Carbon nano-onions: Unique carbon nanostructures with fascinating properties and their potential applications

•Production, structures and physical properties of the onion-like structures are presented.•Structural modification of carbon nano-onions via physical methods is highlighted.•Doping of carbon nano-onions with metals and heteroatoms is presented.•Covalent and non-covalent functionalisation of carbon nano-onions is presented.•Potential applications of carbon nano-onions are highlighted.