Strain, Bubbles, Dirt, and Folds: A Study of Graphene PolymerAssisted Transfer

This is a process where the graphene fi lm is covered in a stabilizing polymer layer and the copper is chemically etched away. The polymer graphene stack can then be placed onto the destination substrate and subsequently the polymer is dissolved in an appropriate solvent. The overwhelming majority of reported cases use Poly(methyl methacrylate) (PMMA) for the polymer and either iron chloride (FeCl 3 ) or ammonium persulfate (APS) solutions for the etchant. [ 3,5–7 ] Herein lies the problem; the FeCl 3 leaves an electrically active residue of metal ions on the graphene [ 8 ] and APS is a crosslinking agent for PMMA which leaves an undissolvable residue. [ 9 ] Both of these effects have signifi cant impact on the electrical and photonic behavior of graphene. In order to address these issues with PMMA in PAT we offer alternatives for use in laboratory settings. This is not to say that other approaches to mitigating the polymer and etchant residue problem have not been suggested. The most signifi cant modifi cation to the PAT process is the addition of a vacuum annealing step. [ 10,11 ] It has been proposed to remove PMMA residue by decomposition and evaporation because at elevated temperatures outside of the infl uence of oxidants PMMA decomposes into radicals which desorb or interact chemically with the PMMA. [ 12 ] Obviously the latter instance is destructive to the PMMA. Chemical approaches have been proposed in which the PMMA residue can be removed by Chloroform, [ 13 ] or the electrical infl uence of the residues can be removed by modifying the PMMA or substrate such that it does not dope the graphene. [ 14,15 ] Further, more aggressive methods have been applied with varying success such as plasma removal of the polymer residue, [ 16 ] annealing in oxygen [ 17 ] and replacing or augmenting the polymer layer with thermal tape or Polydimethylsiloxane (PDMS). [ 7,18 ] While these efforts are excellent additions to the PAT playbook they treat the symptoms and ignore the key problem of polymer incompatibility. In order to address polymer incompatibility, we investigate the interplay between polymer characteristics and CVD graphene during PAT. It has been shown previously that CVD graphene is under signifi cant compressive strain following growth due to a signifi cant mismatch in thermal expansion parameter with the underlying copper. [ 19 ] During growth, this strain is partially mitigated by the formation of thermal expansion folds. [ 20 ]