Electronic band dispersion of graphene nanoribbons via Fourier-transformed scanning tunneling spectroscopy

Abstract

The electronic structure of atomically precise armchair graphene nanoribbons of width N = 7 (7-AGNRs) are investigated by scanning tunneling spectroscopy (STS) on Au(111). We record the standing waves in the local density of states of finite ribbons as a function of sample bias and extract the dispersion relation of frontier electronic states by Fourier transformation. The wavevector-dependent contributions from these states agree with density functional theory calculations, thus enabling the unambiguous assignment of the states to the valence band, the conduction band and the next empty band with effective masses of 0.41 ± 0.08 m e , 0.40 ± 0.18 m e and 0.20 ± 0.03 m e , respectively. By comparing the extracted dispersion relation for the conduction band to corresponding height-dependent tunneling spectra, we find that the conduction band edge can be resolved only at small tip-sample separations and has not been observed before. As a result, we report a band gap of 2.37 ± 0.06 eV for 7-AGNRs adsorbed on Au(111).

On‐Surface Synthesis of Atomically Precise Graphene Nanoribbons

“…Investigations have relied both on area-averaging techniques, such as angle-resolved photoemission spectroscopy (ARPES), which is enabled by parallel alignment of GNRs on vicinal metal surfaces, [ 50,58 ] and on Fourier transform scanning tunneling spectroscopy (FT-STS), which is performed on individual GNRs. [ 19,59 ] While some questions regarding discrepancies between ARPES and FT-STS results remain to be answered, [ 59 ] we note that the FT-STS results of m* = 0.4 m e for the 7-AGNR and m* = 0.1 m e for the 9-AGNR are in good agreement with theoretical predictions. [ 19,59 ] Here, m e is the free electron mass and m * is the effective mass of both the valence and the conduction band.…”

“…[ 19,59 ] While some questions regarding discrepancies between ARPES and FT-STS results remain to be answered, [ 59 ] we note that the FT-STS results of m* = 0.4 m e for the 7-AGNR and m* = 0.1 m e for the 9-AGNR are in good agreement with theoretical predictions. [ 19,59 ] Here, m e is the free electron mass and m * is the effective mass of both the valence and the conduction band. For AGNRs with smaller bandgaps, effective masses are expected to decrease even further as the valence-band maximum and the conduction-band minimum approach the Dirac point of graphene.…”

Resolving Atomic Connectivity in Graphene Nanostructure Junctions

“…Figure 2f shows such a current map (√I, bias voltage = 2 mV) simultaneously acquired with the constant height frequency shift image shown in Figure 2d. The current exhibits high amplitude close to the junction at the original zigzag edge and stretches significantly into the regular 29,30 and (ii) the zigzag ends, where we expect halffilled end states, also called Tamm states. 31,32 The appearance of the zigzag terminus of a free 7-AGNR, namely the Tamm states, strongly depends on the bond configuration of the contributing carbon atoms, for example, the corresponding fingerprint vanishes completely upon a change of only one C atom from sp 2 to sp 3 hybridization.…”

Hierarchical on-surface synthesis and electronic structure of carbonyl-functionalized one- and two-dimensional covalent nanoarchitectures

“…The use of specifically designed organic precursors provides for on-surface synthesis of atomically precise graphene nanoribbons56 with tunable band gap through heteroatom-doping or by modifying their lateral size and edge termination78910111213. In contrast, the growth of long-range ordered two-dimensional (2D) porous graphene in ultra-high vacuum (UHV) is still a major challenge, despite the use of various design concepts and surface reactions1415161718.…”