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van der Waals Heterostructures based on Liquid Phase Exfoliated MoS2 and WS2 nanosheets
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 21 (2020) 1840–1845
www.materialstoday.com/proceedings
ISFM-2018
van der Waals Heterostructures based on Liquid Phase Exfoliated MoS2 and WS2 nanosheets Sneha Sinha, Jyotsna and Sunil K. Arora* Centre for Nanoscience and Nanotechnology, South Campus, Panjab University, Chandigarh-160014, India.
Abstract We have synthesized the van der Waals (vdW) heterostructures of two-dimensional layered materials (MoS2 and WS2) via physical mixing of different proportions of colloidal dispersions containing exfoliated MoS2 and WS2 nanosheets. The synthesized vdW heterostructures exhibited optical and phononic properties distinct from its components. We find that the A- and B- excitonic peak positions in heterostructures are red shifted with respect to the corresponding peak positions of the constituent materials (MoS2 and WS2) and are also dependent on the compositional ratio used to produce the heterostructures. The in-plane (E12g) and out-of-plane (A1g) Raman modes in heterostructures exhibited hardening with respect to the corresponding modes in individual nanosheets of MoS2 and WS2. Our results highlight the role of interlayer interactions in the synthesized MoS2-WS2 heterostructures. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Symposium on Functional Materials (ISFM-2018): Energy and Biomedical Applications. Keywords: Transition metal dichalcogenides, Liquid Exfoliation, vdW Heterostructure.
1. Introduction Transition metal dichalcogenides (TMDs) belong to the large family of two-dimensional layered materials (2DLMs) with strong in-plane bonding (covalent) and weak out-of plane bonding (van der Waals). The weak van der Waals forces are responsible for easy breakdown of bulk single crystal in to mono- and few-layer form by using mechanical or chemical exfoliation. *Corresponding Author. Tel.: +91-7696265266 E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Symposium on Functional Materials (ISFM-2018): Energy and Biomedical Applications.
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The artificial stacking of different 2DLMs (either vertically or laterally) results in an assembly known as van der Waals (vdW) heterostructure [1]. These vdW heterostructures offers new functionality and applications beyond their constituent 2DLMs [1,2]. For example, a staggered (type II) band alignment was predicted and observed for vdW heterostructures based on semiconducting TMDs (MoS2 and WS2) with excitons (bound electrons and holes) localized in individual monolayers [3,4]. Consequently, TMD heterostructures are quite promising for optoelectronic applications [5,6]. One of the major challenges in this subject area to realize the application potential of these vdW heterostructures is to produce them with a method that is scalable and produces clean interfaces. Conventional fabrications of these heterostructures have relied on the low-yield manual exfoliation from the bulk single crystals using scotch tape method and subsequent manual stacking of individual TMD layers, which remain impractical for scaled-up applications. Efforts are in progress to grow layers of individual component 2DLM on top of the other by chemical vapor deposition (CVD) and pulsed laser deposition (PLD) methods [7,8]. Other alternative approaches used to produce vdW heterostructure are mixing colloidal dispersions of 2D nanosheets and selforganizational assembly of a mixture of suspensions containing different 2DLMs [9,10]. Therefore, finding a method to produce these hybrid materials with improved yield and controllable stacking is highly desirable. Herein, we present a simple but effective method to form vdW heterostructure of TMDs (MoS2 and WS2) using colloidal dispersions containing exfoliated MoS2 and WS2 nanosheets. The bulk MoS2 and WS2 powder were exfoliated using mixed solvent strategy under ultrasonic treatment. Through detailed structural studies we show that the structural integrity of the individual component material is retained in the heterostructure however, the optical and phononic properties were affected by the enhanced interlayer coupling at the hetero-interface. 2. Experimental Procedure In order to produce vdW heterostructure of MoS2 and WS2 nanosheets, firstly we produce exfoliated dispersions using mixed solvent strategy. Dispersions containing MoS2 (WS2) nanosheets were prepared by adding 25 mg powder of MoS2 (WS2) (< 2µm, 99%, Sigma Aldrich) in to 5 ml mixture of 30% isopropanol and water solution containing 1% volume of hydrazine monohydrate. The mixture was sonicated in an ultrasonic bath for duration of 4 hours at a temperature of 50°C. The resulting suspensions were centrifuged at 3000 rpm for 30 min. The supernatant was carefully collected and was used for further characterization. To estimate the exfoliation yield, infiltrant was collected and weighed. The exfoliation yield in the dispersions was found to be 1.4 mg ml-1 for MoS2 and 1.38 mg ml-1 for WS2, which is far better than recently reported exfoliation efficiencies using NMP (N-Methyl Pyrollidone) as solvent and surfactant assisted exfoliation methods [11]. MoS2−WS2 heterostructures were prepared by mixing the exfoliated dispersions of MoS2 and WS2 in different volume proportions, and sonicated the resulting solution for 1 hour. The volume of the resulting mixture of colloidal dispersion was reduced further by evaporating the solvent at 65°C. The obtained dispersion was then used for further characterization. The entire work flow is represented in Figure 1.
