Ordered and Atomically Perfect Fragmentation of Layered Transition

Aug 15, 2017 - Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 63...
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Ordered and Atomically Perfect Fragmentation of Layered Transition Metal Dichalcogenides via Mechanical Instabilities

Ming Chen,†,# Juan Xia,‡,# Jiadong Zhou,§,# Qingsheng Zeng,§ Kaiwei Li,† Kazunori Fujisawa,∥ Wei Fu,§ Ting Zhang,† Jing Zhang,† Zhe Wang,† Zhixun Wang,† Xiaoting Jia,⊥ Mauricio Terrones,∥ Ze Xiang Shen,‡ Zheng Liu,*,†,§ and Lei Wei*,† †

School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore School of Physical and Mathematical Sciences, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore § Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore ∥ Department of Physics and Center for 2-Dimensional and Layered Materials, Pennsylvania State University, University Park, Pennsylvania 16802, United States ⊥ Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0111, United States ‡

S Supporting Information *

ABSTRACT: Thermoplastic polymers subjected to a continuous tensile stress experience a state of mechanical instabilities, resulting in neck formation and propagation. The necking process with strong localized strain enables the transformation of initially brittle polymeric materials into robust, flexible, and oriented forms. Here we harness the polymer-based mechanical instabilities to control the fragmentation of atomically thin transition metal dichalcogenides (TMDs). We develop a simple and versatile nanofabrication tool to precisely fragment atom-thin TMDs sandwiched between thermoplastic polymers into ordered and atomically perfect TMD nanoribbons in arbitrary directions regardless of the crystal structures, defect content, and original geometries. This method works for a very broad spectrum of semiconducting TMDs with thicknesses ranging from monolayers to bulk crystals. We also explore the electrical properties of the fabricated monolayer nanoribbon arrays, obtaining an on/off ratio of ∼106 for such MoS2 arrays based field-effect transistors. Furthermore, we demonstrate an improved hydrogen evolution reaction with the resulting monolayer MoS2 nanoribbons, thanks to the largely increased catalytic edge sites formed by this physical fragmentation method. This capability not only enriches the fundamental study of TMD extreme and fragmentation mechanics, but also impacts on future developments of TMD-based devices. KEYWORDS: advanced nanomaterials, transition metal dichalcogenides (TMDs), controlled fragmentation, mechanical instabilities, necking process, hydrogen evolution reaction

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force microscopy (AFM) tip or an in-plane nanomechanical testing system, both the Young’s modulus and the breaking strength of monolayer and multilayer TMDs are measured, demonstrating that TMDs are promising candidates for fabrication of flexible devices.9−14 However, due to the fragile nature of TMDs from the presence of defects and unavoidable imperfections, fragmentation of TMDs is uncontrollable and tends to initiate from these pre-existing random defects, resulting in irregular morphologies, which have essentially limited their studies and applications.

wo-dimensional layered semiconductors, notably the transition metal dichalcogenides (TMDs) have enabled a broad range of studies and applications in electronics,1,2 optoelectronics,3,4 sensing5 and catalysis,6,7 owing to their sizable bandgaps, attractive carrier mobilities and catalytic activities, as well as their mechanical properties. Their mechanical properties, particularly the extreme and fragmentation mechanics, are of both fundamental and technological importance, as they reveal deformation physics on atom-thick layers with impact on various potential applications. For example, the dynamics of cracks in monolayer MoS2 has been studied by popping the membrane using focused electron beam, indicating that the vacancy defects can modulate the toughness.8 Also, with the assistance of an atomic © 2017 American Chemical Society

Received: June 14, 2017 Accepted: August 15, 2017 Published: August 15, 2017 9191

