Reverse Exponential Decay of Electrical Transmission in Nanosized Graphite Sheets

Quantum transport effects in a molecular junction composed of a nanosized graphite sheet and two gold leads are studied on the basis of Landauer's formalism. The formulation for tunneling current by Caroli, Combescot, Nozieres, and Saint-James is extended to incorporate multiple interactions in a metal−insulator connecting region. A large variation of conductance is obtained, depending on the manner of connections between a graphite sheet and two gold chains. Connections between zigzag-edge sites and gold chains have significant transport effects. Graphite sheets of several sizes are studied to increase our understanding of the exponential law of conductance g = g 0 e -γ L, in which L is the molecular length and γ is the damping factor. Reverse exponential law with negative γ is observed in graphite sheets with zigzag edges. That is, the conductance is enhanced with an increase in L . This interesting behavior of conductance is due to the unique nature of the HOMO and LUMO localized in the zigzag-edge regions and the remarkable decrease in the HOMO−LUMO gap with L . Quantum transport effects in graphite sheets with defects such as a disordered zigzag edge are also studied. It is found that the regular zigzag-edge structures lead to effective quantum transport in graphite sheets.