**2. Materials and Methods**

Using the sol-gel method for coating, a certain amount of hafnium acetone was weighed as the raw material, the magnetic stirrer was used to dissolve it in acetic acid until a colloid formed, the hafnium acetone colloid was spirally coated onto the different substrate by a rotary coating machine and then placed on a drying platform. The drying platform was heated from room temperature to 300 ◦C for 10 min, which decomposed hafnium acetone into HfO2 at high temperature. In this paper, there were two samples of different substrates, HfO2/Pt/Ti/SiO2/Si and HfO2/Pt/mica flexible structures. For further discussion, the structures of HfO2/Pt/Ti/SiO2/Si and HfO2/Pt/mica are abbreviated as S1 and S2, respectively, as shown in Figure 1. Both S1 and S2 were annealed at 700 ◦C in air atmosphere for 30 min. After annealing, an Au point electrode with diameter of 0.5 mm was plated on the sample using a small high-vacuum coating machine and a mask template with diameter of 0.5 mm at room temperature for two min to form a top–bottom (TB) electrode structure.

**Figure 1.** Schematic patterns of the HfO2/Pt/Ti/SiO2/Si (S1) and HfO2/Pt/mica (S2) devices.

Current–voltage (*I-V*) and endurance characteristics were measured by the Keithley 2400 s instrument. Atomic force microscopy (AFM) showed the surface morphology of the film, and field emission scanning electron microscopy (FESEM) could clearly observe the thickness of the HfO2 thin film and the layers between substrate and film. Additionally, the phase structures of HfO2 films were analyzed by grazing-incidence X-ray diffraction (GIXRD) with an incident angle of 1◦. Moreover, X-ray photoelectron spectroscopy (XPS) analyses of the HfO2 thin films were carried out using an Escalab 250Xi X-ray photoelectron spectrometer.
