▎ 摘　　要

We show that contrast analysis carried out by standard optical microscopy can be employed as a simple and quick technique to monitor the cleanness of graphene during the process steps required for device fabrication. Graphene flakes deposited by adhesive tape exfoliation can display a strong contrast increase upon processing, due to the organic contamination arising from the diffusion of glue residues over the samples. On the other hand, graphene deposited by an adhesive-free method, such as electrostatic deposition, does not show any contrast variation, suggesting a low degree of contamination. Therefore, the fabrication process of graphene-based devices may be monitored and optimized on the basis of an easy optical inspection.