Piezotronics in two‐dimensional materials

Abstract

The fascinating two‐dimensional (2D) materials are being potentially applied in various fields from science to engineering benefitting from the charming physical and chemical properties on optics, electronics, and magnetism, compared with the bulk crystal, while piezotronics is a universal and pervasive phenomenon in the materials with broking center symmetry, promoting the new field and notable achievements of piezotronics in 2D materials with higher accuracy and sensitivity. For example, 20 parts per billion of the detecting limitations in NO₂ sensor, 500 μm of spatial strain resolution in flexible devices, and 0.363 eV output voltage in nanogenerators. In this review, three categories of 2D piezotronics materials are first introduced ranging from organic to inorganic data, among which six types of 2D inorganic materials are emphasized based on the geometrical arrangement of different atoms. Then, the microscopic mechanism of carrier transport and separation in 2D piezotronic materials is highlighted, accompanied with the presentation of four measured methods. Subsequently, the developed applications of 2D piezotronics are discussed comprehensively including different kinds of sensors, piezo‐catalysis, nanogenerators and information storage. Ultimately, we suggest the challenges and provide the ideas for qualitative–quantitative research of microscopic mechanism and large‐scale integrated applications of 2D piezotronics.

INTRODUCTION

Piezotronics, regulating the carrier transport by piezo-potential,^1–4 has received significant attentions and substantial expansion for intrinsic applications in piezoelectric, piezophototronics, and flexible devices due to the universality and pervasiveness of internal and external strain in the devices and non-center symmetrical semiconductors.^5–12 Simultaneously, two-dimensional (2D) materials were widely explored on optics,^13–24 electronics,^25–33 and magnetism.^34–41 since the first discovery of single atomic thickness graphene. Thus, piezotronics based on 2D materials was presented and developed rapidly from microscopic mechanism, various materials to the practical applications on nanogenerators, sensors, catalysis, information storage, and other flexible devices.^42–54

The entrance of piezotronics from 2D materials was opened thoroughly since the first experimental confirmation from 2D MoS₂ by Hone and Wang groups in 2014, where piezo-voltage and output current were detected only from MoS₂ flakes with odd number of atomic layers.³¹ Then, piezotronics based on 2D materials has been grown into forest since the expansion from black phosphorus (BP),^55–59 hexagonal boron nitride (h-BN),^60–63 transition metal dichalcogenide (TMDCs) and Janus TMDCs,^63–72 II-VI semiconductors,^73–79 III-V,^80–88 Group-III monochalcogenides (III-VI),^89,90 and Janus III-VI,^91,92 Group-IV monochalcogenides (IV-VI),^93–97 Group-V (V-V),⁹⁸ and so on.^99–109 At the same time, the variety of applications of piezotronics from 2D materials were achieved on flexible optoelectronics, piezotronic catalysis, biological medicine, information storage, and so forth.

Herein, we introduce three categories of 2D materials for piezotronics at first, where six kinds of inorganic crystals are distinguished according to the composition and structure of monolayer materials. Then, the reported methods for detecting piezotronic signal and mechanism of 2D piezotronics are concluded and discussed. Additionally, the applications, evolvement, and presented problems are revealed and proposed to promote the advancement of various sensors, nanogenerators, piezo-catalysis and information storage. Finally, the challenges and visions are furnished to expand the depth and breadth of 2D piezotronics. This review is aimed at providing a top view of the development, problem, and prospect of piezotronics in 2D materials.

2D MATERIALS FOR PIEZOTRONICS

Piezotronics is presented in the materials with centrosymmetric breaking, where the centers of gravity from the cations and anions are not coincided when applied an external strain, leading to the generation of piezo-potential in the interface between the semiconductors and the metals. The piezo-potential will modulate the Schottky barrier height (SBH) and output current passing through the interface, resulting in the enhanced or weakened sensitivity factor.⁷⁵ Therefore, 2D materials with non-center symmetry structure have the potential for exploiting piezotronics. We divide the reported 2D materials for piezotronics into three categories: 2D inorganic materials with non-center symmetry structure originally, modulated 2D materials who possess center symmetry structure originally, and 2D organic perovskites.

2D inorganic materials

Reed group calculated and predicted the piezotronic monolayer materials among 1173 2D layer materials from the materials project database with more than 50 000 inorganic crystals.¹⁰⁶ 325 monolayer 2D crystals were identified to be potential 2D piezotronic materials, which lack the center symmetry structure originally. And the reported 2D monolayer piezotronic materials currently can be classified into six categories according to the crystal structure, containing TMDCs and Janus TMDCs, II-VI, III-V, III-VI and Janus III-VI, IV-VI, and V-V (Figures 1 and 2).

