ARTICLES PUBLISHED ONLINE: 28 SEPTEMBER 2014 | DOI: 10.1038/NNANO.2014.222
Lateral epitaxial growth of two-dimensional layered semiconductor heterojunctions Xidong Duan1*†, Chen Wang2†, Jonathan C. Shaw3†, Rui Cheng2, Yu Chen2, Honglai Li1,4, Xueping Wu1,4, Ying Tang1, Qinling Zhang4, Anlian Pan1,4*, Jianhui Jiang1, Ruqing Yu1, Yu Huang2,5 and Xiangfeng Duan3,5* Two-dimensional layered semiconductors such as MoS2 and WSe2 have attracted considerable interest in recent times. Exploring the full potential of these layered materials requires precise spatial modulation of their chemical composition and electronic properties to create well-defined heterostructures. Here, we report the growth of compositionally modulated MoS2–MoSe2 and WS2–WSe2 lateral heterostructures by in situ modulation of the vapour-phase reactants during growth of these two-dimensional crystals. Raman and photoluminescence mapping studies demonstrate that the resulting heterostructure nanosheets exhibit clear structural and optical modulation. Transmission electron microscopy and elemental mapping studies reveal a single crystalline structure with opposite modulation of sulphur and selenium distributions across the heterostructure interface. Electrical transport studies demonstrate that the WSe2–WS2 heterojunctions form lateral p–n diodes and photodiodes, and can be used to create complementary inverters with high voltage gain. Our study is an important advance in the development of layered semiconductor heterostructures, an essential step towards achieving functional electronics and optoelectronics.
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nspired by the recent advancements in graphene, the layered transition-metal dichalcogenides (TMDs; for example, MoS2 , MoSe2 , WS2 and WSe2) have attracted considerable attention as two-dimensional semiconductors with unique layer-numberdependent electronic and optical properties1–22. Exploring the full potential of these layered semiconductors requires precise spatial modulation of the chemical, structural and electronic properties of these two-dimensional atomic crystals to create well-defined heterostructures, much like the traditional semiconductor heterostructures that have essentially defined the foundation of all modern electronic and optoelectronic devices, including transistors, p–n diodes, photovoltaic/photodetection devices, light-emitting diodes and laser diodes. Although graphene–boron nitride (BN)-based twodimensional lateral heterostructures have been reported23–26, the growth of TMD lateral heterostructures is a considerable challenge and has not been realized to date, despite increasing efforts regarding the growth of diverse TMD nanosheets27–40. Here, we demonstrate that compositionally modulated MoS2– MoSe2 and WS2–WSe2 lateral heterostructures can be prepared by in situ modulation of the vapour-phase reactants during growth of these two-dimensional crystals. Raman and photoluminescence mapping studies demonstrate that the resulting heterostructure domains exhibit clear structural and optical modulation. Transmission electron microscopy (TEM) and electron diffraction studies reveal a single crystalline structure, and energy-dispersive X-ray elemental mapping confirms spatial modulation of the chemical compositions in the heterostructure domains. Electrical transport studies demonstrate that the WSe2–WS2 heterojunctions can form lateral p–n diodes with excellent current rectification behaviour and photocurrent generation characteristics, and can be used
to create complementary metal–oxide semiconductor (CMOS) inverters with high voltage gain (up to 24).
Lateral eptaxial growth strategy It has recently been shown that a wide range of TMD materials (for example, MoS2 , MoSe2 , WS2 and WSe2) can be directly grown on a silicon oxide substrate in the form of single- or few-layer, wellfaceted triangular or hexagonal domains or nearly continuous nanosheets27–40. The synthetic process usually involves a thermal chemical vapour deposition (CVD) process, in which the vapour-phase reactants are generated by thermally evaporating selected solid source materials. With this approach, a lateral heterostructure can in principle be produced through the successive growth of a second material (for example, MoSe2 or WSe2) at the edge of an existing domain of a first material (for example, MoS2 or WS2). With a relatively small lattice mismatch (∼4%) between MoS2 and MoSe2 or WS2 and WSe2 , it is possible to produce coherent MoS2–MoSe2 and WS2–WSe2 heterostructures through a lateral epitaxial process (Fig. 1). Although conceptually simple, the growth of TMD lateral heterostructures is a considerable challenge and has not been realized to date. Our studies indicate that simple sequential growth often fails to produce the desired heterostructures because (1) the edge growth front can be easily passivated after termination of the first growth and exposure to ambient conditions, and thus cannot function as the effective nucleation site for subsequent lateral epitaxial growth after transfer into a second growth chamber, and (2) the atomically thin TMD nanosheets are usually too delicate (for example, they are far more volatile than graphene or BN) to withstand the significant changes in growth temperature or conditions41 that are often
1 State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China, 2 Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA, 3 Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA, 4 Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha, Hunan 410082, China, 5 California Nanosystems Institute, University of California, Los Angeles, California 90095, USA; †These authors contributed equally to this work. * e-mail: [email protected]; [email protected]; [email protected]
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Figure 1 | Schematic of lateral epitaxial growth of WS2–WSe2 and MoS2– MoSe2 heterostructures. A triangular domain of WS2 (MoS2) is first grown using a CVD process. The peripheral edges of the triangular domain feature unsaturated dangling bonds that function as the active growth front for the continued addition and incorporation of precursor atoms to extend the twodimensional crystal in the lateral direction. With an in situ switch of the chemical vapour source for WSe2 (MoSe2) in the middle of growth, lateral heteroepitaxial growth can occur at the peripheral active growth front to form WS2–WSe2 (MoS2–MoSe2) lateral heterostructures.
necessary for the sequential growth of a second material to form the desired heterostructures. To grow lateral heterostructures it is therefore essential to design similar synthetic conditions applicable to both materials of interest, and to retain a fresh, unpassivated edge growth front for successive lateral epitaxial growth. To this end, we have designed a thermal CVD process (Supplementary Figs 1 and 2) that allows in situ switching of the vapour-phase reactants to enable lateral epitaxial growth of single- or few-layer TMD lateral heterostructures. For example, to produce a WS2–WSe2 heterostructure (Fig. 1), the WS2 domains are first grown by thermally evaporating a solid WS2 source at the centre hot zone of a 1-inch tube furnace in an argon atmosphere (Supplementary Fig. 1). The peripheral edges of the triangular domains feature unsaturated dangling bonds that function as the active growth front for the continued addition and incorporation of precursor atoms to extend the two-dimensional crystal in the lateral direction. By in situ transfer of the WS2 solid source out of the hot zone and the WSe2 solid source into the hot zone without exposure to ambient conditions, the chemical vapour source is switched from WS2 to WSe2 in the middle of the growth process, enabling lateral heteroepitaxial growth of WSe2 at the peripheral active growth front to produce WS2–WSe2 lateral heterostructures.
