Electronic band gaps of confined linear carbon chains ranging from polyyne to carbyne

摘要

Ultra long linear carbon chains of more than 6000 carbon atoms have recentlybeen synthesized within double-walled carbon nanotubes, and they show apromising new route to one--atom--wide semiconductors with a direct band gap.Theoretical studies predicted that this band gap can be tuned by the length ofthe chains, the end groups, and their interactions with the environment.However, different density functionals lead to very different values of theband gap of infinitely long carbyne. In this work, we applied resonant Ramanexcitation spectroscopy with more than 50 laser wavelengths to determine forthe first time the band gap of long carbon chains encapsulated inside DWCNTs.The experimentally determined band gaps ranging from 2.253 to 1.848 eV follow alinear relation with Raman frequency. This lower bound is the smallest band gapof linear carbon chains observed so far. The comparison with experimental dataobtained for short chains in gas phase or in solution demonstrates the effectof the DWCNT encapsulation, leading to an essential downshift of the band gap.This is explained by the interaction between the carbon chain and the hosttube, which greatly modifies the chain's bond length alternation.