Figure 1. Work flow for the formation of vdW heterostructure from the exfoliated TMD dispersions.
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The X-ray diffraction (XRD) patterns of the as-exfoliated MoS2, WS2 nanosheets and their co-stacked heterostructure were recorded using Panalytical’s X’pert Pro diffractometer with Cu K-alpha-1 radiation. The UVvisible spectra of exfoliated dispersions and their vdW heterostructures were recorded using V-770 (Jasco) spectrophotometer in a matched pair of quartz cuvettes of path length 1cm. The Raman spectroscopy characterization was performed using WiTec Confocal Raman Spectrometer with a 532 nm excitation laser and 1µm diameter laser spot size. The samples for Raman studies were prepared by drop-casting films on Silicon wafers (100) and dried in hot air oven at 60oC. The composition of the films was determined using Energy Dispersive X-ray spectroscopy (EDX) and corresponded well to the initial mixture of the constituent 2DLMs (e.g. for MoS2:WS2 in 1:1 ratio the EDX studies showed a ratio of 1:1.05). TEM studies were performed with the JEOL-JEM 2100 microscope operated at an accelerating voltage of 200 kV. Carbon-coated copper grids (300 mesh) were used to prepare sample for TEM studies. 3. Results and Discussion Prior to discussing the phononic properties of MoS2-WS2 heterostructure, we present details of structural studies performed on MoS2, WS2 nanosheets and their heterostructure. The XRD pattern of exfoliated nanosheets shows a single strong (002) peak at 14.5o while the other peaks are either absent or relatively weak. This confirms that the nanosheets have been exfoliated along the (002) axis. However, in case of MoS2-WS2 heterostructure, a relatively weak and broad diffraction peak at 14.5o is observed, which corresponds to the diffraction from the (002) plane of the constituent nanosheets (Fig. 2a). Figure 2(b) illustrates the low magnification TEM image of the MoS2−WS2 heterostructure (1:1 composition). The high resolution lattice image from the heterostructure along with selected area electron diffraction (SAED) is shown in Figure 2(c). The SAED pattern shows sharp diffraction spots corresponding to constituent MoS2 and WS2 nanosheets. The inter-planar spacing determined from lattice fringe separation (Fig. 2c) is found to be 0.27 nm, which is consistent with (100) plane of 2H-phase of constituent TMDs. Thus, the XRD and TEM studies justify the fact that the structural integrity of the individual TMD component is retained in the synthesised vdW heterostructure.
Figure 2. (a) XRD spectra of exfoliated MoS2, WS2 nanosheets and their vdW heterostructure containing 1:1 proportion of the constituent TMD nanosheets (b) TEM image of the MoS2−WS2 heterostructure (c) High resolution lattice image of the heterostructure along with the SAED pattern shown in the Inset.
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Figure 3 shows the optical absorbance spectra of the exfoliated MoS2, WS2 nanosheets and heterostructures made with different proportions of constituent nanosheets. In case of MoS2, two characteristic peaks A and B were observed at λ = 678 and 617 nm, respectively, which corresponds to the direct transitions (band edge excitons) at the K-point arising due to the spin-orbital coupling [12]. For WS2, the A- and B- excitonic peaks are centered at 632 nm and 527 nm, respectively (Fig. 3a). From the optical absorbance spectra recorded with different volume ratios of MoS2 and WS2 in the vdW heterostructures (Fig. 3b); it was found that there is a clear enhanced contribution from constituent excitonic peaks with positions (red shift) and intensities of the absorption bands altered. The observed red shift in the optical spectra can be attributed to the enhanced interlayer coupling as we move away from single layer limit and the reduced dielectric screening of the Coloumb potential between the charged carriers [5].
