Recent progress in piezoelectric thin films as self-powered devices: material and application

Piezoelectric materials have become a key component in sensors and actuators in many industrial fields, such as energy harvesting devices, self-powered structures, biomedical devices, nondestructive testing, owing to the novel properties including high piezoelectric coefficient and electromechanical coupling factors. Piezoelectric thin films integrated on silicon substrates are widely investigated for their high performance and low manufacturing costs to meet the requirement of sensor networks in internet of things (IoT). The aim of this work is to clarify the application and design structure of various piezoelectric thin films types, synthesis methods


Introduction
Thin film sensors are widely used in aerospace, mechanical manufacturing, civil engineering, mining and other industrial fields (Zhang et al., 2021a;Shalabi et al., 2022;Song et al., 2022).With thickness in range from a few hundred nanometers to tens of microns, thin film sensors can be integrated directly onto surface of component under test without changing environment inside the device, making integrated structure and sensing fabrication easy to implement (Yu et al., 2023;Yue et al., 2023).Using large area film preparation technology, thin film strain resistance is placed on metal elastic substrate, with high precision, good creep, and strong anti-interference ability, etc. (Agarwala et al., 2017;Kirthika et al., 2017;Qiao et al., 2018;Russell et al., 2022).Through the development of new material systems and new physical mechanisms, piezoelectric thin film sensors have made great progress in sensitivity, response range, response time, linearity, hysteresis, and stability, and have shifted from the development of a single sensor to the development and optimization of the system level.Piezoelectric materials play key roles in various electronic devices such as wireless sensor networks, mobile electronics, wearable and implantable biomedical devices.Piezoelectric thin films have found a broad range of lab-on-chip Song et al. 10.3389/fmats.2024.1373040applications.Generated power from piezoelectric sensor can be used to drive low powered electronic devices.By combining hardware and software technologies, piezoelectric film sensors form an intelligent sensing platform for the internet of things (IoT), which will be widely used in health monitoring, home interconnection and intelligent manufacturing, et al. (Yamamoto et al., 2016;Zhang et al., 2021b;Liu et al., 2023a).
Internet of things is considered to be the third major innovation in information technology after the emergence of computers and internet (Fan et al., 2022;Zhang et al., 2023a;Kalyanasundaram Balasubramanian et al., 2023).Through connections between different entities, data is shared and numerous devices on the network are able to interact and collaborate (Zhao et al., 2020a;Chionh et al., 2020;Liu et al., 2023b;Zhang et al., 2023b;Huang et al., 2023;Portilla et al., 2023;Tsakanikas et al., 2023).There are three main layers in architecture of IoT, including perception layer, network layer and application layer (Uslu et al., 2020).The sensing layer obtains data from the external physical world through various means such as sensors or digital cameras, and transmits it through a series of short-range transmission technologies such as radio frequency identification (RFID), industrial fieldbus, Bluetooth and infrared (Liu et al., 2020;Mrabet et al., 2020;Cui et al., 2021;Liu et al., 2022a).With the rapid development of intelligent manufacturing, intelligent transportation, smart city and wearable technology (Atlam et al., 2018;Eini et al., 2021;Lopez-Castaño et al., 2021;Mondal and Rehena, 2022;Debeuckelaere et al., 2023), the IoT has a great demand for the miniaturization, integration, and low power consumption of sensors (Botta et al., 2016;Silvano and Marcelino, 2020).At present, new flexible sensors have been used in medical devices, such as electronic skin, personal medical devices and prosthetics (Hwang et al., 2015;Liao et al., 2015;Chen et al., 2016;Choi et al., 2016;Chortos et al., 2016;Ge et al., 2016;Park et al., 2016;Zang et al., 2016).Recently, the proliferation of new piezoelectric crystals, piezoelectric polymers, and leadfree piezoelectric materials has led to significant improvements in electromechanical coupling response, material properties, and applications.As shown in Figure 1, this work will present the development status of thin film sensors from four aspects: material selection, synthesis, sensor processing technology, and sensor application.

Piezoelectric materials
Piezoelectric materials present a mechanical-electric coupling effect that can cause mechanical bending when electric field applied and can also cause electric charges to build up on the two ends of material when it is bent (Hinchet et al., 2018).Owing to both positive and negative piezoelectric effects, piezoelectric materials can be used as both sensors and actuators (Joseph et al., 2018;Antony Jeyaseelan and Dutta, 2020;Ni et al., 2022).In 1880, brothers Pierre and Jacques Curie demonstrated the direct piezoelectric effect for the first time.They apply mechanical stress to a variety of single crystals such as tourmaline, quartz, topaz and Rochelle salt.The stress applied to the crystal produces a measurable surface charge.However, they did not initially show that it was also possible to generate electric-induced strain.The following year, mathematician Gabriel Lippmann predicted the inverse piezoelectric effect, which the Curie brothers soon confirmed.Through MEMS process, piezoelectric thin films can be integrated on silicon substrate to make tiny sensors and controller.Recently, due to the high piezoelectricity, biocompatibility, and low dielectric constant, bio-piezoelectric materials have become one of the most potential smart materials in biology (Kim et al., 2020;Chen et al., 2023a).In this part, the classification of piezoelectric materials will be introduced in detail.
In this work, we considered the most common crystal structures in piezoelectric applications.From a commercial perspective, polycrystalline ferroelectrics are possible for large-scale production.Due to the arbitrary orientation of crystals in ceramics, symmetry breaking elements must be introduced externally in order to obtain piezoelectric response.Therefore, ferroelectric materials can be used for piezoelectric ceramics.They can be polarized, meaning that their polar axis can be aligned with an external electric field, resulting in the required reversal of symmetry being disrupted.Therefore, among the 20 non centrosymmetric crystal point groups with potential to exhibit piezoelectric properties, piezoelectric ceramic materials only need to consider 10"polar" groups: 1 (triclinic), 2, m (monoclinic), 2 mm (orthorhombic), 3, 3 m (rhombohedral), 4, 4 mm (tetragonal), 6, and 6 mm (hexagonal).Among all possible crystal structures, perovskite structure may be the most common and technically relevant.The chemical composition of perovskite is ABO 3 .This structure can be described as a simple cubic cell with a large cation (a site) at the corner, a small cation (B site) at the center of the body, and oxygen at the center of the face.

