Viscoelastic Transfer of 2D Material Using PDMS


Viscoelastic transfer using polydimethylsiloxane (PDMS) stamps is one of the methods used for the deterministic placement of 2D materials and the fabrication of van der Waals heterostructures. PDMS-based transfer offers a fast and convenient way of depositing monolayer flakes of 2D materials onto virtually any substrate and, unlike other methods, does not require the use of wet chemistry.

This video guide takes you through the full process including:

  • Preparation of the PDMS stamp
  • Mechanical exfoliation of the 2D material
  • Flake search and thickness identification
  • Target substrate cleaning
  • Initial alignment
  • Deterministic Transfer.
Video by Ossila

 

Video Transcript:


Preparation of the PDMS stamp

1. To begin, clean the glass substrates in a UV Ozone Cleaner.

2. Cut the PDMS (product code: PF-3-X4, Gel-Pak) into small squares.

3. Remove the PDMS film with the protective polyethylene coversheet from the polyester substrate and carefully apply it to the glass.

4. Peel off the protective polyethylene coversheet.

 

Exfoliation

1. Cut two pieces of blue tape and put the crystal on one of them. Ensure it is laid flat.

2. Put the second piece of tape on top and press gently with tweezers. Slowly peel the tape off.

3. Continue to distribute the bulk material over the surface of the tape by bringing the two pieces of tape together.

4. Once most of the surface is covered with thin layer of material, remove the bulk crystal from the tape.

5. Additional pieces of tape can be created by transferring some of the prepared material onto a fresh piece of tape.

6. Put one of the pieces of tape on top of the prepared PDMS film and apply gentle pressure with a cotton bud.

7. Peel the blue tape off. The rate of the peel defines the amount of material transferred onto PDMS; a faster peel rate will result in more material being transferred.

 

Flake search and thickness identification

1. Inspect the PDMS substrate using an optical microscope to look for the most translucent flakes.

2. Optical contrast measurements can be used to identify the thickness of the flake by comparing the intensity of light reflected by the flake and the surrounding substrate.

3. The optical contrast of layered 2D materials changes linearly with increasing number of layers.

4. The value of the optical contrast for a monolayer flake will depend on the material, type of substrate, and optical system used.

5. But it should be close to the minimum difference between the adjacent thicknesses.

6. Some 2D materials, such as transition metal dichalcogenides, become fluorescent when their thickness is reduced to a single monolayer.

7. Therefore, a fluorescence microscope can also be used to confirm the monolayer thickness.

 

Target substrate cleaning

1. Place the substrates into a substrate rack.

2. Put the rack into a Hellmanex solution.

3. Place the beaker into a sonicator for 5 minutes.

4. Remove the beaker from the sonicator and take out the rack from the solution.

5. Rinse the substrates in water then place the rack into IPA.

6. Put the rack into the sonicator for an additional 5 mins.

7. Dry the substrates with a nitrogen gun.

8. Clean the substrates in a UV Ozone Cleaner for 5 minutes.

 

Initial alignment

1. Attach the glass slide (PDMS stamp facing down) to the 3-axis micromanipulator. Locate the transferred flake and place it roughly in the centre of the field of view.

2. Put the target substrate onto the substrate holder and fix it using vacuum or a piece of adhesive tape.

3. Adjust the substrate position until the target area is roughly in the centre of the field of view.

4. Place the glass slide above the substrate and bring them close to each other (while maintaining sufficient vertical separation).

5. Focus the microscope on the transferred flake.

6. Draw the outline of the flake in the microscope software. If necessary, mark any flakes in its vicinity that might interfere with the transfer.

 

Deterministic Transfer

1. Start to slowly bring the PDMS stamp down.

2. If the PDMS stamp has shifted, adjust its position and re-align the flake with the outline that you drew.

3. Focus on the substrate and continue lowering the stamp until it comes in contact with the substrate.

4. The point of contact can be identified by the change of the colour and contrast.

5. Continue lowering the glass slide, until the contact line is ~100μm away from the flake.

6. Start to slowly bring the stamp up. A slow peel off rate is critical for a successful transfer.

7. When the contact line gets close to the sample, switch to a higher-magnification lens and keep the peel off rate as slow as possible.

8. Look for a change in colour and contrast in the thin region. This indicates the flake is being successfully transferred.

9. The lack of contrast variation and movement of the flake shows that it is being lifted with the stamp.

10. Once the contact line is at 50 μm away from the flake, the rate of peel off can be increased.

11. Lift the stamp up until it is fully separated from the substrate. Remove the glass slide and inspect the transferred flake.