Aligning the band gap of graphene nanoribbons by monomer doping.

Silicon-based field-effect transistors (FETs) are the building blocks of modern digital logic circuitry and therefore part of virtually every electronic device available today. Over the past decades, continuous downscaling of existing designs has met the rising performance requirements, but as the size of FETs approaches the regime of atomic structures, new concepts are required to maintain the current pace at which microelectronics is developing. At small gate channel lengths, the applicability of quantum mechanical principles results in several so-called short-channel effects (e.g. reduced carrier mobility). Since its experimental realization in 2004, graphene has been discussed intensively as a substitute for doped silicon in FETs because of its high charge carrier mobility and its unsurpassably low thickness. Graphene transistors have even been realized but cannot be put into the “off” state because of the lack of a band gap. However, there are concepts available for opening a band gap, for example, applying strain along the sheet or biasing bilayers of graphene. Also, lateral confinement in quasione-dimensional graphene nanoribbons (GNRs) leads to a band gap, which furthermore is highly sensitive to the width and edge shape of the GNR, thus opening possibilities to tailor the electronic properties of a device. Indeed, FETs built from nanoribbons show much higher on/off-ratios than graphene transistors, which makes them more suitable for integration into logic devices. However, the ability to control the electronic properties is essential: while the size of the gap can be engineered by varying the nanoribbon widths, the alignment of the GNR band structure with respect to the Fermi level of a metal electrode is equally important. Such a shifting of the entire band structure is observed both in two-dimensional graphene as well as in chemically synthesized or lithographically patterned GNRs upon doping, particularly with nitrogen atoms. Using present doping techniques, the distribution of dopant atoms will not be well-defined on the nanoscale and the band gap shift upon nitrogen doping depends on the site of the N atom, that is, the bonding configuration to neighboring carbon atoms. Generally, for doped and pristine GNRs, fabrication remains a challenge as well-established top-down approaches using lithography or unzipping of carbon nanotubes yield relatively wide ribbons with an undetermined edge structure. Particularly for small widths on the order of a few nanometers (where the band gap reaches a technologically relevant size) atomically precise edges are necessary and can be realized using Br-substituted precursor molecules, which are thermally activated on a surface and—in a bottom-up synthesis— covalently assemble to a specific nanostructure. In this study, we employed the latter approach to prepare GNRs with an atomically precise edge structure and doping pattern through polymerization of specific monomers directly on the Au(111) surface and studied the position and size of the band gap of these GNRs with surface-sensitive electron spectroscopies. Besides straight armchair edge GNRs, another type of chevron-shaped nanoribbons has previously been fabricated using an on-surface reaction. In this process adsorption of several layers of 6,11-dibromo-1,2,3,4-tetraphenyl-triphenylene (monomer 1 in Scheme 1) on Au(111) and heating at 250 8C leads to desorption of the second and higher layers as well as halogen dissociation and coupling of the resulting activated biradical monomers, yielding a sterically crowded and hence twisted polyphenylene. In a second heating step at 440 8C, this polymer undergoes a subsequent cyclodehydrogenation reaction providing access to the desired chevronshaped GNR with armchair edges. Selective substitution of the parent monomer 1 with either one or two N atoms provided monomers 2 and 3, respectively, which were used to generate GNRs with different doping levels (exemplarily shown for the doubly N doped GNR 5 in Scheme 1) and accordingly with potentially different electronic structure properties. Synthesis of the new monomers 2 and 3 was accomplished by Diels–Alder reactions of an appropriate cyclopentadienone with either mixed phenylpyridyl-acetylene or bispyridylacetylene, followed by immediate cheletropic CO extrusion (for details see the Supporting Information). [*] C. Bronner, S. Stremlau, A. Haase, Prof. Dr. P. Tegeder Fachbereich Physik, Freie Universit t Berlin Arnimallee 14, 14195 Berlin (Germany) E-mail: [email protected] [email protected]