Unveiling Anharmonicity in Carbon Atomic Wires through UV Resonance Raman Spectroscopy

Carbon atomic wires, a specific type of carbyne (an infinite linear carbon wire), exhibit unique properties that make them promising for applications in various technology fields. Despite their potential, the properties of carbyne are not well understood due to the challenges posed by synthesizing these systems.

Our research team from Politecnico di Milano and Elettra studied carbon atomic wires properties using a technique called resonance Raman spectroscopy. We employed synchrotron-based UV resonance Raman spectroscopy at the IUVS beamline of Elettra to investigate short carbon atomic wires terminated by hydrogen, methyl, or cyano groups. This approach allowed us to collect multi-wavelengths resonance Raman spectra of many carbon atomic wires, as shown in Figure 1, and observe, for the first time, the Raman spectra of size-selected methyl- and cyano-capped chains.

Figure 1: Experimental UV resonance Raman spectra of a) H-, b) CH₃-, and c) CN-capped wires. DFT simulations of Raman spectra for the same wires are depicted in the top panels. First- and second-order experimental Raman spectra are presented in the middle and bottom series of panels, respectively.

We gained valuable insights into the properties of short carbon atomic wires, understanding how they are influenced by size confinement and termination effects. The focus of our study was on the wire vibrational and electronic properties, as well as their anharmonicity, that is, the deviation from the harmonic, “linear” behavior.

Our results showed that the degree of electron conjugation (the interaction of electrons within the wire) increases with the wire length and is influenced by the specific termination. We also discovered that, as the wire length increases, the anharmonic correction of the vibrational energy becomes more significant, see Figure 2. Additionally, we found that the electron-phonon coupling (interaction between electrons and wire vibrations) is sensitive to the chain length and termination, which we determined by studying the vibronic lines of the UV absorption spectra. As the observed increase of anharmonicity and electron-phonon coupling with the chain length is sizable but not remarkable, we concluded that the onset for the carbyne limit is still distant. However, these findings also highlight the potential applications of carbon atomic wires in nanoelectronics and thermoelectricity.

Figure 2: a) Portion of the potential energy surface (PES) of hydrogen-capped wires calculated by DFT, illustrating the growth of anharmonicity with chain length. The dashed curve indicates the ideal harmonic potential. b) Nondimensional anharmonic parameter for each wire determined from calculations.

Overall, this research deepens our understanding of carbon atomic wires and their potential usage in advanced technologies. By studying their properties at the molecular level, scientists can tailor new structures that suit specific applications in fields like optics, electronics, and optoelectronics.

This research was conducted by the following research team:

Pietro Marabotti¹, Matteo Tommasini², Chiara Castiglioni², Sonia Peggiani¹, Patrick Serafini¹, Barbara Rossi³, Andrea Li Bassi¹, Valeria Russo¹, Carlo Spartaco Casari^1
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 - Sinctrotrone Trieste S.C.p.A., Trieste, Italy.

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