Contact resistance is as a function of reverse contact area. Defects in graphene sheets lead to smaller overlapped area in metal/graphene interfaces and therefore increase the contact resistance.

Dear Amjadi,

Thanks very much for your reply. But I am still confused.

I have read a few works in that contact area in graphene was patterned intentionally to reduce contact resistance. 

http://pubs.acs.org/doi/abs/10.1021/nn400671z

http://pubs.acs.org/doi/abs/10.1021/nn405834b

how do you think about those results?

Thanks again.

Jia

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Dear Zhang,

Thanks a lot for the new thinking angle to analysis this question.

Do you have any ideas about the interaction between metal and graphene(with or without defects)?

Beat Regards

Jia

These papers are explaining totally different story. They used etching of graphene edges to enhance the chemical bonding between graphene and metal atoms. Definitely, chemical bonding drastically decreases the contact resistance, as compared with physical bonding. In the case of physical bonding, the graphene defects can increase the contact resistance in the interface area.