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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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. Vibrations could be a cause for a failed alignment, therefore, one must minimize their effect. In this regard, the advantage of the &#8220;hands-free&#8221; 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 &#8220;hands-free&#8221;, 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.

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

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 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 &#8220;top&#8221; ReS2 crystal from the stamp onto a silicon substrate with previously exfoliated &#8220;bottom&#8221; ReS2. Each time we aimed to align the edges at a specific angle. Employing the angular component of the transfer setup, the &#8220;top&#8221; ReS2 flake was placed such that it would be twisted by the specific desired angle with respect to the already present &#8220;bottom&#8221; 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&#176;. 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&#176; &#177; 0.1&#176; (mean &#177; 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&#176; to 90&#176; in increments of 15&#176; (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).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59727/59727fig1.jpg" /> 
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 &#1012; (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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59727/59727fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59727/59727fig2v2.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59727/59727fig2v2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59727/59727fig3.jpg" /> 
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 &#176;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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59727/59727fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59727/59727fig4.jpg" /> 
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 &#176;C. (e) The top and bottom flakes are aligned and brought into contact. (f) At 100 &#176;C, we can slowly raise the top stage which results in the drop-off of the flake from the stamp to the bottom substrate. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59727/59727fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59727/59727fig5.jpg" /> 
Figure 5: ReS2 flakes transferred with a relative angle of 75&#176;. 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&#176; &#177; 0.1&#176;. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59727/59727fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59727/59727fig6.jpg" /> 
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&#176; to 90&#176; in increments of 15&#176;. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59727/59727fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/59727/59727fig7.jpg" /> 
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 &#956;m. (d) AFM image of the twisted MoS2 structure with a line profile (inset) along the direction indicated by the black dashed line. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59727/59727fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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 ...

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9.4K Views.  University of Ottawa. Dit protocol maakt het mogelijk om materialen te maken die voorheen niet in de natuur bestonden, met als doel het blootleggen van voorheen ontoegankelijke fysieke verschijnselen of het ontwikkelen van superieure apparaten voor technologische toepassingen. Het belangrijkste voordeel van deze techniek is dat de apparatuur op afstand en in een omgevingscontrole kan worden bediend, en dit kan het risico op handmatige fouten verminderen en ook de netheid van het monster verbeteren. Bovendien is he...

Video: Fabricating van der Waals Heterostructures with Precise Rotational Alignment

Fabricating Nanogaps by Nanoskiving

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two electrodes-and generating them in practical quantities-is still challenging. The preparation of nanogap electrodes using nanoskiving, which is a form of edge lithography, is a fast, simple and powerful technique. This method is an entirely mechanical process which does not include any photo- or electron-beam lithographic steps and does not require any special equipment or infrastructure such as clean rooms. Nanoskiving is used to fabricate electrically addressable nanogaps with control over all three dimensions; the smallest dimension of these structures is defined by the thickness of the sacrificial layer (Al or Ag) or self-assembled monolayers. These wires can be manually positioned by transporting them on drops of water and are directly electrically-addressable; no further lithography is required to connect them to an electrometer.

The fabrication of electrically addressable, high-aspect-ratio (> 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.

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two ...

