Graphene transfer from Cu substrate - NanoLabStaff/nanolab GitHub Wiki

by Ida Marie Høiaas ([email protected])

Short version:

  • Make scaffold
  • Etch copper
  • Transfer to target substrate
  • Remove PMMA
  • Check graphene

Create a scaffold that protects the graphene during the transfer process graphene transfer 002

This is commonly done by depositing a rather thick layer of PMMA on top of the graphene. If the Cu foil is wrinkled, one can flatten it by covering the sample by a clean wafer, e.g. Si, and pressing gently. Fasten sample to glass cover slide with single-sided kapton tape as a frame covering as little area as possible, to ensure a flat sample and ease the spin coating of PMMA. Typical values for PMMA deposition are:

    PMMA A9 950k
    4000 rpm/s, ramp at 1000 rpm/s^2, 2 minutes
    Bake at 70°C for 2 minutes

graphene transfer 009

This gives a PMMA layer of approximately 1.5 um. After depositing PMMA and baking the sample, remove the glass cover slide.

Remove graphene with oxygen plasma Graphene is normally deposited on both sides of the copper foil, and the first step of the transfer process is to remove graphene from one side if this is the case. This is done by plasma etching the graphene from the side not covered by PMMA in the plasma cleaner with the following parameters:

    2.5 minutes duration
    90% power
    50% O2

It is normally not possible to see any change in the sample before/after plasma treatment without using an electron microscope.

Etch Cu with ammonium persulfate (APS)

APS is a solution that selectively etches copper but not graphene or PMMA. 1 L of deionized water (DIW) with 25-30 g APS powder gives a concentration of 0.1-0.15 M APS. Typically, one can use 3 g APS powder and 100 mL DIW to get a suitable amount of 0.15 M etchant. Etching of the Cu foil takes about 3-4 hours. To further reduce the etching time, one can use a magnetron bar to stir the sample at about 60 rpm. The total etching time is then typically 1-2 hours. graphene transfer 015

The sample is left to float in the solution with the Cu exposed to the etchant. The Cu will be removed after 1-2 hours, and the remaining layers of graphene and PMMA will be a transparent thin foil.

graphene transfer 017

Transfer the graphene/PMMA foil to two subsequent containers with DIW. Let the sample float on top of the second container for about 30 minutes to rinse off remaining copper. To transfer the graphene between the various containers, one can either scoop the sample with a small petri dish or use a plastic film.

Transfer to target substrate After being rinsed in water, the graphene is transferred to the target substrate. This is done by scooping the sample from the DIW with the target substrate. Let the sample dry at room temperature for one hour and then on a hot plate at 80°C for 30 minutes. graphene transfer 044

Remove PMMA Place the sample in acetone overnight to remove the PMMA. Let it dry in room temperature afterwards (rinsing in water increases the chance of tears and mechanical stress in the graphene) for at least 1-2 hours. To further remove transfer residues, it is possible to anneal the sample at 300°C for three hours.

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