Synthesis and Properties of Nanodiamonds Produced by HPHT Carbonization of 1-Fluoroadamantane

Unique carbonization behavior of 1-fluoroadamantane was revealed at pressures 5.5-9 GPa and temperatures up to 1300 degrees C. In course of the carbonization, formation of nanodiamonds was detected at record low temperature of 420 degrees C and pressure of 5.5 GPa. Analysis of sample mass decrease implies that for the samples synthesized at temperatures up to 600-800 degrees C the decomposition reaction proceeds with release of molecular hydrogen, and for those synthesized at higher temperatures with formation of volatile hydrocarbons. At 8-9 GPa, sustained growth of nanodiamonds is observed over whole temperature range studied, while at 5.5 GPa, graphitic sp2 carbon is formed along with nanodiamonds at temperatures above 600 degrees C. It is suggested that the molecular hydrogen formation regime of carbonization is critically important for nanodiamond nucleation. Subsequent growth of nanodiamonds is determined by the pressure-dependent catalytic activity of the C-H-F growth medium. Trans-port properties measured on the 5 nm-nanodiamonds demonstrate p-type semiconductor behavior with activa-tion energy of 0.1 eV at room temperature and unusual non-monotonic pressure dependence of conductivity. Transmission electron microscopy reveals surface-bound sp2-C domains on nanograins; vibrational spectroscopy indicates strong surface hydrogenation of synthesized nanodiamonds. Modeling confirms plausibility for for-mation of metastable surface with reconstructed sp2 and hydrogenated fragments allowing for surface conduc-tivity. Facile synthesis of nanodiamonds from 1-fluoroadamantane can be of great interest for fundamental investigations and practical applications. Our experiments suggest that fluorine-containing fluids may play a significant role in genesis of natural diamond.