*2.4. Characterization*

The structure and functional groups of the surface of the films were analyzed by Fouriertransform infrared spectroscopy (FTIR, 2000 GX spectrometer, Perkin Elmer, Waltham, MA, USA) at 4000 to 400 cm<sup>−</sup>1. The structure of the molecules was determined using a Renishaw micro-Raman spectroscopy system with a laser wavelength of 532 nm (Raman, RENISHAW PLC, Kingswood, UK). Crystalline properties were evaluated by X-ray diffraction (XRD, D8 Advance X-ray diffractometer, Bruker, Germany). The morphology and microstructure of the films were inspected by scanning electron microscopy (SEM, SU-3500, Hitachi, Japan) at an accelerating voltage of 10 kV and by transmission electron microscopy (TEM, JEM-2100F, Tokyo, Japan). The thermal properties of the films were determined from 25 to 800 ◦C at a heating rate of 10 ◦C min−<sup>1</sup> in a nitrogen atmosphere by thermogravimetric analysis (TGA, Metter-Toledo, Switzerland). The composition and chemical state of the elements in the films were determined by X-ray photoelectron spectroscopy (XPS, K-Alpha+ spectrometer, Thermo-Fisher, Waltham, MA, USA).
