS2/Graphene Heterostructures. Sci. Adv. The authors declare no competing financial interest. 2020, 6 (20), No. eaay0761. ■ ACKNOWLEDGMENTS We thank Jaco Geuchies, Yongkang Wang, Chao Zhu, Maksim (13) Luo, D.; Tang, J.; Shen, X.; Ji, F.; Yang, J.; Weathersby, S.; Kozina, M. E.; Chen, Z.; Xiao, J.; Ye, Y.; et al. Twist-Angle-Dependent Ultrafast Charge Transfer in MoS2-Graphene van Der Waals Grechko, Heejae Kim, Sheng Qu, Jiabao Yang, Min Liu, Shuai Heterostructures. Nano Lett. 2021, 21 (19), 8051−8057. Chen, Yunji Lee, Pushpendra Kumar, Arjan Houtepen and (14) Ferrante, C.; Di Battista, G.; López, L. E. P.; Batignani, G.; Samir Al-Hilfi for fruitful discussions. S.F. and L.G. acknowl- Lorchat, E.; Virga, A.; Berciaud, S.; Scopigno, T. Picosecond Energy Transfer in a Transition Metal Dichalcogenide-Graphene Hetero- edge the fellowship support from China Scholarship Council structure Revealed by Transient Raman Spectroscopy. Proc. Natl. (CSC). X.J. acknowledges the financial support by DFG Acad. Sci. U. S. A. 2022, 119 (15), No. e2119726119. through the Excellence Initiative by the Graduate School of (15) Trovatello, C.; Piccinini, G.; Forti, S.; Fabbri, F.; Rossi, A.; De Excellence Materials Science in Mainz (MAINZ) (GSC 266) Silvestri, S.; Coletti, C.; Cerullo, G.; Dal Conte, S. 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