Carbyne Nanocrystal: One-Dimensional van der Waals Crystal.

In terms of carbon-atom hybridization, well-established forms of carbon are the first carbon diamond with three-dimensional sp3-hybridized carbon atoms and the second carbon graphite with two-dimensional sp2-hybridized carbon atoms which have been known and utilized for millennia. Sequentially, there is the third carbon, i.e., carbyne with one-dimensional (1D) sp-hybridized carbon atoms, which would result in an allotrope of carbon. Here, we demonstrate that carbyne nanocrystals (CNCs) are 1D van der Waals crystals (1D-vdWCs) composed of 1D carbon chains with sp-hybridized carbon atoms, and van der Waals action occurs between carbon chains based on an atomic insight into 1D sp-carbon chains. CNCs are synthesized by laser ablation in liquids, and the relevant spectroscopic analyses confirm that CNCs are composed of 1D carbon chains with the alternating carbon-carbon single and triple bonds. The crystal structure of CNCs is determined by X-ray diffraction, transmission electron microscopy (TEM), including selective electron diffraction (SAED), high-resolution TEM (HRTEM), and scanning TEM (STEM) and the corresponding simulations. SAED and HRTEM images reveal the translational symmetry of CNCs, and STEM images show the specific position of the carbon chain in CNCs and the arrangement of atoms on the carbon chain. Experimental data are in good agreement with the simulations, which demonstrate that CNCs are 1D-vdWCs with a hexagonal lattice in which the 1D carbon chain has a kinked structure consisting of an alternating carbon-carbon single bond and a triple bond of eight carbon atoms in a cycle. These findings bring out an emerging era of the third carbon carbyne.