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Origins of Distinct Contact of Pd and Pt Nanolayers on Graphene Q. J. Wang and J. G. Che Surface Physics Laboratory, Fudan University, Shanghai 200433, China Abstract: Based on the first principle calculations, we demonstrate the physical reason that accounts for the distinct contacts between Pd and Pt nanolayers on graphene. Pd monolayer favors hybridization and charge transfer between d xz+dyz and dz2 orbitals and π states of graphene, which leads to a strong hybridization bond between Pd and graphene. On the contrary, stronger interaction between Pt atoms weakens the bond between Pt monolayer and graphene, prevents the hybridization to occur. Introduction: Carbon nanotubes (CNTs) and graphene are widely used to fabricate molecular devices such as field effect transistors (FET). However, it is found that the conductance primarily depends on the contact resistance rather than the channel conductance. Javey et. al. first reported the fabrication of nanotube FETs which exhibit Ohmic contact between CNT and Pd contacts. On the contrary, Pt contacted devices exhibit non-metallic behavior. It is still a puzzle why these two similar metal have different properties on CNTs. Method: Our results were obtained using the Vienna ab initio simulation package (VASP), which is based on the density-functional theory and the projector argumented-plane-wave method. The wave functions were expanded in a plane-wave basis sets with an energy cutoff of 500 eV. The exchangecorrelation potential was approximated with local density approximation (LDA). Figure 1: Models: Pd/Pt monolayer (ML) on graphene. Figure 2: The average distance of metal MLs and graphene layers. red balls = C atoms. white balls = metal atoms. black solid square = Pt layers. red open circle = Pd layers. black dashed line = supercell. Layered metals have the similar average distance between graphene layer and the nearest metal layer. In this model, there are two metal atom on the top site and one on the hollow site. Figure 3: The charge difference of Pd ML-graphene. iso value = 4.5 × 10-2 e/A3 . red iso-surface = decreased charge density. yellow iso-surface = increased charge density. cyan balls = C atoms. purple balls = Pd atoms. The charge difference proved that the dz2 and dxz+dyz states of Pd atoms interact with the π state of graphene layer. Figure 5: (a) band structure of Pd ML. Solid line = dxz+dyz state (b) = iso surface of bonding state of dxz+dyz state at M point. (c) = iso surface of anti-bonding state. Iso value = 5.1 × 102 e/A3. Figure 4: The LDOS of dxz+dyz states of Pd/Pt atoms. (a) = Pd. (b) = Pt. blue line = metal atoms in suspended layer. black line = metal atoms at the top site. red line = metal atoms at the hollow site. In Pd-graphene, the bonding state (-1.7 eV) and anti-bonding state (-0.3 eV) of dxz+dyz state in the suspended Pd layer were broken. Pt has a deeper bonding state of dxz+dyz, which changes little in Pt-graphene. Figure 6: The charge difference of Pd ML with adsorbed H atoms on graphene. iso value = 8.2 × 10-2 e/A3. small balls above Pd ML = H. others: the same as in Figure 3. H atoms get electrons from the antibonding state of dxz+dyz states of Pd atoms, which strengthen the bonding state of dxz+dyz state in Pd ML. As a result, the interaction between Pd ML and graphene was weakened. The layer distance of Pd ML and graphene increased to 3.08 A.