Piezoelectric crystal
Piezoelectric crystals commonly found in various applications include quartz and water-soluble crystals such as sodium potassium tartrate, diammonium ethylene tartrate, dipotassium tartrate, and potassium sulphate (Zu et al., 2016;Lutjes et al., 2021;Dong et al., 2022).For polycrystalline, notable piezoelectric materials include barium titanate, zirconium lead titanate, and lead niobium magnesium oxide, et al. (Okayasu and Watanabe, 2016;Wu et al., 2016;Kumar et al., 2023;Shi et al., 2023).In 1997, large S-E strain was found in lead based relaxor ferroelectric crystals, which is widely regarded as a significant advancement in piezoelectric materials (Xu et al., 2000).Sm-doped lead niobium magnesium oxide-lead titanate crystals exhibit high piezoelectric coefficients exceeding 4,000 pC/N and dielectric constants with value of 12,000.The uniformity of crystals rod properties is enhanced by leveraging the sub-condensation properties of Sm elements during crystal growth.This advancement establishes a solid basis for the development of piezoelectric crystals suitable for high frequency medical ultrasound probes and high precision actuators (Li et al., 2019).
Barium titanate (BaTiO 3 , BT) presents high dielectric properties, which is extensively manufactured as high-frequency circuit components (Karvounis et al., 2020;Chen et al., 2022), as shown in Figure 2A.The thermal properties of BT can be enhanced by the process of Pb and Ca co-dopant (Hasan et al., 2023).The Framework of piezoelectric thin-film sensors, including materials, processing, and applications.
enhanced piezoelectric properties in BT-based ceramics can be achieved by use of ion-pair effect and A/B-site synergistic doping modification (Xie et al., 2022), as seen in Figure 2B.High d 33 value (30.5 pC/N) and Curie temperature (Tc = 657°C) were found in Bi 4 Ti 3 -x (Zn 1/3 Nb 2/3 )xO 12 ceramics.Additionally, the aligned ferroelectric domains demonstrate remarkable temperature stability (Xie et al., 2022).Lead zirconate titanate (PZT) is a solid-state solution consisting of lead titanate (PbTiO 3 ) and lead zirconate (PbZrO 3 ), which is extensively used as transducers materials owing to consistent piezoelectric properties and high Curie temperature (Tan et al., 2019), as shown in Figure 2C.The ceramics exhibit various features, after small amounts of dopant, such as niobium, antimony, tin, manganese, tungsten, et al. (Koh et al., 2022;Chen et al., 2023b;Habeeb Khan et al., 2023).With the protection of environment, lead free piezoelectric materials are widely investigated.Quartz crystals integrated on silicon substrate is shown in Figure 2D. Figure 2E illustrates the schematic structure of AlN crystals (Fei et al., 2018).Commonly used lead-free piezoelectric systems include KNN-BNT, KNN-BT, BNT-BT, BKT-BT, BNT-BT-KNN, and BNT-BKT (Mayamae et al., 2017;Zhang et al., 2022a;Wang et al., 2022;Safari et al., 2023;Tai et al., 2023).The S-E strain in (K, Na) NbO 3 polycrystalline is enhanced by solid solution near phase border, which undergoes a phase transition, as shown in Figure 2F.

