## 2 Materials and Methods
### Research on State-of-the-Art Piezoelectric Materials
Useful lead-free materials are often binary or ternary solid solutions of the following end-member compositions: barium titanate (\(BaTiO_{3}\); \(BT\)), sodium bismuth titanate ((\(Na_{1/2}Bi_{1/2}\)) \(TiO_{3}\); NBT), potassium bismuth titanate ((\(K_{1/2}Bi_{1/2}\)) \(TiO_{3}\); KBT), potassium sodium niobate (\(K_{0.5}Na_{0.5}NbO_{3}\); \(KNN\)), and bismuth ferrite (\(BiFeO_{3}\); \(BFO\)), as mentioned by [20]. Commonly used state-of-the art binary lead-free piezoelectric materials are NBT-KBT, NBT-KNN, NBT-BT, NBT-BFO, KBT-BT, and KNN-BT, as illustrated by the same source [20]. Due to the limitations of the binary systems described above in terms of piezoelectric properties, they cannot replace PZT in all its applications. Therefore, much research has focused on new lead-free compositions that have become even more complex with the use of ternary solid solutions [20]. Piezoelectric polymer materials are also referred to as organic lead-free piezoelectric materials. According to the studies [21; 22; 23; 24], piezoelectric polymers are commonly grouped based on their topology and dipole moment as described in Figure 2: bulk polymers, polymer composites, and voided charged polymers. Bulk polymers are solid polymer films that have the piezoelectric mechanism through their molecular structure and its arrangement. Composite polymers are polymer structures with integrated piezoelectric ceramics from which the piezoelectric effect is generated, according to [25; 26]. These composites make use of the mechanical flexibility of polymers and the high electromechanical coupling of the piezoelectric ceramics [27; 28; 29]. For voided charged polymers, the piezoelectric properties are determined by internal dipoles when the gas voids are charged with an electric field. The gas voids are present in its thin polymer layer. Piezoelectric Thin Film. In the current trend of Industry 4.0, which is characterized by miniaturization and integration, piezoelectric thin-film technology plays an indispensable role. In the development of nanoscale and microscale devices, piezoelectric thin-film technology plays a major role due to the additional functionality provided by electromechanical coupling and their micro-machining capability [30]. This has been proven by the increasing inclusion of piezoelectric thin films as functional materials for microelectromechanical (MEMS) and nanoelectromechanical (NEMS) systems. The study [30] found that the advancement of thin-film piezoelectric materials is strongly related to the development and growth of bulk piezoelectric materials. Almost all of the state-of-the-art bulk piezoelectric materials discussed above can be used as thin-film piezoelectric materials with the help of different production technology, as stated by [31; 32]. The same studies also mentioned that the two basic crystal structures of the thin films are wurtzite and perovskite structures. Under the category of wurtzite crystal structure, non-ferroelectric piezoelectric materials such as \(ZnO\) and \(AlN\) are the common materials and are suitable for high-frequency resonator applications, as asserted by the study.