UV Resonance Raman spectroscopy reveals uniqueness of carbon atomic wires
Carbon atomic wires (CAWs) are one-dimensional linear chains made of carbon atoms. The recent interest in these systems is driven by their superior mechanical, thermal, and optoelectronic properties, which make carbon atomic wires relevant and versatile compounds for a multitude of applications in different fields. The structural simplicity of these wires makes them a useful model system to study fundamental properties of matter, as well as a practical example of the one-dimensional chain described in nearly all physics textbooks. In particular, CAWs are characterized by a collective vibration (or phonon) involving all the carbon-carbon bonds of the chain. Raman spectroscopy is the most powerful technique to investigate this fascinating and fundamental feature, strictly related to the other properties of CAWs due to strong π-electron conjugation. Due to stability issues, CAWs are typically found in low-concentration solutions, making the measurements of their vibrational properties extremely challenging.
In this study, we analyzed the vibrations of a specific class of CAWs, called hydrogen-capped polyynes, through resonance Raman spectroscopy, exploiting selective absorption of UV light (Fig. 1a). Working at the resonance with the Raman probe enabled the signal of the collective vibrations of these chains to be enhanced by several orders of magnitude. Although only a few laboratory-based laser lines can reach the UV range of interest, they cannot provide fine tunability to exactly excite all of the UV absorption peaks of CAWs. On the contrary, the high brightness and tunability of the IUVS beamline allowed the complete UV resonance Raman spectra of the atomic chains to be accessed (Fig. 1b).
The most striking result we obtained is the observation of high-order (namely up to the 5th order) vibrational transitions (overtones) of the fundamental collective vibration, referred to as α mode (Fig. 1b). The most interesting finding, however, is that the intensity of the overtones doesn’t follow the commonly expected behavior, summarized by the “decrease as the order increase” rule of thumb. Instead, the second order is massively enhanced when the chains are excited at the wavelength corresponding to the second absorption maximum. A peculiar behavior of the overtone intensity is also observed when probing the third absorption peak. This phenomenon is not related to the size of the chains, but rather a characteristic feature of this class of CAWs (see Fig. 2a).
Our results provide an excellent benchmark to verify one of the fundamental theories of resonance Raman, i.e., Albrecht’s theory. By calculating the intensity of the overtones with this analytical model (see Fig. 2b), we could nicely predict the enhancement of the second-order peak as well as the other spectral features. By combining these results with previous UV-Vis absorption spectra, we were able to better understand the optoelectronic properties of hydrogen-capped polyynes. In particular, by calculating the Huang-Rhys factor (Fig. 2c), we quantified the size of their electron-phonon coupling that resulted comparable with that of other carbon nanostructures and organic molecules and is strongly correlated with the chain length, representing the short limit of what has been observed in long linear carbon chains (confined carbyne).
In conclusion, carbon atomic wires provide an excellent model system for the analysis of the vibronic excitation and electron-phonon coupling in one-dimensional materials. The effect of anharmonicity is still under investigation to refine theoretical models and get even more precise predictions in the future. Our results and ongoing research on carbon atomic wires at the IUVS beamline will pave the way for future applications in the field of optoelectronic devices and carbon-based electronic. Importantly, our findings may help develop more precise and realistic models describing a wider range of materials than CAWs, e.g., other conjugated systems and 1D systems.
This research was conducted by the following research team:
Pietro Marabotti1, Matteo Tommasini2, Chiara Castiglioni2, Patrick Serafini1, Sonia Peggiani1, Mariagrazia Tortora3, Barbara Rossi3, Andrea Li Bassi1, Valeria Russo1 and Carlo Spartaco Casari1
1 Micro and Nanostructured Materials Laboratory—NanoLab, Department of Energy, Politecnico di Milano, Milano, Italy
2 Department of Chemistry, Materials and Chem. Eng. ‘G. Natta’, Politecnico di Milano, Milano, Italy
3 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
