Ultra-narrow metallic armchair graphene nanoribbons

Abstract

Graphene nanoribbons (GNRs)-narrow stripes of graphene-have emerged as promising building blocks for nanoelectronic devices. Recent advances in bottom-up synthesis have allowed production of atomically well-defined armchair GNRs with different widths and doping. While all experimentally studied GNRs have exhibited wide bandgaps, theory predicts that every third armchair GNR (widths of N=3m+2, where m is an integer) should be nearly metallic with a very small bandgap. Here, we synthesize the narrowest possible GNR belonging to this family (five carbon atoms wide, N=5). We study the evolution of the electronic bandgap and orbital structure of GNR segments as a function of their length using low-temperature scanning tunnelling microscopy and density-functional theory calculations. Already GNRs with lengths of 5 nm reach almost metallic behaviour with ∼100 meV bandgap. Finally, we show that defects (kinks) in the GNRs do not strongly modify their electronic structure.

Figure 1. Bottom-up synthesis of N=5 armchair GNRs.

(a) Reaction scheme of the polymerization of the DBP precursor to atomically well defined N=5 armchair GNRs. (b) Overview STM image after cyclodehydrogenation at 320 °C, showing straight and kinked GNRs (V=500 mV, I=50 pA; scale bar, 10 nm). (c) Zoomed-in STM topography of different ribbon lengths (V=300 mV, I=50 pA; scale bar, 2 nm).

Figure 2. dI/dV spectroscopy and real-space imaging of the GNR wavefunctions.

(a,b) dI/dV spectra acquired on three (a) and five (b) monomer GNR with a metallic tip. Location of the spectra are marked in the STM topographies in the insets and the black curve is measured on Au(111). The red and blue curves are shifted for clarity. (c-f) Experimental constant-height dI/dV maps (c,d) and the corresponding calculated LDOS maps (e,f) for three (c,e) and five (d,f) monomer GNRs at bias voltages corresponding to the positions marked with arrows in a,b.

Figure 3. Energies of molecular orbitals as function of ribbon length.

(a) Energies of the different molecular orbitals (HOMO-2 to LUMO) as a function of the ribbon length. (b) Comparison between experimental and calculated energy gaps as a function of the ribbon length.

Figure 4. Electronic structure of a kinked GNR with four- and five-monomer segments.

(a) STM topography with an overlaid model shows the connection on the kink by a pentagon. (b) dI/dV spectrum acquired at the kink (red line), an Au(111) spectrum (black line) is provided for comparison. (c) Constant-height dI/dV maps at energies labelled by arrows in b. (d) Corresponding simulated LDOS maps.