High-pressure synthesis and optical properties of nanodiamonds obtained from halogenated adamantanes

Following our previous brief communication the carbonization of halogenated adamantanes C10H14Br2 and C10H15Cl under high pressure with the aim to produce nanodiamonds have been studied in details. A fixed pressure of 8 GPa and temperatures up to 2000 K were applied. Carbonization behavior of both precursors is very similar and starts in range of 900-1000 K with formation of about 1-2 nm diamond nanocrystals. With increasing process temperature the size of obtained nanodiamonds slowly grows up to tens of nanometers at 2000 K. The observed behavior provides favorable conditions for the synthesis of the nanodiamonds of the precisely controlled crystal size:Raman spectroscopy, X-ray diffraction and high resolution transmission microscopy were used to monitor the phase composition of the samples and the crystal size of nanodiamonds. Lattice parameter of nanodiamonds, calculated by using the Rietveld analysis of the X-ray diffraction patterns, increases by up to 2-3% as an average crystallite size decreases to the minimal value of 1-2 nm. Based on the experimental results and density functional theory calculations, extremely high increase in the lattice parameter of nanodiamonds is interpreted in terms of the strain exerted on the nanocrystals due to diamond surface reconstruction, particularly on the (110) face. Lowering of the crystal size is accompanied by the red-shift and broadening of the diamond Raman line. The effects are explained in terms of the crystal lattice expansion and the phonon confinement model.