Download Origins of Distinct Contact of Pd and Pt Nanolayers on Graphene

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.