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Nano Energy

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Piezo/triboelectric nanogenerators based on 2-dimensional layeredstructure materials

Sang A. Hana,c, Jaewoo Leea, Jianjian Linb, Sang-Woo Kimc, Jung Ho Kima,⁎

a Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, Squires Way, North Wollongong, NSW2500, Australiab College of Chemistry and Molecular Engineering Qingdao University of Science and Technology, Qingdao 266042, Chinac School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea

A R T I C L E I N F O

Keywords:Piezoelectric nanogeneratorsTriboelectric nanogenerators2-Dimensional layered structure materials

A B S T R A C T

Recently, research on energy harvesting has attracted great attention as a solution to energy depletion andenvironmental problems due to the use of fossil fuels such as coal, natural gas, and oil. To be precise, harvestingtechnology converts the energy sources around us such as solar, heat, and mechanical energy into electricalenergy. It has the advantage of being able to supply and sustain energy on a permanent basis, rather than beingnon-renewable, and it is also eco-friendly. Among the various energy harvesting techniques, nanogeneratorsbased on piezoelectric and triboelectric phenomena can generate electrical energy based on mechanical energysources, which are usually ubiquitous, there are no restrictions due to weather, time, or space, and this tech-nology is also user-friendly. Recently, two-dimensional (2D) materials have been chosen for implementingpiezo/triboelectric nanogenerators. The 2D materials have transparency, flexibility, and a high surface-to-vo-lume ratio. Owing to the very low thickness of the atomic unit, a stacking structure using 2D materials can bealso made to form a very thin device, which is applicable for insertion into the body or wearable electronicdevices. In this review, we summarize the characteristics and research results on piezo/triboelectric energyharvesters based on 2D layered structure materials.

1. Introduction

A number of studies have been conducted over the last few decadesto solve our global problems of energy exhaustion and environmentalpollution caused by the use and depletion of fossil fuels. Fossil fuelssuch as coal, natural gas, and oil have limited reserves and cause manyenvironmental problems. In order to solve these problems, many stu-dies on environmentally friendly alternative energy sources have beenconducted, and energy harvesting has attracted especially great atten-tion as one of many alternatives. Energy harvesting is a renewableenergy technology that converts solar energy, thermal energy, or me-chanical energy, which are wasted energy in the surrounding en-vironment, into electrical energy. Using this technology, we can obtainsustainable energy supplies and replace energy that previously wasbased on non-renewable fossil fuels. Unlike power plants, which payhigh costs for energy sources such as fossil fuels, energy harvestingtechnology has the advantage that it consumes waste energy and haslittle cost.

Energy harvesting technology can be simply classified into

photovoltaic cells, thermoelectric devices, and piezo/triboelectric na-nogenerators, according to the energy source. Among the energy har-vesting technologies, the first to be developed was photovoltaic cellsusing solar/light energy. After the photovoltaic effect was discovered in1839, solar cells were studied with a view to increasing their efficiencyby using two-component materials (GaAs, InP, CdTe, etc.) or organicmaterials, or stacking several layers, or changing their structures [1,2].Even through solar cells are environmentally friendly and the sun is apermanent resource, there is the disadvantage that their electrical en-ergy generation is limited, depending on the weather, time, and space.

Thermoelectric energy harvesting is a technology that convertsthermal energy into electrical energy based on the Seebeck effect. TheSeebeck effect means that the temperature difference between junctionsof two different types of metal or semiconductor in a circuit is con-verted directly to a voltage. This phenomenon occurs because the en-ergy of the free electrons is different, depending on the temperature, sothat the free electrons move to the energy balance state, that is, theenergy is lowered until the equilibrium state is formed, thereby formingthe potential difference. In particular, thermoelectric energy harvesting

https://doi.org/10.1016/j.nanoen.2018.12.081Received 21 September 2018; Received in revised form 6 November 2018; Accepted 26 December 2018

⁎ Corresponding author.E-mail address: jhk@uow.edu.au (J.H. Kim).

Nano Energy 57 (2019) 680–691

Available online 27 December 20182211-2855/ © 2018 Published by Elsevier Ltd.

