On-surface synthesis and characterization of linear and cyclic C6

A novel family of carbon allotropes, known as cyclo[n]carbons, belonging to carbon clusters (Cn), have attracted significant attention from both experimentalists and theoreticians because of the controversial stabilities of different isomers and their chemical structures. Recently, certain cyclocarbons, namely C26, C20, C18, C16, C14, C13, C12 and C10, have been successfully synthesized and characterized on the thin insulating NaCl surface. However, synthesis of even smaller cyclocarbons (n < 10) still remains challenging. The substantial difficulties in synthesizing smaller cyclocarbons are mainly due to their inherent high reactivity and the increased strain. In particular, C6 has been a subject of long-standing debate in theory, as the energy difference between its linear and cyclic forms is very close, which renders the opportunity for the synthesis and characterization of both isomers on the surface. In this study, we successfully generate both linear and cyclic C6 by modulating the thickness of NaCl layers deposited on the Au(111) surface, using tip-induced dehalogenation of hexaiodobenzene (C6I6) molecules at 4.7 K. The linear C6 was generated either on the 1 monolayer (ML) or 2 ML NaCl surface, and identified as a polyynic structure through bond-resolved atomic force microscopy (AFM), verifying the theoretical calculations of a Peierls transition for linear C6 from gas phase to on-surface adsorption. While, the cyclic C6 could be generated on the 2 ML NaCl surface (the smallest cyclocarbon so far), conversely, the cumulenic nature of cyclic C6 is confirmed both experimentally and theoretically. Additionally, voltage pulses can induce cyclic C6 to undergo either an adsorption-configuration transformation from a nearly planar to a tilted geometry, or a structural transformation to the linear form. This work presents a methodology for the on-surface synthesis of small carbon clusters and offers insights into their electronic and geometric structures.