Abstract
Two-dimensional heterojunctions of transition-metal dichalcogenides1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 have great potential for application in low-power, high-performance and flexible electro-optical devices, such as tunnelling transistors5,6, light-emitting diodes2,3, photodetectors2,4 and photovoltaic cells7,8. Although complex heterostructures have been fabricated via the van der Waals stacking of different two-dimensional materials2,3,4,14, the in situ fabrication of high-quality lateral heterostructures9,10,11,12,13,15 with multiple junctions remains a challenge. Transition-metal-dichalcogenide lateral heterostructures have been synthesized via single-step9,11,12, two-step10,13 or multi-step growth processes15. However, these methods lack the flexibility to control, in situ, the growth of individual domains. In situ synthesis of multi-junction lateral heterostructures does not require multiple exchanges of sources or reactors, a limitation in previous approaches9,10,11,12,13,15 as it exposes the edges to ambient contamination, compromises the homogeneity of domain size in periodic structures, and results in long processing times. Here we report a one-pot synthetic approach, using a single heterogeneous solid source, for the continuous fabrication of lateral multi-junction heterostructures consisting of monolayers of transition-metal dichalcogenides. The sequential formation of heterojunctions is achieved solely by changing the composition of the reactive gas environment in the presence of water vapour. This enables selective control of the water-induced oxidation16 and volatilization17 of each transition-metal precursor, as well as its nucleation on the substrate, leading to sequential edge-epitaxy of distinct transition-metal dichalcogenides. Photoluminescence maps confirm the sequential spatial modulation of the bandgap, and atomic-resolution images reveal defect-free lateral connectivity between the different transition-metal-dichalcogenide domains within a single crystal structure. Electrical transport measurements revealed diode-like responses across the junctions. Our new approach offers greater flexibility and control than previous methods for continuous growth of transition-metal-dichalcogenide-based multi-junction lateral heterostructures. These findings could be extended to other families of two-dimensional materials, and establish a foundation for the development of complex and atomically thin in-plane superlattices, devices and integrated circuits18.
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Acknowledgements
This work was supported by the National Science Foundation (NSF) grant DMR-1557434 (CAREER: Two-Dimensional Heterostructures Based on Transition Metal Dichalcogenides). L.B acknowledges the US Army Research Office MURI grant W911NF-11-1-0362 (Synthesis and Physical Characterization of Two-Dimensional Materials and Their Heterostructures) and the Office Naval Research DURIP Grant 11997003 (Stacking Under Inert Conditions). TEM work was performed at the National High Magnetic Field Laboratory, which is supported by the NSF Cooperative Agreement DMR-1157490 and the State of Florida. P.K.S. and H.R.G. thank M. A. Cotta for comments.
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P.K.S. and H.R.G. conceived the idea and designed the experiments. P.K.S. performed the synthesis, Raman and photoluminescence characterization, and related analysis. S.M. and L.B. performed device fabrication, electrical measurements and analysis. Y.X. conducted aberration-corrected STEM imaging with assistance from P.K.S. and H.R.G. H.R.G. carried out TEM data analysis. P.K.S. and H.R.G. analysed the results and wrote the paper with input from L.B., S.M. and Y.X. All authors discussed the results and commented on the manuscript. H.R.G. supervised the project.
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Extended data figures and tables
Extended Data Figure 1 One-pot synthetic approach for sequential edge-epitaxy of TMDs.
a, Schematics of the modified chemical vapour deposition system that allows the alternate switching of carrier gas that generates the selective edge-epitaxial growth for multi-junction heterostructure synthesis. Note that water vapour is introduced by passing the carrier gas through the bubbler. The carrier gas is selected by a three-way valve placed at the entrance of the quartz tube reactor. b, Temperature profile of the furnace, a single heterogeneous source containing both precursors is placed in the high-temperature zone, whereas the substrates are placed downstream at the lower-temperature zone. c, Growth rates for MoSe2 and WSe2 domains as a function of the substrate temperature. The error bars along the y axis denote the mean standard deviation (±δ), and error bars along the x axis represent the average length of a typical growth region denoted in b. d, Atomic ball model, showing the material distribution across the heterostructure in cross-section (top) and plane view (bottom).
Extended Data Figure 2 Growth of single-junction MoSe2–WSe2 lateral heterostructures.
a–d, Optical images of single-junction MoSe2–WSe2 monolayer lateral heterostructures with different WSe2 lateral growth times of 80 s (a), 45 s (b), 30 s (c) and 15 s (d). The inset in d shows the Raman map of the narrow WSe2 shell, which is difficult to visualize in the optical image. e–g, Composite photoluminescence maps corresponding to optical images in a–c, respectively, at 1.53 eV (MoSe2 domain) and 1.6 eV (WSe2 domain). h, Composite Raman map corresponding to the optical image in d, at frequencies 240 cm−1 (MoSe2 domain) and 250 cm−1 (WSe2 domain). i, Low-magnification optical images of the MoSe2–WSe2 single-junction heterostructure shown in b, obtained at different distances from the source precursor as mentioned in Extended Data Fig. 1b, c (regions II to IV). j, k, Photoluminescence spectra of monolayer (1L), bilayer (2L) and trilayer (3L) heterostructures; MoSe2 (j) and WSe2 (k) domains. Scale bars: a–h, 10 μm.
