Kim et al. Nanoscale Research Letters 2012, 7:49 http://www.nanoscalereslett.com/content/7/1/49 transmission electron microscope [TEM] (JEOL Ltd., Akishima, Tokyo, Japan). Two types of thermal analysis were performed on the Ag nanopaste: differential scanning calorimeter [DSC] and thermo-gravimetric analysis [TGA]. A screen printing machine (MT-550TV, Micro- Tec, Chiba City, Chiba, Japan) was used to duplicate the conductive patterns. The Ag nanopaste was printed using a 400-mesh screen mask onto a silicon [Si] substrate passi- vated with SiO2. All printed patterns were dried on a hot plate at 70°C for 10 min, and then sintered in a box-type, muffle furnace (RTA-BRT100, BLS Korea Inc., Seoul, South Korea) for 30 min under a sintering temperature ranging from 150°C to 300°C. A four-point probe method was adopted to measure the electrical resistivity. The apparent density of the screen-printed Ag nanopaste depending on the sintering temperatures was calculated from the precise volume and mass measured using a com- mercial precision scale (JL-180, Chyo Balance Corp., Min- ami-ku, Kyoto, Japan) and a 3-D surface profiler (Nanoview2000, Nanosystem Inc., Daejon City, South Korea), respectively. Figure 1 shows a schematic diagram of scratch testing, and the table in Additional file 1 lists the detailed scratch parameters. The scratch test was car- ried out using a commercial scratch tester (MSTX, CSM Instruments, Needham, MA, USA) equipped with a dia- mond indenter having a tip radius of 10 μm. The micro- structural evolution was investigated using a field emission scanning electron microscope [FE-SEM] (JSM-7401F, JEOL Ltd., Akishima, Tokyo, Japan), and an optical micro- scope [OM] was used to observe the fracture surfaces of the screen-printed Ag nanopaste after the scratch test. Results and discussion Figure 2a, b shows a TEM image and the measured size distribution of the Ag nanopaste, respectively. In the TEM image, most of the Ag nanoparticles have a dia- meter of approximately 25 nm. It was also confirmed that the diameters of the Ag nanoparticles were within Figure 1 A Schematic diagram of scratch testing. Page 2 of 6 Figure 2 A TEM image (a) and size distribution (b) of Ag nanoparticles. 10 and 30 nm with a narrow distribution, as shown in Figure 2b. This distribution of particle size was known to be affected by three major factors, i.e., the generation rate of the Ag embryos, the growth rate of the Ag nano- particles, and the extent of surfactant absorbing or encapsulating if no agglomeration occurred between the nanoparticles [13]. The thermal properties of the Ag nanopaste are shown in Figure 3. The DSC curve had a slightly exothermic slope from 30°C to 200°C at the rate of 10°C/min and one sharp endothermic peak at 230°C. It means that the necking reaction in the Ag nanopaste occurred at the point of the endothermic peak, resulting in the coarsened nanoparticles. Compared with the TGA curve, any moist- ure and solvent in the Ag nanopaste were dried or decomposed thermally from 100°C to 300°C at the rate of 10°C/min. In this temperature range, a maximum rate of weight loss was shown at 225°C, and the total weight loss was approximately 27 wt.%. Based on these results, the sintering temperature range was determined from 150°C to 300°C to investigate the influence of heat treatment on the microstructural evolution and property variation. The microstructural evolution as a function of sinter- ing temperatures was shown in Figure 4. The surface of the Ag nanopaste sintered at 150°C for 30 min exhibited