  Current wafer-scale fabrication methods for graphene-based electronics and
sensors involve the transfer of single-layer graphene by a support polymer.
This often leaves some polymer residue on the graphene, which can strongly
impact its electronic, thermal, and mechanical resonance properties. To assess
the cleanliness of graphene fabrication methods, it is thus of considerable
interest to quantify the amount of contamination on top of the graphene. Here,
we present a methodology for direct measurement of the mass of the graphene
sheet using quartz crystal microbalances (QCM). By monitoring the QCM resonance
frequency during removal of graphene in an oxygen plasma, the total mass of the
graphene and contamination is determined with sub-graphene-monolayer accuracy.
Since the etch-rate of the contamination is higher than that of graphene,
quantitative measurements of the mass of contaminants below, on top, and
between graphene layers are obtained. We find that polymer-based dry transfer
methods can increase the mass of a graphene sheet by a factor of 10. The
presented mass measurement method is conceptually straightforward to interpret
and can be used for standardized testing of graphene transfer procedures in
order to improve the quality of graphene devices in future applications.
