Scalable Transfer of Suspended Two-Dimensional Single Crystals

Page 1 of 24

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

Nano Letters

Scalable Transfer of Suspended Two Dimensional Single Crystals

Bo Li1,‡, Yongmin He1,2‡, Sidong Lei1, Sina Najmaei1, Yongji Gong3, Xin Wang4, Jing Zhang1, Lulu Ma1, Yingchao Yang1, Sanghyun Hong5, 6, Ji Hao5, 6, Gang Shi1, Antony George1, Kunttal Keyshar1, Xiang Zhang1, Pei Dong1, Liehui Ge1, Robert Vajtai1, Jun Lou1, Yung Joon Jung5, 6*, Pulickel M. Ajayan1,* ‡

These authors contributed equally to this work

*

Email:[email protected]; [email protected]

1

Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, U.S.A.,2School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People’s Republic of China, 3Department of Chemistry, Rice University, Houston, Texas 77005, U.S.A., 4 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, U.S.A., 5Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115, U.S.A., 6George J. Kostas Research Institute for Homeland Security Northeastern University, Boston, MA 02115, U.S.A.

Abstract Large-scale suspended architectures of various 2D materials (MoS2, MoSe2, WS2 and graphene) are demonstrated on nanoscale patterned substrates, with different physical and chemical surface properties, such as flexible polymer substrates (polydimethylsiloxane), rigid Si substrates and rigid metal substrates (Au/Ag). This transfer method represents a generic, fast, clean and scalable technique to suspend 2D atomic layers. The underlying principle behind this approach, which employs a capillary-force-free wet-contact printing method, was studied by characterizing the nanoscale solid-liquid-vapor interface of 2D layers with respect to different substrates. As a proof-of-concept, a photodetector of suspended MoS2 has been demonstrated with significantly improved photosensitivity. This strategy could be extended to several other 2D material systems and open the pathway towards better optoelectronic and nanoelectromechnical systems. KEYWORDS: wet-contact printing, capillary-force-free transfer, suspended architectures, van der Waals layered materials, two-dimensional materials, nano-patterned substrates Introduction Two-dimensional (2D) atomically layered materials, also known as van der Waals materials, such as graphene, transition metal dichalcogenides and III-V group compounds have attracted great interests due to their atomically thin structures, versatile electrical and photoelectrical properties and their potentials to achieve thin and flexible devices 1-12. Once a layered material is

ACS Paragon Plus Environment

Nano Letters

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

thinned down to a monolayer, the material itself is a surface and the properties of which are strongly influenced by the substrate due to the changes in atomic arrangement13, defects and impurities14, dielectric constant15,

16

, functional groups17 and even roughness18. Significant

progress has been made towards substrate engineering to actively control the properties of 2D materials. For example, hexagonal boron nitride (h-BN), an atomic flat substrate with a very high dielectric constant, minimizes the influence of surface roughness and defects, and the 2D devices on h-BN show improved mobility compared to their counterparts on SiO2/Si substrate15, 16, 18-21. The property of the substrate as well as that of the 2D materials can be further engineered through chemical modification17. Another extreme direction of substrate engineering is the complete elimination of the influence of supporting substrates, i.e., suspended architectures. Suspended graphene has been demonstrated with significant improvement in electrical properties22-25. For example, Du et al., reported a low temperature mobility of 200,000 cm2 V-1 s-1 for suspended graphene which is much higher than devices on SiO2/Si substrates22. Furthermore, to characterize intrinsic mechanical and thermal transport properties

26, 27

, suspended architectures are ideal for accurate

measurements since it eliminates the force and phonon transfer between nanomaterials and the substrate. In many applications such as microelectromechanical system (MEMS) or nanoelectromechanical system (NEMS), resonators, etc., suspended architectures also represent unique functional units28, 29. Several powerful strategies such as the stamping method26, 30, 31, polymer assisted transfer32-37, water lift off transfer technique38 and face-to-face transfer technology39, have been developed to transfer 2D material to arbitrary flat substrates. However, only some of them are compatible with hierarchical structures patterned substrates to build suspended 2D material architectures.


Page 2 of 24

Page 3 of 24

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

Nano Letters

Especially for those wet methods using solutions, the solution can be trapped between suspended architectures and substrates, and then the evaporation of the trapped solution leads to the capillary force that can easily break the suspended 2D architectures34. Some modifications and additional steps are required to fabricate suspended 2D architectures depending on the properties of the sample and substrate, resulting in more complicated transfer processes26, 31, 34. For example, the stamping method without contamination from the transfer media provides the best sample quality25. However, the stamping method is not compatible with 2D materials obtained by CVD method (e.g., graphene on Cu, MoS2 on SiO2/Si and etc.), because of the strong interaction between materials and donor substrate. Therefore, additional etching steps are required to minimize this interaction26, 31. Another example is PMMA assisted transfer, in which a special substrate (perforated substrate) and annealing processes are required to prevent capillary force as reported by Suk et al34. In their work, both dry and wet methods using PMMA as the transfer media were demonstrated. Despite the great success of large area suspended architectures, the PMMA assisted transfer method has showed several limitations. First, the annealing process at an elevated temperature may damage the 2D materials. Second, the annealing process might not be able to remove PMMA fully, leaving contaminations. Third, delicate operations and multiple transfer steps lead to a long transfer time and complicated processes. In addition, the wet method is not suitable for substrates with micro-wells (in contrast to the perforated substrate with through holes) since the trapped solution in the well could break the suspended film34. In addition to transfer methods, a common method for suspending 2D materials is to selectively remove the supporting substrate by electron beam lithography (EBL) to define the location, followed by hydrofluoric acid etching, photoresist stripping and critical point drying (CPD) to release the 2D materials 22, 24. The CPD process, widely used in MEMS and NEMS, is necessary


