On-surface synthesis of atomically-precise molecular nanomaterials

On-surface synthesis is a powerful tool to fabricate atomically defined carbon-based nanostructures in a bottom-up approach with prospective applications in molecular electronics, optoelectronics, and sensor devices. Structural precision and control are essential as the electronic properties of these nanostructures sensitively depend on their geometry. Using specifically designed organic precursors provides for the on-surface synthesis of atomically precise graphene nanoribbons with tunable band gaps by modifying their lateral size and edge termination, porous graphenes, and nanographenes. In addition, their electronic structure can be further tuned by including heteroatoms or non-six-membered rings.

The growth of long-range ordered two-dimensional (2D) porous graphenes in ultra-high vacuum (UHV) is still a major challenge. The bond flexibility in the molecular building blocks and the irreversibility of the newly formed C-C bond prohibit an error correction during the network formation. Therefore, electronic properties of bottom-up fabricated 2D networks remain experimentally widely unexplored, despite the exciting physical properties such as a half-metallic character or Dirac cones predicted by density-functional theory (DFT). In line with graphene nanoribbons, porous graphene exhibits a band gap that can be controlled by pore size, density, functionalization, and geometry.

Our research explores different coupling reactions and develops strategies to synthesize novel one-dimensional and two-dimensional carbon-based molecular materials on metal and insulating surfaces. We can identify their structure at the atomic scale by using high-resolution scanning probe microscopy at low temperatures (STM/ncAFM at 4K). In addition, we study the local electronic properties of the created one-dimensional and two-dimensional nanostructure using scanning tunneling spectroscopy (STS). Exciting aspects are how the band structure in two-dimensional covalently-linked networks can be tuned by using functional groups and changing the geometry of the networks (Dirac cone engineering). We aim to develop new carbon allotropes in a bottom-up approach, including graphyne-like materials which involve both sp2- and sp-carbon atoms and have many interesting properties, e.g. the potential of direction-dependent Dirac cones.

Our research activities in the field of on-surface synthesis are part of the SFB 953 Synthetic carbon allotropes and RTG 2861 PCL Planar carbon lattices.

Selected publications:

• Planar π-extended cycloparaphenylenes featuring all-armchair edge topology
  F. Xiang, S. Maisel, S. Beniwal, V. Akhmetov, C. Ruppenstein, M. Devarajulu, A. Dörr, O. Papaianina, A. Görling, K.Y. Asharov, S. Maier
  Nat. Chem. 14, 871–876 (2022)
• Metalated Graphyne-Based Networks as 2D Materials: Crystallization, Topological Defects, Delocalized Electronic States and Site-Specific Doping
  Z. Yang, T. Sander, J. Gebhardt, T.A. Schaub, J. Schönamsgruber, H. Soni, A. Görling, M. Kivala, S. Maier
  ACS Nano, 14, 12, 16887-16896 (2020)
• Two-dimensional delocalized states in organometallic bis-acetylide networks on Ag(111)
  Z. Yang, J. Gebhardt, T.A. Schaub, T. Sander, J. Schönamsgruber, H. Soni, A. Görling, M. Kivala, S. Maier
  Nanoscale, 10, 3769-3776 (2018)
• Hierarchical on-surface synthesis and electronic structure of carbonyl-functionalized one- and two-dimensional covalent nanoarchitectures
  C. Steiner, J. Gebhardt, M. Ammon, Z. Yang, A. Heidenreich, N. Hammer, A. Görling, M. Kivala, S. Maier
  Nature Communications, 8, 14765 (2017)
• On-surface synthesis of porous carbon nanoribbons from polymer chains
  M. Ammon, T. Sander, S. Maier
  J. Am. Chem. Soc., 139 (37), 12976–12984 (2017)