Ultra-low doping and local charge variation in graphene measured by Raman: experiment and simulation.

Avoiding charge density variations and impurities in graphene is vital for high-quality graphene-based devices. Here, we demonstrate an optical method using Raman 2D peak-split to monitor charge density variations in the range 1-25e10 cm-2. We compare Raman signatures with electrostatically gated Raman and transport measurements to correlate the 2D peak-split with the charge density on graphene with high precision. We found that the Raman 2D peak-split and peak areas linearly varies with the charge density, where a lower charge density results in a larger 2D peak-split. We simulate Raman 2D spectra under various doping conditions to study the correlation between Raman 2D peak and charge puddles. These simulations give qualitative agreement with experimental results. Our work provides a simple and non-invasive optical method for estimating the doping level, local charge density variation and transport properties of graphene before fabricating graphene devices, with up to two orders of magnitude higher precision than previously reported optical methods.