DOI: 10.1021/acsnano.7b04158 ACS Nano 2017, 11, 9191−9199

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ACS Nano Here we show a route to control the fragmentation of TMDs monolayers by exploiting polymer-based mechanical instabilities. When stretching a thermoplastic polymer, it first extends elastically until the yield stress, and then the consequent downward bending of the stress−strain curve leads to mechanical instabilities within the polymer, thus resulting in neck formation with strong localized strain. The neck forms locally and then propagates along the polymer at a constant speed until the neck extends throughout the entire length of the polymer.15−18 We take advantage of this strong localized yet propagating strain to fragment TMDs monolayers sandwiched between thermoplastic polymers, and demonstrate the formation of well-ordered atomically sharp and thin TMD nanoribbons. We systematically explore the surface morphology and crystal structure of these monolayer nanoribbons, and further demonstrate the universality of this method in several typical TMDs monolayers such as WS2, MoS2, MoSe2, and WSe2. Moreover, to evaluate the potential applications of the nanoribbon arrays by this physical fragmentation method, both field-effect transistor (FET) measurement and electrocatalytic performance for the hydrogen evolution reaction (HER) are carried out. These results may enrich the fundamental study of TMD extreme and fragmentation mechanics and lead to the development of TMD material-based flexible devices for electrical, optoelectronic, sensing and catalytical applications.

RESULTS AND DISCUSSION Fragmentation of WS2 Monolayers via Mechanical Instabilities. Figure 1a illustrates the experimental steps for the formation of a periodic train of TMD nanoribbons. The starting monolayer WS2 (see Methods) is transferred onto a polycarbonate (PC) substrate by conventional poly(methyl methacrylate) (PMMA)-mediated transfer process, followed by a consolidation process (see Methods). After consolidation, the PMMA/WS2 monolayers/PC is mounted in the clamps of a linear travel stage for the following necking process, and the tensile load is measured with a load cell. After an initial linear extension, the mechanical instabilities of the PC film results in the formation of neck, accompanied by relative large amount of strain localized in the necking region.16,18 When the neck encounters the sandwiched WS2 monolayer, the localized strain reduces both the width and the thickness of PC (Figure S1 and S2), which subsequently pinches and fragments the localized WS2 monolayer into a nanoscale ribbon (Figure 1b−d, Figures S2 and S3). As the neck propagates further, the localized strain continues to fragment the monolayers into an ordered array of nanoribbons (Figure 1e,g−j). As depicted above, there is an increase of horizontal spacing and a decrease of longitudinal spacing between these monolayers compared to the original monolayers located on Si/SiO2 surface, as shown in Figure 1f,g. The present observations have the following features. First, fragmentation of WS2 monolayers is size-independent, as the original monolayer sizes from ∼1 μm to ∼100 μm (Figure 1e, Figure S4), thus demonstrating the broad size-range applicability of our necking method. Second, there are wrinkles formed on each monolayer nanoribbon (Figure S3). This is due to the lateral compression of the PC film during the necking process. The transverse strain is then transferred from PC film to WS2 monolayers by adhesive force between PC and WS2 monolayers, which caused the wrinkles of the WS2 monoalyers through compressive delamination.19−22 Third, we find the nanoribbons are parallel to each other and aligned along the

Figure 1. Fragmentation of monolayer WS2 via mechanical instabilities. (a) Schematic of the experimental steps to fragment monolayer WS2. (b) Optical image of a monolayer WS2 during the necking process, highlighting the propagating mechanical instabilities, wherein fragmentation occurs. The triangle represents 1L WS2. (c,d) Fluorescence images of monolayer WS2 during the necking process, corresponding to the dashed rectangles c and d in (b). (d) After necking propagation, WS 2 monolayers are fragmented into well-ordered nanoribbons. (c) Before necking process, WS2 monolayers are maintaining integrity. (e) Fluorescence image of a monolayer WS2 with sample size of ∼100 μm after necking process. (f) Fluorescence images of WS2 monolayers before (located on SiO2/Si surface) and (g) after necking process (drawing speed: 0.1 mm/s). (h−j) Magnified fluorescence images of monolayer WS2 nanoribbons with different orientations, corresponding to (h), (i) and (j) in (f) and (g).

strain direction, as shown in Figure. 1h−j and Figure S5. The observed fragmentation of TMDs monolayers in this work is fundamentally different from previously reported fragmentations as follows: (1) Fragmentation is independent of the defects and crystal structure of the TMDs monolayers. (2) Fragmentation is independent of the original size of the TMDs monolayers (from a few microns to more than 100 μm). This is because that a controlled form of shear-lag 19,20,22−26 fragmentation (SLF) is induced at the necking front (see Supporting Information for more discussion), in other words, the tensile strain is mainly focused on the propagating of necking front,16,18 thereby resulting in a localized brittle fragmentation. Spectroscopic Characteristics of Monolayer WS 2 Nanoribbons. Raman and photoluminescence (PL) spectroscopy are used to map and monitor the strain induced structure evolvement in monolayer nanoribbons. Figure 2a,b are the fluorescence images of two adjacent WS2 monolayers before (located on SiO2/Si surface) and after necking process. It clearly shows that monolayer WS2 triangles break into an ordered train of nanoribbons with almost uniform sizes. The 9192