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has the advantages of being very stable and noiseless because me-chanical forces do not impinge on the device. There are still thedrawbacks of low technical maturity and energy efficiency, however.

The field of piezoelectric materials has been developed since thediscovery of BaTiO3 (BTO) with high piezoelectricity [3]. Piezoelectricceramics have been used in various fields such as acoustics, ultrasound,communications, and measuring instruments. The field of energy har-vesting has been studied since the 1990s, and progress in this field hassince accelerated, especially in the last few years. In the case of energyharvesting using triboelectricity, a generator based on electrostatics,which is usually considered negative in our daily lives, was first re-ported in 2012, and the efficiency was improved by five times in a yearby optimizing the structural efficiency [4]. Unlike photovoltaic cells,which are influenced by weather, time, and space, energy harvestingusing piezo/triboelectric phenomenon can generate electrical energybased on waste energy sources, it is not restricted by space, and hasattracted attention for being user-friendly [5]. Fig. 1 summarizes theclassification of energy harvesting technologies according to the energysource. Each harvester's power, advantages, and disadvantages are alsodescribed.

2. Nanogenerators

2.1. Piezo/triboelectric nanogenerators

In this review, we are focusing on the piezo/triboelectric energyharvesting technology of the various energy harvester classes. Fig. 2summarizes the structure, materials, and characteristics of the piezo/triboelectric nanogenerators. Piezoelectric energy harvesters requirethe use of piezoelectric materials. A piezoelectric material is a materialwhich generates a charge by first generating a polarization in responseto a mechanical stress. It has the advantage of converting vibrations andphysical energy into environmentall friendly electrical energy withmuch higher energy conversion efficiency than for thermoelectrical andphotovoltaic energy. In addition, all the ferroelectric materials arepiezoelectric materials, in which the polarization of the molecularstructure is arranged in a certain direction. This type of material ismainly used for micro/nanoscale products that require precision be-cause of their excellent response to peripheral stimuli. The sourcewhich generates piezoelectric energy for harvesting is mainly bending

and vibration, and materials with piezoelectric characteristics are used,such as quartz [6], lead zirconate titanate (PZT) [7–9], BTO [10–12],etc. Piezoelectric characteristics also can be confirmed in thin filmmaterials that have semiconductor characteristics, such as zinc oxide(ZnO) [13–17], aluminum nitride (AlN) [18–22], gallium nitride (GaN)[23–25], etc., not only in the bulk type piezoelectric materials men-tioned above. Also, it has been confirmed that certain two-dimensional(2D) layered structure materials [26–32] with an atomic thickness havepiezoelectric characteristics. In particular, the piezoelectric propertiesof transition metal dichalcogenides (TMDs) have been reported boththeoretically and experimentally in recent days [28–30,33–35]. Theefficiency of piezoelectric nanogenerator devices is about 25–50%, andthey can be applied in body implantation devices, wireless sensornodes, wearable devices, etc.

Triboelectric energy harvesting is generally based on the electro-static effect generated at the interface between two different materials.This electrostatic phenomenon occurs not only between solids, but alsobetween solids and liquids, between liquids, and between liquids andgases. What we often feel as static electricity is a phenomenon in whichelectric current flows into the human body due to discharge from acharged body to a conductor or from a charged body to the humanbody. For this reason, the charge that is generated by triboelectricityhas been mentioned negatively in scientific studies or technical appli-cations. The Zhong Lin Wang group at the Georgia Institute ofTechnology, however, first reported a triboelectric nanogenerator usingthe triboelectric charge generated by static electricity and fabricated aneffective device in 2012 [36]. Since then, research on triboelectric na-nogenerators has been actively carried out through changing thestructure of devices, using various materials, and so on [4]. Tribo-electric nanogenerators have various driving modes, such as pressure,vibration, rotation, sliding, etc. They generate a potential differencedue to friction between two materials, so they have advantages such asno restriction of usable materials, simple device structure, and low cost.When mechanical force is applied using two different materials, frictionoccurs on the materials’ surfaces. The atoms that make up the materialseach consist of a nucleus that has an electrically positive charge (+)and electrons that have an electrically negative charge (-). When anatom loses electrons, it becomes positive (+), but when an atom gainselectrons, it become negative (-). If two materials that are in the elec-trically neutral state under normal conditions rub against each other,

Fig. 1. Energy harvesting technology classification according to the energy source.

