Transforming Monolayer Transition-Metal Dichalcogenide Nanosheets

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Functional Nanostructured Materials (including low-D carbon)

Transforming monolayer transition metal dichalcogenide nanosheets into one-dimensional nanoscrolls with high photosensitivity Xiangru Fang, Pei Wei, Lin Wang, Xiaoshan Wang, Bo Chen, Qiyuan He, Qiuyan Yue, Jindong Zhang, Weihao Zhao, Jialiang Wang, Gang Lu, Hua Zhang, Wei Huang, Xiao Huang, and Hai Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01856 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Applied Materials & Interfaces

Transforming monolayer transition metal dichalcogenide nanosheets into one-dimensional nanoscrolls with high photosensitivity Xiangru Fang,1 Pei Wei,1 Lin Wang,1 Xiaoshan Wang,1 Bo Chen,2 Qiyuan He,2 Qiuyan Yue,1 Jindong Zhang,1 Weihao Zhao,1 Jialiang Wang,1 Gang Lu,1 Hua Zhang,2 Wei Huang,1,3 Xiao Huang,1* Hai Li1* 1

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu

National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.R. China 2

Center for Programmable Materials, School of Materials Science and Engineering, Nanyang

Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore 3

Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127

West Youyi Road, Xi'an 710072, China *To

whom

correspondence

should

be

addressed.

[email protected]

ACS Paragon Plus Environment

Email:

[email protected];

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract One-dimensional (1D) nanoscrolls derived from two-dimensional (2D) nanosheets own unusual physical and chemical properties that arise from the spiraled 1D morphology and the atomic-thin 2D building blocks. Unfortunately, preparation of large-sized nanoscrolls of transition metal dichalcogenides (TMDCs) remains a big challenge, which greatly restricts the fabrication of single-scroll devices for their fundamental studies and further applications. In this work, we report a universal and facile method, by making use of the evaporation process of volatile organic solvent, to prepare TMDC (e.g. MoS2 and WS2) nanoscrolls with lengths of several tens to one hundred micron from their 2D precursors pre-synthesized by chemical vapor deposition (CVD) on Si/SiO2. Both atomic force microscopy and electron microscopy characterizations confirmed the spirally rolled-up structure in the resulting nanoscrolls. An interlayer spacing of as small as ~0.65 nm was observed, suggesting the strong coupling between adjacent layers, which was further evidenced by the emergence of new breathing mode peaks in the ultralow frequency Raman spectrum. Importantly, compared with the photodetector fabricated from a monolayer MoS2 or WS2 nanosheet, the device based on an MoS2 or WS2 nanoscroll showed the much enhanced performance, respectively, with the photosensitivity greatly increased of up to two orders of magnitude. Our work suggests that turning 2D TMDCs into 1D scrolls is promising in achieving high performances in various electronic/optoelectronic applications, and our general method can be extended to the preparation of large-sized nanoscrolls of other kinds of 2D materials that may bring about new properties and phenomena.

Keywords: transition metal dichalcogenides, chemical vapor deposition, evaporation, nanoscroll,


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monolayer, photosensitivity, spiral

1.

