A Review of Ultrathin Piezoelectric Films

Due to their high electromechanical coupling and energy density properties, ultrathin piezoelectric films have recently been intensively studied as key materials for the construction of miniaturized energy transducers, and in this paper we summarize the research progress. At the nanoscale, even a few atomic layers, ultrathin piezoelectric films have prominent shape anisotropic polarization, that is, in-plane polarization and out-of-plane polarization. In this review, we first introduce the in-plane and out-of-plane polarization mechanism, and then summarize the main ultrathin piezoelectric films studied at present. Secondly, we take perovskite, transition metal dichalcogenides, and Janus layers as examples to elaborate the existing scientific and engineering problems in the research of polarization, and their possible solutions. Finally, the application prospect of ultrathin piezoelectric films in miniaturized energy converters is summarized.

Generally speaking, microscopically, potential piezoelectric materials only originate from materials with a non-centrosymmetric microstructure. The work of Gowoon Cheon et al. [19] describes 325 potential two-dimensional (2D) piezoelectric materials. They are not piezoelectric in the bulk phase but lack central symmetry in the monolayer. There are 32 crystal symmetry groups, among which 21 have broken central symmetry. Surprisingly, 20 of them have directly observed piezoelectric responses. Piezoelectric materials were first discovered in naturally occurring natural products (quartz crystals and Rochelle salt). Monocrystalline, polycrystalline and polymer materials with better piezoelectric properties were discovered or synthesized later [20]. The classification of piezoelectric materials is given in Figure 1. Grounding three different criteria, completely different classification results will be showed. Furthermore, each type of material uses several materials to illustrate [21,22]. Most importantly, our focus is on all the parts of the diagram marked blue, that is to say, the type of polarization is a key factor. In Section 3, in-plane and out-of-plane polarization will be introduced in more detail.
Nevertheless, previous attention and research has focused mainly on materials with broken central symmetry [23][24][25]. Recently, in the reporting of D.-S. Park et al. [26], the piezoelectric phenomenon was realized in a centrosymmetric oxide. Unlike other methods, Nevertheless, previous attention and research has focused mainly on materials w broken central symmetry [23][24][25]. Recently, in the reporting of D.-S. Park et al. [26], piezoelectric phenomenon was realized in a centrosymmetric oxide. Unlike other m ods, this exciting breakthrough was achieved by regulating the inserted oxygen vaca with an applied electric field. In this regard, it also provides a new way to induce pie lectricity. Under the action of a direct current (DC) electric field, the degree of defect gration has been significantly improved, which is represented by the numerical lea dielectric constant. As a result, the achievement leads to the ultra-high voltage electr property of the film [27]. Xiaoqing Yang et al. also used the first-principles method to dict the longitudinal piezoelectric effect of the oxide family AO (A = Cd, Ba, Sr, Ca, etc.) under the action of biaxial strain [28]. After this, it was also proved experiment that the piezoelectric constant is enhanced in the 2D limit [29]. At the macro scale, a st with a particular large gradient is required to cause significant polarization in the m rial, whereas, at the nano scale, even a small strain can do so [30,31]. For example, for star material MoS2 in the 2D monolayer, an independent monolayer material is obtai experimentally through various preparation methods, and its piezoelectric properties characterized [32,33]. These works provide an experimental basis and proof for 2D trathin piezoelectricity. Additionally, the layer odd-even dependence of the piezoele response was also found [33]. Aiming to achieve more adjustable piezoelectricity, a new fabrication method of ditional 2D materials [34] has been developed. At the same time, large piezoelectric sponses have been achieved in the Janus monolayer through first-principles calculat [35]. Typically, the effect of inducing vertical dipoles by breaking the symmetry of the of-plane structure caused this huge response [36]. Of course, this mechanism can als applied to group-III chalcogenide monolayers, such as Ga2SSe, with both inversion br ing and specular symmetry [37], though they still remain thermodynamically stable. V recently, with the continuous progress of scientific and technological means, Boha Mortazavi et al. realized the first-principles study of piezoelectricity on MA2Z4 (M = Mo, W; A = Si, Ge; Z = N, P) monolayer through machine learning [38]. It is enoug raise concerns about new types of ultrathin films. The unique excellent performanc this group can even have a certain competitiveness in various application fields (nan lectronics, optoelectronics and energy conversion nanosystems) [39]. Aiming to achieve more adjustable piezoelectricity, a new fabrication method of traditional 2D materials [34] has been developed. At the same time, large piezoelectric responses have been achieved in the Janus monolayer through first-principles calculations [35]. Typically, the effect of inducing vertical dipoles by breaking the symmetry of the out-of-plane structure caused this huge response [36]. Of course, this mechanism can also be applied to group-III chalcogenide monolayers, such as Ga 2 SSe, with both inversion breaking and specular symmetry [37], though they still remain thermodynamically stable. Very recently, with the continuous progress of scientific and technological means, Bohayra Mortazavi et al. realized the first-principles study of piezoelectricity on MA 2 Z 4 (M = Cr, Mo, W; A = Si, Ge; Z = N, P) monolayer through machine learning [38]. It is enough to raise concerns about new types of ultrathin films. The unique excellent performance of this group can even have a certain competitiveness in various application fields (nanoelectronics, optoelectronics and energy conversion nanosystems) [39]. In this review, we first elaborate the basic working principle of the piezoelectric effect in Section 2, and discuss the piezoelectric properties of thin film materials. Additionally, we also observe that the intrinsic difference of polarization mechanism between thin film and bulk material lies in the difference of anisotropy. Afterwards, we find that there are layer thickness effects and odd-even effects in these materials that already have theoretical and experimental foundations. Hence, we also summarize the physical mechanism grounding the instance of MoS 2 and hexagonal boron nitride (h-BN). Notably, the modes applied by the strain have the potential to induce novel quantum effects, which will be the focus of future research. In Section 3, we classify ultrathin piezoelectric films from an entirely different perspective from most of the previous reviews: ultrathin in-plane piezoelectric films and ultrathin out-of-plane piezoelectric films. Moreover, based on transition metal dichalcogenides (TMDs), Janus structure and novel ultrathin films, the piezoelectric polarization mechanism and modification methods are analyzed. For the purpose of helping the workers who are interested in ultrathin piezoelectric films to clarify the components and their respective characteristics, we compiled the details included in this review. Finally, in Section 4, we summarize the current research progress of ultrathin piezoelectric films and their applications in energy harvesting and conversion and point out that there are still unavoidable defects and limitations in the fabrication of 2D materials within the scope of current technology.

