Janus Monolayer Transition-Metal Dichalcogenides

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Janus Monolayer Transition-Metal Dichalcogenides Jing Zhang,†,∥ Shuai Jia,†,∥ Iskandar Kholmanov,‡ Liang Dong,§ Dequan Er,§ Weibing Chen,† Hua Guo,† Zehua Jin,† Vivek B. Shenoy,§ Li Shi,‡ and Jun Lou*,† †

Department of Materials Science and Nanoengineering, Rice University, Houston, Texas 77005, United States Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States § Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ‡

S Supporting Information *

ABSTRACT: The crystal configuration of sandwiched S− Mo−Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and X-ray photoelectron spectroscopy and confirmed by time-of-flight secondary ion mass spectrometry. Density functional theory (DFT) calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure. KEYWORDS: Janus SMoSe, sulfurization, Raman, TOF-SIMS, HER ince the discovery of graphene in 2004,1 two-dimensional (2D) materials have been attracting increasing attention due to the many interesting properties originating from the bulk to monolayer transition. Among the 2D family, transition-metal dichalcogenides (TMDs) have been most widely studied; MoS2 as the representative TMD material, its electrical,2 optical3 as well as other physical properties4,5 are well understood. By controlling the stoichiometric ratio of chemical vapor deposition grown MoSxSe2−x,6 WSxSe2−x,7 and MoxW1−xS28 alloys, the optical and electrical properties of the monolayers can be tuned. Various combinations of out-of-plane TMD heterojunctions have been prepared by transfer methods. MoS2/WS2,9 MoS2/MoSe2,10 MoS2/WSe2,11 and MoSe2/ WSe212 out-of-plane and in-plane heterojunctions have also been grown by the CVD method. The heterojunctions, especially the in-plane ones, exhibit interesting physical properties due to the presence of an atomically sharp interface.13 In this work we demonstrate a type of TMD structure possessing an unconventional asymmetry sandwich construction that is distinctively different from both types of heterojunctions. As shown in Figure 1b and Figure 2a, the socalled “Janus” SMoSe consists of three layers of atoms, namely, sulfur, molybdenum, and selenium from the top to the bottom.

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© 2017 American Chemical Society

Unlike the randomly alloyed SMoSe, the Janus SMoSe is highly asymmetric along the c-axis direction. The polarized chemical construction potentially derives an intrinsic electric field inside the crystal and physical properties such as the Zeeman-type spin splitting.14 We have managed to reproducibly obtain monolayer Janus SMoSe flakes by well-controlled sulfurization of monolayer MoSe2. The monolayer MoSe2 flakes were first grown by the CVD method.15 Briefly, MoO3 powder as the Mo source was placed in a porcelain boat at the center of the heating zone. Sulfur powder as the S source was placed upstream of the heating zone. Monolayer MoSe2 flakes started to crystallize and grow at around 800 °C on a SiO2/Si substrate facing the MoO3 powder under the protection of ultrahigh purity argon. The asgrown monolayer MoSe2 exhibits typical Raman and high photoluminescence (PL)16,17 signals as shown in Figure 1c and d. The sulfurization of the top layer Se was realized by a controlled substitutional reaction with vaporized sulfur in a typical CVD setup (Figure 1a). Briefly, the monolayer MoSe2 Received: May 8, 2017 Accepted: August 3, 2017 Published: August 3, 2017 8192

DOI: 10.1021/acsnano.7b03186 ACS Nano 2017, 11, 8192−8198

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Figure 1. Reactions under different growth temperature. (a) Schematic illustration of the reaction setup. (b) Proposed reaction mechanism for the sulfurization of monolayer MoSe2 on a SiO2/Si substrate at different temperatures. Between the monolayer and the substrate is the van der Waal interaction. (c, d) Raman and PL (under 532 nm diode laser excitation) spectra of MoSe2, Janus SMoSe, and MoS2 corresponding to the diagram in (b).

