Nanoscale electromechanical and energy harvesting devices based on few-layer and monolayer two-dimensional (2D) materials with non-symmetric configuration have received enormous attention in recent years. Specifically, piezoelectric and triboelectric devices based on 2D materials for energy harvesting, physical/chemical sensing, healthcare, and optoelectronics applications have been a growing interest. In this chapter, the typical preparation methods of 2D-layered materials, such as exfoliation methods and chemical vapor phase deposition (CVD), will be discussed first. Then, various characterization techniques by atomic microscopic analysis for 2D materials will be provided briefly. Finally, future aspects of developing 2D piezoelectric and triboelectric devices and their potential applications will be introduced.
- energy harvesting
- two dimensional materials
2D materials have been creating a renaissance in many scientific areas since the adventure of graphene in 2014 . Recently, the family of 2D-layered materials has been profoundly growing up, including transition metal dichalcogenide (TMD) [2, 3], transition metal carbide (TMC) , and graphene-based materials [5, 6], as shown in Figure 1 . They cover the whole range of material properties from insulators, semiconductors, metals to superconductors, offering a broad portfolio of material’s solutions with extraordinary chemical and physical properties for wide applications in promising energy and sensing application technologies [8, 9, 10, 11].
For example, piezoelectricity, which is a unique material characteristic, allows effective conversion of ambient mechanical energy into electricity or vice versa. Previously, various TMDs have been applied to fabricate piezoelectric devices owing to their non-centrosymmetric, large piezoelectric coefficients, and layered crystal structure . Prof. Alyörük and his co-workers theoretically predict the piezoelectric constants in various kinds of TMDs . Moreover, Professor Wu’s group first demonstrated that a monolayer molybdenum disulfide (MoS2) is capable of generating piezoelectric response and piezotronics applications . Meanwhile, Professor Kim’s group also reported that bilayer tungsten diselenide (WSe2) is a suitable candidate for next generation piezoelectric nanogenerators (PENG) . More importantly, Prof. Li’s group have measured the multi-directional piezoelectricity of indium selenide (In2Se3) and demonstrated its potential as a biomechanical energy harvesting device for PENG applications . As shown in Figure 2 , a 2D material-based PENG is consisted of a monolayer MoS2 with the multi-layer metal electrode Cr/Pd/Au.
Briefly, the basic sequence of the working principle of the as-fabricated PENG is shown in Figure 2 . Three stages are involved in charge generation process by external mechanical stress, which are initial, stretched, and released states. First, the mechanical strain is applied to the few-layer MoS2 on the PET substrate for different bending radius. Then, the as-fabricated piezoelectric device is coupled to an external load resistor, forming a total electric circuit loop to investigate piezoelectric response. In addition, several attempts of utilizing 2D materials in piezoelectric devices have been found in previous sensor network, including strain sensors , pressure sensors , and gas sensors . Nevertheless, some challenges may still remain for 2D material-based piezoelectric devices, including material selection, device reliability, and low electric output power.
Recently, the rise of triboelectric nanogenerators (TENGs) starts a new route to generating electricity from ambient mechanical energy. The TENGs are operated at the basis of well-known contact electrification effect, which were first invented and explained in 2012 ( Figure 3 ; ). Within the contact electrification effect, two different materials become mutually charged after it comes into contact with each other. Two electrically charged material causes an electrostatic potential difference, driving the induced electrons to flow via outer circuit loop to provide electricity.
Toward the development of TENGs, 2D materials also become one of the key features. For instance, Prof. Wang’s group successfully introduced a monolayer MoS2 as an electron acceptor layer to capture triboelectric charges that result in significant output enhancement ( Figure 4 ; ). In addition, Prof. Kim’s group studied the triboelectric series with various promising 2D materials paving the way for future design rule of 2D material-based TENGs ( Figure 5 ; ).
In this chapter, the electromechanical property of 2D materials and their subsequent applications in energy harvesting and sensing fields will be addressed adequately. To begin with, a variety of fabrication methods to prepare 2D materials are briefly described. Sequentially, the exciting progresses of these materials made in both energy harvesting and sensing applications, especially for piezoelectricity, triboelectricity, and multi-functional sensing designs, are explored and discussed. Furthermore, future prospects and further developments in above-mentioned research fields based on 2D materials are also commented.
