The cumulenic linear C5 and its coupling-reaction products.

Linear carbon (Cn), an elusive sp-hybridized carbon allotrope, has long attracted interest for its remarkable properties and debated structures. Here, we report the synthesis of an uncapped linear C5 via tip-induced dehalogenation and ring-opening reaction of C5Br6, with its cumulenic structure confirmed by atomic force microscopy. We further demonstrate the tip-induced coupling of linear C5 to form longer chains (e.g., C10, C15). By applying higher voltage pulses to C5Br6 (also C6Br6), various carbon chains, including C9, C10, C13, C14, C15, C17, C18, C21, C23, could be synthesized. Even-numbered chains (C10, C14 and C18) adopt polyynic structures due to a Peierls transition on NaCl, whereas for odd-numbered chains, C9 adopts a cumulene-like structure, and longer ones exhibit hybrid structures with terminal triple bonds and cumulene-like interiors. Scanning tunneling spectroscopy reveals smaller transport gaps for odd- than even-numbered chains, consistent with Peierls theory, and decreasing gaps with increasing chain length.