Piezoelectric polymers and composites
Polar polymers like polyvinylidene fluoride (PVDF) are good examples of polymer piezoelectric materials, which show low sound resistance, and can be made into thin parts (Wang et al., 2023a).Polymers can be used directly in actuator and sensor, owing to electrical and mechanical energy conversion (Takahashi and Tadokoro, 1980;Koseki et al., 2012;Bouad et al., 2022;Zhang et al., 2023c).Due to big dipole moments, β phases PVDF exhibit high dielectric and piezoelectric properties (Nasir et al., 2006;Satapathy et al., 2011;Huang et al., 2021).The d 33 value can reach up to 62pC/N in β-phase PVDF films (Huang et al., 2021).Polytetrafluoroethylene (PTFE) is resistant to acids, alkalis, and a wide range of organic solvents (Dhanumalayan and Joshi, 2018;Niu et al., 2022).Neutral PTFE particles (d > ∼1-5 µm) can be turned into piezoelectric electrets using an easy-to-use ultrasound process at faster rate than piezoelectric catalysts (Wang et al., 2021).
Compared with the piezoelectric polymer, the inorganic piezoelectric ceramic particles have high piezoelectric constant and low breakdown strength.Therefore, through adding piezoelectric ceramics powder into polymers, the composite shows high piezoelectric coefficient and low poling field (Hema Malini et al., 2022).Compared with individual piezoelectric components, piezoelectric composite materials can overcome the temperature Crystalline unit cell and microstructure of piezoelectric materials.(A)-I: Non centrosymmetric unit cell structure of BaTiO 3 for tetragonal phase (below Curie temperature), and centrosymmetric unit cell structure of BaTiO 3 for cubic phase (above Curie temperature).(A)-II and III: Variation of cell volume at different temperatures in BaTiO 3 crystals (Karvounis et al., 2020).(B)-I, II, and III: Schematic of BT-based crystal structure and ferroelectric domain walls.(B)-IV and V: Schematic migration model of the charge carriers (Xie et al., 2022).(C)-I: Relationships between polarization and applied electric field in PZT.(C)-II and III: Microstructure of as-deposited PZT thin film (Tan et al., 2019).(C)-IV: Schematic diagram of Mn 3+ valence change to generate oxygen vacancies and strengthen pinning (Chen et al., 2023b).(C)-V: Comparison of Mn-doped PSN-PZT among other commercially PZT-based ceramics (Hao et al., 2019).(D): Two different microstructures in α-quartz (100) film on silicon substrate (Zhang et al., 2019).boundary of piezoelectric polymers and the inherent brittleness of inorganic piezoelectric biomaterials, while also allowing for large-scale manufacturing.The addition of barium titanate particles contributes the formation of β-phase in PVDF membrane (Khan et al., 2021), as shown in Figure 3A.Guan measured heel pressure in a new hybrid film made of lead zirconate titanate powder and micro fibrillated cellulose (PZT/MFC), which is a bendable film made with a polarization process (Guan et al., 2022), as presented in Figure 3B.Li made synthetic fabric films out of mixtures of PVDF and cellulose acetate (CA).Electrostatically spun PVDF/CA fiber membranes (EFMs) were used to make flexible nanogenerators (Li et al., 2022a), as shown in Figure 3C.For TiO 2 /SiO 2 /polymethyl methacrylate (PMMA)/PVDF composites, the β -phase content in the PVDF matrix increase from 19% to 43% as TiO 2 or SiO 2 contents increase from 0% to 5%, which made piezoelectric properties of PVDF better (Zhao et al., 2012;Li et al., 2013).A flexible ZnO/PVDF hybrid piezoelectric films with TiO 2 particles addition were used to make nanogenerator, showing a voltage is 2.3 times higher than that of pure PVDF nanogenerator (Kim et al., 2018).In a three-phase hybrid nanogenerator out of PVDF, ZnO, and BT nanorods, the output increase from 3 V to 12 V (Sabry and Hussein, 2019).The addition of nanoparticles can improve the crystallinity and the mount of β phase of the fiber films (Fakhri et al., 2019;Ye et al., 2021).Kar made a new bendable piezoelectric nanogenerator based on two-dimensional SnO 2 nanosheets and PVDF composites, and presented a high output rate with value of 16.3% (Kar et al., 2019).A threedimensional hybrid nanostructure of MnO2/Gr/multi-walled carbon nanotubes was also used to enhance the electrical properties of PVDF (Yang et al., 2018).Recently, nano-piezoelectric materials, including quantum dots, two-dimensional materials and topological insulators, have been added to PVDF to improve the power output of self-powered wearable devices and achieve excellent output performance (Hoque et al., 2017;Biswas et al., 2019;Bagchi et al., 2020;Shi et al., 2020;Saikh et al., 2021;Sarkar et al., 2023a;Sarkar et al., 2023b).A 2D halide chalcogenide transverse heterostructures prepared by liquid-phase epitaxy, which strongly suppressed the in-plane ionic diffusion in 2D halide chalcogenides by doping rigid π-conjugated organic ligands (Shi et al., 2020).

Bio-piezoelectric materials
Piezoelectric biomaterials are naturally suited for coupling mechanical and electrical forces in biological systems for realtime sensing, actuation, and power generation in vivo; however, large-scale synthesis and alignment of piezoelectric phases in bio-piezoelectric thin films still a major challenge (Swagata et al., 2017;Hoque et al., 2018;Lay et al., 2021;Sarkar et al., 2021;Das et al., 2022;Mondal et al., 2022).Piezoelectric biomaterials are low symmetry, highly ordered structures, lacking inversion centers.Therefore, linear electromechanical coupling is an inherent functional characteristic of most biomolecules.Piezoelectricity has been demonstrated in various biomaterials, such as wood and bone, as well as fibrillar proteins such as collagen, chitin, and elastin, which exist in highly ordered crystalline molecular forms in mammalian tissues.The classical piezoelectric principle has been applied to similar uniaxial oriented bioactive polymers, such as poly (lactic acid) (PLLA), poly (lactic acid) γ-Benzylglutamic acid (PBG) and cellulose 3.These biodegradable polymers have been used as piezoelectric implants to promote pure and composite forms of bone formation.As is well known, the surface charge and wettability of the scaffold control the interaction between the material cell interfaces.The shape of the bracket can be controlled through various manufacturing processes, and the additional functionalization of the material can be used to fix biochemical substances.The morphological characteristics, piezoelectric constant, and ferroelectricity of piezoelectric materials can be modified to meet specific requirements.Nanostructured PZT in the form of nanoribbons or nanowires has been developed and applied in the construction of biomedical and energy harvesting devices.However, due to its cytotoxicity, the application of PZT ceramics in tissue engineering is limited.Therefore, lead-free piezoelectric ceramics have been developed to reduce people's concerns about lead exposure to toxic environments.Two simple, portable, cost-effective, biocompatible, and environmentally friendly piezoelectric nanogenerators (PENGs) were designed using naturally biodegradable mud volcanic clay from the Andaman and Nicobar Islands in India.The output voltage of MPENG and BPENG is ∼85V, and the short-circuit current is ∼1.6 μA.The output voltage is∼125 V, and the short-circuit current is ∼1.9 μA.The power density is 4115 respectively μW/cm 3 and 7187 μ W/cm 3 (Das et al., 2022).Recently, a generator can produce electrical signals when squeezed by body movement, which will lead to a wide range of uses for muscle-powered electromechanical treatments (Chorsi et al., 2019).A self-assemble method using lysine as a piezoelectric generator has presented a way to make a product through chemical qualities of a material, as shown in Figure 3D.This fast self-assembly technology could greatly cut the cost of these kinds of gadgets and make them much easier to get and use.
A biodegradable poly nanofiber was designed to get chondrocytes and cartilage tissue grow back, which can create piezoelectric effect when stressed or loaded in a joint (Yang et al., 2021), as shown in Figure 3E.Without extra battery, cartilage can be regrown by implanting the disposable PLA support and producing microcurrents while walking.After 1-2 months of exercise, hyaline cartilage grown back in rabbits with serious osteoarticular flaws that had received piezoelectric scaffold grafts.The fully healed cartilage tissue was surrounded by a large number of chondrocytes and type II collagen.An active self-assemble method to shape piezoelectric biomaterial films is presented in Figure 3F.Nanoconfinement caused uniform nucleation, which will get rid of dependence on interface of films (Liu et al., 2022b).During the process of van der waals exfoliation, ultrathin films are thinned to effective piezoelectric domain thickness (Zhang et al., 2023d).

