Composition-Modulated Two-Dimensional Semiconductor Lateral

Dec 19, 2016 - Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States. §College of Material...
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Composition-Modulated Two-Dimensional Semiconductor Lateral Heterostructures via Layer-Selected Atomic Substitution Honglai Li,† Xueping Wu,† Hongjun Liu,† Biyuan Zheng,† Qinglin Zhang,† Xiaoli Zhu,† Zheng Wei,§ Xiujuan Zhuang,† Hong Zhou,† Wenxin Tang,§ Xiangfeng Duan,‡ and Anlian Pan*,† †

Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, School of Physics and Electronic Science, and State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha, Hunan 410082, P. R. China ‡ Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States § College of Materials Science and Engineering, Chongqing University, Chongqing 400030, P. R. China S Supporting Information *

ABSTRACT: Composition-controlled growth of two-dimensional layered semiconductor heterostructures is crucially important for their applications in multifunctional integrated photonics and optoelectronics devices. Here, we report the realization of composition completely modulated layered semiconductor MoS2−MoS2(1−x)Se2x (0 < x < 1) lateral heterostructures via the controlled layer-selected atomic substitution of pregrown stacking MoS2, with a bilayer located at the center of a monolayer. Through controlling the reaction time, S at the monolayer MoS2 at the peripheral area can be selectively substituted by Se atoms at different levels, while the bilayer region at the center retains the original composition. Microstructure characterizations demonstrated the formation of lateral heterostructures with a sharp interface, with the composition at the monolayer area gradually modulated from MoS2 to MoSe2 and having high-quality crystallization at both the monolayer and the bilayer areas. Photoluminescence and Raman mapping studies exhibit the tunable optical properties only at the monolayer region of the as-grown heterostructures, which further demonstrates the realization of high-quality composition/bandgap modulated lateral heterostructures. This work offers an interesting and easy route for the development of high-quality layered semiconductor heterostructures for potential broad applications in integrated nanoelectronic and optoelectronic devices. KEYWORDS: layered semiconductor, transition-metal dichalcogenides, lateral heterostructures, tunable compositions, atomic substitution wo dimensional (2D) atomic crystal materials,1−3 especially transition-m etal dichalcogenides (TMDs),4−11 have attracted considerable interest recently due to their atomically thin geometry structure and unique electronic and optical properties for potential applications in integrated optoelectronic devices and systems.12−22 Semiconductor heterostructures with spatially modulated bandgaps and sharp composition interfaces are important for high-performance device applications.23−25 In the past several years, controlled growth of TMD atomic crystal semiconductor heterostructures has received more and more attention.26−36 For example, Gong et al. have shown the growth of high-quality vertical and in-plane WS2−MoS2 heterostructures with light emissions broadly tuned in both vertical and lateral directions.26 Duan et al. have reported the lateral growth of MoS2−MoSe2 and WS2−WSe2 heterostructures, based on which atomic p−n diodes and inverters have been achieved.27

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© 2016 American Chemical Society

Li et al. have reported the two-step epitaxial growth of the lateral WSe2−MoS2 heterojunction with an atomically sharp interface, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch.28 The optoelectronic properties of semiconductor heterostructures are directly related to the energy band diagram at their interfaces.37 Controlled growth of atomic crystal semiconductor heterostructures with band gap engineered interfaces is particularly important for their further broad applications. However, to the best of our knowledge, high quality 2D semiconductor heterostructures with continuously modulated composition or band gap have never been reported. Received: November 10, 2016 Accepted: December 19, 2016 Published: December 19, 2016 961

DOI: 10.1021/acsnano.6b07580 ACS Nano 2017, 11, 961−967

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Figure 1. Contrastive characters of Se substitution in monolayer and bilayer MoS2 nanosheets, respectively. (a, d) Optical images of monolayer and bilayer MoS2 nanosheets after substitution, respectively. Insets: corresponding AFM images with section analysis along the black lines. Annealing temperature-related PL spectra of the monolayer (b, c) and bilayer (e, f) before and after annealing for 1, 3, and 5 min. (g and h) Annealing temperature-dependent bandgap values and compositions of the two sheets after annealing for 5 min.

Figure 2. Schematic diagram of the preparation of the lateral heterostructured MoS2−MoS2(1‑x)Se2x nanosheet by the Se substitution in a designed stacking MoS2 nanosheet.