Extended Data Figure 3 Optical properties of single-junction MoSe2–WSe2 lateral heterostructure.
a, b, Raman spectra of MoSe2 and WSe2 domains from a single-junction MoSe2–WSe2 monolayer lateral heterostructure using 514 nm (a) and 613 nm (b) laser excitation. c, Raman spectra at the interfaces of the single-junction MoSe2–WSe2 lateral heterostructure. d, e, Composite Raman intensity maps at a frequency of 240 cm−1 (MoSe2 domain, d), 250 cm−1 (WSe2 domain, e). f, Position map corresponding to the optical image in Extended Data Fig. 2a. g, Photoluminescence spectra of MoSe2, WSe2 domains and at the interface of the MoSe2–WSe2 single-junction monolayer lateral heterostructure shown in Extended Data Fig. 2a. h–j, Photoluminescence intensity maps at 1.53 eV (MoSe2 domain, h) and 1.6 eV (WSe2 domain, i); the composite is shown in j. k, l, Peak width (in eV) map (k) and position (in eV) map (l) corresponding to the optical image in Extended Data Fig. 2a. Scale bars: d–l, 10 μm.
Extended Data Figure 4 Optical properties of multi-junction MoSe2–WSe2 lateral heterostructure.
a, Typical SEM image of a five-junction MoSe2–WSe2 monolayer lateral heterostructure corresponding to Fig. 1b. b, Optical microscope image of a large area of a five-junction MoSe2–WSe2 lateral heterostructure, corresponding to Fig. 1c showing the conformal growth of respective MoSe2 or WSe2 domains. c, d, Raman intensity map of five-junction MoSe2–WSe2 lateral heterostructure corresponding to Fig. 1b, g, at frequencies 250 cm−1 (c, WSe2 domain) 240 cm−1 (d, MoSe2 domain). e, Composite Raman map image at 250 cm−1 and 240 cm−1. f, Optical image of a three-junction MoSe2–WSe2 monolayer lateral heterostructure corresponding to the inset of Fig. 1i. g, Raman peak position mapping between 236–255 cm−1. h, Composite photoluminescence intensity mapping at 1.53 eV (MoSe2 domain) and 1.6 eV (WSe2 domain). Scale bars: f–h, 10 μm.
Extended Data Figure 5 Effect of water vapour and H2 on the solid sources (MoX2 and WX2).
a, Raman spectral evolution of the MoO2 oxide phase from both MoSe2 and MoS2 solid sources upon reaction with a constant flow of N2 + H2O vapour for more than 20 min at 1,060 °C (Supplementary Table 2). b, Raman spectral evolution of different oxide phases of WX2 upon reaction with different reactive gas environment for more than 20 min at 1,060 °C as follows. Only Ar + H2 (5%) through H2O (200 s.c.c.m.); the Raman spectra is composed of WSe2, most likely a Se-deficient surface as well as a mixture of complex oxide phases as indicated by the broad peak around 800 cm−1 (1); first partially oxidized by N2 + H2O (5 min) followed by Ar + H2 (5%) through H2O (200 s.c.c.m.) for 10 min. The dominant phase observed in the Raman spectra is WO236,37,38,39,42 (2); completely oxidized by N2 + H2O flow for 20 min—the dominant phase observed in the Raman spectra is W20O58 (3). c, d, Reduction of different metal oxide (MoO3 and WO3) and selenide (MoSe2 and WSe2) solid sources as a function of reaction time and carrier gases: in N2 + H2O (c) and Ar + H2 (5%) (d) flow conditions at 1,060 °C. It can be observed that the weight loss of MoO3 (38.5% in 2 min) is very rapid compared to that of WO3 (1% in 2 min). In contrast, the reduction rate of MoSe2 and WSe2 solid precursors are almost linear during H2 exposure at high temperatures. During oxidation by H2O, however, the weight loss of MoSe2 is two and five times faster than that of WSe2 and WO3 respectively. e–h, A direct visualization of the reaction of MoSe2 can be gained from the change in colour of the source precursor under different conditions: bulk powder of MoSe2 (e); after reaction in Ar + H2 (5%) through H2O (200 s.c.c.m.) (f); after reaction in N2 through H2O (200 s.c.c.m.); the chocolate brown indicates the MoO2 phase (g); the shiny surface indicates the presence of metallic molybdenum reduced from MoX2 along with the MoO2 phase (h). i–l, Different oxide phases of WX2 upon reaction with different reactive gas environment for more than 20 min at 1,060 °C. Bulk powder of WSe2 (i); only Ar + H2 (5%) through H2O (200 s.c.c.m.) (j, corresponding to spectrum 1 in b); first partially oxidized by N2 + H2O (5 min) followed by Ar + H2 (5%) through H2O (200 s.c.c.m.) for 10 min (chocolate brown, k, corresponding to spectrum 2 in b)36,37,38,39,42; completely oxidized by N2 + H2O flow for 20 min—the dominating phase observed in the Raman spectra is W20O58 (blue-violet, l, corresponding to spectrum 3 in b). The insets of l show the high magnification image (left) and the materials in an alumina boat (right).
