Fabricating van der Waals Heterostructures with Precise Rotational Alignment

Engineering

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Summary

In this work we describe a technique that is used to create new crystals (van der Waals heterostructures) by stacking ultrathin layered 2D materials with precise control over position and relative orientation.

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Boddison-Chouinard, J., Plumadore, R., Luican-Mayer, A. Fabricating van der Waals Heterostructures with Precise Rotational Alignment. J. Vis. Exp. (149), e59727, doi:10.3791/59727 (2019).

Abstract

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We demonstrate not only lateral control but, importantly, also control over the angular alignment of adjacent layers. The core of the technique is represented by a home-built transfer setup which allows the user to control the position of the individual crystals involved in the transfer. This is achieved with sub-micrometer (translational) and sub-degree (angular) precision. Prior to stacking them together, the isolated crystals are individually manipulated by custom-designed moving stages that are controlled by a programmed software interface. Moreover, since the entire transfer setup is computer controlled, the user can remotely create precise heterostructures without coming into direct contact with the transfer setup, labeling this technique as “hands-free”. In addition to presenting the transfer set-up, we also describe two techniques for preparing the crystals that are subsequently stacked.

Introduction

Research in the burgeoning field of two-dimensional (2D) materials began after researchers developed a technique which enabled the isolation of graphene1,2,3 (an atomically flat sheet of carbon atoms) from graphite. Graphene is a member of a larger class of layered 2D materials, also referred to as van der Waals materials or crystals. They have strong covalent intralayer bonding and weak van der Waals interlayer coupling. Therefore, the technique for isolating graphene from graphite can also be applied to other 2D materials where one can break the weak interlayer bonds and isolate single layers. One key development in the field was the demonstration that just as the van der Waals bonds holding adjacent layers of two-dimensional materials together can be broken, they can also be put back together2,4. Therefore, crystals of 2D materials can be created by controllably stacking together layers of 2D materials with distinct properties. This spurred a great deal of interest, as materials previously inexistent in nature can be created with the goal of either uncovering formerly inaccessible physical phenomena4,5,6,7,8,9 or developing superior devices for technology applications. Therefore, having precise control over stacking 2D materials has become one of the main goals in the research field10,11,12.

In particular, the twist angle between adjacent layers in van der Waals heterostructures was shown to be an important parameter for controlling material properties13. For example, at some angles, the introduction of a relative twist between adjacent layers can effectively electronically decouple the two layers. This was studied both in graphene14,15 as well as in transition metal dichalcogenides16,17,18,19. More recently, it was surprisingly found that it can also alter the state of matter of these materials. The discovery that bilayer graphene oriented at a “magic angle” behaves as a Mott insulator at low temperatures and even a superconductor when the electron density is properly tuned has sparked great interest and a realization of the importance of the angular control when fabricating layered van der Waals heterostructures13,20,21.

Motivated by the scientific opportunities opened up by the idea of tuning the properties of novel van der Waals materials by adjusting the relative orientation between the layers, we present a home-built instrument along with the procedure to create such structures with angular control.

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Protocol

1. Instrumentation for the transfer procedure

  1. In order to visualize the transfer process, utilize an optical microscope that can operate under bright-field illumination. Since the typical sizes of the 2D crystals are 1–500 µm2, equip the microscope with 5x, 50x, and 100x long working distance objectives. The microscope must also be equipped with a camera that connects to a computer (Figure 1a).
  2. Use separate manipulators to individually control the position of the two crystals that are about to be stacked. Employ manipulators that are programmable and controllable by a computer to minimize vibrations during the transfer procedure.
    NOTE: The manipulators responsible for the movement of the top substrate holder (Figure 1b-c) need only move in the X, Y, and Z direction. Importantly, the manipulators responsible for controlling the bottom substrate holder (Figure 1e-f) are also able to rotate by any angle ϴ (translational and rotational motion).
  3. In order to attach the samples onto the top stage manipulators, fabricate custom sample holders that can support a glass slide; the top crystal will be placed on the glass slide (Figure 1d).
  4. For the bottom manipulators, place a flat heating element in a machined glass-ceramic holder (Figure 1g) and affix it to the rotating stage. Connect the heating element to a power supply and a temperature controller.
  5. Program the controllers with an instrumentation software (e.g., LabVIEW) to control the relative position of the manipulators (Figure 2).
    1. To perform the necessary motions, program the software with the following capabilities: individually or simultaneously move the manipulators; read, save and retrieve the position of each manipulator; easily adjust the speed of the manipulators, and autofocus the bottom stage. Build-in safety features to prevent any possible collisions between the sample and the lens.

