Name: Fabricating van der Waals Heterostructures with Precise Rotational Alignment (Video) | JoVE Journal
Uploaded: 2019-07-05T13:00:00.000+00:00
Duration: 9 min 25 s
Description: Learn to fabricate van der Waals heterostructures with precise rotational alignment using ultrathin 2D materials. This technique enables sub-degree angular control for exploring novel physical phenomena.

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5 minutes

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

1. Instrumentation for the transfer procedure
<ol>
	<li>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&#8211;500 &#181;m2, 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).</li>
	<li>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 &#1012; (translational and rotational motion).</li>
	<li>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).</li>
	<li>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.</li>
	<li>Program the controllers with an instrumentation software (e.g., LabVIEW) to control the relative position of the manipulators (Figure 2).
	<ol>
		<li>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.</li>
	</ol>
	</li>
</ol>
2. Mechanical exfoliation of a 2D crystal.
<ol>
	<li>Prepare a substrate for the mechanical exfoliation procedure.
	<ol>
		<li>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.</li>
		<li>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.</li>
	</ol>
	</li>
	<li>Mechanically exfoliate the crystal onto the substrate.
	<ol>
		<li>Using tweezers, carefully remove a portion of the crystal and place it on a piece of semiconductor-grade adhesive tape.</li>
		<li>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.</li>
		<li>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.</li>
	</ol>
	</li>
	<li>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.</li>
	<li>Use an optical microscope to examine the exfoliated flakes. Estimate their thickness by assessing the flake&#8217;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.</li>
</ol>
3. PMMA-PVA method for fabricating van der Waals heterostructures (top substrate preparation).
<ol>
	<li>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).
	<ol>
		<li>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&#39;s user manual. 
		NOTE: When using the spin coater, it is suggested to do so under a fume hood while wearing proper PPE.</li>
		<li>Directly place the substrate on a hot plate and bake it uncovered in air at 75 &#176;C for 5 min.</li>
		<li>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).</li>
		<li>Directly place the substrate on a hot plate and bake it uncovered in air at 75 &#176;C for 5 min.</li>
		<li>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).</li>
		<li>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).</li>
		<li>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.</li>
		<li>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).</li>
		<li>Lower a glass slide and adhere it to the polymer by pressing it against the exposed tape.</li>
	</ol>
	</li>
</ol>
4. Polydimethylsiloxane (PDMS) stamping method for fabricating van der Waals heterostructures (top substrate preparation).
<ol>
	<li>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.</li>
	<li>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).
	<ol>
		<li>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).</li>
		<li>Directly place the substrate on a hot plate and bake it uncovered in air at 75 &#176;C for 5 min.</li>
		<li>Remove the substrate from the hot plate and place pieces of adhesive tape along its edges to create a tape frame.</li>
		<li>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).</li>
		<li>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.</li>
		<li>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.</li>
		<li>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.</li>
	</ol>
	</li>
</ol>
5. Transferring flakes from the top substrate to the bottom substrate using the PMMA-PVA method (Figure 3e-h).
<ol>
	<li>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 &#8220;bottom&#8221; 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).</li>
	<li>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.</li>
	<li>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.</li>
	<li>Heat the bottom substrate to 75 &#176;C for better adhesion of PMMA to the bottom substrate. The PMMA will detach from the glass slide (Figure 3f).</li>
	<li>Clean the bottom substrate following step 2.3 to remove the PMMA film (Figure 3g-h).</li>
</ol>
6. Transferring flakes from the top substrate to the bottom substrate using the stamping method (Figure 4d-f).
<ol>
	<li>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 &#8220;bottom&#8221; crystal. Also, place the top substrate (the glass slide with PDMS from step 4.2.5&#8211;4.2.6) into the top substrate holder of the transfer setup (Figure 4d).</li>
	<li>Heat the bottom substrate to 100 &#176;C then follow steps 5.2&#8211;5.4 to align and bring into contact the top crystal with the bottom flake (Figure 4e).</li>
	<li>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).</li>
</ol>

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</ol>

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

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. ...

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 &#8220;magic angle&#8221; 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.

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

fabricating van der waals heterostructures with precise rotational alignment

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

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 &#8220;hands-free&#8221;. In addition to presenting the transfer set-up, we also describe two techniques for preparing the crystals that are subsequently stacked.

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 ...

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.

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We ...

fabricating-van-der-waals-heterostructures-with-precise-rotational

Research

JoVE Journal

Engineering

The fabrication of electrically addressable, high-aspect-ratio (&gt; 1000:1) metal nanowires separated by gaps of single nanometers using either sacrificial layers of aluminum and silver or self-assembled monolayers as templates is described. These nanogap structures are fabricated without a clean room or any photo- or electron-beam lithographic processes by a form of edge lithography known as nanoskiving.

Fabricating Nanogaps by Nanoskiving

A method for permanently bonding two silicon wafers so as to realize a uniform enclosure is described. This includes wafer preparation, cleaning, RT bonding, and annealing processes. The resulting bonded wafers (cells) have uniformity of enclosure ~1%1,2. The resulting geometry allows for measurements of confined liquids and gasses.

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Atomically Traceable Nanostructure Fabrication

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Probe Type II Band Alignment in One-Dimensional Van Der Waals Heterostructures Using First-Principles Calculations

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In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

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Optimized Fabrication Procedure for High-Quality Graphene-based Moir&#233; Superlattice Devices

This study presents a residue-free fabrication methodology for producing single flakes of two-dimensional materials and assembling them into complex heterostructures using only van der Waals interactions. The technique eliminates the need for external substances and specific experimental conditions, enabling complex heterostructure assemblies through bottom-up, top-down, and modular stacking processes.

Residue-Free Fabrication of van der Waals Heterostructures of Two-Dimensional Materials

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 μm diameter indium wires separated by ~100 μm, which exhibit a terahertz plasmonic response.

Fabricating Metamaterials Using the Fiber Drawing Method

We demonstrate how to determine the size distribution of semiconductor nanocrystals in a quantitative manner using Raman spectroscopy employing an analytically defined multi-particle phonon confinement model. Results obtained are in excellent agreement with the other size analysis techniques like transmission electron microscopy and photoluminescence spectroscopy.

Characterization of Nanocrystal Size Distribution using Raman Spectroscopy with a Multi-particle Phonon Confinement Model

Learn to fabricate van der Waals heterostructures with precise rotational alignment using ultrathin 2D materials. This technique enables sub-degree angular control for exploring novel physical phenomena.

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

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