  Graphene nanoribbons (GNRs) are one-dimensional (1D) structures that exhibit
a rich variety of electronic properties1-17. Therefore, they are predicted to
be the building blocks in next-generation nanoelectronic devices.
Theoretically, it has been demonstrated that armchair GNRs can be divided into
three families, i.e., Na = 3p, Na = 3p + 1, and Na = 3p + 2 (here Na 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 p1,3-6. 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
investigated7,9,10,13,17. Here, we studied electronic structures of the
armchair GNRs with atomically well-defined widths ranging from Na = 6 to Na =
26 by using scanning tunnelling microscope (STM). Our result demonstrated
explicitly that all the studied armchair GNRs exhibit semiconducting gaps due
to quantum confinement and, more importantly, the observed gaps as a function
of Na are well grouped into the three categories, as predicted by
density-functional theory calculations3. Such a result indicated that we can
tune the electronic properties of the armchair GNRs dramatically by simply
adding or cutting one carbon dimer line along the ribbon width.
