Peng et al. Nanoscale Research Letters 2012, 7:559 http://www.nanoscalereslett.com/content/7/1/559 (set process), Au cations could migrate from the interface of the anodic Au electrode to connect the broken region from the top region of the conducting bridge via the reduction/crystallization process, enabling the switching of resistive state (Figure 4b, b1 and c1). The higher the positive bias is applied, the more Au cations migrated to the cathode can be achieved (Figure 4b, b2). Finally, a complete bridge connected by two Au electrodes contain- ing defective ZnO and Au NPs was observed at an ap- plied bias of >4.6 V, namely a threshold voltage of LRS where an obvious increasing of current could be obtained (Figure 4b, b3 and Figure 4c, c2). In addition, the grain boundaries of the polycrystalline ZnO grains also pro- mote the formation of the conducting bridge since Au ions preferentially accumulate at grain boundaries [20]. In the reset process, a reverse bias opposite the bias polarity of the set process is necessary to switch the de- vice from LRS to HRS, as shown in Figure 5a. The cor- responding device configuration with the direction of the current is shown in the inset of Figure 5a, for which an in situ dynamic reset process was recorded, as shown in Figure 5b (b1 to b3) at different biases as indicated by b1 to b3 in the corresponding I-V curve of Figure 5a. The schematics of the reset processes are shown in Figure 5c (c1 to c2). As the reverse bias increases, the (a) 0.6 ) A m (0.4 t n e r ru0.2 C i b3 b2 b1 0 -1 Reset -2 -3 -4 Voltage(V) -5 (b1) CCuurrrreenntt (b2) Current (b3) Current + - Au Au Au 50 nm AZunNOPsLRS ZnO 50 nm 50 nm ZnO HRS Au Au Au Au (c1) ZnO Bridge Current Au Au (c2) ZnO Bridge Current Au Figure 5 I-V curve of An/ZnO/Au operated at a reset 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 breakdown of the conducting bridge near the anode associated with the migration of Au NPs (c). Page 5 of 6 current increases where the migration of Au cations from anode to cathode via the conducting bridge would occur again until a current saturation was observed at the reversed bias >4.5 V, indicating an occurrence of the reset processes (Figure 5b, b1 and b2, and Figure 5c, c1). The corresponding TEM image during the current sat- uration as shown in Figure 5b (b3) provides us with a distinct evidence that the migration of Au cations finally results in a discontinuity (broken region) near the inter- face of the anode and the narrow neck region, as marked by a dashed circle and arrow in Figure 5c, c2. The results are also consistent with our previous observation in the set process (Figure 4b, b1). The formation of the conducting bridge from our in situ observation, which is opposite the conventional formation of the conducting bridge via the ECM mechanism, can be explained as due to the different diffusivity of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting bridge. For the conventional ECM mechanism, the matrix is typically composed of a solid electrolyte with a low melting point, for which cations, after an ionization process from the ac- tive electrode, can easily diffuse through the electrolyte to be reduced at the cathode with the wide base and then terminated at the anode with the narrow neck, resulting in the formation of the conducting bridge with the conical shape. In the typical ECM mechanism, the rate limit is determined by the reduction process at the interface of the cathode rather than the ion transport inside the elec- trolyte film. For the Au/ZnO/Au case as an example in our study, the rate limit is determined by the speed of dif- fusion for Au cations transported inside the ZnO film be- cause the ZnO layer can be considered as a dense film, thereby limiting the diffusion of Au cations. Once Au cations ionized from Au electrode transport into the ZnO film, they will be reduced into Au NPs immediately, lead- ing to the wide base of the conducting bridge near the anode and the formation of the narrow neck near the cathode, which is consistent with the results observed by other groups in Ag/SiO2/Pt [21] and Ag/ZrO2/Pt systems [22]. Furthermore, an issue regarding the formation of a conducting bridge on the surface of the ZnO or inside the ZnO film and the role of oxygen vacancies inside the ZnO for the formation and dissolution of the Au-conducting bridge have to be investigated systematically. Conclusions In summary, we have designed a specific TEM chip for the in situ observation of ReRAM operations, including set and reset processes based on Au/ZnO/Au system in- side a TEM. An obvious bipolar resistive switching be- havior with a nanogap distance of 190 nm could be achieved in air with forming, set, and reset voltages of approximately 8.5, 4.6, and ~4.8 V, respectively. A clear