PbI2–MoS2 Heterojunction: van der Waals Epitaxial Growth and

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 4203−4208

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PbI2−MoS2 Heterojunction: van der Waals Epitaxial Growth and Energy Band Alignment Junting Xiao,† Jinxin Liu,† Kuanglv Sun,† Yuan Zhao,† Ziyi Shao,† Xiaoliang Liu,† Yongbo Yuan,† Youzhen Li,† Haipeng Xie,† Fei Song,‡ Yongli Gao,§ and Han Huang*,†

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†

Hunan Key Laboratory of Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, P.R. China ‡ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 239 Zhangheng Road, Pudong New Area, Shanghai 201204, P.R. China § Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, United States S Supporting Information *

ABSTRACT: van der Waals (vdW) epitaxy offers a promising strategy without lattice and processing constraints to prepare atomically clean and electronically sharp interfaces for fundamental studies and electronic device demonstrations. Herein, PbI2 was thermally deposited at high-vacuum conditions onto CVD-grown monolayer MoS2 flakes in a vdW epitaxial manner to form 3D−2D heterojunctions, which are promising for vdW epitaxial growth of perovskite films. X-ray diffraction, X-ray photoemission spectroscopy, Raman, and atomic force microscopy measurements reveal the structural properties of the highquality heterojunctions. Photoluminescence (PL) measurements reveal that the PL emissions from the bottom MoS2 flakes are greatly quenched compared to their as-grown counterparts, which can be ascribed to the band alignment-induced distinct interfacial charge-transfer behaviors. Strong interlayer excitons can be detected at the PbI2/MoS2 interface, indicating an effective type II band alignment, which can be further confirmed by ultraviolet photoemission spectroscopy measurements. The results provide a new material platform for the application of the vdW heterojunctions in electronic and optoelectronic devices.

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optoelectronic devices.11−15 It is well-known that PbI2 is an important precursor for lead halide perovskites.16−19 While monolayer PbI2 has an indirect band gap over 3.72 eV, multilayer PbI2 has a direct band gap of 2.38 eV.20 Recent valence band X-ray photoemission spectroscopy (XPS) results show that the valence band maximum (VBM) of ultrathin PbI2 single crystal is located at the binding energy of ∼0.76 eV below the Fermi level (EF), suggesting that it tends to be a ptype semiconductor.21 In contrast, transition-metal dichalcogenides (TMDCs), the famous 2D material family following graphene, usually show contrary properties. For example, MoS2 shows an indirect-to-direct band gap transition when the number of layers is reduced to a monolayer and is an n-type semiconductor because of the existing sulfur vacancies. Recently, the vdW heterojunctions of PbI2 and monolayer TMDCs have been intensively explored. The multilayer PbI2− WS2 heterostructure constructed by dry transferring the exfoliated monolayer WS2 onto solution-processed multilayer PbI2 single crystal flakes has a type II band alignment, while that with MoS2 has a type I band alignment.22 However, the

an der Waals (vdW) epitaxy offers a promising strategy without lattice and processing constraints to prepare atomically clean and electronically sharp interfaces for fundamental studies and electronic device demonstrations.1−4 It has been well-realized in two-dimensional (2D) atomic crystals via either chemical vapor deposition/physical vapor deposition (CVD/PVD) methods5−7 or layer-by-layer stacking methods8,9 and intensively investigated. Furthermore, MoS2 flakes were reported to be good templates for solution-phase vdW epitaxial growth of MAPbI3 perovskite films.10 Energy band alignment at the interface plays key roles in semiconductor heterojunctions. For straddling (type I) band alignment, the photogenerated carriers in the wider band gap layer can efficiently transfer into the narrow band gap one, giving rise to an increased carrier population and enhanced photoluminescence emission, which has the potential for lightemitting applications. In contrast, for staggered (type II) band alignment, photogenerated electrons and holes can transfer across the interface and be separated at different layers because of the band offset, making these heterojunctions ideal for lightharvesting and photodetection. Layered lead iodide (PbI2) consists of three atomic planes covalently bonded in the sequence of I−Pb−I repeating units stacking along the c-axis and shows strong light adsorption and emission as well as good potential for applications in © XXXX American Chemical Society

