Internal Structure of Diamond Nanocrystals by Modeling and PDF Analysis
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Internal Structure of Diamond Nanocrystals by Modeling and PDF Analysis S. Stelmakh1, W. Palosz2, S. Gierlotka1, K. Skrobas1 and B. Palosz1 1 2
Institute of High Pressure Physics PAS, ul. Sokolowska 29/37, 01-142 Warsaw, Poland Brimrose Corporation, Sparks, Md 21152, USA
ABSTRACT The structure of nanocrystalline diamond was approximated by spherical nanograins assuming that the grain core with a perfect crystal lattice is surrounded by a sequence of shells with (essentially) identical atomic architecture but with altered density. We call such a model a nanocrystal with density modulated waves. To examine the effect of density modulation present in nanograins, we built atomistic models of nanodiamond grains and compared the average values of inter-atomic distances calculated for the grains with density waves to those calculated for grains with the perfect, diamond crystal lattice. We show that the atomic structure of nanodiamond can be best described by a model where, between the inner core and the surface layer, three density waves with intermittent compressive and tensile strains exist. The sequence of the density waves is preserved in all examined nanodiamond samples without regard to chemical treatment and vacuum annealing (at up to 1200oC). INTRODUCTION A model of nanodiamond often recalled in the literature comprises a crystalline diamond core with a perfect lattice which is surrounded by an amorphous shell with mixed sp2/sp3 bonds and/or onion-like graphite shell [1,2]. A presence of internal pressure caused by a surface stress modified upon chemical treatment, and the curvature of nanoparticles is also considered (e.g. [3]). A strong evidence of the presence of distortion(s) in the diamond lattice comes from examination of compressibility of nano-diamond powder [4] and from NMR studies [5]. In 2006 we suggested that individual nanograins may have a core-shell structure and, using the concept of the apparent lattice parameter, alp(Q), we proposed a model of nano-diamond where the inner core with a perfect lattice is surrounded by a surface shell about 3 Å thick. The shell preserves the diamond structure but has interatomic distances larger by about 3-5% relative to the crystalline grain interior [6]. A more sophisticated model of internal structure of nanodiamonds was also developed, although only qualitative estimation of the density distribution was given [7-9]. In this paper a quantitative description of models of diamond nanograins is presented. EXPERIMENTAL DETAILS Commercial UD96 and UD90 powders from Microdiamant AG, and the same powders purified by different chemical treatment procedures [10] or vacuum pre-annealed (2h at temperatures up to 1200oC) were examined using large-Q neutron diffraction technique with Qmax up to 35 Å-1 (station NPDF at LANSCE). G(r) function was derived using the PDFgetN
application [11] and further elaborated with the PDFgui program [12]. Atomistic spherical models of diamond nanocrystals and their corresponding theoretical diffraction patterns were obtained with the nano