Introduction

Owing to their outstanding physical and chemical properties, two-dimensional (2D) nanomaterials have attracted great attention over the past years.1-7 Recently, it has been reported that conversion of 2D nanosheets into one-dimensional (1D) nanostructures such as nanofibers, nanotubes and nanoscrolls,8-27 is a promising way to further extend their applications in battery, sensing, filtration and photocatalytic processes.11, 28, 29 These kinds of 1D nanostructures not only retain the excellent properties of their 2D hosts, but also display novel properties that arise from their 1D geometric arrangement. Amongst these 1D nanostructures, nanoscrolls prepared from spirally rolling up 2D nanosheets are especially attractive, as based on theoretical predictions, they are anticipated to show unusual electronic, mechanical and optical properties compared with their host nanosheets or thin films, due to their unique tubular structures, open ends and adjustable interlayer distances.11, 16, 30-32 As the most representative 2D material, graphene-based nanoscrolls have been intensively investigated, thanks to the large-area and large-size preparation of graphene on substrates as well as the scalable synthesis of graphene derivatives in solution. Importantly, graphene-based nanoscrolls have demonstrated enhanced performance compared with their 2D counterpart in various fields such as supercapacitors,28 batteries,29 flexible electronic devices27 and sensing.15, 26 In contrast to graphene, transition metal dichalcogenides (TMDCs) have wider band gaps and larger on/off ratios, which make them ideal candidates in the fields of flexible electronics,33 valleytronics,34 electro-mechanics35 and straintronics.36 Therefore, TMDC nanoscrolls are expected to exhibit promising properties and functions. However, up to now, only few reports have explored the formation of MoS2 nanoscrolls.20,



22, 24, 37, 38

For example, the edge of a fractured monolayer MoS2 can be partially rolled up by the

temperature difference-induced strain during the chemical vapor deposition (CVD) process.20 Similarly, weak argon plasma treatment can roll up the edges of CVD-grown MoS2 to form short straight scrolls or scroll kinks with lengths less than 1 µm.22 Liquid-exfoliated MoS2 nanosheets can also form short scrolls with lengths less than 1 µm by being mixed with amine-functionalized amphiphilic organic molecules in solution.24, 37, 38 Aforementioned MoS2 nanoscrolls are usually less than 1 µm or partially scrolled on edges, making it extremely difficult to fabricate single-scroll devices for further characterizations and applications. Therefore, exploring an effective approach to obtain high-quality MoS2 nanoscrolls with lengths larger than 10 µm is of great importance for fundamental research and practical applications.

In this work, we presented a versatile method to prepare high-quality MoS2 and WS2 nanoscrolls with lengths of several tens to one hundred micron in large scale. After monolayer MoS2 or WS2 nanosheets were grown by CVD, a drop of volatile organic solvent was deposited onto them. During the evaporation of the organic solvent, the MoS2 or WS2 nanosheets were rolled up rapidly till forming straight MoS2 or WS2 nanoscrolls. Straight spiraled MoS2 and WS2 nanoscrolls were clearly observed in atomic force microscopy (AFM) and scanning electron microscopy (SEM) images. Transmission electron microscopy (TEM) characterization shows the close-packed structure of nanoscroll. Ultralow frequency (ULF) Raman spectroscopy characterization indicates new breathing modes are observed in MoS2 nanoscrolls. Compared with the monolayer nanosheets, the MoS2 and WS2 nanoscrolls showed much improved photosensitivity by two orders of magnitude.

2.

Results and discussion


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The whole experimental process for preparing MoS2 and WS2 nanoscrolls is schematically illustrated in Scheme 1. We take MoS2 as an example to illustrate it in detail. After monolayer MoS2 nanosheets were grown on SiO2/Si by CVD (steps a-b in Scheme 1), a drop of volatile organic solvent (ethanol) is deposited on the substrate to cover the MoS2 nanosheets (step c in Scheme 1). As ethanol has low surface tension and is amphiphilic, it wets MoS2 and SiO2/Si substrate completely and a very thin layer was formed near the contact line during evaporation (magnified inset shown in Scheme 1d).39,

40

Since ethanol is volatile, it evaporates quickly under ambient conditions and

temperature gradient is produced near the contact line,41 resulting in surface tension gradient near the contact line.42 It has been reported that such surface tension gradient can induce fluid flow (convection, Marangoni flow) in the ethanol thin layer.40, 43 Therefore, the resulting fluid flow near the contact line could roll up the edge of MoS2 nanosheet. As-curled MoS2 nanosheet continues the rolling process with the movement of receding contact line until complete MoS2 scroll is formed finally (magnified inset shown in Scheme 1e). Figures S1-S2 in the Support Information (SI) show the detailed process to transform the monolayer MoS2 and WS2 nanosheets into the nanoscrolls during ethanol evaporation. Due to the random direction of fluid flow in ethanol thin layer, various scrolling behaviors of MoS2 and WS2 nanosheets were observed in our experiments (Figures S1-S2 and Video S1 in SI).

