Wafer-Scale van der Waals Heterostructures with Ultraclean Interfaces

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Surfaces, Interfaces, and Applications

Wafer-scale van der Waals heterostructures with ultraclean interfaces via the aid of viscoelastic polymer Stephen Boandoh, Frederick Osei-Tutu Agyapong-Fordjour, Soo Ho Choi, Joo Song Lee, Ji-Hoon Park, Hayoung Ko, Gyeongtak Han, Seok Joon Yun, Sehwan Park, Young-Min Kim, Woochul Yang, Young Hee Lee, Soo Min Kim, and Ki Kang Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16261 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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ACS Applied Materials & Interfaces

Wafer-scale van der Waals Heterostructures with Ultraclean Interfaces via the aid of Viscoelastic Polymer Stephen Boandoh,†tFrederick Osei-Tutu Agyapong-Fordjour,† Soo Ho Choi,‡ Joo Song Lee,§ JiHoon Park,#,┴ Hayoung Ko,§ Gyeongtak Han,┴ Seok Joon Yun, #Sehwan Park, # Young-Min Kim,#,┴ Woochul Yang,‡ Young Hee Lee, #,┴ Soo Min Kim,*,§ and Ki Kang Kim*,†

†Department

of Energy and Materials Engineering, Dongguk University, Seoul, 04620, Republic

of Korea ‡Department

§Institute

of Physics, Dongguk University, Seoul, 04620, Republic of Korea

of Advanced Composite Materials, Korea Institute of Science and Technology (KIST),

Wanju-Gun, 55324, Republic of Korea # Center

for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), 16419,

Republic of Korea ┴ Department

of Energy Science, Sungkyunkwan University, Suwon, 16419, Republic of Korea

*Corresponding

author E-mail: [email protected], [email protected] 1 ACS Paragon Plus Environment

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

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KEYWORDS van der Waals heterostructure; glass transition temperature; viscoelastic polymer support layer; conformal contact; ultraclean interface

2 ACS Paragon Plus Environment

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ABSTRACT

Two-dimensional (2D) van der Waals (vdW) heterostructures exhibit novel physical and chemical properties, allowing the development of unprecedented electronic, optical, and electrochemical devices. However, the construction of wafer-scale vdW heterostructures for practical applications is still limited due to the lack of well-established growth and transfer techniques. Herein, we report a method for the fabrication of wafer-scale 2D vdW heterostructures with ultraclean interface between layers via the aid of freestanding viscoelastic polymer support layer (VEPSL). The low glass transition temperature (Tg) and viscoelastic nature of VEPSL ensures absolute conformal contact between 2D layers, enabling the easy pick-up of layers and attaching to other 2D layers. This eventually leads to the construction of random sequence 2D vdW

heterostructures

such

as

molybdenum

disulfide/tungsten

disulfide/molybdenum

diselenide/tungsten diselenide/hexagonal boron nitride. Furthermore, VEPSL allows the conformal transfer of 2D vdW heterostructures onto arbitrary substrates, irrespective of surface roughness. To demonstrate the significance of the ultraclean interface, fabricated molybdenum disulfide/graphene heterostructure employed as an electrocatalyst, yielded excellent results of 73.1 mV∙dec-1 for Tafel slope and 0.12 kΩ of charge transfer resistance, which are almost twice lower than that of impurity-trapped heterostructure.



