The dance of coronene: molecular twisting, lifting and curling
The process of surface-assisted cyclodehydrogenation of polycyclic aromatic hydrocarbons has recently been adopted as one of the most effective, versatile, and flexible strategies for bottom-up synthesis of fullerenes, nanographenes, and graphene nanoribbons. The large number of possible precursors is the key for tailoring and controlling the structural properties of carbon networks via polymerization reactions.
In this study we have shown that thermally assisted cyclodehydrogenation of coronene (C24H12) on Ir(111) takes place through sequential steps that involve large changes of the molecule's pristine configuration. For a comprehensive characterization of the reaction process we used several techniques, namely fast and high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, ultraviolet photoelectron spectroscopy, angle resolved photoemission spectroscopy, temperature programmed desorption, and low energy electron diffraction. The experiments have been performed at the Surface Science Laboratory and at the SuperESCA and BaDElPh beamlines of Elettra, and supported by theoretical simulations at the University College London, UK.
By comparing experimental and ab initio density functional theory results, in particular for the evolution of the C1s core level as a function of temperature (see Figure 1), we were able to evidence that the coronene molecules adsorbed on Ir(111) undergo major conformational changes during surface-assisted dissociation. As reported in Figure 2, they initially tilt upwards with respect to the surface, still keeping their planar configuration, and subsequently experience a rotation, which changes the molecular axis orientation. Upon lifting, the internal carbon-carbon strain is relieved. As the dehydrogenation proceeds, the molecules experience a progressive increase in the average interatomic distance, and gradually settle to form dome shaped nano-graphene flakes, which represent the precursors for graphene formation.
Figure 1. (a) Temperature dependent C 1s core level spectra, shown as a two-dimensional intensity plot, acquired after coronene deposition at T=300 K. (b) Comparison between selected experimental and theoretically calculated C1s core-level spectra (red curves).
Figure 2. (a) Illustration of final states side and top views of C24Hn through the 12 sequential C-H bond cleavages that bring coronene molecules to the nanodome configuration. Different colors correspond to different C-metal substrate heights. The colors are rescaled in each step to emphasize the geometrical differences in each configuration (the height scale in Åis also reported for each step of the reaction).
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Our results provide important insight into the complex mechanism of molecular break-up which is known to become more complex with increasing molecular size, degrees of freedom for molecular motion, and competition with other surface processes, such as desorption. Moreover the novel mechanism could have implications in the synthesis of new graphene-based nanostructured materials with applications in spintronics, molecular memories and sensors. |
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This research was conducted by the following research team:
Davide Curcio1, Luca Omiciuolo1, Monica Pozzo2, Paolo Lacovig3, Silvano Lizzit3, Naila Jabeen1,4,5, Luca Petaccia3, Dario Alfè2and Alessandro Baraldi1,3,6
2 Department of Earth Sciences, Department of Physics and Astronomy, Thomas Young Centre@UCL, London Centre for Nanotechnology, University College London, London, United Kingdom
3 Elettra-Sincrotrone Trieste, Trieste, Italy
4 International Centre for Theoretical Physics, Trieste, Italy
5 Nanosciences & Catalysis Division, National Centre for Physics, Islamabad, Pakistan
6 IOM-CNR, Laboratorio TASC, Trieste, Italy
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Reference
Davide Curcio, Luca Omicuolo, Monica Pozzo, Paolo Lacovig, Silvano Lizzit, Naila Jabeen, Luca Petaccia, Dario Alfè and Alessandro Baraldi.“Molecular Lifting, Twisting and Curling during Metal-Assisted Polycycilc Hydrocarbon Dehydrogenation” Journal of the American Chemical Society 138, 3395, (2016), doi: 10.1021/jacs.5b12504This paper was selected for the Cover of JACS, Vol. 113, Issue 10, 2016 |
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