The past decades have witnessed the emergence of lowdimensional carbon-based nanostructures owing to their unique properties and various subsequent applications. It is of fundamental importance to explore ways to achieve atomically precise fabrication of these interesting structures. The newly developed on-surface synthesis approach provides an efficient strategy for this challenging issue, demonstrating the potential of atomically precise preparation of low-dimensional nanostructures. Up to now, the formation of various surface nanostructures, especially carbon-based ones, such as graphene nanoribbons (GNRs), kinds of organic (organometallic) chains and films, have been achieved via on-surface synthesis strategy, in which in-depth understanding of the reaction mechanism has also been explored. This review article will provide a general overview on the formation of one-dimensional carbon-based nanostructures via on-surface synthesis method. In this review, only a part of the on-surface chemical reactions (specifically, CÀ X (X=Cl, Br, I) and CÀ H activation reactions) under ultra-high vacuum conditions will be covered. CÀ X and CÀ H Activation of sp-CarbonOn-surface dehalogenative/dehydrogenative homocoupling reactions of precursors functionalized with alkynyl groups have manifested great potential for the fabrication of low-dimensional carbon-based nanostructures involving acetylenic scaffolds, such as carbyne, graphyne and graphdiyne. Sun [8] et al. reported the successful formation of one-dimensional wires with acetylenic scaffolding via on-surface CÀ Br activation of sphybridized carbon atoms. They designed and synthesized the precursor 4,4'-di(bromoethynyl)-1,1'-biphenyl (bBEBP), and then deposited it on Au(111) substrate at RT. After annealing tõ 320 K, one-dimensional chains were formed on the surface. These chains were composed of two kinds of alternating protrusions. Thus, an organometallic chain structure combined with CÀ AuÀ C was proposed (as shown in Figure 1a), with the DFT model being in good agreement with the STM image. Further annealing to~425 K leaded to Au atoms released from the organometallic chains, and consequently, CÀ C coupled molecular chains with acetylenic linkages were obtained (as shown in Figure 1b). The disappearance of dot protrusions between biphenyl groups can be observed from the STM images. Additionally, the feasibility to incorporate acetylenic scaffoldings into two-dimensional networks was also be demonstrated in this work. Liu et al. analogously synthesized graphdiyne zigzag chains and macrocycles. Statistical results showed that macrocycles were preferred to form in low coverage of organometallic intermediates. [9] These works act as a supplement to our understandings of on-surface dehalogenative homocoupling reactions, and moreover, make CÀ X activation of sp-hybridized carbon a suitable strategy to synthesize highquality nanostructures with acetylenic linkages.Glaser coupling, discovered by Glaser [10] in 1869, can also be applied to the preparation of chain structures...

Carbon allotropes comprising sp-hybridized carbon atoms have been investigated for decades for their molecular structure. One of the unsolved mysteries is whether they should take a linear or cyclic configuration in condensed phases due to the lack of atomistic characterizations. Herein, we designed a molecule with a C6 skeleton as a model system to address this issue, which was achieved by eliminating Br atoms from hexabromobenzene (C6Br6) molecule on the Ag(111) substrate via thermal treatment. It is found that the C6 ring intermediate resulting from complete debromination is energetically unstable at room temperature based on theoretical calculations. It subsequently transforms into the C6 polyynic chain via a ring-opening process and ultimately polymerizes into the organometallic polyyne, whose triyne structural unit is revealed by bond-resolved noncontact atomic force microscopy. Theoretical calculations demonstrated an energetically favorable pathway in which the ring-opening process occurs after complete debromination of C6Br6. Our study provides a platform for the synthesis of elusive carbon-rich materials.

As a bottom-up strategy, on-surface synthesis shows great potential in constructing novel carbon-based nanostructures. Herein, on-surface reactions of 1,1,2,2-tetraiodoethylene and 1,1,4,4-tetrabromo-1,2,3-butatriene were systematically explored. A combination of scanning tunneling microscopy and noncontact atomic force microscopy demonstrates that it is feasible to achieve acetylenic and diacetylenic organometallic polyyne (i.e., acetylenic Ag−carbyne and diacetylenic Cu−carbyne) on Ag(110) and Cu(110) surfaces, respectively. Interestingly, after annealing the sample, the acetylenic Ag−carbyne was transformed to diacetylenic Ag−carbyne on Ag(110), while an opposite transformation process occurred between diacetylenic and acetylenic Cu−carbyne on Cu(110). The DFT calculations indicate that such a lattice dependence of skeleton reconstruction originates from the thermodynamic stability of diacetylenic Ag−carbyne/acetylenic Cu−carbyne on the corresponding surfaces.

From the structure point of view, there are a number of ways of tiling a carbon sheet with different polygons, resulting in prospects of tailoring electronic structures of lowdimensional carbon nanomaterials. However, up to now, the experimental fabrication of such structures embedded with periodic nonhexagon carbon polygons, especially ones with more than three kinds, is still very challenging, leaving their potential properties unexplored. Here we report the bottom-up synthesis of a nanoribbon composed of 4−5−6−8-membered rings via lateral fusion of polyfluorene chains on Au(111). Scanning probe microscopy unequivocally determines both the geometric structure and the electronic properties of such a nanoribbon, revealing its semiconducting property with a bandgap of ∼1.4 eV on Au(111). We expect that this work could be helpful for designing and synthesizing complicated carbon nanoribbons.

Stereoselective synthesis of geometrically pure isomers is challenging and important in a variety of chemistry related disciplines and applications. The recently developed on‐surface synthesis strategy provides us opportunities in the fabrication of specific nanostructures which remain challenging by conventional solution chemistry. In this work, we choose alkenyl gem‐dibromides as molecular precursors, and demonstrate the formation of specific trans‐cumulene products via C−C couplings on Cu(110). This study supplements the database of on‐surface synthesis strategies by introducing the alkenyl gem‐dibromide group on the surface. More importantly, it provides a way to stereoselective synthesis of specific isomers involving C−C double bonds, which may be of substantial utility in stereochemistry.