Synthesis of piezoelectric thin films
There are many ways to make piezoelectric thin films, such as vacuum evaporation, sputtering coating, chemical vapor deposition, molecular beam epitaxy, and sol-gel method.Here, a list of the most common ways to make thin films, such as AlN, KNN, ZnO, PZT, and PVDF, et al.

AlN piezoelectric films
AlN thin films is a hot topic of research in RF field because they are often used in high frequency sound resonators.These films can be made by blasting with a magnetron.Zhao reported a high-quality bendable AIN piezoelectric film made with micro-and nano-fabrication technology (Zhang et al., 2022b).The Mo/AIN/Al structure on silicon {100} was made by blasting, and then ion etching was used to remove the silicon used as a support to get a bendable Mo/AIN/Al sandwich film (Zhao et al., 2020b).Wen used an RF magnetron sputtering method with aluminum-rich AlN (Al-AlN) targets to make AlN films on Si surfaces with a low number of defects (Terai et al., 2023).Al vacancies and O impurity defects in AlN films can be cut down with the help of modulation in AlN films.Patidar used a highly detailed oblique angle deposition method to make c-axis oriented AlN (0002) films with reactive metal ions that were timed with HiPIMS (Wen et al., 2022).As shown in Figure 4A, combining HiPIMS with a small substrate bias of only −30 V greatly improves the crystalline quality and texture of the films.Process gas doping and point defect formation can be further reduced by synchronizing the negative substrate bias with Al.The films have a clear out-of-plane pattern and regular grain polarization.
One of the best ways to enhance piezoelectric response of AlN thin films is to mix it with other elements to make conformal films.By mixing transition metals (TMs, TM = Sc, Cr, Sr, Mo, Ru, and Rh, etc.), TM-N bonds become weak and move the TM atoms closer to the centers of the three nearby N atoms.It was found that the place of the TMs was strongly linked to their group number (Akiyama et al., 2009a;Akiyama et al., 2009b;Luo et al., 2009;Liu et al., 2013;Mayrhofer et al., 2015;Hu et al., 2018;Manna et al., 2018;Yanagitani and Jia, 2019;Fiedler et al., 2021;Feng et al., 2022;Lv et al., 2023a;Patidar et al., 2023;Zha et al., 2023).For Mo dopant, piezoelectric coefficient d 33 of AlN: Mo (3.46%) films reached to 7.33 pm/V (Zha et al., 2023).In the past few years, research has been done to figure out how to make high-quality mixed epitaxial films of AlN.Using nanopatterned AlN/sapphire templates with regular hexagonal holes, the dislocation, etch pit density in AlN heteroepitaxial thin films was reduced to about 104 cm -2 , which is close to value in AlN bulk crystals, by controlling the separation and grouping of the columns (Namikawa et al., 2023).
High performance PZT films were obtained on fluorine-doped tin oxide (FTO)-coated aluminum borosilicate glass (AG) surfaces using a modified sol-gel method (Lee et al., 2021).Liu made Pb  (Li et al., 2023a).(E): Schematic of fabrication procedure of KNN/PVDF-based nanogenerator (Abdullah et al., 2021).(F): Schematic of synthesis and fabrication of Sr x Zn 1-x O nanostructured humidity sensor (Li et al., 2018).(G): Schematic of transparent ZnO thin films grown on glass substrates (Murthy et al., 2023).(H): Schematic illustration of PVDF/PZT NPs fiber films preparation (Sapkota et al., 2022).(I): Schematic preparation of Cu x O/PVDF films (Gupta et al., 2019).4B.The Pb(Zr 0.52 Ti 0.48 )O 3 films have been changed by Pb(Zr x Ti 1-x )O 3 crystalline seed islands with different Zr/Ti ratios, presenting higher dielectric constant, lower coercive electric field, and less leakage current density.Furthermore, PbZr 0.52 Ti 0.48 O 3 thin films were successfully made by the sol-gel method at ultra-low temperature (450 °C) in an oxygen plasma-assisted environment in Ref. (Liu et al., 2022c), which can be used to make modern CMOS devices.