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Figure 2. Spectroscopic characteristics of monolayer WS2 in different process steps. (a,b) Fluorescence images of WS2 monolayers before and after necking process (drawing speed: 0.2 mm/s), respectively. (c,d) Raman/Photoluminescence (PL) mapping images (mapping at the E′(Γ) + 2LA(M) mode and the strongest PL peak from the A exciton) of the monolayer WS2 nanoribbons shown in (b). (e) Representative Raman and PL spectra of monolayer WS2 in different process steps, namely original (blue), after transfer (red), after consolidation (green) and after necking process (pink). (f) Frequencies of E′(Γ) + 2LA(M) and A′1 modes/peak energy of monolayer WS2 in different process steps.

processes, which is because a local uniaxial strain is introduced into monolayers during the PMMA-mediated transfer process.29 After necking process, the E mode blue-shifts to 352.6 cm−1, owing to the stress relaxation of tungsten (W)-sulfur (S) bonds.30 Furthermore, the strain during the whole process has less effect on the A1′ mode compared with E mode, attributing to the out-of-plane vibration feature of A1′ mode.29 The observed slight shift of E mode and A′1 mode during the whole process indicates that the crystal structure remains high quality. The similar evolvement process can be also found in PL spectra as shown in Figure 2e. After transfer and consolidation, the PL peak (A exciton) shows an evident red shift (Figure 2e,f). This phenomenon is consistent with previous results.29 After necking process, the released local strain results in a widening of bandgap30 and finally causes the blue shift of PL peak. Microscopic Characteristics of Monolayer WS2 Nanoribbons. The atomic structure and quality of the monolayer WS2 nanoribbons are then visualized and examined by annular dark field scanning transmission electron microscopy (ADFSTEM). Figure 3a shows a resulting WS2 nanoribbon with a width of ∼0.8 μm. Figure 3b−d are the ADF-STEM images in the middle and along the edge of this nanoribbon, respectively. The corresponding fast Fourier transformation (FFT) patterns illustrate a typical 6-fold symmetry of crystal structure, which

corresponding Raman and PL maps of the nanoribbons are shown in Figure 2c and 2d, respectively. The Raman and PL maps are integrated in terms of E′(Γ) + 2LA(M) peak (∼350 cm−1) and A exciton (∼630 nm) to study the crystal structure and morphology of the monolayer WS2. The alternate blue and white-color Raman/PL intensities represent the nanoribbons and voids, respectively. Figure 2e illustrates the Raman and PL single spectra of the monolayer WS2 from as-grown to after-necking state. We first investigate the change of the Raman spectra of monolayer WS2. In detail, the original WS2 monolayer deposited on Si/SiO2 surface presents two Raman feature peaks at 417.6 and 352.5 cm−1 (Figure 2f), corresponding to A′1 mode due to out-ofplane vibrations and E mode due to in-plane vibrations, respectively. Specifically, the E mode contains E′(Γ) and 2LA(M) with close proximity, where both modes represent inplane atomic vibrations yet with W and S atoms moving in the same (E′(Γ)) or opposite direction (2LA(M)).27,28 Therefore, we simply nominate the modes E′(Γ) + 2LA(M) as E mode hereinafter and discuss their peak evolution feature together at different processes. We also performed Lorentzian fitting to separate the individual contributions of E′(Γ) and 2LA(M) mode, as shown in Supplementary Figure S6. The E mode is softened to 349.2 cm−1 after transfer and consolidation 9193

