The cumulenic linear C5 and its coupling-reaction products
Authors: Xu Wei , Sun Luye , Guo Yuan , Xiang Wenzhi 
Published: February 2025

Abstract

Linear carbon (Cn), an elusive sp-hybridized carbon allotrope, has attracted much interests both experimentally and among theorists for decades due to remarkable properties and significant controversy. Similar to recently popular sp-hybridized cyclo[n]carbon, linear carbon may also have two distinct structural forms, i.e., cumulene for consecutive double bonds, or polyyne for alternating triple and single bonds. Peierls transition theory predicted that for infinite linear Cn, polyyne should be more stable than cumulene, and meanwhile opening a band gap at Fermi level. Due to its very high reactivity, synthesis and characterization of linear carbon are exceptionally challenging. Previous works have tried different methods, especially heavy end-capping groups or confined into carbon nanotube, to synthesize carbon chains, however, atomically precise synthesis of uncapped linear carbon, and real-space characterization of its detailed structure remains very rare. Especially, its cumulenic form, is proved to be even harder to synthesize compared to polyynic one. In solution, till now, the longest cumulene known to date is n = 9 (also with protected group at both ends). Here, we report the synthesis of an uncapped linear C5 via tip-induced dehalogenation and ring-opening reaction of a fully halogenated precursor, C5Br6, and characterization of it cumulenic structure by bond-resolved atomic force microscopy (AFM). Furthermore, we demonstrate that it is feasible to achieve tip-induced cascade reactions among linear C5, leading to the successful synthesis of longer carbon chains, for example, C10, C15. Inspired by that, utilizing the tip to apply larger voltage pulses (~4.5 V), carbon chains with various lengths, including C9, C10, C13, C14, C15, C17, C18, C21, could be efficiently synthesized, which provide us an ideal system to study the influence of number of carbon atoms and odevity effect on the structure of linear carbon, and moreover, to investigate the relationship between structures and related electronic properties. For the even-numbered chains (i.e., C10, C14 and C18), the polyynic structure emerges, due to the occurrence of Peierls transition within such finite carbon chains induced by NaCl surface, similar to the case of linear C6. Interestingly, for the odd-numbered chain, i.e., C9, it presents a cumulene-like structure (with rather small bond length alternations). While, for longer ones, i.e., C13, C15, C17, and C21, the major moieties are still cumulene-like structures and the both ends present triple bond characteristics. Scanning tunneling spectroscopy (STS) measurements further reveal that the transport gaps of odd-numbered chains are obviously smaller than that of even-numbered chains, validating the structures assigned above, and the transport gaps gradually decrease with increasing number of carbon atoms. Our findings exhibit an on-surface method for atomically precise synthesis of a series of linear carbon, rendering further investigation of their unique structures and electronic properties.