Buckyballs, or C60 molecules, were first discovered at Rice University in 1985 but how they form has perplexed scientists ever since. Buckyballs are created at high temperatures of over 2000 °C. One theory, known as the hot giant hypothesis suggests that the carbon atoms first come together in their thousands into curved 2D graphite sheets, which then wrap up to form single-layer giant fullerenes. Heat then distorts these giant fullerenes, shrink wrapping them into ever smaller fullerenes by the removal of carbon atoms so that just a tiny C60 molecule remains.
This "hot evolution" is so rapid that it has been impossible to observe it in experiments – until now that is. Jianyu Huang of Sandia National Laboratories in New Mexico and colleagues at Rice University in Houston obtained their images using an ingenious technique that involves creating a controllable heat bath inside a nanotube 10 nm wide. This allowed them to observe single giant fullerenes gradually shrinking. The images thus confirm the theory and previous computer simulations that Boris Yakobson’s team at Rice University had made.
At temperatures of around 2000 °C, the giant fullerenes reduce a hundred times in size, while the fullerene shell remains intact. The fullerene simply goes from being a slightly polygonized shape to a nearly spherical shape with a smaller diameter. The number of carbon atoms in the giant fullerene decreases linearly with time until the small sub-buckyball cage appears (see figure).
Modelling performed by the Sandia–Rice team also showed that carbon atoms are removed mainly from the weakest binding energy pentagon sites, which leads to a constant evaporation rate.
The work suggests a possible route to tailor the structure of fullerenes to the desired size for applications, say Huang and co-workers Yakobson, Feng Ding and Kun Jiao.
The researchers reported their results in Physical Review Letters.
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