2.1 Preparation of 2D materials: exfoliation methods
Traditional approaches to extract single- and few-layer-thick 2D materials from their bulk solids are based on exfoliation methods, which can be categorized into mechanical exfoliation and chemical exfoliation . For mechanical exfoliation (ME) process, it has been widely adopted in preparing diverse 2D materials, such as graphene , phosphorene  ( Figure 6 ), and borophenes . The ME process contains several advantages, making them promising for small-scale devices and fundamental researches. For instance, layed Materials prepared from ME process are commonly crystalline and the preparation process is usually rapid. However, the disadvantages of ME method are also obvious, limiting its large-scale production and future applications, such as low material yield, area uniformity, and layer-to-layer asymmetry. As one would like to obtain a certain 2D material with only a few layer or even monolayer, the ME process is relatively time-consuming and inefficient.
2.2 Preparation of 2D materials: chemical vapor deposition (CVD)
In contrast to the exfoliation methods, chemical vapor deposition method (CVD) has been profoundly investigated to produce 2D-layered thin film on desirable substrates via the chemical reaction of volatile precursors in recent years . 2D materials grown by CVD methods have been demonstrated to obtain scalable size, controllable thickness, and high crystallinity. Take the tin disulfide (SnS2) as an example, according to Yang et al. , SnS2 nanosheets could be synthesized on SiO2 substrates by CVD method using Sn and sulfur as the precursors. The as-synthesized SnS2 flakes could be ranged from 50 to 70 μm in lateral dimensions. This synthesis method can produce ultrathin and highly crystalline SnS2 flakes. Meanwhile, it is noted that the 2D materials with heterostructure can also be produced by multiple CVD process. Revannath et al. presented a p-MoS2/n-MoS2 vertical heterostructure by a multiple step CVD process, where the molybdenum oxide (MoO3) and sulfur were served as precursors. The as-fabricated heterostructure can be further employed for future light-emitting diode (LED) applications . As discussed above, one can found that 2D materials produced by CVD techniques possess several advantages, such as good quality, high yield, and uniform dimensions. Moreover, 2D material-based heterostructures can also be obtained by multiple CVD process, which is crucial to both research and industrial applications  ( Figure 7 ).
2.3 Piezoelectricity in 2D materials
Piezoelectricity was first discovered in 1880, which is due to the accumulated electric charge polarization of materials in response to applied mechanical stress. Previously, bulk materials have been reported to possess the piezoelectric effect, including crystals and polymers. Accordingly, in 2D materials, piezoelectricity is usually attributed to the non-symmetric structure to generate polarization charges in response to the externally applied mechanical stimuli . Recently, owing to the continuous growth of wearable, flexible, healthcare, and artificial intelligent robots industry, the market demands for nanoscale and multi-functional sensing devices have become critical, especially for human-machine interface interaction and remote healthcare monitoring. Under these circumstances, 2D piezoelectric materials with their ultrathin geometry, excellent electromechanical response, and other unique physical properties are suitable candidates and of great importance. Moreover, to directly observe piezoelectricity inside 2D materials, piezoresponse force microscopy (PFM) method is widely implemented, which is based on the converse piezoelectric effect [32, 33] ( Figure 8 ).
2.4 Piezoelectric devices based on 2D materials
As mentioned above, after successful inspection of piezoelectricity in 2D materials, a series of correlated applications, including field-effect transistors (FET), sensors, catalytic reactions, optoelectronics, and energy storage, are emerged [34, 35, 36, 37, 38]. Herein, we will briefly review the device application of 2D piezoelectric materials, especially for energy harvesting device development. For example, in 2017, Muralidharan et al. present a mechano-electrochemical device configuration based on sodiated black phosphorus (BP) nanosheets, where this device is capable of harvesting low frequency mechanical energy at 0.01 Hz  ( Figure 9 ).
In addition, Lee et al. have also developed a monolayer WSe2 piezoelectric nanogenerator (PENG), which can provide an output voltage of 45 mV ( Figure 9 ). The output electrical signal will only appear during the moment of stretching and releasing from external strain. This proves the concept of using this device to generate electricity from mechanical stimuli. Moreover, one can also see that the output voltage and current increase with increasing tensile strain. Furthermore, the as-fabricated PENG can sustain stable electrical output even after 3 hours, demonstrating its excellent device reliability.