Nano Letters

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

to eliminate the capillary force between the suspended unit and the substrate during the evaporation of the aqueous solution. However, multiple lithography processes may not only introduce contaminations to the suspended materials25, 40, but also easily break the ultrathin film. The sample quality can be greatly improved through a lithography-free method by combining the stamping method and shadow mask deposition technique developed by Bao et al25. However, the delicate alignment processes in both steps of the lithography-free method also present a barrier for scalable fabrication. Thus, a facile, clean and scalable method to achieve large-scale suspended 2D materials is essential, especially for building nanoscale devices. In this paper, we demonstrate large-scale suspended 2D materials over various nanoscale patterned substrates with the combination of chemical vapor deposition (CVD) growth and a wet-contact printing method. The wet-contact printing method is not only lithography-free, but also ultra-fast and scalable to substrates with various nanopatterns, and physical/chemical natures. The fundamental transferring mechanism at the nanoscale solid-liquid-vapor (SLV) interface has been elucidated. Finally, as a proof-of-concept, a photodetector with improved photoresponse was built by directly suspending MoS2 onto interdigitated Ag/Au electrodes. CVD-grown 2D materials were used in the scalable transfer process for building 3D architectures of the suspended 2D single crystal films. Unlike the top-down mechanical exfoliation method where the thickness and surface area of samples vary, the CVD method produces large-scale single crystals with controlled thickness, atomic ratio and even stacking order which are crucial for the scalable application of such materials7, 41-43. In this work, largescale MoS2, MoSe2 and WS2 sheets have been directly grown on a Si donor substrate with 285nm-thick thermal oxide. Also, graphene single crystal was grown on Cu substrate first and then transferred to donor substrate using a PMMA assisted transfer method44.


Page 4 of 24

Page 5 of 24

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

Nano Letters

We have developed a wet-contact printing method, as shown in Figure 1a and Figure S1. In this approach, 2D materials can be perfectly transferred from a donor substrate (SiO2/Si) to a receiving substrate with nanoscale patterns to achieve suspended architectures. Triangular shaped MoS2 monolayers are shown as an example. First, the donor substrate with the 2D material (represented by the red triangle) was brought in contact with the nanopatterned receiving substrate. Diluted HF acid solution (16%) was dropped on the sample for 30 second to 1 min to remove the SiO2 sacrificial layer and release the 2D materials to the receiving substrate. Then, the donor substrate was detached and the residual HF solution residing on the receiving substrate was removed by tilting the substrate or pressing a tissue on the edge of the droplet. The wet contact printing method can deliver several unique features. (I) This is a wet method but no CPD process is required leading to a very fast transfer process (The whole process can be finished in 1-2 minutes). It exhibits the potential of being integrated into continuous processes. (II) HF solution is not reacting with 2D materials and can be evaporated completely leaving no contaminations or defects. (III) This method has shown great scalability in terms of the materials (PDMS, Si and Au) and pattern design (nanostrips and nanoholes) of receiving substrates. Figure 1b to 1d show the representative suspended MoS2 single crystals on different (soft and hard) nanopatterned substrates: PDMS nanostrips (Figure 1b), Si nanostrips (Figure 1c) and Si nanoholes (Figure 1d). Figure 1b highlights the complete transfer of various MoS2 structures including monolayer and bilayer MoS2 with different shapes to the target PDMS substrate with arrays of nanostrips (width x = 350 nm, center distance y = 640 nm and height z = 180 nm). The AFM images (Figure 1e and 1h) further demonstrate a nearly perfect transfer of the triangular MoS2 monolayer to a PDMS substrate. Clear contrast between suspended MoS2 regions and the bottom of the supporting substrate can be defined from the topography (Figure 1e) and the line


