Carbon-based nanomaterials such as carbon nanotubes and graphene nanoribbons occupy a special place in nanoscience, and particularly in nanoelectronics. This is largely due to their unusual properties deriving from electron confinement that can open a bandgap with controlled quantum states. Consequently, carbon nanomaterials have emerged as promising building blocks for novel devices. Control over specific electronic properties implies ultimate (i.e. atomic) structural precision, and we aim at achieving this precision by a bottom-up synthesis approach based on the assembly of molecules on a metallic template using on-surface chemical reactions.
Spatial confinement in graphene nanoribbons leads to the opening of a bandgap that depends sensitively on the details of their atomic structure. To obtain a bandgap suitable for room temperature applications, the width of the ribbons needs to be reduced to well below 5 nm. In addition, ribbon width and edge structure need to be atomically precise, which cannot be achieved by current top-down methods such as lithography or etching. To meet these challenges we have developed, in collaboration with the group of Prof. Klaus Müllen (MPI Mainz, Germany), versatile on-surface synthesis routes to the bottom-up fabrication of atomically precise graphene nanoribbons. Building on these novel strategies, we aim at developing a versatile fabrication toolbox to GNRs of different widths, shapes and edge structures. With the atomically precise materials at hand, we explore their rich physics and chemistry by means of various experimental approaches. On the other hand, we work towards the application of graphene nanoribbons as the active material in prototypical electronic devices, which involves high-throughput fabrication as well as the development and optimization of suitable substrate transfer and contacting techniques.
Many potential applications of carbon nanotubes would strongly benefit from the availability of samples containing only one distinct structural form, i.e. single-chirality nanotubes. Despite tremendous efforts to understand and control the growth of carbon nanotubes, structural selectivity has been a long standing challenge. We investigate a seeded growth approach where specifically designed molecular precursors ‘fold’ into the desired nanotube end caps by a thermally induced on-surface reaction. In a second step these end caps are then elongated to singly capped nanotubes using a surface-catalyzed growth step. Our current focus is on the on-surface synthesis of a range of end caps that allow for the seeded growth of nanotubes of different chiralities. In another line of research, we investigate the influence of intrinsic and artificially introduced structural defects on the electronic properties of carbon nanotubes. On more technological grounds, we have a long standing expertise in the development of field emission sources based on carbon nanotubes. To this end, we also develop Scanning Anode Field Emission Microscopes (SAFEM) for the detailed characterization of planar field emission cathodes.
1D / 2D polymers
The on-surface synthesis approach can also be applied to the fabrication of low-dimensional polymeric materials. Our main goals are the design of 1D and 2D polymers with specific electronic properties and the fabrication of precise templates for the selective anchoring of molecular building blocks. An interesting target is so-called porous graphene, i.e. a 2D sheet of graphene with pores whose size and location is precisely defined by the design of the precursor monomer. In contrast to the semimetal graphene, porous graphene exhibits a band gap that can be tuned by the size and density of the pores. Manipulation of the electronic properties can further be achieved by incorporating elements other than carbon, or non-graphene bond topologies, in the 1D or 2D nanostructures. For example, replacing a C-C group by the isoelectric B-N can create “electronic voids”. To extend our toolbox of on-surface chemistry beyond aryl-aryl coupling and cylcodehydrogenation, we also study other reactions such as cycloaddition, cyclodehydrofluorination or cyclotrimerization, to name only a few.