Peng et al. Nanoscale Research Letters 2012, 7:559 http://www.nanoscalereslett.com/content/7/1/559 Page 4 of 6 forming. We found that a conical path with different contrasts could be observed in the TEM image, which is expected from higher electron scattering behaviors due to a higher atomic number, containing Au NPs confirmed from the EDS spectrum as shown in Add- itional file 1: Figure S2 (ESI) by a local reduction- oxidation process from Au cations after an initial forming breakdown process. The results are similar to an electro- chemical metallization (ECM) mechanism where the con- ducting bridge with the conical shape could be formed with a wide base from the cathode and a narrow neck near the anode [19]. Although, the formation of conducting bridge with a conical shape in the Au/ZnO/Au system agrees with the ECM mechanism, the formed positions of the conducting bridge for the wide base and the narrow neck are different from the typical formation of a conduct- ing bridge via the ECM mechanism. In our case, the wide base of the conducting bridge was found near the anode, while the narrow neck was found near the cathode. Before the set process, a broken region near the cathode as shown in Figure 4b (image b1) could be clearly observed, which limits the current flow, providing us a distinct evidence of HRS. After the high-enough bias was applied Figure 2 TEM images of the Au/ZnO/Au device with a nanogap of 190 nm. (a) Before and (b) after forming in a vacuum of <10−9 Torr. (c) The corresponding I-V behavior where the broken behavior could be seen. Figure 3 SEM images of the An/ZnO/Au (a) before and (b) after forming in air. (c) The corresponding I-V behavior shows the bipolar switching characteristic with forming, set, and reset voltages of approximately 8.5, 4.6, and 4.8 V, respectively. Figure 4 I-V curve of An/ZnO/Au operated at a set process (a). The corresponding real-time images at b1, b2, and b3 in the I-V curve were recorded as (b1) to (b3) (b). (c1) and (c2) show the schematics of the conducting bridge formation associated with migration of Au NPs (c).