FIGURE 1. Piezoelectric coefficients and crystal structures of monolayer TMDCs and Janus TMDCs (A), II-VI (B), III-V (C), III-VI and Janus III-VI (D), IV-VI (E), V-V (F)

FIGURE 2. Piezoelectric coefficients and bandgap of monolayer TMDCs (A), II-VI (B), III-V (C,D), III-VI and Janus III-VI (E,F), IV-VI (G,H), V-V (I)

TMDCs and Janus TMDCs

Monolayer TMDCs are usually marked as MX₂, where M represents transition metal elements and X indicates chalcogen atoms.^63–65 Monolayer piezotronic TMDCs have the hexagonal crystal structure and D_3h symmetry with the space group $p\overline{6}m2$.^64,71,110 Density functional theory was calculated and the experiment was performed to confirm the piezoelectric coefficient and bandgap of TMDCs to be the range of 2.12–13.54 pm V⁻¹,^63–65,70 and 0.52–2.1 eV,^8,65,66,111 respectively (Figure 2A). Meanwhile, the piezoelectric coefficient of TMDCs is increased with increasing the atomic number of chalcogen atoms or decreasing the atomic number of transition metal atoms.⁶⁴

Janus TMDCs are used to be presented as MXY, where X ≠ Y, M is transition metal elements, X and Y indicate chalcogen atoms.^67,71,112 For different atomic radius and electronegativities of X and Y atoms, bond lengths from M-X and M-Y are different from each other and symmetry along vertical direction is broken, resulting in a lower symmetry than monolayer TMDCs. The site symmetry and space group of monolayer Janus TMDCs are C_3v and P3m1,^71,92,112 respectively. The range of piezoelectric coefficients of in-plane (d₁₁) and out-of-plane (d₃₁) are 2.26–5.30 pm V⁻¹ and 0.007–0.30 pm V⁻¹,^71,92 respectively.

II-VI

Monolayer II-VI semiconductors are expressed as MX, where M is II atoms and X is VI atoms.⁷⁹ Monolayer II-VI semiconductors have a honeycomb structure with non-centrosymmetric atom arrangement.^77,78 Piezoelectric coefficient and bandgap of monolayer II-VI were calculated to be the range of −1.16 to 26.7 pm V⁻¹,^70,73,79 and 0.66–5.34 eV,^76–78 respectively (Figure 2B). Piezoelectric coefficients were predicted to be enhanced by decreasing the atomic number of M (M = Ba, Mg, Zn, Cd) or increasing the atomic number of M atoms (M = Ca, Sr, Ba).^63,73,79

III-V

Monolayer III-V semiconductors are also shown as MX, where M is III atoms and X is V atoms.⁸⁶ The crystal structure can be divided into in-plane hexagonal and non-plane hexagonal, where those with in-plane structure are stable in the air. The space group and symmetry of monolayer III-V with in-plane structure are $p\overline{6}m2$ and D_3h,^80,81,86 respectively (Figure 1C, i). And the piezoelectric coefficient and bandgap of monolayer III-V are from 0.09 to 5.5 pm V⁻¹,^63,84 and 0.31 to 5.9 eV,^81,84 respectively (Figure 2C,D). While the space group and symmetry of III-V semiconductors with non-plane structure are Pmn2₁ and C_2v.^80,85 For the non-plane structure, the piezoelectric coefficient of monolayer III-V semiconductors has two kinds: in parallel (d₁₁) and perpendicular (d₃₁) planes, which are in the scope of 0.02–1.50 pm V⁻¹ and 0.02–0.57 pm V⁻¹, respectively.^63,82,84 The bandgap is 0.74–2.49 eV.^81,83,85 Besides, the piezoelectric coefficients of d₁₁ and d₃₁ increase with decreasing the atomic number of V atoms or increasing the atomic number of III atoms except for AlN, GaN, and InN.

III-VI and Janus III-VI

III-VI semiconductors can be represented as MX or MMXX (M = Ca, In; X = S, Se, Te). Space group and symmetry belong to $p\overline{6}m2$ and D_3h, respectively.⁸⁹ The piezoelectric coefficient of d₁₁ is in the range of 1.12–1.98 pm V⁻¹.^70,87 The bandgap is from 1.32 to 2.36 eV.^91,92 When M or X is replaced by other atoms, III-VI becomes Janus III-VI, termed as M₂XX′ or MM′X₂ (M, M′ = Ca, In; X, X′ = S, Se, Te).⁹¹ Janus III-VI belongs to layered hexagonal structure with site symmetry D₃.⁹¹ The piezoelectric coefficients of d₁₁ and d₃₁ are from 1.91 to 8.47 pm V⁻¹ and from 0.07 to 0.46 pm V⁻¹, respectively.^91,92 And the bandgap is from 0.89 to 2.03 eV (Figure 2E,F).^91,92