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the electrons of one material are transferred to the other material, sothat the two materials become (+) or (-), respectively. The “tribo-electric series”, is a series of materials arranged in a sequence thatshows how easy it is to lose or gain electrons. It is mentioned in manyscientific articles [4,37–39] and makes it possible to select a set offriction materials with large output performance based on this se-quence. Recent studies have shown that nylon and, human skin arepositive, while polytetrafluoroethylene (PTFE), perfluoroalkoxy alkane(PFA), and polydimethylsiloxane (PDMS) are negative materials whenrubbed. Triboelectric energy harvesters are environmentally friendlyand have high energy efficiency and high output performance, so they

are mainly used for self-generating sensors that need high outputpower.

2.2. Operating mechanism

Fig. 3a shows the principle of a piezoelectric energy harvestingdevice using a piezoelectric material to generate electrical energy. Thedipoles present in the piezoelectric material without mechanical energyare vertically arranged on the surface of the device by poling. When thedevice is bent by an external force, a piezoelectric potential is formedinside the material due to deformation of the device. The electrons flow

Fig. 2. Comparison of piezoelectric and tribo-electric nanogenerators. The piezo/triboeletricnanogenerators are compared in terms of de-vice structure, energy source, materials, andcharacteristics. aLead zirconate titanate,bBarium titanate, cPolyvinylidene fluoride,dPolytetrafluoroethylene, ePerfluoroalkoxy al-kane, fPolydimethylsiloxane.

Fig. 3. Driving mechanisms of (a) piezoelectric nanogenerator and (b) triboelectric nanogenerator.

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through the external circuit to balance the potential created by thedipole and eventually accumulate on the upper electrode. When thestress acting on the device disappears and it returns to the initial state,the charge also returns to its starting place through the circuit. Throughthis repetitive process of applying and removing pressure to the device,positive and negative electric signals are generated alternatively[40–43].

Static electricity is a very common phenomenon in our environ-ment, and it occurs when two different materials rub again each other.The sign of the static charge is determined by the combination of ob-jects. The polarity of the electrical charge becomes higher when ma-terials are combined that are far from each other in the above-men-tioned "triboelectric series". The general mechanism of triboelectricenergy harvesting is shown in Fig. 3b. In the initial state withoutpressure or force, the triboelectric nanogenerator (TENG) is in a neutralstate, where there is no electric potential difference between the twomaterials. When pressure or force is applied to the TENG, the upper andlower materials rub against each other. When two different materialsare in contact, the surface is charged by triboelectrification. When thetwo materials are separated, the charge is accumulated in the upper andlower electrodes via the electrostatic induction phenomenon, and cur-rent flows through the external circuit until charge balance is achieved.When the two materials are brought close to each other, the compen-sating charges which are accumulated disappear, so that the currentflow is in the opposite direction to what occurred the first time. Con-tinuous current flows between both electrodes through repeated contactand separation processes [44–48]. As long as the two contact materialshave different electron affinity, the charge transfer phenomenon canhappen once the two surfaces make a contact and thus triboelectricitycan exist in the external circuit, even if friction does not actually occurin the generation of triboelectricity in the TENG.

3. Materials: two-dimensional (2D) layered structure materials

There are various materials for application in piezo/triboelectricenergy harvesting devices, and herein, we will discuss piezo/tribo-electric nanogenerators based on 2D layered structure materials. Thediscovery of the most representative 2D material, graphene, has creatednew excited interest in the field of materials science [49–52]. The in-terest in graphene has been so explosive that countless research projectshave been conducted [53–57], and new interest in various layeredstructure materials has been aroused. Various 2D materials that havebeen highlighted since the discovery of graphene have shown manytypes of applicability [58–64]. The basic characteristics of 2D materialsare transparency, flexibility, and having a high surface to volume ratio,which can make it possible to obtain very thin devices, even if stackingstructures are used, due to the atomic layer thickness, as shown inFig. 4.