Figure 3. (a) Optical absorbance spectra of exfoliated MoS2 and WS2 nanosheets (b) Absorbance spectra for vdW heterostructures revealing red shift of A- and B- excitonic peaks upon co-stacking of the constituent TMD nanosheets. Label A and B on curves corresponds to the excitonic peaks ‘A’ and ‘B’.
Raman spectroscopy studies were carried out on TMD nanosheets drop-casted on to Si (100) substrates and were used to determine the number of monolayers in the exfoliated nanosheets. The number of monolayers in exfoliated nanosheets of MoS2 and WS2 could vary and so the Raman spectra tend to spatially vary. Figure 4(a) shows a typical Raman spectrum observed for the exfoliated nanosheets of MoS2 and WS2. Exfoliated few layered nanosheets of MoS2 and WS2 have shown characteristic E12g and A1g Raman modes located at 385 and 409.95 cm-1 for MoS2 and 358 and 422.2 cm-1 for WS2, respectively (Fig. 4a). It is known that the E12g mode origins from the opposite vibration of two S atoms with respect to the Mo (W) atom in the basal plane and represents the interlayer displacements of Mo (W) and S atoms [12-14]. The A1g mode represents the out-of-plane vibrations of metalchalcogen bonds along c axis, providing information on the strength of the interaction between the adjacent layers [12-14]. The frequency difference, ∆, between E12g and A1g modes was found to be 25 cm-1 for MoS2 and 64.2 cm-1 for WS2 which suggests that the nanosheets are 4-5 monolayers thick. The Raman spectra for vdW heterostructures (made with different proportions of MoS2 and WS2) reveals that the greater contribution is from the TMD component which is in greater proportion (Fig. 4b). The vdW heterostructure (1:1 composition) exhibits the signature peaks corresponding to the Raman modes from constituent nanosheets of WS2 and MoS2, respectively (Fig. 4b). The blue-shift in A1g mode of MoS2 (WS2) film upon heterostructure formation was found to be 0.37 (0.41) cm-1, respectively. Similarly, the blue-shift in E12g modes of MoS2 (WS2) film upon heterostructure formation was found to be 0.17 (0.20) cm-1, respectively.
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Figure 4. (a) Raman spectra observed for the exfoliated nanosheets of MoS2 and WS2 (b) Raman spectra of vdW heterostructures with different volume proportions of constituent MoS2 and WS2 nanosheets.
Generally, the blue shift of A1g and red shift of E12g mode with an increase in the number of layers (< 6) has been reported for pure TMD thin films [13,14]. The blue shift in A1g mode with increase in the number of TMD layers can be understood from the fact that the presence of interlayer coupling and weak van der Waals forces suppresses the out-of-plane atomic vibration, resulting in higher force constant of lattice vibration [12-14]. The red shift of E12g mode with increasing sample thickness is expected to be due to the dielectric screening of the long-range Coulomb forces [12-14]. However, the electron–phonon interaction, strain and temperature also have significant effects on phonon frequencies [15,16]. In our case, it is interesting to note that both E12g and A1g modes of MoS2 stiffen by coupling with WS2 or vice-versa. This can be understood if we consider an increase in the interlayer interaction across the hetero-interface. The strong interlayer interaction between MoS2 and WS2 nanosheets can surpass the Coulomb interactions and strengthen both in-plane and out-of-plane effective restoring forces acting on the atoms resulting in the higher frequencies of both E12g and A1g modes [5,15,16]. 4. Conclusion We successfully synthesized the vdW heterostructures of MoS2 and WS2 via physical mixing of MoS2 and WS2 nanosheets dispersions (obtained through liquid phase exfoliation). We find that in the heterostructure form, the optical properties (red shift for the A- and B- exciton peak positions) and phononic properties (E12g and A1g Raman modes) are affected by the compositional ratio used to produce the heterostructure. Our results highlight the dominant role of interlayer interactions in the synthesized MoS2-WS2 heterostructure as compared to constituent nanosheets. Our study successfully demonstrates the use of a simple, cost effective and scalable method of producing vdW heterostructure which can be adopted for production of vdW heterostructures of other 2DLMs. Acknowledgement This work was financially supported by the DST-INSPIRE fellowship from DST, Govt. of India. We would like to thank Sophisticated Analytical Instrumentation Facility (SAIF), Chandigarh and DST-PURSE Central Facility for providing the technical support for the research work. We would also like to acknowledge Institute for Nanoscience and Technology (INST), Mohali for extending their help in the Raman and TEM measurements.