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Figure 3. ADF-STEM characterization of monolayer WS2 nanoribbons. (a) Low-magnification ADF-STEM image of a resulting monolayer WS2 nanoribbon with the sharp edges and the formation of wrinkles after necking process. Inset: schematic draw of this nanoribbon. (b−d) High resolution ADF-STEM images of the nanoribbon (flat area), corresponding to the green, blue and yellow rectangles in (a). Insets of (b− d) are the corresponding fast Fourier transform (FFT) images. A lattice spacing of 0.27 nm assigned to the (100) plane is shown in (b). (e) High resolution ADF-STEM image of the nanoribbon (wrinkled area), corresponding to the light-blue rectangle in (a). It shows the overlay of three monolayers. (f) Zoomed-in image in e exhibits a periodic moiré pattern. Inset of f is the corresponding FFT image. (g) Image of the edge of the wrinkled area with different numbers of layer (1L, 2L and 3L), corresponding to the purple rectangle in (a). (h,i) High resolution ADF-STEM images of sections in g enclosed in red and light-green rectangles. Inset of (h) is the corresponding FFT image. The red dashed lines in (c), (d), (i) show either armchair edges or zigzag edges.

different drawing speeds. Similar Raman/PL evolvements are observed in these monolayer TMDs (MoS2, MoSe2, and WSe2), as shown in Figure S8−S10. Several features of the mechanical instabilities induced fragmentation of TMDs are also highlighted in Figure 4. First, as shown in Figure 4c, the average width W̅ of the monolayer MoS2 and WS2 nanoribbons (∼0.72 μm) are relatively larger than that of MoSe2 and WSe2 (∼0.54 μm). This can be explained by the formula W̅ ∝ (E3Dγh)1/3,19 where E3D is the Young’s modulus, γ is the surface energy and h is the thickness (monolayer thickness: ∼0.7 nm, Figure S11) (see Supporting Information for more disscussion). Take MoS2 and MoSe2 as an example, the Young’s modulus E3D is 270 ± 100 GPa10 (264 ± 18 GPa)11 for monolayer MoS2, 177 ± 9.3 GPa13 for monolayer MoSe2. The DFT calculated lowest surface energy of MoS2 and MoSe2 are 1.83 J m−2 and 1.55 J m−2, respectively.13 Thus, it can be inferred from the above formula that W̅ MoS2 > W̅ MoSe2, showing good agreement with the experimental results. Second, the drawing speed is found to have little effect on the average width W̅ (Figure 4c), possibly due to the similar strain induced at the necking front. Third, we also observe that WSe2 multilayers (2L, 3L, and bulk) are also fragmented, as shown in Figure 4e.

confirms that the formation of WS2 nanoribbons exhibits high crystalline quality. Two different cracking edges, armchair and zigzag edges, are found after the necking process. Particularly, the cracks in monolayer WS2 prefer to propagate along the zigzag edges, owing to the relative lower surface energy of zigzag edges.13,31 Furthermore, the nearly perfect straight brittle fragmentation of monolayer WS2 is observed after necking process, as revealed from the highly ordered edges of the nanoribbon in Figure 3c, d and i. The atomic structure of the wrinkles is also characterized, as shown in Figure 3e−h. As expected, the folded region exhibits different moiré patterns for 2-layer (2L) and 3L WS2. Corresponding FFT analyses clearly show different sets of hexagonal spots, which are similar to the previous observation of folded edge of monolayer WS2 and MoS2 after CVD growth.32,33 Fragmentation of Atomically Thin TMDs via Mechanical Instabilities. This simple technique also applies to a series of typical TMD monolayers. As shown in Figure 4 and Figure S7−S10, these different TMD monolayers are all fragmented into well-ordered nanoribbons, thus demonstrating the universality of our necking method. Figure 4a,b shows the stress−strain curves of PC/atom-thin TMDs/PMMA at 9194

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Figure 4. Fragmentation of atomically thin TMDs via mechanical instabilities. (a) Stress−strain measurements of PC/WS2 monolayers/ PMMA at different drawing speeds. (b) Stress−strain measurements of PC/TMDs monolayers/PMMA at drawing speed of 0.1 mm/s. (c) Width of fragmented TMDs monolayers at different drawing speeds. (d,e) Optical images of mono- to multilayer thick WSe2 film before and after necking process, respectively. (f,g) Raman and PL mapping images (mapping at the A′1 + E′ mode and the strongest PL peak from the A exciton) of the section in (e) enclosed in a red solid rectangle. The dashed white parallelogram in (g) shows the grain boundary in the monolayer WSe2.

