Phase Transition in Monoatomic Carbon Chains

=======
Abstract
=======
Mono-atomic chains of sp-carbon atoms, or carbynes, represent ultimate one-dimensional crystals. Besides significant fundamental interest they are highly promising for applications in nano-electronics and photonics. Fabrication of free-standing carbyne samples is extremely challenging because of instability of linear atomic chains. We have succeeded in stabilizing linear carbon chains by gold nano-particles. X-ray and electron diffraction studies reveal formation of van der Waals nanocrystals composed by polyyne chains of carbon attached to gold nanoparticles by their ends. These nanocrystals having a shape of threads may be deposited on a substrate and oriented by application of the external electric field. The low-temperature photoluminescence spectra of these structures reveal sharp excitonic resonances accompanied by satellite peaks associated to positively and negatively charges trions. Excitons are characterized by radiative decay times ranging between 800 and 1200 ps. We confront the data on the exciton energy and oscillator strength dependences on the lengths of the straight parts of carbon chains with the results of tight binding and DFT calculations. This analysis leads to a surprising conclusion: in contrast to single free standing polyyne chains which remain direct-gap semiconductors independently on their lengths, the van der Waals crystals composed by such chains exhibit a semiconductor-semimetal transition. The transition occurs at the critical length of the straight part of the chains of 42 atoms.

===============
About the Speaker
===============
Dr. Stella Kavokina's research spans nanomaterials, metasurfaces, laser fabrication, and low dimensional quantum systems. She has contributed to the development of hybrid plasmonic nanostructures, Tamm plasmon systems, and nanocomposite metamaterials for photonic and energy applications. More recently, her work has focused on carbynes—monoatomic carbon chains—where her group achieved major advances in their stabilization and integration, including the observation of excitonic resonances, opening new prospects for quantum nanoelectronics and nanophotonics.