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INTRODUCTION The possibility of assembling various layered two-dimensional (2D) materials into vertical van der Waals (vdW) heterostructure opens up a new area of electronics, optoelectronics and electrochemical devices due to their unprecedented physical and chemical properties.1-9 For instance, atomic-thick graphene (Gr)/hexagonal boron nitride (hBN)/Gr tunneling1, 10 or coulomb drag11 devices can be realized with this new device architecture. Distinct vdW heterostructure devices have recently been demonstrated via “proof-of-concept” by combining mechanical exfoliation of bulk 2D materials with “pick-up”-“drop-down” method, but this technique is limited to only a few micrometer-sized samples.1, 11-16 For practical applications, the construction of largearea vdW heterostructure is highly necessary; hence techniques to synthesize or transfer vdW heterostructures require extensive investigations. Synthesis of large-area vdW heterostructure via chemical vapor deposition (CVD) still remains a challenge due to lack of well-established layerby-layer growth techniques.17 Alternatively, layer-by-layer transfer techniques have been proposed for large-scale sequential stacking of 2D materials into desired vdW heterostructures with diverse supporting layers of thermal release tape, polydimethylsiloxane (PDMS) and Ni thin film.18-24 Some of these methods require sophisticated equipment and special processing skills. Furthermore, the construction of vdW heterostructures on only flat arbitrary substrates has been demonstrated to date. Among the several transfer methods25-28, poly (methyl methacrylate) (PMMA) transfer is widely employed for large-area transfer.29 However; this method is associated with several disadvantages such as: 1) residual impurities between 2D layers, 2) damage of 2D materials during the transfer process and 3) limitation of transfer onto hydrophobic 2D material surfaces. Residual impurities typically generated by the incomplete removal of PMMA are trapped during subsequent stacking 4 ACS Paragon Plus Environment

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of other layers onto the contaminated 2D layer.30 Also, the damage to 2D materials during the transfer process are as a result of the non-conformal contact between 2D layers and target substrates due to the use of hard plastic PMMA.31-32 Furthermore, the hydrophobic surface of weakly bonded 2D materials to insulating substrate induces delamination from the substrate during the wet-process.33 This inevitably means that, for an effective transfer of any 2D material irrespective of its surface hydrophobicity, a dry transfer with freestanding support layer is necessary to avoid delamination from the growth substrate during the pick-up process.34 The aforementioned issues have necessitated the development of robust wafer-scale assembly techniques, to maximize the enormous potential of unique vdW heterostructure devices. Here, we report a “pick-up”-“drop-down” strategy for the wafer-scale assembly of vdW heterostructures with ultraclean interface via the aid of freestanding viscoelastic polymer support layer (VEPSL). The working principle of this transfer process is based on the viscoelasticity and low glass transition temperature (Tg) of the polymer, which allows VEPSL to behave as an elastic solid on which stacked 2D materials form intimate contact. This enables an easy and conformal transfer to large-scale, rough, groove and terrace-like substrates, which has not been attained with conventional thermal release tape approach.20 The freestanding VEPSL helps in the “pick-up”“drop-down” of any 2D layer onto subsequent 2D layers. Polyethylene (PE) as VEPSL is employed for the effective layer-by-layer transfer of wafer-scale ten-layer Gr film. Furthermore, the use of only the topmost PE layer facilitates the construction of randomly stacked vdW heterostructure (i.e. molybdenum disulfide (MoS2)/tungsten disulfide (WS2)/molybdenum diselenide (MoSe2)/tungsten diselenide (WSe2)/hBN) with ultraclean interfaces between 2D layers on a large scale by sequential transfer. The ultraclean interfaces in vdW heterostructure are confirmed with cross-sectional scanning transmission electron microscope (STEM). The



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significance of ultraclean interface of vdW heterostructures is demonstrated with MoS2/Gr electrocatalyst for hydrogen evolution reaction. MoS2/Gr with ultraclean interface exhibits better performance in terms of Tafel slope (73.1 mV∙dec-1) and low charge transfer resistance (0.12 kΩ) due to the increased interaction and effective charge transfer between 2D layers compared to MoS2/Gr with contaminated interface prepared by the conventional PMMA method with almost twice as high values of Tafel slope (115.8 mV∙dec-1) and charge transfer resistance (0.23 kΩ).

RESULTS AND DISCUSSION Figure 1a illustrates a scheme for the large-scale assembly of 2D materials for the construction of vdW heterostructures on arbitrary substrates. PE as VEPSL was spin-coated onto the intended topmost layer of 2D material for the assembly, followed by curing (90 ̊C) above Tg in an oven to induce conformal contact between VEPSL and 2D material (see Methods for details). The freestanding VEPSL/2D film is detached from the growth substrate and dried (Figure 1b). The resultant film is then placed onto the next 2D layer for stacking. Attaching and detaching after curing is repeated to build desired stacked vdW heterostructures. Eventually, the VEPSL is removed in hot chlorobenzene at 180 °C for 10 minutes or at 90 °C for 30 minutes. (see Supporting Information, Figure S1). Via the VEPSL method, ten-layers of monolayer Gr, grown by CVD were stacked and transferred onto a 4-inch SiO2/Si wafer (Figure 1c). The ratio of 2D-band over G-band intensity (I2D/IG) in the Raman spectra of varying Gr layers assembled 1, 2, 3, 4, 5 and 10 layers, is observed to gradually decrease as a function of number of Gr layers (Figure 1d and see Supporting Information, Figure S2), which is in good agreement with previous observation.35 The results imply a successful construction of 10 L Gr film on a wafer-scale, via sequential stacking of monolayer Gr with the VEPSL method.


