Semiconductor-to-metal transition in carbon-atom wires driven by sp2 conjugated endgroups

TL;DR

This study explores how conjugated endgroups can induce a transition from semiconducting to metallic behavior in carbon-atom wires, using synthesis, spectroscopy, and DFT calculations to guide the design of tunable carbon nanomaterials.

Contribution

It demonstrates that specific chemical design of endgroups and charge transfer control are essential for modulating the electronic properties of sp-carbon wires.

Findings

Charge transfer causes structural change from polyyne to cumulene.

Modulation of properties requires tailored endgroup design.

Hybrid sp-sp2 systems can be engineered for tunable electronic responses.

Abstract

Novel bis(biphenyl)-capped polyynes have been synthesized to investigate the modulation of the electronic and optical properties of sp-hybridized carbon-atom wires (CAWs) capped with {\pi}-conjugated $sp^{2}$ endgroups. Raman and Surface Enhanced Raman spectroscopy (SERS) investigation of these systems and Density Functional Theory (DFT) calculations reveal structural changes from polyyne-like with alternating single-triple bonds towards cumulene-like with more equalized bonds as a consequence of the charge transfer occurring when wires interact with metallic nanoparticles. While polyynes have semiconducting electronic properties, a more equalized system tends to a cumulene-like structure characterized by a nearly metallic behavior. The possibility to drive a semiconductor-to-metal transition has been investigated by systematic DFT calculations on a series of CAWs capped with different conjugated endgroups revealing that the modulation of the structural, electronic and vibrational properties of the sp-carbon chain towards the metallic wire cannot be simply obtained by using extended {\pi}-conjugated $sp^{2}$ carbon endgroups, but require a suitable chemical design of the endgroup and control of charge transfer. These results provide useful guidelines for the design of novel $sp-sp^2$ hybrid carbon nanosystems with tunable properties, where graphene-like and polyyne-like domains are closely interconnected. The capability to tune the final electronic or optical response of the material makes these hybrid $sp-sp^2$ systems appealing for a future all-carbon-based science and technology.