Piezoelectric Effect and Polarization Mechanisms
The piezoelectric effect results from the uneven distribution of ions in the crystal structure in some materials [40]. "Electric charge that accumulates in response to applied mechanical stress in materials that have non-centrosymmetric crystal structures" is defined as piezoelectricity [41]. A piezoelectric material is one that can achieve energy collection and conversion through the piezoelectric effect. Typically, the dipoles are randomly distributed in the crystal structure and call for an equilibrium state of electroneutrality on a macroscopic level. In these materials, though, electrons and holes move in opposite directions towards the metal-semiconductor interface due to a piezoelectric potential, which is generated by an external stimulus (polarization). The behavior by which the dipole tends to the same random direction is known as the piezoelectric polarization effect [42]. The connection between elasticity and electrical behavior is made successfully from then.
The piezoelectric effect can be modulated by regulating the interface properties of the electron transport as well as photoelectric properties [43]. The piezoelectric polarization effect can be divided into direct effect and inverse effect. The schematic diagram of the two working mechanisms is shown in Figure 2. Direct piezoelectric effects occur when the existing dipole balance in the crystal structure is disturbed due to the application of mechanical stress (stretching or compression) or vibrations. This disturbance results in an unbalanced distribution of charge, contributing to a surface charge density that is able to be captured by the electrode [44]. In this process, mechanical energy is converted to electric energy [45]. Conversely, the conversion of electrical energy into mechanical energy is an inverse piezoelectric effect. The whole process can be described as applying an electric field of a certain magnitude to the piezoelectric material. This effect causes a mechanical displacement within the material. The intrinsic equation of the two effects can be expressed as bellow [41]: Inverse Effect : where T stands for the stress, d stands for the piezoelectric constant, S stands for the strain, D stands for the electric displacement, E stands for the electric field intensity, s stands for the mechanical flexibility, and ε stands for the dielectric constant of the material. The four piezoelectric coefficients d ij , e ij , g ij , h ij are expressed as [46]: these two terms after the equal sign correspond to direct and inverse piezoelectric effects, respectively.
these two terms after the equal sign correspond to direct and inverse piezoelectric effects, respectively. Taking 2D TMDs as an example, the 2H phase structure has the symmetry of 3ℎ space group. The in-plane and out-of-plane piezoelectric coefficients 11 , 31 and the elastic stiffness coefficients 11 , 31 satisfy the following relation [ Piezoelectric energy conversion and collection devices usually work with a direct piezoelectric effect, and the types are mainly the 33, 11 and 31 modes; these are shown in Figure 2a-c. In the 33 and 11 modes, the applied stress is in the same direction as the generated voltage, while in the 31 mode the applied stress is axial, and the voltage is vertical. The piezoelectric output is under the control of the operation mode. The 33 mode has excellent voltage output, while a superior manifestation aspect to high current output is found in the 31 mode [49,50]. On the other hand, the inverse piezoelectric effect is put into use in piezoelectric actuators, as shown in Figure 2d.
In various polarization mechanisms [51,52], it is common that the layer thickness effect and odd-even effect are reported [53]. As a matter of fact, they are attributed to changes in interlayer coupling and symmetry. Even layers samples have inversion sym- Taking 2D TMDs as an example, the 2H phase structure has the symmetry of D 3h space group. The in-plane and out-of-plane piezoelectric coefficients d 11 , d 31 and the elastic stiffness coefficients c 11 , c 31 satisfy the following relation [47]: With regard to 2D materials with C 2v space group symmetry, this relationship becomes [48]: Piezoelectric energy conversion and collection devices usually work with a direct piezoelectric effect, and the types are mainly the 33, 11 and 31 modes; these are shown in Figure 2a-c. In the 33 and 11 modes, the applied stress is in the same direction as the generated voltage, while in the 31 mode the applied stress is axial, and the voltage is vertical. The piezoelectric output is under the control of the operation mode. The 33 mode has excellent voltage output, while a superior manifestation aspect to high current output is found in the 31 mode [49,50]. On the other hand, the inverse piezoelectric effect is put into use in piezoelectric actuators, as shown in Figure 2d.
In various polarization mechanisms [51,52], it is common that the layer thickness effect and odd-even effect are reported [53]. As a matter of fact, they are attributed to changes in interlayer coupling and symmetry. Even layers samples have inversion symmetry, while odd layers samples destroy that. For example, direct and inverse piezoelectric effects have been experimentally confirmed in a single and few layers of MoS 2 . Nevertheless, in-plane piezoelectricity only exists in odd layers, and decreases rapidly with increasing number of layers. The reason for the phenomenon is that the response of the opposite direction layers is cancelled out. Meanwhile, any strain or electric field perpendicular to its surface will theoretically produce a zero piezoelectric response [30].

Piezoelectric Thin Films
The single layer of piezoelectric film is prepared experimentally, and the piezoelectricity constant is enhanced in the 2D limit [29,32,33]. These results and data mean the nano-scale ultrathin piezoelectric films are not only limited to theoretical calculation, but also signal a new era in the field of piezoelectric research. Tables 1 and 3 show the piezoelectric coefficients of 2D piezoelectric film measured under the inverse piezoelectric effect and the direct piezoelectric effect, respectively. We find that several of these materials, such as ZnO, MoS 2 and Zr 2 P 2 BrCl, show strong anisotropic piezoelectric polarization effects. This property is useful in that can be used to achieve different energy conversion effects by applying stretching or compression effects in different directions. In order to screen, distinguish and store different energy signals, it is often necessary to use anisotropic materials to design devices.

