Based on first-principles calculations, we propose an exchange-transfer mechanism to understand the distinctively different behaviors of Pd and Pt contacts on graphene. The feature of the mechanism is that the $\pi$ electrons on the graphene transferring to the Pd $d_{xz}+d_{yz}$ orbital are largely compensated by the electrons from the Pd $d_{z^2}$ orbital. This mechanism causes more interaction states and transmission channels between the Pd and graphene. Most importantly, the mechanism keeps enough $\pi$ electrons on the graphene. We show that a tensile strain in the Pd layer, necessary to match the graphene lattice, plays a key role in stimulating this exchange transfer when Pd covers on graphene, while a similar strain in the Pt layer does not cause such a mechanism.

• Received 9 February 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.066802