Received: June 8, 2019 Accepted: July 11, 2019 Published: July 11, 2019 4203

DOI: 10.1021/acs.jpclett.9b01665 J. Phys. Chem. Lett. 2019, 10, 4203−4208

Letter

The Journal of Physical Chemistry Letters PbI2−MoS2 heterobilayer is theoretically predicted to exhibit a type II band alignment, and the VBM (conduction band minimum, CBM) is localized on MoS2 (PbI2), which leads MoS2 (PbI2) to be p- (n-)type semiconductor,23 in contrast to the above discussion. The PbI2−WS2 heterobilayer synthesized via a two-step CVD strategy has a type I band alignment, while the PbI2−WSe2 has a type II band alignment.24,25 The reported contradictions call for more investigations on this topic. In this Letter, the multilayer PbI2/MoS2 vdW heterojunctions were fabricated through thermal deposition of PbI2 at high-vacuum conditions onto CVD-grown monolayer MoS2 flakes and were systematically investigated using optical microscopy (OM), atomic force microscopy (AFM),26 X-ray diffraction (XRD), XPS, ultraviolet photoemission spectroscopy (UPS),27 and Raman and photoluminescence (PL) spectroscopy.28 XRD and AFM measurements reveal the lamellar stacking of PbI2 on monolayer MoS2 in a vdW epitaxy manner. XPS measurements confirm the stability of the heterojunction in ambient conditions. PL measurements reveal that the PL emissions from the bottom MoS2 flakes are greatly quenched compared to their as-grown counterparts, and strong interlayer excitons can be detected at the PbI2/MoS2 interface, indicating an effective type II band alignment, which was further confirmed by UPS measurements. The PL quenching can be ascribed to the band alignment-induced distinct interfacial charge-transfer behaviors. PbI2/MoS2 vdW heterojunctions were ex situ prepared via a two-step method, as illustrated in Figure 1. First, monolayered

Figure 1b.31,32 The substrate−source distance is ∼20 cm. Because of the layered nature of MoS2 and PbI2, a vdW epitaxy is expected. Figure 1c displays the side view of the proposed model of the PbI2/MoS2 heterojunctions. The interlayer distance for PbI2 is 0.69 nm. The in-plane lattice constant for MoS2 (PbI2) is 0.316 nm (0.456 nm). A model of 2 × 2 PbI2 cells on top of 3 × 3 MoS2 cells is proposed, as shown in the top view in Figure 1d, which is in accordance with recent reports.23,24 Previous density functional theory (DFT) calculations suggest that the stacking shown in Figure 1d is the most stable one with a negligible binding energy of 0.234 eV per unit, in the same magnitude as other vdW crystals, and the PbI2/MoS2 heterobilayer has a type II band alignment with the VBM localized on MoS2.23 The qualities of the as-grown MoS2 samples were characterized using OM, AFM, and Raman spectroscopy. MoS2 flakes in OM images have a triangular appearance with straight edges, as shown in Figure 2a. The typical AFM image of a monolayered MoS2 flake in Figure 2b shows a smooth top surface, and the corresponding line profile reveals a height of 0.8 nm, in agreement with previous reports.33 Figure 2c shows a typical Raman spectrum from MoS2 flakes. Two characteristic Raman peaks located at 382.4 (E12g) and 402.7 cm−1 (A1g) can be observed. It is well-known that the frequency difference between them increases with the number of layers because of the interlayer vdW interactions and the corresponding dielectric screening.34 The frequency difference for the asgrown MoS2 flakes is ∼20.3 cm−1, confirming a monolayer. The inset shows a 2D Raman intensity mapping image around 402.7 cm−1 of a MoS2 flake. The uniform contrast indicates that the MoS2 possesses a high crystalline degree. All the results confirm the high quality of the CVD-grown MoS2 in a monolayer nature. The physiochemical properties of the vertical PbI2/MoS2 vdW heterostructure were characterized using XRD, XPS, AFM, PL, and Raman spectroscopy, respectively. Figure 3a shows the XRD pattern of the PbI2 films thermally deposited onto MoS2 partially covered SiO2/Si. Similar to on graphene/ PET,7 the four diffraction peaks can be assigned to the (00l) planes of PbI2 (JCPDS: 73-1750), and the (001) peak with a full width at half-maximum (fwhm) of 0.209° dominates. The interlayer distance is calculated to be 0.687 nm. No other peaks observed suggests that the PbI2 films on both monolayer MoS2 flakes and SiO2 have a prior growth in the [001] direction in the most common 2H polytype, that is, lamellar stacking along the c-axis of the I−Pb−I sandwich layer, and the sharpness of the diffraction peaks also confirms the good crystallinity of PbI2 films. This is in good agreement with the

Figure 1. (a) Illustration of the CVD growth of monolayer MoS2 on SiO2/Si. (b) Schematic of the thermal evaporation of PbI2. (c) Side and (d) top views of the vertical PbI2/MoS2 vdW heterostructure.