2. Mechanical exfoliation of a 2D crystal.

  1. Prepare a substrate for the mechanical exfoliation procedure.
    1. Submerge 1 cm x 1 cm squares of silicon/silicon oxide (Si/SiO2) wafers into a beaker filled with acetone and place the beaker into an ultrasonic cleaner for 10 min.
    2. Individually remove the wafers from the beaker with tweezers and rinse them with isopropanol (IPA) then dry them with a nitrogen (N2) gun.
      NOTE: When working with acetone and IPA, it is suggested to do so under a fume hood while wearing the proper PPE.
  2. Mechanically exfoliate the crystal onto the substrate.
    1. Using tweezers, carefully remove a portion of the crystal and place it on a piece of semiconductor-grade adhesive tape.
    2. Take a second piece of adhesive tape and firmly press it against the initial tape with crystal then peel away the two pieces of tape. After repeating several times, many thinner pieces of crystal will be found on the tape.
    3. Press the adhesive tape with the thin 2D crystals onto a freshly cleaned substrate such that the crystal is in direct contact with the substrate and peel away the tape to leave exfoliated flakes on the substrate.
  3. To remove any residual adhesive, place the resulting samples (substrates with 2D crystals exfoliated onto their surface) in a beaker filled with acetone for 10 min. Remove the samples using tweezers, rinse them with IPA, and dry with a N2 gun.
  4. Use an optical microscope to examine the exfoliated flakes. Estimate their thickness by assessing the flake’s optical contrast with the substrate22. Image the flakes using atomic force microscopy (AFM) in tapping mode (see the Table of Materials) to better quantify the surface morphology and to measure the flake thickness.

3. PMMA-PVA method for fabricating van der Waals heterostructures (top substrate preparation).

  1. Prepare the top substrate for the transfer procedure by exfoliating the crystal on a poly(methyl methacrylate) (PMMA) film attached to a glass slide (Figure 3a-d).
    1. Follow the procedure described in step 2.1. to obtain a clean substrate. Spin coat a layer of polyvinyl alcohol (PVA) on the substrate at 3,000 rpm for 1 min by following the protocol described in the instrument's user manual.
      NOTE: When using the spin coater, it is suggested to do so under a fume hood while wearing proper PPE.
    2. Directly place the substrate on a hot plate and bake it uncovered in air at 75 °C for 5 min.
    3. Spin coat a layer of PMMA on the substrate from step 3.1.2 by following a procedure similar to the one in step 3.1.1, but this time set the spinning parameters to an angular velocity of 1,500 rpm for 1 min (Figure 3a).
    4. Directly place the substrate on a hot plate and bake it uncovered in air at 75 °C for 5 min.
    5. Remove the substrate from the hot plate and place pieces of adhesive tape along its edges to create a tape frame. Then, mechanically exfoliate a 2D crystal on the PMMA surface by following step 2.2 (Figure 3b).
    6. Use sharp tweezers to separate the PMMA from the PVA by slowly peeling back the tape frame. The PMMA layer and exfoliated crystal along with the tape frame will detach from the PVA and Si/SiO2 substrate (Figure 3c).
    7. Invert the tape frame and place it on a machined support such that the crystal is facing downwards (Figure 3d).
      NOTE: This support enables the user to place the tape frame under an optical microscope to inspect the exfoliation onto PMMA and identify a flake with the desired thickness and geometry.
    8. Use sharp tweezers and the optical microscope to place a small washer (0.5 mm inner radius) precisely on the PMMA film such that it surrounds the desired flake (Figure 3d).
    9. Lower a glass slide and adhere it to the polymer by pressing it against the exposed tape.

4. Polydimethylsiloxane (PDMS) stamping method for fabricating van der Waals heterostructures (top substrate preparation).