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References [1] Geim K., Grigorieva I. V., van der Waals heterostructures, Nature 2013; 499:419–425. [2] Li M. Y., Chen C. H., Shi Y., Li L. J., Heterostructures based on two-dimensional layered materials and their potential applications, Materials Today 2016; 19:322–335. [3] Kos′ mider K., Rossier J. F., Electronic properties of the MoS2-WS2 heterojunction, Phys. Rev. B 2013; 87:075451. [4] Hill H. M., Rigosi A. F., Rim K. T., Flynn G. W., Heinz T. F., Band alignment in MoS2/WS2 transition metal dichalcogenide heterostructures probed by Scanning Tunneling microscopy and spectroscopy, Nano Lett. 2016; 16:4831-4837. [5] He J. G., Hummer K., Franchini C., Stacking effects on the electronic and optical properties of bilayer transition metal dichalcogenides MoS2, MoSe2, WS2, and WSe2, Phys. Rev. B 2014; 89:075409. [6] Liu Y., Weiss N. O., Duan X., Cheng H. C., Huang Y., Duan X.,van der Waals heterostructures and devices, Nature Reviews Materials 2016; 1:16042. [7] Dong R., Kuljanishvili I., Progress in fabrication of transition metal dichalcogenides heterostructure systems, J. Vac. Sci. Technol. B 2017; 35:030803. [8] Gong Y., Lin J., Wang X., Shi G., Lei S., Lin Z., Zou X., 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. [9] Jeffery A. A., Nethravathi C., Rajamathi M., Two-dimensional nanosheets and layered hybrids of MoS2 and WS2 through exfoliation of ammoniated MS2 (M = Mo,W), J. Phys. Chem. C 2014; 118:1386−1396. [10] Pesci F. M., Sokolikova M. S., Grotta C., Sherrell P. C., Reale F., Sharda K., Ni N., Palczynski P., Mattevi C., MoS2/ WS2 heterojunction for photoelectrochemical water oxidation, ACS Catal. 2017; 7:4990−4998. [11] Smith R. J., King P. J., Lotya M., Wirtz C., Khan U., De S., O'Neill A., Duesberg G.S., Grunlan J.C., Moriarty G., Chen J., Wang J., Minett A.I., Nicolosi V., Coleman J. N., Large-scale exfoliation of inorganic layered compounds in aqueous surfactant solutions, Adv. Mater. 2011; 23:3944–3948. [12] Chhowalla M., Shin H. S., Eda G., Loh K. P., Zhang H., The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets, Nat. Chem. 2013; 5:263–275. [13] Lee C., Yan H., Brus L. E., Anomalous lattice vibrations of single- and few-layer MoS2, ACS Nano 2010; 4:2695–2700. [14] Berkdemir A., Gutiérrez H. R. , Botello-Méndez A.R., Identification of individual and few layers of WS2 using Raman spectroscopy, Sci Rep. 2013; 3:1755. [15] F. Wang, J. Wang, S. Guo, J. Zhang, Z. Hu, J. Chu, Tuning coupling behavior of stacked heterostructures based on MoS2, WS2, and WSe2, Sci. Rep. 2017; 7:44712 . [16] Huo N., Wei Z., Meng X., Kang J., Wu F., Li S. S., Interlayer coupling and optoelectronic properties of ultrathin two-dimensional heterostructures based on graphene, MoS2 and WS2, J. Mater. Chem. C 2015; 3:5467-5473.
ScienceDirect Materials Today: Proceedings 21 (2020) 1840–1845
www.materialstoday.com/proceedings
ISFM-2018
van der Waals Heterostructures based on Liquid Phase Exfoliated MoS2 and WS2 nanosheets Sneha Sinha, Jyotsna and Sunil K. Arora* Centre for Nanoscience and Nanotechnology, South Campus, Panjab University, Chandigarh-160014, India.
