Energy gaps of atomically precise armchair graphene sidewall nanoribbons

Abstract

Theoretically, it has been demonstrated that armchair Graphene nanoribbons (GNRs) can be divided into three families, i.e., ${N}_{a}=3p,\phantom{\rule{0.16em}{0ex}}{N}_{a}=3p+1$, and ${N}_{a}=3p+2$ (here ${N}_{a}$ is the number of dimer lines across the ribbon width and $p$ is an integer), according to their electronic structures, and the energy gaps for the three families are quite different even with the same $p$. However, a systematic experimental verification of this fundamental prediction is still lacking, owing to very limited atomic-level control of the width of the armchair GNRs investigated. Here, we studied electronic structures of the armchair GNRs with atomically well-defined widths ranging from ${N}_{a}=6$ to ${N}_{a}=26$ by using a scanning tunneling microscope. Our result demonstrated explicitly that all the studied armchair GNRs exhibit semiconducting gaps and, more importantly, the observed gaps as a function of ${N}_{a}$ are well grouped into the three categories, as predicted by density-functional theory calculations. Such a result indicated that the electronic properties of the armchair GNRs can be tuned dramatically by simply adding or cutting one carbon dimer line along the ribbon width.