First Principles Study Of Metal / Of Graphene Contact With Nature

With interesting electronic,thermal,and mechanical properties,carbon nanotubes (CNT) have potential applications for electronic devices.A key problem concerned is the contacts of devices to electrodes.Based on first principles calculations,in this work we study the contact properties concerned with two experimental observations,which have not been understood up to now:1.Why do Pd and Pt with electronic iso-structures have distinctively different behaviors when coating on CNTs? 2.If it is traced to that the interaction of Pd/CNT is stronger than that of Pt/CNT,why do Ti/CNT have a poor conductance although there is a stronger interaction between Ti and CNT than that between Pd and CNT?These observations are of experimental nature to underline the importance of a thorough theoretical investigation.In theoretical aspects,however,simulating atoms coating on a CNT is still a great challenge.The contacts between CNTs and electrodes are conventionally simulated by either end-contact or side-contact.However,metal coated on CNTs should realistically be neither side-contact nor end-contact;rather, CNTs should be surrounded by metal atoms,a dirty system without translation symmetry. It is difficult that due to slight difference between iso-structure Pd and Pt,a model such as side-contact and end-contact could not reveal the physical origins of the core problems.The actual atomic structure at the contact is unclear.Therefore,it is instructive to extract the different interaction involved when Pd and Pt are coated on CNTs from various effects.As a simplification,we study the contact of Pd and Pt on graphene instead of on CNTs,in order to provide a useful guidance for these experiments.On the other hand,as an unrolled CNT,the graphene itself is also a rapidly rising star of the family of carbon nano-materials with interesting electronic,thermal,and mechanical properties for potential applications and technological advance in electronic device miniaturization.Those contacts with electrodes are also critically important and have attracted increasing attention.Our calculations show that the optimized average distance of the metal to the graphene layer is 2.49 and 3.33 Afor Pd and Pt with the adsorption energy of 0.54 and 0.26 eV/unit cell,respectively.Up to five monolayers(ML) of Pd and Pt on graphene, the calculated distances show the similar difference between Pd and Pt on graphene. To understand its physical mechanism,we examine the charge transfer for one ML Pd on graphene.Analyzing the orbital symmetry one can conclude that electrons transfer between the Pd and graphene layer.The grapheneπorbital plays a bridge role in the electron transfer between the Pd and graphene layer.The electrons on theπorbital of graphene transferring to the d_xz+d_yz orbital of Pd are largely compensated by the electrons on the d_z2 orbital of Pd.We denote the mechanism as exchange transfer. Doubtless,this exchange-transfer mechanism increases the interaction states and transmission channels between the Pd and graphene layer.Most importantly,the mechanism keeps enoughπelectrons on the graphene.However,in Pt/graphene,this mechanism is suppressed by the strong hybridiza-tion of d_xz+d_yz orbitals between neighboring Pt atoms.The feature of these distinctively different behaviors is thus the difference of the interaction competition between metal-metal and metal-carbon:it is shown that the interatomic distances of the Pt atoms inside the unit cell with a 2.3%tensile strain are already contracted.It is found that the reduction of interatomic distances of the metal atoms starts from 1.0%tensile strain in the Pt monolayer,while for the case of Pd,there is no tendency to contract up to 6.4%tensile strain in the Pd monolayer.This indicates that in quite a wide range of the strain,the interaction competition between Pt-Pt and Pt-C favors the Pt-Pt interaction for Pt/graphene,while the Pd-C interaction is more favorable than the Pd-Pd interaction for Pd/graphene.Therefore,the physics obtained using the commensurate(8 carbon vs. 3 metal atoms) model,in which the Pt and Pd layer have a 2.3%and 3.5%tensile strain when coating on graphene,respectively,will not change,since the interaction competition between Pt-Pt and Pt-C(Pd-Pd and Pd-C) plays a key role in causing the exchange transfer mechanism.In order to confirm this conclusion,we deposited H on the hollow site of the Pd layer.After full relaxation,the distance of Pd and graphene increases to 3.08 A. Analyzing the electron distribution indicates that the electrons in the d_xz+d_yz orbitals of Pd transfer to H atoms.That is,the d_xz+d_yz antibonding orbitals lose electrons, leading to stronger bonding states,decreasing the chemical reactivity of the Pd layer and turning off the exchange-transfer mechanism.We emphasize here that a strong interaction does not definitely mean a better conduction.It is found that the interaction of Ti/CNT is much stronger than that of Pd/CNT;our calculations also show adsorption energy of 3.52 and 0.54eV/unit cell for Ti/graphene and Pd/graphene,respectively.However,the conductance of Pd/CNT is better than that of Ti/CNT,as observed experimentally.Therefore,the calculated charge distribution shows that in contrast to the case of Pd/graphene,Ti loses its d_xz+d_yz electrons when coating on graphene,while its d_z2 orbitals obtain electrons.There are strong bonding between d_z2 orbitals of Ti and p_z orbitals of C.The calculated band structures show that theπ-bonding and anti-bonding states of the graphene near the Fermi level are already destroyed,decreasing the conductance of Ti/graphene.In contrast,in the case of Pd/graphene,theπstates of graphene keep almost unchanged when Pd coating on the graphene.Therefore,although the interaction between Ti and C is much stronger than that between Pd and C,the conductance of Ti/CNT is poorer than that of Pd/CNT,since the strong interaction between Ti and C destroys theπstates of CNT for conductance.