Abstract We have synthesized the van der Waals (vdW) heterostructures of two-dimensional layered materials (MoS2 and WS2) via physical mixing of different proportions of colloidal dispersions containing exfoliated MoS2 and WS2 nanosheets. The synthesized vdW heterostructures exhibited optical and phononic properties distinct from its components. We find that the A- and B- excitonic peak positions in heterostructures are red shifted with respect to the corresponding peak positions of the constituent materials (MoS2 and WS2) and are also dependent on the compositional ratio used to produce the heterostructures. The in-plane (E12g) and out-of-plane (A1g) Raman modes in heterostructures exhibited hardening with respect to the corresponding modes in individual nanosheets of MoS2 and WS2. Our results highlight the role of interlayer interactions in the synthesized MoS2-WS2 heterostructures. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Symposium on Functional Materials (ISFM-2018): Energy and Biomedical Applications. Keywords: Transition metal dichalcogenides, Liquid Exfoliation, vdW Heterostructure.
1. Introduction Transition metal dichalcogenides (TMDs) belong to the large family of two-dimensional layered materials (2DLMs) with strong in-plane bonding (covalent) and weak out-of plane bonding (van der Waals). The weak van der Waals forces are responsible for easy breakdown of bulk single crystal in to mono- and few-layer form by using mechanical or chemical exfoliation. *Corresponding Author. Tel.: +91-7696265266 E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Symposium on Functional Materials (ISFM-2018): Energy and Biomedical Applications.
S. Sinha et al. / Materials Today: Proceedings 21 (2020) 1840–1845
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The artificial stacking of different 2DLMs (either vertically or laterally) results in an assembly known as van der Waals (vdW) heterostructure [1]. These vdW heterostructures offers new functionality and applications beyond their constituent 2DLMs [1,2]. For example, a staggered (type II) band alignment was predicted and observed for vdW heterostructures based on semiconducting TMDs (MoS2 and WS2) with excitons (bound electrons and holes) localized in individual monolayers [3,4]. Consequently, TMD heterostructures are quite promising for optoelectronic applications [5,6]. One of the major challenges in this subject area to realize the application potential of these vdW heterostructures is to produce them with a method that is scalable and produces clean interfaces. Conventional fabrications of these heterostructures have relied on the low-yield manual exfoliation from the bulk single crystals using scotch tape method and subsequent manual stacking of individual TMD layers, which remain impractical for scaled-up applications. Efforts are in progress to grow layers of individual component 2DLM on top of the other by chemical vapor deposition (CVD) and pulsed laser deposition (PLD) methods [7,8]. Other alternative approaches used to produce vdW heterostructure are mixing colloidal dispersions of 2D nanosheets and selforganizational assembly of a mixture of suspensions containing different 2DLMs [9,10]. Therefore, finding a method to produce these hybrid materials with improved yield and controllable stacking is highly desirable. Herein, we present a simple but effective method to form vdW heterostructure of TMDs (MoS2 and WS2) using colloidal dispersions containing exfoliated MoS2 and WS2 nanosheets. The bulk MoS2 and WS2 powder were exfoliated using mixed solvent strategy under ultrasonic treatment. Through detailed structural studies we show that the structural integrity of the individual component material is retained in the heterostructure however, the optical and phononic properties were affected by the enhanced interlayer coupling at the hetero-interface. 2. Experimental Procedure In order to produce vdW heterostructure of MoS2 and WS2 nanosheets, firstly we produce exfoliated dispersions using mixed solvent strategy. Dispersions containing MoS2 (WS2) nanosheets were prepared by adding 25 mg powder of MoS2 (WS2) (< 2µm, 99%, Sigma Aldrich) in to 5 ml mixture of 30% isopropanol and water solution containing 1% volume of hydrazine monohydrate. The mixture was sonicated in an ultrasonic bath for duration of 4 hours at a temperature of 50°C. The resulting suspensions were centrifuged at 3000 rpm for 30 min. The supernatant was carefully collected and was used for further characterization. To estimate the exfoliation yield, infiltrant was collected and weighed. The exfoliation yield in the dispersions was found to be 1.4 mg ml-1 for MoS2 and 1.38 mg ml-1 for WS2, which is far better than recently reported exfoliation efficiencies using NMP (N-Methyl Pyrollidone) as solvent and surfactant assisted exfoliation methods [11]. MoS2−WS2 heterostructures were prepared by mixing the exfoliated dispersions of MoS2 and WS2 in different volume proportions, and sonicated the resulting solution for 1 hour. The volume of the resulting mixture of colloidal dispersion was reduced further by evaporating the solvent at 65°C. The obtained dispersion was then used for further characterization. The entire work flow is represented in Figure 1.
