▎ 摘　　要

The ability to control the nature of charge carriers in single-molecule junctions is of paramount importance to the design of molecular scale CMOS logic circuits. One major limitation is that the present methods for tuning the polarity of the charge carriers in molecular junctions always rely on chemical modifications of certain moieties of the central molecule. Here we investigate this issue by computing the electronic transport properties of graphene-porphine-graphene (GPG) molecular junctions with the nonequilibrium Green's function formalism combined with density functional theory. Our calculations show that the dominant charge carriers in these GPG junctions can be tuned from holes to electrons by simply varying the number of C-C covalent bonds formed at the porphine-graphene interfaces. When porphine is doubly fused to graphene, it exhibits p-type transport behavior; in contrast, the triply fused one displays n-type behavior. The cause of this is that distinct linkage motifs lead to a rearrangement of the molecular levels participating in charge transport. It is the first time that such effect is shown without using a chemical modification of the porphine. In addition, hydrogen tautomerization reaction in porphine induces a dramatic conductance switch for the doubly fused GPG junctions, suggesting promising applications of this system as two-level molecular switch or memory unit with binary digits.