and gate dielectric (SiO2) thickness of 270 nm. Figure 5b shows output characteristics (Ids−Vds) of the FET under different gate voltages ranging from 0 to 50 V. The fabricated FET device shows an on/off ratio of 106, indicating that the fabricated MoS2 nanoribbon arrays by this facile physical fragmentation method are suitable for practical nanoelectronic applications. Furthermore, MoS2 is a promising electrocatalyst for electrochemical hydrogen production. It is well established that HER efficiency depends strongly on the number of active sites, for instance, exposed edges in MoS2. Thus, introduction of edges to create more active sites is an effective way to improve the catalyst activity.6,7,34,35 In view of this, we explore the HER activity of the fabricated monolayer MoS2 nanoribbons by our necking method. As shown in Figure 5e, large enhancement of HER performance is detected in the monolayer MoS2 nanoribbons. The polarization curve recorded on monolayer MoS2 nanoribbons exhibits a current density of 1.63 mA cm−2 at a voltage of 0.4 V versus RHE. Its current density is nearly 3.4 times higher than 0.37 mA cm−2 of original

Especially, large area of 2L WSe2 breaks into well-ordered ribbons. It is worth mentioning that this phenomenon has also been observed in other three types of TMD multilayers. In addition, as expected, the average width W̅ of these ribbons is proportional to the number of layers, showing a good agreement with the above formula. Fourthly, our necking method also applies to the monolayers with grain boundary, as shown in Figure 4g. These atomically sharp 2L ribbons and monolayer nanoribbons depicted in Figure 4e and 4g indicate the broad applicability of our necking method. Field-Effect Transistors and Enhanced Electrochemical Performance for Hydrogen Evolution Reaction Based on Fabricated Monolayer MoS2 Nanoribbons. To investigate the electrical properties of the nanoribbons, we fabricate bottom-gated field-effect transistors (FETs) based on the monolayer MoS2 nanoribbons. Figure 5a plots the transfer characteristic (Ids−Vgs) of the FET at Vds = 1 V. Inset of Figure 5a shows the optical image of the fabricated FET device with the channel width W ∼ 10 μm and channel length L ∼ 2 μm, 9195

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Figure 5. Electrical properties of the fabricated monolayer MoS2 nanoribbons and its electrochemical performance for hydrogen evolution reaction (HER). (a) Transfer characteristic (Ids−Vgs) of the FET at Vds = 1 V, showing an on/off ratio of 106. Inset shows the fabricated FET device based on MoS2 nanoribbon arrays. The Si/SiO2 substrate with the SiO2 thickness of 270 nm is used as bake-gate. Source/drain (S/D) metal contacts are defined by electron beam lithography, and Cr/Au (5/50 nm) are deposited by thermal evaporation followed by the lift off process. The channel width W ∼ 10 μm and channel length L ∼ 2 μm. (b) Output characteristics (Ids−Vds) of the FET under different gate voltages ranging from 0 to 50 V. (c,d) Schematic representations illustrate the function of the edges of monolayer MoS2 triangle and monolayer MoS2 nanoribbons as the active sites for HER. (e,f) HER polarizing curves and Tafel plots for supporting Au (cyan), monolayer MoS2 triangles (red), and monolayer MoS2 nanoribbons (black).

CVD-grown monolayer MoS2 triangles. This improvement in electrocatalytic activity is ascribed to the largely increased edge areas of monolayer MoS2 nanoribbons and the fact to use this physical fragmentation method instead of chemical etching, as schematically illustrated in Figure. 5c,d. Moreover, as shown in Figure 5f, the Tafel slope of the monolayer MoS2 nanoribbons is 109 mV dec−1, similar to 104 mV dec−1 of the origin monolayer MoS2 triangles, which reveals that both the active sites located in origin and the resulting edges areas are active for HER activity. The Tafel slope of 109 mV dec−1 indicates that HER on monolayer MoS2 nanoribbons is through the Volmer−Heyrovsky reaction as the rate-determining step. Discussion. The propagating neck may induce some additional cracks in the nanoribbons, possibly due to the mechanical property discrepancies between PC and PMMA, such as elongation36,37 and adhesion force.38 This can be further minimized by adopting the same substrate materials, which is also an effective method to realize perfect nanoribbon arrays. Moreover, wrinkles are induced after the necking process, which may affect the uniformity of the optoelectronic performance when applying these monolayer nanoribbons to devices. A slight stretching in the perpendicular direction to the original drawing direction or localized heating can be employed to remove wrinkles. Since the proposed method relies on a