Figure 1. Work flow for the formation of vdW heterostructure from the exfoliated TMD dispersions.
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The X-ray diffraction (XRD) patterns of the as-exfoliated MoS2, WS2 nanosheets and their co-stacked heterostructure were recorded using Panalytical’s X’pert Pro diffractometer with Cu K-alpha-1 radiation. The UVvisible spectra of exfoliated dispersions and their vdW heterostructures were recorded using V-770 (Jasco) spectrophotometer in a matched pair of quartz cuvettes of path length 1cm. The Raman spectroscopy characterization was performed using WiTec Confocal Raman Spectrometer with a 532 nm excitation laser and 1µm diameter laser spot size. The samples for Raman studies were prepared by drop-casting films on Silicon wafers (100) and dried in hot air oven at 60oC. The composition of the films was determined using Energy Dispersive X-ray spectroscopy (EDX) and corresponded well to the initial mixture of the constituent 2DLMs (e.g. for MoS2:WS2 in 1:1 ratio the EDX studies showed a ratio of 1:1.05). TEM studies were performed with the JEOL-JEM 2100 microscope operated at an accelerating voltage of 200 kV. Carbon-coated copper grids (300 mesh) were used to prepare sample for TEM studies. 3. Results and Discussion Prior to discussing the phononic properties of MoS2-WS2 heterostructure, we present details of structural studies performed on MoS2, WS2 nanosheets and their heterostructure. The XRD pattern of exfoliated nanosheets shows a single strong (002) peak at 14.5o while the other peaks are either absent or relatively weak. This confirms that the nanosheets have been exfoliated along the (002) axis. However, in case of MoS2-WS2 heterostructure, a relatively weak and broad diffraction peak at 14.5o is observed, which corresponds to the diffraction from the (002) plane of the constituent nanosheets (Fig. 2a). Figure 2(b) illustrates the low magnification TEM image of the MoS2−WS2 heterostructure (1:1 composition). The high resolution lattice image from the heterostructure along with selected area electron diffraction (SAED) is shown in Figure 2(c). The SAED pattern shows sharp diffraction spots corresponding to constituent MoS2 and WS2 nanosheets. The inter-planar spacing determined from lattice fringe separation (Fig. 2c) is found to be 0.27 nm, which is consistent with (100) plane of 2H-phase of constituent TMDs. Thus, the XRD and TEM studies justify the fact that the structural integrity of the individual TMD component is retained in the synthesised vdW heterostructure.
Figure 2. (a) XRD spectra of exfoliated MoS2, WS2 nanosheets and their vdW heterostructure containing 1:1 proportion of the constituent TMD nanosheets (b) TEM image of the MoS2−WS2 heterostructure (c) High resolution lattice image of the heterostructure along with the SAED pattern shown in the Inset.
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Figure 3 shows the optical absorbance spectra of the exfoliated MoS2, WS2 nanosheets and heterostructures made with different proportions of constituent nanosheets. In case of MoS2, two characteristic peaks A and B were observed at λ = 678 and 617 nm, respectively, which corresponds to the direct transitions (band edge excitons) at the K-point arising due to the spin-orbital coupling [12]. For WS2, the A- and B- excitonic peaks are centered at 632 nm and 527 nm, respectively (Fig. 3a). From the optical absorbance spectra recorded with different volume ratios of MoS2 and WS2 in the vdW heterostructures (Fig. 3b); it was found that there is a clear enhanced contribution from constituent excitonic peaks with positions (red shift) and intensities of the absorption bands altered. The observed red shift in the optical spectra can be attributed to the enhanced interlayer coupling as we move away from single layer limit and the reduced dielectric screening of the Coloumb potential between the charged carriers [5].
Figure 3. (a) Optical absorbance spectra of exfoliated MoS2 and WS2 nanosheets (b) Absorbance spectra for vdW heterostructures revealing red shift of A- and B- excitonic peaks upon co-stacking of the constituent TMD nanosheets. Label A and B on curves corresponds to the excitonic peaks ‘A’ and ‘B’.
