Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor

The recent discovery of graphene1,2,3 has led to many advances in two-dimensional physics and devices4,5. The graphene devices fabricated so far have relied on SiO2 back gating1,2,3. Electrochemical top gating is widely used for polymer transistors6,7, and has also been successfully applied to carbon nanotubes8,9. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to 5×1013 cm−2, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer8,10 in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used SiO2 back gate, which is usually about 300 nm thick11. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.