Promoting the assembly of carbon onions: An atomistic approach

Atomistic simulations at two levels, classical and quantum-mechanical, are performed to probe the binding possibilities of the smallest multi-shelled concentric fullerenes, known as "carbon onions". We focus on the binding behavior of adjacent carbon onions and promote their binding through the addition of vacancies, as well as through doping with boron and nitrogen atoms. Molecular dynamics (MD) simulations are used to address the effect of different conditions of temperature and pressure on the binding of the onions and the thermal stability of the assembled structure. At a smaller scale, density-functional theory (DFT) based calculations reveal the electronic structure of the coalesced carbon onions, their charge density and frontier orbitals. The effect of van der Waals forces is also evaluated using a tight-binding scheme. Our main finding is that binding of adjacent carbon onions is promoted through the addition of vacancies and/or dopants on the outer surface of the carbon onions. The results are evaluated with respect to the relative distance between the adjacent carbon onions, the number of vacancies, and the amount or type of doping. We aim to optimize the conditions for assembling these nanoscale building blocks and understand their corresponding electronic properties in view of their potential in nano-assembling novel functional nanomaterials. (a) Two adjacent small carbon onions repel each other, while the carbon onions merge when tuning the external conditions and introducing vacancies on the structures as revealed from quantum mechanical (b) and classical simulations (c), respectively