Abstract
Motivated by their importance in chemistry, physics, astronomy and materials science, we investigate routes to the formation of large polycyclic aromatic hydrocarbon (PAH) molecules and the fullerene C60 from specific smaller PAH building blocks. The behaviour of selected PAH molecules under electron (using transmission electron microscopy, TEM) and laser irradiation is examined, where four specific PAHs—anthracene, pyrene, perylene and coronene—are assembling into larger structures and fullerenes. This contrasts with earlier TEM studies in which large graphene flakes were shown to transform into fullerenes via a top-down route. A new combined approach is presented in which spectrometric and microscopic experimental techniques exploit the stabilisation of adsorbed molecules through supramolecular interactions with a graphene substrate and enable the molecules to be characterised and irradiated sequentially. Thereby allowing initiation of transformation and characterisation of the resultant species by both mass spectrometry and direct-space imaging. We investigate the types of large PAH molecule that can form from smaller PAHs, and discuss the potential of a “bottom-up” followed by “top-down” mechanism for forming C60.
Highlights
There is widespread interest in planar aromatic and fullerenic carbon macromolecules, and in transformations between these molecular forms
In this paper we report electron- and photon-promoted selfassembly experiments in which small planar aromatic molecules ranging in size from anthracene (C14H10) to coronene (C24H12), adsorbed on a graphitic surface, are exposed to an electron beam or laser radiation, in which chemical evolution to form larger Polycyclic aromatic hydrocarbon (PAH) and C60 is probed by transmission electron microscopy and mass spectrometry
TEM is a useful tool for probing transformations of carbon species with a vacuum chamber pressure of approx. 10−12 mbar and high-energy electrons to initiate transformations. It has been shown by Chuvilin et al that under these conditions graphene flakes can undergo transformations that result in the formation of fullerene molecules (Chuvilin et al, 2010)
Summary
There is widespread interest in planar aromatic and fullerenic carbon macromolecules, and in transformations between these molecular forms. The “top-down” mechanism in this case involves UV-photolysis-initiated dehydrogenation of PAHs containing approximately 70 carbon atoms to form small graphene flakes (Goroff, 1996), loss of a carbon atom, followed by pentagon formation within the structure This has been explored theoretically in a study carried out to calculate IR spectra of intermediates in the fullerene formation process to make comparison with IR interstellar emission features (Galué, 2014). PAHs have affinity for graphene (Björk et al, 2010; Dappe et al, 2015) and readily form stable supramolecular complexes (e.g., PAHs adsorbed on graphene by π - π interactions), which allows us to study the molecules both by TEM and by matrix-assisted laser desorption/ ionisation time-of-flight mass spectrometry (MALDI-TOF MS) This combined approach enables the correlation of data from the local-probe and bulk-probe analysis methods for the first time, and reveals important aspects of the molecular polycondensation process, including intermediate species, and determine mechanistic details linking specific molecular precursors with formation of large PAHs and C60. Our new methodology was designed and implemented to utilise both of the aforementioned experimental techniques to enable characterisation of species and initiation of reactions within an identical location of a sample, so that transformation processes and molecular speciation could be carried out at each step of the experiment
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