MoS2 flakes were CVD-grown on SiO2 (300 nm)/Si substrates.29,30 Then, multilayer PbI2 was thermally evaporated onto the CVD-grown MoS2, as schematically demonstrated in

Figure 2. CVD grown MoS2 flakes. OM (a) and AFM (b) images of the as-grown MoS2 flakes in regular triangular shape with sharp edges and smooth and flat surfaces. The inset height profile across a step edge of a MoS2 flake in panel b shows a thickness of 0.85 nm, indicating a monolayer. (c) Typical Raman spectrum taken on a MoS2 flake and the corresponding 2D Raman mapping image (inset) of a MoS2 flake, showing the high quality and uniformity. 4204

DOI: 10.1021/acs.jpclett.9b01665 J. Phys. Chem. Lett. 2019, 10, 4203−4208

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Figure 3. Characterizations of the vertical 20 nm (nominal) PbI2/MoS2 vdW heterostructure. (a) XRD pattern. (b) Pb 4f and I 3d core-level XPS spectra. (c) OM image. Insets: typical AFM images taken off, across the edge of, and on a heterostructure, as well as a height profile across the step edge. (d) Raman spectra collected from PbI2/MoS2 vdW heterostructure (red) and PbI2/SiO2 (blue). (e) PL spectra collected from as-grown MoS2 flake (upper panel) and PbI2/MoS2 vdW heterostructure (lower panel). Insets: corresponding PL intensity mapping image and PbI2 related PL spectrum (excited by 337 nm laser).

active A2u phonons. Those located at 166.7 and 213.9 cm−1 are assigned to the acoustic Eu and A2u phonons. The frequency difference between the two MoS2-related Raman peaks is 20.5 cm−1, in line with that of as-grown MoS2 of 20.3 cm−1, suggesting the interfacial interaction with PbI2 is weak. This is slightly different from the case in ref 22, where the frequency difference between the two MoS2-related Raman peaks is enlarged by 1.5 cm−1 because of the strong interlayer interaction. To further investigate the interlayer interaction in the vdW PbI2/MoS2 heterojunctions, PL measurements on as-grown MoS2 and PbI2/MoS2 were performed in the MoS2 PL emission range. The PL spectrum of as-grown MoS2 shown in the upper panel in Figure 3e can be fixed using the WiRE software by three peaks, corresponding to excitons B (1.99 eV), A0 (1.84 eV), and A− (1.81 eV), confirming the n-type semiconductor of MoS2. The uniform contrast in the inserted PL intensity map around the A0 exciton confirms the high quality of the MoS2 flake. For the PbI 2 /MoS 2 vdW heterojunction, in addition to the total intensity being drastically reduced to 1/5 of that of as-grown MoS2, one more peak at the lower energy side (1.73 eV) is required to fit the PL spectrum. There are many possibilities for this broader shoulder, such as PbI2-induced new tail states or below-gap states. It is reported that a defect-induced peak suddenly appears when temperature decrease to below 150 K for MoS2 grown on sapphire.36 However, our temperature-dependent PL results show no such trend, as shown in Figure 4a. Furthermore, the Raman spectra for MoS2 before and after PbI2 deposition show no obvious difference, as shown in Figures 2c and 3d, indicating no obvious change in MoS2. This rules out the possibility of PbI2-induced new tail states or below-gap states in MoS2. Similar to the case of PbI2/WSe2,24 we attributed this to the interlayer exciton (I), which is the energy difference between the CBM of PbI2 and VBM of MoS2 in a type II heterojunction. Although CBM and VBM in monolayer MoS2 are at the K point while for PbI2 should at the