As shown in Figure 1b, the top and bottom layer Se atoms of monolayer MoSe2 on SiO2/Si substrate are located under different chemical environments, namely, under atmosphere pressure and in the van der Waals gap, respectively. While the gaseous sulfur molecules (mainly S8, possibly pyrolyse to smaller molecules) approach, the top layer Se atoms are in direct contact with sulfur and could be quickly substituted once the thermal driving force overcomes the thermodynamic barrier. However, in order to substitute the bottom layer Se atoms, the gaseous sulfur molecules have to diffuse into the van der Waals gap first. Once a selenium atom is substituted, it has to diffuse out to facilitate further sulfurization. Assuming the sulfur partial pressure is identical, the diffusion process becomes the rate-limiting step, and the temperature directly determines the reaction rate. We have shown in Figure 1b that below 750 °C and over 850 °C the Raman peaks are similar to MoSe2 and MoS2, respectively, indicating that sulfur does not substitute selenium below a certain temperature and is able to diffuse into the van der Waals gap once the temperature is high enough. Similar observation has been made on the sulfurization of monolayer WSe2 as depicted in Figure S2. The tendency to form the Janus structure has been shown in similar sulfurization processes but under high-vacuum conditions.7,18,19 The high vacuum potentially lowers the diffusion energy barrier of sulfur and facilitates the sulfurization process, resulting in a reduced temperature required for complete sulfurization of the selenides (∼700 °C). We attribute our reproducible synthesis of the Janus SMoSe to the fact that atmosphere pressure is maintained during the reaction. The atmospheric pressure significantly enlarges the stable temperature window for the top layer atom substitution reaction. The sulfurization time, on the other hand, did not lead to the selenium substitution in the bottom layer, implying that only temperature and pressure played crucial roles in this selective sulfurization process. The Janus SMoSe triangular flake looks identical to MoSe2 when observed under the optical microscope (Figure 2b). The morphology of the flake was checked by atomic force microscopy (AFM), and the profile shows the flake thickness was MoS2, in general agreement with the experimental results of SeMoS > SMoSe > MoSe2 > MoS2. The only exception is the relative efficiencies of SMoSe and MoSe2. Theoretical results show that they have similar HER efficiencies, while experimentally SMoSe has a better efficiency. The discrepancy may arise from the densities of single S- and Se-vacancies, which affect the electronic and catalytic properties of TMD monolayers. In the supercells we simulate, a universal vacancy density of 6.25% in an S or Se atomic layer is used, regardless of the materials. However, under experimental conditions, the single S- and Se-vacancies may vary in MoS2, SMoSe (SeMoS), and MoSe2. Therefore, a further experimental investigation on the defect microstructure of the TMD samples is needed to address this discrepancy. In conclusion, a 2D monolayer with an asymmetric structure, Janus SMoSe, has been synthesized by a controlled sulfurization process. The S−Mo−Se trilayer atomic structure exhibits unique Raman vibration peaks that can be distinguished from pure MoS2 and MoSe2 and randomly alloyed MoSxSe2−x. The experimental characteristic Raman peaks agree well with predictions of DFT simulations. TOF-SIMS analysis further confirms that the Janus SMoSe is composed of the S−Mo−Se trilayer structure. Both Raman mapping and TOF-SIMS element mapping/XPS profile analysis indicate that the monolayer is uniformly sulfurized and the asymmetric structure distributes all over the Janus monolayer. DFT simulation was used to predict the bandgap of the Janus SMoSe, and the value is consistent with the experimental results derived from the PL spectroscopy. It is also suggested that an intrinsic electric field potentially exists perpendicular to the inplanar direction. High basal plane HER catalytic activity is discovered for the Janus structures, and DFT calculation implies that the activity may originate from the synergistic effects of the intrinsic defects and structural strain in the Janus structure.

MATERIALS AND METHODS Experimental Details. All the chemicals are purchased from Sigma-Aldrich and used as received without further purification. The CVD procedures are carried out using a MTI tube furnace. The Raman and PL spectra are collected with Renishaw inVia confocal Raman microscope. The TEM images are taken with JEOL 2100F microscope. The XPS spectra are collected with PHI Quantera instrument. The devices are fabricated with FEI Quanta 400 SEM. The TOF-SIMS data are collected with a IONTOF TOF.SIMS.5 instrument. The electronic properties of such asymmetric 2D monolayer is investigated by measuring the field-effect transistor (FET) based on it. FET devices on Janus SMoSe monolayers were fabricated using an ebeam lithography method. The fabricated device (inset in Figure S6) has a channel length of 6 μm and width of 3.6 μm. At positive backgate voltage (Vg) region, the output characteristics (increased drain current (Id) vs Vg) indicates the n-type behavior of the Janus SMoSe monolayer. The mobility of the device is calculated (see details in Supporting Information) to be 6.2 × 10−2 cm−2 V−1 s−1, and the on/ off ratio is 102. The low mobility of the Janus SMoSe can potentially be attributed to the heat treatment accompanying the sulfurization reaction. During the process, the residual oxygen in the CVD environment often lead to the creation of vacancies on the monolayer structure24,25 and reduction in the carrier mobility.26 The basal plane HER polarization curves of the monolayer TMDs were collected using a modified three-electrode method (Figure S9).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03186. Descriptor of the HER efficiency by DFT calculation; HER device fabrication and measurement details; survey XPS of Janus SMoSe; Raman and PL spectra of WSe2 sulfurization; Raman spectra of MoSe2 with increasing layer numbers with and without sulfurization; Raman vibration modes of SMoSe; charge density distribution of SMoSe; TOF-SIMS of MoSe2; XPS profile analysis of the SMoSe; FET device performance of SMoSe; images of a typical HER measurement device; PL of SMoSe before and after annealing; preparation of SeMoS configuration from SMoSe; experimental and theoretical Raman peaks of MoSe2, SMoSe, and MoS2 monolayers (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 8196

