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 ...

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S. J., Steele, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Folded+MoS2+layers+with+reduced+interlayer+coupling.">Folded MoS2 layers with reduced interlayer coupling.</a> Nano Research. 7 (4), 572-578 (2014).</li><li>van der Zande, A. 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Tuning superconductivity in twisted bilayer graphene Available from: <a target="_blank" href="https://arxiv.org/abs/1808.07865">https://arxiv.org/abs/1808.07865</a> (2018)</li><li>Blake, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Making+graphene+visible.">Making graphene visible.</a> Applied Physics Letters. 91 (6), 063124 (2007).</li><li>Lin, Y. -. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-Layer+ReS2:+Two-Dimensional+Semiconductor+with+Tunable+In-Plane+Anisotropy.">Single-Layer ReS2: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy.</a> ACS Nano. 9 (11), 11249-11257 (2015).</li><li>Chenet, D. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In-Plane+Anisotropy+in+Mono-+and+Few-Layer+ReS2+Probed+by+Raman+Spectroscopy+and+Scanning+Transmission+Electron+Microscopy.">In-Plane Anisotropy in Mono- and Few-Layer ReS2 Probed by Raman Spectroscopy and Scanning Transmission Electron Microscopy.</a> Nano Letters. 15 (9), 5667-5672 (2015).</li><li>Masubuchi, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Autonomous+robotic+searching+and+assembly+of+two-dimensional+crystals+to+build+van+der+Waals+superlattices.">Autonomous robotic searching and assembly of two-dimensional crystals to build van der Waals superlattices.</a> Nature Communications. 9 (1), 1413 (2018).</li><li>Ling, X., Wang, H., Huang, S., Xia, F., Dresselhaus, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+renaissance+of+black+phosphorus.">The renaissance of black phosphorus.</a> Proceedings of the National Academy of Sciences of the United States of America. 112 (15), 4523-4530 (2015).</li><li>Huang, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Layer-dependent+ferromagnetism+in+a+van+der+Waals+crystal+down+to+the+monolayer+limit.">Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit.</a> Nature. 546, 270 (2017).</li><li>Kim, H. 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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 &#8805;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&#176; travel with step size of 4.091 &#956;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 &#956;s - 2 s, color camera</td>
		</tr>
		<tr>
			<td>Molybdenum disulfide (MoS2)</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 (ReS2)</td>
			<td>HQ Graphene</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Si/SiO2 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

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 ...

Watch this Scientific Journal Video about Fabricating van der Waals Heterostructures with Precise Rotational Alignment at JoVE.com

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

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 ...

