Graphene Transfer
- Graphene grown on copper and spin coat 495K A8 PMMA and bake at 180degreesC for 90seconds.
- Etch copper/graphene/PMMA stack in FeCl3 solution.
- Use a clean glass slide (rubbed with Acetone/Ethanol and IPA and then blow dried) to transfer graphene/PMMA stack from FeCl3 solution to a beaker of DI water.
(I normally leave the side in its own beaker of DI water during the transfer process to keep it clean of contaminants and any residue it collects).
- Continue to float the graphene/PMMA stack in successive beakers of DI water, leaving it for 3-5mins in each fresh beakers until you feel the water is clear (Normally takes 3-4 dips).
- Now float the graphene/PMMA on concentrated HCl acid solution (solution of 1:1 DI water:HCL is the minimum needed) and leave for 2-3 minutes (HCl has limited effect on graphene, so a longer etch is fine if you need to ensure the Fe based contamination on the underside of graphene is completely dissolved away. PLEASE keep the solution labelled and covered in the fume hood, so it maybe re-used or we will run out of HCl very quickly!!
- Once HCl treatment has been completely float the stack again on successive beakers of DI water (2-3) to wash off any left over HCl solution or residue.
- Now transfer the graphene/PMMA stack onto a clean Si chip (10min acetone with 10min IPA sonication and 90% power 300 second O2 plasma clean) by suspending the Si chip in the DI water and pushing it towards the stack, allowing surface tension to draw it onto the Si as you pull it out of the water line.
- If transferred badly you can try and smoothen out the film by dipping the Si/Graphene/PMMA stack in IPA solution allowing the solution to draw underneath the graphene/PMMA layer, but not letting it float off.
- Then allow the Si/Graphene/PMMA stack to dry out in the fume hood for 24hours (this will evaporate any trapped water/solvent slowly, so rapid evaporation does not break the graphene film).
- Following this bake the stack at 150degreesC for 3minutes, to cement the graphene attachement to the Si substrate.
PMMA Removal
There are several solvent options that will work to remove PMMA. The traditional acetone will work, but will leave residue unless you use heat over a long time period (eg. 3 days). The alternatives suggested here give quicker and overall cleaner results. I have trialled all the solvents suggested by literature and have listed them here, just incase you don't have successful results with my method and would like other options to try.
Heating the solvent 5-10degrees below that of its boiling (normally 40-50degreesC) will increases solvent dissolution time, but in my experience isn't a great advantage.
Using a fresh beaker of solvent halfway through the process acts to increase the activity of the solution and increase PMMA removal tendency.
My method:
- Place the Si/Graphene/PMMA stack in 20ml of Toluene (in a covered beaker) for 2hours at room temperature, followed by a further 2hours in fresh toluene solution.
This has produced the clean results as I have demonstrated recently. It is worth noting however that my results have been subject to some inconsistent reproducibility and unpolished/rougher copper will give more contoured graphene that retains PMMA more effectively. Therefore some samples made need slightly longer times in the solvent. Greenish residue in the centre of the stack is known to be Fe based residue from the FeCl3 and if so means the HCl transfer step was not carried out for long enough or at high enough concentration.
Should you experience problems with Toluene there are many other solvents that are effective PMMA removers that do not seem to degrade graphene and work following the same procedure as outlined above:
- Dichloroethane (DCE) (can mix 1:1 with acetone too)
- Dichloromethane (DCM)
(ref: Efficient removers for poly(methylmethacrylate))
- Acetic Acid
(ref: Graphene Transfer with Reduced Residue)
- Chloroform
(ref: Toward Intrinsic Graphene Surfaces: A Systematic Study on Thermal Annealing and Wet-Chemical Treatment of SiO2-Supported Graphene Devices)
(Anisole seems less effective than these other alternatives, but will remove PMMA too)
All these solvents are currently stocked in our lab.