IV-VI

Four types of structures are contained in monolayer IV-VI semiconductors, which are indicated by MX or MX₂ (M = Sn, Ge; X = Se, S). For the presentation of MX, puckered or hexagonal crystal structures were reported (Figure 1E, i–iii). One of the puckered structures is alternated by the zigzag line (Figure 1E, i), like black phosphorene, with C_2v site symmetry and Pmn2₁ space group,^80,95,96 And piezoelectric coefficient d₁₁ and bandgap of this MX are 75.43–250.28 pm V⁻¹ and 0.77–1.37 eV.⁹⁴ The other puckered structure of MX shows a rectangular Wigner-Seitz cell alternated by the armchair direction with piezoelectric coefficient d₁₁ and bandgap in the range of 20.7–91.56 pm V⁻¹ and 1.62–1.86 eV (Figure 1E, ii). Simultaneously, the MX with hexagonal structure (Figure 1E, iii) presents that piezoelectric coefficient d₁₁ and bandgap are from −5.65 to −4.63 pm V⁻¹ and 2.21 to 2.47 eV,⁹⁶ respectively. While MX₂ shows D_2d site symmetry and $p\overline{4}m2$ space group (Figure 1E, iv).^95,97 The piezoelectric coefficient e₁₄ and bandgap of MX₂ are 3.45–3.81 × 10⁻¹⁰ C m⁻¹ and 0.54–1.44 eV (Figure 2G,H).⁹⁷

V-V

Monolayer V-V semiconductors possess α-phase and β-phase induced by their flexible structures and special symmetry.¹¹³ The space group and symmetry of α-phase and β-phase are calculated to be C_2v, Pmn2₁ and C_3v, P3m1,^80,98 respectively. The piezoelectric coefficient d₁₁ and bandgap of α-phase are 6.94–243.45 pm V⁻¹ and 0.29–2.18 eV,⁹⁸ while those of β-phase are 0.67–4.83 pm V⁻¹ and 1.49–1.98 eV (Figure 2I).⁹⁸ Both piezoelectric coefficients d₁₁ from α-phase and β-phase of V-V materials increase with increasing the atomic number of V cation.

Modulated 2D materials

The piezotronic engineering has been developed in graphene-like 2D materials by modulating the interfaces or defects in 2D materials, which possess a center symmetric structure originally. Three techniques were explored for changing center symmetric structure to non-center symmetry: interface interaction,¹¹⁴ atomic adsorption,^115–122 and introducing defects (Figure 3).^123,124

FIGURE 3. A, Schematic diagram of the piezoelectric activity of monolayer graphene on the SiO2 substrate. B,C, Schematic of the PFM measurement and piezo-response of graphene supported on SiO2 substrate and suspended graphene. Reproduced with permission.114 Copyright 2015, spring nature. D, Schematic diagram of different absorbed atoms on graphene. E, Dependence of equibiaxial strain on electric field for different absorbed position of Li+ on graphene. F, Dependence of equibiaxial strain on electric field for different absorbed atoms on graphene. Reproduced with permission.116 Copyright 2012, American Chemical Society. G,H, Schematic of circular (G) and triangular (H) defects in graphene. I, Dependence polarization on the external strain of graphene with triangular defects. Reproduced with permission.123 Copyright 2012, American Institute of Physics

The 2D materials with center symmetry structure originally could generate piezo-response with the modulation from interfaces. Kholkin group observed piezotronic response in graphene with the chemical interaction between graphene and oxygen atoms from SiO₂ substrate (Figure 3A–C).¹¹⁴ The piezo-response from graphene supported by SiO₂ substrate is four times than that of suspended one, indicating the interaction between graphene and substrate induces the change of interface properties of graphene, which leads to non-center symmetry and piezotronic material of graphene.

Selective surface adsorption of atoms on non-piezotronic graphene could also break center symmetry and generate piezotronic effects. Reed group found absorbed Li⁺ atoms on graphene could produce non-center symmetric graphene and piezo-response (Figure 3D–F).¹¹⁶ The absorbed position on graphene does not affect the piezoelectric coefficient significantly. While the species of absorbed atoms has a strong influence on the piezoelectric coefficient d₃₁.

Non-center symmetric internal defects in graphene can induce piezotronic effect simultaneously. Sharma et al. introduced circular and triangular defects in graphene by electron beam irradiation (Figure 3G–I).¹²³ The graphene with circular defect does not show piezo-response, while that with triangular defects gives a piezoelectric coefficient of 0.124 C m⁻². Then, Kelany et al. built two hole-like defects in monolayer graphene with changing the symmetry of graphene from D_6h into D_3h and C_2v, respectively.¹¹⁸ Both types of defects present the piezoelectric response, where the piezoelectric value is as large as 5.6 ± 0.4 × 10⁻¹⁰ C m⁻¹.