WS2–WSe2 lateral heterostructures
The resulting WS2–WSe2 lateral heterostructure domains predominantly exhibit a well-faceted equilateral triangular geometry (Fig. 2a and inset) with some truncated triangles or hexagons. Atomic force microscopy (AFM) studies show that these two-dimensional domains typically have a thickness of one to a few nanometres (Fig. 2a), with a small number of thicker domains up to 10–50 nm in thickness (Supplementary Fig. 3). To probe the spatial structural and optical modulation in the WS2–WSe2 heterostructures, we conducted micro-Raman and micro-photoluminescence (micro-PL) studies using a confocal Raman microscope. Importantly, the Raman spectra (excited by a 514 nm laser at 50 µW) taken from the centre and peripheral regions of a triangular domain clearly show distinct features. The Raman spectrum from the centre region has two prominent peaks at 419.3 cm−1 and 355.4 cm−1 (blue line in Fig. 2b), corresponding to the A1g and E12g resonance modes of WS2 (refs 42, 43), while the Raman spectrum from the peripheral region has a single peak at 256.1 cm−1 (green line in 2
DOI: 10.1038/NNANO.2014.222
Fig. 2b), in agreement of the A1g resonance mode of WSe2 (ref. 43). These micro-Raman studies demonstrate the co-existence of two distinct materials within the same triangular domain. The Raman mapping studies can further reveal the spatial modulation within the triangular domain, with the central part consisting a smaller triangular domain of WS2 (Fig. 2d) and the peripheral region composed of WSe2 (Fig. 2e). A composite Raman mapping image shows seamless lateral integration of WS2 and WSe2 in the heterostructure domain (Fig. 2f). Similarly, micro-PL studies also show highly distinct photoluminescence peaks at ∼665 nm for the centre part and ∼775 nm for the peripheral part (Fig. 2c), consistent with the near band-edge emission from WS2 and WSe2 , respectively20,42,44. The photoluminescence mapping studies (Fig. 2g–i) have features similar to those of Raman mapping studies, further confirming the formation of WS2–WSe2 lateral heterostructures. Importantly, these Raman and photoluminescence mapping studies indicate that there is no apparent overlap or gap between the WS2 and WSe2 signals (within the limit of optical resolution), suggesting that the WS2 inner triangle and WSe2 surrounding areas are laterally connected, although the sharpness of the lateral heterostructures cannot be determined accurately based on these optical studies. To investigate the crystalline and microstructure modulation within the WS2–WSe2 lateral heterostructures we conducted TEM and electron diffraction studies. For the TEM studies we focused on smaller heterostructure domains to provide a complete analysis of the entire domain. The low-resolution TEM image of a multilayer triangular domain in Fig. 3a does not show an obvious heterostructure interface. To better determine the lateral heterostructure interface we therefore used high-angle annular dark-field (HAADF) TEM to image the triangular heterostructure domain (Fig. 3b). The HAADF TEM is sensitive to atomic number and shows a more clearly differentiable contrast in the triangular domain, with the slightly darker inner triangle corresponding to the WS2 region, and the outer brighter region corresponding to WSe2. A selected area electron diffraction (SAED) pattern taken across the heterostructure interface (Fig. 3c) apparently shows a single set of hexagonally arranged diffraction spots that can be indexed to the hexagonal symmetry of the [001] zone plane of WS2 or WSe2 lattice structures. The observation of a single set of diffraction pattern suggests that the WS2 or WSe2 regions in the triangular heterostructure domain exhibit nearly the same lattice structure and the same lattice orientation. A magnified view of the diffraction spots (the circular insets in Fig. 3c) reveals that each diffraction spot consists of a pair of diffraction peaks. Careful analysis of these diffraction peaks yields (100) lattice plane spacings of 2.70 Å and 2.81 Å, in agreement with the values for WS2 and WSe2 , within instrumental error45. A lattice-resolved TEM image around the interface also further confirms the single crystalline structure. Importantly, the lattice fringes appear to be continuous from the WS2 region to the WSe2 region, indicating that the heterostructure is nearly coherent (Fig. 3d). To further unambiguously determine the chemical modulation across the lateral heterostructure interface, we conducted elemental mapping of the triangular domain using energy dispersive X-ray spectroscopy (EDS). These elemental mapping images show that W is nearly uniformly distributed throughout the entire triangular domain (Fig. 3e), while S and Se show obvious modulation within the heterostructure domain, with S localized in the centre and Se in the peripheral areas (Fig. 3f,g), respectively. A composite EDS mapping image of the S and Se atoms further confirms seamless lateral integration of the WS2 and WSe2 in the heterostructure domain (Fig. 3h). EDS linescan profiles of the elemental distribution across the junction clearly show the opposite modulation of S and Se across the interface (Fig. 3i). The junction is not atomically abrupt, but shows a transition from WS2 to WSe2 phases over a length scale of 35 ± 5 nm. This gradual interface might be attributed to a relatively slow switching of the vapour-phase reactants in the thermal
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Figure 2 | AFM, Raman and photoluminescence characterization of WS2–WSe2 lateral heterostructures. a, AFM image of a triangular domain with a thickness of 1.2 nm. Inset: optical image of a triangular domain. Scale bars, 5 µm. b, Raman spectra of a heterostructure domain. The blue curve is obtained from the centre region, and shows the characteristic Raman peaks of WS2. The green curve is obtained from the peripheral region, and shows the characteristic Raman peaks of WSe2. c, Photoluminescence spectra of a heterostructure domain. The orange curve is obtained from the centre region and shows the characteristic photoluminescence peak of WS2 , and the red curve is obtained from the peripheral region, and shows the characteristic photoluminescence peak of WSe2. d, Raman mapping at 419 cm−1 (WS2 A1g signal), demonstrating that WS2 is localized at the centre region of the triangular domain. Scale bar, 5 µm. e, Raman mapping at 256 cm−1 (WSe2 A1g signal), demonstrating that WSe2 is located in the peripheral region of the triangular domain. f, Composite image consisting of Raman mapping at 256 cm−1 and 419 cm−1, showing no apparent overlap or gap between the WS2 and WSe2 signals, demonstrating that the WS2 inner triangle and WSe2 peripheral areas are laterally connected. g,h, Photoluminescence mapping images at 665 nm and 775 nm, showing characteristic photoluminescence emission of WS2 and WSe2 in the centre and peripheral regions of the triangular domain, respectively. Scale bar, 5 µm. i, Composite image consisting of photoluminescence mapping at 665 nm and 775 nm, demonstrating the formation of WS2–WSe2 lateral heterostructures, consistent with Raman mapping studies. Scale bars in d,g apply to e,f,h,i.
CVD process and/or ion diffusion across the interface during the high-temperature synthetic process.
