Regioselective hydrogenation of graphene nanoribbons investigated by scanning tunnelling microscopy

Abstract

On surface synthesis of graphene nanoribbons (GNRs), with well-defined width, edge, and with the introduction of dopants, may open new possibilities in future electronic devices owing to the GNRs tunable electronic structure. However, recent reports have shown that the band gap of ribbons doped with heteroatoms (such as boron, nitrogen and sulphur) remains unchanged in magnitude in the majority of cases. To purposefully tailor the band gap, this research focuses on the hydrogenation of GNRs. The premise is based on previous theoretical calculations that studied the GNR’s electronic properties and show an opening of the band gap.

In this thesis, the hydrogenation mechanism and the electronic properties of hydrogenated GNRs were investigating experimentally primarily by low temperature Scanning Tunnelling Microscopy and Spectroscopy (LT-STM/STS). Gas-phase Density Functional Theory (DFT) calculations were also used to support the experimental findings. 7-Armchair GNRs (7-AGNRs) were prepared on the Au(111) surface, then were hydrogenated by exposure to activated hydrogen atoms. Vibrational spectroscopies were performed to verify the results of the hydrogenation process. The sequence of hydrogenation steps for 7-AGNRs is determined. Since the desired edge hydrogenation of 7-AGNRs could not be obtained, (3,1)-GNRs were subsequently selected for hydrogenation. This could allow to create periodic hydrogenation patterns on zigzag segments. The growth mechanism for the fabrication of (3,1)-GNRs from different precursors was studied through temperature-programmed growth (TPG). This allows to establish a time-saving and reproducible recipe. After hydrogenating the (3,1)-GNRs, a combination of STM imaging, along with DFT calculations, offers additional insights into the preferential reaction sites. This confirms the concept of regioselectivity in hydrogenation. Moreover, STS data show the evolution in electronic properties upon hydrogenation, opening up the possibilities of fine-tuning the electronic properties of the graphene nanoribbons via this chemical modification.