proposed model in Figure 1c and reveals the good quality of the vdW PbI2/MoS2 heterojunctions. High-resolution XPS was implemented to analyze the chemical status of elements of the PbI2. Figure 3b presents the narrow-scanned XPS spectrum of Pb 4f and I 3d. The peaks with binding energies of 138.8 and 143.7 eV below EF are ascribed to Pb 4f7/2 and 4f5/2, and those at 619.8 and 631.2 eV are assigned to I 3d5/2 and 3d3/2, which are in good agreement with the previous reports for bulk PbI2.21 The atomic ratio is calculated to be 1:2.03, confirming that the PbI2 is intact during thermal deposition and storage in air. The OM image shown in Figure 3c displays that PbI2 grows smoothly on the substrate. The inserted three AFM images (left, PbI2/ SiO2; middle, across the boundary; right, PbI2/MoS2/SiO2) reveal the different growth behaviors of PbI2. While bright protrusions on SiO2 indicate that PbI2 nucleates into clusters on SiO2, the flat and smooth top surface of PbI2 on MoS2 suggests a layer-by-layer growth, consistent with the model proposed in Figure 1d. Moreover, no PbI2 grain boundary was observed on individual MoS2 flakes. This can be attributed to the dangling-bond-free surface of MoS2 being able to facilitate the migration of atoms and accelerate the growth of PbI2 grains along the lateral direction. The overlapped height profile across the boundary of MoS2 shows that the surface roughness of PbI2 on MoS2 is about 1/5 of that of PbI2 on SiO2 (300 nm)/ Si. Therefore, like graphene, MoS2 enabled the low-temperature growth of high-quality PbI2 in a vdW epitaxial mode. To investigate the vibrational properties of PbI2/MoS2 vdW heterojunctions, Raman and PL measurements were carried out. Figure 3d shows the Raman spectra of PbI2 on a SiO2/Si substrate with (red) and without (blue) MoS2, respectively. Both spectra contain five PbI2-related Raman-active peaks at identical positions, in good agreement with previous reports.35 Raman peaks located at 69.9 and 94.3 cm−1 are assigned to Raman-active Eg (in-plane) and A1g (out-of-plane) modes in 2H polytype of PbI2, respectively. The one located at 108.7 cm−1 is assigned to the longitudinal optical branch of infrared4205

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that of the in-plane Eg phonon.37 Figure S1a shows the same trend, confirming the PbI2 films in different thickness. The thickness-dependent PL spectra shown in Figure S1b reveal the existence of an addition shoulder at the lower-energy side for all samples, indicating that it is the interlayer exciton. Because of the data taken from different samples, the peak positions are slightly different. To obtain more comprehensive phonon properties of PbI2 and MoS2 in the vdW heterojunctions, temperature-dependent PL and Raman measurements were carried out. Figure 4a shows the corresponding temperature-dependent PL spectra from 120 to 300 K. Each spectrum can be fitted using four peaks, as shown in the lower panel in Figure 3e. The positions of all four exciton states red-shifted as the temperature increasing attributed to the increased electron−phonon interaction as well as slight changes in the bonding length, which is very similar to that in conventional semiconductors.38 A standard semiconducting band gap model is adopted to quantify the observations: ÄÅ ÉÑ ÅÅ ÑÑ ℏω Å Å Eg (T ) = Eg (0) − SℏωÅÅcoth − 1ÑÑÑÑ ÅÅÇ ÑÑÖ 2KBT

Figure 4. (a) Temperature-dependent PL spectra of PbI2/MoS2 vdW heterostructure. Plotted PL (b) and Raman (c) peak position evolutions as a function of temperature.

gamma point, DFT calculations show CBM and VBM of the MoS2/PbI2 vdW heterostructure are localized on PbI2 and MoS2, respectively.23 Thus, the formed interlayer exciton is a momentum-direct one with emission. The difference between A0 and I-exciton is experimentally measured to be ∼110 meV. The inserted PL spectrum from PbI2 films excited by a 337 nm laser in the lower panel shows a sharp peak centered at 2.42 eV, containing two main peaks located at 2.35 and 2.43 eV. The high-energy emission is generally attributed to recombination of free excitons generated by the band-to-band transition, while the lowerenergy one is assigned as a bound exciton emission due to the intrinsic defects of PbI2 films such as dangling bonds from the grain boundary or surface roughness defects.37 The much more intensive emission from the high-energy one and the narrow fwhm indicate the high crystalline quality of the deposited PbI2. To further confirm this additional exciton arises from the PbI2/MoS2 interlayer, thickness-dependent Raman and PL measurements were performed. The results are shown in panels a and b of Figure S1, respectively. It is well-known that the ratio of Raman intensity (I(A1g)/I(Eg)) can be used to quickly determine the thickness of 2D PbI2 nanosheets because the out-of-plane A1g vibration intensity decreases faster than

