Synthesis and properties of nanodiamonds produced by HPHT carbonization of 1-fluoroadamantane

Abstract

Unique carbonization behavior of 1-fluoroadamantane was revealed at pressures 5.5–9 GPa and temperatures up to 1300 °C. In course of the carbonization, formation of nanodiamonds was detected at record low temperature of 420 °C and pressure of 5.5 GPa. Analysis of sample mass decrease implies that for the samples synthesized at temperatures up to 600–800 °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 °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. Transport properties measured on the 5 nm-nanodiamonds demonstrate p-type semiconductor behavior with activation 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 formation of metastable surface with reconstructed sp2 and hydrogenated fragments allowing for surface conductivity. 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.