Theoretical study of vertical van der Walls metal-porphyrin and metal free-porphyrin junctions

Abstract

Abstract This paper provides a theoretical study of the thermoelectric properties of a vertical graphene/porphyrins/graphene architecture. It presents the details of calculating the conductance of metal-porphyrins, which is found to be enhanced by manipulating the metal central atom of the organic 125 porphyrin framework over the family N i , Z n , F e I I , and C o I I . The results demonstrate that even when there is no direct inter-molecular coupling, indirect inter-molecular interactions mediated by the graphene electrodes produce quantum interference effects in the electronic structure of the molecular junction. These junctions are all observed to be HOMO-dominated, meaning that their Seebeck exhibit the same sign and similar behavior. The resulting single-molecule thermopowers range from almost +50 μV/K for both Z n - and N i -porphyrin to +77 μV/K and +85 μV/K for C o - and F e -porphyrin, respectively. For these geometries, the effect of the metal complex with porphyrin on the conductance of the junctions can be seen. An extra resonance appeared in the HOMO-LOMO gap, and it can be disappeared or be shifted closer to Fermi energy to create a new path for the electron transmission. It only depends on the type of metal coordinating at the porphyrin center. Introducing such a new technique for designing high conductance and thermopower opens a door for high thermoelectric performance materials.