MoS2–MoSe2 lateral heterostructures To assess further the generality of the lateral epitaxy strategy for other two-dimensional heterostructures we also synthesized MoS2–MoSe2 lateral heterostructure nanosheets using a similar strategy. The MoS2–MoSe2 lateral heterostructures were grown using MoO3 , S and Se powder as the solid source precursors, with the vapour-phase S first reacting with MoO3 to form MoS2 domains, followed by the introduction of Se for successive epitaxial growth of MoSe2 (Supplementary Fig. 2). Note that MoS2–MoSe2 can only be obtained when S and Se vapour are switched in situ without exposing the MoS2 nuclei to ambient conditions. An optical microscope image of a representative MoS2–MoSe2 lateral heterostructure domain shows a well-faceted triangular geometry (Fig. 4a, inset). AFM studies show a flake thickness of 0.8 nm, indicating a monolayer structure (Fig. 4a). The Raman spectrum taken at the centre region shows characteristic resonance peaks of monolayer MoS2 , with the out-of-plane
A1g mode at 402.7 cm−1 and the in-plane E12g mode at 381.5 cm−1 (Fig. 4b). These values are slightly redshifted when compared with other CVD-grown MoS2 , which might be attributed to slight alloy formation with Se atoms35,36. The Raman spectrum taken in the peripheral region shows clear peaks at 234.5 cm−1 and 284.9 cm−1, corresponding to the out-of-plane A1g mode and the in-plane E12g mode of MoSe2 (Fig. 4b). Also note that there is an additional peak at 251 cm−1 adjacent to the A1g mode of MoSe2 , which corresponds to slightly S-alloyed MoSe2 and is consistent with previous reports35,36. The Raman mapping image at 403 cm−1 clearly shows that the MoS2 signal is localized in the centre triangle (Fig. 4d), and the mapping image at 235 cm−1 demonstrates that the MoSe2 signal is only present in the outer periphery of the triangular heterostructure domain (Fig. 4e). Photoluminescence spectra taken in the centre and peripheral regions show characteristic emission peaks corresponding to the near band-edge emission of monolayer MoS2 (680 nm) and monolayer MoSe2 (790 nm) (Fig. 4c)3,4,20. The MoS2 emission is slightly redshifted compared to the reported photoluminescence emission from exfoliated MoS2 (653–660 nm)3,4, which may be attributed to slight alloy formation (∼10%),
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Figure 3 | Structural and chemical modulation in WS2–WSe2 lateral heterostructures. a, TEM image of a triangular domain of a WS2–WSe2 lateral heterostructure. Scale bar, 200 nm. b, HAADF TEM image of the heterostructure domain, showing the slightly darker inner triangle corresponding to the WS2 region and the slightly brighter outer region corresponding to WSe2. Scale bar, 200 nm. c, Electron diffraction pattern taken across the heterostructure interface, with each diffraction spot consisting of a pair diffraction peaks (see insets for magnified views), with indexed lattice spacings of 2.70 Å for WS2 and 2.81 Å for WSe2 , respectively. d, High-resolution TEM image showing a highly crystalline structure with continuous lattice fringes across the WS2–WSe2 heterostructure interface. Scale bar, 5 nm. e–g, EDS elemental mapping images of W, S and Se atoms, showing a relatively uniform distribution of W, and clear modulation of S and Se in the triangular domain. h, Composite EDS mapping image of S and Se atoms, showing seamless lateral integration of WS2 and WSe2 in the heterostructure domain. Scale bars (e–h), 200 nm. i, EDS linescan profiles of S and Se distributions, showing opposite modulation across the heterostructure interface.
consistent with previous reports of MoS2–2xSe2x alloys35,36. Photoluminescence mapping images of MoS2 at 680 nm (Fig. 4f ) and MoSe2 at 790 nm (Fig. 4g) further confirm the formation of the lateral heterostructure across the triangular domain. The crystallinity of the MoS2–MoSe2 heterostructures was also examined using TEM (Fig. 4h) and SAED. The SAED patterns taken from either side of MoS2–MoSe2 heterostructure interface display a typical six-fold symmetry with the same spatial arrangement and the same spatial orientation (Fig. 4i,j). Analysis of the SAED patterns yields lattice spacings of 2.70 Å and 2.78 Å, corresponding to the (100) lattice planes of the MoS2 and MoSe2 crystal structures, respectively45. Finally, the EDS linescans of the S and Se distribution clearly show opposite modulations of S and Se atoms across the MoS2–MoSe2 heterostructure interface (Fig. 4k), with an interface transition over a length scale of ∼40 nm (comparable to that of the WS2–WSe2 heterostructure interface). These studies confirm the formation of MoS2–MoSe2 lateral heterostructures within the monolayer nanosheet.
Electronic characterization and heterostructure devices The above discussions clearly demonstrate the successful growth of lateral heterostructures of two-dimensional layered TMDs. The formation of such two-dimensional heterostructures with well-defined 4
composition or electronic modulation can open up exciting opportunities for the creation of a series of functional electronic and optoelectronic devices. Before constructing functional devices across the heterostructure interface, we first characterized the basic electrical transport properties of WS2 and WSe2 , respectively. To this end, WS2 and WSe2 field-effect transistors (FETs) were fabricated on a Si/SiO2 substrate, with a Ti/Au thin film as the source– drain contacts for WS2 , a Au thin film as the contacts for WSe2 , and the silicon substrate as the backgate electrode. To electrically access the inner WS2 triangle without shorting to the peripheral WSe2 , a 50-nm-thick Al2O3 thin film was deposited on part of the peripheral WSe2 to insulate it from the contact electrodes for the WS2. Figure 5a,b shows the Ids–Vds characteristics for different backgate voltages for WS2 and WSe2 , respectively. A linear Ids–Vds relationship is clearly observed for both WS2 and WSe2 , indicating the metal contact barrier does not significantly affect the intrinsic device behaviour. The formation of low-barrier contacts for both WS2 and WSe2 is very important for probing the intrinsic electronic and optoelectronic characteristics of the lateral heterojunction devices. The Ids–Vds plots at varying backgate voltages show that the current increases with increasing positive gate voltage for WS2 (Fig. 5a), indicating n-type semiconductor behaviour. In contrast, the current increases with increasing negative gate voltage for
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Figure 4 | Growth and characterization of MoS2–MoSe2 lateral heterostructures. a, AFM and optical microscope images (inset) of a triangular MoS2–MoSe2 monolayer domain, with an AFM linescan indicating a step height of ∼0.8 nm, corresponding to the monolayer MoS2–MoSe2 domain. Scale bars, 2 µm. b, Raman spectra taken from the centre (MoS2) and peripheral (MoSe2) areas of a triangular heterostructure domain. c, Photoluminescence emission spectra obtained from the centre (MoS2) and peripheral (MoSe2) areas of a triangular heterostructure domain. d,e, Raman spectroscopy mapping images of the A1g mode of MoS2 at 403 cm−1 (d) and A1g mode of MoSe2 at 235 cm−1 (e). Scale bars, 2 µm. f,g, Photoluminescence mapping images of the centre MoS2 region with emission at 680 nm (f) and the peripheral MoSe2 region with emission at 790 nm (g). Scale bars, 2 µm. h, Low-resolution TEM image of a monolayer heterostructure. The white dotted line roughly defines the boundary between MoS2 and MoSe2. Scale bar, 200 nm. i,j, SAED patterns taken from the MoS2 region (i) and the MoSe2 region (j) of a lateral heterostructure. k, EDS linescan profiles across the MoS2–MoSe2 heterostructure interface, demonstrating the opposite modulations of S and Se concentration.
WSe2 (Fig. 5b), demonstrating p-type behaviour. It is also noted that n-type WS2 is in the normally ‘off’ state at zero gate, with a relatively large positive turn-on voltage (∼40 V), indicating low intrinsic electron doping, while the p-type WSe2 is normally ‘on’, suggesting a relatively high hole-doping concentration. We also evaluated the carrier mobility in these synthetic materials based on FET measurements (Supplementary Fig. 4). The electron mobility in WS2 is typically in the range of 10–20 cm2 V−1 s−1 and the hole mobility in WSe2 is in the range of 30–100 cm2 V−1 s−1, both of which are comparable to the best values for synthetic materials reported to date. With well-defined p-type characteristics in WSe2 and n-type characteristics in WS2 , the WSe2–WS2 lateral heterostructure forms a natural heterojunction p–n diode (Fig. 5c, inset). Electrical transport studies across a heterojunction device show obvious current rectification behaviour in the Ids–Vds plot (Fig. 5c), with current only able to pass through the device when the p-type WSe2 is positively biased. The observation of current rectification clearly demonstrates a p–n diode is formed within the WSe2–WS2 lateral heterostructure. The ultrathin nature of the heterojunction allows gate tunability of the diode characteristics. The diode output characteristics (Ids–Vds) under different backgate voltage show that the output current increases with increasing positive gate voltage, suggesting that the n-type WS2 is partly limiting charge transport in the device. This is consistent with the electrical
transport properties of WS2 and WSe2 , in which WS2 is much lower doped and therefore dominates the charge transport across the lateral heterojunction p–n diode. Such p–n diodes form the fundamental basis for many functional devices, including photodiodes. To this end, we also investigated the photocurrent response of our lateral heterojunction p–n diodes as photodiodes. The Ids–Vds measurements of a multilayer heterojunction p–n diode with and without laser illumination (514 nm, 30 nW) show a clear photovoltaic effect with an open-circuit voltage of ∼0.47 V and short-circuit current of ∼1.2 nA (Fig. 5d). In general, the photoresponse exhibits a rapid temporal response beyond our experimental time resolution of 100 µs (Fig. 5d, inset), demonstrating that the photoresponse indeed originates from photocarrier generation and separation rather than any other extrinsic effects. Based on the photocurrent response and total input laser power, we can determine the external quantum efficiency of the photon-to-electron conversion to be ∼9.9%. After determining the optical absorption of the multilayer heterostructure device (∼23%), we can estimate an internal quantum efficiency of ∼43%. Finally, the photocurrent mapping clearly shows that the photocurrent response is localized to the lightly doped WS2 and WS2–WSe2 interface region near the centre of the triangular domain (Fig. 5e), confirming that the depletion layer is largely localized to the lightly doped WS2 in the lateral heterojunction diodes.