Frontiers in
After fast thermal annealing at 620°C, a high energy storage density of 10.0 J/cm 3 was obtained in PbZr 0.52 Ti 0.48 O 3 (PZT)/PbZrO 3 (PZ) hybrid films on a LaNiO 3 /SiO 2 /Si substrate synthesized by sol-gel method (Li et al., 2023a).Rhun grown thick PZT films on platinum-coated silicon plates using sol-gel method, and move PZT films and ITO electrodes onto glass substrates (Yang et al., 2023).In visual range, the average amount of light that get through PZT stacks on glass was 70%, which makes it possible to make clear piezoelectric motors on glass for high-performance haptic devices and other new uses, like self-cleaning or making smart windows.

KNN piezoelectric films
Several methods are used to make KNN-based piezoelectric films, such as alkaline methods, pulsed laser methods, and solgel methods (Bai et al., 2011;Khorrami et al., 2017;Akmal et al., 2018;Sharma et al., 2019;Fast et al., 2020;Kovacova et al., 2020;Abdullah et al., 2021;Cheng et al., 2022;Nair et al., 2022).Because of the advances in flexible piezoelectric nanogenerators, alkali metal niobates have received a lot of attention and are considered an environmentally friendly choice for lead-based piezoelectric materials.Abu used potassium sodium niobate (KNN) to make an energetic film (Fast et al., 2020), as shown in Figure 4D.It was used to make copper plates for a piezoelectric nanogenerator (PENG).It was also shown that KNN-based energy films could be used for multipurpose uses (like force and pressure sensors) and lead-free energy harvesting.As shown in Figure 4E, Nair made a lead-free foldable piezoelectric nanogenerator out of KNN-PVDF nanocomposites (Abdullah et al., 2021).Cheng investigated the strengthening effect of CuO-doped KNN-based ceramics, and found that Cu doping strongly upset the ferroelectric ordering (Nair et al., 2022).An idea of using solution synthesis to keep the alkali chemistry of the Mn-doped KNN films uniform was given in Ref. (Cheng et al., 2022).A uniform grain size of 80 nm and a leakage current density of 2.8 * 10 −8 A/cm 2 under an electric field of up to 600 kV/cm were obtained in chemically uniform KNN thin films.

ZnO piezoelectric films
ZnO thin films can be made by blasting with an RF magnetron (Li et al., 2018;Sonklin et al., 2022;Algün et al., 2023;Cuadra et al., 2023;Kahveci et al., 2023;Murthy et al., 2023;Toma et al., 2023).{002} oriented Li-doped ZnO thin films on SiO 2 /Si was obtained using RF magnetron sputtering, when the sputtering power was 220 W and the Li-doped concentration was 5% (Sonklin et al., 2022).As shown in Figure 4F, nanoparticles of undoped zinc oxide (ZnO) and Sr x Zn 1-x (x = 0.01, 0.02, 0.03, 0.04, and 0.10) were synthesized using the sol-gel method (Algün et al., 2023), as shown in Figure 4F.As shown in Figure 4G, high crystallinity ZnO thin films were synthesized by a spray pyrolysis method, show antibacterial qualities when exposed to UV light (Cuadra et al., 2023).Transparent conductive films were made by sputtering Gadoped and (Ga + Nd)-doped ZnO films with an RF magnetron (Toma et al., 2023).It was found that undoped ZnO has a resolution of 85% in the visible range.Using sol-gel dip coating method, Murthy putted thin plates of aluminum and rubidium-doped ZnO on glass surfaces (Murthy et al., 2023), and found that the quality of crystals gets worse because of lattice stress after Rb particles added to a ZnO host lattice.

PVDF piezoelectric films
There are several ways to make PVDF films, such as electrostatic spinning, solution casting, solution casting, spin coating method, electro-spinning technique, stretching, vacuum evaporation, and homogenization, etc (Jin et al., 2021;Yen et al., 2022;Das et al., 2023;Ghosh et al., 2023;Mishra et al., 2023;Sarkar et al., 2024).The scratch spraying method was used to make polyvinylidene fluoride-trifluoro ethylene (PVDF-TrFE) copolymers (Singh et al., 2018;Bhunia et al., 2019;Gupta et al., 2019;Sapkota et al., 2022).In first step, cobalt ferrite (CoFe 2 O 4 ) nanoparticles were made using acoustic chemistry.In second step, cobalt ferrite nanoparticles with different weight percentages (0, 2.5, 5, and 10%) were added to PVDF-TrFE to make nanocomposites.As shown in Figure 4H, an electrostatic spinning process was employed to make polyvinylidene difluoride matrix fiber membranes with modified lead zirconate titanate nanoparticles (PZT NPs) (Yuan et al., 2023).The inserted particles changed the distribution of polarized electric field, which helped to polarize PVDF.As shown in Figure 4I, thin films substrate is made of flexible hollow fiber polymer membranes sprayed with thin layers of photocatalyst (Zakria et al., 2023).Copper oxide/polyvinylidene fluoride thin film hollow fiber membranes (Cu x O/PVDF TF HFM) were made by spraying Cu x O on PVDF hollow fiber membranes (HFM) with a radio frequency (RF) Frontiers in Materials 08 frontiersin.orgmagnetron.As much as 91% of the BPA was taken out of the cleaned wastewater by irradiation.After three rounds in a row, the efficiency of recycling hit about 71.3%.Flexible nanocomposite films were made by adding BFO powder to copolymers PVDF-TrFE and PVDF-HFP, showing high output voltage (Tripathy et al., 2023).Dielectric constant was reached up to 40 in an independent flexible hybrid film made of polyvinylidene fluoride (PVDF) and molybdenum disulphide (MoS 2 ) nanoflakes synthesized by sol-gel method, which was to be 5 times more than that of pure PVDF (Jangra et al., 2023).A lead-free, facile, low-cost, sol-gel-processed reduced graphene oxide (rGO)/P(VDF-TrFE) nanocomposite with multipurpose capability demonstration as a piezoelectric nanogenerator (PENG) and hybrid piezoelectric triboelectric nanogenerator (HPTENG) devices is presented in Ref.171.The maximum output power densities of hybrid piezo-triboelectric and piezoelectric devices are 0.28 W/cm3 and 0.34 mW/cm3, respectively.The triboelectric device demonstrates the direct illumination of 45 blue light-emitting diodes, which are connected in series to collect the mechanical energy generated by repeated finger taps (Bhunia et al., 2019).
4 Applications in self-powered IoT devices

