Analysis of density waves in CdSe, SiC, and diamond nanocrystals by application of NanoPDF software package to experimental Pair Distribution Functions

S. Stelmakh, W. Palosz, S. Gierlotka, K. Skrobas, B. Palosz
Institute of High Pressure Physics PAS, PL

Keywords: nanodiamond, CdSe quantum dots, PDF analysis

Summary:

Introduction of the apparent lattice parameter concept for interpretation of diffraction patterns of nano-materials led us previously to tentative core-shell models of SiC, diamond and GaN. Analysis of Pair Distribution Functions obtained from same diffraction data shows more features than those which could be explained by a single-shell model. That led us to a multi-shell model with “wavy” atomic density. Here we present models of CdSe, diamond, and SiC nanocrystals developed for experimental G(r) functions obtained from diffraction data measured in the range up to Q≈20Å̄̄̄̄ˉ¹ (Ag-source), and for neutron data measured at LANSCE, Los Alamos. The data were analyzed using software package NanoPDF and lead us to models with several concentric density maxima and minima present between the grain center and its surface (which we call the “density waves”). Our atomistic models consist of the uniform grain core surrounded by 2-3 spherical shells (density waves), each with the thickness on the order of 0.3 nm. Atomic densities inside the shells may differ from that of bulk by about 5% for diamond and by up to 20% for CdSe. The exact figures vary depending on the crystallite size, the surface treatment, and the presence of overlayers.