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Figure 5 | Electrical characterization and functional devices from WS2–WSe2 lateral heterojunctions. a, Ids–Vds output characteristics of a WS2 FET at various backgate voltages (indicated in the plot) show increasing current with increasing positive gate voltage, demonstrating n-type behaviour. b, Ids–Vds output characteristics of a WSe2 FET at various backgate voltages (indicated in the plot) show decreasing current with increasing positive gate voltage, demonstrating p-type behaviour. c, Gate-tunable output characteristics of a lateral WSe2–WS2 heterojunction p–n diode. The gate voltage varies from 80 to 20 V in steps of 10 V, as indicated. Inset: optical image of a heterojunction p–n diode device. The orange dashed line outlines the triangular heterostructure domain and the white dashed rectangle outlines the 50 nm Al2O3 deposited on WSe2 to insulate the WS2 contact electrodes. Scale bar, 2 µm. d, Experimental output (Ids–Vds) characteristics of the lateral WSe2–WS2 heterojunction p–n diode in the dark (black line) and under illumination (red line; wavelength, 514 nm; power, 30 nW). Inset: temporal photocurrent response under periodic on/off laser illumination through a mechanical chopper. e, Scanning photocurrent mapping image of the lateral WSe2–WS2 heterojunction p–n diode showing that the photoresponse is localized at the WSe2–WS2 interface and the lower-doped WS2 region near the centre of the triangular domain. The orange dashed line outlines the triangular heterostructure domain. The yellow solid line outlines the gold electrodes. Scale bar, 2 µm. f, A CMOS inverter obtained by integrating a p-type WSe2 and n-type WS2 FET, showing the expected inverter function with a voltage gain as large as 24. The black curve is the output–input curve and the red curve indicates the voltage gain. Inset: Image and circuit diagram of the WSe2–WS2 CMOS inverter. Scale bar, 2 µm.
The preparation of WS2–WSe2 lateral heterostuctures with both p- and n-type characteristics can also allow us to construct many other functional devices. For example, a CMOS inverter (a logic NOT gate) can be constructed by integrating a p-channel WSe2 and an n-channel WS2 transistor in series across the heterojunction interface (Fig. 5f, insets). To reduce the required gate voltage, the CMOS inverter was fabricated using 20 nm HfO2 as the gate dielectrics. The output–input (Vout–Vin) voltage response of the inverter shows constant high voltage output at low input (black curve in Fig. 5f ). When the input is increased to ∼1.5 V, the output voltage is quickly switched to nearly 0 V and maintains a low state at higher input voltages. Significantly, the CMOS inverter can readily exhibit a high voltage gain. Differentiation of the measured Vout–Vin relation reveals a voltage gain as large as 24 (red curve in Fig. 5f ). Such a large gain can be particularly beneficial for the interconnection of arrays of logic circuits for functional electronic applications, without requiring signal restoration at each stage.
Conclusion In summary, we have successfully demonstrated the growth of lateral heterojunctions of two-dimensional layered semiconductors using a lateral heteroepitaxial approach. Although lateral heterojunctions between h-BN and graphene (or between intrinsic and substitutionally doped graphene) have been reported previously23–26,46, they usually exhibit limited functions due to the semimetal nature of graphene and the insulator nature of h-BN. The realization of a lateral heterojunction in two-dimensional layered semiconductors can open up exciting opportunities for the creation 6
of a wide range of functional devices, ranging from complementary logic circuits, photovoltaics and photodetectors, to light-emitting diodes and laser diodes. Beyond the typical two-dimensional layered semiconductors, this approach may be extended to the creation of heterojunctions of many other two-dimensional materials with diverse physical properties, including superconducting (for example, NbSe2), magnetic (for example, CrSe2) and topologically insulating (for example, Bi2Se3 and Bi2Te3) materials. It thus has the potential to enable seamless integration of these highly distinct physical properties, creating well-defined spatial modulation of chemical compositions and electronic properties in these two-dimensional atomic crystals. With further development in future studies, the lateral epitaxy approach may be extended to the growth of superlattice structures of two or more two-dimensional layered materials with greater complexity and more functions, and the size of the lateral ‘interface’ in such atomically thin heterostructures may eventually be reduced to a single atomic line between two distinct two-dimensional materials to form an ‘interline’ (versus ‘interface’). With quantum confinement in two dimensions at this interline and the importance of the interface in semiconductor heterojunctions and topological insulators, the formation of the interline in ultrathin lateral heterostructures could open up a new door to exotic physics and/or totally new device concepts.
Methods
Synthesis. The lateral heterostructures were synthesized in home-built CVD systems using thermally evaporated vapour-phase reactants from solid source materials at
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atmospheric pressure. The CVD systems were designed such that the chemical vapour sources could be interchanged in situ by mechanically shifting the selected solid source materials into and out of the hot zone (Supplementary Figs 1 and 2), which was essential to retain the active growth front for sequential heteroepitaxial growth. For the growth of WS2–WSe2 , WS2 and WSe2 powders were directly used as the solid source, and for the growth of MoS2–MoSe2 lateral heterostructures, MoO3 and elemental S and Se powders were used as the solid source. Characterization. The microstructures and morphologies of the nanostructures were characterized by optical microscope, AFM and TEM. The micro-Raman and micro-PL studies were conducted using a Horiba LabRAM HR Evolution confocal Raman system with Ar ion laser (514 nm) excitation. The WS2–WSe2 heterojunction devices were fabricated using electron-beam lithography followed by electron-beam deposition of metal thin films. Ti/Au (5/50 nm) were used for the contact electrodes for WS2 , and Au (50 nm) for WSe2. Electrical transport measurements were conducted in a Lakeshore probe station, with a computercontrolled analog-to-digital converter and a low-noise current pre-amplifier. Scanning photocurrent measurements were conducted with the same Horiba confocal Raman system combined with the same electrical measurement system.
Received 12 June 2014; accepted 1 September 2014; published online 28 September 2014
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Acknowledgements The authors acknowledge the Nanoelectronics Research Facility (NRF) at UCLA for technical support. The authors thank N.O. Weiss for preparing the schematics in Fig. 1. A.P. acknowledges support from the National Basic Research Program of China (no. 2012CB932703) and the National Natural Science Foundation of China (11374092). J.J. and R.Y. acknowledge support from the National Natural Science Foundation of China (21025521, 21221003). Y.H. acknowledges a National Institutes of Health Director’s New Innovator Award Program (1DP2OD007279). X.D. acknowledges support by the National Science Foundation (CAREER award no. 0956171).
