Investigation of relaxation of nanodiamond surface in real and reciprocal spaces

The structure of a nanodiamond powder with an average grain size of 5 nm was investigated using large-Q neutron diffraction. Both Bragg scattering and PDF analysis were employed. The effect of annealing under vacuum at temperatures up to 1200 degrees C was studied. The studies lead to a tentative model of nanocrystalline diamond, where the core with a perfect diamond lattice is surrounded by a shell of compressed diamond lattice, and this core-shell structure is enveloped in a non-diamond carbon. The non-diamond envelope of nanograins, a gas-like carbon, is stable up to 1000 degrees C and transforms into a graphite phase (an onion-type structure) at about 1200 degrees C. The amount of non-crystalline carbon in the powder annealed below 1000 degrees C is about 10%. In the sample annealed at 1200 degrees C a graphite-type carbon, with a total of about 15% of SP2 bonds is formed. (c) 2006 Elsevier B.V. All rights reserved.