Ultrathin In-Plane Piezoelectric Films
With the development of energy equipment towards miniaturization, the critical scale of transducers needs to be reduced. The scale of ultrathin films in this review is defined to the nanometer scale. Most 2D materials have intrinsic piezoelectric polarization. We introduce the piezoelectric properties of TMDs, h-BN and black phosphorus (BP). A schematic of each of these structures is shown in Figure 3.
Ultrathin piezoelectric films are expected to be widely used as a result of their stresselectric conversion characteristics. Graphene has the longest history of study among 2D materials [103][104][105], and therefore was the material in which researchers initially tried to achieve the piezoelectric effect. Grounding the piezoelectric mechanism, the effect calls for the breaking of central symmetry. Swapnil Chandratre and Pradeep Sharma achieved the piezoelectric effect in 2D materials for the first time by digging triangular holes in graphene [106]. Shortly after, Mitchell T. Ong et al. chemically modified graphene with hydrogen and fluorine adsorbed on different position, and induced both in-plane and out-of-plane piezoelectric effects [107], as shown in Figure 4.
Ultrathin nanosheets of layered TMDs have tunable electronic structures, so as to be attractive for piezoelectric energy harvesting. TMDs can be represented as MX 2 (M = Cr, Mo, W, Nb, Ta and X = S, Se, Te) [64]. Monolayer TMDs are constructed by a metal plane surrounded by two dichalcogenide planes [108]. Hexagonal structured monolayer TMDs are usually semiconductors and belong to D 3h space group, the breaking of whose symmetry leads to piezoelectricity [109]. Duerloo et al. first predicted that 2D TMD materials are piezoelectric by the first-principles method [66]. MoS 2 is the most widely studied TMD and it is worth going into more detail on. Monolayer MoS 2 generates intrinsic electric dipoles due to the displacement of cationic Mo atoms and anionic S atoms when subjected to external strains. Additionally, the inherent electric dipoles lead to a voltage between the two sides of MoS 2 . The piezoelectric property of monolayer MoS 2 is only restricted to the in-plane (d 11 ) direction rather than the out-of-plane (d 33 ) direction, since the symmetry feature along the vertical z-axis of monolayer MoS 2 stays the same as in the initial state. Furthermore, Wu [71] and Zhu [33] et al. experimentally realized piezoelectricity in monolayer MoS 2 . Zhu et al. measured a piezoelectric coefficient of e 11 = 2.9 × 10 -10 C m −1 in a free-standing single layer of MoS 2 . Additionally, they observed a finite and zero piezoelectric response in odd and even numbers of layers, respectively, which is in sharp contrast to bulk piezoelectric materials [33]. This is because the intrinsic difference of the polarization mechanism between thin film and bulk material relies on the difference of anisotropy. The measurement results are shown in Figure 5b. This is because symmetry is broken only in odd layers, while it is restored in even layers. The measurement method is shown in Figure 5a. The MoS 2 film was indented with a scanning 9 of 24 atomic force microscopy (AFM) probe, aiming to convert the in-plane stress to an out-of-plane force. The induced stress then changed the load on the tip and the curvature of the cantilever, which could be measured by the deflection of a laser beam [33]. Additionally, in the study of Wu et al., as shown in Figure 6, MoS 2 was placed on a polyethylene terephthalate (PET) flexible substrate [71]. Uniaxial strain was applied to the MoS 2 in the manner of mechanically bending the substrate. Therefore, periodic stretching and releasing of the substrate can generate piezoelectric outputs in external circuits with alternating polarity [71]. However, in principle, the exfoliated monolayer MoS 2 lacks mechanical durability. Ju-Hyuck Lee et al. fabricated bilayer WSe 2 with a mechanical durability of up to 0.95% of strain via turbostratic stacking [ Figure 7a]. This form can increase the degrees of freedom in the bilayer symmetry and finally lead to non-centrosymmetry in the bilayers [ Figure 7b] [72,110]. The density function theory (DFT) simulation results are shown in Figure 7c.