Figure 1a shows the optical microscopy (OM) image of CVD-grown triangle shaped MoS2 nanosheet. As shown in AFM image (Figure 1b), as-grown MoS2 nanosheet has clean surface and a height of 0.9 nm, indicating it is monolayer. As shown in OM image (Figure 1c), monolayer MoS2 nanosheet was transformed into straight nanoscroll after the ethanol thin layer onto it was completely evaporated. It has been reported that the thickness of MoS2 nanosheet is correlated to its color in OM



image.44 Both ends of the nanoscroll show light blue color, implying these parts might have lower height compared with that of the central part of nanoscroll, which shows gray blue color. As shown in Figure 1d, the parts of nanoscroll with different color have different heights. The top and bottom parts of nanoscroll with light blue color show the height of 28.9 and 18.8 nm (Figure S3 in SI), respectively. While the central part with gray blue color shows a height of 51.5 nm (Figure S3 in SI). As shown in the magnified AFM images (Figures 1e-f), MoS2 nanoscroll with spiraled structure can be clearly observed, which is similar to the schematic inset shown in Figure 1d. From the steps shown in the central part of nanoscroll, the MoS2 nanosheet should roll and fold for 10 cycles to form the close-packed nanoscroll (Figure 1e). The corner of MoS2 nanosheet is clearly indicated by the white arrow shown in Figure 1e, implying the scroll process starts from the left corner to right corner to form the good scroll structure. The bottom part of nanoscroll is consisting of MoS2 nanosheet folded by 4 cycles (Figure 1f). Many MoS2 nanoscrolls with similar structure have also been observed (Figures S4-S8 in SI), confirming our proposed formation of nanoscrolls. SEM measurement shows the smooth surface and round shaped structure of MoS2 nanoscroll (Figure S9 in SI). As shown in TEM images (Figures 1g-h), the center of MoS2 nanoscroll shows darker contrast compared with the outer part, indicating more MoS2 layers are close-packed in the center. The high-resolution TEM image clearly shows the layered structure and the distance between neighboring layers is ~ 0.65 nm (Figure 1h), which is consistent with the thickness of monolayer MoS2, further confirming the layer-by-layer stacking of MoS2 nanosheet in the nanoscroll.

Figures 2a-b show the ULF and high frequency Raman spectra of MoS2 nanosheet and nanoscroll, respectively. There is no notable peak in the ultralow frequency (ULF) range (< 50 cm-1) for MoS2 nanosheet, further confirming it is monolayer (Figure 2a).45 However, there are three broad peaks


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located at 21, 26 and 38 cm-1 in the ULF Raman spectrum of MoS2 nanoscroll, respectively, which are disappeared under cross polarization, indicating they are layer breathing (LB) mode peaks.46 These results indicate that there is LB coupling between the concentric layers of MoS2 nanoscroll. In ଵ the high frequency range, MoS2 nanoscroll shows Eଶ௚ and ‫ܣ‬ଵ௚ peaks located at 380 and 403 cm-1,

respectively. Compared to monolayer MoS2 nanosheet, the Eଵଶ௚ peak clearly red-shifts for 4 cm-1, ଵ while ‫ܣ‬ଵ௚ peak slightly blue-shifts for 1 cm-1. It has been reported that the Eଶ௚ peak shows blue ଵ shift under uniaxial and biaxial strains.47 Similarly, the Eଶ௚ peak of MoS2 nanotube also shows blue ଵ shift compared to bulk MoS2.48 While the red shift of Eଶ௚ peak is found on top of MoS2 wrinkle,