2.5 Triboelectricity in 2D materials
Triboelectric charging is a well-known electrical charging phenomenon of materials, which has been studied for more than 2500 years . The triboelectric charging phenomenon occurs at two different materials, which come into contact and separate with each other. Owing to the charge transfer during contact, charges of opposite signs accumulate on the surface of each material, thereby developing static electricity or so-called triboelectricity. In addition to conventional thin-film and bulk materials, recently, 2D materials, such as TMDs and graphene (GR), have also been found to exhibit considerable triboelectricity [41, 42, 43]. Generally, Kelvin probe force microscope (KPFM) and Scanning Kelvin Probe microscopy (SKPM) have been utilized into characterizing surface potential and surface work function. The KPFM method allows one to measure and compare the surface potentials of the dielectrics before and after friction, which is highly correlated to the triboelectricity ( Figure 10 ).
2.6 Triboelectric devices based on 2D materials
To further understand and evaluate the triboelectricity inside 2D materials, several attempts have been made to fabricate triboelectric devices based on 2D materials. For instance, Kim et al. reported the first flexible, transparent TENG device using graphene . The as-fabricated TENGs were able to power commercial LEDs by using the electrical power output generated from TENG without any other external energy source. Meanwhile, Dong et al. reported high-performance TENG device by new materials, such as fluorinated MXene, and successfully demonstrated to both rigid and flexible TENGs applications. Furthermore, these MXene-based TENGs can be further integrated into accessories, wrist bands, and textiles  ( Figure 11 ).
For 2D materials, future challenges and aspects for fundamental research and industrial applications, especially for energy harvesting and sensing field, may be summarized as follows. Though various kinds of 2D materials have been explored to possess either piezoelectricity or triboelectricity, most of them are not well investigated owing to the difficult material preparation process via exfoliation or CVD method. To be more specific, several key factors still need to be further understood in both 2D piezoelectric and triboelectric materials. In 2D piezoelectric materials, first, it has been well known that the band structure could be affected by piezoelectric field created by strain; therefore, the influence of strain on band structure of 2D materials should be thoroughly investigated. Second, optimize synthetic methods of 2D piezoelectric materials are required, playing a key role for further improving output characteristics of piezoelectric devices. Meanwhile, as for 2D triboelectric materials, compared with traditional triboelectric materials, they have shown great advantages, such as ultrathin film feasibility, flexibility, and process compatibility with large array devices, advancing its applications as future wearable sensors and human-machine bridging interface. Nevertheless, further efforts still need to be made in the following aspects. First, an entirely qualitative and quantitative characterization of 2D triboelectricity should be implemented to understand the triboelectric charge transferring process. Moreover, 2D triboelectric materials in a large scale with low costs, high uniformity, and low-temperature synthesis process should be achieved, which will be beneficial for next-generation flexible device developments.
This work was supported by the Ministry of Science and Technology (MOST) of Taiwan. Professor Chuan-Pei Lee especially thanks the financial support of MOST, under grant numbers 107-2113-M-845-001-MY3).
Conflict of interest
The authors declare no competing financial interests.