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To show the versatility of VEPSL method for the transfer of insulating hBN and semiconducting transition mental dichalcogenides (TMdCs, MX2: M = Mo and W, X = S, Se), randomly stacked vdW heterostructure is constructed sequentially with various 2D materials. Figure 1e presents the optical

image

of

stacked

monolayer

TMdCs

and

multilayer

hBN;

in

a

MoS2/WS2/WSe2/MoSe2/hBN vdW heterostructure (see Supporting Information for the growth methods of TMdCs and hBN). While it is relatively difficult to distinguish the individual layers in the optical image (Figure 1e), Raman mapping images of E12g mode MoS2 near 382 cm-1, E12g mode of WS2 near 350.8 cm-1, A1g mode of MoSe2 near 238.7 cm-1, A1g mode of WSe2 near 249.6 cm-1 and E2g mode of hBN near 1370 cm-1, respectively, clearly shows the presence of each 2D layer in Figure 1f-j. The representative Raman spectrum of the vdW heterostructure shown in Figure 1k illustrates the overlapping of Raman modes of each 2D layer in the heterostructure. No 2D materials are observed on the growth substrate after the pick-up process, implying conformal contact occurs between 2D layers during the stacking process (see Supporting Information, Figure S3). The characteristic peaks in Raman spectra are preserved, whereas the characteristic PL peaks were severely quenched (see Supporting Information, Figure S4). Similar trends with diverse heterostructures such as WSe2/WSe2/MoS2, WSe2/MoS2, and WSe2/WS2 are observed (see Supporting Information, Figure S5).36-38 This indicates that the physical structures of 2D layers in vdW heterostructure are preserved, while the electronic structures are changed via layer-layer interaction. Furthermore, ethyl cellulose (EC) (see, Supporting Information for details) as alternative VEPSL was employed and shown to yield excellent results for the construction of vdW heterostructure (See Supporting Information, Figure S6). It should be emphasized that the VEPSL method which includes the dry process, is suitable for transfer of any material, regardless of its hydrophobicity to diverse substrates, which is one of the limitations of the conventional wet-



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PMMA method (see Supporting Information, Figure S7). This enables the construction of random MoS2/WS2/WSe2/MoS2/hBN vdW heterostructure irrespective of growth methods. Furthermore, the versatility of the transfer process is shown with transfer onto quartz substrates for UV-visible spectroscopic analysis. The UV-visible spectrum of MoS2/hBN/Gr heterostructure clearly depicts the respective peaks of the individual 2D materials in the vdW heterostructure (see Supporting Information, Figure S8). The interfaces between stacked CVD grown 2D layers in a constructed vdW heterostructure were carefully characterized with cross-sectional STEM. Figure 2a is the schematic illustration of multilayer hBN/four layer (4L) WS2/1L Gr/multilayer hBN vdW heterostructure (Figure 2a). Figure 2b and c are the annular bright field (ABF) and dark field (ADF) cross-sectional scanning transmission electron microscope (STEM) images of the assembled vdW heterostructure, respectively. The 4L WS2 and 1L Gr sandwiched between multilayer hBN films (top and bottom layers) are clearly seen. Since only the topmost hBN layer came in contact with the VEPSL, all the interfaces in the vdW heterostructure are extremely clean over the whole region (see Supporting Information, Figure S9). The presence of each 2D material is confirmed by the elemental profiles measured via energy dispersive X-ray spectrometry (EDS) in Figure 2d, further supporting the successful construction of vdW heterostructure. As is often the case with transfer of Gr and related 2D materials, ensuring the continuity of 2D film without tearing after transfer is the most important issue for future device applications. VEPSL method provides a solution to this problem by aiding conformal contact of 2D materials to terrace-like substrates (Figure 3a). To attain absolute contact, a comprehensive strain in the support polymer layer is necessary.39-40 During curing, thermal energy higher than Tg induces increase in free volume with sliding or movement of groups or branches of polymer chain,41-42


