Edge-Epitaxial Growth of 2D NbS2-WS2 Lateral Metal-Semiconductor Heterostructures

Dr. Y. Zhang, L. Yin, J. Chu, Dr. T. A. Shifa, Dr. F. Wang, X. Zhan, Prof. Z. Wang, Prof. J. He CAS Center for Excellence in Nanoscience CAS Key Laboratory of Nanosystem and Hierarchical Fabrication National Center for Nanoscience and Technology Beijing 100190, China E-mail: wangzx@nanoctr.cn; hej@nanoctr.cn L. Yin, Dr. T. A. Shifa, Dr. F. Wang, Y. Wen, Prof. Z. Wang, Prof. J. He University of Chinese Academy of Science No.19A Yuquan Road, Beijing 100049, China J. Chu State Key Laboratory of Electronic Thin Films and Integrated Devices University of Electronic Science and Technology of China Chengdu 610054, China J. Xia key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201803665.

www.advmat.de www.advancedsciencenews.com metal-semiconductor heterostructure maybe also be promising in high-speed integrated circuits and microwave technologies, considering its high-frequency characteristics. Additionally, the ultrathin body thickness of 2D configuration can bring metallic TMDs high mechanical flexibility, which makes the pliable/ wearable device systems capable. Hence, the direct synthesis of 2D metal-semiconductor TMD heterostructures is crucial for the practical applications of TMD-based electronic and optoelectronic devices.
Bulk metallic TMD crystals are commonly synthesized via chemical vapor transport techniques [36][37][38] and their thin flakes are obtained by mechanical exfoliation method. [19] Very few reports about the uniform and controllable growth of monolayer metallic TMDs have been made so far by chemical vapor deposition (CVD) method. [17,20,39] Although lots of interesting physics and electronic properties have been explored from 2D TMD semiconductors and its heterostructures obtained via one-step, two-step, and even multistep growth processes, [40][41][42][43][44][45][46][47] the synthesis of 2D metallic TMDs and establishment of Van der Waals (vdW) heterostructures between metallic and semiconductor TMDs are sparsely reported. [48,49] These have not only hindered the exploration of their electronic/magnetic properties but also greatly impeded their practical applications in electronic devices.
Having in mind the astonishing research progress on CVD grown high-quality and large domain MoS 2 , WS 2 flakes, etc., [50][51][52] and also speculating the similar structure of NbS 2 with a complementary electronic properties, it is reasonable to hypothesize that the rational synthesis of NbS 2 -WS 2 heterostructures could lead us one step forward in the area of 2D materials . In this study, we present a direct synthesis of monolayer NbS 2 with a large domain size via an ambient pressure CVD (APCVD) method. More importantly, we realize a controllable epitaxial growth of NbS 2 -WS 2 lateral heterostructures via a facile "two-step" CVD route. Transmission electron microscopic studies show perfect atomic structures and clear chemical modulation with distinct interfaces. Moreover, Raman and photoluminescence (PL) spectroscopic characterizations reveal the controlled spatial modulation within the concentric triangular domain such that the central part is composed of WS 2 triangle and the peripheral region consisting of NbS 2 . Using our CVD-grown NbS 2 -WS 2 lateral heterostructures, we fabricated field-effect transistors (FETs) wherein the electrical transport measurements demonstrate explicit Schottky junction features with well-defined rectification, which indicate the potential application in electronic devices. We believe that the findings in our work pave a promising way for the property investigations and application developments of 2D metallic materials and 2D metal-semiconductor TMD heterostructures.
