Carbon nano-onions are multi-shelled nanoparticles that consist of concentric graphitic layers and a hollow core. The largest have 70 concentric shells. The typical diameter of these structures is 3 to 5 nm and they can be regarded as belonging in the giant fullerens family. Ideally the center shell is a C60 molecule. Carbon nano-onions were discovered in 1980 and their highly symmetric structure gives them extra abilities compared to other carbon nanostructures, e.g nanotubes, nanodiamonds, etc. The number of carbon atoms they consist of can be altered and lead to different physical and chemical properties that may serve various applications. The most stable allotrope of carbon (at least for small objects) is not planar graphite but closed shells, owing chiefly to the need for satisfying the dangling bonds at the edge of the network.

The synthesis of carbon nano-onions is possible through the following methods: irradiation of carbon black particles, carbon arc plasma or plasma chemical vapor deposition or dc arc-discharge, thermal annealing of nanodiamond particles, shock-wave treatment or high-energy electron bombardment on carbon soots or diamond clusters or carbon implantation into copper or silver substrate. Current techniques can only be used to prepare a small quantity of carbon onions with a mass of by-products, which are hardly separated from the product.

The fabrication of carbon nanomaterials usually calls for expensive vacuum systems to generate plasmas and yields are disappointingly low. High-quality spherical carbon nano-'onions' can be produced in large quantities without the use of vacuum equipment. The nanoparticles, which have C60 cores surrounded by onion-like nested particles, are generated by an arc discharge between two graphite electrodes submerged in water. This technique is economical and environmentally benign, and produces uncontaminated nanoparticles which may be useful in many applications.Researchers of China synthesized large-scale carbon onions by liquid arc method.

Laser irradiation of amorphous silicon carbide

Onion-like carbon clusters embedded in crystalline SiC were observed Researchers of Germany after laser-induced crystallization of amorphous SiC. The carbon onions have a size in the range of 5–30 nm. The shells are defective with interplanar distances about 0.37 nm.

Ultra fast laser ablation

Carbon onions can be formed experimentally using ultra fast laser ablation. in the process the amount of argon is critical to the formation of well ordered onions in two different ways. First, sufficient argon is required to encourage atoms to cluster and form large enough precursors. Second, the level of argon has to be low enough to allow the carbon precursors to anneal at high enough temperatures for long enough to order the precursors into carbon onions. It can be assumed that spherical onions form from the outer layer first through a process of rearrangement of atoms to form concentric spheres of curved graphite.

Photoelectron spectroscopy has been used to produce high temperature phase transformation of nanodiamonds (ND). Depending on the temperature of annealing various particles with a diamond core covered by nanometer sized fullerene-like shells and onion like carbon (OLC) were formed.