Effective characterization of polymer residues on two-dimensional materials by Raman spectroscopy

Abstract

Large-area two-dimensional (2D) materials grown by chemical vapor deposition need to be transferred onto a target substrate for real applications. Poly(methyl methacrylate) as a supporting layer is widely used during the transfer process and removed after finishing it. However, it is a challenge to diminish the polymer layer completely. It is necessary to readily characterize the polymer residues on 2D materials to facilitate the removal process. Here, we report a method that characterizes the polymer residues on 2D materials by tracking the presence of G-band of amorphous carbons (a-Cs) in the Raman spectrum after forming carbonized a-Cs through thermal annealing. The (13)C-graphene is employed to separate the Raman signal G-band between (12)C-a-Cs and (13)C-graphene in the Raman spectrum. The residence of the polymer residues is clearly confirmed by the different Raman signals of two different isotopes ((12)C and (13)C) due to differences in mass. Our effective method recognizes that while the polymer residue is not easily removed on graphene, those on hexagonal boron nitride and molybdenum disulfide are almost diminished under optimum thermal annealing conditions. Our method will not only contribute to the development of a new transfer process, but also help to achieve a clean surface of 2D materials.

Transfer of large-scale two-dimensional semiconductors: challenges and developments

“…For this reason, they deserve to be highlighted in this review. Citing the well-known problems of using PMMA-based transfer methodologies, specifically polymer residues which degrade performance (see for example [22,51,52,92,153]), Zhang et al used cellulose acetate (CA) as a support layer to transfer CVD-grown TMDs onto a SiO 2 /Si substrate [66]. To avoid the problems associated with using the conventional hot NaOH etching method to detach the film (such as cracks or wrinkles from bubbles), the authors used a combination of NH 4 F and HF (known as buffered oxide etch) which works at room temperature.…”

“…A tried-and-tested methodology was developed which could simply be duplicated for use in transferring exfoliated films and, thereafter, larger-area TMD films for initial characterization. However, the removal step invariably leaves residues which are hard to remove with post-transfer cleaning procedures such as ultra-high vacuum (UHV) annealing [50,51].…”

A systematic study of the controlled generation of crystalline iron oxide nanoparticles on graphene using a chemical etching process

“…The main disadvantage of applying these supporting polymer layers to graphene is their removal during the final steps of the transfer procedure. Dissolving the polymer, for example by using hot acetone, typically leaves residue on the graphene monolayer [ 22 , 32 – 34 ]. As graphene is close to or even atomically thin, even very minor amounts of contamination can affect its electronic properties [ 22 , 33 – 34 ].…”

Electronic properties of CVD graphene: The role of grain boundaries, atmospheric doping, and encapsulation by ALD

“…However, the effect of PMMA and atmospheric contamination is difficult to separate as PMMA acts as an absorbent layer for vapors, significantly increasing their effect . Moreover, polymer residues have been observed to preferentially adhere along GBs , again highlighting how such contamination effects interlink with the graphene microstructure.…”