Author contributions X.D. designed the research. X.D.D. synthesized the WS2–WSe2 heterostructures and conducted the initial Raman, photoluminescence and TEM characterizations and data analysis. J.C.S. synthesized the MoS2–MoSe2 heterostructures and conducted the relevant Raman characterizations. C.W. and R.C. conducted the Raman characterizations, device fabrication, characterization and data analysis. Y.C. conducted the TEM studies and data analysis, A.P., H.L. and X.W. contributed to the CVD set-up. A.P., Y.T. and Q.Z. contributed to Raman and photoluminescence studies. J.J., R.Y., A.P., Y.H. and X.D. supervised the research. X.D., X.D.D. and J.C.S. co-wrote the paper. All authors discussed the results and commented on the manuscript.
Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to X.D.D., A.P. and X.D.
Competing financial interests
The authors declare no competing financial interests.
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Lateral epitaxial growth of two-dimensional layered semiconductor heterojunctions Xidong Duan1*†, Chen Wang2†, Jonathan C. Shaw3†, Rui Cheng2, Yu Chen2, Honglai Li1,4, Xueping Wu1,4, Ying Tang1, Qinling Zhang4, Anlian Pan1,4*, Jianhui Jiang1, Ruqing Yu1, Yu Huang2,5 and Xiangfeng Duan3,5* Two-dimensional layered semiconductors such as MoS2 and WSe2 have attracted considerable interest in recent times. Exploring the full potential of these layered materials requires precise spatial modulation of their chemical composition and electronic properties to create well-defined heterostructures. Here, we report the growth of compositionally modulated MoS2–MoSe2 and WS2–WSe2 lateral heterostructures by in situ modulation of the vapour-phase reactants during growth of these two-dimensional crystals. Raman and photoluminescence mapping studies demonstrate that the resulting heterostructure nanosheets exhibit clear structural and optical modulation. Transmission electron microscopy and elemental mapping studies reveal a single crystalline structure with opposite modulation of sulphur and selenium distributions across the heterostructure interface. Electrical transport studies demonstrate that the WSe2–WS2 heterojunctions form lateral p–n diodes and photodiodes, and can be used to create complementary inverters with high voltage gain. Our study is an important advance in the development of layered semiconductor heterostructures, an essential step towards achieving functional electronics and optoelectronics.
I
nspired by the recent advancements in graphene, the layered transition-metal dichalcogenides (TMDs; for example, MoS2 , MoSe2 , WS2 and WSe2) have attracted considerable attention as two-dimensional semiconductors with unique layer-numberdependent electronic and optical properties1–22. Exploring the full potential of these layered semiconductors requires precise spatial modulation of the chemical, structural and electronic properties of these two-dimensional atomic crystals to create well-defined heterostructures, much like the traditional semiconductor heterostructures that have essentially defined the foundation of all modern electronic and optoelectronic devices, including transistors, p–n diodes, photovoltaic/photodetection devices, light-emitting diodes and laser diodes. Although graphene–boron nitride (BN)-based twodimensional lateral heterostructures have been reported23–26, the growth of TMD lateral heterostructures is a considerable challenge and has not been realized to date, despite increasing efforts regarding the growth of diverse TMD nanosheets27–40. Here, we demonstrate that compositionally modulated MoS2– MoSe2 and WS2–WSe2 lateral heterostructures can be prepared by in situ modulation of the vapour-phase reactants during growth of these two-dimensional crystals. Raman and photoluminescence mapping studies demonstrate that the resulting heterostructure domains exhibit clear structural and optical modulation. Transmission electron microscopy (TEM) and electron diffraction studies reveal a single crystalline structure, and energy-dispersive X-ray elemental mapping confirms spatial modulation of the chemical compositions in the heterostructure domains. Electrical transport studies demonstrate that the WSe2–WS2 heterojunctions can form lateral p–n diodes with excellent current rectification behaviour and photocurrent generation characteristics, and can be used
to create complementary metal–oxide semiconductor (CMOS) inverters with high voltage gain (up to 24).
Lateral eptaxial growth strategy It has recently been shown that a wide range of TMD materials (for example, MoS2 , MoSe2 , WS2 and WSe2) can be directly grown on a silicon oxide substrate in the form of single- or few-layer, wellfaceted triangular or hexagonal domains or nearly continuous nanosheets27–40. The synthetic process usually involves a thermal chemical vapour deposition (CVD) process, in which the vapour-phase reactants are generated by thermally evaporating selected solid source materials. With this approach, a lateral heterostructure can in principle be produced through the successive growth of a second material (for example, MoSe2 or WSe2) at the edge of an existing domain of a first material (for example, MoS2 or WS2). With a relatively small lattice mismatch (∼4%) between MoS2 and MoSe2 or WS2 and WSe2 , it is possible to produce coherent MoS2–MoSe2 and WS2–WSe2 heterostructures through a lateral epitaxial process (Fig. 1). Although conceptually simple, the growth of TMD lateral heterostructures is a considerable challenge and has not been realized to date. Our studies indicate that simple sequential growth often fails to produce the desired heterostructures because (1) the edge growth front can be easily passivated after termination of the first growth and exposure to ambient conditions, and thus cannot function as the effective nucleation site for subsequent lateral epitaxial growth after transfer into a second growth chamber, and (2) the atomically thin TMD nanosheets are usually too delicate (for example, they are far more volatile than graphene or BN) to withstand the significant changes in growth temperature or conditions41 that are often
1 State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China, 2 Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA, 3 Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA, 4 Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha, Hunan 410082, China, 5 California Nanosystems Institute, University of California, Los Angeles, California 90095, USA; †These authors contributed equally to this work. * e-mail: [email protected]; [email protected]; [email protected]
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Figure 1 | Schematic of lateral epitaxial growth of WS2–WSe2 and MoS2– MoSe2 heterostructures. A triangular domain of WS2 (MoS2) is first grown using a CVD process. The peripheral edges of the triangular domain feature unsaturated dangling bonds that function as the active growth front for the continued addition and incorporation of precursor atoms to extend the twodimensional crystal in the lateral direction. With an in situ switch of the chemical vapour source for WSe2 (MoSe2) in the middle of growth, lateral heteroepitaxial growth can occur at the peripheral active growth front to form WS2–WSe2 (MoS2–MoSe2) lateral heterostructures.
necessary for the sequential growth of a second material to form the desired heterostructures. To grow lateral heterostructures it is therefore essential to design similar synthetic conditions applicable to both materials of interest, and to retain a fresh, unpassivated edge growth front for successive lateral epitaxial growth. To this end, we have designed a thermal CVD process (Supplementary Figs 1 and 2) that allows in situ switching of the vapour-phase reactants to enable lateral epitaxial growth of single- or few-layer TMD lateral heterostructures. For example, to produce a WS2–WSe2 heterostructure (Fig. 1), the WS2 domains are first grown by thermally evaporating a solid WS2 source at the centre hot zone of a 1-inch tube furnace in an argon atmosphere (Supplementary Fig. 1). The peripheral edges of the triangular domains feature unsaturated dangling bonds that function as the active growth front for the continued addition and incorporation of precursor atoms to extend the two-dimensional crystal in the lateral direction. By in situ transfer of the WS2 solid source out of the hot zone and the WSe2 solid source into the hot zone without exposure to ambient conditions, the chemical vapour source is switched from WS2 to WSe2 in the middle of the growth process, enabling lateral heteroepitaxial growth of WSe2 at the peripheral active growth front to produce WS2–WSe2 lateral heterostructures.