Graphene with a 2D structure composed only of carbon atoms hasexcellent properties, such as large surface area [65–67], easy mod-ification [68,69], chemical stability [70], high mechanical strength[71–73], high thermal conductivity [74–76], and excellent lighttransmittance [77–79]. As a result, it has been applied in various fields,such as transparent displays [80,81], electrode materials for secondarybatteries [82–84], photovoltaic cells [85–88], automobiles, and thelighting industry. Graphene is also attracting attention as a strategiccore material to drive the growth of related industries. In addition tographene, there are various types of 2D materials that form layeredstructures, such as hexagonal boron nitride (h-BN), transition metaldichalcogenides (TMDs), transition metal trichalcogenides (TMTs),metal phosphorous trichalcoogenides (MPTs), and metal mono-chalcogenides (MMCs).

Hexagonal boron nitride (h-BN) features strong sp2-hybridizedbonding of nitrogen and boron atoms, so that there are no unsaturatedbonds on the surface and the structure is flat on the atomic level.Similar to graphene, it is transparent, flexible, and has excellent

mechanical properties [89–92]. In addition, it has attracted attention asa material that can be used as a substrate suitable for 2D material re-search [93,94] and applications, due to its excellent thermal con-ductivity [95–97] while remaining electrically insulating. Transitionmetal chalcogenides (TMDs) can be simply expressed by the formulaMX2, where M is the transition metal and X is the chalcogen element.These can be formed as various kinds of semiconducting materials ac-cording to the M and X elements [98,99]. Due to the changes in the dorbitals of the M elements, p orbitals of the X elements, and hybridorbitals, they each have their own conduction band, electrical con-ductivity band gap, and band-gap electronic structures respectively[100]. It is possible to change the electronic structure of TMDs from anindirect transition band-gap to a direct one due to the change in theinteraction between orbitals as the thickness of the TMDs compoundbecomes thinner [101–103]. This effect is expected to be utilized inoptical devices in which direct transition characteristics are essential.Furthermore, it is possible to realize heterojunction materials havingvarious band structure junctions by using a stack of TMDs compoundshaving different characteristics electronic structures, and the chargetransport properties can be improved by controlling the band structure.

The above-mentioned 2D layered structure materials have piezo-electric characteristics even at atomic thickness. The 2D piezoelectricmaterials are very important as materials for energy harvester devel-opment for wearable devices because they have high piezoelectriccoefficients and flexibility in the planar direction, unlike the conven-tional piezoelectric materials, and they are thus suited for variety ofapplications such as actuators, sensors, transducers, and energy har-vesters in the fields of piezotronics, nanorobotics, and nanoelec-tromechanical systems [33,104–109]. In addition, these properties en-able them to generate various electrical signals due to theirtransformation characteristics as well as the high-efficiency piezo-electric characteristics that they show by reacting sensitively to theshape deformation of the material.

Hundreds of 2D materials have been reported in previous studies[110–112]. Depending on their in-plane structure, they can be mainlydivided into three groups: III-V group materials, metal dichalcogenides,and monochacogenides, depending on their in-plane structure. Fig. 5shows the potential 2D material groups and their piezoelectric constantvalues that can give them piezoelectric properties [30].