Paton KR, Varrla E, Backes C, Smith RJ, Khan U, O’Neill A, et al. Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids. Nature Materials. 2014; 13:624
Butler SZ, Hollen SM, Cao L, Cui Y, Gupta JA, Gutiérrez HR, et al. Progress, challenges, and opportunities in two-dimensional materials beyond graphene. ACS Nano. 2013; 7:2898
Pham VP, Yeom GY. Recent advances in doping of molybdenum disulfide industrial applications and future prospects. Advanced Materials. 2016; 28:9024
Shahzad F, Alhabeb M, Hatter CB, Anasori B, Man Hong S, Koo CM, et al. Electromagnetic interference shielding with 2D transition metal carbides (MXenes). Science. 2016; 353:1137
Chen P-Y, Liu M, Valentin TM, Wang Z, Spitz Steinberg R, Sodhi J, et al. Hierarchical metal oxide topographies replicated from highly textured graphene oxide by intercalation templating. ACS Nano. 2016; 10:10869
Pham VP, Jang HS, Whang D, Choi JY. Direct growth of graphene on rigid and flexible substrates: Progress, applications, and challenges. Chemical Society Reviews. 2017; 46:6276
Tan C, Cao X, Wu X-J, He Q , Yang J, Zhang X, et al. Recent advances in ultrathin two-dimensional nanomaterials. Chemical Reviews. 2017; 117:6225
Lee C-P, Lin C-A, Wei T-C, Tsai M-L, Meng Y, Li C-T, et al. A paper-based electrode using a graphene dot/PEDOT: PSS composite for flexible solar cells. Nano Energy. 2015; 18:109
Yang PK, Chang W-Y, Teng P-Y, Jeng S-F, Lin S-J, Chiu P-W, et al. Fully transparent resistive memory employing graphene electrodes for eliminating undesired surface effects. Proceedings of the IEEE. 2013; 101:1732
Chen K-S, Balla I, Luu NS, Hersam MC. Emerging opportunities for two-dimensional materials in lithium-ion batteries. ACS Energy Letters. 2017; 2:2026
Mao S, Chang J, Pu H, Lu G, He Q , Zhang H, et al. Two-dimensional nanomaterial-based field-effect transistors for chemical and biological sensing. Chemical Society Reviews. 2017; 46:6872
Li H, Shi Y, Chiu M-H, Li L-J. Emerging energy applications of two-dimensional layered transition metal dichalcogenides. Nano Energy. 2015; 18:293
Alyörük MM, Aierken Y, Çakır D, Peeters FM, Sevik C. Promising piezoelectric performance of single layer transition-metal dichalcogenides and dioxides. Journal of Physical Chemistry C. 2015; 119:23231
Wu WZ, Wang L, Li YL, Zhang F, Lin L, Niu SM, et al. Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics. Nature. 2014; 514:470
Lee JH, Park JY, Cho EB, Kim TY, Han SA, Kim TH, et al. Reliable piezoelectricity in bilayer WSe2 for piezoelectric nanogenerators. Advanced Materials. 2017; 29:1606667
Xue F, Zhang J, Hu W, Hsu W-T, Han A, Leung S-F, et al. Multidirection piezoelectricity in mono-and multilayered hexagonal α-In2Se3. ACS Nano. 2018; 12:4976
Qi J, Lan YW, Stieg AZ, Chen JH, Zhong YL, Li LJ, et al. Piezoelectric effect in chemical vapour deposition-grown atomic-monolayer triangular molybdenum disulfide piezotronics. Nature Communications. 2015; 6:7430
Park M, Park YJ, Chen X, Park Y-K, Kim M-S, Ahn J-H. MoS2 -based tactile sensor for electronic skin applications. Advanced Materials. 2016; 28:2556
He QY, Zeng ZY, Yin ZY, Li H, Wu SX, Huang X, et al. Fabrication of flexible MoS2 thin-film transistor arrays for practical gas-sensing applications. Small. 2012; 8:2994
Fan FR, Tian ZQ , Wang ZL. Flexible triboelectric generator. Nano Energy. 2012; 328:328
Wu C, Kim TW, Park JH, An H, Shao J, Chen X, et al. Nhanced triboelectric nanogenerators based on MoS2 monolayer nanocomposites acting as electron-acceptor layers. ACS Nano. 2017; 11:8356
Seol M, Kim S, Cho Y, Byun K-E, Kim H, Kim J, et al. Triboelectric series of 2D layered materials. Advanced Materials. 2018; 30:1801210
Fiori G, Bonaccorso F, Iannaccone G, Palacios T, Neumaier D, Seabaugh A, et al. Electronics based on two-dimensional materials. Nature Nanotechnology. 2014; 9:768