Extended Data Figure 6 Growth of multi-junction MoS2–WS2 lateral heterostructure.
a–d, Optical images of MoS2–WS2 monolayer lateral heterostructures: single-junction (a), two-junction (b), three-junction (c), five-junction (d). e, Typical low-magnification optical image of the five-junction structure shown in d. f, SEM images of the three-junction MoS2–WS2 lateral heterostructure shown in c. g, SEM image of a three-junction single island (Fig. 2b). h, Typical photoluminescence spectra from MoS2 and WS2 domains of the three-junction heterostructure shown in g. The strong photoluminescence intensity compared with that of the Raman A1g mode (over 300 times greater) indicates the monolayer nature as well as high optical quality of the as-grown heterostructure.
Extended Data Figure 7 Optical properties of MoS2–WS2 lateral heterostructures.
a–c, Composite photoluminescence intensity mapping of single-junction (a), two-junction (b) and three-junction (c) MoS2–WS2 monolayer lateral heterostructures corresponding to the optical images in Extended Data Fig. 6a–c, respectively, at 1.84 eV (MoS2 domain) and 1.97 eV (WS2 domain). d, Raman intensity mapping at frequency 351 cm−1 (d, WS2 domain), 405 cm−1 (e, MoS2 domain). f, Photoluminescence position mapping corresponding to the optical image in Fig. 2a. g, SEM image of a three-junction MoS2–WS2 monolayer lateral heterostructure island. The high magnification image of the boxed region, shown in the right panel, shows the lateral connectivity between respective domains of MoS2 or WS2. Scale bars: a–g, 10 μm.
Extended Data Figure 8 Growth of three-junction MoS0.64Se1.36–WSe1.32S0.68 lateral alloy heterostructure.
a, Optical image of a three-junction MoS2(1−x)Se2x–WS2(1−y)Se2y monolayer lateral heterostructure. b, The corresponding low magnification optical image of the heterostructure shown in a. c, d, Raman (c) and photoluminescence (d) spectra corresponding to the optical image in a between points 1–4. e, Normalized photoluminescence contour colour plot along the direction perpendicular to the interfaces, as shown in the optical image in the inset. f, g, Photoluminescence intensity maps at 1.61 eV (f, MoS0.64Se1.36 domain) and 1.71 eV (g, WSe1.32S0.68 domain) corresponding to the optical image in Fig. 3a. h–k, Raman intensity maps (Fig. 3a) at frequency 400.5 cm−1 (A1g(S–Mo) modes, h); 264 cm−1 (A1g(Se–Mo) modes, i); 404 cm−1 (A1g(S–W) mode, j); and 256 cm−1 (A1g(Se–W) mode, k). Scale bars: f–k, 10 μm.
Extended Data Figure 9 Growth of three-junction MoSe0.96S1.04–WSe0.92S1.08 lateral alloy heterostructure.
a, b, Low-magnification optical images of three-junction MoS2(1−x)S2x–WS2(1−y)Se2y monolayer lateral heterostructure (corresponding to the optical image in Fig. 3b). c, d, Typical large-area SEM image (c) and high magnification SEM image (d) of a single island showing the presence of different growth rates along the vertex and the axial directions. The MoS2(1−x)S2x growth along the vertex direction is less than that of the axial direction.
Extended Data Figure 10 Optical properties of multi-junction MoSe0.96S1.04–WSe0.92S1.08 lateral heterostructure.
a, SEM image of a three-junction MoS2(1−x)S2x–WS2(1−y)Se2y monolayer lateral heterostructure. Scale bar, 2 μm. b, c, Raman (b) and photoluminescence (c)spectra of points 1 to 4; and interfaces. d, Normalized photoluminescence spectra from a line scan perpendicular to the three junctions, regions 1 to 4 in a, as indicated in the inset of Fig. 3e (λexc = 633 nm). e–g, Raman intensity maps corresponding to the optical image in Fig. 3b, at frequencies 412 cm−1 (A1g(S–W) modes, e); 402 cm−1 (A1g(S–Mo) mode, f) and 354 cm−1 (E2g(S–W) modes, g). h, Raman position mapping between 399–417 cm−1. There is a thin line of MoS2(1−x)S2x between the WS2(1−y)Se2y strip along the vertex direction which could not be resolved during the Raman mapping. i, j, Photoluminescence intensity map, corresponding to the optical image in Fig. 3b, at 1.67 eV (MoSe0.96S1.04 domain, i) and 1.8 eV (WSe0.92S1.08 domain, j). Scale bars: e–j, 10 μm.
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Sahoo, P., Memaran, S., Xin, Y. et al. One-pot growth of two-dimensional lateral heterostructures via sequential edge-epitaxy. Nature 553, 63–67 (2018). https://doi.org/10.1038/nature25155
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DOI: https://doi.org/10.1038/nature25155
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