**4. Conclusions**

In summary, an Au/HfO2/Pt/Ti/SiO2/Si device and an Au/HfO2/Pt/mica device were fabricated by the sol-gel method. As a popular research material, the S1 device structure has been thoroughly studied. At present, the breakthrough point was whether the HfO2 with a flexible structure would have the same performance as the typical device. Herein, quite a few advantages and disadvantages of flexible HfO2 devices have been identified by analyzing the differences between the S1 and S2 devices. The Od intensity of XPS spectra for the S2 device was lower than for the S1 device, which indirectly illustrates that the HRS/LRS ratio of the S2 device was lower. Meanwhile, the *I*-*V* characteristic also demonstrated the difference in off/on ratio. Nevertheless, HRS/LRS ratio of the S2 device also reached 50, which is enough to illustrate the potential application of flexible HfO2 device and that they are worth further study. For the Au/HfO2/Pt/mica device, the conduction mechanism was dominated by Ohmic conduction in LRS, and Ohmic conduction and SCLC conduction together in HRS. There is no doubt that the CF model can perfectly illustrate this conduction mechanism. The potential problem is the poor fatigue characteristics of the HfO2-mica-based RRAM, which cannot be solved at present, but we hope to solve effectively in the future.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/8/1124/s1, Figure S1: Current–Voltage plots of repeated samples, Figure S2: The FESEM pattern of the surface for S1 device, Figure S3: The FESEM pattern of the surface for S2 device.

**Author Contributions:** Conceptualization, C.-F.L. and X.-G.T.; Data curation, C.-F.L.; Formal analysis, C.-F.L., L.-Q.W. and H.T.; Funding acquisition, X.-G.T.; Investigation, C.-F.L.; Methodology, X.-G.T.; Project administration, X.-G.T.; Resources, Y.-P.J., Q.-X.L., W.-H.L. and Z.-H.T.; Writing—original draft, C.-F.L.; Writing—review & editing, C.-F.L., X.-G.T., L.-Q.W. and H.T.

**Funding:** This research was funded by "the National Natural Science Foundation of China (Grant Nos. 11574057, 51604087and51702055)", "the Guangdong Provincial Natural Science Foundation of China (Grant No. 2016A030313718)", and "the Science and Technology Program of Guangdong Province of China (Grant Nos. 2016A010104018 and 2017A010104022)".

**Conflicts of Interest:** The authors declare no conflict of interest.
