1.
Introduction

Nanowires, as a kind of one-dimensional (1D) nanostructure, have been studied for many years. As early as 1995, William et al. has synthesized InP, InAs, and GaAs nanowhiskers by a solution–liquid–solid method^[1]. In the following decades, more and more studies on nanowires have been reported, such as electrical properties^[2-4], synthesis methods^[5-8], theoretical prediction^[9-12] and so on. Due to the quantum size effect and edge effect, the 1D materials show many different properties compared to 2D nanosheets. For example, zigzag graphene nanoribbons with a narrow width are antiferromagnetic semiconductors, while a semiconductor-to-metal transition can be observed as the width increases^[13]. Recent reports show that 1D InSe nanowires, synthesized by a physical vapor transport system or chemical vapor deposition (CVD) method, exhibit high photoresponsivity and fast response speed due to the enhanced density of states^{[10, 14]}.



The problem of lattice mismatch is often faced when 2D materials are combined into a heterostructure, but the appearance of 1D nanowires solves this problem. Recently, heterostructures composed of 1D materials and 2D materials have attracted much attention due to their promising application in electronic and optoelectronic devices. For instance, Qin et al. reported the synthesis of 1D Mo₆Te₆?2D MoTe₂ heterostructures via the molecular beam epitaxy (MBE) method, which provides a new approach to synthesize 1D semimetallic nanowires^[15]. Moreover, 2D/1D heterostructures have exhibited excellent device performances in many reports, such as high on/off ratio^[16], wide gate tunability^[17], better performance in lithium-ion batteries^[18] and remarkable hydrogen evolution reaction (HER) activity^[19]. However, previous studies of 2D/1D heterostructures are mainly focused on vertical stacking. Controllable synthesis of 2D/1D lateral heterostructures with tunable width is rarely reported.



Herein, we successfully synthesize 2D/1D WS₂/WSe₂ (WS₂/MoS₂) lateral heterostructures via a home-built bidirectional airflow CVD reactor in which the width of the WSe₂ (MoS₂) can be controlled by controlling air flow direction and growth time. Scanning electron microscope (SEM) and atomic force microscopy (AFM) images show that the width of the WSe₂ can be tuned from 11 nm to 4 μm by precisely controlling the growth time. The interface of the 2D/1D heterostructures is clear and sharp. We also control the width of MoS₂ in the same way. Our research provides a new method for the synthesis of 1D nanowires and lays the foundation for the future study of 2D/1D lateral heterostructures.

2.
Methods

2.1
Synthesis of 2D WS₂

WS₂ nanosheets were first synthesized as substrates in our experiment. Different from the traditional CVD system, our reactor is equipped with four switches, which can pass two directions of air flow. At first, the WS₂ powders were used as precursor and placed in the center of the heat zone. Meanwhile, the SiO₂ (285 nm)/Si was placed at the low temperature zone. Next, 600 sccm Ar gas was introduced from the substrate to the precursor (green arrow, reverse flow) for about 20 min to clean the air in the quartz tube. Then, the flow rate was reduced to 75 sccm and the furnace was heated to 1200 °C in 40 min. When the temperature reached 1200 °C, we changed the direction of Ar gas to forward (red arrow) and kept it for 4 min. At last, the reactor was cooled to room temperature.

2.2
Synthesis of 2D/1D WS₂/WSe₂ heterostructures

At first, the WSe₂ powders, as the precursor, were placed in the center of the heat zone. The SiO₂/Si substrate with WS₂ nanosheets was placed in the low temperature zone to grow heterostructures. Next, 600 sccm Ar was introduced at about 20 min. Then, the flow rate was reduced to 120 sccm and the furnace was heated to 1150 °C in 35 min. When the temperature reached to 1150 °C, the direction of Ar gas was changed and kept for 0–40 s. Then we used magnets and quartz hooks to pull the boat out quickly and turned the air flow back into the reverse direction. At last, the reactor was cooled to room temperature.