RESULTS Figure 1a shows the real-color optical image of a MoS2 nanosheet, and the inset gives the corresponding atomic force microscopy (AFM) image, revealing the monolayer nature of the sheet. Parts b and c of Figure 1 give the PL spectra of this sample excited with a 488 nm argon ion laser before and after annealing under Se vapor for 1, 3, and 5 min at a temperature below 730 and 740 °C, respectively, which shows that the PL spectra for the annealing time below 730 °C remain the same as that before annealing (0 min), while the peak wavelength is gradually red-shifted with increasing annealing time at 740 °C, from ∼670 nm (before annealing, pure MoS2) to 714 nm. The results indicate that the monolayer sample is chemically stable at a relatively low temperature below 730 °C, while selenium atomic substitution can take place in this sample at 740 °C, and the substitution rate is increased with elevation of the annealing time. By contrast, similar experiments were conducted for bilayer MoS2 nanosheets. Figure 1d gives the optical image of a typical bilayer MoS2 with the corresponding AFM image (inset), and parts e and f of Figure 1 give the corresponding PL before and after annealing in the Se atmosphere, with the same annealing times as those conducted for the monolayer sample.

It was reported that the pregrown TMD monolayer nanosheets can be easily selenized and sulfurized with a simple annealing approach, which provides a simple method for the synthesis of composition modulated 2D layered semiconductor alloys.38 In this work, we find that the selenylation of layered MoS2 is highly dependent on the layer number, with selenium substitution temperature of the monolayer greatly decreased more than that of the bilayer and multilayer. Based on this finding, we realized the composition completely modulated MoS2−MoS2(1−x)Se2x (0 < x < 1) lateral heterostructures via the controlled layer-selected atomic substitution of pregrown stacking MoS2 nanosheets composed of a bilayer located at the center of a monolayer. Scanning transmission electron microscopy (STEM), photoluminescence (PL), and Raman scattering measurements demonstrate the realization of the atomic layered lateral heterostructures. The achieved heterostructures display high-quality crystallization and a very sharp interface, with the composition at the monolayer area completely modulated from MoS2 to MoSe2, accompanying the continuously tuned PL from 668 to 760 nm. These composition-modulated 2D semiconductor lateral heterostructures may find potential applications in integrated nanoelectronics and nanophotonics. 962

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Figure 3. (a) Optical image of the successfully grown stacking MoS2 nanosheets. (b) Typical TEM image of the obtained nanosheet after Sesubstitution at 750 °C for 1 min and (c) the corresponding TEM−EDX profiles recorded at two positions of different thickness (1, 2) in the sheet. (d) EDX line scan profiles for the different detected elements: S and Se, respectively, across the interface of the stacking MoS2 after substitution. (e) The HRTEM image taken from the interfacial regions (scale bars, 2 nm). Insets of (e): the SAED patterns taken from the monolayer and bilayer positions of the nanosheet after substitution. (f) HAADF−STEM image taken across the interfacial regions between the monolayer and the bilayer after substitution at 750 °C for 3 min (scale bar: 2 nm).

It is interesting to find that the bilayer sample is always chemically stable when the annealing temperature is below 800 °C and the selenium substitution begins to take place at 810 °C since the peak wavelength of the PL spectra is red-shifted with increasing annealing time. Figures 1g gives the annealing temperature dependent PL peak energy of both the monolayer and the bilayer samples after annealing for 5 min, respectively, and Figure 1h shows the corresponding annealing temperaturedependent Se molar fraction x converted from the PL spectra using Vegard’s law,15 which more clearly demonstrates that the Se substitution temperature of the bilayer MoS2 (810 °C) is much higher than that of the monolayer MoS2 (740 °C). For both samples above their respective substitution temperature, the substitution rate is gradually increased with increasing annealing temperature. Inspired by the great difference of the Se substitution temperature from the monolayer and the bilayer MoS2, composition-tuned lateral MoS2−MoS2(1−x)Se2x heterostructures can, in principle, be obtained by layer-selected Se substitution of stacking MoS2 nanosheets with the monolayer at the peripheral and the bilayer at the center, as shown schematically in Figure 2. When the stacking MoS2 nanosheets are exposed in the Se atmosphere at an appropriate temperature, the Se atoms can only react with the peripheral monolayer MoS2 and substitute their S atoms, keeping the composition of the central bilayer MoS2 unchanged, thus resulting in the formation of MoS2−MoS2(1−x)Se2x lateral heterostructures. The composition or substitution rate in the monolayer region of the heterostructures can be controlled by the substitution time and realize the tunability of the

