Kim et al. Nanoscale Research Letters 2012, 7:49 http://www.nanoscalereslett.com/content/7/1/49 Page 5 of 6 Figure 8 Optical micrographs of the scratched Ag nanopaste surface sintered at various temperatures. (a, e) 150°C, (b, f) 200°C, (c, g) 250°C, and (d, h) 300°C. from 150°C to 300°C. To rationalize these experimental results, the microstructural evolution and variation of density were investigated as a function of sintering tem- perature. The Ag nanopaste sintered at higher tempera- tures showed the accelerated condition. The calculated apparent density of the Ag nanopaste increased from 6.08 g/cm3 at 150°C to 7.46 g/cm3 at 300°C. It was con- cluded that the printed Ag films sintered at higher tem- peratures became more densely packed, which resulted in the lower electrical resistivity and the stronger adhe- sion of the printed Ag nanopaste. Additional material Additional file 1: The parameters of scratch test. A table listing the detailed scratch parameters. Abbreviations DSC: differential scanning calorimeter; FE-SEM: field emission scanning electron microscope; OM: optical microscope; TEM: transmission electron microscope; TGA: thermo-gravimetric analysis Acknowledgements This work was supported by the World Class University program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (Grant No. R32-2009-000-10124-0). Author details 1SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, Republic of Korea 2School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, 440- 746, Republic of Korea Authors’ contributions KSK carried out the density measurement and scratch test and wrote the manuscript. YK carried out the two thermal analysis of the Ag nanopaste and participated in the screen printing. SBJ participated in the design and coordination of this research. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 1 September 2011 Accepted: 5 January 2012 Published: 5 January 2012 References 1. Yoo H, Shin H, Lee M: Direct patterning of double-layered metal thin films by a pulsed Nd:YAG laser beam. Thin Solid Films 2010, 518:2775-2778. 2. Kim KS, Lee YC, Ahn JH, Jung SB: The effect of heat treatment on flexibility of silver nanopaste circuit screen-printed on polyimide. IEEE NMDC2010: Topics in Nanostructure Characterization. IEEE Nanotechnology Materials and Devices Conference (NMDC)2010; October 2010 Monterey: IEEE; 2010, 306. 3. Ko SH, Pan H, Grigoropoulos CP, Luscombe CK, Frechet JMJ, Poulikakos D: All-inkjet-printed flexible electronics fabrication on a polymer substrate by low-temperature high-resolution selective laser sintering of metal nanoparticles. Nanotechnology 2007, 18:345202-345209.