2D organic–inorganic hybrid perovskites

Organic–inorganic hybrid perovskites have developed into a new type of 2D piezotronic material.^125–133 The organic–inorganic hybrid perovskites have already made into monolayer 2D sheets in 2015 (Figure 4A,B).¹²⁵ Mitzi group first detected the piezo-response from (PMA)₂PbBr₄, (PEA)₂PbI₄, (NMA)₂PbBr₄, (NEA)₂PbI₄, and (NEA)₂PbBr₄ by piezo-response force microscopy (PFM) and confirmed the non-center symmetric structure of those organic–inorganic hybrid perovskites (Figure 4C).¹²⁶ Then, Xiong group measured the piezo-amplitude from (ATHP)₂PbBr₄ and (CHA)₂PbBr₄ perovskites and confirmed the broken inversion symmetry by second harmonic generation (SHG).¹²⁷ In short, organic–inorganic hybrid perovskites are being explored as a new 2D piezotronic material.

FIGURE 4. A,B, Optical (A) and AFM image (B) of monolayer (C4H9NH3)2PbBr4 perovskites. Reproduced with permission.125 Copyright 2015, American Association for the Advancement of Science. C, Schematic diagram illuminates piezo-response on driving voltage and frequency (inset). Reproduced with permission.126 Copyright 2017, American Chemical Society. D, PFM images of (ATHP)2PbBr4. E,F, PFM resonance frequency (E) and piezoelectric amplitude (F) of (ATHP)2PbBr4 and (CHA)2PbBr4. Reproduced with permission.127 Copyright 2020, American Chemical Society

BASICS OF PIEZOTRONICS FOR 2D MATERIALS

Experimental measurement of piezo-response of 2D materials is important for developing piezotronics of 2D materials. Four methods have been reported to detect piezo-response: bending and releasing of flexible devices,^31,134–136 atomic force microscopy (AFM),^137–142 PFM,^143–148 and lateral excited scanning probe microscopy (SPM) (Figure 5A–D).^36,52,149

FIGURE 5. Methods for detecting piezo-response, A, Bending and releasing of flexible devices. Reproduced with permission.31 Copyright 2014, Springer Nature. B, AFM. Reproduced with permission.137 Copyright 2014, Springer Nature. C, PFM. Reproduced with permission.145 Copyright 2016, American Association for the Advancement of Science. D, SPM. Reproduced with permission.149 Copyright 2018 Elsevier Ltd. E,F, Modulated SBH in the interface between metal and n-type 2D materials, when applied the tensile strain (E) and compressive strain (F). Reproduced with permission.44 Copyright 2018, Elsevier Ltd. G,H, Regulated energy band structure and depletion zone in p-n junction with n-type 2D materials as piezotronic materials, when applied the tensile strain (G) and compressive strain (H). Reproduced with permission.44 Copyright 2018, Elsevier Ltd

The mechanism of piezotronics from 2D materials is similar to the modulated output current through the interfaces between metal and semiconductor,^{31,44,150–154} or p-n junction.^44,155–157 For the interface between metal and semiconductor, we take n-type semiconductor for example (Figure 5E,F), where Schottky contact is created. The output current is decided by the SBH, which is modulated by the piezo-potential. The piezo-potential is induced by the center deviation of positive and negative charges when applied the external strain.¹⁵⁸ The negative piezo-potential around the interface increases the SBH and decreases the output current, while the positive piezo-potential around the interface reduces the SBH and enhances the output current, when applied external strain.⁴⁴ On the other hand, for the interface of p-n junction (Figure 5G,H), we take n-type 2D materials as the piezotronic materials. The negative piezo-potential repels electrons and increases the energy band height in n-type interface, resulting in broadening the depletion zone in n-type semiconductor and reducing the output current. While the positive piezo-potential attracts electrons and reduces the energy band height in n-type interface, leading to narrowing the depletion zone in n-type semiconductor and enhancing current when applied the external strain.⁴⁴

APPLICATIONS OF PIEZOTRONICS FROM 2D MATERIALS

According to the mechanism of piezotronics from 2D materials, the output current will be regulated to increase or decrease when applied the external strain. Thus, the gauge factor for related sensors will be enhanced or reduced, causing scrambling research of diverse applications, such as various sensors,^158–180 nanogenerators,^181–192 piezo-catalysis,^193–195 information storage,^196–199 and so on.

Sensors

For the superiority of enhanced responsiveness of sensors combined with piezotronics, compared with those of Ohmic contact, trace sensors based on 2D piezotronic materials have been developed to enhance the sensitivity and lower the detected limitation. Such as, strain, toxic gas, pulse blood pressure, photodetector, humidity, and so on.

