  Reports of metallic behavior in two-dimensional (2D) systems such as high
mobility metal-oxide field effect transistors, insulating oxide interfaces,
graphene, and MoS2 have challenged the well-known prediction of Abrahams, et
al. that all 2D systems must be insulating. The existence of a metallic state
for such a wide range of 2D systems thus reveals a wide gap in our
understanding of 2D transport that has become more important as research in 2D
systems expands. A key to understanding the 2D metallic state is the
metal-insulator transition (MIT). In this report, we demonstrate the existence
of a disorder induced MIT in functionalized graphene, a model 2D system.
Magneto-transport measurements show that weak-localization overwhelmingly
drives the transition, in contradiction to theoretical assumptions that
enhanced electron-electron interactions dominate. These results provide the
first detailed picture of the nature of the transition from the metallic to
insulating states of a 2D system.