2.3
Synthesis of 2D/1D WS₂/MoS₂ heterostructures

Similarly, MoS₂ powders were placed in the center of the heat zone and the SiO₂/Si substrate with WS₂ nanosheets was placed at the downstream. Next, 600 sccm Ar was introduced. Then, the flow rate was reduced to 120 sccm and the furnace was heated to 1190 °C in 40 min. When the temperature reached to 1190 °C, the direction of Ar gas was changed and kept for 0.5–3 min. Then we used magnets and quartz hooks to pull the boat out quickly and turned the air flow back into the reverse direction. At last, the reactor was cooled to room temperature.

2.4
Material characterization

The Raman and PL spectra images were conducted via a laser micro-Raman spectrometer (Renishaw Invia). The width of the heterostructure was measured by AFM (Bruker model: Dimension ICON) and SEM (TESCAN MIRA3).

3.
Result and discussion

As shown in Fig. 1, we modified a bidirectional flow CVD process to precisely control the growth process that has been reported^[20]. In this system, reverse gas flow (green arrow) was introduced during the heating stage, which can clean the quartz tube and avoid unwished for source deposition. Compared with the forward flow, the reverse flow passes through the substrate at a lower temperature, which can avoid thermal etching in the heating process. The direction of air flow was transformed (red arrow) quickly when reaching the growth temperature. Moreover, when the growth process stops, the substrate was pulled out and the airflow reverses again. For 1D WSe₂ growth, when we pull the boat out and reverse the direction of the gas, the growth process can be terminated completely and immediately.






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Figure1.
(Color online) Schematic of a modified bidirectional flow CVD system.




Because the heat resistance of WS₂ is better than that of WSe₂, we choose WS₂ as the growth substrate. As shown in Fig. 2(a), monolayer WS₂ that we synthesized in the first step is about 100 μm. The uniform surface and sharp edge are suitable for the epitaxy of the second step. Fig. 2(b) shows that PL peak of monolayer WS₂ locates at 640 nm, which is consistent with previous study^{[20, 21]}. As shown in Fig. 2(c), there are two distinct peaks at 357 and 419 cm^–1, which can be assigned to the E' and A₁' modes, respectively^[22]. The sharp edge and uniform surface of WS₂ facilitate epitaxial growth. The PL mapping images at 640 nm and Raman mapping images at 357 cm^–1 are shown in Figs. 2(e) and 2(f). The uniform color represents the high quality and uniformity of monolayer WS₂, which laid a good foundation for the next step of growth.






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Figure2.
(Color online) (a) Optical image of monolayer WS₂. (b) PL spectrum of WS₂. (c) Raman spectrum of WS₂. (d) Optical image of WS₂ in the yellow rectangle of (a). (e) PL mapping image of monolayer WS₂. (f) Raman mapping image of monolayer WS₂.




As shown in Fig. 3, the width of WSe₂ of the WS₂/WSe₂ heterostructures can be sequentially tuned by controlling the air direction and growth time. When the growth time is controlled at 5 s, the WSe₂ nanoribbons can be grown at the edge of WS₂ with a width of 90 nm (Fig. 3(a)), which means that the 2D/1D WS₂/WSe₂ heterostructure is synthesized. With the prolongation of growth time, the width of WSe₂ is increased to a few hundred nanometers (Figs. 3(b)–3(d)). As shown in Figs. 3(e) and 3(f), the width of WSe₂ can be tuned larger than 1 μm when the growth time is 30 s. Moreover, it can be observed that the WS₂/WSe₂ heterostructures are clean and uniform, which are good for application.






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Figure3.
(Color online) AFM images of WS₂/WSe₂ lateral heterostructure when the growth time are (a) 5 s, (b) 7 s, (c) 9 s, (d) 11 s, (e) 30 s and (f) 40 s.