Ultrathin In-Plane Piezoelectric Films
With the development of energy equipment towards miniaturization, the critic scale of transducers needs to be reduced. The scale of ultrathin films in this review is d fined to the nanometer scale. Most 2D materials have intrinsic piezoelectric polarizatio We introduce the piezoelectric properties of TMDs, h-BN and black phosphorus (BP). schematic of each of these structures is shown in Figure 3.  materials [103][104][105], and therefore was the material in which researchers initially tried to achieve the piezoelectric effect. Grounding the piezoelectric mechanism, the effect calls for the breaking of central symmetry. Swapnil Chandratre and Pradeep Sharma achieved the piezoelectric effect in 2D materials for the first time by digging triangular holes in graphene [106]. Shortly after, Mitchell T. Ong et al. chemically modified graphene with hydrogen and fluorine adsorbed on different position, and induced both in-plane and outof-plane piezoelectric effects [107], as shown in Figure 4.  [107]. Reprinted with permission from [107]. Copyright 2013 American Chemical Society.
Ultrathin nanosheets of layered TMDs have tunable electronic structures, so as to be attractive for piezoelectric energy harvesting. TMDs can be represented as MX2 (M = Cr, Mo, W, Nb, Ta and X = S, Se, Te) [64]. Monolayer TMDs are constructed by a metal plane surrounded by two dichalcogenide planes [108]. Hexagonal structured monolayer TMDs are usually semiconductors and belong to 3ℎ space group, the breaking of whose symmetry leads to piezoelectricity [109]. Duerloo et al. first predicted that 2D TMD materials are piezoelectric by the first-principles method [66]. MoS2 is the most widely studied TMD and it is worth going into more detail on. Monolayer MoS2 generates intrinsic electric dipoles due to the displacement of cationic Mo atoms and anionic S atoms when subjected to external strains. Additionally, the inherent electric dipoles lead to a voltage between the two sides of MoS2. The piezoelectric property of monolayer MoS2 is only restricted to the in-plane ( 11 ) direction rather than the out-of-plane ( 33 ) direction, since the symmetry feature along the vertical z-axis of monolayer MoS2 stays the same as in the initial state. Furthermore, Wu [71] and Zhu [33] et al. experimentally realized piezoelectricity in monolayer MoS2. Zhu et al. measured a piezoelectric coefficient of 11 = 2.9 × 10 -10 C m −1 in a free-standing single layer of MoS2. Additionally, they observed a finite and zero piezoelectric response in odd and even numbers of layers, respectively, which is in sharp contrast to bulk piezoelectric materials [33]. This is because the intrinsic difference of the polarization mechanism between thin film and bulk material relies on the difference of anisotropy. The measurement results are shown in Figure 5b. This is because symmetry is broken only in odd layers, while it is restored in even layers. The measurement method is shown in Figure 5a. The MoS2 film was indented with a scanning atomic force microscopy (AFM) probe, aiming to convert the in-plane stress to an out-of-plane force. The induced stress then changed the load on the tip and the curvature of the cantilever, which could be measured by the deflection of a laser beam [33]. Additionally, in the study of Wu et al., as shown in Figure 6, MoS2 was placed on a polyethylene terephthalate (PET) flexible substrate [71]. Uniaxial strain was applied to the MoS2 in the manner of mechanically bending the substrate. Therefore, periodic stretching and releasing of the substrate can generate piezoelectric outputs in external circuits with alternating polarity [71]. However, in principle,  The AFM probe changes the fold degree of the film and deflects the laser beam, so that the stress can be obtained through the load change on the cantilever beam [33]. Reproduced with permission from [33]. Copyright 2014, Springer Nature. (b) The measured piezoelectric response of MoS2 is odd-even dependent as the number of layers increases [33]. Reproduced with permission from [33]. Copyright 2014, Springer Nature.  The AFM probe changes the fold degree of the film and deflects the laser beam, so that the stress can be obtained through the load change on the cantilever beam [33]. Reproduced with permission from [33]. Copyright 2014, Springer Nature. (b) The measured piezoelectric response of MoS 2 is odd-even dependent as the number of layers increases [33]. Reproduced with permission from [33]. Copyright 2014, Springer Nature.
There has been a boom in research about in-plane piezoelectric 2D materials. Yang et al. calculated the piezoelectric coefficient of monolayer MX (M = Sn or Ge, X = Se or S) and found that the in-plane piezoelectric coefficient d 11 of MX was the biggest among 2D materials [68]. This result is two orders of magnitude larger than that of routinely used piezoelectric semiconductors, such as ZnO and monolayer MoS 2 . This is because monolayer group IV monochalcogenides have a unique "puckered" C 2v symmetry, as well as electronic structure.
Monolayer h-BN is a wide-band insulator with the same crystal structure as graphene. Unlike graphene, monolayer h-BN is piezoelectric due to the asymmetry introduced into the structure by the two sublattices (B and N). The elastic and piezoelectric effects were theoretically calculated using Born's long-wavelength theory [111]. In 2020, Yang Nan et al. studied hexagonal boron nitride nanosheets through molecular dynamics simulations. Their simulation results indicated that the piezoelectric constants of boron nitride nanosheets depend very strongly on the macroscopic shape (Figure 8), while being nearly independent of the macroscopic size ( Figure 9) [101]. In addition, when strain is applied to h-BN, some novel quantum effects, such as the quantum spin Hall phase [112], are born. These novel quantum effects, when combined with piezoelectric polarization effects, are bound to prompt researchers to ponder more interesting scientific questions. Figure 5. (a) The schematic diagram of a device for measuring the in-plane piezoelectric stress. Two hydrogen silsesquioxane (HSQ) columns are under the MoS2 film, which are on a SiO2/Si substrate with an Au electrode on the upper side. The AFM probe changes the fold degree of the film and deflects the laser beam, so that the stress can be obtained through the load change on the cantilever beam [33]. Reproduced with permission from [33]. Copyright 2014, Springer Nature. (b) The measured piezoelectric response of MoS2 is odd-even dependent as the number of layers increases [33]. Reproduced with permission from [33]. Copyright 2014, Springer Nature.  There has been a boom in research about in-plane piezoelectric 2D materials. Yang et al. calculated the piezoelectric coefficient of monolayer MX (M = Sn or Ge, X = Se or S) and found that the in-plane piezoelectric coefficient d11 of MX was the biggest among 2D materials [68]. This result is two orders of magnitude larger than that of routinely used piezoelectric semiconductors, such as ZnO and monolayer MoS2. This is because monolayer group IV monochalcogenides have a unique "puckered" symmetry, as well as elec-     Black phosphorus (BP) is a layered material that can be obtained by mechanical stripping. The piezoelectric properties of BP are demonstrated due to its non-centrosymmetric crystal structure [113]. In addition, surface-oxidization is an effective way to enhance the piezoelectricity of black phosphorene, since this method can break the structural symmetry. By first-principles methods, the calculated piezoelectric coefficients d 11 for surface-oxidized BP are manifested as 88.54 pm V −1 , which is comparable to those of group-IV monochalcogenides and larger than that of 2D h-BN and MoS 2 [77]. However, this method of stripping or traditional chemical and physical vapor deposition can likely introduce defects or other impurities, which can interfere with the results of the study. It is worth further consideration by researchers.
There are many kinds of means to control the properties of 2D materials, such as substitutional doping, staking, surface adatoms, or defects to break the centrosymmetry of pristine materials [114]. However, these strategies of regulating mainly affect the outof-plane piezoelectric properties, which will be discussed later. The in-plane piezoelectric properties are little affected by these methods in principle, since the intrinsic asymmetry of materials is the factor most important to in-plane piezoelectric properties. Unconventionally, in 2022, Park et al. induced high levels of piezoelectricity in centrosymmetric oxides [26]. They used a DC electric field to rearrange the oxygen vacancies in the CeO 2-x (CGO) thin film. The crystallographic symmetry was broken and piezoelectric effects were induced in centrosymmetric materials. The piezoelectricity is measured by applying an additional alternating current (AC) electric field (E AC ). When the AC electric field is in the same direction as DC, oxygen vacancies are pushed up, which causes the material to expand. Conversely, when the AC electric field is in the opposite direction to DC, oxygen vacancies are pushed oppositely [27]. Their results show that the piezoelectric coefficients (d 33 ) can reach up to nearly 200,000 pm V −1 at a frequency of 10 mHz under a DC electric field of 1 MV cm −1 . This data is particularly significant, being two orders of magnitude larger than the ferroelectric oxide. They also discovered that the field-induced redistribution of oxygen vacancies could induce a cubic-to-tetragonal structural transition [26], as shown in Figure 10b. This inspired people to apply this method to more materials or look for new mechanisms to generate piezoelectricity. changes of different size boron nitride nanosheets when strained along the armchair direction and (c) zigzag direction [101]. Reprinted with permission from [101]. Copyright 2018 Royal Society of Chemistry.
Black phosphorus (BP) is a layered material that can be obtained by mechanical stripping. The piezoelectric properties of BP are demonstrated due to its non-centrosymmetric crystal structure [113]. In addition, surface-oxidization is an effective way to enhance the piezoelectricity of black phosphorene, since this method can break the structural symmetry. By first-principles methods, the calculated piezoelectric coefficients 11 for surface-oxidized BP are manifested as 88.54 pm V −1 , which is comparable to those of group-IV monochalcogenides and larger than that of 2D h-BN and MoS2 [77]. However, this method of stripping or traditional chemical and physical vapor deposition can likely introduce defects or other impurities, which can interfere with the results of the study. It is worth further consideration by researchers.
There are many kinds of means to control the properties of 2D materials, such as substitutional doping, staking, surface adatoms, or defects to break the centrosymmetry of pristine materials [114]. However, these strategies of regulating mainly affect the outof-plane piezoelectric properties, which will be discussed later. The in-plane piezoelectric properties are little affected by these methods in principle, since the intrinsic asymmetry of materials is the factor most important to in-plane piezoelectric properties. Unconventionally, in 2022, Park et al. induced high levels of piezoelectricity in centrosymmetric oxides [26]. They used a DC electric field to rearrange the oxygen vacancies in the CeO2-x (CGO) thin film. The crystallographic symmetry was broken and piezoelectric effects were induced in centrosymmetric materials. The piezoelectricity is measured by applying an additional alternating current (AC) electric field (EAC). When the AC electric field is in the same direction as DC, oxygen vacancies are pushed up, which causes the material to expand. Conversely, when the AC electric field is in the opposite direction to DC, oxygen vacancies are pushed oppositely [27]. Their results show that the piezoelectric coefficients ( 33 ) can reach up to nearly 200,000 pm V −1 at a frequency of 10 mHz under a DC electric field of 1 MV cm −1 . This data is particularly significant, being two orders of magnitude larger than the ferroelectric oxide. They also discovered that the field-induced redistribution of oxygen vacancies could induce a cubic-to-tetragonal structural transition [26], as shown in Figure 10b. This inspired people to apply this method to more materials or look for new mechanisms to generate piezoelectricity.  [26]. From [26]. Reprinted with permission from AAAS. (b) The phase transition of CGO from cubic to tetragonal phase under an EDC [26]. From [26]. Reprinted with permission from AAAS.  [26]. From [26]. Reprinted with permission from AAAS. (b) The phase transition of CGO from cubic to tetragonal phase under an EDC [26]. From [26]. Reprinted with permission from AAAS.