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eventually resulting in the enhancement of the interaction between polymer/2D and terrace-like substrates.39-40 The viscoelastic nature of PE coupled with its extremely low Tg of -35 °C43 will facilitate the rapid softening and conformal attachment to the underlying substrates, at curing temperatures above Tg. Though the Tg of PE is negative, the optimum curing temperature was attained at 90 °C. This implies that, at 90 °C, there is adequate movement of groups or branches of PE chains to facilitate conformal attachment to arbitrary substrates. Figure 3b shows the asgrown Gr film with PE, transferred onto a pre-patterned rough substrate. The absence of air gaps between PE/Gr and the underlying pre-patterned rough substrate is evident of conformal contact on the rough substrate (top image of Figure 3b). The PE layer can then be removed, leaving a perfectly transferred Gr on the pre-patterned rough surface as shown in the bottom image of Figure 3b. A further tilted-view in SEM reveals that the transferred Gr is indeed continuous even on the sides of the terraces (see Supporting Information, Figure S10). For comparison, PMMA which has higher Tg around 136 °C is employed.44 Several air gaps can be seen from the SEM images of the PMMA/Gr/pre-patterned rough substrate at even 120 °C curing due to incomplete wetting at relatively lower temperature, compared to Tg of PMMA (top image of Figure 3c). Curing of the PMMA sample was carried out at 120 °C because most of the reports of transfer with PMMA employ curing temperatures between 90-120 °C.18-19, 27 After removal of the PMMA layer, the underlying Gr is observed to be severely damaged as shown in the bottom image of Figure 3c. To further improve conformal contact, thermal energy over the Tg of PMMA is required. However, when curing was performed at 160 °C above Tg of PMMA, the PMMA layer becomes difficult to remove after transfer (see Supporting Information, Figure S11). Therefore, for conformal transfer of Gr and related 2D materials onto arbitrary substrates irrespective of surface roughness, a viscoelastic support layer with a low Tg is highly desirable.



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One of the promising applications of noble vdW heterostructure is as electrocatalyst for water splitting.45-46 MoS2 is chosen as the active layer, due to it being a better candidate material among TMdCs, for hydrogen evolution reaction (HER). However, the catalytic activity of MoS2 is limited to only the S-edges.47 The limited catalytic activity of MoS2 can be significantly enhanced when coupled with other 2D materials such as Gr, in a vdW heterostructure, as a result of the sandwich structure making the basal plane of MoS2 close to the thermo-neutral (ΔGH ~ 0).48 To evaluate the catalytic performance of MoS2/Gr vdW heterostructure for water splitting, MoS2/Gr with ultraclean-interface (MoS2/Gr_UI) was prepared with the VEPSL method to serve as the electrocatalyst on a glassy carbon (GC) as the working electrode (Figure 4a). For comparison, only MoS2 layer and contaminated-interface MoS2/Gr electrocatalyst (MoS2/Gr_CI) were prepared using the conventional layer-by-layer wet-PMMA stacking technique on a GC electrode (Figure 4b). To increase the interaction between MoS2/Gr and GC, as well as between 2D layers, thermal annealing was carried out at 600 °C,49 because without annealing, all the samples are delaminated during HER measurements (see Supporting Information, Figure S12 and S13). Figure 4c-e and 4f-h present the optical and corresponding Raman mapping images of Gr and MoS2 for MoS2/Gr_UI and MoS2/Gr_CI after annealing at 600 °C, respectively. The optical image of MoS2/Gr_UI shows a continuous film with some multilayer MoS2 islands on MoS2/Gr/SiO2/Si, without any noticeable change in surface morphology after thermal annealing (Figure 4b and see Supporting Information, Figure S14a and b). However, discontinuous and rolled-up MoS2 film on Gr in MoS2/Gr_CI can be observed with the PMMA stacked electrocatalyst (Figure 4f and see Supporting Information, Figure S14c and d), which might be attributed to the evaporation of methyl methacrylate (MMA) from thermal decomposition of PMMA residues trapped between MoS2 and Gr during the thermal annealing process.50 In addition, Raman mapping


