Nucleation and growth of carbon onions synthesized by ion implantation at high temperatures

Resonant nuclear reaction analysis and high-resolution transmission electron microscopy experiments were performed on silver substrates implanted at high temperatures (500'700 °C) by 90'120 keV carbon ions. These characterizations revealed that three different carbon components were synthesized during the implantation process. At the beginning of the implantation process (e.g., for low doses), an amorphous carbon component forms onto the silver surface and at the silver grain boundaries. The nucleation and growth of the carbon onions take place inside the bulk of the metallic substrates when the carbon concentration increases. We also observe the formation of empty carbon nanocapsules which is attributed to a carbon precipitation around nanometer-sized silver grains, the so-encapsulated Ag atoms surprisingly leaving out from the carbon cage. Comparison between carbon concentration profiles and carbon onions sizes allows us to discuss the growth mechanism. It is proposed that two different regimes act: whereas radiation-enhanced diffusion could be involved only for implantation depths lower than the average projected range of the implanted carbon ions, a simple thermal regime governs the growth of the carbon onions in depth.