One of the major challenges in the realization of electronic devices consisting of 2D materials produced using chemical vapour deposition (CVD) is the transfer of the 2D material from the growth catalyst to a technologically useful substrate. Transfer most commonly involves the use of PMMA polymer as a supporting layer that stabilizes graphene and other 2D materials during transfer between the two substrates. PMMA transfer residue is a critical technological limitation to utilizing the extreme electronic properties that graphene inherently holds. Hence, researchers have devised numerous methods to remove PMMA contamination, although removal is rarely fully effective.
In a most recent paper, just accepted in ACS Applied Nano Materials, scientists show that an argon gas cluster ion beam can be used to clean the surface of graphene with great success. The energy of the argon atoms is finely tuned to prevent damage of graphene, while removing the polymer material. The cleaning process is monitored in situ through the use of integrated X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy in the same chamber where cleaning takes place.
The methods used to observe the sample during cleaning allow for a surface chemistry analysis, as well as monitoring doping and strain. All results indicate that contamination is removed by argon ion treatment, yielding chemically clean graphene surfaces with less crystal lattice strain. There is little change in doping during the process.
This new work demonstrates the importance of applying concurrent characterization during surface cleaning, since past work has shown that contamination due to exposure to air can skew the results with standalone measurements performed in sequence. The scientists further showed that thermal cleaning of PMMA yields less contamination than cleaning with solvent chemicals.
The ion beam cleaning method can be used to treat entire graphene wafers at a time, which is important for industrial acceptance of this process, marking a step towards integration of CVD graphene into electronic device architectures in mass production. The technique could be modified for cleaning of other 2D materials, such as semiconducting transition metal dichalcogenides.