Ultrathin Out-of-Plane Piezoelectric Films
With the systematic study of piezoelectricity in 2D materials, a variety of correlated novel piezoelectric devices have been successively fabricated in the field of energy harvesting, actuators, strain-tuned electronics, and optoelectronics. Most 2D materials, as shown in Figure 11a,b, have only in-plane piezoelectricity, which limits their applications in vertically integrated nanoelectromechanical systems [64]. Due to the limitation of external conditions and structural stability, it is difficult to make advanced functional devices with piezoelectric materials. Herein, Table 2 shows that the piezoelectric properties of common materials in different polarization directions are experimentally confirmed [54,115]. If 2D materials can obtain large out-of-plane piezoelectricity, they will be more widely used in a variety of piezoelectric applications. It has stimulated more and more research on innovative regulatory means, such as doping, construction of Janus structures and so on, because of the puzzle of how to obtain out-of-plane polarization.

Ultrathin Out-Of-Plane Piezoelectric Films
With the systematic study of piezoelectricity in 2D materials, a variety of correlated novel piezoelectric devices have been successively fabricated in the field of energy harvesting, actuators, strain-tuned electronics, and optoelectronics. Most 2D materials, as shown in Figure 11a,b, have only in-plane piezoelectricity, which limits their applications in vertically integrated nanoelectromechanical systems [64]. Due to the limitation of external conditions and structural stability, it is difficult to make advanced functional devices with piezoelectric materials. Herein, Table 3 shows that the piezoelectric properties of common materials in different polarization directions are experimentally confirmed [54,115]. If 2D materials can obtain large out-of-plane piezoelectricity, they will be more widely used in a variety of piezoelectric applications. It has stimulated more and more research on innovative regulatory means, such as doping, construction of Janus structures and so on, because of the puzzle of how to obtain out-of-plane polarization. Figure 11. (a,b) The piezoelectric coefficients of some common two-dimensional (2D) materials [54]. Reproduced with permission from [54]. Copyright 2018, Springer Nature. Table 3. Relevant information of the experimentally confirmed piezoelectric 2D materials [54]. Reproduced with permission from [54]. Copyright 2018, Springer Nature.

Out-Of-Plane Piezoelectricity Obtained by Controlling Structures
Out-of-plane piezoelectricity is usually obtained by controlling the crystal structure of the 2D materials, such as through defect engineering, deformation or doping. TMDs are ideal candidates as low-dimensional piezoelectric materials, owing to their structural non-centrosymmetry [33]. Due to in-plane inversion symmetry breaking, it was proved through experimental characterization and theoretical calculation that only intrinsic in- Figure 11. (a,b) The piezoelectric coefficients of some common two-dimensional (2D) materials [54]. Reproduced with permission from [54]. Copyright 2018, Springer Nature. Table 2. Relevant information of the experimentally confirmed piezoelectric 2D materials [54]. Reproduced with permission from [54]. Copyright 2018, Springer Nature.

Out-of-Plane Piezoelectricity Obtained by Controlling Structures
Out-of-plane piezoelectricity is usually obtained by controlling the crystal structure of the 2D materials, such as through defect engineering, deformation or doping. TMDs are ideal candidates as low-dimensional piezoelectric materials, owing to their structural non-centrosymmetry [33]. Due to in-plane inversion symmetry breaking, it was proved through experimental characterization and theoretical calculation that only intrinsic inplane piezoelectricity exists in TMDs [64,66,117]. From this, breaking inversion symmetry in an out-of-plane direction is an efficient method to realize the out-of-plane piezoelectric polarization [115]. It is noted that the layered TMDs have the natural advantage of being established into out-of-plane piezoelectric samples through crystal structure transformation.

Deformation
While only in-plane piezoelectricity exists on the crystal symmetry, it is also possible to generate out-of-plane piezoelectricity by way of bringing strain to the gradient. The schematic diagram of different degrees of corrugation is shown in Figure 12a. As long as the 2D TMDs deform along the out-of-plane direction, the internal flexoelectricity polarization occurs along this axial direction. In this sense, the out-of-plane piezoelectricity of 2H phase MoTe 2 can be modulated by either controlling the roughness of the substrate or the thickness of MoTe 2 sample [118], as shown in Figure 12b.
polarization [115]. It is noted that the layered TMDs have the natural advantage of being established into out-of-plane piezoelectric samples through crystal structure transformation.