Uniform monolayer metallic NbS 2 triangles were grown on sapphire (Al 2 O 3 (0001)) substrates via an APCVD system in a three-zone furnace. Niobium pentoxide (Nb 2 O 5 ) and Sulfur (S) powder were chosen as the reaction precursors. The Nb 2 O 5 powder, mixed with a little amount of Sodium chloride (NaCl), was placed at the center of quartz tube onto which the growth substrate (sapphire(Al 2 O 3 (0001))) was laid facing down. The sulfur powder was then placed at the upstream region. The reaction condition was set in such a way that the temperature of the upstream region was raised to 150-200 °C, and the sulfur vapor was carried by Ar gas flow (mixed by H 2 ) toward the center to react with Nb 2 O 5 at 790 °C and deposit NbS 2 on sapphire. More details of the growth experiments are provided in the Experimental Section and Supporting Information ( Figure S1). Figure 1a schematically depicts the typical structure model of monolayer NbS 2 seen from top and side views, which is similar to that of MoS 2 . [9] A typical scanning electronic microscopy (SEM) image in Figure 1b demonstrates the achievement of uniform NbS 2 triangles on sapphire. And the maximum edge length of the as-grown NbS 2 triangle can be as large as 100 µm, confirmed by the inset optical image. Furthermore, X-ray photoelectron spectroscopy (XPS) results and Raman spectra of as-grown monolayer NbS 2 were also obtained and supplemented in Figures S2 and S3 (Supporting Information), fully confirming its chemical component. Figure 1c gives a representative atomic force micro scopy (AFM) image and demonstrates that the thickness of as-grown NbS 2 triangles is ≈1.0 nm, indicating its monolayer nature. Besides, the as-grown NbS 2 is proved to be air-stable ( Figure S4, Supporting Information). Afterward, the NbS 2 -WS 2 lateral heterostructure was fabricated by sequentially growing each components on sapphire (Al 2 O 3 (0001)) substrates through a "two-step" CVD route, as schematically illustrated in Figure 1d. In the first step, large-scale monolayer WS 2 triangles were synthesized on the sapphire substrate via an APCVD method. Briefly, WO 3 powder, mixed with a little amount of NaCl powder, was used as growth precursor and sulfurized by sulfur vapor to obtain a large scale WS 2 monolayer at 730 °C (details of the growth process are elaborated in Figure S5 in the Supporting Information and the Experimental Section). Subsequently, monolayer WS 2 triangles supported by sapphire substrate were in turn utilized for the preparation of NbS 2 layers. In the second growth step, the APCVD method for the NbS 2 growth (mentioned above) was again adopted to eventually realize the epitaxial growth of NbS 2 . Notably, there exists a short interruption before the second growth step for material transition. Generally, the edges or grain boundaries of primary synthesized WS 2 are easily passivated by some impurities under an ambient condition. Merely, the absorbent molecules can be removed by the NbS 2 growth process at a relatively high growth temperature (≈790 °C) under the S atmosphere. So no additional treatment is necessary between the two-step growth processes. Figure 1e shows SEM image of monolayer WS 2 triangle obtained from the first step growth. The XPS and AFM results are shown in Figures S6 and S7 (Supporting Information). The morphology of the lateral heterostructure grown as triangular NbS 2 -WS 2 on sapphire substrate can be clearly visualized from the SEM image in Figure 1f. These two concentric triangular regions with a slightly different image contrast marked by yellow and blue triangles reveal a transparent interface between WS 2 (centric) and NbS 2 (peripheral) (the elements' distribution is presented in Figure S16, Supporting Information). The AFM image (inset of Figure 1f) indicates that the heterostructure possesses a smooth surface with a thickness of 10 nm for NbS 2 and a height of ≈1.0 nm for WS 2 . Notably, by controlling the growth parameters, the thickness of NbS 2 within the heterostructure can be as thin as about ≈5 nm, shown in Figure S9 (Supporting Information). Furthermore, the chemical composition of NbS 2 -WS 2 lateral heterostructure www.advmat.de www.advancedsciencenews.com was determined by X-ray photoelectron spectroscopy (XPS) characterization ( Figure 1g). It is apparent that the heterostructure constitutes elements of Nb, W, and S. The chemical states of Nb 3d 5/2 and 3d 3/2 can be identified from the peaks at binding energies of 203.5 and 207.5 eV, respectively. The 4f 7/2 and 4f 5/2 states of W are corroborated from the peaks at binding energies ≈32.0 and 34.3 eV whereas those at binding energies ≈161.5 and 162.5 eV are meant for 2p 3/2 and 2p 1/2 states of S, respectively, which are in agreement with the spectra of NbS 2 and WS 2 . [41] Besides, by further designing and controlling the growth parameters, NbS 2 /WS 2 vertical heterostructures were also successfully obtained via Van der Waals epitaxial growth. And the coverage of upper NbS 2 film can be efficiently controlled by tuning the growth condition. Much more detailed information about their morphologies and optical properties of the NbS 2 /WS 2 vertical heterostructures is shown in Figure S10-S12 (Supporting Information). In brief, these results demonstrate that we realize the epitaxial growth of NbS 2 -WS 2 heterostructures in both lateral and vertical ways via a "two-step" CVD method.
