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

The recent discovery of graphene^1,2,3 has led to many advances in two-dimensional physics and devices^4,5. The graphene devices fabricated so far have relied on SiO₂ back gating^1,2,3. Electrochemical top gating is widely used for polymer transistors^6,7, and has also been successfully applied to carbon nanotubes^8,9. 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 layer^8,10 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.