  1. Prepare a polypropylene carbonate (PPC) solution for spin coating by mixing three parts PPC crystal with seventeen parts anisole. This is done by letting the solution mix in a stirrer for approximately 8 h or until the solution is homogeneous.
  2. Prepare the top substrate for the transfer procedure by exfoliating crystal on a PPC film and by then placing it on a PDMS stamp attached to a glass slide (Figure 4a-d).
    1. Follow the procedure in step 2.1. to obtain a clean substrate. Spin coat a layer of PPC on the substrate at 3,000 rpm for 1 min (Figure 4a).
    2. Directly place the substrate on a hot plate and bake it uncovered in air at 75 °C for 5 min.
    3. Remove the substrate from the hot plate and place pieces of adhesive tape along its edges to create a tape frame.
    4. Mechanically exfoliate the 2D layered crystal on the PPC coated substrate by following step 2.2 and use the optical microscope to identify a flake with the desired thickness and geometry. (Figure 4b).
    5. Use scissors or a surgical blade to cut a piece of PDMS into a 2 mm x 2 mm square and place it in an oxygen plasma etcher for 2 min at 50 W and 53.3 Pa.
    6. At the end of the cycle, press a glass slide on the PDMS stamp to bond the two together. Place the glass slide and PDMS stamp back into the oxygen plasma etcher to undergo the same cycle. Remove the glass slide when the cycle has ended.
    7. Using tweezers, carefully peel back the tape frame and pick-up the PPC film with the exfoliated crystal (Figure 4c) and place it on the PDMS stamp such that the desired flake is located on the stamp.

5. Transferring flakes from the top substrate to the bottom substrate using the PMMA-PVA method (Figure 3e-h).

  1. Place a substrate on the bottom stage of the transfer setup. On this substrate, identify the position of the desired flake. This flake will be the “bottom” crystal. Also, place the top substrate (the glass slide from step 3.1.9) into the top substrate holder of the transfer setup (Figure 3e).
  2. Using a low-magnification objective (5x), bring the bottom substrate into focus and center the desired flake. Slowly lower the top substrate until it enters the depth of field of the objective. Adjust the lateral position and the rotational alignment of the two flakes.
  3. Employ a higher-magnification objective (50x) and continue to lower the top substrate while adjusting the flake alignment. Lower the top substrate until the top flake entirely contacts the bottom flake. Contact is noticeable by a sudden change of color.
  4. Heat the bottom substrate to 75 °C for better adhesion of PMMA to the bottom substrate. The PMMA will detach from the glass slide (Figure 3f).
  5. Clean the bottom substrate following step 2.3 to remove the PMMA film (Figure 3g-h).

6. Transferring flakes from the top substrate to the bottom substrate using the stamping method (Figure 4d-f).

  1. Place a substrate on the bottom stage of the transfer setup. On this substrate, identify the position of the desired flake. This flake will be the “bottom” crystal. Also, place the top substrate (the glass slide with PDMS from step 4.2.5–4.2.6) into the top substrate holder of the transfer setup (Figure 4d).
  2. Heat the bottom substrate to 100 °C then follow steps 5.2–5.4 to align and bring into contact the top crystal with the bottom flake (Figure 4e).
  3. Once complete contact is made between the two flakes (Figure 4e), slowly raise the top substrate. This results in the drop-off of the top flake from the stamp to the bottom substrate (Figure 4f).

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Representative Results

To illustrate the outcomes and effectiveness of our procedure we present a sequence of angle-controlled stacks of rhenium disulfide (ReS2) thin crystals. To emphasize that the described method can also be applied to atomically thin layers, we also exemplify the construction of two relatively twisted monolayers of molybdenum disulfide (MoS2).

To demonstrate the angular alignment capabilities of the transfer setup we use rhenium disulfide (ReS2). Because of its in-plane anisotropic lattice structure, this crystal mechanically exfoliates as elongated bars with well-defined edges23,24. This makes it a perfect candidate for the demonstration of angular alignment. Using the PDMS stamping method described in the protocol, we transferred a “top” ReS2 crystal from the stamp onto a silicon substrate with previously exfoliated “bottom” ReS2. Each time we aimed to align the edges at a specific angle. Employing the angular component of the transfer setup, the “top” ReS2 flake was placed such that it would be twisted by the specific desired angle with respect to the already present “bottom” ReS2.

Figure 5 shows an example of a transfer where a top flake of ReS2 was placed on a bottom ReS2 flake with an intended relative angle of 75°. The optical micrographs of the bottom and top crystals are shown in Figure 5a and Figure 5b respectively. Using the PDMS stamping method described in the protocol, a stack was fabricated and the resulting new crystal is shown in Figure 5c. Atomic force microscopy (AFM) was used to image the stack in Figure 5d; this demonstrates that the twist angle between the top and bottom flakes measured 74.6° ± 0.1° (mean ± SD). The error bar was calculated from the uncertainty in precisely defining the edges of the ReS2 in the micrographs. To further display the angular precision of the transfer setup we repeated this process for several other samples with intended relative angles ranging from 15° to 90° in increments of 15° (Figure 6).