Atomic resolution transmission electron microscope (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were further performed on transferred samples to investigate the detailed atomic structures of monolayer NbS 2 and NbS 2 -WS 2 lateral heterostructure. Figure 2a shows a high-resolution TEM (HRTEM) image of NbS 2 , displaying a perfect atomic arrangement. The (100) lattice plane spacing (d 100 ) extracted from Figure 2a can be revealed as ≈0.291 nm; thus the in-plane lattice constant (a 0 ) should be ≈0.336 nm. Figure 2b shows the corresponding selective area electron diffraction (SAED) pattern with only one set of diffraction spots, revealing the single crystalline nature of NbS 2 and its high crystal quality. On the basis of SAED study, we can also determine the (100) lattice plane spacing of NbS 2 to be 0.291 nm, consistent with the one observed from the HRTEM result. Energy-dispersive spectroscopy (EDS) mapping was also employed along with TEM on the transferred samples to identify the chemical composition.   Raman and PL spectra were further conducted to examine the sequential distribution of the chemical composition as well as the local optical properties within the NbS 2 -WS 2 lateral heterostructures. Figure 3a shows the Raman spectra acquired at different regions within NbS 2 -WS 2 heterostructure domain as marked by black and red dots in the SEM image (inset of Figure 3a). The Raman spectrum from the central part presents two characteristic peaks at 352 cm −1 (E 2g ) and 417 cm −1 (A 1g ) (black line in Figure 3a), in agreement with that of monolayer WS 2 . Interestingly, the peripheral part (red line in Figure 3a) exhibits quite different features, whose two Raman modes located at 332 cm −1 (E 2g ) and 378 cm −1 (A 1g ) correspond to the typical Raman modes of NbS 2 layers. [31,32] These Raman spectra indicate high quality of the two materials in concentric triangular domain and emphasize the formation of NbS 2 -WS 2 lateral heterostructure without compositional alloying. Figure 3b displays the PL spectra collected from WS 2 and NbS 2 regions. The centric WS 2 region exhibits a very strong PL characteristic peak at ≈625 nm, in accordance with an exciton emission of monolayer WS 2 (black line in Figure 3b). However, the peripheral NbS 2 has no PL peak (red line in Figure 3b) due to its metallic nature. Note that the emission around 700 nm originates from the sapphire substrate. Hence, PL spectra also confirm the formation of NbS 2 -WS 2 lateral heterostructure. In order to further solidify our observations made above, the Raman and PL spectra of as-grown WS 2 and WS 2 after "step 2" in APCVD was studied for the sake of comparisons (black line and red line in Figure 3c). The peak positions of two prominent peaks for monolayer WS 2 have no deviation. Raman spectra in Figure 3c reveal the same characteristic peaks of WS 2 with no Raman shifts. Similarly, the typical PL peaks for WS 2 at both conditions also possess the same position and intensity as shown in Figure 3d. These results suggest that WS 2 region in the NbS 2 -WS 2 lateral heterostructure domain still maintain its intrinsic structure with no doping possibility by other atoms or damage after "step 2" growth.  To further investigate the transport properties of obtained NbS 2 crystals, electronic devices based on NbS 2 were fabricated on sapphire substrates. Figure 4a shows a typical optical image of constructed device of NbS 2 with a thickness of ≈2 nm (shown as inset AFM image). Figure 4b demonstrates a linear I ds -V ds curve collected at room temperature with a resistance of ≈2100 Ω, indicating an Ohmic contact between NbS 2 and electrodes. Figure 4c shows the temperature-dependent resistance of the NbS 2 device, exhibiting a decreasing resistance from 200 to 30 K and then an increasing resistance from 30 to 2 K. The temperature of the minimum resistance (T m ) was observed as ≈30 K and higher than that of bulk NbS 2, which may be due to electron-electron interaction enhanced in low dimensional system. [33] These results confirm the metallic nature of NbS 2 crystals, in a good agreement with DFT calculation. [34] The metallic ground state of NbS 2 crystals is also evidenced by the PL spectra without signal ( Figure S3, Supporting Information).