composition or energy band diagram in these achieved heterostructures. The stacking MoS2 nanosheets were synthesized through a traditional chemical vapor deposition (CVD) route with a 300 nm SiO2/Si wafer as the substrate (see the Materials Preparation section for details). As shown in Figure 3a (optical image), the obtained stacking MoS2 nanosheets have a welldefined triangular shape, with a small triangle located at the center of a large triangle. The thickness-dependent contrast can distinguish between the monolayer and bilayer regions of the sheet. After the pregrown stacking nanosheets were annealed in Se atmosphere for different times, the microstructure and composition characterizations of the obtained samples were conducted with transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX). Figure 3b shows the TEM image of a representative stacking nanosheet after atomic substitution at 750 °C for 1 min, which keeps a good appearance characteristic as that of before annealing. Figure 3c plots the EDX spectroscopy spectra collected from two positions in the two regions, respectively (dots 1 and 2 in Figure 3b), which reveal that position 1 (peripheral monolayer region) is composed of considerable Se, S, and Mo elements (the detected Cu element originates from the copper grid), while position 2 (central bilayer region) mainly consists of S and Mo elements with negligible Se elements detected. The elemental analyses indicate that the sheet is a lateral heterostructure with monolayer MoS2(1−x)Se2x alloy at the peripheral region and bilayer MoS2 at the central region. EDX line scan profiles of the elemental distribution along the black line in Figure 3b clearly show the opposite 963

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Figure 4. (a) Optical image of a typically stacking MoS2 nanosheet after substitution. (b) Annealing time-related PL spectra of the stacking lateral heterostructures. (c) Annealing time-related bandgap values and compositions of the composition modulated heterostructures. (d−f) Wavelength-selected PL mapping of a stacking nanosheet substituted for 5 min in the spectral regions of 675−705 nm, 745−775 nm, and the combination, respectively.

Figure 4a is the real-color image of a stacking sheet after substitution, and Figure 4b shows the annealing time dependent PL spectra at the monolayer and bilayer regions of the stacking sheet, respectively. The black spectra were collected from the monolayer region substituted at a temperature of 750 °C for 0, 1, 3, 5 min, respectively, while the red spectra were collected from the bilayer region. It can be seen that all of the spectra collected from the monolayer region (black) reveal single emission bands, with the peak wavelength gradually red-shifted when the substituted time is increased. In contrast, the spectra from the bilayer region (red) almost keep the same peak position over time. The highly distinct PL peaks for the central region and peripheral region demonstrate that the composition-tuned layered semiconductor lateral heterostructures are successfully achieved. Figure 4c shows the annealing time dependent PL peak energy (trilateral) and the correspondingly induced Se molar fraction x (square) of the monolayer and bilayer regions after the annealing, respectively, which further demonstrates that the S in the monolayer region can be gradually substituted by Se at 750 °C, while the bilayer region is very stable at this temperature. In addition, the Se molar fraction x induced from the PL observation is highly consistent with those from the direct EDX analysis (Table S1), further supporting the finding of the different atomic substitution behaviors in the two regions with different layers. PL mapping studies can further reveal the optical modulation within the triangular nanosheet. Taking the substituted time of 5 min into account, Figure 4d,e gives the wavelength-selected PL emission mapping of the examined nanosheet in the spectral regions of 675−705 and 745−775 nm, respectively. Obviously, the short wavelength region (675−705 nm) is located at the center of the nanosheet, while the long wavelength region (745−775 nm) is located at the periphery of sheet. A PL mapping image composed of the short wavelength region and

modulation of elements Se and S (Figure. 3d). The Se content is decreased while the S content is increased across the interface region from the monolayer to the bilayer, which further demonstrates the lateral heterostructure feature of the stacking sheet after atomic substitution. Figure 3e gives the corresponding high-resolution TEM (HRTEM) image across the interface of the nanosheet, which demonstrates that the structure is highly crystallized, with the measured lattice plane spacings of 2.72 and 2.70 Å at the monolayer and bilayer regions, respectively, in agreement with the (100) plane spacing of the composition tunable sheets. The insets of Figure 3e are the selected area electron diffraction (SAED) patterns of the sheet at the two regions. The obvious diffraction intensity contrast verifies the layer number difference of the stacking nanosheet. Both patterns show clearly defined single sets of diffraction spots, which further demonstrate the high quality of the substituted nanosheets.39 The atomic arrangement of the lateral heterostructure is clearly resolved in high angle annular dark field (HAADF)−STEM imaging. The HAADF−STEM image in Figure 3f shows the interfacial regions between the monolayer and the bilayer (brighter region) after annealing at 750 °C for 3 min. The bright spots (indicated by the green arrow) in the monolayer region corresponding to Mo atoms are nearly uniformly distributed, while the contents of S2 atoms (indicated by the yellow arrow) and S + Se atoms (indicated by the red arrow) in this region show obvious contrast, which agrees with the previous report in the monolayer MoS2(1−x)Se2x alloy.40 However, the spots in the bilayer region are of the same brightness, which is in agreement with the atomic structure of bilayer MoS2,41 and no Se atom was detected. The above results clearly demonstrate the realization of high-quality MoSSe (monolayer)−MoS2 (bilayer) lateral heterostructures with an atomic-level sharpened interface. PL spectra were used to characterize the compositiondependent optical modulation of these lateral heterostructures. 964