In fact, there are some narrower nanoribbons at 0–3 s that are difficult to be characterized by AFM, so we conducted scanning electron microscopy (SEM) to characterize their widths. As shown in Fig. 4, the 11 nm wide WSe₂ can be grown at the edge of WS₂, which represents that an ultra-narrow nanowire was synthesized.






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Figure4.
SEM image of the 2D/1D WS₂/WSe₂ lateral heterostructure.




To prove the universality of this approach, 2D/1D WS₂/MoS₂ lateral heterostructures were also synthesized by using the same method. Fig. 5(a) shows that the width of MoS₂ is about 4 μm when the growth time is 3 min. When the growth time is reduced to 1 min, the width of MoS₂ is reduced to 300 nm (Fig. 5(b)). As shown in Fig. 5(c), the width of MoS₂ is reduced to 200 nm with the growth time reduced to 30 s. These results show that our method can control the width of a 2D/1D heterostructure simply and effectively.






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Figure5.
(Color online) AFM phase image of WS₂/MoS₂ lateral heterostructure when the growth time are (a) 3 min, (b) 1 min, and (c) 30 s.




The Raman and PL analyses were conducted to further study the spectroscopy and photoluminescence performance of the as-grown lateral heterostructure. Fig. 6(a) shows the Raman spectra at three different locations of the WS₂, WS₂/MoS₂ interface and MoS₂. Two distinct peaks at 357 and 419 cm^–1 in the Raman spectrum of WS₂ (blue line) are consistent with those mentioned above. The Raman spectrum from the peripheral region display two peaks at 384 and 405 cm^?1 (red line in Fig. 6(a)), in agreement with the E' and A₁' resonance modes of MoS₂^[23], respectively. As expected, four peaks appear in the corresponding Raman spectrum of the WS₂/MoS₂ interface, indicating mixed Raman signals of both WS₂ and MoS₂. The PL spectra (Fig. 6(b)) acquired from the monolayer WS₂ (blue line) and MoS₂ (red line) show strong peaks at wavelengths of 633 and 670 nm, respectively, corresponding to the direct excitonic transition energy in monolayer WS₂ and MoS₂. The mixed peaks at 634 and 668 nm of the interface (black line) can be attributed to WS₂ and MoS₂, respectively. In the same way, the Raman and PL spectra of the WS₂/WSe₂ heterostructure are shown in Figs. 6(c) and 6(d). The Raman spectrum of WS₂/WSe₂ shows three distinct peaks at 257, 356, 413 cm^–1, which are consistent with the Raman spectrum of monolayer WSe₂ and WS₂. The mixed PL signals (Fig. 6(d)) also can be observed at the interface of WS₂/WSe₂.






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Figure6.
(Color online) Raman and PL spectrum of WS₂/MoS₂ and WS₂/WSe₂ heterostructures. (a) Raman spectrum of the WS₂/MoS₂ heterostructure. (b) PL spectrum of the WS₂/MoS₂ heterostructure. (c) Raman spectrum of the WS₂/WSe₂ heterostructure. (d) PL spectrum of the WS₂/WSe₂ heterostructure.

4.
Conclusion

In summary, we successfully synthesized 2D/1D WS₂/WSe₂ (WS₂/MoS₂) lateral heterostructures by a bidirectional flow CVD reactor. WSe₂ and MoS₂ with different widths can be epitaxially grown on the edge of WS₂. The width of the WSe₂ can be tuned from 11 nm to 4 μm by precisely controlling the growth time. AFM and SEM images show that the interface of the 2D/1D heterostructures is clear and smooth. Our investigation provides a new method for the preparation of ultranarrow 1D nanoribbons and breaks new ground in the future study of the 2D/1D lateral heterostructures.

Acknowledgements

The authors at Hunan University acknowledge the support from National Natural Science Foundation of China (No. 51872086), the Hunan Key Laboratory of Two-Dimensional Materials (Grant No. 2018TP1010) and the Innovative Research Groups of Hunan Province (Grant No. 2020JJ1001) for the work conducted at Hunan University.