Energy harvesting
Gathering energy from surrounding environment, like mechanical vibration, heat, fluid flow, electromagnetic radiation in form of light and radio waves (RF), and energy from body can provide clean power to run electronic devices like wireless sensor networks, mobile electronics, and wearable and implantable biomedical devices (Shirvanimoghaddam et al., 2016;Liu et al., 2018;Won et al., 2018;Ali et al., 2019).Mechanical energy is the most common type of energy that can be turned into useful power (Wang, 2012;Hu et al., 2019;Karan et al., 2019;Sun et al., 2019;Yan et al., 2019).Piezoelectric energy harvesting is a very easy way to turn mechanical energy in the environment into electrical energy.Because piezoelectric effect is based on intrinsic polarization of the material and does not need a separate voltage source, a magnetic field, or contact with another material, like electrostatic, electromagnetic, and friction electrical energy harvesting (Xie and Wang, 2015;Wang et al., 2018).Compared to other energy harvesting methods, their density output and voltage output are 3-5 times higher (Kim et al., 2011;Wu et al., 2015;Shi et al., 2018;Chen et al., 2019;Guan et al., 2020).Piezoelectric units are easy to integrate to microelectromechanical systems (Madinei et al., 2016;Zhou et al., 2020).It has been used in many fields, such as buildings, transportation, wireless electronics, MEMS, the Internet of Things (IoT), personal and internal healthcare devices (Siang et al., 2018).
Nurettin suggested a cantilever energy harvester using PIN-PMN-PT crystals, showing high power output with value of 102 W/m 3 .The designed energy generator worked well to power wireless devices for tracking and sending data, which could help to give power supply of IoT systems in a safe way (Gao et al., 2020).As shown in Figure 5A, a droplet-based generator (DEG) for gathering energy from the natural environment has been designed (Li et al., 2022b).The self-capacitance effect of upper electrode enables an ultra-high instantaneous peak output power with value of 765 W/m 2 .As shown in Figure 5B, Petritz presents an energy harvesting system used ferroelectric polymer transducers and organic diodes (Petritz et al., 2021).These components are seamlessly integrated onto ultrathin substrates with 1 µm in thickness.
The progress of CMOS has significantly contribution to selfpowered direct current (DC-type) energy harvesters operating at low input voltages, enhancing the overall performance of these energy harvesters (Wang and Li, 2016).Lei presented a surface engineering approach for transparent conductive membranes using self-assembled monolayers (SAMs) in conjunction with silver nanowires (AgNWs) for triboelectric nanogenerators (TENGs) and self-powered pressure sensors (Lei et al., 2023).The enhanced.
TENG has a notable capability to function as a pressure sensor array (4 × 4 pixels) for trajectory tracking, with a high sensitivity of 221 V-kPa-1.As shown in Figure 5C, Wu developed sensors using composite thin-film infrared sensor arrays for imaging human hand (Wu et al., 2023), indicating potential use of self-powered infrared sensor in wearable non-visual sensing and smart sensing applications.As shown in Figure 5D, piezoelectric sensors were designed for real-time monitoring of meteorological wind and rain (Li et al., 2023b).The ZnO@CF/PVDF composite thin film PNG demonstrates a maximum output power of 7.9 μW under an external load of 10 MΩ.Furthermore, the remarkable washing resistance, longevity exceeding 50,000 cycles, and sustained stability over a period of 12 months make the composite film as a viable candidate for deployment of weather sensor in autonomous vehicles.Finally, self-powered physiological monitoring devices have the capability to consistently monitor and transmit electrocardiogram (ECG), blood pressure, temperature, and exercise parameters of the human body (Yan et al., 2023).

