Electronic structure of confined carbyne from joint wavelength-dependent resonant Raman spectroscopy and density functional theory investigations

TL;DR

This study combines wavelength-dependent resonant Raman spectroscopy and DFT calculations to explore the excited states of ultralong carbyne chains inside carbon nanotubes, revealing multiple resonances and electronic state characteristics.

Contribution

It provides new insights into the excited electronic states of ultralong carbyne chains, which were previously inaccessible experimentally, through combined spectroscopic and computational analysis.

Findings

Identification of three resonances in Raman spectra.

Prediction of two optically allowed electronic states separated by 0.14-0.22 eV.

Correlation of vibrational overtones with electronic state properties.

Abstract

Carbyne, i.e. an infinitely long linear carbon chain (LCC), has been at the focus of a lot of research for quite a while, yet its optical, electronic, and vibrational properties have only recently started to become accessible experimentally thanks to its synthesis inside carbon nanotubes (CNTs). While the role of the host CNT in determining the optical gap of the LCCs has been studied previously, little is known about the excited states of such ultralong LCCs. In this work, we employ the selectivity of wavelength-dependent resonant Raman spectroscopy to investigate the excited states of ultralong LCCs encapsulated inside double-walled CNTs. In addition to the optical gap, the Raman resonance profile shows three additional resonances. Corroborated with DFT calculations on LCCs with up to 100 C-atoms, we assign these resonances to a vibronic series of a different electronic state. Indeed, the calculations predict the existence of two optically allowed electronic states separated by an energy of 0.14-0.22 eV in the limit of an infinite chain, in agreement with the experimental results. Furthermore, among these two states, the one with highest energy is also characterized by the largest electron-vibration couplings, which explains the corresponding vibronic series of overtones.