Zhang et al. Nanoscale Research Letters 2012, 7:312 http://www.nanoscalereslett.com/content/7/1/312 Page 5 of 6 Figure 5 TEM images, atomic model and FFT pattern. (a) Typical TEM images of the 3-D superlattice of Pd NPs with the size of about 8 nm. (b) TEM image of the region indicated by the box in (a). (c) Atomic model with the close-packed structure. (d) FFT pattern of the region indicated by the box in (b). image from the region indicated by the box in Figure 5a. The distance between two rows of NPs is 6.8 nm, which is 0.87 times to the size of the NPs. Figure 5c shows the atomic modeﬃl with close-packed structure. The lattice distance is p3=2 to the diameter of the atoms. The data we measured in the TEM image matched the atomic model well, which further proved the close-packed structure of the superlattice. The corresponding FFT pattern (Figure 5d) is similar with selected area electron diffraction pattern of fcc structure along [111] zone axis, indicating that the 3-D superlattice crystals are almost perfectly aligned according to the fcc close-packed struc- ture. These results are in good agreement with the infor- mation gotten from the SEM images. Understanding of sedimentation and aggregation behavior of NPs would be significant to predict their environmental transport and fate [27]. The self-assemble phenomenon observed in the present case may also helpful for the study of fur- ther application. Conclusions In summary, monodisperse Pd NPs were prepared via a facile solvothermal synthesis using organic amine as solution and stabilizing agent. The size of the Pd NPs could be controlled from about 5 to 10 nm by an improved stepwise seed-mediated method. The as- prepared Pd nanoparticles could self assemble to well- shaped superlattice crystal without size selection process. Organic amine was proved to play a critical role in the formation of monodisperse Pd NPs. Abbreviations FT-IR: Fourier transform infrared; Pd(acac)2: palladium(II) acetylacetonate; Pd NPs: Pd nanoparticles; SEM: scanning electron microscopy; TEM: transmission electron microscope. Competing interests The authors declare that they have no competing interests. Authors’ contributions LZ and ZJ conceived and designed the experiments, analyzed the data and co-wrote the paper. LW performed some of the electron microscopy experiments. ZX gave the advice and guided the research. All authors discussed the results, read and approved the final manuscript. Acknowledgments This work was supported by the National Basic Research Program of China (grant no. 2011CBA00508), the National Natural Science Foundation of China (grant no. 21171141), and the National Fund for Fostering Talents of Basic Science (grant no. J1030415).