Nano Letters

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

profile (Figure 1f). For Si nanostrips patterned substrate (x = 275 nm, y = 605 nm, z = 70 nm), in spite of a small portion of missing strips, the overall triangular shaped suspended MoS2 could be well defined in both SEM image and AFM images (Figure 1c, 1f and 1i). The suspended MoS2 flakes over Si nanoholes (diameter d = 290 nm, center distance between nearest holes y = 700 nm, z = 350 nm) are shown in Figure 1d, 1g and 1j. In order to verify the clean and nondestructive nature of our method, we have performed indentation test on MoS2 flakes suspended on Si nanoholes using AFM. As shown in Figure 1k, the Young’s modulus measured from the indentation test is 355.9 ± 60.5 GPa with a flake thickness of 10 nm. This value is very close to or even higher than the results from exfoliated MoS2 (330 GPa), which is believed to be the cleanest crystal of MoS2 with minimal defects45, 46. To acquire a fundamental insight into this transfer method, it is essential to look into the nanoscale solid-liquid-vapor (SLV) interface between the solution and substrate. For patterned substrates, there are two distinct wetting modes: Cassie-Baxter mode and the Wenzel mode. In the Cassie-Baxter mode, the solution is suspended over the nanopatterns leaving air-pockets between the droplet and the bottom surfaces of the substrate and a notable example is the Lotus leaf surface where the solution is strongly repelled by the superhydrophobic surface. The wetting behavior of the Cassie-Baxter mode can be predicted by theoretical calculations from the Cassie-Baxter equation47. The relation between apparent contact angle, (θi) (measured on nanopatterned substrates) and true contact angle, 𝜃!,! , (measured on the flat substrate) is given by the Cassie-Baxter equation as follows, cos 𝜃! = 𝜑 (cos 𝜃!,! + 1) − 1, (𝑒𝑞. 1)


Page 6 of 24

Page 7 of 24

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

Nano Letters

where 𝜑 is the ratio of substrate that is in contact with the solution over the projected area (A0)3. In the Wenzel mode, the solution completely wets the surface of the substrates and the relation between the apparent contact angle (θi), and true contact angle, 𝜃!,! , can be described by the Wenzel equation based on the modified Young’s equation48, cos 𝜃! = 𝑟 cos 𝜃!,! , (𝑒𝑞. 2) where r is the ratio of the actual surface area (A) that is in contact with the solution and the projected area (A0). 𝜑 = 𝑥/𝑦 and 𝑟 = (𝑦 + 2𝑧)/𝑦 for nanostrips and 𝜑 = (2 3𝑦 ! − 𝜋𝑥 ! )/ 2 3𝑦 ! and 𝑟 = ( 3𝑦 ! + 2𝜋𝑥𝑧)/ 3𝑦 ! for nanoholes. The parameters of nanostructures are defined in Figure 2a. Clearly, an ideal model would be sandwiching the 2D materials at a Cassie-Baxter interface between the solution and patterned substrate. If air-pockets are trapped between 2D materials and the bottoms of substrate, capillary force that usually drags the suspended architecture down to the bottom of substrate can be eliminated. Therefore, we first studied the composition of SLV interface (Cassie-Baxter interface only, Wenzel interface only or a mixture of two) by contact angle measurement. We have also realized that the stability or the composition of SLV interface is also influenced by the initial energy input from the droplet. For example, Patankar demonstrated that by simply elevating the droplet, a transition from Cassie-Baxter to Wenzel mode could be initiated49. Similarly, the disturbances during the transferring process (i.e., detaching donor substrate and removing HF solution) might also influence the SLV interface. Therefore, we tried to mimic the disturbance during the transferring process by changing the height of droplets (the distance between head of needle and substrate) from 2 to 20 mm, as shown in Figure 2b. For h = 2 mm, the droplet is very gently deposited. For h = 20 mm, the


Nano Letters

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

droplet has a momentum when hitting the substrate. Flat Si substrate treated by HF acid solution has a true contact angle (measured by sessile droplet technique) of 𝜃!,! = 70° for both droplet heights. However, PDMS with low surface energy (19.8 mJ/m2) and much lower Young’s modules shows different true contact angles 𝜃!,! with respect to different droplet heights, 𝜃!,! =100.2° and 116° corresponding to h = 20 mm and 2 mm respectively (Figure 2b). As we expected, for all of the nanopatterned substrates, the initial disturbance can significantly influence the SLV interface and instantly decrease the apparent contact angles (𝜃! ), as shown in Figure 2c. We have compared the experimental result of Si nanoholes with theoretical predictions where the calculated 𝜃!,!"##$%!!"#$%& (red solid line) and 𝜃!,!"#$"% (black dashi line) are plotted with respect to the center distance, y (Figure S2). At h = 2 mm, 𝜃! (91°) is close to

𝜃!,!"##$%!!"#$%& (106.5°) suggesting a predominant suspended SLV interface (Cassie-Baxter interface, see Figure S2a). A migration of 𝜃! towards a smaller value can be found with an elevated droplet (h = 20 mm), however, the value (𝜃! = 64°) is still far above the predicted 𝜃!,!"#$"% (~ 0°) for a completely wetted surface (Wenzel mode, black dash line). This suggests an intermediate state where SLV interface could compose both the Wenzel interface and CassieBaxter interface. The results suggests that with minimum disturbance from external environments, 2D monolayer would be sandwiched at the SLV interface with predominately a Cassie-Baxter interface leading to well defined suspended architectures which is supported by the SEM images of suspended MoS2 on Si nanoholes (Figure 1d). For anisotropically patterned substrates such as nanostrips, the contact angle measurements are more complicated and the apparent contact angles were measured in both front view (𝜃!,! ) and side view (𝜃!,∥ ). The solution wet the nanostrips in two distinct directions where the contact angle