which has a curved surface with the maximum tensile uniaxial strain of ~ 2%.49 MoS2 nanoscroll ଵ also has curved surface like a spiral. Therefore, the large red shift of Eଶ௚ peak of MoS2 nanoscroll

could also be attributed to the curved surface under tensile uniaxial strain. Moreover, such red shift ଵ of Eଶ௚ peak have also been found in short MoS2 nanoscrolls prepared in solution38 and by argon

plasma treatment,22 further confirming the formation of MoS2 scroll by our method. The PL peak of MoS2 nanosroll shows a red shift of 14 nm compared to that of monolayer MoS2. Meanwhile, the photoluminescence (PL) peak intensity of MoS2 nanoscroll is largely decreased to one-sixth of that of monolayer MoS2, indicating the MoS2 nanoscroll shows similar PL characteristic to that of multilayer MoS2. Besides ethanol, we also tried to roll up MoS2 nanosheets by using other volatile organic solvents, such as acetone, ethanediol and dichloromethane. As shown in Figure S10 in SI, MoS2 nanoscrolls were successfully prepared after the evaporation of different solvents. In addition, graphene oxide nanosheets can also be scrolled by using ethanol (Figure S11 in SI), further indicating the versatility of our method.

CVD-grown WS2 nanosheets were also involved to investigate whether WS2 nanoscrolls can be



prepared. As shown in Figure 3a, triangle shaped WS2 nanosheet with size of several tens of micron was grown by CVD. AFM measurement shows that the height of as-grown WS2 nanosheet is 0.76 nm (Figure 3b), indicatng it is monolayer. Figures 3c-d show the OM and AFM images of a straight WS2 nanoscroll with length of ~ 35 µm, which is transformed from a triangle shaped monolayer WS2 nanosheet and marked by a red dashed box shown in Figure 3c. According to the scrolling process shown in Scheme S1 in SI, the WS2 nanosheet is proposed to scroll from the right corner to left corner of the triangle along the horizontal direction. The schematic nanoscroll is shown as an inset in Figure 3c to demonstrate the structure of WS2 nanoscroll. AFM measurement indicates that the heights of top, middle and bottom parts of WS2 nanoscroll are 66.1, 175.5 and 327.5 nm (Figure 3d and Figure S12 in SI), respectively. The bottom part of scroll is rolled up from the longest part of nanosheet and thus forms the highest nanoscroll. For the top part of nanoscroll, it is rolled up from the WS2 nanosheet with a length of several microns, thus the height is quite lower compared to that of bottom part. SEM characterization also indicates that the WS2 nanoscroll is transformed from the triangle shaped nanosheet (Figure S13 in SI). TEM characterization shows that WS2 nanoscroll has tubular structure with open end (Figure 3e). As shown in magnified TEM image (Figure 3f), the interlayer spacing of WS2 nanoscroll is ~ 0.69 nm, indicating it is consisting of closely wrapped monolayer WS2 nanosheets.

Raman and PL spectroscopy were used to characterize WS2 nanoscroll and monolayer nanosheet, respectively. As shown in Figure 4a, CVD-grown monolayer WS2 shows a weak peak at 28 cm-1 in the ULF range, which is assigned to transvers acoustic (TA) mode.50 As previously reported,51 CVD-grown monolayer WS2 shows a strong PL peak at 630 nm (Figure 4b). Similar to multilayer nanosheet, WS2 nanoscroll also shows two strong Raman peaks located at ~ 28 and 45 cm-1 in the


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ULF range, which are assigned to TA and longitudinal acoustic (LA) modes (Figure 4a),50 respectively. The TA mode is greatly decreased under cross polarization while LA mode is polarization independent, which is consistent with previously reported result.50 Moreover, there is no detectable lateral in-plane interaction in WS2 nanoscroll as no shear mode peak is observed in the ULF range. Similar to MoS2 nanoscroll, WS2 nanoscroll shows a quite weaker PL peak at 638 nm compared with that of monolayer WS2 (630 nm).