Using the PMMA-PVA procedure as described in the protocol, the transfer setup is successfully used to create a structure consisting of two monolayer flakes of molybdenum disulfide (MoS2). The individual monolayers are exfoliated onto PMMA (Figure 7a) and Si/SiO2 respectively (Figure 7b). Our transfer procedure results in the structure presented in the optical micrograph in Figure 7c. Its morphology is further characterized by atomic force microscopy, confirming the thickness and relative position of the stacked MoS2 monolayer (Figure 7d).

Figure 1
Figure 1: Components of the transfer setup. (a) The optical microscope equipped with the camera and three long working distance objectives. (b) The microscope with the top manipulators which are capable of moving in the XYZ (translational) directions. (c) The top manipulators. (d) A custom machined arm which fastens to the top manipulators and is used to hold a glass slide (top sample). (e) The microscope with the bottom manipulators which can move in XYZ (translational) and ϴ (rotational). (f) The bottom manipulators. (g) A custom thermal insulating bottom stage holder (left) which is directly attached to the rotating stage. The temperature controlled flat heating element is shown on the right. Please click here to view a larger version of this figure.

Figure 2
Figure 2: Instrumentation software front panel responsible for controlling the manipulators. The front panel is divided into two sections; the left section controls the bottom stage whereas the right section controls the top manipulators. The user can move the manipulators individually or simultaneously, read out the position of each manipulator, save positions, adjust the speed of the manipulators, and automatically focus. Please click here to view a larger version of this figure.

Figure 3
Figure 3: PMMA-PVA substrate preparation and transfer procedure. (a) A bare Si/SiO2 substrate is first spin coated with PVA followed by PMMA. (b) A tape frame is placed on the substrate to hold down the polymer during mechanical exfoliation. (c) The tape frame is peeled back to pick-up the PMMA layer with the exfoliated crystal. (d) The tape frame is inverted and placed on a machined support followed by an inspection under an optical microscope. A washer is placed on the polymer film such that it surrounds the desired flake. (e) A glass slide is adhered to the tape frame and is placed into the top manipulator of the transfer setup. A previously prepared substrate with exfoliated crystal is placed on the bottom stage. (f) The top and bottom flakes are aligned and brought into contact. By heating the bottom stage to 75 °C, the PMMA detaches from the glass slide leaving (g) the top flake, the PMMA film and the washer on the bottom substrate. (h) The washer is removed and the PMMA is washed away resulting in a heterostructure. Please click here to view a larger version of this figure.

Figure 4
Figure 4: PDMS stamping preparation and transfer procedure. (a) A bare Si/SiO2 substrate is spin coated with PPC. (b) A tape frame is placed on the substrate to hold down the polymer during mechanical exfoliation. (c) The tape frame is peeled back to pick-up the PPC layer with the exfoliated crystal. (d) The glass slide is placed into the top manipulator of the transfer setup. A previously prepared substrate with exfoliated crystal is placed on the bottom stage and heated to 100 °C. (e) The top and bottom flakes are aligned and brought into contact. (f) At 100 °C, we can slowly raise the top stage which results in the drop-off of the flake from the stamp to the bottom substrate. Please click here to view a larger version of this figure.

Figure 5
Figure 5: ReS2 flakes transferred with a relative angle of 75°. Optical micrographs of (a) a ReS2 crystal mechanically exfoliated on a Si/SiO2 substrate, (b) a ReS2 crystal mechanically exfoliated on a Si/SiO2/PPC substrate, (c) the resulting structure after transfer. (d) An AFM topographic map of the final structure corresponding to the boxed area in (c). The measured angle between the ReS2 flakes is 74.6° ± 0.1°. Please click here to view a larger version of this figure.

Figure 6
Figure 6: ReS2 flakes transferred at specific relative angles. (a-f) Optical micrographs of ReS2 flakes transferred with the intention of forming angles ranging from 15° to 90° in increments of 15°. Please click here to view a larger version of this figure.

Figure 7
Figure 7: Twisted MoS2 bilayer. Optical micrographs of (a) the top MoS2 flake on a PMMA film, (b) the bottom MoS2 flake on a Si/SiO2 substrate, (c) and the twisted MoS2 structure. All scale bars in the optical images measure 5 μm. (d) AFM image of the twisted MoS2 structure with a line profile (inset) along the direction indicated by the black dashed line. Please click here to view a larger version of this figure.

