The resonant Raman spectroscopy of Carbon atomic wires

For the first time we performed a resonance Raman study on size-selected sp-hybridized carbon atomic wires by tuning the excitation light to exactly match the vibronic absorption. To do these experiments we exploited the tunable UV light of ELETTRA Synchrotron radiation facility in Trieste (Italy).
The resonant Raman spectroscopy of Carbon atomic wires
Carbon atomic wires in the class of carbon nanostructures 

1) Could you briefly outline the key findings of your paper? 

Reply: Carbon atomic wires are 1-dimensional chains made of sp-hybridized carbon atoms and represent the lacking carbon allotrope Carbyne. As finite systems they feature optical electronic and vibrational properties strongly modulated by the length (i.e., number of carbon atoms). The absorption spectrum is characterized by strong vibronic features in the UV range. Raman and Surface enhanced Raman spectroscopy are widely used for investigating these systems. For the first time we performed a resonance Raman study by tuning the excitation light to exactly match the vibronic features in the UV (i.e., 200-270 nm). To do these experiments we exploited the tunable UV light of ELETTRA Synchrotron radiation facility in Trieste (Italy) at the IUVS beamline equipped to perform resonance Raman spectroscopy. We observed Raman peaks characteristic of the sp-carbon wires up to the fifth order (scattering of one photon with 5 vibrational quanta) outlining a peculiar effect in the intensity of these peaks, not following the usual decreasing monotonic trend. Such behavior has been explained by using Albrecht’s theory of Resonance Raman. Moreover, our results allowed us to retrieve the detailed electronic and vibrational structure of size-selected wires made by 8,10 and 12 carbon atoms and to estimate the electron-phono coupling which turns out to be strong and size-dependent.  

Experimental set-up for UV Resonance Raman at IUVS beamline at ELETTRA Synchrotron in Trieste, Italy

2) What is your role in this work?

Reply: This work has been done in the framework of an ERC consolidator grant (EspLORE, grant n° 724610). As PI of EsplORE, I conceived and designed the project, and I supervised the team. P. Marabotti conducted the experiments and analyzed the data. Prof. C. Castiglioni and prof. M. Tommasini worked on the theory. Dr. Barbara Rossi as beam scientist at ELETTRA supported the experimental work. All authors discussed the results and commented on the manuscript. 


3) What was the genesis of this paper?  How did you come to this particular problem?

Reply: Our team has been focused on the synthesis of carbon atomic wires and their characterized by Raman spectroscopy. However, Raman is difficult to perform due to the low concentration of these wires in solution. We were looking for a UV laser source to perform resonance Raman, indeed when you irradiate with light that is absorbed by your sample the Raman signal is strongly intensified by this resonance effect. Our carbon wires have very specific absorption wavelength and it turned out extremely difficult to find a tunable UV laser source to perform resonance Raman at the specific absorption of each of the wires we synthesized. Extensive research led us to ELETTRA synchrotron in Trieste where IUVS beamline is equipped to perform Raman with a tunable UV light in the range we needed. Our proposal was accepted and beamtime allocated for this experiment.


4) What is the most empowering implication of your results?

Reply: Our results have some important implications for the understanding of the electronic and vibrational properties of carbon wires. Firstly, through a quite simple analytical model, we show how Albrecht’s theory of resonance Raman nicely explain our results. Albrecht’s theory nicely explains the peculiar change of intensity of higher-order peaks when the wire is irradiated with light matching the different vibronic absorption maxima. Moreover, we were able to obtain the value of the Huang-Rhys factor, which indicated the degree of electron-phono coupling. We found that this value is strongly dependent on the wire length and increases for longer wires. These systems show a very high electron-phono coupling, competitive with other carbon nanostructures or organic molecules, thus making carbon wires appealing for optoelectronic applications.  

5) How have carbon atomic wires been uniquely instrumental to enabling these results?

Reply: Carbon atomic wires are unique for their extremely large conjugation effects strongly connecting the structure with the electronic and vibrational properties. This makes short carbon wires extremely sensitive to size effects. Indeed, these systems feature extremely high tunability of properties by just controlling the length and the termination. Hydrogen terminated systems, the ones we investigated in this work, represent the simplest model of the carbyne, the lacking allotrope of carbon after graphite and diamond, with important size effects. Our study was made possible by the unique combination of clear and neat vibronic peaks and very large frequency of the main vibration of the wire that is present on carbon atomic wires.     

 6) Can you describe the main challenges associated to the preparation of this manuscript? Any anecdotes you’d like to share with us?

Reply: The experimental data were solid, but at the beginning we had no clear explanations for the strange intensity changes we observed for higher order peaks. We discussed with theoreticians at the Dept. of Chemistry Materials and Chemical Engineering in our Institution, and they performed a theoretical analysis showing very similar trend that we observed. This was very exciting, and we started discussing how to obtain information on the carbon wires properties. By reading a paper by Lei Shi and Thomas Pichler and their coworkers in which they estimated the Huang-Rhys factor for a very long wires encapsulated in a carbon nanotube (the so called confined carbyne) we decided to analyse our data to extract Huang-Rhys factor for size selected wires in the short range where the size effects are relevant. The final manuscript was ready more than one year after the experimental campaign done at ELETTRA synchrotron.

7) Anything that stroke you as particularly surprising, unexpectedly pleasant/unpleasant during the peer review process?

Reply: we submitted the manuscript to Nature Communication because we believed in the novelty and possible wide interest. The review process was unexpectedly quite smooth, and we were very pleased by the highly positive evaluation of our work.     

8) Which is the development in the field of carbon nanomaterials that you would like to see in the next 10 years?

Reply: linear carbon structures including short carbon atomic wires, polyynes, cumulenes and confined carbyne have appealing properties for many applications. Development in the synthesis of stable wires together with the deepening of the knowledge on these systems will allow to look for use in advanced applications. Today there are still debates on the stability and on the existence of carbyne, even the nomenclature is not clear and unique. In the next 10 years I would like to see linear carbon as a widely known and investigated nanostructure as fullerenes, nanotubes and graphene and possibly adopted in synergy with those and other carbon nanomaterials for all-carbon solutions to some technological challenges we are facing today.   

9) And now, what’s next?

Reply: we are going on in the investigation of these systems. We have already done another beamtime experiment at ELETTRA with data we are analyzing right now. We can synthesize carbon wires up to 26 carbon atoms in length and with different terminations. Now we are focusing on the effect of the termination on the properties of these systems. In parallel we are looking for an experimental set-up able to perform UV Resonance Raman in a wider range (i.e., above 270 nm) to allow us to investigate longer wires (see image below).

UV-vis spectra of our samples of carbon atomic wires 

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