We found the interaction between the CVD-grown MoS2 and WS2 nanosheets and SiO2/Si substrate plays an important role in preparing nanoscrolls. If the interaction is too strong, only the edges of MoS2 or WS2 nanosheets are rolled up (Figure S14 in SI). Sometimes partially scrolled MoS2 or WS2 nanostructures are also observed, which could also be attributed the strong interaction between nanosheets and substrate (Figures S15-S17 in SI). Appropriate interaction can be tuned by controlling the growth temperature during the CVD process, that is, the distance of SiO2/Si substrate from the center of heating zone. In our study, the optimized distance is 4 cm downstream away from the center of furnace. We also found that the morphology of MoS2 and WS2 nanosheet is another important parameter for preparing nanoscrolls. For the isolated triangle shaped nanosheet, usually a single spiraled nanoscroll is formed from it (Figure 1d, Figure 3d and Figures S18-S19 in SI). While the MoS2 nanoscrolls and scroll kinks with length up to several tens of micron can be fabricated by dropping ethanol droplet onto CVD-grown continuous monolayer MoS2 film (Figure S20 in SI). It has been reported that there is fracture in large-sized MoS2 nanosheets and continuous film after CVD growth.20, 22 During the evaporation of ethanol, the fluid flow in ethanol thin layer near contact line would roll up the MoS2 film from the fracture, thus the long MoS2 nanoscrolls are fabricated.



CVD-grown monolayer MoS2 and WS2 nanosheets have been reported for effective photodetection.

52

It is curious to investigate whether MoS2 and WS2 nanoscrolls show good

performance in photodetection. Besides photoresponsivity, photosensitivity is also an important parameter to evaluate the performance of photodetector.53-55 Photosensitivity is usually described by the photocurrent-to-dark-current ratio (PDR),54, 55 which is defined by the following equation,

PDR=Iphoto/Idark

(1)

Figures 5a-b show the PDRs of photodetectors based on the MoS2 nanosheet and nanoscroll at a bias voltage of 0.1 V under blue and red illumination (Figure S21 in SI), respectively. Figure 5a shows the PDR of MoS2 nanosheet-based photodetector under a 633 nm laser (red light). The photocurrent was increased by ~ 3 times compared with the dark current under a power density of 22.5 mW/mm2 (Figure S22a in SI). However, the MoS2 nanoscroll-based photodetector showed a PDR of ~ 46 (red curve in Figure 5a), which is 10 times higher than that of the MoS2 nanosheet-based photodetector (black curve in Figure 5a), implying the MoS2 nanoscroll presents a high sensitivity at the same condition. In addition, the response and recovery time of MoS2 nanoscroll-based photodetector are 9.5 and 3.5 s (Figure S23a in SI),56 respectively, which are much shorter than those of MoS2 nanosheet-based photodetector (60 and 9.5 s, respectively, Figure S23b in SI). Interestingly, the MoS2 nanoscroll-based photodetector also showed improved photosensitivity under 405 nm (blue light) and 520 nm (green light, see Figure S24 in SI) laser. Figure 5b shows the PDRs of photodetectors based on MoS2 nanosheet and nanoscroll under the blue light with a power density of 2.56 mW/mm2, respectively. MoS2 nanoscroll-based photodetector showed a PDR of ~ 400 (blue curve in Figure 5b), which is ~ 100 times higher than that of MoS2 nanosheet-based