This protocol allows for the creation of materials previously nonexistent in nature, with the goal of uncovering formerly-inaccessible physical phenomena or developing superior devices for technological applications. The main advantage of this technique is that the equipment can be operated remotely and in an environment control, and this can reduce the risk of manual error and also it can improve the sample cleanliness. Furthermore, the system is equipped with a rotational stage, and this allows the user to reach sub-degree angular alignment between the two flakes.As we are working with thin crystals and small surface areas, identification of an appropriate sized flake can be challenging to the untrained eye. Additionally, the crystal's small size can easily lead to misalignment during the transfer procedure. Therefore it is recommended to execute these steps meticulously.Many steps in the protocol have particular details that are difficult to explain but they're much simpler to follow when presented visually. In order to visualize the transfer process, utilize an optical microscope that can operate under bright-field illumination. 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. Equip two separate manipulators, shown here, to individually control the position of the crystals. Fabricate custom sample holders that can support a glass slide.This sample holder will be used to position the top crystal. For the bottom manipulators, place a flat heating element in a machined glass ceramic holder and affix it to the rotating stage. Then, connect the heating element to a power supply and a temperature controller.Submerge a one-by-one-centimeter square from a silicon silicon-oxide wafer in a beaker filled with acetone and then place the beaker into an ultrasonic cleaner. After 10 minutes, use a pair of tweezers to remove the wafer from the beaker and rinse it with isopropanol, then dry the wafer using a nitrogen gun. Next, carefully remove a portion of the crystal and place it on a piece of semiconductor-grade adhesive tape.Take a second piece of adhesive tape and firmly press it against the initial tape holding the crystal. Peel away the two pieces of tape, repeating several times, to produce many thin pieces of crystal. 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.To remove any residual adhesive, place the resulting sample into a beaker filled with acetone. After 10 minutes, remove the sample, rinse with isopropanol and dry with a nitrogen gun. Examine the exfoliated flakes using an optical microscope to estimate their thickness by assessing the flake's optical contrast with the substrate.Then, use AFM in tapping mode to better quantify the surface morphology and to measure the flake thickness. Prepare the top substrate for the transfer procedure by exfoliating the crystal on a glass slide with an attached polymethylmethacrylate film. After obtaining a clean substrate as previously shown, place the substrate on a spin coater and cover it with polyvinyl alcohol.Spin the substrate at 3, 000 RPM for one minute to produce a layer approximately 450 nanometers thick. Then, place the substrate directly onto a hot plate and bake it uncovered at 75 degrees Celsius for five minutes. Once cool, place the substrate back onto the spin coater and cover it with polymethylmethacrylate.Spin coat an 820-nanometer-thick layer of PMMA onto the substrate at 1, 500 RPM for one minute. After spin coating, place the substrate back onto the hot plate and bake it uncovered at 75 degrees Celsius. After five minutes, 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 onto the PMMA surface as shown in the previous section. Use a sharp pair of tweezers to separate the PMMA from the PVA, slowly peeling back the tape frame. The PMMA layer and exfoliated crystal, along with the tape frame, will detach from the PVA in the silicon silicon-oxide-wafer substrate.Next, invert the tape frame and place it on a machined support so that the crystal is facing downwards. Place the sample under an optical microscope and use a pair of sharp tweezers to place a small washer precisely on the PMMA film so that it surrounds the desired flake. Then, lower a glass slide and adhere it to the polymer by pressing it against the exposed tape.Place the prepared substrate on the bottom stage of the transfer setup. On this substrate, identify the position of the desired flake. This flake will become the bottom crystal.Then, place the top substrate into the top substrate holder of the transfer setup. Using a low magnification objective, bring the bottom substrate into focus and center the desired flake. Slowly lower the top substrate until it can be seen by the objective.Then, adjust its lateral position and the rotational alignment of the two flakes. When the flakes are close to being aligned as desired, switch to a 50x objective and continue to lower the top substrate, while adjusting the flake alignment. Then, once aligned, lower the top substrate until the top flake entirely contacts the bottom flake.The contact is noticeable due to a sudden change of color. When contact is made, heat the bottom substrate to 75 degrees Celsius for better adhesion of the PMMA to the bottom substrate. The PMMA will detach from the glass slide.Then, clean the bottom substrate to remove the PMMA film. To use the stamping method of flake transfer, first place the substrate on the bottom stage of the transfer setup. On this substrate, identify the position of the desired flake, which will be the bottom crystal.Next, place the top substrate into the holder of the transfer setup and heat the bottom substrate to 100 degree Celsius. Then, align and bring the top crystal into contact with the bottom flake. Once complete contact is made between the two flakes, slowly raise the top substrate.This results in the drop-off of the top flake from the stamp to the bottom substrate. Crystals of rhenium disulfide are ideal for demonstrating angular alignment, because it mechanically exfoliates as elongated bars with well-defined edges. Here, the top flake was placed at a 75 degree angle to the bottom crystal using the PDMS stamping method shown in this video.Atomic force microscopy was used to precisely measure the twist angle between the top and bottom flakes. Using the PMMA PVA procedure, the transfer setup was successfully used to create a structure consisting of two monolayer flakes of molybdenum disulfide. The individual monolayers are shown here exfoliated onto PMMA and silicon silicon-dioxide.The demonstrated procedure results in the structure shown here through an optical microscope. More details, such as the thickness and the relative position of the stacked monolayer can be confirmed using AFM. The most important parts of this procedure are the lateral and rotational alignment of the flakes.Other transfer methods exist and they follow a similar protocol, and different polymers and two-dimensional crystals can be used to fabricate particular structures. And of course, additional cleaning methods can be used to ensure pristine crystal surfaces and interfaces. When using chemicals such as PMMA, acetone and others, be sure to wear the proper personal protective equipment and to carry out the steps in a well-ventilated environment, such as a fume hood.

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9.3K Views.  University of Ottawa. This protocol allows for the creation of materials previously nonexistent in nature, with the goal of uncovering formerly-inaccessible physical phenomena or developing superior devices for technological applications. The main advantage of this technique is that the equipment can be operated remotely and in an environment control, and this can reduce the risk of manual error and also it can improve the sample cleanliness. Furthermore, the system is equipped with a rotational stage, and this al...