Deformation
While only in-plane piezoelectricity exists on the crystal symmetry, it is also possible to generate out-of-plane piezoelectricity by way of bringing strain to the gradient. The schematic diagram of different degrees of corrugation is shown in Figure 12a. As long as the 2D TMDs deform along the out-of-plane direction, the internal flexoelectricity polarization occurs along this axial direction. In this sense, the out-of-plane piezoelectricity of 2H phase MoTe2 can be modulated by either controlling the roughness of the substrate or the thickness of MoTe2 sample [118], as shown in Figure 12b.  [118]. Reprinted with permission from [118]. Copyright 2018 American Chemical Society. (b) The deformation of MoTe2 film driven by Au fold was observed by atomic force microscopy (AFM). Waveform diagram of band excitation (BE) with Vac. The relationship between the amplitude of piezoelectric response force microscope (PFM) and the magnitude of alternating current (AC) voltage [118]. Reprinted with permission from [118]. Copyright 2018 American Chemical Society.

Defect Engineering
Studies to date have demonstrated the possibility of generating an out-of-plane piezoelectric property in TMDs through defect engineering [114]. Still, it is a challenge to effectively modulate surface corrugation due to the limited modulation of substrate roughness or thickness. Thus comes the question of whether flexoelectricity can be generated at atomic scales or not. If so, out-of-plane piezoelectricity will be generated regardless of the roughness or thickness. The surface vacancy of a Te atom is generated by the thermal annealing process, which contributes to greater surface curvature and thus produces greater out-of-plane piezoelectricity [119].
Furthermore, out-of-plane piezoelectricity in TMDs with layered structure can be formed by using ion beams to engineer the defects. For instance, the formation of Te defects caused by helium ion beam irradiation on the multilayer MoTe2 actually leads to the out-of-plane piezoelectricity [120].

Doping
Chemical doping is also a new choice for obtaining out-of-plane piezoelectricity. Growing the material on the substrate can achieve this aim experimentally. The growth of graphene on SiO2 substrate causes the chemical interaction between graphene and SiO2 surface, thus realizing periodic doping. This interaction can induce band gap opening and dipole moment polarization of the graphene layer. Hence, it grants the material out-of-  [118]. Reprinted with permission from [118]. Copyright 2018 American Chemical Society. (b) The deformation of MoTe 2 film driven by Au fold was observed by atomic force microscopy (AFM). Waveform diagram of band excitation (BE) with Vac. The relationship between the amplitude of piezoelectric response force microscope (PFM) and the magnitude of alternating current (AC) voltage [118]. Reprinted with permission from [118]. Copyright 2018 American Chemical Society.

Defect Engineering
Studies to date have demonstrated the possibility of generating an out-of-plane piezoelectric property in TMDs through defect engineering [114]. Still, it is a challenge to effectively modulate surface corrugation due to the limited modulation of substrate roughness or thickness. Thus comes the question of whether flexoelectricity can be generated at atomic scales or not. If so, out-of-plane piezoelectricity will be generated regardless of the roughness or thickness. The surface vacancy of a Te atom is generated by the thermal annealing process, which contributes to greater surface curvature and thus produces greater out-of-plane piezoelectricity [119].
Furthermore, out-of-plane piezoelectricity in TMDs with layered structure can be formed by using ion beams to engineer the defects. For instance, the formation of Te defects caused by helium ion beam irradiation on the multilayer MoTe 2 actually leads to the out-of-plane piezoelectricity [120].

Doping
Chemical doping is also a new choice for obtaining out-of-plane piezoelectricity. Growing the material on the substrate can achieve this aim experimentally. The growth of graphene on SiO 2 substrate causes the chemical interaction between graphene and SiO 2 surface, thus realizing periodic doping. This interaction can induce band gap opening and dipole moment polarization of the graphene layer. Hence, it grants the material out-ofplane piezoelectricity [55]. In electronics manufacturing and energy harvesting applications, it is possible to use piezoelectric effects along the vertical direction in 2D TMDs.

Out-of-Plane Piezoelectricity in Janus Structures Conventional TMDs Janus Structures
Another atomic-scale approach has been proposed to induce out-of-plane piezoelectricity through constructing asymmetric TMD monolayers, known as conventional Janus structures. In addition to the most common MoSSe [65], NiXY (X/Y = Cl, Br, I; X = Y) is also an example. The structure diagram is shown in Figure 13a-c. In this regard, NiXY is a novel piezoelectric ferromagnetic material with these two unique properties. These Janus monolayer materials, such as NiClBr and NiBrI, have strong in-plane as well as out-of-plane piezoelectricity. It is found that NiClI is a ferrovalley material with magnetic anisotropy, which is an ideal material for ultrathin piezoelectric devices [121].

Conventional TMDs Janus Structures
Another atomic-scale approach has been proposed to induce out-of-plane piezoelectricity through constructing asymmetric TMD monolayers, known as conventional Janus structures. In addition to the most common MoSSe [65], NiXY (X/Y = Cl, Br, I; X≠Y) is also an example. The structure diagram is shown in Figure 13a-c. In this regard, NiXY is a novel piezoelectric ferromagnetic material with these two unique properties. These Janus monolayer materials, such as NiClBr and NiBrI, have strong in-plane as well as out-ofplane piezoelectricity. It is found that NiClI is a ferrovalley material with magnetic anisotropy, which is an ideal material for ultrathin piezoelectric devices [121]. Reprinted from [121], with the permission of AIP Publishing. (c) Side view of NiClI. Reprinted from [121], with the permission of AIP Publishing. (d) Top view of the GeP-GaS monolayer. Reprinted from [123], copyright 2020, with permission from Elsevier. (e) Side view of the GeP-GaS monolayer. Reprinted from [123], copyright 2020, with permission from Elsevier.

Janus Derivatives
As a matter of fact, Janus can not only be based on TMDs, but also be built in typical single-layer structures with hexagonal primitive cells or a buckled honeycomb monolayer. A series of Janus derivatives are obtained by substituting atoms (GeP-GaS, SiP-AlS, and SnP-InS). The structure diagram is shown in Figure 13d,e. This type of structure lacks mirror symmetry, leading to a large out-of-plane piezoelectricity [123]. Single-layer SiN-GaS also exhibits considerable out-of-plane piezoelectricity [124]. Janus M2SeX (M = Ge, Sn; X = S, Te) have both large in-plane piezoelectric coefficients 11 (up to 345.08 pm/V) and out-of-plane piezoelectric coefficients 31 (up to 3.83 pm/V) [125]. As well as the larger surface piezoelectricity, Janus derivatives can also combine a variety of properties to offer opportunities for the manufacture of multifunctional devices. The research of the Janus structure of X2PAs monolayers shows that the out-of-plane piezoelectricity and flexible properties play an important part in improving the performance of multifunctional Reprinted from [122], copyright 2019, with permission from Elsevier. (b) Top view of NiClI. Reprinted from [121], with the permission of AIP Publishing. (c) Side view of NiClI. Reprinted from [121], with the permission of AIP Publishing. (d) Top view of the GeP-GaS monolayer. Reprinted from [123], copyright 2020, with permission from Elsevier. (e) Side view of the GeP-GaS monolayer. Reprinted from [123], copyright 2020, with permission from Elsevier.