Strain sensor

The basic principle of 2D piezotronics is to transform mechanical signal to electrical signal under the external strain. Therefore, strain sensor is the basic sensing in 2D piezotronics sensors. Zhang et al. investigated the conductivity of MoS₂ devices with AFM tip contacting at the center and near the edge of the triangular sample (Figure 6A–C).¹³⁸ When contacting at the center of MoS₂, the interaction can be equaled to compressive stress which induces negative piezo-charges in the interface, resulting in enhanced conductivity of MoS₂. While contacting near the edge of MoS₂ leads to reduced conductivity for the positive piezo-charges in the interface. The highest gauge factor of those AFM tip contacting is more than 1000 larger than those of conventional metal sensors. Then, Hu et al. found grain boundaries in monolayer MoS₂ can enhance piezotronic effect for generating polarization along both sides of the grain boundaries (Figure 6D–F).¹⁶² Hu group also realized high spatial strain resolution of ~500 μm in 2D In₂Se₃ device, where gauge factor of the strain sensor is 237 with uniaxial strain from −0.39% to 0.39%, which is two orders of magnitude higher sensitivity than graphene-based strain sensors.¹⁶⁴ Another outstanding contribution is piezotronic strain-gated OR logic gates achieved by Wang group. Four states, “00”, “01”, “10”, “11” are identified by the output current in ultrathin ZnO piezotronics transistor with 2 nm channel length. “00” state is without strain and “11” state is the simultaneous stress on source and drain electrodes (Figure 6G–I).¹⁶³ Those qualitative research have pioneered, and quantitative research is needed to promote practical applications in future.

FIGURE 6. A,B, I–Vb characteristics of MoS2 device with AFM tip contacting the center (A) and near the edge (B) of triangular film. C, Current response on different strain. Reproduced with permission.138 Copyright 2015, Spring Nature. D, Schematic diagram of measured current with grain boundary. E, Current density of monolayer MoS2 without and with grain boundaries. F, Output current under different strain. Reproduced with permission.162 Copyright 2020, American Chemical Society. G–I, Strain-gated OR logic gates of ZnO piezotronics transistors. Reproduced with permission.164 Copyright 2018, American Chemical Society

Gas sensor

For the large surface area and higher sensitivity improved by piezotronics, toxic gas sensors have been researched in full swing.^160,166,167 Wang group achieved highly sensitive NO₂ sensor with monolayer MoS₂ and reduced the detected limitation to 20 parts per billion (ppb) combined with piezotronics. The sensitivity is enhanced to 671% and the response time reduced to 16 s with 0.67% tensile strain when the concentration of NO₂ is 400 ppb (Figure 7A–C).¹⁶⁶ Then, Xue group confirmed the higher response of NH₃ sensor by piezotronics in Au-MoSe₂ composites.¹⁶⁷ Compared with the sensor of MoSe₂, the Au decorated MoSe₂ sensor provided a Schottky contact, where electrons tend to transport to Au for higher work function. When NH₃ is absorbed to Au-MoSe₂ composites, the electrons transported to Au will be released to react with NH₃ to generate NO, which increased the output current and sensitivity (Figure 7D–F).

FIGURE 7. A, Schematic diagram illuminated the measurement for NO2 sensor based on monolayer MoS2 flexible devices. B, Dependence of gauge factor of NO2 sensor on external strain with different NO2 concentration. C, The response and recovery time of the NO2 sensor without illumination and strain (I), and with 4 mW cm−2 red LED illumination and 0.67% tensile strain (II). Reproduced with permission.166 Copyright 2018, Elsevier BV and Science China Press. D, Schematic illustration of self-powered NH3 sensor driven by MoS2-flake based piezoelectric nanogenerator. E, Schematic of NH3 sensing mechanism and energy band structure Au-MoSe2 composites. F, Dynamic resistance change of MoSe2 and Au-MoSe2 film sensor with different NH3 concentration at room temperature. Reproduced with permission.167 Copyright 2019, Elsevier Ltd. G, Schematic diagram illuminating pulse and breath sensors based on α-In2Se3 device. H, Pulse signals monitored by flexible α-In2Se3 device. I, Three breathing states monitored by flexible α-In2Se3 device. Reproduced with permission.170 Copyright 2019, American Chemical Society

Micro-vibration sensor

2D materials provide an important step in the development of ultra-thin flexible electronics, miniaturization, and wearable devices,^168,169 while piezotronics gives a higher sensitivity. Thus, artificial intelligence micro-vibration sensors based on 2D piezotronics were exploited significantly in recent years. Ahh et al. fabricated the MoS₂-based tactile sensor with graphene electrodes and showed perfect mechanical flexibility with the strain at 1.98%.⁵³ And Hu et al. reported the piezotronics based on α-In₂Se₃ flexible devices and achieved 1 order of magnitude higher piezotronic output voltage than the values of reported 2D piezotronic materials, where the output piezotronic voltage is 0.363 V for a 7-layer α-In₂Se₃ device under 1% strain.¹⁷⁰ The flexible devices were applied to monitor the pulse and breath signals (Figure 7G–I). The monitored pulsed signal gives two peaks obviously, responding to left ventricular ejected blood wave and reflected wave from the lower body. Besides, three types of breathing states were also monitored, corresponding to normal, ragged, and deep states, respectively. For all the cases, the output signals suggest the micro-vibration sensor based on 2D piezotronics has a good responsivity, while the temporal and spatial resolutions should be considered to further sensing.