where Eg(0) is the band gap value at 0 K, S a parameter describing the strength of the electron−phonon coupling, and ℏω the average phonon energy involving the electron−phonon interaction. It is found that this model perfectly fits temperature dependence of all four excitons, as shown in Figure 4b. The extracted parameters are summarized in Table S1 of the Supporting Information, which are consistent with previous reports.39 Figure 4c plots the Raman peak position as a function of temperature for the two MoS2-related peaks (A1g and E12g) as well as the two PbI2-related peaks (A1g and Eg). The corresponding temperature coefficients are −0.0115, −0.0139, −0.0040, and −0.0171 cm−1 K−1, respectively, consistent with previous reports.35 To directly reveal the energy band alignment, MoS2 and PbI2 on SiO2 were investigated using UPS. Figure 5a (c) displays the extended valence band spectrum of CVD-grown MoS2 (PbI2 with nominal thickness of 10 nm on SiO2/Si). The VBM for MoS2 and PbI2 are measured to be about 1.24 and 1.99 eV below EF, indicating both MoS2 and PbI2 on SiO2 are

Figure 5. UPS characterizations. (a and c) Valence regions of UPS spectra close to the Fermi level. The values indicate valence band edges. (b and d) The secondary electron cutoff regions of the UPS spectra. The values indicate the work function. (e) The corresponding type II band alignment for PbI2/MoS2 vdW heterostructure with the illustrative transfer of photoexited carriers within the heterojunctions. The separation of excitons leads to the photoluminescence quenching. 4206

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Epitaxy through Graphene Enables Two-Dimensional Material-Based Layer Transfer. Nature 2017, 544, 340−343. (4) Liu, Y.; Huang, Y.; Duan, X. F. Van Der Waals Integration before and beyond Two-Dimensional Materials. Nature 2019, 567, 323−333. (5) Yang, W.; Chen, G. R.; Shi, Z. W.; Liu, C. C.; Zhang, L. C.; Xie, G. B.; Cheng, M.; Wang, D. M.; Yang, R.; Shi, D. X.; et al. Epitaxial Growth of Single-Domain Graphene on Hexagonal Boron Nitride. Nat. Mater. 2013, 12, 792−797. (6) Gong, Y. J.; Lin, J. H.; Wang, X. L.; Shi, G.; Lei, S. D.; Lin, Z.; Zou, X. L.; Ye, G. L.; Vajtai, R.; Yakobson, B. I.; et al. Vertical and InPlane Heterostructures from WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135−1142. (7) Zhang, J. C.; Huang, Y. C.; Tan, Z. J.; Li, T. R.; Zhang, Y. C.; Jia, K. C.; Lin, L.; Sun, L. Z.; Chen, X. W.; Li, Z. Z.; et al. LowTemperature Heteroepitaxy of 2D PbI2 /Graphene for Large-Area Flexible Photodetectors. Adv. Mater. 2018, 30, 1803194. (8) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722−726. (9) Kang, K.; Lee, K. H.; Han, Y.; Gao, H.; Xie, S.; Muller, D. A.; Park, J. Layer-by-Layer Assembly of Two-Dimensional Materials into Wafer-Scale Heterostructures. Nature 2017, 550, 229−233. (10) Tang, G. Q.; You, P.; Tai, Q. D.; Yang, A. N.; Cao, J. P.; Zheng, F. Y.; Zhou, Z. W.; Zhao, J.; Chan, P. K. L.; Yan, F. Solution-Phase Epitaxial Growth of Perovskite Films on 2D Material Flakes for HighPerformance Solar Cells. Adv. Mater. 2019, 1807689. (11) Wang, Y. G.; Gan, L.; Chen, J. N.; Yang, R.; Zhai, T. Y. Achieving Highly Uniform Two-Dimensional PbI2 Flakes for Photodetectors via Space Confined Physical Vapor Deposition. Sci. Bull. 2017, 62, 1654−1662. (12) Han, M. M.; Sun, J. M.; Bian, L. Z.; Wang, Z.; Zhang, L.; Yin, Y. X.; Gao, Z. F.; Li, F. L.; Xin, Q.; He, L. B. Two-Step Vapor Deposition of Self-Catalyzed Large-Size PbI2 Nanobelts for HighPerformance Photodetectors. J. Mater. Chem. C 2018, 6, 5746−5753. (13) Zheng, W.; Zhang, Z. J.; Lin, R. C.; Xu, K.; He, J.; Huang, F. High-Crystalline 2D Layered PbI2 with Ultrasmooth Surface: LiquidPhase Synthesis and Application of High-Speed Photon Detection. Adv. Electron. Mater. 2016, 2, 1600291. (14) Sun, L.; Wang, C. R.; Xu, L.; Wang, J. L.; Chen, X. S.; Yi, G. C. Millimeter-Sized PbI2 Flakes and Pb5S2I6 Nanowires for Flexible Photodetectors. J. Mater. Chem. C 2018, 6, 7188−7194. (15) Zhong, M. Z.; Zhang, S.; Huang, L.; You, J. B.; Wei, Z. M.; Liu, X. F.; Li, J. B. Large-Scale 2D PbI2 Monolayers: Experimental Realization and Their Indirect Band-Gap Related Properties. Nanoscale 2017, 9, 3736−3741. (16) Liu, M. Z.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (17) Wan, F.; Qiu, X. C.; Chen, H.; Liu, Y. Q.; Xie, H. P.; Shi, J.; Huang, H.; Yuan, Y. B.; Gao, Y. L.; Zhou, C. H. Accelerated Electron Extraction and Improved UV Stability of TiO2 Based Perovskite Solar Cells by SnO2 Based Surface Passivation. Org. Electron. 2018, 59, 184−189. (18) Yan, J. Q.; Lin, S. Y.; Qiu, X. C.; Chen, H.; Li, K. M.; Yuan, Y. B.; Long, M. Q.; Yang, B. C.; Gao, Y. L.; Zhou, C. H. Accelerated Hole-Extraction in Carbon-Electrode Based Planar Perovskite Solar Cells by Moisture-Assisted Post-Annealing. Appl. Phys. Lett. 2019, 114, 103503. (19) Liu, P.; Liu, X. L.; Lyu, L.; Xie, H. P.; Zhang, H.; Niu, D. M.; Huang, H.; Bi, C.; Xiao, Z. G.; Huang, J. S. Interfacial Electronic Structure at the CH3NH3PbI3 /MoOx Interface. Appl. Phys. Lett. 2015, 106, 193903. (20) Toulouse, A. S.; Isaacoff, B. P.; Shi, G. S.; Matuchová, M.; Kioupakis, E.; Merlin, R. Frenkel-like Wannier-Mott Excitons in FewLayer Pb I2. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 165308.