WS2–WSe2 lateral heterostructures
The resulting WS2–WSe2 lateral heterostructure domains predominantly exhibit a well-faceted equilateral triangular geometry (Fig. 2a and inset) with some truncated triangles or hexagons. Atomic force microscopy (AFM) studies show that these two-dimensional domains typically have a thickness of one to a few nanometres (Fig. 2a), with a small number of thicker domains up to 10–50 nm in thickness (Supplementary Fig. 3). To probe the spatial structural and optical modulation in the WS2–WSe2 heterostructures, we conducted micro-Raman and micro-photoluminescence (micro-PL) studies using a confocal Raman microscope. Importantly, the Raman spectra (excited by a 514 nm laser at 50 µW) taken from the centre and peripheral regions of a triangular domain clearly show distinct features. The Raman spectrum from the centre region has two prominent peaks at 419.3 cm−1 and 355.4 cm−1 (blue line in Fig. 2b), corresponding to the A1g and E12g resonance modes of WS2 (refs 42, 43), while the Raman spectrum from the peripheral region has a single peak at 256.1 cm−1 (green line in 2
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Fig. 2b), in agreement of the A1g resonance mode of WSe2 (ref. 43). These micro-Raman studies demonstrate the co-existence of two distinct materials within the same triangular domain. The Raman mapping studies can further reveal the spatial modulation within the triangular domain, with the central part consisting a smaller triangular domain of WS2 (Fig. 2d) and the peripheral region composed of WSe2 (Fig. 2e). A composite Raman mapping image shows seamless lateral integration of WS2 and WSe2 in the heterostructure domain (Fig. 2f). Similarly, micro-PL studies also show highly distinct photoluminescence peaks at ∼665 nm for the centre part and ∼775 nm for the peripheral part (Fig. 2c), consistent with the near band-edge emission from WS2 and WSe2 , respectively20,42,44. The photoluminescence mapping studies (Fig. 2g–i) have features similar to those of Raman mapping studies, further confirming the formation of WS2–WSe2 lateral heterostructures. Importantly, these Raman and photoluminescence mapping studies indicate that there is no apparent overlap or gap between the WS2 and WSe2 signals (within the limit of optical resolution), suggesting that the WS2 inner triangle and WSe2 surrounding areas are laterally connected, although the sharpness of the lateral heterostructures cannot be determined accurately based on these optical studies. To investigate the crystalline and microstructure modulation within the WS2–WSe2 lateral heterostructures we conducted TEM and electron diffraction studies. For the TEM studies we focused on smaller heterostructure domains to provide a complete analysis of the entire domain. The low-resolution TEM image of a multilayer triangular domain in Fig. 3a does not show an obvious heterostructure interface. To better determine the lateral heterostructure interface we therefore used high-angle annular dark-field (HAADF) TEM to image the triangular heterostructure domain (Fig. 3b). The HAADF TEM is sensitive to atomic number and shows a more clearly differentiable contrast in the triangular domain, with the slightly darker inner triangle corresponding to the WS2 region, and the outer brighter region corresponding to WSe2. A selected area electron diffraction (SAED) pattern taken across the heterostructure interface (Fig. 3c) apparently shows a single set of hexagonally arranged diffraction spots that can be indexed to the hexagonal symmetry of the [001] zone plane of WS2 or WSe2 lattice structures. The observation of a single set of diffraction pattern suggests that the WS2 or WSe2 regions in the triangular heterostructure domain exhibit nearly the same lattice structure and the same lattice orientation. A magnified view of the diffraction spots (the circular insets in Fig. 3c) reveals that each diffraction spot consists of a pair of diffraction peaks. Careful analysis of these diffraction peaks yields (100) lattice plane spacings of 2.70 Å and 2.81 Å, in agreement with the values for WS2 and WSe2 , within instrumental error45. A lattice-resolved TEM image around the interface also further confirms the single crystalline structure. Importantly, the lattice fringes appear to be continuous from the WS2 region to the WSe2 region, indicating that the heterostructure is nearly coherent (Fig. 3d). To further unambiguously determine the chemical modulation across the lateral heterostructure interface, we conducted elemental mapping of the triangular domain using energy dispersive X-ray spectroscopy (EDS). These elemental mapping images show that W is nearly uniformly distributed throughout the entire triangular domain (Fig. 3e), while S and Se show obvious modulation within the heterostructure domain, with S localized in the centre and Se in the peripheral areas (Fig. 3f,g), respectively. A composite EDS mapping image of the S and Se atoms further confirms seamless lateral integration of the WS2 and WSe2 in the heterostructure domain (Fig. 3h). EDS linescan profiles of the elemental distribution across the junction clearly show the opposite modulation of S and Se across the interface (Fig. 3i). The junction is not atomically abrupt, but shows a transition from WS2 to WSe2 phases over a length scale of 35 ± 5 nm. This gradual interface might be attributed to a relatively slow switching of the vapour-phase reactants in the thermal
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Figure 2 | AFM, Raman and photoluminescence characterization of WS2–WSe2 lateral heterostructures. a, AFM image of a triangular domain with a thickness of 1.2 nm. Inset: optical image of a triangular domain. Scale bars, 5 µm. b, Raman spectra of a heterostructure domain. The blue curve is obtained from the centre region, and shows the characteristic Raman peaks of WS2. The green curve is obtained from the peripheral region, and shows the characteristic Raman peaks of WSe2. c, Photoluminescence spectra of a heterostructure domain. The orange curve is obtained from the centre region and shows the characteristic photoluminescence peak of WS2 , and the red curve is obtained from the peripheral region, and shows the characteristic photoluminescence peak of WSe2. d, Raman mapping at 419 cm−1 (WS2 A1g signal), demonstrating that WS2 is localized at the centre region of the triangular domain. Scale bar, 5 µm. e, Raman mapping at 256 cm−1 (WSe2 A1g signal), demonstrating that WSe2 is located in the peripheral region of the triangular domain. f, Composite image consisting of Raman mapping at 256 cm−1 and 419 cm−1, showing no apparent overlap or gap between the WS2 and WSe2 signals, demonstrating that the WS2 inner triangle and WSe2 peripheral areas are laterally connected. g,h, Photoluminescence mapping images at 665 nm and 775 nm, showing characteristic photoluminescence emission of WS2 and WSe2 in the centre and peripheral regions of the triangular domain, respectively. Scale bar, 5 µm. i, Composite image consisting of photoluminescence mapping at 665 nm and 775 nm, demonstrating the formation of WS2–WSe2 lateral heterostructures, consistent with Raman mapping studies. Scale bars in d,g apply to e,f,h,i.
CVD process and/or ion diffusion across the interface during the high-temperature synthetic process.
