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

We aim to develop a residue-free fabrication method for 2D heterostructures that avoid exposure to any residues and temperature changes while ensuring the devices remain clean and high quality.

Transferring individual 2D flakes for multi-stack heterostructure is technically challenging and polymers or solvents often leave residue at the interface, disrupting clean stacking between layers.

We established a novel method that enables precise 2D heterostructure assembly using only Van der Waals forces through bottom-up, top-down, and motor stacking techniques.

We plan to apply this method to various applications such as optical or gas sensors that require complex heterostructures with clean and high-quality interfaces.

[Narrator] To begin, prepare a piece of bulk crystal using a cotton swab and a razor blade. Attach the prepared crystal onto the adhesive surface of a pre-exfoliation stamp and place another pre-exfoliation stamp on top of the crystal to form a sandwich configuration. Gently exfoliate the sandwich crystal and repeat this process three to five times to obtain a clean and uniform submicron thick crystal. Now place the pre-exfoliated crystal onto the sample stage and secure the tape stamp with a tilt angle of at least five degrees onto the magnetic plate of the stamp manipulator using a magnet. Align the tape stamp above the pre-exfoliated crystal by adjusting the sample stage. Attach the tape stamp to the top surface of the crystal by adjusting the stamp manipulator in the negative Z direction and gently exfoliate the thin residue-free region by adjusting the stamp manipulator in the positive Z direction. Adhere the residue-free region firmly to the substrate by adjusting the stamp manipulator in the negative Z direction. Then exfoliate the single flake by adjusting the stamp manipulator in the positive Z direction. Pick up the molybdenum disulfide flake with the MoS2 residue-free stamp and release it onto the HBN flake to assemble the structure. Pick up another HBN flake and release it on the MoS2 and HBN structure. Next, pick up the HBN flake using the MoS2 residue-free stamp. Use the picked up HBN flake to cover the MoS2 flake and then pick up the MoS2 flake with the already picked up HBN flake, forming a HBN and MoS2 structure. Finally, release the combined structure onto another HBN flake and then release the hetero B onto the hetero A. The symmetrical hexagonal atomic structure of residue-free MoS2 was confirmed using high resolution transmission electron microscopy and selected area electron diffraction pattern analysis. Energy dispersive X-ray spectroscopy mapping revealed strong molybdenum and sulfur signals and minimal carbon and oxygen, confirming the absence of polymer residue and oxidation on the molybdenum disulfide flakes. Atomic force microscopy analysis showed that the thicknesses of transferred HBN and MoS2 flakes were 18.34 nanometers and 5.27 nanometers respectively. The root means square roughness values of 0.143 nanometers for HBN and 0.153 nanometers for MoS2 confirmed excellent flatness with minimal surface irregularities. Raman spectroscopy analysis revealed peak positions at approximately 1,365 and 383 inverse centimeters for HBN and MoS2 respectively, indicating strain-free transfer. Distinct manipulation methods allowed successful assembly of the heterostructures via both bottom-up and top-down stacking. The assembled hetero structures exhibited randomly distributed gas-filled blisters at the hetero interface without any applied force. Application of increasing forces from 30 to 1,000 nanonewtons progressively reduced blister size and count.