n-type semiconductors, which can be attributed to the presence of S or I vacancies.40,41 The corresponding work functions for MoS2 and PbI2 deduced from the secondary electron cutoff, shown in Figure 5b,d, are 4.38 and 4.36 eV. Taking the band gap of 2.13 and 2.38 eV for monolayer MoS242 and multilayer PbI2 into account, respectively, the diagram of energy band alignment of multilayer PbI2 on CVDgrown monolayer MoS2 on SiO2/Si is proposed in Figure 5d. Because of charge transfer from SiO2 substrate to MoS2, we speculate that the inset of a monolayer of MoS2 between PbI2 and SiO2 will slightly change the work function of PbI2. The CBM of MoS2 is located above that of PbI2. Therefore, for the vdW heterojuction, the VBM and CBM are localized on MoS2 and PbI2, respectively, and a type II band alignment is built, consistent with the above PL results. In summary, we thermally deposited PbI2 onto CVD-grown monolayer MoS2 flakes to form vdW heterojunctions at highvacuum conditions, which were characterized using XRD, XPS, AFM, PL, and Raman spectroscopy. Different with previous reports, a type II band alignment is found for such vdW heterojunctions. Our findings reveal that the band alignment type is heavily dependent on the substrates and preparation methods.

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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.9b01665. Experimental section, additional characterization data including thickness-dependent Raman and PL measurements of PbI2/MoS2 vdW heterostructure, and summarized parameters from experimental data fitting in Figure 4b (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Hunan Key Laboratory of Supermicrostructure and Ultrafast Process, School of Physics and Electronics, Central South University, No. 932, South Lushan Road, Changsha City, Hunan Province, P.R. China. ORCID

Youzhen Li: 0000-0001-8994-3627 Yongli Gao: 0000-0001-9765-5246 Han Huang: 0000-0003-0641-1962 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation (NSF) of China (Grant No. 11874427) and that from NSF of Hunan province (Grant No. 2016JJ1021).

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REFERENCES

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The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.9b01665 J. Phys. Chem. Lett. 2019, 10, 4203−4208