MoS2–MoSe2 lateral heterostructures To assess further the generality of the lateral epitaxy strategy for other two-dimensional heterostructures we also synthesized MoS2–MoSe2 lateral heterostructure nanosheets using a similar strategy. The MoS2–MoSe2 lateral heterostructures were grown using MoO3 , S and Se powder as the solid source precursors, with the vapour-phase S first reacting with MoO3 to form MoS2 domains, followed by the introduction of Se for successive epitaxial growth of MoSe2 (Supplementary Fig. 2). Note that MoS2–MoSe2 can only be obtained when S and Se vapour are switched in situ without exposing the MoS2 nuclei to ambient conditions. An optical microscope image of a representative MoS2–MoSe2 lateral heterostructure domain shows a well-faceted triangular geometry (Fig. 4a, inset). AFM studies show a flake thickness of 0.8 nm, indicating a monolayer structure (Fig. 4a). The Raman spectrum taken at the centre region shows characteristic resonance peaks of monolayer MoS2 , with the out-of-plane
A1g mode at 402.7 cm−1 and the in-plane E12g mode at 381.5 cm−1 (Fig. 4b). These values are slightly redshifted when compared with other CVD-grown MoS2 , which might be attributed to slight alloy formation with Se atoms35,36. The Raman spectrum taken in the peripheral region shows clear peaks at 234.5 cm−1 and 284.9 cm−1, corresponding to the out-of-plane A1g mode and the in-plane E12g mode of MoSe2 (Fig. 4b). Also note that there is an additional peak at 251 cm−1 adjacent to the A1g mode of MoSe2 , which corresponds to slightly S-alloyed MoSe2 and is consistent with previous reports35,36. The Raman mapping image at 403 cm−1 clearly shows that the MoS2 signal is localized in the centre triangle (Fig. 4d), and the mapping image at 235 cm−1 demonstrates that the MoSe2 signal is only present in the outer periphery of the triangular heterostructure domain (Fig. 4e). Photoluminescence spectra taken in the centre and peripheral regions show characteristic emission peaks corresponding to the near band-edge emission of monolayer MoS2 (680 nm) and monolayer MoSe2 (790 nm) (Fig. 4c)3,4,20. The MoS2 emission is slightly redshifted compared to the reported photoluminescence emission from exfoliated MoS2 (653–660 nm)3,4, which may be attributed to slight alloy formation (∼10%),
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Figure 3 | Structural and chemical modulation in WS2–WSe2 lateral heterostructures. a, TEM image of a triangular domain of a WS2–WSe2 lateral heterostructure. Scale bar, 200 nm. b, HAADF TEM image of the heterostructure domain, showing the slightly darker inner triangle corresponding to the WS2 region and the slightly brighter outer region corresponding to WSe2. Scale bar, 200 nm. c, Electron diffraction pattern taken across the heterostructure interface, with each diffraction spot consisting of a pair diffraction peaks (see insets for magnified views), with indexed lattice spacings of 2.70 Å for WS2 and 2.81 Å for WSe2 , respectively. d, High-resolution TEM image showing a highly crystalline structure with continuous lattice fringes across the WS2–WSe2 heterostructure interface. Scale bar, 5 nm. e–g, EDS elemental mapping images of W, S and Se atoms, showing a relatively uniform distribution of W, and clear modulation of S and Se in the triangular domain. h, Composite EDS mapping image of S and Se atoms, showing seamless lateral integration of WS2 and WSe2 in the heterostructure domain. Scale bars (e–h), 200 nm. i, EDS linescan profiles of S and Se distributions, showing opposite modulation across the heterostructure interface.
consistent with previous reports of MoS2–2xSe2x alloys35,36. Photoluminescence mapping images of MoS2 at 680 nm (Fig. 4f ) and MoSe2 at 790 nm (Fig. 4g) further confirm the formation of the lateral heterostructure across the triangular domain. The crystallinity of the MoS2–MoSe2 heterostructures was also examined using TEM (Fig. 4h) and SAED. The SAED patterns taken from either side of MoS2–MoSe2 heterostructure interface display a typical six-fold symmetry with the same spatial arrangement and the same spatial orientation (Fig. 4i,j). Analysis of the SAED patterns yields lattice spacings of 2.70 Å and 2.78 Å, corresponding to the (100) lattice planes of the MoS2 and MoSe2 crystal structures, respectively45. Finally, the EDS linescans of the S and Se distribution clearly show opposite modulations of S and Se atoms across the MoS2–MoSe2 heterostructure interface (Fig. 4k), with an interface transition over a length scale of ∼40 nm (comparable to that of the WS2–WSe2 heterostructure interface). These studies confirm the formation of MoS2–MoSe2 lateral heterostructures within the monolayer nanosheet.
Electronic characterization and heterostructure devices The above discussions clearly demonstrate the successful growth of lateral heterostructures of two-dimensional layered TMDs. The formation of such two-dimensional heterostructures with well-defined 4
composition or electronic modulation can open up exciting opportunities for the creation of a series of functional electronic and optoelectronic devices. Before constructing functional devices across the heterostructure interface, we first characterized the basic electrical transport properties of WS2 and WSe2 , respectively. To this end, WS2 and WSe2 field-effect transistors (FETs) were fabricated on a Si/SiO2 substrate, with a Ti/Au thin film as the source– drain contacts for WS2 , a Au thin film as the contacts for WSe2 , and the silicon substrate as the backgate electrode. To electrically access the inner WS2 triangle without shorting to the peripheral WSe2 , a 50-nm-thick Al2O3 thin film was deposited on part of the peripheral WSe2 to insulate it from the contact electrodes for the WS2. Figure 5a,b shows the Ids–Vds characteristics for different backgate voltages for WS2 and WSe2 , respectively. A linear Ids–Vds relationship is clearly observed for both WS2 and WSe2 , indicating the metal contact barrier does not significantly affect the intrinsic device behaviour. The formation of low-barrier contacts for both WS2 and WSe2 is very important for probing the intrinsic electronic and optoelectronic characteristics of the lateral heterojunction devices. The Ids–Vds plots at varying backgate voltages show that the current increases with increasing positive gate voltage for WS2 (Fig. 5a), indicating n-type semiconductor behaviour. In contrast, the current increases with increasing negative gate voltage for
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WSe2 (Fig. 5b), demonstrating p-type behaviour. It is also noted that n-type WS2 is in the normally ‘off’ state at zero gate, with a relatively large positive turn-on voltage (∼40 V), indicating low intrinsic electron doping, while the p-type WSe2 is normally ‘on’, suggesting a relatively high hole-doping concentration. We also evaluated the carrier mobility in these synthetic materials based on FET measurements (Supplementary Fig. 4). The electron mobility in WS2 is typically in the range of 10–20 cm2 V−1 s−1 and the hole mobility in WSe2 is in the range of 30–100 cm2 V−1 s−1, both of which are comparable to the best values for synthetic materials reported to date. With well-defined p-type characteristics in WSe2 and n-type characteristics in WS2 , the WSe2–WS2 lateral heterostructure forms a natural heterojunction p–n diode (Fig. 5c, inset). Electrical transport studies across a heterojunction device show obvious current rectification behaviour in the Ids–Vds plot (Fig. 5c), with current only able to pass through the device when the p-type WSe2 is positively biased. The observation of current rectification clearly demonstrates a p–n diode is formed within the WSe2–WS2 lateral heterostructure. The ultrathin nature of the heterojunction allows gate tunability of the diode characteristics. The diode output characteristics (Ids–Vds) under different backgate voltage show that the output current increases with increasing positive gate voltage, suggesting that the n-type WS2 is partly limiting charge transport in the device. This is consistent with the electrical
transport properties of WS2 and WSe2 , in which WS2 is much lower doped and therefore dominates the charge transport across the lateral heterojunction p–n diode. Such p–n diodes form the fundamental basis for many functional devices, including photodiodes. To this end, we also investigated the photocurrent response of our lateral heterojunction p–n diodes as photodiodes. The Ids–Vds measurements of a multilayer heterojunction p–n diode with and without laser illumination (514 nm, 30 nW) show a clear photovoltaic effect with an open-circuit voltage of ∼0.47 V and short-circuit current of ∼1.2 nA (Fig. 5d). In general, the photoresponse exhibits a rapid temporal response beyond our experimental time resolution of 100 µs (Fig. 5d, inset), demonstrating that the photoresponse indeed originates from photocarrier generation and separation rather than any other extrinsic effects. Based on the photocurrent response and total input laser power, we can determine the external quantum efficiency of the photon-to-electron conversion to be ∼9.9%. After determining the optical absorption of the multilayer heterostructure device (∼23%), we can estimate an internal quantum efficiency of ∼43%. Finally, the photocurrent mapping clearly shows that the photocurrent response is localized to the lightly doped WS2 and WS2–WSe2 interface region near the centre of the triangular domain (Fig. 5e), confirming that the depletion layer is largely localized to the lightly doped WS2 in the lateral heterojunction diodes.