Photodetector

Piezo-potential promotes separation and transport of photogenerated electrons and holes and increases the output current, causing the enhanced photo-response and developed piezophototronics of 2D materials.^171–177 Those piezophototronic sensors can be divided into two types of contact: metal and 2D materials, and p-n junctions. For the contact of metal and 2D materials, MoS₂,^22,171,178 WSe₂,¹⁷² α-In₂Se₃,¹⁷³ γ-InSe,¹⁷⁴ and In_1−xSn_xSe,¹⁷⁵ were investigated widely. It is interesting that by repairing the defect in MoS₂ flexible phototransistor, a prominent piezophototronic sensor was realized, where 5.6-fold enhancement of responsivity is achieved under a 0.42% tensile strain (Figure 8A–C).¹⁷¹ For the p-n junction, homogeneous and heterogeneous p-n junction were also developed (Figure 8D–F).¹⁷⁶ MoS₂ homogeneous p-n junction was realized by Wang group.¹⁷⁷ The enhancement of photoresponsivity and detectivity under 0.51% strain gives 619% and 319% compared with those without external strain. Heterogeneous p-n flexible photodiode was achieved by Li group with vertically stacking multilayer p-WSe₂ and monolayer n-MoS₂.¹⁷⁶ The optimized photoresponsivity increases by 86% under −0.62% strain along armchair direction of MoS₂.¹⁷⁷

FIGURE 8. A, Optical image of the flexible piezophototronic detector fabricated with monolayer MoS2 on the PET substrate. B,C, Strain dependence of photocurrent (B) and photoresponsivity (C) on different illumination intensities. Reproduced with permission.171 Copyright 2018, American Chemical Society. D, Schematic diagram of monolayer MoS2 homogenous p-n junction. E, Ids–Vds characteristics of MoS2 homogenous p-n junction with different tensile strains. F, Photoresponsivity of MoS2 homogenous p-n junction under different strains and excited intensities. Reproduced with permission.176 Copyright 2018, IOP Publishing Ltd. G, Schematic diagram of MoS2/metal flexible device for humidity detection. H, Dependence of changed relative current on external strain with different relative humidity (RH). I, Dependence of current response on various strains and RH at a bias voltage of 10 V. Reproduced with permission.178 Copyright 2018, American Chemical Society

Humidity sensor

Humidity detection has been realized in MoS₂/metal junction.^115,170 When H₂O is absorbed to monolayer MoS₂, electrons in MoS₂ are captured by H₂O molecular, resulting in reduced carrier concentration and output current. While applied the external strain to the flexible device, the piezo potential is generated along zigzag direction, which modulating SBH of MoS₂/metal interface and output current. The maximum current variation could reach 2048% with the humidity changing from 63% to 5% under 0.61% tensile strain (Figure 8G–I).¹⁷⁸ However, the chemical reaction between absorbed H₂O and MoS₂ will degrade 2D MoS₂ gradually, therefore, the stability and durability should be deliberated for humidity sensor in future.

Nanogenerator

Energy harvesting based on 2D piezotronics provides a breakthrough of flexible self-powered systems for the atomic layer thickness, excellent mechanical performance, and piezotronic properties of 2D materials.^181–184 When applied the tensile or compressive strain in 2D flexible devices, piezo-potential is created in the interface of 2D materials and metal electrodes, where electrons and holes in 2D materials are attracted to opposite polarity piezo-potential and current is produced in the external circuits.¹⁸⁴ When the alternating strain is applied, the alternating current and voltage are generated.¹⁸⁵ Thus, nanogenerator, converting mechanical energy into electrical energy, is established. Wu group first reported the 2D nanogenerator based on MoS₂ with odd number of atomic layers and achieved 5.08% conversion efficiency of mechanical to electrical energy from monolayer MoS₂ nanogenerator (Figure 9A–C).³¹ The piezotronic output from monolayer MoS₂ nanogenerator is directly connected to the crystal orientation for different piezoelectric coefficients along zigzag and armchair directions.^31,143 At the same time, MoS₂ nanogenerators in series or in parallel showed consistent enhancements in output voltages or currents, suggesting potential practical application for powering nanodevices.

FIGURE 9. A, Output current along armchair and zigzag directions of monolayer MoS2 nanogenerators. B, Output voltage of four monolayer MoS2 nanogenerators in parallel. C, Output current of four monolayer MoS2 nanogenerators in series. Reproduced with permission.31 Copyright 2014, Springer Nature. D, Dependence of output voltage of monolayer WSe2 (m-WSe2) and transferred bilayer WSe2 (tb-WSe2) nanogenerators on external strain. E, Output voltage of m-WSe2 and tb-WSe2 nanogenerators with strain at 0.57% and 0.95%. F, The durability test of m-WSe2 and tb-WSe2 nanogenerators with the strain at 0.89%. Reproduced with permission.191 Copyright 2017, Wiley-VCH. G, Schematic of MoS2 nanopores as nanogenerators. H, Schematic diagram of the self-powered MoS2 transistor, where the current between drain (D) and source (S) is supplied by a MoS2 nanopore nanogenerators and the gate (G) source is supported by the other MoS2 nanopore nanogenerators. I, Transfer curve of the self-powered MoS2 transistor. Reproduced with permission.192 Copyright 2016, Springer Nature