Page 8 of 24

Page 9 of 24

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

Nano Letters

measurement in the front view (𝜃!,! ) is usually larger than that in the side view (𝜃!,∥ ) because the alternating solid-vapor interfaces present barriers to the solution propagating in the direction perpendicular to the alignment direction of the nanostrips50-54. In the case of Si nanostrips (Figure S2b), a downward shift can be defined in both 𝜃!,! and 𝜃!,∥ along with the increasing droplet height suggesting an increased ratio of the Wenzel interface. However, even for h = 20 mm, both the 𝜃!,! and 𝜃!,∥ are above 𝜃!,!"#$"% suggesting a mixed SLV interface. The measurements on PDMS substrates also suggest transition from Cassie-Baxter interface to Wenzel interface (see supporting information, Figure S2c and S2d). In order to further clarify this unique SLV interface, we have employed a labelling technology by adding Au nanoparticles (40 nm in diameter) into the HF solution as trajectory tracking while performing the identical transferring process over nanostrips patterned PDMS and Si substrates, as shown in Figure 3a. As a result, the Au nanoparticles on the surfaces can be used as marker for HF acid solution locale. For PDMS nanostrips substrate, we could clearly identify a CassieBaxter interface47,

55

, where the solution is suspended over the nanopatterned substrate as

evidenced by the fact that Au nanoparticles were only found on the top of the PDMS nanostrips as shown in the AFM image in Figure 3b. This suggests that no solution is trapped between 2D materials and bottom of the substrate (Figure 3c), and thereby no capillary force exists leading to suspended 2D architectures. However, for Si nanostrips, nanoparticles can be found both on the top of nanostrips (bright lines) and at the bottom of the trenches (dark lines) (Figure 3d), suggesting at least some part of the solution cannot maintain a Cassie-Baxter interface and in some regions, transition to completely wetting takes place48. These results are consistent with contact angle measurement in Figure 2c and Figure S2b. A close observation near the suspended MoS2 reveals that Au nanoparticles are located on the top of the MoS2 (Figure 3d), with only a


Nano Letters

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

Page 10 of 24

few exceptions where the Au nanoparticles were attached to the edge of MoS2 from the bottom (Figure S3). The nanoparticles attached from the bottom show much less contrast and brightness compared to those located on the top of MoS2 (Figure S3). Some of Au particles can be also found in the trench very close to the edge of suspended MoS2 suggesting that penetration of solution stops at the entrance of the air-pocket underneath the suspended MoS2 (Figure 3e). This phenomenon suggests even when MoS2 is surrounded by a Wenzel interface, no solution can indeed penetrate into the air-pocket underneath the MoS2, as shown in a cross-sectional schematic right under the MoS2 (Figure 3e and inset). The reason for strong wetting resistance at the entrance of the air-pocket can be interpreted by considering the wetting process of an airpocket with a nano-rectangular cross section (330 nm × 200 nm). There are four surfaces along the air-pocket, three Si surfaces and one MoS2 surface, and both Si and MoS2 (surface energy of 40.47 mJ/m2 with a water contact angle of 97.8°56) deliver high surface energy. This brings great difficulties for the HF acid solution to penetrate underneath the MoS2. As a result, it is reasonable to believe that a long nanoscale pocket covered by a large-scale MoS2 monolayer can prevent the water from penetrating underneath. This finding is extremely meaningful which highlight the scalability of wet contact print method over substrates with higher surface energy where the Wenzel mode is dominant.

We believe the conformal contact between 2D materials and receiving substrate at the beginning of the transferring process plays a key role. It provides a kinetic barrier to prevent the solution from penetrating into the interface and air-pocket between 2D materials and receiving substrate. This is supported by an experiment in which we transferred a MoS2 monolayer with big particles presented at the center to Si nanostrips. It is interesting to find a missing part near the big particle


Page 11 of 24

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

Nano Letters

in the center as shown in Figure S4a. Still, well defined suspended MoS2 can be found on the edge of the large flake (Figure S4b). We have observed this in a series of samples. This suggests that the existence of a big particle prevents the two substrates from conformal contact and the MoS2 region that is not in contact with Si can be detached away when removing the solution. This is especially true for rigid substrates. On the other hand, flexible PDMS substrate which can deform and enable conformal contact shows higher success rate in obtaining suspended architectures (Figure 1b).

