Exploring Aromaticity Effects on Electronic Transport in Cyclo[n]carbon Single-Molecule Junctions

Cyclo[n]carbon (C_n) is one member of the all-carbon allotrope family with potential applications in next-generation electronic devices. By employing first-principles quantum transport calculations, we have investigated the electronic transport properties of single-molecule junctions of C_n, with n = 14, 16, 18, and 20, connected to two bulk gold electrodes, uncovering notable distinctions arising from the varying aromaticities. For the doubly aromatic C₁₄ and C₁₈ molecules, slightly deformed complexes at the singlet state arise after bonding with one Au atom at each side; in contrast, the reduced energy gaps between the highest occupied and the lowest unoccupied molecular orbitals due to the orbital reordering observed in the doubly anti-aromatic C₁₆ and C₂₀ molecules lead to heavily deformed asymmetric complexes at the triplet state. Consequently, spin-unpolarized transmission functions are obtained for the Au-C_14/18-Au junctions, while spin-polarized transmission appears in the Au-C_16/20-Au junctions. Furthermore, the asymmetric in-plane π-type hybrid molecular orbitals of the Au-C_16/20-Au junctions contribute to two broad but low transmission peaks far away from the Fermi level (E_f), while the out-of-plane π-type hybrid molecular orbitals dominate two sharp transmission peaks that are adjacent to E_f, thus resulting in much lower transmission coefficients at E_f compared to those of the Au-C_14/18-Au junctions. Our findings are helpful for the design and application of future cyclo[n]carbon-based molecular electronic devices.