Since then, 2D nanogenerators based on various 2D materials have been researched passionately, such as MoSe₂,⁴⁷ α-In₂Se₃,⁴⁹ BP,⁵⁵ hollow 2D MoS₂ shells,¹⁸⁵ BN,¹⁸⁶ ZnO,^187,188 bilayer WSe₂,¹⁹¹ multi-pores MoS₂,¹⁹² and so on. The bilayer WSe₂ nanogenerator indicates that stacking direction of TMDCs with even number of atomic layers determines symmetry and piezotronic performance (Figure 9D–F).¹⁹¹ Another significant work is nanopowered generator by monolayer MoS₂ nanopores, designed by Radenovic group (Figure 9G–I).¹⁹² Two liquids with different concentrations are separated by MoS₂ nanopores, therefore, a chemical potential gradient was formed so that the ions could travel across the nanopore spontaneously. For the surface charges around the nanopores, passing ions will be selected according to charge polarity, resulting in a net osmotic current. The output voltage and current of this nanogenerator have been successfully powered a MoS₂ transistor. All those pioneering works have laid the foundation for practical applications of nanogenerators, the integrated and wearable nanogenerators need be scheduled in future.

Piezo-catalysis

Photocatalytic water splitting and degradation of organic pollutants are considered the most promising solution to energy crisis and environmental pollution. The suitable bandgap of 2D materials for absorbing visible light from sunlight establishes the photocatalytic foundation, and piezotronics enhances photocatalytic performance. MoS₂, WSe₂, CdS, and WS₂ have been applied to catalytic water splitting or degrade organic pollutants with the application of ultrasonic vibration.^193–195 The piezo-potential induced by ultrasonic vibration encourages the separation of holes and electrons in 2D materials, which reacted with water and dissolved O₂ to generate •OH and •O₂⁻ radicals to degrade pollutants (Figure 10D–F).¹⁹⁵ However, the enhanced performance by piezotronics calculated in articles was also contained the separation of various radicals and degraded small molecules promoted by ultrasonic vibration natively, which increase the photocatalytic behaviors simultaneously. And the significant lattice bending of multilayer nanosheets from MoS₂, WS₂, and WSe₂ should be also considered, which inducing piezo-potential initially and might influence the piezo-catalytic mechanism and performance simultaneously.

FIGURE 10. A, Schematic diagram illustrated photo-/piezo-catalytic mechanism of CdS nanosheet. B,C, Time depended catalytic performance with (B) and without (C) light. Reproduced with permission.193 Copyright 2020, Springer Nature. D, Schematic showing the piezo-/photo-catalytic mechanism of FETCP/MoS2 nanosheet. E, Catalytic degradation of FETCP/MoS2 nanosheet with different condition. F, Cycling degradation tests of FETCP/MoS2 nanosheet. Reproduced with permission.195 Copyright 2019, Elsevier BV

Information storage

Information storage is the basic engineering for artificial intelligence, where memristors are considered to be the perfect candidates for nonvolatile memories and artificial intelligence.^196–199 Badhulika et al. fabricated piezotronic memristor based on graphene/MoS₂ 2D nanohybrid, where graphene improves the mobility for conductivity.¹⁹⁶ The graphene/MoS₂ memory shows excellent stability with resistive switching 500 cycles and endurance with data retention of 10⁴ s under 10⁴ On/Off ratio (Figure 11). Ag ions from the top Ag electrode penetrate to the Cu electrodes gradually by increasing the voltage between Ag and Cu electrodes, leading to the “On” state. When applied the external strain, the negative potential is created in the interface between graphene and MoS₂, causing reduced SBH, enhanced voltage distribution and electric field intensity between MoS₂ and Ag electrode. Thus, the drift velocity of Ag ions increases and the voltage for “On” state is reduced with increasing the external strain.

FIGURE 11. A, Schematic of graphene/MoS2 flexible device. B,C, Repeatability (B) and retention (C) of graphene/MoS2 flexible device. D, Schematic illuminates resistive switching mechanism. E, I–V curves of graphene/MoS2 flexible device under different strain. F, Energy band diagram with piezotronic effect of graphene/MoS2 flexible device. Reproduced with permission.196 Copyright 2019, American Chemical Society

Information storage is a new direction for 2D piezotronics. Careful design and lots of attention should be paid to exploit the performance vigorously. For example, reduced the “On” state voltage by reasonable designing the thickness and quality of MoS₂. Designing arrayed devices with different “On” state voltage are applied to store various information with avoiding serial interference.

