Exploring Graphene Nanoribbons Using Scanning Probe Microscopy and Spectroscopy
Graphene nanoribbons (GNRs) are strips of graphene, featuring narrow widths at the nanometer scale. A GNR may be considered as a structure cut out of graphene, which is a two dimensional honeycomb lattice of sp2 carbon atoms. Cutting graphene in different ways may be understood as imposing different boundary conditions on graphene, and therefore the electronic structures of GNRs are dependent on their geometries. Fascinating properties of graphene nanoribbons ranging from width-dependent semiconducting energy gaps to localized edge magnetization are predicted in theory. These properties, together with their ultra-thin nature, give GNRs great potential in future electronic applications. This dissertation focuses on the fundamental relations between the geometry and the electronic structure of GNRs, and explores bottom-up strategies to synthesize GNRs via molecular self-assembly.
Using scanning tunneling microscopy (STM) and spectrocopy (STS), chiral and ultra-narrow armchair GNRs and width-modulated GNR heterojunctions were studied. The localized edge states in chiral GNRs derived from unzipping carbon nanotubes were explored and evidence is shown that these states are spin-polarized. We further modified the chiral GNR edges with hydrogen plasma, and determined both the terminal hydrogen-bonding structure and the edge electronic structure by combining STM and ab initio simulation. Bandgap tuning of bottom-up synthesized armchair GNRs was demonstrated via development of a new molecular building block. We find that the energy gap of wider N = 13 armchair GNRs is 1.4 ± 0.1 eV, 1.2 eV smaller than the bandgap of a narrower N = 7 armchair GNR. In addition, width-modulated GNR heterojunctions were obtained by fusing segments of two different molecular building blocks, and were characterized to possess electronic structure similar to type I semiconductor junctions.
As an effort to develop an alternative route toward synthesis of GNRs, we imaged and studied single-molecule enediyne chemical reactions on metallic surfaces with non-contact atomic force microscopy (nc-AFM). This bond-resolved imaging technique allows us to extract an unparalleled insight into the chemistry involved in complex enediyne cyclization cascades on surfaces.