Figure 4 demonstrates the success of the wet-contact printing method on other 2D materials: MoSe2 (the first row, Figure 4a to 4c), WS2 (the second row, Figure 4d to 4f), and graphene the third row, Figure 4g to 4h) on both Si and PDMS (the third column) nanostrips patterned substrates. The first column shows the titled angle view SEM images of suspended 2D architectures on Si nanostrips. The second column shows the optical images of suspended architectures on PDMS nanostrips. The Raman spectra of 2D material on SiO2/Si substrate (before transfer) and on Si and PDMS nanopatterned substrates are summarized in the third column. The corresponding photoluminescence spectra of all the 2D material presented in this paper before and after transferring are summarized in Figure S5 showing representative peaks of these 2D materials. The surface energy of graphene is measured to be 46.7 mJ/m2 with a water contact angle of 127°57. Even though the surface energy of monolayer WS2 is unavailable, in the bulk format, it is hydrophobic with a water contact angle of 93°. Therefore, the mechanism regarding water resist air pocket is still validated. The success of different suspended 2D materials demonstrates the wet-contact printing method is a universal method to fabricate suspended architectures from 2D materials. Considering the fact that the surface properties of the


Nano Letters

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

Page 12 of 24

substrate as well as 2D materials can be further tuned by chemical reactions, SAM coating and etc., this method could be extended to arbitrary substrates and many other 2D materials. Therefore, this method paves the way to the large-scale fabrication of nanoscale electronics based on 2D materials.

To further demonstrate the unique advantages of these suspended 2D materials by our wetcontact printing method, a state-of-art photodetector based on this structure was demonstrated. First, we designed an interdigitated nanopatterned electrodes (180 nm Ag on the bottom and 20 nm Au on the top as anti-oxidation layer, 375 nm in width and 750 nm in center-to-center spacing) on hafnium oxide (HfO2 = 200nm)/Si substrate. Here, The HfO2 is employed as a dielectric material with high dielectric constant and is resistant to HF acid. As shown in Figure 5a and 5b, these nanopatterned electrodes are uniform, which can serve as a target substrate to suspend MoS2 films for direct photoelectronic measurements. The suspended MoS2 based photodetector was prepared by directly transferring MoS2 onto these nanopatterned electrodes by the wet-contact printing method (Figure 5b). It should be noted that device fabrication such as placing contact pads on suspended 2D materials is of great challenge. Bao et al., managed to deposit electrodes on suspended graphene using a patterned Si shadow mask through a delicate alignment process24. Unfortunately, this method needs highly accurate alignment tools and is time consuming. The method we present here is a one-step method where the device can be directly suspended on the nanoscale interdigitated electrodes. The photoresponses of suspended MoS2 under an illumination with a 543 nm He-Ne laser with tunable powers are shown in Figure 5c. The device area, A, is 6.85 µm2 and the responsivities at 0.5 V are calculated to be 35.6 and 26.4 A/W for 130 mW/cm2 (50 µW) and 260 mW/cm2 (100 µW) respectively. Correspondingly,


Page 13 of 24

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

Nano Letters

unsuspended MoS2 photodetector (device area, A = 40.2 µm2) show lower responsivity (0.5 V) of 6.4 and 4.7A/W for 130 mW/cm2 and 260 mW/cm2 respectively (Figure 5d and Figure S6). The improvement in the photoelectrical performance could be attributed to the suspended MoS2 architecture, which can eliminate the effects of substrates such as phonon scattering, polarization active sites, and electron-hole recombination centers created by surface defects.

To confirm the non-destructive nature of our method, we have compared the optical property of transferred MoS2 obtained by our method with PMMA assisted transfer methods on the same flat substrate. The narrower optical band gap (PL spectra) of samples obtained by wet-contact printing suggests the best sample quality (Figure S7). In addition, the influence of HF acid solution (16%) on the properties of 2D materials is another concern. It is worth emphasizing that the etching time in our method is only ~ 1 minute which is much shorter than the several hours etching period in KOH solution as widely adopted for transferring transition metal dichalcogenides41,

58

. Our previous study on monolayer MoS2 using scanning transmission

electron microscopy suggests that short term HF etching will not affect the crystal structure of MoS241. To further verify the influence of HF etching, we transferred MoS2 to flat PDMS substrate with different HF etching time and compared the PL spectra. No apparent band gap change can be monitored from 1 to 3 min (Figure S8a and S8b). Further electrical measurements suggest the resistance of the MoS2 sample did not encounter significant change (less than 1 %) during the short period of HF etching (Figure S8c).

In summary, for the first time, large-scale suspended 2D material architectures have been demonstrated on nanoscale patterned substrates using CVD grown 2D materials and a wet-


Nano Letters

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

Page 14 of 24

contact printing method. The transferring method represents a fast, clean and scalable technique to suspend 2D architectures. The unique SLV interface at nanoscale warrants its great feasibility to substrates with distinct surface properties and nanopatterns. Together with the direct formation of suspended photodetectors on interdigitated electrodes, this wet-contact printing method demonstrates great potential in large-scale fabrication of suspended 2D devices. This strategy could not only greatly improves the scalability of 2D suspended device fabrication, but also be extended to other nanomaterials and a wide range of substrates. Therefore, our research will open pathway towards high performance electrical, optoelectrical and nanoelectromechnical systems.