physical breakup mechanism, it significantly broadens the repertoire of usable materials and is highly versatile. For example, other thermoplastic polymers39,40 such as poly(ether imide) (Figure S12), polyethylene terephthalate, polysulfone and poly(ether sulfone) are applicable in this method. After the necking process, the width of the nanoribbons, the size and distribution of the wrinkles, and the spacing between the nanoribbons may vary with the polymers used, since the induced necking strain, Poisson’s ratio and the elongation are different. Additionally, iterative necking processes can further downsize the width of the nanoribbons, and we can modify the morphology of these nanoribbons from “long strip” pattern into “square” or “triangular” pattern by controlling the necking directions, thus realizing various TMDs’ shapes, which allows their applications such as high sensitivity sensors5 or nonlinear optics.

CONCLUSIONS Controlling the fragmentation of TMD materisl are of both fundamental and technological importance in extreme and fragmentation mechanics. We have demonstrated here a facile and versatile method to control the fragmentaion of atomically thin TMD materials and fabricate an ordered arrays of TMD nanoribbons by harnessing the polymer-based mechanical 9196

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The PC film was subsequently dissolved using dichloromethane and 2propanol. Annular Dark Field Scanning Transmission Electron Microscopy. Samples were prepared for ADF-STEM imaging using the PMMA-assisted transfer method. After transfer of nanoribbons on Si/ SiO2 wafer, PMMA was spin coated onto the nanoribbons at the speed of 3000 rpm for 60 s, followed by baking at 170 °C for 5 min using a hot plate. Then the sample was carefully placed on the HF surface (49% HF:H2O = 1:4) for ∼30 s, followed by rinsing in DI water six times. Then a TEM grid was used to fish out the sample, and the sample was air-dried and the PMMA was washed off with acetone and 2-propanol. STEM was carried out by Titan3 G2/TEM 60−300 (FEI) operated at 80 keV. In order to reduce irradiation damage, beam current was kept below 40 pA. A high-angle annular dark field (HAADF) detector was used to record ADF signal. While the ADF signal from light element was weaker for HAADF, low-angle condition (LAADF) was used to obtain clear signal from sulfur atoms. For the most images in the text, the Gaussian Blur filter was applied to reduce the noise and enhance the visibility of detailed structure. Electrochemical Characterization. Electrochemical measurements were carried out in a conventional three-electrode electrochemical cell using a Bio-Logic VMP3 potentiostat. An Ag/AgCl (3.5 M H2SO4) electrode and a Pt foil were used as the reference electrode and the counter electrode, respectively. The supported MoS2 served as the working electrode. The catalytic performance was measured using linear sweep voltammetry between 0.2 V and −0.5 V versus reversible hydrogen electrode (RHE) with a scan rate of 5 mV/s. Before each testing, a flow of nitrogen gas was pumped more than a half hour to eliminate dissolved oxygen. All potentials were referenced to a RHE by a value of (0.209 + 0.059 pH) V.

instabilities. This method works for a very broad spectrum of semiconducting TMDs (WS2, MoS2, MoSe2, and WSe2) with thicknesses ranging from monolayers to bulk crystals. Fieldeffect transistors (∼on/off ratio of 106) and enhanced electrochemical performance for hydrogen evolution reaction are demonstrated based on the fabricated monolayer MoS2 nanoribbons. This method possesses the advantage in the formation of sharp, clean and ordered edges benefiting from the nature of brittle fragmentation, thus offering a substantial impact on developments of TMD material-based flexible devices for electrical, optoelectronic, sensing and catalytical applications.