Further, to characterize the electrical transport properties and performance of the obtained NbS 2 -WS 2 lateral vdW heterostructures, series of FETs were fabricated on Si substrate with SiO 2 of 300 nm. Figure 4d schematically illustrates the process of device construction. First, the concentric triangular NbS 2 -WS 2 lateral heterostructure should be etched by reactive ion etching technique, obtaining an NbS 2 -WS 2 lateral heterostructure ribbon. Subsequently, Cr/Au thin films were thermally evaporated as electrodes deposited on NbS 2 region and WS 2 region separately. The detailed fabrication procedure is given in the Experimental Section. Figure 4e shows an optical image of our fabricated FET device based on the NbS 2 -WS 2 lateral heterostructure (the thickness of NbS 2 part is shown in Figure S15, Supporting Information). The transfer characteristic curve (I ds -V gs ) of this device (Figure 4f) shows a typical n-type behavior with an on-off ratio of 10 5 . The field-effect mobility is calculated to be 0.14 cm 2 V −1 s −1 (L = 9.3 µm, W = 10.9 µm, V ds = 1 V), which is comparable with reported CVD-grown heterostructure result. [38] The corresponding output characteristic (I ds -V ds ) in Figure 4g depicts that the I ds decreases with the V gs varying from 80 to 30 V. Most importantly, the gate-tunable output curves demonstrate explicit current rectification behavior, different from that of pure WS 2 ( Figure S16  www.advmat.de www.advancedsciencenews.com confirming the formation of Schottky junction across the interface. Even though the complicated device fabrication processes should influence our device properties, our predicted result will be realized after optimizing the technology in our next work. The finding obtained here tangibly corroborates that NbS 2 -WS 2 lateral heterostructures broaden the potential application in electronic devices.
In summary, we have demonstrated the controllable CVDgrowth of monolayer metallic NbS 2 triangles with their domain sizes reaching up to millimeter scale. More importantly, we realize the epitaxial growth of NbS 2 -WS 2 heterostructures in both vertical and lateral ways via a "two-step" CVD method. To our best knowledge, it is a direct synthesis of lateral metal-semiconductor heterostructure reported for the first time. Transmission electron microscope studies have shown the perfect atomic structures and diffraction spots without rotation, highly indicating the clear interface and epitaxial growth behavior of NbS 2 -WS 2 lateral heterostructures. Raman and PL studies have further demonstrated the obvious chemical modulation and clear interfaces at the NbS 2 -WS 2 lateral heterostructure. Field effect transistors based on the NbS 2 -WS 2 lateral heterostructure domains were fabricated. The electronic transport performance has exhibited a well-defined rectification. This work makes a significant step forward, which could expand the range of new heteromaterials and provide more potential applications in electronic devices.

Experimental Section
Synthesis of Monolayer NbS 2 and NbS 2 -WS 2 Lateral Heterostructures: An APCVD system was used for the NbS 2 synthesis. A three-zone furnace system was adopted in which Niobium pentoxide (Nb 2 O 5 ) powder (Alfa Aesar, purity 99%) and sulfur (S) powder (Alfa Aesar, purity 99.5%) were used as precursors. Two quartz boats containing S, Nb 2 O 5 , and substrates were loaded into the tube from upstream to downstream with the temperature of 190 and 790 °C, respectively. The substrates were faced downward on the Nb 2 O 5 powder (0.5 g) mixed with a little amount of sodium chloride (NaCl (0.05 g)). Prior to heating, the furnace tube was purged with 400 sccm Ar for 30 min and then with 100 sccm Ar and 1-10 sccm H 2 to create a preferable growth atmosphere for the growth. The NbS 2 growth lasted for 15 min and finally NbS 2 monolayer was obtained on sapphire substrates.