Nicolosi V, Chhowalla M, Kanatzidis MG, Strano MS, Coleman JN. Liquid exfoliation of layered materials. Science. 2013; 340:1420
Dhanabalan SC, Ponraj JS, Guo Z, Li S, Bao Q , Zhang H. Emerging trends in phosphorene fabrication towards next generation devices. Advancement of Science. 2017; 4:1600305
Penev ES, Artyukhov VI, Ding F, Yakobson BI. Polymorphism of two-dimensional boron. Advanced Materials. 2012; 24:4956
Chang Y-H, Zhang W, Zhu Y, Han Y, Pu J, Chang J-K, et al. Tin disulfide; an emerging layered metal dichalcogenide semiconductor: Materials properties and device characteristics. ACS Nano. 2014; 8:8582
Yang Y-B, Dash JK, Littlejohn AJ, Xiang Y, Wang Y, Shi J, et al. Large single crystal SnS2 flakes synthesized from coevaporation of Sn and S. Crystal Growth & Design. 2016; 16:961
Nikam RD, Sonawane PA, Sankar R, Chen Y-T. Epitaxial growth of vertically stacked p-MoS2/n-MoS2 heterostructures by chemical vapor deposition for light emitting devices. Nano Energy. 2017; 32:454
Cai Z, Liu B, Zou X, Cheng H-M. Chemical vapor deposition growth and applications of two-dimensional materials and their heterostructures. Chemical Reviews. 2018; 118:6091
Hinchet R, Khan U, Falconi C, Kim S-W. Piezoelectric properties in two-dimensional materials: Simulations and experiments. Materials Today. 2018; 21:611
Gomez A, Gich M, Carretero-Genevrier A, Puig T, Obradors X. Piezo-generated charge mapping revealed through direct piezoelectric force microscopy. Nature Communications. 2017; 8:1113
Cui C, Xue F, Hu W-J, Li L-J. Two-dimensional materials with piezoelectric and ferroelectric functionalities. npj 2D Materials and Applications. 2018; 2:18
Xue Y, Zhang Q , Wang W, Cao H, Yang Q , Fu L. Opening two-dimensional materials for energy conversion and storage: A concept. Advanced Energy Materials. 2017; 7:1602684
Rao CNR, Gopalakrishnan K, Maitra U. Comparative study of potential applications of graphene, MoS2, and other two-dimensional materials in energy devices, sensors, and related areas. ACS Applied Materials & Interfaces. 2015; 7:7809
Varghese SS, Varghese SH, Swaminathan S, Singh KK, Mittal V. Two-dimensional materials for sensing: Graphene and beyond. Electronics. 2015; 4:651
Zeng M, Xiao Y, Liu J, Yang K, Fu L. Exploring two-dimensional materials toward the next-generation circuits: From monomer design to assembly control. Chemical Reviews. 2018; 118:6236
Deng DH, Noveselov KS, Fu Q , Zheng NF, Tian ZQ , Bao XH. Catalysis with two-dimensional materials and their heterostructures. Nature Nanotechnology. 2016; 11:218
Muralidharan N, Li M, Carter RE, Galioto N, Pint CL. Ultralow frequency electrochemical−mechanical strain energy harvester using 2D black phosphorus nanosheets. ACS Energy Letters. 2017; 2:1797
Lowell J, Rose-Innes AC. Contact electrification. Advances in Physics. 1980; 29:947
Kim KN, Jung YK, Chun J, Ye BU, Gu M, Seo E, et al. Surface dipole enhanced instantaneous charge pair generation in triboelectric nanogenerator. Nano Energy. 2016; 26:360-370
Kim S, Kim TY, Lee KH, Kim TH, Cimini FA, Kim SK, et al. Rewritable ghost floating gates by tunnelling triboelectrification for two-dimensional electronics. Nature Communications. 2017; 8:15891
Zheng C, Zhang Q , Weber B, Ilatikhameneh H, Chen F, Sahasrabudhe H, et al. Direct observation of 2D electrostatics and ohmic contacts in template-grown graphene/WS2 heterostructures. ACS Nano. 2017; 11:2785
Kim S, Gupta MK, Lee KY, Sohn A, Kim TY, Shin K-S, et al. Transparent flexible graphene triboelectric nanogenerators. Advanced Materials. 2014; 26:3918
Dong Y, Mallineni SSK, Maleski K, Behlow H, Mochalin VN, Rao AM, et al. Metallic MXenes: A new family of materials for flexible triboelectric nanogenerators. Nano Energy. 2018; 44:103