## 1 Introduction  
Recently, there has been a growing interest in energy-harvesting devices, and particularly piezoelectric ones that scavenge the vibrations, force, or pressure changes from the ambient. The advanced nanomaterials and technologies allow such devices to be very thin, lightweight, compact, and even flexible [1; 2; 3]. The potential applications of such microdevices could be for the self-sufficient power supply of biomedical microsensors attached to the human body and reacting to blood pressure changes [4], in the security sector as a part of the tracking devices [5], in everyday life by incorporatingthem in clothes and shoes (wearable piezoelectric harvesters) for activation from the body movements, implanted underneath the keyboard of a PC/laptop [6], etc. A key moment in the development and optimization of the thin film piezoelectric energy harvestings was the enhancement of their relatively low conversion efficiency. This problem has deepened in the last few years due to replacing the most efficient piezoelectric coating of lead-zirconium titanate (PZT) [7] with lead-free materials, which are less effective. Vibration-activated microgenerators can provide a power output of \(\sim\)4 uW for the range of low frequencies that correspond to human motion, and up to 800 uW if they are stimulated by the high frequencies of machine motions [8]. Zinc oxide (ZnO) is among the environmentally friendly and cost-effective materials that exhibit piezoelectric properties. It is characterized with a piezoelectric coefficient of about 12 pC/N, a bandgap of 3.37 eV at room temperature, and relative dielectric permittivity of \(\epsilon_{r}\) = 8.75 [9]. Different growing techniques have been used to deposit thin nanostructured ZnO films, because it was found that the piezoelectric coefficient is much higher for nanostructures than bulk wurtzite structures. However, most of the films' deposition methods are not compatible with the conventional microdevices fabrication technology or cannot be applied to the flexible substrates due to elevated temperatures or unwanted chemical reactions. Such methods include, for example, hydrothermal [10], the reduction of ZnO powder by H\({}_{2}\)[11], and the seedless chemical approach [12]. From the other side, there are available data for the application of chemical vapor deposition (CVD) [13] and Radiofrequency (RF) sputtering [14]. A variety of harvesting designs with incorporated ZnO films have been proposed in the literature; however, different problems have arisen, such as low electromechanical coupling, low dielectric permittivity, high dielectric losses, and high mechanical instability at bending, twisting and/or stretching. These problems prevent the commercial implementation of ZnO, and further optimization is required. One of the successful approaches to improve ZnO piezoelectric behavior is to form nanostructures such as nanowires, nanorods, nanoflowers, nanobelts, and other three-dimensional (3D) film structures that can be tuned by the growing mode in order to facilitate the polarization of the media [15; 16; 17]. Some of these microsystems have narrow frequency ranges (from few tenth of Hz to few KHz) that are usually limited to the resonance frequency [18]. [PERSON] et al. [19] demonstrated a fully stand-alone, self-powered sensor device, powered by a ZnO nanowire array on a flexible Kapton substrate. The mechanical bending of this microgenerator resulted in an output voltage of about 350 mV and a current density of 125 nA/cm\({}^{2}\). [PERSON] et al. [20] achieved a high effective piezoelectric coefficient for ZnO nanobelts that varied with the frequency between 14.3 pm/V and 26.7 pm/V, which confirmed the possible application of these nanostructures as nanosensors and nanoactuators. Some articles are devoted to the investigation of the size effect on the piezoelectric response for ZnO nanowires and nanobelts, which is also important for energy-harvesting devices [21; 22].