Fabricating van der Waals Heterostructures with Precise Rotational Alignment

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.