Chemical vapour deposition is a good technique for producing large-area samples of monolayer graphene. However, to make electronic devices, the material – which is grown on metal substrates such as copper – then needs to be transferred onto an insulating support, such as SiO2 or hexagonal boron nitride. Most researchers employ a polymer like PMMA (poly(methylmethacrylate)) in the transfer step but it is difficult to remove all traces of the polymer afterwards – even using strong solvents like acetone.

The residual PMMA and, in particular, the H2O/O2 molecules adsorbed on the polymer, inevitably contaminate the graphene surface. They act as scattering sites, p-doping the material by an electron transfer reaction and reducing charge carrier mobilities in the material to modest values of around 1000–5000 cm2/(V·s). Extremely high charge carrier mobility is a very important physical characteristic of pristine graphene, and one of the reasons why it is often dubbed the “wonder material”.

Lower polymer concentrations are better

A team led by Rodney Ruoff and Deji Akinwande began by transferring CVD-grown monolayer graphene onto a SiO2 substrate using three different PMMA solution concentrations – of 10, 40 and 80 mg/mL in chlorobenzene. Next, they fabricated back-gated field-effect transistors (FETs) using the transferred graphene samples and characterized each one using X-ray photoelectron spectroscopy, atomic force microscopy and Raman spectroscopy. The analyses confirmed that the lowest concentration of PMMA solution left the least polymer residue on the graphene surface.

Compensating for p-type doping

In a separate experiment, the Texas team dipped the transistors in formamide overnight and left the devices to dry. This process left traces (albeit short-lived) of formamide in the PMMA residue film – a treatment that appeared to temporarily improve the room-temperature charge carrier mobility of the graphene FETs in air by more than 50%.

“The improvement comes thanks to the –NH2 functional groups in formamide donating electrons to the graphene,” explained team member Ji Won Suk. “The technique could help compensate for the p-type doping induced by residual polymer contaminants if perhaps combined with an appropriate barrier that would prevent the formamide from evaporating from the PMMA film. We will now also be looking at other molecules with the same property-enhancing effects as formamide but that do not evaporate.”

According to the researchers, the results could help in making improved, high-performance carbon-based devices in the future. “Polymer-based transfer of graphene still needs to be better understood,” Won Suk told, “and fabricating real-world graphene devices that retain the excellent properties of pristine graphene still remains a huge challenge for us.”

The results are presented in Nano Letters.