First, the III-V group of the periodic table includes BN, AlN, andGaN. They do not have a high theoretical piezoelectric coefficientcompared to other groups. This is mainly due to the monolayer struc-ture of III-V groups, as shown in Fig. 5b. When strain is applied, due tothe asymmetric structure, polarization occurs, and the piezoelectricphenomenon is simultaneously generated. The degree of stretching inthe lateral direction is very small, however, as shown in the Fig. 5b.Second, the metal dichalcogenide group includes molybdenum disulfide(MoS2), tungsten disulfide (WS2), tungsten diselenide (WSe2), andmolybdenum diselenide (MoSe2). They have a structure in which atransition metal atom is sandwiched between two chalcogen atomlayers, as shown in Fig. 5c. Due to this structure, when strain is appliedin the lateral direction, the piezoelectric coefficients of the metalchalcogenide group may be slightly larger than for the III-V group.Thus, they can have piezoelectric constants of about 2–13 pm/V. Third,the monochalcogenide group, represented by tin sulfide (SnS) andgermanium sulfide (GeS), has a hexagonal wurtzite structure in thestable state. The lateral structure of the monolayer monochalcogenidesis shown in Fig. 5d. Interestingly this is a very strong structure in termsof the strain in the lateral direction, so they can have a piezoelectricconstant of about 75–250 pm/V. This is an enormous number as com-pared to the III-V group and the metal dichalcogenide group. This isalso comparable to the piezoelectric constant of a typical bulk piezo-electric material. Even though the problem of large-area synthesis stillneeds to be solved in the future, 2D materials have sufficient potentialfor piezoelectric devices.

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4. Piezoelectric nanogenerators based on 2D layered structurematerials

4.1. Intrinsic piezoelectric properties

Experimental results on the piezoelectric properties of two-dimen-sional (2D) materials have been reported very recently. The 2D piezo-electric semiconductor material is a key factor for the energy harvesterbecause it needs to have a high piezoelectric constant and flexibility in

the plane direction, unlike the conventional piezoelectric materials. Forthis reason, the 2D piezoelectric semiconductor is not only well suitedfor wearable devices, but also reacts very sensitively to the shape de-formation of the material, enabling the generation of various electricalsignals due to its transformation characteristics as well as its high-ef-ficiency piezoelectric characteristics. The piezoelectric properties of 2Dmaterials were first reported in theoretical calculations [30,33]. Ex-perimental results on the piezoelectric properties of 2D materials weremore recently reported, however, in 2014 by Zhong Lin Wang's group

Fig. 4. Characteristics and applications of 2-dimensional layered structure materials.

Fig. 5. (a) Configurations of two-dimensional material groups and their piezoelectric coefficients. Atomic structures of (b) the III-V group, (c) the metal dichalco-genide group, and (d) the monochalcogenide group.

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[109]. They reported the piezoelectric characteristics of MoS2 amongthe 2D piezoelectric materials. Monolayer MoS2 was obtained on a Sisubstrate using the physical exfoliation method, which is one of theeasiest ways to obtain 2D materials. The thickness was precisely con-firmed by atomic force microscopy (AFM) and Raman spectroscopy.Also, the crystallographic orientations of MoS2 flakes were analyzed bya second-harmonic generation (SHG) process. After measuring thesecond harmonic perpendicular to the polarization, the monolayerMoS2 was transferred to a flexible polyethylene terephthalate (PET)substrate, and a Cr/Pd/Au (1 nm/20 nm/50 nm) electrode was de-posited. Fig. 6a shows the flexible device fabricated by the abovemethod, and based on this, the current and voltage output performancewere measured. Fig. 6b shows the voltage and current output perfor-mance of monolayer MoS2. As the MoS2 is synthesized, the crystal or-ientation of the MoS2 can be divided into the armchair and zigzag di-rections. When the deformation is applied to the armchair direction,positive current and voltage output appear at increasing strain. Incontrast, negative output is observed at reduced strain, convertingmechanical energy into electricity. The output performance increaseswith the applied strain, so that the current of this device reached 27 pA,and the voltage reached 18mV under 0.64% strain. As a result of theoutput under various strain conditions, it was confirmed that the cur-rent and voltage outputs increase with increasing strain. Based on this,the load resistance was measured at 0.53% strain, and it was arguedthat the MoS2 based piezoelectric device operated in a stable manner. Inaddition, the piezoelectric output of monolayer, 2, 3, 4, 5, and 6 layer,and bulk MoS2 was measured. The piezoelectric characteristics are notshown for even layer numbers, and the output performance is decreased

as the layer number is increased, but quite interestingly, the piezo-electric characteristics appear only in odd layers. This work is the firstexperimental paper to prove the piezoelectric properties of 2D mate-rials.

