The mechanism of direct laser writing of graphene features into graphene oxide films involves photoreduction and thermally assisted structural rearrangement

Photoreduction of graphene oxide (GO) to reduced graphene oxide (rGO) under ambient conditions was evaluated for three laser processing approaches: 800 nm fs pulses, 248 nm ns pulses, and 788 nm continuous wave (CW) illumination. Resultant features were compared using Raman, XPS, optical profilometry and SEM. The most effective approach for photoreduction and graphenization with minimal defects was nanosecond processing (0.12 J cm(-2); 40 pulses overlapped). The Raman 2D band intensity confirmed a graphene-like structure. The ablation threshold for GO (248 nm, 5 ns) is similar to 10 mJ cm(-2). Laser photoreduction with CW 788 nm yielded similar oxygen removal, but conversion to a graphene-like structure was poorer, and more defects were introduced. Femtosecond pulse illumination resulted in oxygen removal from the surface but the transformation to an sp(2) graphene-like structure was not observed. This result suggests that the photochemical reduction of GO is not thermally mediated, but the structural reorganization from sp(3) to sp(2) requires heat deposition into the material. This is the first such comparison on a consistent GO substrate under comparable ambient conditions, and the results provide insight into the fundamental mechanism and the utility of the method for direct laser writing electronically active materials. (C) 2015 Elsevier Ltd. All rights reserved.