CONCLUSIONS AND PROSPECT

In this review, we present the 2D piezotronic materials from inorganic to organic crystals with the original or modulated centrosymmetric structure. Four kinds of experimental measurement methods and mechanism of 2D piezotronics are described, containing bending and releasing of flexible devices, AFM, PFM, and SPM. Various applications were also exploited to improve the sensitivity, limitation, resolution, and durability, such as different sensors, nanogenerators, piezo-catalysis, and information storage. Although the preliminary work has laid the foundation for future research, challenges are always existed in 2D piezotronics. Efforts might be engaged as follows:

• Excepting the piezo-potential induced by the external strain, the electric band structure of 2D materials is also sensitive to pressure engineering,^200,201 which influences the transport of electrons significantly. The output performance of 2D piezotronics devices is decided by SBH between 2D materials and electrodes,²⁰² determined by the energy of conduction band minimum or valence band maximum for n-type or p-type 2D materials, respectively. However, the external strain also modulates bandgap and energy band extremum, further, the conductivity type and output currents.^203–205 For example, the bandgap of monolayer MoS₂ will be decreased from 1.883 to 1.690 eV under 1.49% compressive strain.²⁰³ The valence band maximum of monolayer MoS₂ shifts from K point to Γ point when applying compressive strain, while conduction band minimum shifts from K to the point between K and Γ points.²⁰⁴ Besides, the conductivity type of monolayer TMDC could change from semiconductor to metal under 11% external strain.²⁰⁵ The changed bandgap and shifted extreme point in reciprocal space have a great influence on output voltage and currents, which might be calculated in present experiments. Thus, modulated output voltage and current in 2D piezotronics should be considered the energy band engineering simultaneously.
• The various applications are the preliminary, qualitative, quantitative research, and comparative exploration in vertical and horizontal need be launched. For example, for the piezo-catalysis, the types and concentration of defects in 2D materials should be considered. For the various types of sensors, statistical results should be performed for the inhomogeneous crystal structure and local designed devices in single sheet of 2D materials.
• The separation and transport of electrons and holes in different defects by piezotronics need to be calculated and confirmed by experiments exhaustively. Defects are the intrinsic properties in 2D materials, which affect the carrier transport, electronic and optical properties significantly.²⁰⁶ The well-known defects are cationic or anionic vacancies, dislocations, and grain boundaries, and so on, which are confirmed by first-principles calculations and observed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM).^207–209 The concentration, distribution, and orientation of point defects influence the separation and transport of carrier, resulting in anisotropic electrical performance with the same type of defect and different conductivities along the same crystal orientation with different types of defects.^210–212 Meanwhile, the external strain can change the distribution and types of defects.^213–216 First, the tensile strain could lower the activation energy of vacancy migration and induce the migration of vacancies,^213,214 leading to the redistribution of vacancies, furtherly, changing the point defects to dislocation. Second, the external strain favors the generation of vacancies in 2D materials,²¹⁵ resulting in the increasing concentration of vacancies. Third, the orientation of line defects also depends on the mechanical strain,²¹⁶ which might rotate the orientation of dislocation. Therefore, external strain could alter the concentration, distribution, and types of defects, which might influence the separation and transport of carrier and electrical performance. However, the changed electrical performance induced by external strain might be calculated in present piezotronics experiments. Thus, the carrier separation and transport related to defects in piezotronics need to be calculated and confirmed by experiments.
• For the purpose of final practical application, arrayed flexible devices with 2D piezotronics should be exploited with avoiding serial interference. The refraction, diffraction, boundary reflection, and photon scattering in 2D materials and arrayed devices might cause serial interference and influence the resolution of flexible devices during piezo-phototronics experiments.^217,218 At the same time, the carrier generation and diffusion might lead to electrical serial interference in piezotronics, which is connected to the size, spacing, and distribution of single device, buffer layer thickness, and epitaxial layer thickness of arrayed devices.^219,220 In order to avoid the serial interference, independent electrodes for any single device in arrayed devices are needed, which are difficult for design and fabrication of arrayed devices. Thus, reasonable design of flexible arrayed devices is necessary to reduce the serial interference.
• The growth of 2D materials needs further development. Large area 2D materials and heterojunction materials with the order of magnitude from centimeter to meter are urgently needed. And the 2D piezotronics based on heterojunction should be explored in full swing.
• The experimental method of bending and releasing of flexible devices for 2D piezotronics should be paid much attention because the method is closer to practical applications. And the measured method of 2D piezotronics need to be expanded furtherly.

ACKNOWLEDGMENTS

The authors thank the support of National Natural Science Foundation of China (Nos. 11704081, U20A20166, 61675027, 61805015, and 61804011), National Key R&D project from Minister of Science and Technology, China (2016YFA0202703), Natural Science Foundation of Beijing Municipality (Z180011), Natural Science Foundation of Guangxi Province (Nos. 2020GXNSFAA297182 and 2017GXNSFBA198229), Shenzhen Science and Technology Program (Grant No. KQTD20170810105439418), and the Fundamental Research Funds for the Central Universities.

CONFLICT OF INTEREST

The authors declare no conflict of interest.