Raman spectroscopy studies were carried out on TMD nanosheets drop-casted on to Si (100) substrates and were used to determine the number of monolayers in the exfoliated nanosheets. The number of monolayers in exfoliated nanosheets of MoS2 and WS2 could vary and so the Raman spectra tend to spatially vary. Figure 4(a) shows a typical Raman spectrum observed for the exfoliated nanosheets of MoS2 and WS2. Exfoliated few layered nanosheets of MoS2 and WS2 have shown characteristic E12g and A1g Raman modes located at 385 and 409.95 cm-1 for MoS2 and 358 and 422.2 cm-1 for WS2, respectively (Fig. 4a). It is known that the E12g mode origins from the opposite vibration of two S atoms with respect to the Mo (W) atom in the basal plane and represents the interlayer displacements of Mo (W) and S atoms [12-14]. The A1g mode represents the out-of-plane vibrations of metalchalcogen bonds along c axis, providing information on the strength of the interaction between the adjacent layers [12-14]. The frequency difference, ∆, between E12g and A1g modes was found to be 25 cm-1 for MoS2 and 64.2 cm-1 for WS2 which suggests that the nanosheets are 4-5 monolayers thick. The Raman spectra for vdW heterostructures (made with different proportions of MoS2 and WS2) reveals that the greater contribution is from the TMD component which is in greater proportion (Fig. 4b). The vdW heterostructure (1:1 composition) exhibits the signature peaks corresponding to the Raman modes from constituent nanosheets of WS2 and MoS2, respectively (Fig. 4b). The blue-shift in A1g mode of MoS2 (WS2) film upon heterostructure formation was found to be 0.37 (0.41) cm-1, respectively. Similarly, the blue-shift in E12g modes of MoS2 (WS2) film upon heterostructure formation was found to be 0.17 (0.20) cm-1, respectively.
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Figure 4. (a) Raman spectra observed for the exfoliated nanosheets of MoS2 and WS2 (b) Raman spectra of vdW heterostructures with different volume proportions of constituent MoS2 and WS2 nanosheets.
Generally, the blue shift of A1g and red shift of E12g mode with an increase in the number of layers (< 6) has been reported for pure TMD thin films [13,14]. The blue shift in A1g mode with increase in the number of TMD layers can be understood from the fact that the presence of interlayer coupling and weak van der Waals forces suppresses the out-of-plane atomic vibration, resulting in higher force constant of lattice vibration [12-14]. The red shift of E12g mode with increasing sample thickness is expected to be due to the dielectric screening of the long-range Coulomb forces [12-14]. However, the electron–phonon interaction, strain and temperature also have significant effects on phonon frequencies [15,16]. In our case, it is interesting to note that both E12g and A1g modes of MoS2 stiffen by coupling with WS2 or vice-versa. This can be understood if we consider an increase in the interlayer interaction across the hetero-interface. The strong interlayer interaction between MoS2 and WS2 nanosheets can surpass the Coulomb interactions and strengthen both in-plane and out-of-plane effective restoring forces acting on the atoms resulting in the higher frequencies of both E12g and A1g modes [5,15,16]. 4. Conclusion We successfully synthesized the vdW heterostructures of MoS2 and WS2 via physical mixing of MoS2 and WS2 nanosheets dispersions (obtained through liquid phase exfoliation). We find that in the heterostructure form, the optical properties (red shift for the A- and B- exciton peak positions) and phononic properties (E12g and A1g Raman modes) are affected by the compositional ratio used to produce the heterostructure. Our results highlight the dominant role of interlayer interactions in the synthesized MoS2-WS2 heterostructure as compared to constituent nanosheets. Our study successfully demonstrates the use of a simple, cost effective and scalable method of producing vdW heterostructure which can be adopted for production of vdW heterostructures of other 2DLMs. Acknowledgement This work was financially supported by the DST-INSPIRE fellowship from DST, Govt. of India. We would like to thank Sophisticated Analytical Instrumentation Facility (SAIF), Chandigarh and DST-PURSE Central Facility for providing the technical support for the research work. We would also like to acknowledge Institute for Nanoscience and Technology (INST), Mohali for extending their help in the Raman and TEM measurements.
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