Origins of distinctly different behaviors of Pd and Pt contacts on graphene

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.