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alloy while the composition at the central bilayer region remains in its original form. The lateral heterostructures with tunable compositions can give composition-related optical modulations with the PL peak positions broadly tunable at the periphery while fixed at the center. These composition-tuned lateral heterostructures could find significant applications in 2D fundamental physical research and the construction of functional electronic and photoelectric devices.

the long one (Figure 4f) shows a seamless lateral integration, further demonstrating the feature of the lateral heterostructure. The formation of composition-tuned lateral heterostructures can further be confirmed by the composition-dependent vibration modes observed from the micro-Raman measurements. Figure 5a plots the normalized annealing time-

METHODS Materials Preparation. The stacking MoS2 nanosheets were synthesized through a CVD route on the 300 nm SiO2/Si substrate, with sulfur and MoO3 powders as the source materials.15 Before heating, an Ar gas flow was introduced into the system to eliminate the air, and then the furnace was rapidly heated to 830 °C while the pressure inside the system was kept at 200−300 Torr. After 10 min of growth, the furnace was naturally cooled to room temperature. The substitution reaction was also took place through a common CVD route. A boat with Se powder was placed upstream, and another boat covered with Si/SiO2 wafers adhering pregrown MoS2 nanosheet was placed at the heating zone of a quartz tube. Ar mixed with 5% H2 gas was first introduced into the system at a fast rate (120 sccm, 30 min) to purge the oxygen from the chamber before the furnace was heated. The temperature in the center of the furnace was then rapidly heated to the reaction temperature, with the region of Se powder at 260 °C, keeping the pressure inside the tube at about 75 Torr. Then the furnace was naturally cooled to room temperature. TEM and Optical Characterizations. The microstructure was characterized by AFM (Bruker Multimode 8), TEM (Tecai F20, voltage: 300 kV) equipped with an EDS detector, and HAADF-STEM (FEI Titan G2, 60−300 keV). Before the survey of the TEM results, the nanosheets were transferred onto grid of copper using a PMMA (Mw = 950 K, 4 wt %, AR-P 679.04, Allresist)-mediated nanotransfer method (speed: 3000 rpm, 1 min).15 The PL and Raman measurements were performed with the confocal μ-PL system (WITec, alpha-300). A 488 nm argon ion laser (power: about 30 mW, spot size: 1−2 μm) was used to characterize the structural and optical modulation of the sheet.

Figure 5. (a, b) Annealing time-related Raman spectra collected from the monolayer and bilayer regions of the stacking lateral heterostructure, respectively. (c−e) Frequency-selected Raman mapping of a stacking nanosheet substituted for 5 min at 403 cm−1, 260 cm−1, and the combination, respectively.

dependent Raman spectra of the peripheral monolayer region shown in Figure 4a. The results show that the intensities of Se− Mo related modes are gradually enhanced, while the intensities of S−Mo related modes are gradually attenuated. Meanwhile, all of the vibration modes are increasingly shifted to the low frequency with the substituted time. All of the above observations show good agreement with the continuously composition-tuned MoS2(1−x)Se2x alloy. On the other hand, the annealing time-dependent Raman spectra of the central bilayer region are shown in Figure 5b. Similar to the PL spectra, the Raman spectra here are constant over time. The obvious difference of Raman spectra collected from the two regions after various substitution times reveals that the composition of the lateral heterostructure is tuned gradually. Parts c−e of Figure 4 give the frequency-selected Raman mapping of the examined nanosheet at 260 cm−1, 403 cm−1, and the combination, respectively. Similar to those of PL mapping studies, the low frequency (260 cm−1) is located at the center of the nanosheet, while the high frequency (403 cm−1) is located at the periphery of sheet, and a seamless lateral integration is also demonstrated. The above results further confirm the formation of MoS2−MoS2(1−x)Se2x lateral heterostructures.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07580. Schematic for the experiment setup and the analysis of the atomic substitution rate (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xiangfeng Duan: 0000-0002-4321-6288 Anlian Pan: 0000-0003-3335-3067 Notes

The authors declare no competing financial interest.

CONCLUSIONS In summary, lateral composition-tuned atomic-layered heterostructures have been successfully prepared through an effective control of the layer-dependent atomic substitution process. Both microstructure and spectral characterizations demonstrate that the achieved nanosheets after substitution are lateral heterostructures, with the composition at the peripheral monolayer region being continuously tuned to the ternary

ACKNOWLEDGMENTS We are grateful to the NSF of China (Nos. 51525202, 61574054, 61505051, and 61474040), the Hunan province science and technology plan (Nos. 2014FJ2001 and 2014TT1004), and the Aid program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province. 965

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DOI: 10.1021/acsnano.6b07580 ACS Nano 2017, 11, 961−967

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DOI: 10.1021/acsnano.6b07580 ACS Nano 2017, 11, 961−967