Chemistry

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

In recent years &pi;-conjugated organic semiconductors have emerged as the active material in a number of diverse applications including large-area, low-cost displays, photovoltaics, printable and flexible electronics and organic spin valves. Organics allow (a) low-cost, low-temperature processing and (b) molecular-level design of electronic, optical and spin transport characteristics. Such features are not readily available for mainstream inorganic semiconductors, which have enabled organics to carve a niche in the silicon-dominated electronics market. The first generation of organic-based devices has focused on thin film geometries, grown by physical vapor deposition or solution processing. However, it has been realized that organic nanostructures can be used to enhance performance of above-mentioned applications and significant effort has been invested in exploring methods for organic nanostructure fabrication.	
A particularly interesting class of organic nanostructures is the one in which vertically oriented organic nanowires, nanorods or nanotubes are organized in a well-regimented, high-density array. Such structures are highly versatile and are ideal morphological architectures for various applications such as chemical sensors, split-dipole nanoantennas, photovoltaic devices with radially heterostructured "core-shell" nanowires, and memory devices with a cross-point geometry. Such architecture is generally realized by a template-directed approach. In the past this method has been used to grow metal and inorganic semiconductor nanowire arrays. More recently &pi;-conjugated polymer nanowires have been grown within nanoporous templates. However, these approaches have had limited success in growing nanowires of technologically important &pi;-conjugated small molecular weight organics, such as tris-8-hydroxyquinoline aluminum (Alq3), rubrene and methanofullerenes, which are commonly used in diverse areas including organic displays, photovoltaics, thin film transistors and spintronics.	
Recently we have been able to address the above-mentioned issue by employing a novel "centrifugation-assisted" approach. This method therefore broadens the spectrum of organic materials that can be patterned in a vertically ordered nanowire array. Due to the technological importance of Alq3, rubrene and methanofullerenes, our method can be used to explore how the nanostructuring of these materials affects the performance of aforementioned organic devices. The purpose of this article is to describe the technical details of the above-mentioned protocol, demonstrate how this process can be extended to grow small-molecular organic nanowires on arbitrary substrates and finally, to discuss the critical steps, limitations, possible modifications, trouble-shooting and future applications.

We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

In  recent years &pi;-conjugated  organic semiconductors have emerged as the active material in a number of  diverse applications including large-area, ...

Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned and bonded silicon wafers. Unlike confinements in porous materials often used for these types of experiments3, bonded wafers provide predesigned uniform spaces for confinement. The geometry of each cell is well known, which removes a large source of ambiguity in the interpretation of data.
Exceptionally flat, 5 cm diameter, 375 &#181;m thick Si wafers with about 1 &#181;m variation over the entire wafer can be obtained commercially (from Semiconductor Processing Company, for example). Thermal oxide is grown on the wafers to define the confinement dimension in the z-direction. A pattern is then etched in the oxide using lithographic techniques so as to create a desired enclosure upon bonding. A hole is drilled in one of the wafers (the top) to allow for the introduction of the liquid to be measured. The wafers are cleaned2 in RCA solutions and then put in a microclean chamber where they are rinsed with deionized water4. The wafers are bonded at RT and then annealed at ~1,100 &#176;C. This forms a strong and permanent bond. This process can be used to make uniform enclosures for measuring thermal and hydrodynamic properties of confined liquids from the nanometer to the micrometer scale.

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.

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned ...

Atomically Traceable Nanostructure Fabrication

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

We report a protocol for combining the atomic metrology of the Scanning Tunneling Microscope for surface patterning with selective Atomic Layer Deposition and Reactive Ion Etching. Using a robust process involving numerous atmospheric exposures and transport, 3D nanostructures with atomic metrology are fabricated.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process ...

Fabrication of Low Temperature Carbon Nanotube Vertical Interconnects Compatible with Semiconductor Technology

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive thin-films. Due to the low growth temperature, the process allows integration with modern low-&#954; dielectrics and some flexible substrates. The process is compatible with standard semiconductor fabrication, and a method for the fabrication of electrical 4-point probe test structures for vertical interconnect test structures is presented. Using scanning electron microscopy the morphology of the CNT bundles is investigated, which demonstrates vertical alignment of the CNT and can be used to tune the CNT growth time. With Raman spectroscopy the crystallinity of the CNT is investigated. It was found that the CNT have many defects, due to the low growth temperature. The electrical current-voltage measurements of the test vertical interconnects displays a linear response, indicating good ohmic contact was achieved between the CNT bundle and the top and bottom metal electrodes. The obtained resistivities of the CNT bundle are among the average values in the literature, while a record-low CNT growth temperature was used.

A method for the growth of low temperature vertically-aligned carbon nanotubes, and the subsequent fabrication of vertical interconnect electrical test structures using semiconductor fabrication is presented.

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive ...

Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition metal dichalcogenides (TMDs) MoS2 and WS2 can be prepared on sapphire substrates. By controlling the metal film thicknesses, good layer number controllability, down to a single layer of TMDs, can be obtained using this growth technique. Based on the results obtained from the Mo film sulfurized under the sulfur deficient condition, there are two mechanisms of (a) planar MoS2 growth and (b) Mo oxide segregation observed during the sulfurization procedure. When the background sulfur is sufficient, planar TMD growth is the dominant growth mechanism, which will result in a uniform MoS2 film after the sulfurization procedure. If the background sulfur is deficient, Mo oxide segregation will be the dominant growth mechanism at the initial stage of the sulfurization procedure. In this case, the sample with Mo oxide clusters covered with few-layer MoS2 will be obtained. After sequential Mo deposition/sulfurization and W deposition/sulfurization procedures, vertical WS2/MoS2 hetero-structures are established using this growth technique. Raman peaks corresponding to WS2 and MoS2, respectively, and the identical layer number of the hetero-structure with the summation of individual 2D materials have confirmed the successful establishment of the vertical 2D crystal hetero-structure. After transferring the WS2/MoS2 film onto a SiO2/Si substrate with pre-patterned source/drain electrodes, a bottom-gate transistor is fabricated. Compared with the transistor with only MoS2 channels, the higher drain currents of the device with the WS2/MoS2 hetero-structure have exhibited that with the introduction of 2D crystal hetero-structures, superior device performance can be obtained. The results have revealed the potential of this growth technique for the practical application of 2D crystals.

Through the sulfurization of pre-deposited transition metals, large-area and vertical 2D crystal hetero-structures can be fabricated. The film transferring and device fabrication procedures are also demonstrated in this report.

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition ...

A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on high-density data storage and low-power consumption capabilities. However, the high-quality oxide based nonvolatile memory on flexible substrates is often constrained by the material characteristics and the inevitable high-temperature fabrication process. In this paper, a protocol is proposed to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica. The versatile deposition technique and measurement method enable the fabrication of flexible yet single-crystalline non-volatile memory elements necessary for the next generation of smart devices.

In this paper, we present a protocol to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica.

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on ...

Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared on transparent membranes which are very fragile and difficult to manipulate. We present processes for the fabrication of samples with magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for Lorentz transmission electron microscopy and magnetic transmission x-ray microscopy studies. The samples are prepared on silicon nitride membranes and the fabrication consists of a spin coating, UV and electron-beam lithography, the chemical development of the resist, and the evaporation of the magnetic material followed by a lift-off process forming the final magnetic structures. The samples for the Lorentz transmission electron microscopy consist of magnetic nanodiscs prepared in a single lithography step. The samples for the magnetic x-ray transmission microscopy are used for time-resolved magnetization dynamic experiments, and magnetic nanodiscs are placed on a waveguide which is used for the generation of repeatable magnetic field pulses by passing an electric current through the waveguide. The waveguide is created in an extra lithography step.

A protocol for the fabrication of magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for transmission electron microscopy (TEM) and magnetic transmission x-ray microscopy (MTXM) studies is presented.

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared ...

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

Computational tools based on density-functional theory (DFT) enable the exploration of the qualitatively new, experimentally attainable nanoscale compounds for a targeted application. Theoretical simulations provide a profound understanding of the intrinsic electronic properties of functional materials. The goal of this protocol is to search for photocatalyst candidates by computational dissection. Photocatalytic applications require suitable band gaps, appropriate band edge positions relative to the redox potentials. Hybrid functionals can provide accurate values of these properties but are computationally expensive, whereas the results at the Perdew-Burke-Ernzerhof (PBE) functional level could be effective for suggesting strategies for band structure engineering via electric field and tensile strain aiming to enhance the photocatalytic performance. To illustrate this, in the present manuscript, the DFT based simulation tool VASP is used to investigate the band alignment of nanocomposites in combinations of nanotubes and nanoribbons in the ground state. To address the lifetime of photogenerated holes and electrons in the excited state, nonadiabatic dynamics calculations are needed.

Calculations performed by the Vienna Ab initio Simulation Package can be used to identify the intrinsic electronic properties of nanoscale materials and predict the potential water-splitting photocatalysts.

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

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