## 3 Results and Discussion  
Figure 2 shows the X-ray diffraction (XRD) spectrum of the zinc oxide nanobranched structure, which was similar to that of the wurtzite structure. Although some of the peaks, such as (200) and (103) corresponding to the ZnO crystalline structure are missing from the pattern, the peaks of some main phases responsible for the piezoelectric response, such as (100), (102) and (110) presented, and indicated a non-centrosymmetric structure for ZnO [27]. A modified pattern could be ascribed to the amorphous polymer-contained electrode, which could serve as a base for the ZnO film sputtering without an auxiliary seed sublayer. This is also the reason for randomly oriented ZnO nanobranches grown on the amorphous substrate [25]. For the nanobranched ZnO, there is a slight shift of the highest intensity peak from 30\({}^{\circ}\) toward lower angles 2\(\theta\) of 27\({}^{\circ}\) when compared with other ZnO nanostructures (nanowires, nanorods, etc.). This is evidence for an increase in the nanoformations spacing (distance between planes), which induces uniform tensile strain in the nanobranches due to the dislocated growth of the side dendrites [28]. The strain is calculated to be 0.94 \(\times\) 10\({}^{-3}\), following the formula \(\epsilon=\frac{\beta}{4 Lang}\), where \(\beta\) is the measured broadening of the major diffraction line peak at an angle of 2\(\theta\), at half its maximum intensity, and \(\theta\) is the Bragg diffraction angle of the XRD peak. Nevertheless, this value is much lower than some cases reported in the literature, which were obtained as a result of lattice unit cell expansion due to high deposition temperature or doping process; as a result, it can be assumed that the electromechanical performance of the film will not be affected [29]. Moreover, it is  
Figure 1: Schematics of (**a**) a layered configuration of the flexible polymer-based piezoelectric harvester (**left**) and photo of the sample (**right**); (**b**) lab-made combined vibrational setup and cyclic bend tester (1—beam, 2—sample under test, 3—weight detector, 4—tiller bending the beam, 5—electromotor activating the tiltler); (**c**) sample configuration at static bent conditions with a certain radius of curvature. suggested that such low density film could be easy polarizable, and the dipoles formation and voltage generation processes will be strongly alleviated, which will cause dielectric losses to decrease. The surface morphology of the ZnO nanophranched film was characterized by using scanning electron microscopy (SEM), and the image is shown in Figure 3. The film consists of long wires with a length of few micrometers, a diameter of \(\sim\)150-200 nm, and side grown dendrites, all forming configuration that resemble leaf. Each of the side growths has a different length, varying from 500 nm to 1.5 \(\upmu\)m, but their thickness seems to be one and the same: approximately 100 nm. All nanoformations are merged into a loose network. The exact mechanism of the side dendrites formation should be further examined. The branched structure has no preferable growth direction and no alignment; they are randomly oriented. This is in good agreement with the XRD pattern, and indicates relatively poor crystallinity. The reason could be the non-crystalline substrate/polymer electrode, which is the base for further revealing of the nanostructure. In our previous study [30], we showed that a very similar ZnO texture can be used to enhance the piezoelectric response of unit piezoelectric film thickness, due to the contribution of the dendrites to the process of electrical poles formation. Further, the structural and morphological results are correlated with dielectric studies. It was also proved that such an element generates piezoelectric voltage at more than one frequency of the mechanical stimulus and can work in a non-resonance mode.