Janus Derivatives
As a matter of fact, Janus can not only be based on TMDs, but also be built in typical single-layer structures with hexagonal primitive cells or a buckled honeycomb monolayer. A series of Janus derivatives are obtained by substituting atoms (GeP-GaS, SiP-AlS, and SnP-InS). The structure diagram is shown in Figure 13d,e. This type of structure lacks mirror symmetry, leading to a large out-of-plane piezoelectricity [123]. Single-layer SiN-GaS also exhibits considerable out-of-plane piezoelectricity [124]. Janus M 2 SeX (M = Ge, Sn; X = S, Te) have both large in-plane piezoelectric coefficients d 11 (up to 345.08 pm/V) and out-of-plane piezoelectric coefficients d 31 (up to 3.83 pm/V) [125]. As well as the larger surface piezoelectricity, Janus derivatives can also combine a variety of properties to offer opportunities for the manufacture of multifunctional devices. The research of the Janus structure of X 2 PAs monolayers shows that the out-of-plane piezoelectricity and flexible properties play an important part in improving the performance of multifunctional sensing and controlling of nanodevices [126]. The piezoelectric ferromagnetism material Fe 2 IX (X = Cl and Br) combines piezoelectricity, topological properties and ferromagnetism orders. In this sense, it provides a potential platform for multi-functional spintronic devices with a large gap and high T C (429 K) [127].
Janus materials also have tribo-piezoelectricity. The sliding of the in-plane interlayer leads to a remarkable enhancement in the vertical piezoelectricity aspect. In the process of Janus bilayer sliding, the tribological energy will convert into electrical energy. Hence, it brings a new perspective to creating new piezoelectric nanogenerators [122]. The construction of a Janus structure can fully demonstrate the modification of traditional 2D ultrathin films.

Out-of-Plane Piezoelectricity in Multi-Element Transition Metal Materials
If simple Janus can achieve structure asymmetry, can more complex structures do the same? The answer is, of course, yes. In 2D materials, a quantity of three-and fourmembered compound monolayers are non-centrosymmetric, such as MXenes, lithiumbased ternary chalcogenides and so on. A schematic of each of these structures is shown in Figure 14. Due to the uneven charge distribution caused by non-mirror symmetry, these multi-element transition metal materials have strong out of plane piezoelectricity.
spintronic devices with a large gap and high TC (429 K) [127].
Janus materials also have tribo-piezoelectricity. The sliding of the in-plane interlayer leads to a remarkable enhancement in the vertical piezoelectricity aspect. In the process of Janus bilayer sliding, the tribological energy will convert into electrical energy. Hence, it brings a new perspective to creating new piezoelectric nanogenerators [122]. The construction of a Janus structure can fully demonstrate the modification of traditional 2D ultrathin films.