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images of Gr for both samples (Figure 4c and f) appear quite uniform (Figure 4d and g), indicating the whole region is completely covered with Gr without any damage. Meanwhile, Raman mapping image of MoS2 for MoS2/Gr_UI sample, as shown in Figure 4e, appear to preserve the high uniformity of the stacked MoS2/Gr_UI except for the spot labeled (M), which results from the presence of multilayer MoS2 islands. However, several regions without MoS2 for MoS2/Gr_CI sample (Figure 4h and see Supporting Information, Figure S15) are clearly seen, which further supports the observation made in Figure 4f that, the top MoS2 layer is severely damaged by the evaporation of MMA during thermal annealing. Representative Raman spectra of selected spots (I, II, III and IV) in Figure 4b and f reveal that, only the topmost MoS2 layer for MoS2/Gr_CI sample is damaged, whiles the underlying Gr layer remains intact (Figure 4i). The catalytic activity of MoS2/Gr and only monolayer MoS2 was evaluated with the linear sweep voltammetry (LSV) (Figure 4j).51 As can been seen from the LSV plots, the MoS2/Gr_UI outperforms MoS2/Gr_CI with a near zero onset potential and a characteristic steeper polarization curve. In addition, MoS2/Gr vdW heterostructure recorded higher performance than only MoS2 layer, as expected.52 An impressive Tafel slope of ~73 mv∙dec-1 for MoS2/Gr_UI is achieved (Figure 4k), which is the highest value among the investigated electrocatalysts. Even though MoS2/Gr film has limited exposed S-edges as active sites, its Tafel slope is comparable to the reported values of MoS2 with high S-edges.47 This might be attributed to activation of the basal plane of MoS2 as active sites by changing the electronic structure of MoS2/Gr heterostructure due to the induced built-in electric field between layers as hypothesized in a previous study.45 This implies that vdW heterostructures are more promising electrocatalysts than single 2D materials.53 Figure 4l presents the electrochemical impedance spectroscopy (EIS) of studied electrocatalysts.52 The charge transfer resistance is extracted from the Randle circuit model as shown in the inset of



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Figure 4l.48 MoS2/Gr_UI exhibits lower charge transfer resistance than MoS2/Gr_CI. The higher charge transfer resistance of MoS2/Gr_CI is as a result of the presence of residual impurities between MoS2 and Gr, as well as the damage of MoS2 film. In addition, we found that thermal annealing helps improve conformal contact and the cyclic stability over 3000 cycles of the MoS2/Gr_UI electrocatalyst (see Supporting Information, Figure S12, and S16-S17). CONCLUSION In summary, a versatile “pick-up”-“drop-down” technique with freestanding VEPSL has been developed for the large-scale assembly of vdW heterostructures. With the aid of the low Tg of a viscoelastic polymer, wafer-scale vdW heterostructures with ultraclean interface and conformal transfer have been realized. The viscoelastic polymer further facilitates conformal contact on arbitrary substrates irrespective of surface roughness (smooth, grooved or terrace-like), resulting in transfer of continuous 2D film. The use of only the topmost VEPSL for the “pick-up”-“dropdown” transfer enables ultraclean interfaces to be attained between 2D layers in vdW heterostructure. The electrocatalytic activity of vdW heterostructures with ultraclean and contaminated interface revealed that, the ultraclean interface of the studied heterostructure yielded excellent catalytic results, due to its high uniformity, enhanced charge transfer as a result of increased interaction between layers, whereas contaminated interface of stacked 2D layers results in severe damage and the increase of charge transfer resistance, resulting in low catalytic activity. The high cyclic stability of the assembled electrocatalyst further supports the conformal contact with the electrode, preventing the damage of the constructed electrocatalyst during HER even in hush bubbling conditions. Our strategy with freestanding VEPSL as a “pick-up”-“drop-down” technique ensures clean interface, conformal contact, ease of scalability and extreme


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reproducibility for the construction of diverse vdW heterostructures, paving the way for the fabrication of reliable and attaining desired device performance in future large-scale applications.

