ZnO Si3N4 SiO2 Si Au ZnO Si3N4 SiO2 Si Au + 100 nm A-u Au cathode (a) (c) Si3N4 SiO2 Si PMMA ZnO Si3N4 SiO2 Si (b) (d) EBL Coating of Au electrode (30 nm) (e)Ini-ns- sitituu TTEEMMhhooldledrer (f) Etching hole (KOH) and coating of ZnO layer (25 nm) ZnO film Au anode 10 µm Peng et al. Nanoscale Research Letters 2012, 7:559 http://www.nanoscalereslett.com/content/7/1/559 Page 3 of 6 Figure 1 Schematics of the TEM chip fabrication. (a) Deposition of 80/20-nm Si3N4/SiO2 on Si substrate by LPCVD, (b) KOH chemical etching from the back side of the Si substrate, followed by deposition of 25-nm-thick ZnO layer by radio frequency magnetron sputtering, (c) E-beam lithography of PMMA to define position of the electrodes, (d) deposition of Au by metal deposition and lift-off processes, (e) detailed configuration of the TEM chip after fabrication, and (f) the corresponding SEM and TEM images of the device. observed after the hard breakdown is ascribed to the opening of the conducting channel between the two Au electrodes due to the generation of defects, such as oxy- gen vacancies or Zn interstitials after the bias was ap- plied, thereby a higher conductivity [18]. A high current due to high conductivity across two Au electrodes can readily lead to a significant migration of Au ions from the electrode, which agrees with the observation from the TEM image as shown in Figure 2b. Overall processes are irreversible owing to the lack of supplementary oxy- gen from the ambient to compensate the oxygen vacan- cies inside the ZnO layer in the reverse bias. Another device with the same fabrication processes was measured in air in order to prove the importance of environmental issue. The corresponding TEM images of the samples before and after the forming bias are shown in Figure 3a,b. Obviously, a typical bipolar switching I-V behavior could be observed after the forming process with forming, set, and reset voltages of approximately 8.5, 4.6, and 4.8 V, respectively, as shown in Figure 3c. Notably, the migration of Au ions from anode to cathode could be observed with a conical shape, indicating the formation of a conducting bridge (Figure 3b). The findings provide us the important in- formation that the formation of a conducting bridge/ path is more favorable in the ambient environment ra- ther than in vacuum. To understand the detailed formation of the conduct- ing bridge between two electrodes, including the set and reset processes, the device after the forming process was loaded into the TEM for the in situ observation at differ- ent operation states. Therefore, a series of dynamic behaviors of the set and reset processes could be recorded simultaneously at different times with different biases. In order to ensure the accuracy of data recording, we have operated the device at set and reset processes several times. In the set process, the bias was applied into the device to switch the device from HRS to LRS, with which the sudden increase of current could be observed, as shown in Figure 4a. The inset shows the detailed configuration of the device and the direction of the applied current inside the in situ TEM. Figure 4b (b1 to b3) reveals the TEM images of dynamic switching behaviors from HRS to LRS at different applied biases, as indicated in the I-V curve of Figure 4a. The corre- sponding schematics of the set processes are shown in Figure 4c (c1 and c2). Obviously, a substantial number of Au NPs in the ZnO layer can reduce the threshold voltages (set voltage) for the subsequent growth of the conducting bridge compared with that for the initial