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photodetector (black curve in Figure 5b and Figure S22c in SI). MoS2 nanoscroll-based photodetector showed slight shorter response time than that of MoS2 nanosheet-based photodetector under blue light (27 and 60 s, respectively). However, the recovery time of MoS2 nanoscroll-based photodetector is 0.7 s (Figure S23c in SI), which is two orders of magnitude shorter than that of MoS2 nanosheet-based photodetector (95s, see Figure S23d in SI). Meanwhile, MoS2 nanoscroll-based photodetector showed linear response under illumination with various power density (Figure S25 in SI), indicating the reliability of MoS2 nanoscroll-based photodetector. Furthermore, WS2 nanoscroll and nanosheet-based photodetectors were also fabricated to investigate their photosensitivities. As shown in Figure 5c, WS2 nanoscroll-based photodetector shows a PDR of ~ 72, which is 8 times higher than that of monolayer WS2 nanosheet-based photodetector (PDR of ~ 9, see Figure S22b in SI) under red light. Similar to MoS2 nanoscroll, WS2 nanoscroll-based photodetector also shows an improved sensitivity to blue light compared with that of WS2 nanosheet-based photodetector. As shown in Figure 5d, WS2 nanoscroll-based photodetector has a PDR of ~ 650 while monolayer WS2-based photodetector has a PDR of ~ 16 (Figure S22d in SI). Moreover, both WS2 nanoscroll and nanosheet showed quick response to incident light. The response and recovery time of them are less than 2 s both under red and blue light (Figure S26 in SI), indicating the better optoelectronic property of WS2 nanostructures. Monolayer MoS2 and WS2 nanosheets are both semiconductors with direct band gap and have excellent optoelectronic performance.53, 56 In addition, both monolayer MoS2 and WS2 nanosheets have good transmittance for visible light.57 Therefore, it is possible for light to permeate the concentric layers of MoS2 and WS2 nanoscroll and the photocurrent could be generated in each layer of nanoscroll to improve the photoresponse. Meanwhile, monolayer MoS2 or WS2 nanosheet has large surface to volume ratio,



thus the performance of MoS2 or WS2 nanosheet-based photodetector is sensitive to the surrounding environment. It has been reported that the adsorbates on MoS2 nanosheets in ambient conditions, such as H2O and O2 molecules, largely decrease the photoresponse and mobility of MoS2 nanosheet-based devices.58, 59 Compared to monolayer nanosheet, MoS2 and WS2 nanoscrolls have quite small surface to volume ratio. Therefore, the influence of adsorbates on the performance of MoS2 and WS2 nanoscrolls-based photodetectors can be mostly avoided, thus the MoS2 and WS2 nanoscroll-based photodetectors show better performance compared to those of MoS2 and WS2 nanosheet-based photodetectors. Moreover, the 1D structure of nanoscroll could confine the electron transportation along the 1D axis,60 which will further improve the photosensitivity of nanoscroll.

3.

Conclusion

In summary, high-quality MoS2 and WS2 nanoscrolls with length of several tens of micron have been successfully prepared from CVD-grown monolayer MoS2 and WS2 nanosheets in large scale. During the evaporation of volatile organic solvent, such as ethanol, the edge of monolayer MoS2 and WS2 nanosheet was firstly rolled up by the surface tension gradient induced fluid flow near the contact line. As-formed curled MoS2 or WS2 nanosheet was continued being rolled up with the movement of receding ethanol thin layer. AFM and OM measurements showed that the spiraled MoS2 and WS2 nanoscrolls have a height of several tens to hundreds of nanometer with length up to several tens of micron. High-resolution TEM image showed that such nanoscroll was consisted of close-wrapped monolayer nanosheet. New breathing mode peaks appeared in the ULF Raman spectra of MoS2 nanoscrolls, indicating the existence of interlayer LB coupling. Polarization independent LA mode was found in the ULF Raman spectra of WS2 nanoscrolls, implying the


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ଵ different vibrational properties of nanoscrolls from monolayer. Meanwhile, the Eଶ௚ peak of MoS2

nanoscroll red-shifts for 4 cm-1, which could be attributed to the curved surface of nanoscroll. Compared to monolayer nanosheets, MoS2 and WS2 nanoscrolls showed much better photosensitivities and larger PDRs. MoS2 nanoscrolls showed much shorter response and recovery time compared with those of MoS2 nanosheets. Our method not only provide a versatile way to obtain nanoscrolls from various 2D nanosheets but also shed light on the potential optoelectronic applications of nanoscrolls.