METHODS Wafer-scale vdW heterostructures construction with freestanding VEPSL. Molten polyethylene (Mw: 48,000) in cyclohexanone (PMAX-6000, DongYang P-MAX Chemtech) was used as the freestanding VEPSL. The VEPSL is referred to as “freestanding” because, at 18 µm thick, the film is tough and flexible enough to be picked up with a tweezer, dried, and attached to arbitrary substrates. Typically, the viscous PMAX is diluted in a 75:25 (PMAX: Toluene (anhydrous 99.8%, Sigma Aldrich)) ratio, to serve as the transfer support. The “pick-up”-“drop-down” technique begins with the coating of thick VEPSL and curing at 90 °C for 10 minutes on the 2D material intended to be the topmost layer in the vdW heterostructure. The freestanding VEPSL/2D material is then delaminated from the growth substrate, suspended on clean distilled water, picked up and dried by N2 gun or in a vacuum desiccator. The dried VEPSL/2D material is then attached at 90 °C to the next 2D material to be picked up in an oven. The “pick-up”-“drop-down” step is then repeated for the stacking of several 2D materials until vdW heterostructures with desired layer numbers of 2D materials are obtained. The stacked 2D materials are then attached at 90 °C for 1 hour to a target substrate and the VEPSL removed in hot chlorobenzene at 180 °C for 10 minutes or at 90 °C for 30 minutes to avoid thermal expansion coefficient mismatch between stacked 2D layers.

VEPSL assembly of MoS2/Gr electrocatalyst.



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VEPSL is coated on as-grown MoS2 at a rate of 2500 revolution per minute and cured at 90 °C in an oven for 10 minutes. The VEPSL/MoS2 is then detached in 30 % hydrogen fluoride (HF) in distilled water and cleaned by suspending over clean distilled water. The freestanding VEPSL/MoS2 is then picked up with a tweezer, dried with a N2 gun or in a vacuum desiccator and attached to Gr/Copper (Cu) at 90 °C for 30 minutes. Cu is then etched with FeCl3 (TFB, Transene Company, Inc) and the resulting VEPSL/MoS2/Gr film cleaned, dried and attached to a GC electrode at 90 °C for 30 minutes. VEPSL is then removed in hot chlorobenzene at 180 °C for 10 minutes or 90 °C for 30 minutes.

Wet-PMMA stacking of MoS2/Gr electrocatalyst PMMA was spun onto Gr/Cu and cured for 10 minutes at 90 °C. The Cu foil was then etched in FeCl3 (TFB, Transene Company, Inc) and the film cleaned by suspending over clean distilled water. The cleaned film was scooped onto a pre-cleaned SiO2/Si substrate and cured in the oven for 1 hour at 90 °C. PMMA was then removed in hot acetone. To transfer MoS2 onto the transferred Gr on SiO2/Si, PMMA was spun onto the as-grown MoS2/SiO2/Si and cured at 90 °C. The PMMA/MoS2 was detached in 30 % HF solution and cleaned by suspending over clean distilled water. The cleaned PMMA/MoS2 was then scooped with transferred Gr/SiO2/Si. Curing of the resulting PMMA/MoS2/Gr/GC was carried out in the oven for 90 °C for 1 hour after which the PMMA was removed in hot acetone for 10 minutes to obtain the desired MoS2/Gr/SiO2/Si.

Electrochemical Test. A three-electrode system was adopted to compare the electrochemical performance of MoS2/Gr_CI and MoS2/Gr_UI and samples were transferred onto glassy carbon plate using PMMA and VEPSL as support layers respectively. They were subsequently tested in


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0.5M H2SO4 electrolyte at 5 mVs-1. Platinum was used as a counter electrode and Ag/AgCl, saturated KCl as reference electrode. All potentials in this work are reported vs RHE using the equation; E(vs RHE) = E ( vs Ag/AgCl, saturated KCl) + 0.197 + 0.0592*pH Characterization. The morphology and uniformity of the assembled vdW heterostructures were analyzed via optical microscopy (Nikon LV-IM, Nikon). Micro-Raman spectroscopy and Photoluminescence (XperRam100, Nanobase) measurement were performed to confirm the uniformity and increased interaction between layers with an excitation energy of 2.41 eV. Fouriertransform infrared (FTIR) spectroscopy (FTIR-7600, Lambda Scientific) and X-ray photoelectron microscopy (K-alpha, Thermo Fisher Scientific) were used for the stability characterization of the MoS2/Gr before and after HER measurement. UV-Visible spectrophotometry (Cary 50, Varian) was used to characterize the optical property of assembled MoS2/hBN/Gr. ADF-STEM (JEMARM200F, Jeol) was employed to analyze the interface of the assembled vdW heterostructures.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Details on growth of TMdCs, hBN, Gr and the stacking