For the synthesis of NbS 2 -WS 2 lateral heterostructure, monolayer WS 2 flakes were first synthesized via APCVD method. S powder was placed at the upstream of furnace, tungsten trioxide (WO 3 ) powder (Alfa Aesar, purity 99.9%) and sapphire (Al 2 O 3 (0001)) substrates were successively placed at the center of furnace. The evaporating temperature of S and WO 3 were 150 and 730 °C, respectively. Prior to heating, the furnace tube was purged with 400 sccm Ar for 30 min and the reaction time was about 10 min. Finally, WS 2 monolayer was obtained on sapphire substrates. The sapphire substrate, onto which the WS 2 monolayers were grown, was used as substrate for the second step growth of metallic NbS 2 . Eventually, epitaxially grown lateral NbS 2 -WS 2 heterostructure was obtained.
Transfer of As-Grown NbS 2 and NbS 2 -WS 2 Lateral Heterostructures: The as-grown samples were spin-coated with poly(methyl methacrylate) (PMMA; 495 K, A4, Microchem Company) at a speed of 1500 rpm for 45 s followed by drying at 180 °C for 10 min. Then the samples supported by PMMA film were lifted up by tweezers under the water and it was then collected by a target substrate. Finally, the PMMA film was removed via dissolution with acetone for about 30 min and dried by the flowing N 2 gas.

Characterizations of As-Grown NbS 2 and NbS 2 -WS 2 Lateral
Heterostructures: The morphologies of NbS 2 and NbS 2 -WS 2 lateral heterostructures were characterized by optical microscope (OM, Olympus BX51M), Hitach S-4800 scanning electron microscope, Tecnai F20 transmission electron microscope, and Titan G2 60-300 Probe Cs Corrector HRSTEM. STEM-EDX elemental mapping was characterized by Tecnai F20. The thickness of obtained samples was characterized by atomic force microscope (Bruker Icon). The temperature-dependent Raman and PL spectra were obtained from confocal microscope-based Raman spectrometer (Renishaw InVia, 514 nm excitation laser). X-ray photoelectron spectroscopy was tested on ESCALAB 250 Xi.
Fabrications and Measurement of the As-Grown NbS 2 and NbS 2 -WS 2 Devices: The as-grown NbS 2 flakes on sapphire were fabricated into devices without transfer. The standard electron-beam lithography (EBL) processing was used to define the source/drain electrodes. To avoid charge accumulation on the insulating sapphire substrate during EBL, a layer of conductive coating (AR-PC, 5090.02) was spin-coated on the sapphire substrate with NbS 2 flakes before spin-coating PMMA (495 K). Subsequently, Cr/Au metal layer (Cr: 8 nm, Au: 50 nm) was deposited to form electrodes by thermal evaporation. Room-temperature measurements were performed with a semiconductor parameter analyzer (Keithley Model 4200-SCS). Temperature dependence of resistivity was performed using the Physical Properties Measurement Systems (Quantum Design) with temperature ranging from 200 to 2 K.
The as-grown NbS 2 -WS 2 lateral heterostructures were transferred from sapphire substrate to SiO 2 /Si for the fabrication of FET devices. After this, a layer of PMMA was spin-coated on the substrate followed by a 3 min bake at 120 °C. Standard EBL technique was employed to prepare the source/drain electrodes. Then 8 nm Cr and 50 nm Au were thermally evaporated as electrodes and followed by lift-off process with acetone. All of the measurements were performed on a probe station (Lakeshore, TTP4) with vacuum of 10 −6 torr. The data were collected by Keithley 4200 semiconductor parameter analyzer.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.