After this study, the piezoelectric properties of various 2D materialsinvestigated through ab initio prediction [30] or of free-standing MoS2[29] have been actively studied. Kim et al. measured the piezoelectriccoefficient of monolayer MoS2 grown by the chemical vapor deposition(CVD) method using piezoresponse force microscopy (PFM) in 2017[113]. Fig. 6c shows an optical image of monolayer MoS2 with a tri-angular shape grown by CVD and a schematic diagram of a PFM formeasuring the piezoelectric constant of MoS2. A PFM is a type of AFMthat enables imaging and manipulation of the piezoelectric/ferro-electric domains of a material. The cantilever deflection is detected bythe standard split photodiode detector method and demodulated by alock-in amplifier (LiA). In conventional PFM, the piezoelectric char-acteristics of the material are analyzed by measuring in the verticaldirection, but in this work, studies on the PFM were conducted in thelateral direction. MoS2 has a hexagonal crystal structure, and Mo or Satoms can be arranged along each side of the MoS2 flake. The MoS2 hasan armchair or a zigzag structure according to the atomic orientation,and thus the piezoelectric coefficient is not expected to be similar formeasurements along these two different directions. Unexpectedly, thepiezoelectric coefficients can be obtained from the slope of the solid linerepresenting the fitted linear equation, since there is a linear relation-ship between the electrical and mechanical states. The force-distancecurve can suggest a method of measuring the lateral piezoelectriccoefficient, d11, according to previous studies. The piezoelectric

Fig. 6. Study of the intrinsic piezoelectric characteristics of 2D piezoelectric materials. (a) Piezoelectric nanogenerator device based on monolayer MoS2 exfoliated bythe mechanical method. (b) Top to bottom: applied strain as a function of time, piezoelectric outputs of monolayer MoS2 in the armchair and zigzag directions. (c)Optical image of triangle shaped monolayer MoS2 grown by the CVD method on Si substrate, and schematic diagram of a PFM system to measure the piezo response.Schematic illustration, piezo response, and voltage output performance of (d) armchair direction MoS2 and (e) zigzag direction MoS2.

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constant of MoS2 can be determined by comparing and calculating theknown piezoelectric response of α-quartz. Fig. 6d and e show schematicimages of the armchair and zigzag orientations of MoS2, and the cor-responding results of measuring the piezoelectric response of the α-quartz, armchair, and zigzag MoS2. In details, the d11 of the MoS2 in thearmchair direction was measured to be 3.78 pmV−1, and in the zigzagdirection, it was measured to be 1.38 pmV−1. The experimental resultsobtained in this study are consistent with previously reported simula-tions [33]. Based on these results, a piezoelectric nanogenerator(PENG) was fabricated, and the output performance was measured. Themeasured output characteristics were measured at maximum of 20mVand, 30 pA in the armchair direction, and 10mV and 20 pA in thezigzag direction under 0.5 Hz and 0.48% strain, respectively. As a re-sult, it is meaningful that the piezoelectric constant, which varies ac-cording to the direction of atomic orientation of the MoS2 grown byCVD, is measured and analyzed by introducing the lateral PFM method.