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of vdW heterostructure with PMMA and ethyl cellulose are provided, together with Figure S1S17. AUTHOR INFORMATION Corresponding Author E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS. S.M.K. acknowledges support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2018R1A2B2002859) and the Korea Institute of Science and Technology (KIST) Institutional Program. K.K.K. acknowledges support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry

of

Science,

ICT

&

Future

Planning

(2015R1C1A1A02037083)

and

(2018R1A2B2002302) and the Dongguk University Research Fund of 2017 (S-2017-G000100013). Y.-M.K. acknowledges financial support by the Institute for Basic Science (IBS-R011D1) and Creative Materials Discovery Program (NRF-2015M3D1A1070672) through the NRF grant.


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FIGURES


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Figure 1. Tg assisted construction of wafer scale 2D van der Waals (vdW) heterostructures with freestanding VEPSL. (a) Schematic illustration of layer-by-layer stacking of 2D materials with freestanding VEPSL. (b) Photograph of freestanding VEPSL for large-scale assembly of diverse 2D layers (c) Photograph of wafer-scale assembled 10 L monolayer CVD Gr on 4-inch SiO2 wafer. (d) Ratio of 2D-band over G-band intensity as a function of number of Gr layers. (e, f-j)

Optical

and

corresponding

Raman

mapping

images

of

assembled

MoS2/WS2/MoSe2/WSe2/hBN. E12g mode of MoS2 near 382 cm-1, E12g mode of WS2 near 350.8 cm-1, A1g mode of MoSe2 near 238.7 cm-1, A1g mode of WSe2 near 249.6 cm-1 and E2g mode of hBN near 1370 cm-1 are used for each Raman mapping image. (k) Representative Raman spectra



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of MoS2, WS2, MoSe2, WSe2, and hBN with their corresponding stacked spectrum. The inset indicates zoomed-in Raman spectra near 1370 cm-1 for hBN.


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Figure 2. Interface Characterization of VEPSL assembled hBN/WS2/Gr/hBN. (a) Atomic layers of vertically assembled 2D materials. (b) Annular bright field (ABF) cross-sectional scanning transmission electron microscopy (STEM) image of assembled hBN/WS2/Gr/hBN. (c) Corresponding annular dark field (ADF) STEM image of the selected area in (b). (d) STEM-EDS line profile along the yellow-dashed line in (c).



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Figure 3. Conformal transfer onto rough substrates. (a) Schematic illustration of near-perfect wetting via stacking above Tg of VEPSL. (b and c) SEM images of Gr on a pre-patterned substrate before and after removal of VEPSL and PMMA, respectively.


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Figure 4. HER activity of assembled MoS2/Gr electrocatalyst. (a, b) Schematic illustrations of ultraclean and impurity-trapped interface of MoS2/Gr vdW heterostructure, assembled with VEPSL and wet-PMMA stacking methods (denoted as MoS2/Gr_UI and MoS2/Gr_CI), respectively. ((c, f), (d, g) and (e, h)) Optical and corresponding Raman images of MoS2/Gr_UI and MoS2/Gr_CI after annealing at 600 °C, respectively. G-band of Gr near 1583 cm-1 and E12g of MoS2 near 384 cm-1 for Raman mapping image are used. (i) Representative Raman spectra of selected spots (I, II, III and IV) in (c) and (f), respectively. (j) Linear sweep voltammetry curves of GC, MoS2, MoS2/Gr_UI and MoS2/Gr_CI on GC, respectively. (k and l) Tafel plot and electrochemical impedance spectroscopy (EIS) of studied electrocatalysts, respectively.



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Table of contents (TOC)


Wafer-Scale van der Waals Heterostructures with Ultraclean Interfaces

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