Kim et al. Nanoscale Research Letters 2012, 7:49 http://www.nanoscalereslett.com/content/7/1/49 Page 4 of 6 Figure 5 The apparent density of the Ag nanopaste. Figure 7 The critical friction force of the Ag nanopaste. Figure 6 reveals the influence of the sintering tem- perature on the electrical resistivity of the Ag nanopaste. The electrical resistivity dramatically decreased with the increasing sintering temperature, implying a conduction pathway between the Ag nanoparticles due to the inter- particle neck formation and growth. This is because the mechanism of electrical conduction in a metal nano- paste features the point-to-point contact between the conductive nanoparticles [15]. In other words, when the conductive nanoparticles have been necked to a suffi- cient extent, the film of the printed nanopaste becomes relatively well conductive despite of its porosity. The Ag nanopaste sintered at 300°C showed the lowest electrical resistivity of 1.89 μΩ·cm. Figure 7 exhibits the adhesion strength of the Ag nanopaste printed on the Si substrate as a function of sintering temperature. A critical friction force is the force needed to pull out a film on a substrate. Overall, Figure 6 The electrical resistivity of the Ag nanopaste. the critical friction force increased linearly with an increasing sintering temperature. As previously seen in Figure 4b, the nanoparticles on the surface of the Ag nanopaste formed clusters with a diameter of 130 to approximately 180 nm due to interparticle necking at a sintering temperature of 200°C. The clusters grew larger with the three-dimensional interconnections as the sin- tering temperature increased, which increased from around 300 nm at 250°C (Figure 4c) to 600 nm at 300°C (Figure 4d). Therefore, as the sintering temperature increased, the clusters were connected more strongly, and hence, the surface area between the Ag nanopaste and the Si substrate increased, resulting in the higher friction force. In order to investigate the scratched surface of the Ag nanopaste, the scratches were observed using the OM. The panorama images illustrate the direction of scratch testing, and the morphologies of the entire scratches are shown in Figure 8a, b, c, d. As the sintering temperature increased, larger parts of the Ag nanopaste were pulled out due to the stronger connections in the Ag clusters. Figure 8e, f, g, h indicates the exact starting points to pull the film out from the substrate. The different fracture modes were identified depending on the sintering tem- perature. The printed Ag nanopaste sintered at 300°C exhibited a fracture behavior like a bulk Ag film. Conclusions The characteristics of thin printed patterns are domi- nated by the heat treatment applied. The influence of sin- tering temperature on the adhesion of the screen-printed Ag nanopaste was investigated. The scratch test, which is to measure the critical friction force of the film, was sug- gested to be a suitable method to evaluate the adhesion of printed patterns. Overall, the critical friction force increased by 43% as the sintering temperature increased