0D sp-carbon nanostructures were hard to isolate and characterize due to their instability in condensed phases. A large number of experimental and theoretical studies on carbon clusters have been applied to their molecular structures and electronic properties ^[1][37]. Most importantly, a fundamental and controversial question is still open: the configuration of cyclocarbons is polyynic (alternating single and triple bonds of different lengths) or cumulenic (consecutive double bonds) ^[2][3][29,38]. Initial laser desorption mass spectrometry has been reported to obtain evidence for the size-selective growth of fullerenes through the coalescence of cyclo[n]carbons, the molecular carbon allotropes consisting of monocyclic rings with a certain number (n) of carbon atoms ^[4][39]. Three carbon oxides C_n(CO)_n/3 (Figure 1a–c) with n = 18, 24, and 30 were prepared as precursors to the cyclocarbons cyclo-C₁₈, C₂₄, and C₃₀ (Figure 1d), respectively. Mass spectroscopy pointed out that the coalescence of cyclo-C₃₀ was accompanied by the formation of predominantly buckminsterfullerene (C₆₀), while the smaller C₂₄O₆ and C₃₂O₈ preferentially produced fullerene C₇₀ instead. This work provides not only new insights into the mechanism of fullerene formation but also a protocol to obtain cyclocarbons. However, the structure of cyclocarbon, whether it is cumulene or polyyne, was not explained. Recently, cyclo[18]carbon (C₁₈) was synthesized with atomic precision via atom manipulation and characterized for the first time. By using low-temperature STM-AFM, carbon monoxide was eliminated from the precursor (Figure 1e), a cyclocarbon oxide molecule C₂₄O₆, on a bilayer sodium chloride (NaCl) on Cu(111) at 5 K ^[5][40] resulting in the formation of a single C₁₈ molecule. The nc-AFM image of the intact precursor was shown in Figure 1f and characterization of C₁₈ by high-resolution AFM revealed a polyynic structure of C₁₈ with defined positions of alternating triple and single bonds, which was observed as bright lobes under nc-AFM contrast shown in Figure 1g. The AFM contrast evidenced the structure of C₁₈ on NaCl with the defined positions of C≡C triple bonds, which is supported by the simulated image in Figure 1h. This work provides the first example of a real-space characterization of cyclo[18]carbon, which shows a polyynic form rather than a cumulenic structure using a combination of experimental and theoretical tools.

Innovative carbon clusters with manifold architectures involving sp-carbon can be prepared by using rationally designed halogenated hydrocarbons. For example, a nanostructure involving diyne moieties was fabricated via on-surface assisted homocoupling reaction starting from 1,3-bis(2-bromoethynyl)benzene (shortened as BBEB, Figure 1i) on Au(111) ^[6][41]. The submolecular resolution observations confirmed the formation of the organometallic intermediates (Figure 1j) and graphdiyne-like macrocycles after demetallization (Figure 1k). It can be concluded that on-surface synthesis has strong advantages in the preparation of 0D carbon structures. Therefore, more attention should be devoted to the on-surface synthesis and its mechanism for the discovery of novel 0D carbon nanostructures comprising of sp-carbons.

A plethora of novel two-dimensional carbon nanostructures are flourishing, such as nanoporous graphene, nonbenzenoid carbon allotropes, etc., especially the nanostructures involving sp-hybridized carbons. The C–C triple bond is of great advantage for avoidance of fluctuation arising from cis-trans isomerization which is different from olefinic bond ^[7][8][58,59]. What is more, the graphyne comprising of sp- and sp²-carbon atoms has been predicted to have a crystalline state, which was also predicted to be one of the most stable carbon phases containing acetylenic groups as a major structural component. Additionally, graphyne was calculated to be a semiconductor with a bandgap of 1.2 eV ^[9][60], which demonstrated their possible applications in electronic devices. However, the atomic precise fabrication of graphyne is limited by its high chemical reactivity. Zhang et al. developed a method to grow extended 2D graphdiyne-like networks using terminal alkynes on Ag(111) via homocoupling reaction, as demonstrated in Figure 2a–c ^[10][61]. The STM image in Figure 2b revealed a closer inspection of irregular, open-porous networks. The inset showed a magnified image of the associated honeycomb unit, and a corresponding model. The formation of covalent bonds was substantiated by complementary XPS measurements. The absence of the low energy shoulder peak at 283.7 eV (a typical binding energy for methylacetylide) revealed a resulting covalent structure, which is contradictory to that proposed by the organometallic binding mechanism when compared with the simulated XPS spectrum of the organometallic dimer. Other impressive efforts have also been devoted to synthesizing graphyne-like nanostructures. Similarly, Zhang et al. have synthesized highly regular single-layer alkynyl-silver organometallic networks at the micrometer scale via gas-mediated surface reaction (Figure 2d) ^[11][62]. Different from the previous strategy through thermal induction combined with catalyzation via substrate, terminal alkyne radicals were obtained via oxygen gas mediated deprotonation on Ag(111), and the activation procedure was confirmed by XPS spectroscopy. The peak at 283.6 eV evidenced the strong interaction between the alkynyl groups and the substrates. There was an absence of the O 1s signature in the XPS spectrum after dosing O₂, ruling out adsorbed oxygen species and other intermediates containing oxygen. The results indicated that both gas species and substrate play crucial roles in the particular activation process. This work provides a versatile fabrication procedure featuring high chemoselectivity without poisoning the surface. Aside from dehydrogenation, debromination of sp- and sp³-carbon was explored, which greatly avoids the byproducts generated at elevated temperatures. As demonstrated in Figure 2e–g, Xu group applied the on-surface synthesis protocol by introducing dehalogenative homocouplings of alkynyl bromides on Au(111), which results in the formation of an organometallic intermediate and subsequent release of gold atoms at elevated temperature ^[12][51]. Furthermore, they synthesized C≡C triple-bonded structural motif by dehalogenative homocouplings of tribromomethyl-substituted arenes, as illustrated in Figure 2h ^[13][52]. Notably, the organometallic intermediates were not formed in this process. This work provides a new protocol to convert sp³-carbon atoms to sp-hybridized ones through pre-designated molecular precursor. Moreover, Figure 2i,j demonstrate two potential precursors that could be considered to synthesize graphdiyne and graphyne, respectively.