Author Contributions B.L. conceived the idea and designed the experiments. Y.H. and B.L. performed transferring, SEM imaging, Raman and Photoluminescence spectroscopy and contact angle measurement, S.L. performed the electrical and optoelectrical measurements. S.N., Y.G., J.Z, L.M. synthesized the materials. X.W. performed the nanoindentation measurement. Y.Y. performed the AFM imaging. X.Z. prepared the reference samples using PMMA assisted transfer. A.G. G.S. and J.H. fabricated the nanoscale substrates. S.H. contributed to the development of the wetting model. The manuscript was written by B.L., Y.H., R.V., Y.J.J, and P.M.A with comments and input from all authors.

Acknowledgement This work is supported by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA. Y.H. acknowledges the financial support from China Scholarship Council. Y.J.J. acknowledges the financial support from National Science Foundation-ECCS grant (1202376), US Army under grant W911NF-10-2-0098, subaward 15-215456-03-00. We acknowledge Dr. Matin Amani and Dr. Madan Dubey from Army Research Laboratory for providing the HfO2 substrate.


Page 15 of 24

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

Nano Letters

Supporting Information Methods, water wetting of patterned substrates, influence of conformal contact, PL spectra of suspended 2D materials, unsuspended MoS2 photodetector, demonstration of non-destructive nature of wet-contact printing method and the influence of HF etching, mechanical measurement. This material is available free of charge via the Internet at http://pubs.acs.org.


Nano Letters

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

Page 16 of 24

References 1. Novoselov, K. S.; Geim, A. K.; Morozov, S.; Jiang, D.; Zhang, Y.; Dubonos, S.; Grigorieva, I.; Firsov, A. Science 2004, 306, (5696), 666-669. 2. Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z.; Storr, K.; Balicas, L. Nat Mater 2010, 9, (5), 430-435. 3. Lei, S.; Ge, L.; Liu, Z.; Najmaei, S.; Shi, G.; You, G.; Lou, J.; Vajtai, R.; Ajayan, P. M. Nano Lett 2013, 13, (6), 2777-2781. 4. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Nat Nanotechnol 2012, 7, (11), 699-712. 5. Song, L.; Ci, L. J.; Lu, H.; Sorokin, P. B.; Jin, C. H.; Ni, J.; Kvashnin, A. G.; Kvashnin, D. G.; Lou, J.; Yakobson, B. I.; Ajayan, P. M. Nano Lett 2010, 10, (8), 3209-3215. 6. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Phys Rev Lett 2010, 105, (13), 136805. 7. Wang, X.; Gong, Y.; Shi, G.; Chow, W. L.; Keyshar, K.; Ye, G.; Vajtai, R.; Lou, J.; Liu, Z.; Ringe, E. ACS Nano 2014, 8, (5), 5125-5131. 8. Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; LópezUrías, F.; Crespi, V. H.; Terrones, H.; Terrones, M. Nano Lett 2012, 13, (8), 3447-3454. 9. Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; Ajayan, P. M. ACS Nano 2014, 8, 1263–1272. 10. Lei, S.; Sobhani, A.; Wen, F.; George, A.; Wang, Q.; Huang, Y.; Dong, P.; Li, B.; Najmaei, S.; Bellah, J.; Ajayan, P. M. Adv Mater 2014, 26, 7666-7672. 11. Lee, J.-H.; Lee, E. K.; Joo, W.-J.; Jang, Y.; Kim, B.-S.; Lim, J. Y.; Choi, S.-H.; Ahn, S. J.; Ahn, J. R.; Park, M.-H. Science 2014, 344, (6181), 286-289. 12. Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. Nature 2015, 520, (7549), 656-660. 13. Zhou, S.; Gweon, G.-H.; Fedorov, A.; First, P.; De Heer, W.; Lee, D.-H.; Guinea, F.; Neto, A. C.; Lanzara, A. Nat Mater 2007, 6, (10), 770-775. 14. Dolui, K.; Rungger, I.; Sanvito, S. Phys Rev B 2013, 87, (16), 165402. 15. Yankowitz, M.; Xue, J.; Cormode, D.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; Jacquod, P.; LeRoy, B. J. Nat Phys 2012, 8, (5), 382-386. 16. Yang, W.; Chen, G.; Shi, Z.; Liu, C.-C.; Zhang, L.; Xie, G.; Cheng, M.; Wang, D.; Yang, R.; Shi, D. Nat Mater 2013, 12, (9), 792-797. 17. Najmaei, S.; Zou, X.; Er, D.; Li, J.; Jin, Z.; Gao, W.; Zhang, Q.; Park, S.; Ge, L.; Lei, S. Nano Lett 2014, 14, (3), 1354-1361. 18. Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Nat Nanotechnol 2010, 5, (10), 722-726. 19. Xue, J.; Sanchez-Yamagishi, J.; Bulmash, D.; Jacquod, P.; Deshpande, A.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; LeRoy, B. J. Nat Mater 2011, 10, (4), 282-285. 20. Huang, Z.; He, C.; Qi, X.; Yang, H.; Liu, W.; Wei, X.; Peng, X.; Zhong, J. J Phys D Appl Phys 2014, 47, (7), 075301. 21. Gillen, R.; Robertson, J.; Maultzsch, J. Phys Status Solidi (b) 2014, 251, 2620-2625. 22. Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Nat Nanotechnol 2008, 3, (8), 491-495. 23. Adam, S.; Das Sarma, S. Solid State Commun 2008, 146, (9), 356-360. 24. Bolotin, K. I.; Sikes, K.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. Solid State Commun 2008, 146, (9), 351-355.