Out-of-Plane Piezoelectricity in Multi-Element Transition Metal Materials
If simple Janus can achieve structure asymmetry, can more complex structures do the same? The answer is, of course, yes. In 2D materials, a quantity of three-and four-membered compound monolayers are non-centrosymmetric, such as MXenes, lithium-based ternary chalcogenides and so on. A schematic of each of these structures is shown in Figure 14. Due to the uneven charge distribution caused by non-mirror symmetry, these multi-element transition metal materials have strong out of plane piezoelectricity. Figure 14. The schematic diagram of the ultrathin out-of-plane films [86,89]. (a) Top and side view of Sc2CO2 MXene. Reprinted from [86], copyright 2019, with permission from Elsevier. (b) A Sc2CO2 MXene piezoelectric cantilever. Reprinted from [86], copyright 2019, with permission from Elsevier. (c) Top view of the Zr2P2BrCl monolayer. Reprinted from [89], copyright 2023, with permission from Elsevier. (d,e) Side view of the Zr2P2BrCl monolayer from x-z plane and z-y plane. Reprinted from [89], copyright 2023, with permission from Elsevier. Strikingly, M2CO2 (M = Sc, Y, La) has an in-plane piezoelectricity comparable to 2H-MoS2 as well as extraordinary out-of-plane piezoelectricity. The piezoelectric coefficient 31 of Sc2CO2 MXene reaches 0.78 pm V −1 . At 2.5% strain, 0.1 V vertical piezoelectric voltage can only be produced in such ultrathin Sc2CO2 monolayer [86]. Similarly, BiCrX3(X = S, Se, and Te) is a kind of piezoelectric ferromagnetism material with both in-plane and out-of-plane polarization at room temperature. Additionally, this is another breakthrough in high-performance piezoelectric materials [128].
The out-of-plane piezoelectric coefficients 31 of 2D monolayer Li-based ternary chalcogenides LiMX2 (M = Al, Ga, In; X = S, Se, Te), such as LiAlSe2, LiGaTe2 and LiAlTe2, reach up to 0.61, 0.70 and 0.83 pm/V, respectively. This is owing to the unique double-buckled stacking structure of these LiMX2 monolayers [87]. Later, it was found that phase structure ( -LialS2 and -LialSe2) also has an excellent out-of-plane piezoelectric coefficient; even the number is Figure 14. The schematic diagram of the ultrathin out-of-plane films [86,89]. (a) Top and side view of Sc 2 CO 2 MXene. Reprinted from [86], copyright 2019, with permission from Elsevier. (b) A Sc 2 CO 2 MXene piezoelectric cantilever. Reprinted from [86], copyright 2019, with permission from Elsevier. (c) Top view of the Zr 2 P 2 BrCl monolayer. Reprinted from [89], copyright 2023, with permission from Elsevier. (d,e) Side view of the Zr 2 P 2 BrCl monolayer from x-z plane and z-y plane. Reprinted from [89], copyright 2023, with permission from Elsevier.
Strikingly, M 2 CO 2 (M = Sc, Y, La) has an in-plane piezoelectricity comparable to 2H-MoS 2 as well as extraordinary out-of-plane piezoelectricity. The piezoelectric coefficient d 31 of Sc 2 CO 2 MXene reaches 0.78 pm V −1 . At 2.5% strain, 0.1 V vertical piezoelectric voltage can only be produced in such ultrathin Sc 2 CO 2 monolayer [86]. Similarly, BiCrX 3 (X = S, Se, and Te) is a kind of piezoelectric ferromagnetism material with both in-plane and outof-plane polarization at room temperature. Additionally, this is another breakthrough in high-performance piezoelectric materials [128].
The out-of-plane piezoelectric coefficients d 31 of 2D monolayer Li-based ternary chalcogenides LiMX 2 (M = Al, Ga, In; X = S, Se, Te), such as LiAlSe 2 , LiGaTe 2 and LiAlTe 2 , reach up to 0.61, 0.70 and 0.83 pm/V, respectively. This is owing to the unique double-buckled stacking structure of these LiMX 2 monolayers [87]. Later, it was found that γ phase structure (γ-LialS 2 and γ-LialSe 2 ) also has an excellent out-of-plane piezoelectric coefficient; even the number is twice as high as that of β phase structure [88]. When compared with the piezoelectric coefficients of other materials as shown in Figure 15, it is evident that the γ-LiMX 2 structures are very promising materials for high-performance piezoelectric nanodevices. Zr 2 P 2 BrCl monolayer is of particular interest for its ferroelasticity, controllable anisotropic properties along the in-plane direction and outstanding out-of-plane piezoelectricity. These monolayers have a strong out-of-plane piezoelectricity since the heterogeneous charge distribution results from its broken mirror symmetry. The piezoelectric coefficient d 33 = 129.705 pm V −1 of Zr 2 P 2 BrCl monolayer is actually significant, which is two orders of magnitude higher than MoSTe multilayers. It can even be a new alternate material for the design of memory devices, robot bionic skin or multipurpose nanodevices [89]. twice as high as that of phase structure [88]. When compared with the piezoelectric coefficients of other materials as shown in Figure 15, it is evident that the -LiMX2 structures are very promising materials for high-performance piezoelectric nanodevices. Figure 15. (a) Relationship between piezoelectric coefficient d11 and polarizability of LiMX2 monolayer [87]. Reprinted from [87], copyright 2021, with permission from Elsevier. (b) Comparison of the piezoelectric coefficient 31 of LiMX2 single layer with reported piezoelectric materials [87]. Reprinted from [87], copyright 2021, with permission from Elsevier.
Zr2P2BrCl monolayer is of particular interest for its ferroelasticity, controllable anisotropic properties along the in-plane direction and outstanding out-of-plane piezoelectricity. These monolayers have a strong out-of-plane piezoelectricity since the heterogeneous charge distribution results from its broken mirror symmetry. The piezoelectric coefficient 33 = 129.705 pm V −1 of Zr2P2BrCl monolayer is actually significant, which is two orders of magnitude higher than MoSTe multilayers. It can even be a new alternate material for the design of memory devices, robot bionic skin or multipurpose nanodevices [89].
In addition to the materials mentioned above, the low-layer CuInP2S6 (CIPS) nanosheet also exhibits great performance. CIPS is a ferroelectric material with a high 33 piezoelectric Figure 15. (a) Relationship between piezoelectric coefficient d 11 and polarizability of LiMX 2 monolayer [87]. Reprinted from [87], copyright 2021, with permission from Elsevier. (b) Comparison of the piezoelectric coefficient d 31 of LiMX 2 single layer with reported piezoelectric materials [87]. Reprinted from [87], copyright 2021, with permission from Elsevier.
In addition to the materials mentioned above, the low-layer CuInP 2 S 6 (CIPS) nanosheet also exhibits great performance. CIPS is a ferroelectric material with a high d 33 piezoelectric coefficient of 17.4 pm V −1 . The data is superior to that of other reported 2D piezoelectric films to date. The intense out-of-plane piezoelectricity in the ultrathin CIPS contributes to the integration of nanoscale energy transducers, and thus eventually the production of nanogenerators [63]. A CIPS-based piezotronics device under compressive stress is shown in Figure 16. charge distribution results from its broken mirror symmetry. The piezoelectric coefficient 33 = 129.705 pm V −1 of Zr2P2BrCl monolayer is actually significant, which is two orders of magnitude higher than MoSTe multilayers. It can even be a new alternate material for the design of memory devices, robot bionic skin or multipurpose nanodevices [89].
In addition to the materials mentioned above, the low-layer CuInP2S6 (CIPS) nanosheet also exhibits great performance. CIPS is a ferroelectric material with a high 33 piezoelectric coefficient of 17.4 pm V −1 . The data is superior to that of other reported 2D piezoelectric films to date. The intense out-of-plane piezoelectricity in the ultrathin CIPS contributes to the integration of nanoscale energy transducers, and thus eventually the production of nanogenerators [63]. A CIPS-based piezotronics device under compressive stress is shown in Figure 16. Figure 16. The schematic diagram of piezoelectric device based on CuInP2S6 (CIPS) [63]. Reprinted from [63], copyright 2022, with permission from Elsevier. Figure 16. The schematic diagram of piezoelectric device based on CuInP 2 S 6 (CIPS) [63]. Reprinted from [63], copyright 2022, with permission from Elsevier.

Concluding Remarks
The intrinsic differences of ultrathin films, thick films and bulk materials regarding polarization due to gradually reduced anisotropy are explored in the first part of this review. When the thickness of material gradually decreases to several atoms thick, the effective piezoelectric polarization will enhance significantly. Secondly, several promising materials with strong anisotropy, such as ZnO, MoS 2 and Zr 2 P 2 BrCl monolayer, are screened by summarizing the piezoelectric coefficient, polarization direction and preparation quality of the ultrathin films. It is noted that there are great difficulties in the acquisition and preparation of ultrathin piezoelectric films. The available methods have inevitable shortcomings and limitations, such as introducing a large number of defects, which is also a great challenge faced by the large family of 2D materials. Thirdly, based on the traditional piezoelectric hexagonal boron nitride structure, the effect of layer thickness, strain modes and sizes on polarization, and the novel quantum effects induced by it, such as topological state, quantum Hall effect, etc., are analyzed in detail. Particularly, the polarization mechanism of a few-atom-thick material is demonstrated by taking the symmetrically polarized monolayer TMDs as an example. In addition, to illustrate the construction and modification of a 2D piezoelectric material module, the Janus structure is taken as an example. Through this review, we have found that ultrathin piezoelectric films have made rapid progress and become a promising structure in the development of miniaturized energy conversion equipment.

Conflicts of Interest:
The authors declare no conflict of interest.