Cleaving single crystals in situ under ultra-high vacuum conditions provides a reliable and straightforward approach to prepare clean and atomically well-defined surfaces. Here, we present a versatile sample cleaver to efficiently prepare ionic crystal surfaces under ultra-high vacuum conditions, which is suitable for preparation of softer materials, such as alkali halides, and harder materials, such as metal oxides. One of the advantages of the presented cleaver design is that the cleaving blade and anvil to support the crystal are incorporated into the device. Therefore, no particularly strong mechanical manipulator is needed, and it is compatible with existing vacuum chambers equipped with an xyz-manipulator. We demonstrate atomically flat terraces and the atomic structure of NaCl(001), KBr(001), NiO(001), and MgO(001) cleavage planes prepared in situ under ultra-high vacuum conditions and imaged by low-temperature non-contact atomic force microscopy.

Graphyne-based two-dimensional (2D) carbon allotropes feature extraordinary physical properties; however, their synthesis as crystalline single-layered materials has remained challenging. We report on the fabrication of large-area organometallic Ag-bis-acetylide networks and their structural and electronic properties on Ag(111) using low-temperature scanning tunneling microscopy combined with density functional theory (DFT) calculations. The metalated graphyne-based networks are robust at room temperature and assembled in a bottom-up approach via surface-assisted dehalogenative homocoupling of terminal alkynyl bromides. Large-area networks of several hundred nanometers with topological defects at domain boundaries are obtained due to the Ag-acetylide bonds' reversible nature. The thermodynamically controlled growth mechanism is explained through the direct observation of intermediates, which differ on Ag(111) and Au(111). Scanning tunneling spectroscopy resolved unoccupied states delocalized across the network. The energy of these states can be shifted locally by the attachment of a different number of Br atoms within the network. DFT revealed that free-standing metal-bis-acetylide networks are semimetals with a linear band dispersion around several high-symmetry points, which suggest the presence of Weyl points. These results demonstrate that the organometallic Ag-bis-acetylide networks feature the typical 2D material properties, which make them of great interest for fundamental studies and electronic materials in devices.

AbstractDemonstrated here is a supramolecular approach to fabricate highly ordered monolayered hydrogen‐ and halogen‐bonded graphyne‐like two‐dimensional (2D) materials from triethynyltriazine derivatives on Au(111) and Ag(111). The 2D networks are stabilized by N⋅⋅⋅H−C(sp) bonds and N⋅⋅⋅Br−C(sp) bonds to the triazine core. The structural properties and the binding energies of the supramolecular graphynes have been investigated by scanning tunneling microscopy in combination with density‐functional theory calculations. It is revealed that the N⋅⋅⋅Br−C(sp) bonds lead to significantly stronger bonded networks compared to the hydrogen‐bonded networks. A systematic analysis of the binding energies of triethynyltriazine and triethynylbenzene derivatives further demonstrates that the X3‐synthon, which is commonly observed for bromobenzene derivatives, is weaker than the X6‐synthon for our bromotriethynyl derivatives.

The electronic structure of surface-supported organometallic networks with Ag-bis-acetylide bonds that are intermediate products in the bottom-up synthesis of graphdiyne and graphdiyne-like networks were studied. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal a frontier, unoccupied electronic state that is delocalized along the entire organometallic network and proves the covalent nature of the Ag-bis-acetylide bonds. Density-functional theory (DFT) calculations corroborate the spatial distribution of the observed delocalized state and attribute it to band mixing of carbon and silver atoms combined with n-doping of the metal surface. The metal-bis-acetylide bonds are typical metal-organic bonds with mixed character containing covalent and strong ionic contributions. Moreover, the organometallic networks exhibit a characteristic graphene-like band structure with linear band dispersion at each K point.

We demonstrate the on-surface synthesis of porous carbon nanoribbons on Ag(111) via a preprogrammed isomerization of conformationally flexible polymer chains followed by dehydrogenation reactions using thermal annealing. The carbon chains are fabricated by polymerization of prochiral 1,3,5-tris(3-bromophenyl)benzene (mTBPB) directly on the surface using an Ullmann-type reaction. At room temperature, mTBPB partially self-assembles in halogen-bonded 2D networks, which transform into organometallic chains and rings after debromination. The chain and ring formation is facilitated by conformational switching from a C3h to Cs symmetry of mTBPB via rotation of m-phenylene units. The high conformational selectivity toward Cs-conformers is templated by the twofold coordination to Ag adatoms. After thermally induced covalent-linking through aryl-aryl coupling, well-ordered nanoporous chains are created. Finally, the rotation of single phenylene units in combination with dehydrogenation cross-linking reactions within the polymer chains leads to the unexpected formation of porous carbon nanoribbons. We unveil the reaction mechanism in a low-temperature scanning tunneling microscopy study and demonstrate that the rotation of m-phenylene units is a powerful design tool to promote structural control in the synthesis of cyclic covalent organic nanostructures on metal surfaces.

The synthesis of non-conjugated, carbon-rich building blocks is described, based on a basic scaffold of triethynylmethanol (TEtM). The substitution of the ethynyl groups can be easily varied (including R3 Si, H, Br), and this allows structural tuning for stabilization, synthetic derivatization, and adsorption on Ag(111) or Au(111). X-ray crystallography helps to explain the surprising stability of the selected derivatives in the solid state, and an unusual form of hydrogen bonding is identified from these analyses. Preliminary efforts to achieve surface-based reactions on Ag(111) and Au(111) are outlined experimentally and computationally.