METHODS WS2 (WSe2). WS2 (WSe2) monolayers were grown by chemical vapor deposition (CVD) method under atmospheric pressure. In a typical growth, WO3 powder (5 mg), in an alumina boat was located at the middle of a tube furnace, while sulfur (selenium) elemental powder (2 g) in another alumina boat was upwind to the WO3. A piece of Si wafer with 285 nm SiO2 top layer was suspended on WO3 boat with polished surface down. Argon gas of 80 standard cubic centimeters per minute (sccm) and Ar/H2 of 80/15/sccm were used as carrier gas for the synthesis of WS2 and WSe2, respectively. The system was heated up to 800 °C (900 °C) with a heating rate of 50 °C/min. After the system maintained at 800 °C (900 °C) for 15 min while the sulfur (selenium) was fixed at 200 °C (300 °C), the system cooled down naturally. MoS2 (MoSe2). MoS2 (MoSe2) monolayers were grown by CVD method under atmospheric pressure. In a typical growth, MoO3 powder (5 mg), in an alumina boat was located at the middle of a tube furnace, while sulfur (selenium) elemental powder (2 g) in another alumina boat was upwind to the MoO3. A piece of Si wafer with 285 nm SiO2 top layer was suspended on MoO3 boat with polished surface down. Argon gas of 60 sccm was used as carrier gas, and provided an inert atmosphere as well. The system was heated up to 750 °C with a heating rate of 50 °C/min. After the system maintained at 750 °C for 15 min while the sulfur (selenium) was fixed at 200 °C (300 °C), the system cooled down naturally. Sample Preparation for Necking Process. The process starts from spin-coating a poly(methyl methacrylate) (PMMA, glass transition temperature Tg = 105 °C)41 film onto atom-thin TMDs, followed by baking at 170 °C for 5 min using a hot plate. Next, the PMMA/atom-thin TMDs/SiO2/Si is floated on the surface of a HF solution (49% HF:H2O = 1:4) for ∼30 s. After the etching, the PMMA/atom-thin TMDs are rinsed with deionized water and transferred onto a 125 μm thick polycarbonate (PC, Tg = 145 °C)42 substrate, followed by baking at 50 °C for 10 min using a hot plate. After the completion of the transfer process, the sandwiched structure PMMA/atom-thin TMDs/PC is consolidated at 150 °C for 30 min in a vacuum oven to ensure the tight attachment of atom-thin TMDs between PMMA and PC. Fluorescence Characterization. Fluorescence images were obtained by an Olympus fluorescence microscope with a 100 W Mercury lamphouse and a U-MWGS3 mirror cube. The excitation wavelength was between 510−550 nm. Fluorescence images are adjusted for better contrast. Raman and PL Characterization. Witec CRM200 backscattering Raman system equipped with a solid-state YAG laser at 2.33 eV (532 nm) was used with a proper and consistent power avoiding sample heating (1.5 mW). The size of the laser beam was around 300 nm, which was focused on the sample using an objective (Olympus, 100×/ NA0.95). Transfer of Nanoribbons on Various Substrates. After necking process, PMMA was removed using acetone and 2-propanol. The PC/ nanoribbons were then placed on Au or Si/SiO2 wafer surface, followed by baking at 150 °C for 2 h in a vacuum oven, making the PC/nanoribbons adhere to the Au or Si/SiO2 surface (Figure S14).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04158. Additional information regarding Optical/Raman/PL characterization of WS2, MoS2, MoSe2, WSe2 before and after necking process, discussion on the fracture mechanism of atomically thin TMDs (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ze Xiang Shen: 0000-0001-7432-7936 Lei Wei: 0000-0003-0819-8325 Author Contributions #

M. Chen, J. Xia, and J. Zhou contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported in part by the Singapore Ministry of Education Academic Research Fund Tier 2 (MOE2015-T2-1066, MOE2015-T2-2-010, and MOE2015-T2-2-007) and Singapore Ministry of Education Academic Research Fund Tier 1 (RG85/16 and RG164/15), Nanyang Technological University (Start-up grant M4081515: Lei Wei), and the Singapore National Research Foundation under NRF RF Award No. NRF-RF2013-08. M.T. and K.F. acknowledge support from the U.S. Army Research Office under MURI grant W911NF-11-1-0362. 9197

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ACS Nano

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DOI: 10.1021/acsnano.7b04158 ACS Nano 2017, 11, 9191−9199

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DOI: 10.1021/acsnano.7b04158 ACS Nano 2017, 11, 9191−9199