4.

Experimental Section

4.1 Growth of monolayer MoS2 and WS2 nanosheets by chemical vapor deposition.

The CVD process was performed in a single heating-zone tube furnace under ambient pressure. 2 mg MoO3 powder (99.9%, Aladdin) was placed on the center of a quartz boat, which was located 4 cm downstream away the center of heating zone. After the 300 nm Si/SiO2 substrate was treated by the seeding promoter perylene-3, 4, 9, 10-tetracarboxylic acid tetra-potassium salt (PTAS),61 it was put face-down on top of the quartz boat. Another quartz boat with 0.2 g sulfur powder (99.9%, Aladdin) was placed upstream of the tube. The growth temperature was controlled at 710 °C and the growth time was kept for 5 minutes with 40 sccm N2 was used as carrying gas. For the CVD growth of WS2, 0.4 g sulfur powder and 2 mg WO3 (99.9%, Aladdin) were used as precursor. The CVD process is same to the that of MoS2 growth except the temperature was controlled at 770 °C.

4.2 Fabrication of MoS2 and WS2 nanoscrolls.

After the 300 nm Si/SiO2 substrate with CVD-grown monolayer MoS2 or WS2 nanosheets was cut into 5*5 mm chip, a drop of 5 µl ethanol was deposited onto it. During the evaporation of ethanol,



the MoS2 or WS2 nanosheets will be rolled up until nanoscrolls were formed finally. The formation of nanoscroll is usually completed within 2 min. Figures S1-S2 in SI demonstrate the detailed scrolling processes of MoS2 nanosheets and WS2 nanosheets, respectively.

4.3 OM, AFM, SEM, TEM, Raman and PL characterizations.

The optical microscope (Axio Scope A1, Zeiss), atomic force microscope (Dimension ICON with Nanoscope V controller, Bruker), scanning electron microscope (JEOL JSM-6700) and transmission electron microscope (JEOL JEM-2100) were used to characterize as-prepared nanoscrolls. In addition, Raman and photoluminescence spectra of as-grown MoS2 and WS2 nanosheets as well as nanoscrolls were collected on a LabRAM HR Evolution Raman spectrometer (Horiba Jobin Yvon) with a 532 nm laser focused through a 100× objective lens.

4.4 Device fabrication and measurement.

Photodetectors based on MoS2 or WS2 nanosheets and nanoscrolls were fabricated as follows. 50 nm thick gold and 10 nm thick chromium were deposited on CVD-grown nanosheets and as-prepared nanoscrolls by using TEM grid (300 mesh) as mask in a thermal evaporator, respectively. Keithley 4200 semiconductor characterization system was used to monitor the real-time current change of as-prepared devices under red, blue and green laser (λ=633nm, 405nm and 520nm), respectively. The power densities for red laser are 22.5, 17.5, 10.25 and 5.5 mW/mm2, respectively. The power density for blue laser is 2.56 mW/mm2. The power density for green laser is 1.13 mW/mm2.

Acknowledgement


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This work was supported by the National Natural Science Foundation of China (Grant No. 21571101, 51322202), the Natural Science Foundation of Jiangsu Province in China (Grant No. BK20161543, BK20130927), the Joint Research Fund for Overseas Chinese, Hong Kong and Macao Scholars (Grant No. 51528201), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 15KJB430016). H.Z. thanks the support from MOE under AcRF Tier 2 (ARC 19/15, No. MOE2014-T2-2-093; MOE2015-T2-2-057; MOE2016-T2-2-103; MOE2017-T2-1-162) and AcRF Tier 1 (2016-T1-001-147; 2016-T1-002-051; 2017-T1-001-150), NTU under Start-Up Grant (M4081296.070.500000) in Singapore. We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy (and/or X-ray) facilities. Corresponding Author ∗E-mail: [email protected]; [email protected] Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available. In-situ observation of formation process of MoS2 and WS2 nanoscrolls. OM, AFM and SEM images of MoS2 and WS2 nanoscrolls transformed from isolated triangle shaped monolayer nanosheet and continuous film. MoS2 nanoscrolls fabricated by dropping acetone, ethanediol and dichloromethane. OM and AFM images of partially scrolled MoS2 and WS2 nanoscrolls. OM images of photodetectors based on MoS2 nanoscroll and nanosheet. Plots of photocurrent-to-dark-current ratio of photodetectors based on MoS2 and WS2 nanosheets under red, blue and green laser. Plots of response and recovery time of photodetectors based on MoS2 as well as WS2 nanoscroll and nanosheet. Plot of real-time photocurrent of photodetector based on MoS2 nanoscroll under 633 nm laser with different power density. Schematic illustration of the process for transforming a straight MoS2 or WS2 nanoscroll from a triangle shaped monolayer nanosheet. This material is available free of charge via the Internet at XXX.



