Zhang et al. Nanoscale Research Letters 2012, 7:312 http://www.nanoscalereslett.com/content/7/1/312 Page 3 of 6 Figure 1 TEM images of monodisperse Pd nanoparticles prepared in different organic amines. (a) Oleylamine, (b) octadecylamine, (c) n-octylamine and (d) butylamine. mL). Thus, the Pd NPs could grow bigger and bigger after solvothermal process again and again. Figure 3b-d shows the TEM images and size distribution of Pd NPs prepared via second and third growths. The average sizes of the Pd NPs have been finely tuned to 7.8 and 9.5 nm, while the uniformity was well kept. It should be pointed out that the growth parameters, such as the concentra- tion of Pd(acac)2, the temperature and the reaction time, Figure 2 FT-IR spectra of the samples. Samples were prepared in (a) oleylamine, (b) octadecylamine, (c) n-octylamine and (d) butylamine. should be well controlled in these seed-mediated growth to avoid new nuclear phenomenon, especially in further growth circles. Otherwise, the size of products will be in a larger range. The uniformity of as-prepared Pd NPs can also be confirmed by their self-assemble three-dimensional superlattices. As it is well known, a narrow particle size distribution is required to obtain long-range superlattice ordering. Just by diffusing 0.2-mL ethanol into the reacted mixture, micrometer-sized 3-D superlattice crys- tals could be easily prepared in the present case without further size selection process. Figure 4a shows the SEM image of superlattice crystals formed by as-prepared Pd NPs with an average size of 7.8 nm, indicating the for- mation of well-shaped crystals with a size of several micrometers, and most of them were octahedron or dis- torted octahedron. Figure 4b is a single superlattice crys- tal with octahedron shape, all of the exposed surfaces (Figure 4c) were arranged by close-packed nanocrystals similar with the atom arrangement in the {111} surfaces of face-centered cubic (fcc) metal crystals. A closer look of some step surface (Figure 4d) revealed that crystals were self assembled by close-packed monodisperse Pd nanocrystals in the sequence of ABCABC. . ., and thus formed an fcc structure. In the formation of superlattice crystals, the individual nanoparticles can be considered