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Figure 5 | Electrical characterization and functional devices from WS2–WSe2 lateral heterojunctions. a, Ids–Vds output characteristics of a WS2 FET at various backgate voltages (indicated in the plot) show increasing current with increasing positive gate voltage, demonstrating n-type behaviour. b, Ids–Vds output characteristics of a WSe2 FET at various backgate voltages (indicated in the plot) show decreasing current with increasing positive gate voltage, demonstrating p-type behaviour. c, Gate-tunable output characteristics of a lateral WSe2–WS2 heterojunction p–n diode. The gate voltage varies from 80 to 20 V in steps of 10 V, as indicated. Inset: optical image of a heterojunction p–n diode device. The orange dashed line outlines the triangular heterostructure domain and the white dashed rectangle outlines the 50 nm Al2O3 deposited on WSe2 to insulate the WS2 contact electrodes. Scale bar, 2 µm. d, Experimental output (Ids–Vds) characteristics of the lateral WSe2–WS2 heterojunction p–n diode in the dark (black line) and under illumination (red line; wavelength, 514 nm; power, 30 nW). Inset: temporal photocurrent response under periodic on/off laser illumination through a mechanical chopper. e, Scanning photocurrent mapping image of the lateral WSe2–WS2 heterojunction p–n diode showing that the photoresponse is localized at the WSe2–WS2 interface and the lower-doped WS2 region near the centre of the triangular domain. The orange dashed line outlines the triangular heterostructure domain. The yellow solid line outlines the gold electrodes. Scale bar, 2 µm. f, A CMOS inverter obtained by integrating a p-type WSe2 and n-type WS2 FET, showing the expected inverter function with a voltage gain as large as 24. The black curve is the output–input curve and the red curve indicates the voltage gain. Inset: Image and circuit diagram of the WSe2–WS2 CMOS inverter. Scale bar, 2 µm.
The preparation of WS2–WSe2 lateral heterostuctures with both p- and n-type characteristics can also allow us to construct many other functional devices. For example, a CMOS inverter (a logic NOT gate) can be constructed by integrating a p-channel WSe2 and an n-channel WS2 transistor in series across the heterojunction interface (Fig. 5f, insets). To reduce the required gate voltage, the CMOS inverter was fabricated using 20 nm HfO2 as the gate dielectrics. The output–input (Vout–Vin) voltage response of the inverter shows constant high voltage output at low input (black curve in Fig. 5f ). When the input is increased to ∼1.5 V, the output voltage is quickly switched to nearly 0 V and maintains a low state at higher input voltages. Significantly, the CMOS inverter can readily exhibit a high voltage gain. Differentiation of the measured Vout–Vin relation reveals a voltage gain as large as 24 (red curve in Fig. 5f ). Such a large gain can be particularly beneficial for the interconnection of arrays of logic circuits for functional electronic applications, without requiring signal restoration at each stage.
Conclusion In summary, we have successfully demonstrated the growth of lateral heterojunctions of two-dimensional layered semiconductors using a lateral heteroepitaxial approach. Although lateral heterojunctions between h-BN and graphene (or between intrinsic and substitutionally doped graphene) have been reported previously23–26,46, they usually exhibit limited functions due to the semimetal nature of graphene and the insulator nature of h-BN. The realization of a lateral heterojunction in two-dimensional layered semiconductors can open up exciting opportunities for the creation 6
of a wide range of functional devices, ranging from complementary logic circuits, photovoltaics and photodetectors, to light-emitting diodes and laser diodes. Beyond the typical two-dimensional layered semiconductors, this approach may be extended to the creation of heterojunctions of many other two-dimensional materials with diverse physical properties, including superconducting (for example, NbSe2), magnetic (for example, CrSe2) and topologically insulating (for example, Bi2Se3 and Bi2Te3) materials. It thus has the potential to enable seamless integration of these highly distinct physical properties, creating well-defined spatial modulation of chemical compositions and electronic properties in these two-dimensional atomic crystals. With further development in future studies, the lateral epitaxy approach may be extended to the growth of superlattice structures of two or more two-dimensional layered materials with greater complexity and more functions, and the size of the lateral ‘interface’ in such atomically thin heterostructures may eventually be reduced to a single atomic line between two distinct two-dimensional materials to form an ‘interline’ (versus ‘interface’). With quantum confinement in two dimensions at this interline and the importance of the interface in semiconductor heterojunctions and topological insulators, the formation of the interline in ultrathin lateral heterostructures could open up a new door to exotic physics and/or totally new device concepts.
Methods
Synthesis. The lateral heterostructures were synthesized in home-built CVD systems using thermally evaporated vapour-phase reactants from solid source materials at
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atmospheric pressure. The CVD systems were designed such that the chemical vapour sources could be interchanged in situ by mechanically shifting the selected solid source materials into and out of the hot zone (Supplementary Figs 1 and 2), which was essential to retain the active growth front for sequential heteroepitaxial growth. For the growth of WS2–WSe2 , WS2 and WSe2 powders were directly used as the solid source, and for the growth of MoS2–MoSe2 lateral heterostructures, MoO3 and elemental S and Se powders were used as the solid source. Characterization. The microstructures and morphologies of the nanostructures were characterized by optical microscope, AFM and TEM. The micro-Raman and micro-PL studies were conducted using a Horiba LabRAM HR Evolution confocal Raman system with Ar ion laser (514 nm) excitation. The WS2–WSe2 heterojunction devices were fabricated using electron-beam lithography followed by electron-beam deposition of metal thin films. Ti/Au (5/50 nm) were used for the contact electrodes for WS2 , and Au (50 nm) for WSe2. Electrical transport measurements were conducted in a Lakeshore probe station, with a computercontrolled analog-to-digital converter and a low-noise current pre-amplifier. Scanning photocurrent measurements were conducted with the same Horiba confocal Raman system combined with the same electrical measurement system.
Received 12 June 2014; accepted 1 September 2014; published online 28 September 2014
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Acknowledgements The authors acknowledge the Nanoelectronics Research Facility (NRF) at UCLA for technical support. The authors thank N.O. Weiss for preparing the schematics in Fig. 1. A.P. acknowledges support from the National Basic Research Program of China (no. 2012CB932703) and the National Natural Science Foundation of China (11374092). J.J. and R.Y. acknowledge support from the National Natural Science Foundation of China (21025521, 21221003). Y.H. acknowledges a National Institutes of Health Director’s New Innovator Award Program (1DP2OD007279). X.D. acknowledges support by the National Science Foundation (CAREER award no. 0956171).
Author contributions X.D. designed the research. X.D.D. synthesized the WS2–WSe2 heterostructures and conducted the initial Raman, photoluminescence and TEM characterizations and data analysis. J.C.S. synthesized the MoS2–MoSe2 heterostructures and conducted the relevant Raman characterizations. C.W. and R.C. conducted the Raman characterizations, device fabrication, characterization and data analysis. Y.C. conducted the TEM studies and data analysis, A.P., H.L. and X.W. contributed to the CVD set-up. A.P., Y.T. and Q.Z. contributed to Raman and photoluminescence studies. J.J., R.Y., A.P., Y.H. and X.D. supervised the research. X.D., X.D.D. and J.C.S. co-wrote the paper. All authors discussed the results and commented on the manuscript.
Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to X.D.D., A.P. and X.D.
Competing financial interests
The authors declare no competing financial interests.
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