▎ 摘　　要

Width modulation has long been proven to be an effective method for band-gap engineering of graphene nanoribbons (GNRs). The success of bottom-up GNR synthesis technology has led to the realization of width modulation within a nanoribbon, which is regarded as a GNR heterojunction (HJ). However, the HJ "shape" does not guarantee an ideal HJ band structure. We report a first-principles investigation of GNR HJs composed of narrow and wide GNR segments. Two possible coupling modes exist when two GNR segments with different widths are connected. We find that the electronic states near the Fermi level are localized in the small band-gap segment for one coupling mode, whereas they are almost equally distributed in the two segments for the other one. Consequently, only the former mode can produce a HJ band structure. The underlying mechanisms are analyzed using two hypothetical experiments. The delocalized electronic states are generated by the interface edge C-C bonds. The stabilities of the two coupling modes are verified by molecular dynamics simulations. To exclude other factors and demonstrate the universality of this finding, GNR HJs with varied lengths and widths are also studied. These investigations provide insights into GNR HJ constructions.