DOI: 10.1021/acsnano.7b03186 ACS Nano 2017, 11, 8192−8198

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Jing Zhang: 0000-0002-4706-8630 Li Shi: 0000-0002-5401-6839 Author Contributions ∥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS J.L. gratefully acknowledges the support from the AFOSR grant FA9550-14-1-0268, the Welch Foundation grant C-1716, and the Army Research Office grant W911NF-16-1-0447. L.S. acknowledges the support from the NSF award EFRI-1433467. The theoretical work is supported primarily by contract W911NF-16-1-0447 from the Army Research Office (V.B.S.) and also by grants EFMA-542879 (D.E) and CMMI-1363203 (L.D.) from the U.S. National Science Foundation. We appreciate the great help from Dr. B. Chen with the XPS measurements and analysis. REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (2) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (3) Splendiani, A.; Sun, L.; Zhang, Y. B.; Li, T. S.; Kim, J.; Chim, C. Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271−1275. (4) Cai, L.; He, J. F.; Liu, Q. H.; Yao, T.; Chen, L.; Yan, W. S.; Hu, F. C.; Jiang, Y.; Zhao, Y. D.; Hu, T. D.; Sun, Z. H.; Wei, S. Q. VacancyInduced Ferromagnetism of MoS2 Nanosheets. J. Am. Chem. Soc. 2015, 137, 2622−2627. (5) Zeng, H. L.; Dai, J. F.; Yao, W.; Xiao, D.; Cui, X. D. Valley Polarization in MoS2 Monolayers by Optical Pumping. Nat. Nanotechnol. 2012, 7, 490−493. (6) Gong, Y. J.; Liu, Z.; Lupini, A. R.; Shi, G.; Lin, J. H.; Najmaei, S.; Lin, Z.; Elias, A. L.; Berkdemir, A.; You, G.; Terrones, H.; Terrones, M.; Vajtai, R.; Pantelides, S. T.; Pennycook, S. J.; Lou, J.; Zhou, W.; Ajayan, P. M. Band Gap Engineering and Layer-by-Layer Mapping of Selenium-Doped Molybdenum Disulfide. Nano Lett. 2014, 14, 442− 449. (7) Duan, X. D.; Wang, C.; Fan, Z.; Hao, G. L.; Kou, L. Z.; Halim, U.; Li, H. L.; Wu, X. P.; Wang, Y. C.; Jiang, J. H.; Pan, A. L.; Huang, Y.; Yu, R. Q.; Duan, X. F. Synthesis of WS2xSe2−2x Alloy Nanosheets with Composition-Tunable Electronic Properties. Nano Lett. 2016, 16, 264−269. (8) Lin, Z.; Thee, M. T.; Elías, A. L.; Feng, S.; Zhou, C.; Fujisawa, K.; Perea-López, N.; Carozo, V.; Terrones, H.; Terrones, M. Facile Synthesis of MoS2 and MoxW1-xS2 Triangular Monolayers. APL Mater. 2014, 2, 092514. (9) 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.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M. Vertical and in-Plane Heterostructures from WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135−1142. (10) Duan, X. D.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Li, H. L.; Wu, X. P.; Tang, Y.; Zhang, Q. L.; Pan, A. L.; Jiang, J. H.; Yu, R. Q.; Huang, Y.; Duan, X. F. Lateral Epitaxial Growth of Two-Dimensional Layered Semiconductor Heterojunctions. Nat. Nanotechnol. 2014, 9, 1024−1030. (11) Li, M. Y.; Shi, Y. M.; Cheng, C. C.; Lu, L. S.; Lin, Y. C.; Tang, H. L.; Tsai, M. L.; Chu, C. W.; Wei, K. H.; He, J. H.; Chang, W. H.; Suenaga, K.; Li, L. J. Epitaxial Growth of a Monolayer WSe2-MoS2 Lateral P-N Junction with an Atomically Sharp Interface. Science 2015, 349, 524−528. 8197

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DOI: 10.1021/acsnano.7b03186 ACS Nano 2017, 11, 8192−8198