▎ 摘　　要

We studied the adsorption of atomic hydrogen on monolayer graphene and quasi free monolayer graphene, epitaxially grown on SiC(0001). By means of in-situ surface transport measurements, a metal-insulator transition was found on both n- and p-type doped two dimensional electron systems. The detailed analysis of the temperature dependent resistivity revealed that even ultra-low concentrations (n(H) approximate to 10(12) cm(-2)) of locally chemisorbed H-clusters act as effective scattering centers for the propagating electrons and limit the mean-free path L-0 proportional to 1/root n(H). Despite the weak disorder due to adsorption, strong localization was found. The activation energy for destroying the phase coherence within the system is around 30 meV. Our analysis rules out the formation of a band insulator or even a "bad metal" due to adsorption of hydrogen.