Bio-medical sensors
Due to the pressing demands of industry, flexible sensors have been significant investigated and focused on strain sensors for accurate motion detection, particularly in relation to human muscles (Gullapalli et al., 2010).Photodetectors designed for biomonitoring, such as continuous glucose monitoring, have experienced substantial growth in research efforts (Chung et al., 2019;Teymourian et al., 2020;Zhou et al., 2021).In addition, there is emerging interest in the advancement of temperature and humidity sensors (Park et al., 2012;Huang et al., 2013;Pandey et al., 2014;Nakajima and Tsuchiya, 2015;Fujita et al., 2016;Nakajima et al., 2017;Nakajima and Tsuchiya, 2020;Shin et al., 2020;Trudeau et al., 2020).There has been a notable surge in use of flexible gas sensors (Monereo et al., 2011;Alrammouz et al., 2018;Fioravanti and Carotta, 2020;Sugahara et al., 2020) and electrochemical sensors (Huang et al., 2011;Santos et al., 2014;Manjakkal et al., 2020) on purpose of real-time monitoring.This monitoring approach involves the analysis of human breath and sweat to effectively identify pre-symptomatic conditions with a high level of accuracy.Furthermore, flexible magnetic sensors have been extensively investigated for muscles and brain applications (Satake et al., 2019).(Chao et al., 2023).
Frontiers in Materials 10 frontiersin.orgAs shown in Figure 5E, Shin presented a novel conceptual framework for artificial skin using negative temperature coefficient (NTC) materials (Alrammouz et al., 2018).Utilizing a network of physiological temperature sensors to evaluate the performance of temperature-sensitive artificial skin in the measurement of exhaled breath temperature, the early advancement of pathogenic conditions inside the respiratory system was identified.
On purpose of healthcare monitoring, Nakajima produced thermistor-on-a-polyimide sheet thin-film sensor arrays using strontium-and nickel-doped chalcogenide SmMnO 3 (Nakajima and Tsuchiya, 2020), as shown in Figure 5F.At bending angles of 60°and 20°, thermistor sensor exhibited notable resilience when subjected to a bending angle of 60°and a minimal bending radius of 500 µm.During a bending test including over 1,000 cycles, the observed temperature variation remained within a narrow range of 0.1°C.Sun developed a humidity sensor with a flexible design using multi-walled carbon nanotubes (MWCNTs) that were securely affixed to the surface folds of a natural latex membrane (Sun et al., 2023).The sensor has a rapid reaction time of 0.7 s and a broad humidity detection range (0%-100%) due to the weak contact link between multi-walled carbon nanotubes (MWCNTs) and water molecules.Consequently, it can effectively detect human breathing at a frequency of 1 Hz.As shown in Figure 5G, Lee presented a cutting-edge development in the form of an ultrathin self-powered artificial skin (SPAS) that relies on the utilization of piezoelectric nanogenerators (Lee et al., 2014).The sensor is capable of gathering and storing elastic deformation energy produced as by bending and stretching movements of skin.Rim shown a novel approach including a conformal biosensor for fabrication of highly responsive field effect transistors (FETs) based on 2O 3 (Rim et al., 2015).Ultrathin films with a thickness of 3.5 nm were fabricated, exhibiting high density and uniformity over a large surface area.Wang fabricated a flexible strain sensor by incorporating silver nanowires onto carbon blackmodified electrostatically spun thermoplastic polyurethane fabric sheets, demonstrating high level of sensitivity with a strain factor over 16,000 (Wang et al., 2023b).It also possesses a wide strain range, spanning from 360.0% to 1%.Chao developed a permeable epidermal sensor by arranging conductive MXene nanosheets and silver nanowires on electrostatically spun elastic substrate (Chao et al., 2023).
As shown in Figure 5H, He presented a method for fabrication of robust nanocomposite organic hydrogel (NCO hydrogel) strain sensors and their integration into a flexible wearable device (He et al., 2023).The NCO hydrogel was affixed to substrate and linked to Bluetooth module in order to developing a portable wearable device for surveillance of human motion.Flexible sensors possess a broad spectrum of potential applications for monitoring and tracking human health in real-time (Luo et al., 2023).Nevertheless, the primary material used in flexible sensors is thin films.

Flexible RFID
Several challenges still need to be addressed in field of industry, particularly the design of flexible micro-nano mechanical sensors and measurement of various physical quantities such as pressure, piezoelectricity, and strain.Additionally, the synthesis of sensitive materials, conductive inks, large-area printed electronics for mass production, require further resolution.
The prevailing technology for authentication in the internet of things (IoT) is radio frequency identification (RFID).RFID is a wireless, non-contact automatic identification technology that enables the identification of specific targets, and reading and writing of relevant data through radio signals.Wang designed a detection system that utilized reeds and switch-based proximity sensors (Wang et al., 2023c).The integration of sensors with distinct passive RFID tags has been shown in Figure 6A.RFID technology can be effectively used in field of structural health monitoring (SHM), a critical aspect in guaranteeing operational safety, such as pipelines, storage tanks, aero planes, ships, and automobiles (Baumbauer et al., 2020;Nesser et al., 2023).As shown in Figure 6B, high sensitivity strain measurement data can be wireless transferred by RFID.As shown in Figure 6C, Tekcin designed a pliable sensor that relies on an inductive-capacitive (LC) circuit and a parallel-plate capacitive sensing unit (Tekcin et al., 2022).The piezoresistive properties were enhanced through the addition of tailored fractures.Lv presented a novel flexible sensor integrated pyramidal micropatterns with ion-gel nanofibers, enabling it to effectively detect both normal and tangential forces (Lv et al., 2023b).As shown in Figure 6D, Kanazawa presented a flexible sensor designed for measuring the distribution of wind pressure (Kanazawa and Ushijima, 2020).To enhance the mechanical mobility of resistive strain sensor matrix against wind, a suspension structure was integrated into a plastic sheet.The use of mechanically functionalized substrates gives an opportunity for the advancement of flexible electronics.Chen designed anisotropic magneto resistive (AMR) sensors on flexible substrates (Chen and Zhang, 2023).The AMR sensors demonstrated a maximum bending radius of around 2.3 cm under mechanical bending, illustrating the practicality of producing compact AMR sensors on flexible substrates for detecting magnetic fields in nonplanar scenarios, as shown in Figure 6E.