Page 17 of 24

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

Nano Letters

25. Bao, W.; Liu, G.; Zhao, Z.; Zhang, H.; Yan, D.; Deshpande, A.; LeRoy, B.; Lau, C. N. Nano Res 2010, 3, (2), 98-102. 26. Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, (5887), 385-388. 27. Balandin, A. A. Nat Mater 2011, 10, (8), 569-581. 28. Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, (5811), 490-493. 29. Bao, W.; Miao, F.; Chen, Z.; Zhang, H.; Jang, W.; Dames, C.; Lau, C. N. Nat Nanotechnol 2009, 4, (9), 562-566. 30. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. P Natl Acad Sci USA 2005, 102, (30), 10451-10453. 31. Lee, G.-H.; Cooper, R. C.; An, S. J.; Lee, S.; van der Zande, A.; Petrone, N.; Hammerberg, A. G.; Lee, C.; Crawford, B.; Oliver, W. Science 2013, 340, (6136), 1073-1076. 32. Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Nano Lett 2008, 9, (1), 30-35. 33. Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Nature 2009, 457, (7230), 706-710. 34. Suk, J. W.; Kitt, A.; Magnuson, C. W.; Hao, Y.; Ahmed, S.; An, J.; Swan, A. K.; Goldberg, B. B.; Ruoff, R. S. ACS Nano 2011, 5, (9), 6916-6924. 35. Dean, C.; Young, A.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. Nat Nanotechnol 2010, 5, (10), 722-726. 36. Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E. Science 2009, 324, (5932), 1312-1314. 37. Gurarslan, A.; Yu, Y.; Su, L.; Yu, Y.; Suarez, F.; Yao, S.; Zhu, Y.; Ozturk, M.; Zhang, Y.; Cao, L. ACS Nano 2014, 8, (11), 11522-11528. 38. Lee, Y.-H.; Yu, L.; Wang, H.; Fang, W.; Ling, X.; Shi, Y.; Lin, C.-T.; Huang, J.-K.; Chang, M.-T.; Chang, C.-S. Nano Lett 2013, 13, (4), 1852-1857. 39. Gao, L.; Ni, G.-X.; Liu, Y.; Liu, B.; Neto, A. H. C.; Loh, K. P. Nature 2014, 505, (7482), 190-194. 40. Lau, C. N.; Bao, W. Z.; Velasco, J. Mater Today 2012, 15, (6), 238-245. 41. Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X. L.; Shi, G.; Lei, S. D.; Yakobson, B. I.; Idrobo, J. C.; Ajayan, P. M.; Lou, J. Nat Mater 2013, 12, (8), 754-759. 42. Gong, Y.; Shi, G.; Zhang, Z.; Zhou, W.; Jung, J.; Gao, W.; Ma, L.; Yang, Y.; Yang, S.; You, G. Nat Commun 2014, 5,3193. 43. Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.; Yakobson, B. I. Nat Mater 2014, 13, 1135–1142. 44. Liu, Z.; Ma, L. L.; Shi, G.; Zhou, W.; Gong, Y. J.; Lei, S. D.; Yang, X. B.; Zhang, J. N.; Yu, J. J.; Hackenberg, K. P.; Babakhani, A.; Idrobo, J. C.; Vajtai, R.; Lou, J.; Ajayan, P. M. Nat Nanotechnol 2013, 8, (2), 119-124. 45. Castellanos-‐Gomez, A.; Poot, M.; Steele, G. A.; van der Zant, H. S.; Agraït, N.; Rubio-‐ Bollinger, G. Adv Mater 2012, 24, (6), 772-775. 46. Castellanos-Gomez, A.; Poot, M.; Steele, G. A.; van der Zant, H. S. J.; Agrait, N.; RubioBollinger, G. Nanoscale Res Lett 2012, 7, 1-4. 47. Cassie, A.; Baxter, S. T Faraday Soc 1944, 40, 546-551. 48. Wenzel, R. N. Ind Eng Chem 1936, 28, (8), 988-994. 49. Patankar, N. A. Langmuir 2004, 20, (17), 7097-7102. 50. Chen, Y.; He, B.; Lee, J.; Patankar, N. A. J Colloid Interf Sci 2005, 281, (2), 458-464.