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Discussion

The home-built transfer setup presented here offers a method for building novel layered materials with both lateral and rotational control. Compared to other solutions described in the literature10,25, our system does not require complex infrastructure, yet it achieves the goal of controlled alignment of 2D crystals.

The most critical step in the procedure is that of aligning and placing the top crystal in contact with the bottom one. Vibrations could be a cause for a failed alignment, therefore, one must minimize their effect. In this regard, the advantage of the “hands-free” set-up presented here is that the user does not risk introducing vibrations caused by the manual handling of the manipulators. Further improvements can be achieved by placing the set-up in an environment with lower vibrations or on a table equipped with vibration damping mechanisms.

As rotational alignment is becoming a more and more important parameter to consider when creating van der Waals heterostructures, the rotational capability of this transfer setup is one of its strengths. The fact that the optical microscope is limited in resolving the edges of both crystals represents main the limitation in the alignment precision.

Not all 2D materials are inert in air. Crystals such as black phosphorus (BP)26 or chromium triiodide (CrI3)27,28 are known to degrade upon exposure to air. Therefore, to create heterostructures using these materials, and preserve the pristine interface, the transfer procedure would have to take place in an inert environment such as inside a glovebox. Because the transfer instrumentation presented here is “hands-free”, it can be operated in a glovebox where these inert materials could be used.

Lastly, the stacking method presented here can have broader applicability and can be extended to situations when two crystals or a crystal and a substrate need to be precisely aligned laterally and rotationally.

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Disclosures

The authors have nothing to disclose. The authors have no competing financial interests.

Acknowledgments

The authors acknowledge funding from University of Ottawa and NSERC Discovery grant RGPIN-2016-06717 and NSERC SPG QC2DM.

Materials

Name Company Catalog Number Comments
5X objective lens Nikon Metrology MUE12050 23.5 mm working distance and 0.15 numerical aperture
50X objective lens Nikon Metrology MUE21500 19 mm working distance and 0.4 numerical aperture
100X objective lens Nikon Metrology MUE21900 4.5 mm working distance and 0.8 numerical aperture
Acetone Sigma-Aldrich 270725 Purity ≥99.90%
Adhesive tape Ultron Systems, Inc.
Anisole MicroChem
Atomic force microscope Bruker Dimension Icon We typicall use the ScanAsyst mode
Bottom stage rotation manipulator Zaber Technologies X-RSW60A-PTB2 360° travel with step size of 4.091 μrad
Bottom stage X manipulator Zaber Technologies X-LSM025A-PTB2 25 mm travel with step size of 47.625 nm
Bottom stage Y manipulator Zaber Technologies X-LSM025A-PTB2 25 mm travel with step size of 47.625 nm
Bottom stage Z manipulator Zaber Technologies X-VSR40A-KX14A 40 mm travel with step size of 95.25 nm
Isopropanol Sigma-Aldrich 563935 Purity 99.999%
LabVIEW software National Instruments
Macor McMaster-Carr 8489K238
Microscope camera Zeiss 426555-0000-000 5 megapixel, 47 fps live frame rate, exposure time of 100 μs - 2 s, color camera
Molybdenum disulfide (MoS2) HQ Graphene
Optical breadboard Thorlabs, Inc. MB4545/M
Optical microscope Nikon Metrology LV150N
Oxygen plasma etcher Plasma Etch, Inc. PE-50
PDMS stamp Gel-Pak PF-20-X4
PMMA 950 A6 MichroChem Corp. M230006 0500L1GL
Polypropylene carbonate Sigma-Aldrich 389021-100g
PVA Partall #10 Composites Canada
Rhenium disulfide (ReS2) HQ Graphene
Si/SiO2 substrate Nova Electronics Materials HS39626-OX
Spin coater Laurell Technologies WS-650-23
Temperature controller Auber Instruments SYL-23X2-24 Controls the temperature of the bottom stage via a J type thermocouple
Top stage controller unit Mechonics CF.030.0003
Top stage X manipulator Mechonics MS.030.1800 18 mm travel with step size of 11 nm
Top stage Y manipulator Mechonics MS.030.1800 18 mm travel with step size of 11 nm
Top stage Z manipulator Mechonics MS.030.3000 30 mm travel with step size of 11 nm
Ultrasonic bath Elma Schmidbauer GmbH Elmasonic P 30 H

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