Conclusion
The emergence of self-powered flexible electronic systems represents a huge paradigm shift in the future of electronics in the field of human friendliness or human integration.In particular, by capturing permanent energy and generating electricity from abundant mechanical energy, such systems are of direct interest for applications in sensor networks and wearable electronics.The rapid development of wearable devices and portable electronic systems has raised the demand for transferable, adaptable, wearable, flexible, mechanically robust and stable energy sources that can provide the required power from renewable sources.In this work, we highlight the latest advances and achievements in field of piezoelectric thin film used as self-powered sensors.Firstly, piezoelectric energy conversion materials are the core of sensor function.Crystalline structure, morphology and electrical properties in several kinds of piezoelectric thin films, especially high-performance lead-based films, lead free films, and biofilms, are reviewed.In addition, piezoelectric properties of films doped with various elements are summarized and the corresponding synthesis processes are analyzed.Second, manufacturing process of thin film sensors is outlined, followed by a brief overview of techniques used in microelectromechanical system (MEMS) processing.Furthermore, industrial practices regarding batch fabrication of MEMS are analyzed.Thirdly, the application scenarios of latest thin film sensors used in internet of things are introduced, including energy harvesting, biosensors, RF sensors, et al.It highlights the significant performance improvements presented by these advanced thin film sensors in different application areas.Finally, the existing thin film technique is subject to several restrictions, including challenges in industrial preparation and processing, as well as issues related to dimensional accuracy, among others.Future research should consider improving film quality and developing ecologically sustainable film production methods.Piezoelectric self-powered nanogenerators can be used in wearable electronic products, Frontiers in Materials 12 frontiersin.org

FIGURE 3 (
FIGURE 3 (A)-I, II, and III: Conceptual illustration of BaTiO 3 -PVDF composite structure before and after stretching, including the applied uniaxial stretching mechanism, polymer matrix elongation, and conversion of α and γ phase PVDF to β phase.(A)-IV and V: SEM images of composite films before and after stretching (Khan et al., 2021).(B): Structure of thin films mixed of PZT powder and MFC (Guan et al., 2022).(C)-I: Chemical interactions between the CA and PVDF molecular chains.(C)-II: Picture of PVDF/CA PENG.(C)-III and IV: SEM images of pure PVDF and PVDF/CA (Guan et al., 2022).(D)-I: Schematic synthesis approach of piezoelectric glycine-PVA films over a large area.(D)-II: Photographs of wafer-sized as-grown film (left) and largely curved film showing the flexibility (right).(D)-III and IV: Cross-sectional SEM image and EDS mapping of a sandwich-structured films.(D)-V, VI, and VII: Crystallization process of glycine-PVA sandwich thin films.(E)-I: Schematic of bio-organic films printer and the synthesis of β-glycine nanocrystalline films.(E)-II: Photographs of films on 4-inch silicon wafer.(E)-III: Films on flexible gold-coated polyethylene terephthalate (PET) substrate (Yang et al., 2021).(F)-I and II: PFM image of SIS films.(F)-III: Cross-sectional SEM image of untreated SIS and peeled-off SIS films.(F)-IV: Picture of SIS ultrathin films (Liu et al., 2022b).

FIGURE 5 (
FIGURE 5 (A)-I: Application of SCE-DEG on building's roofs.(A)-II: Output voltage of SCE-DEG (Li et al., 2022b).(B)-I: Photograph of ultra-flexible P(VDF:TrFE)70:30-based transducer.(B)-II, III, and IV: Electrical properties of ferroelectric layers (Petritz et al., 2021).(C)-I: Thermoelectric conversion test system.(C)-II to V: Electrical properties of various PPM films (Wu et al., 2023).(D)-I: Structures of flexible ZnO@CF/PVDF composite films.(D)-II: Strain-stress curve of ZnO@CF/PVDF composite films (Li et al., 2023b).(E)-I: NiO epidermal temperature sensors attached at various facial positions.(E)-II: Continuous monitoring of breathing.(E)-III: Real-time monitoring of hyperventilation recorded in conjunction with SpO 2 change during incremental exercise test (Shin et al., 2020).(F)-I: Voiceprint and humidity signals during speaking of different volunteers.(F)-II: Response of sensors to different frequencies of human respiration.(F)-III: Voiceprint and humidity response signal of sensors in noisy environment (Nakajima and Tsuchiya, 2020).(G)-I: Schematic diagram of bent SPAS.(G)-II and III: Output voltage of BG ZnO NRs from bending parallel and vertically along rubbing direction.(G)-IVto VII: Output voltage and current density in devices connected in forward and reverse directions(Lee et al., 2014).(H)-I: Schematic fabrication of flexible breathable electronic sensor by elaborately assembling conductive MXene nanosheets and AgNWs.(H)-II and III: Performance of flexible breathable sensors before and after therapy.(H)-IV: NIR thermal images of wrist bending(Chao et al., 2023).

FIGURE 6 (
FIGURE 6 (A): Multiple smart screen-printed flexible RFID enabled self-powered sensor tags (Wang et al., 2023c).(B)-I and II: Photographic of designed LC sensor.(B)-III: SEM image of electrodes on sensors.(B)-IV: Resistance variation of cracked Cr/Au film under strain steps.(B)-V: Application of supersensitive passive RFID strain sensors (Nesser et al., 2023).(C): Experimental apparatus for impedance measurement (Tekcin et al., 2022).(D)-I: A kind of wind pressure distribution sensor.(D)-II: Deflection of suspended structure and resistance recorded by single sensor for different wind pressures (Kanazawa and Ushijima, 2020).(E)-I and II: Micro-structures of PNG flexible composite films and experimental instrument.(E)-III: Current output of the devices(Lv et al., 2023b).
Marco et al., 2023)3 thin films with islands of Pb(Zr x Ti 1-x )O 3 solid species using the sol-gel method (DiMarco et al., 2023), as shown in Figure (