Nano Letters

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

Page 18 of 24

51. Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. Langmuir 2005, 21, (3), 911-918. 52. Zhao, Y.; Lu, Q.; Li, M.; Li, X. Langmuir 2007, 23, (11), 6212-6217. 53. Chung, J. Y.; Youngblood, J. P.; Stafford, C. M. Soft Matter 2007, 3, (9), 1163-1169. 54. Xia, D.; Brueck, S. Nano Lett 2008, 8, (9), 2819-2824. 55. Quéré, D. Ann Rev Mater Res 2008, 38, 71-99. 56. Gaur, A. P.; Sahoo, S.; Ahmadi, M.; Dash, S. P.; Guinel, M. J.; Katiyar, R. S. Nano Lett 2014, 14, 4314−4321. 57. Wang, S.; Zhang, Y.; Abidi, N.; Cabrales, L. Langmuir 2009, 25, (18), 11078-11081. 58. Tongay, S.; Fan, W.; Kang, J.; Park, J.; Koldemir, U.; Suh, J.; Narang, D. S.; Liu, K.; Ji, J.; Li, J. Nano Lett 2014, 14, (6), 3185-3190.


Page 19 of 24

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

Nano Letters

Figures

Figure 1. (a) The schematic of wet-contact printing method. (b) Suspended MoS2 on nanostrips pattered PDMS substrate (width x = 350 nm, center distance y = 700 nm and height z = 200 nm). (c) Suspended MoS2 on nanostrips pattered Si substrate (x = 275 nm , y = 605 nm, z = 70 nm). (d) Suspended MoS2 on hexagonal nanoholes patterned Si substrates (diameter d = 290 nm, center distance between nearest holes y = 700 nm, z = 350 nm). AFM images of suspended MoS2 triangle on PDMS nanostrips with topography (e) and a line profile (h), Si nanostrips with topography (f) and a line profile (i) and Si nanoholes with topology (g) and a line profile (j). (k) Force-displacement curve from AFM indentation measurement. Scalar bars: (b) 40 µm (c) 2 µm (d) 1 µm.


Nano Letters

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

Page 20 of 24

Figure 2. Contact angle measurements of nanopatterned substrates. (a) Schematic of nanoholes and nanostrips. For nanoholes, x is diameter of the nanoholes, y is the center distance of the nanoholes with hexagonal arrays, z is the height. For nanostrips, x is width of the nanostrips, y is the center distance of the nanoholes with hexagonal arrays, z is the height. (b) Schematic of contact angle measurement with changing the height of droplet (h = 2 and 20 mm) and measurement of true contact angle on flat PDMS and Si substrates with respect to different droplet conditions. (c) Contact angle measurement of Si nanostrips, PDMS nanostrips and Si nanoholes with different height. For Si nanoholes, the substrate is isotropic and there is no different between front view contact angle and side view contact angle.


Page 21 of 24

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

Nano Letters

Figure 3. Diagnose the nanoscale Solid-liquid-vapor interface during transferring process using Au nanoparticles. (a) Schematic of mixing Au nanoparticle (diameter = 40 nm) in HF acid solution. (b) The distribution of Au nanoparticle on PDMS nanostrips. (c) The schematic of Cassie-Baxter interface with suspended droplet. (d) The distribution of Au nanoparticle on Si nanostrips and near suspended MoS2. The brighter strips are top surfaces of the Si nanostrips and the dark strips are the bottom surface of Si nanotrenches between the nanostrips. Scalar bar, 500 nm. (e) The schematic of wetting condition near the MoS2 and the cross sectional view of SLV interface right under suspended MoS2.


Nano Letters

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

Page 22 of 24

Figure 4. Suspended 2D materials on Si and PDMS substrates. Titled angle SEM images of MoSe2 (a), WS2 (d) and graphene (g) on Si nanostrips Scalar bars: (a) 2 µm (d) 500 nm (g) 200 nm. Optical images of MoSe2 (b), WS2 (e) and graphene (h) on PDMS nanostrips. Scalar bars: (b) 40 µm (e) 10µm (h) 10 µm. Raman spectrums of MoSe2 (c), WS2 (f), graphene (i) on PDMS, SiO2, and Si nanostructures, respectively.


Page 23 of 24

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

Nano Letters

Figure 5. Photodetector based on suspended MoS2. (a) The schematic of suspended MoS2 on Ag/Au interdigitated electrodes built on HfO2 (200 nm)/Si substrate and the AFM image of Ag/Au interdigitated electrodes. (b) Tilted angle SEM image on suspended MoS2 on interdigitated electrodes. Scalar bars: 500 nm (c) Photoresponse of suspended MoS2 excited by 543 nm laser at different powers (50 and 100 µW). Device area, A, is 6.85 µm2. (d) Photoresponse of non-suspended MoS2 excited at the same conditions. Device area, A, is 40.2 µm2.


Nano Letters

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

Table of Contents Wet-‐Contact Printing Method

Patterned Substrate2D Materials

HF solution

Suspended 2D Materials

Contact

MoS2 on PDMS

Etching SiO2

MoSe2 on Si

Dry WS2 on Si


Page 24 of 24

Scalable Transfer of Suspended Two-Dimensional Single Crystals

Recommend Documents