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Scheme 1. Schematic illustration of the process for preparing MoS2 and WS2 nanoscrolls. (a-b) CVD growth of monolayer MoS2 or WS2 nanosheets. (c) A drop of ethanol is deposited on MoS2 or WS2 nanosheets. (d) The edges of MoS2 or WS2 nanosheets are rolled up by the fluid flow near the contact line. (e) Complete MoS2 or WS2 nanoscrolls are formed with the movement of receding ethanol thin layer.


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Figure 1. (a-b) Optical Microscope (OM) and corresponding AFM images of the CVD-grown monolayer MoS2 nanosheet, respectively. (c-d) OM and corresponding AFM images of MoS2 nanoscroll transformed from monolayer nanosheet. Inset in (d) shows the schematic structure of MoS2 nanoscroll. The dashed lines and text shown in (d) indicate the heights of top, middle and bottom parts of MoS2 nanoscroll. (e-f) Magnified AFM images of top and bottom parts of MoS2 nanoscroll shown in (d), respectively. Close-wrapped structure can be clearly observed in (e-f). (g) TEM image of as-prepared MoS2 nanoscroll. (h) High-resolution TEM image of MoS2 nanoscroll shown in (g). Inset shows the layered structure of MoS2 nanocroll.



Figure 2. Raman spectra of MoS2 nanoscroll and nanosheet under the XX and XY polarization at (a) ଵ ultralow frequency and (b) the Eଶ௚ and ‫ܣ‬ଵ௚ mode regions, respectively. (c) Photoluminescence

spectra of MoS2 nanoscroll and nanosheet, respectively.


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Figure 3. (a-b) Optical Microscope (OM) and corresponding AFM images of the CVD-grown monolayer WS2 nanosheet, respectively. (c-d) OM and corresponding AFM images of WS2 nanoscroll transformed from monolayer nanosheet. Inset in (c) shows the schematic structure of WS2 nanoscroll. The dashed lines and text shown in (d) indicate the heights of top, middle and bottom parts of WS2 nanoscroll. (e) TEM image of as-prepared WS2 nanoscroll. (f) High-resolution TEM image of WS2 nanoscroll shown in (e). The layered structure of WS2 nanocroll can be clearly observed in (f).



Figure 4. (a) Raman spectra of WS2 nanoscroll and nanosheet under the XX and XY polarization at ultralow frequency region, respectively. (b) Photoluminescence spectra of WS2 nanoscroll and nanosheet, respectively.


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Figure 5. Plots of photocurrent-to-dark-current ratios (PDRs) of photodetectors based on MoS2 nanosheets and nanoscrolls under (a) 633 nm and (b) 405 nm laser with power density of 22.5 and 2.56 mW/mm2, respectively. Plots of PDRs of photodetectors based on WS2 nanosheets and nanoscrolls under (c) 633 nm and (d) 405 nm laser with power density of 22.5 and 2.56 mW/mm2, respectively.



TOC


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Transforming Monolayer Transition-Metal Dichalcogenide Nanosheets

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