Theoretical study on the mechanisms of formation of primal carbon clusters and nanoparticles in space

Abstract

We present a theoretical study of assembling clusters and nanoparticles in space from primordial aggregations of unbound carbon atoms. Geometry optimization and SCC-DFTB dynamics methods are employed to predict carbon clusters, their time evolution and stability. The initial density of the aggregates is found to be of primary importance for the structure of the clusters. Aggregates with low initial density yield clusters with an approximately equal prevalence of sp and sp² hybridization with almost missing sp³. Higher initial density results in sp²-dominant molecules, resembling the carbon skeleton of polycyclic aromatic hydrocarbons (PAHs). Larger initial aggregations result in sp²-dominant polymers. Such materials are highly porous and possess a similarity to laterally bound nanotubes. Some clusters resemble fullerene building blocks. We employed metadynamics to model the inter-fragment coupling of such structures and predict the formation of spheroid nanoparticles, closely resembling fullerenes. One such structure has the lowest binding energy per atom among the studied molecules. All zero-dimensional forms, obtained by the simulations, conform to the experimentally detected types of molecules in space. The theoretical IR spectrum of the nanoparticles closely resembles that of fullerene C₇₀ and therefore such imperfect structures may be mistaken for known fullerenes in experimental infrared (IR) telescope studies.