The synthesis of ultrananocrystalline diamond (UNCD) films has been accomplished in the laboratory making use of carbon dimer, C2, produced by fragmentation of C60. Interestingly, UNCD had been found in several primitive carbonaceous chondrites where it is the largest constituent of the presolar grain fraction in this class of meteorites. During the late stages of red giant stellar evolution, so-called planetary nebulae with lifetimes of ~ 104 years form. These objects have been known for over 200 years. However, their progenitors, called protoplanetary nebulae (PPNs) with lifetimes of ~ 103 years, were discovered only within the last 25 years. Many PPNs such as the Egg Nebula have intense C2, absorption bands, which are strongly correlated with prominent 21 micron emission features. It is suggested that the synthesis of UNCD in the laboratory and in the cosmos proceeds on the basis of new growth and nucleation mechanisms involving the insertion of C2 into carbon-carbon and carbon-hydrogen bonds. The carrier of the intense 21 micron emission is almost certainly a carbon containing material, and the possibility that UNCD is the carrier will be discussed. Carbon dimer, the smallest all-carbon molecule, is extremely reactive and can give rise to complex chemistries involving H, O, N, and other elements. The PPNs may, therefore, be viewed as cradles of chemistry providing insights into the earliest stages of cosmic chemical evolution. Extensive characterization studies on laboratory-produced UNCD lead to the conclusion that phase-pure diamond is synthesized with a microstructure consisting of randomly oriented 3-10 nm crystallites. By adjusting the noble gas/hydrogen ratio in the gas mixture, a continuous transition from micro- to ultrananocrystallinity is achieved. Up to 10% of the total carbon in the UNCD films is located at 2- to 4-atom-wide grain boundaries. Because the grain boundary carbon is pi-bonded, the mechanical, electrical, and optical properties of these films are profoundly altered. A number of applications in fields as diverse as tribology, cold cathodes, corrosion resistance, electrochemical electrodes, as well as SAW and MEMS devices are being pursued.
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