4.2. Enhanced piezoelectric properties

Theoretical and experimental studies on the intrinsic piezoelectricproperties of two-dimensional (2D) materials have been steadily pro-gressing. In recent years, studies have been reported which haveslightly different results from the theory of piezoelectric properties of2D materials, as well as studies that can offer guidance in preventingthe degradation of piezoelectric properties due to the inevitable defectsin the synthesis process. In general, multi-layer TMDs show sig-nificantly reduced or eliminated piezoelectricity because continuousgrowth of multi-layer TMDs leads to a stable stacking structure withalternating polarization directions in neighboring layers [29,109,114].Lee et al. have reported simulations and experimental observations ofpiezoelectricity in mono- and bilayer WSe2 synthesized by CVD [115].The WSe2 was grown by the CVD method with a size of 30–50 µm andtransferred to a PET substrate by the general wet transfer method. First,the piezoelectric constant of the monolayer WSe2 was measured by thelateral PFM method to be 3.26 ± 0.3 pmV−1. It was confirmed thatthis is similar to the theoretical value [33]. Also, the PENG outputperformances were measured at 45mV and 100 pA under 0.39% strainand a 40mm/s strain rate. Next, the piezoelectric properties were in-vestigated using bilayer WSe2 fabricated by two methods. The firstsample was a bilayer WSe2 (db - WSe2) film grown directly on a sap-phire substrate, and the second was a bilayer WSe2 (tb - WSe2) obtainedby transferring monolayer WSe2 on another monolayer WSe2 film. Ingeneral, the bilayer WSe2 with Bernal stacking loses its piezoelectricitydue to its centrosymmetric structure, unlike the monolayer WSe2, be-cause the polarity is completely canceled in the stacking mode, AA'(Fig. 7a) [116,117]. According to the relative stability of the differentstacking modes, db-WSe2 has a mix of different stacking modes. Thestability of the different stacking structures determines the ratio of eachstacking structure in the mixed state, which ultimately affects the pie-zoelectric properties of the bilayer. Density functional theory (DFT)calculations for the five stacking modes shown in Fig. 7a were per-formed. It has been confirmed that the stacking structures AA', AB, andAB' have relatively lower energy than the A′B and AA stacking struc-tures, and most of the Bernal stacking has the AA 'structure due to itsstability.

On the other hand, tb-WSe2 fabricated using the transfer methodshowed piezoelectricity due to an increase in the degrees of freedom inbilayer symmetry. When there was random stacked by the transfermethod, it mitigated the geometric relationship between the two layers,so that various stacking structures were possible, which increases theasymmetry. This can be confirmed by the lateral PFM results in Fig. 7b.The tb-WSe2 has a smaller piezoelectric coefficient than α-quartz, but itclearly shows piezoelectric characteristics. Based on this, a piezoelectricgenerator was fabricated, and its output performance was evaluated.Fig. 7c shows the piezoelectric output of PENGs based on monolayerWSe2 and tb-WSe2. The output voltage for monolayer WSe2 is 90 mV

under a 0.64% strain condition. When the strain exceeds 0.64%, how-ever, the output performance is degraded. On the other hand, the tb-WSe2 based PENG showed 85mV output at a high strain of 0.95%. Thisis because bilayer TMDs materials have superior mechanical propertiescompared with monolayer TMDs materials due to their high Young'smodulus and the interlayer sliding effect of the bilayer TMDs materials[118,119]. This study shows that bilayer TMDs materials have piezo-electric properties and operate in a very stable manner even under highstain.

For practical applications, CVD is one of the large area synthesismethods used for 2D materials, but defects inevitably occur during thesynthesis process. The free electrons induced by these defects generate apotential screening effect that eliminates some of the piezoelectric po-tential generated by mechanical deformation, thereby reducing thepiezoelectric effect. For example, sulfur (S) pores act as n-type carriersin MoS2 and generate a screening effect. To improve the piezoelectricproperties of MoS2, the n-type carrier density should be lowered. Hanet al. improved the piezoelectric properties of MoS2 by effectivelycontrolling the sulfur vacancies by heat treatment with additional H2Sin the in-situ synthesis of monolayer MoS2 by the CVD method [120], ascan be seen in Fig. 7d. The characteristic changes from before to aftersulfur treatment of MoS2 were precisely evaluated. Fig. 7e shows thepiezoelectric response of α-quartz, pristine MoS2, and sulfur (S)-treatedMoS2 using piezoresponse force microscopy (PFM). The piezoelectriccoefficient of pristine MoS2 was calculated to be 3.06 ± 0.6 pmV−1,but it shows unstable characteristics. On the other hand, in the case ofS-treated MoS2, the piezoelectric coefficient is 3.73 ± 0.2 pmV−1,which is close to the theoretical value [33]. Additionally, the change inelectrical properties between pristine MoS2 and S-treated MoS2 wereconfirmed by the curve of the drain-source current vs. the gate-sourcevoltage (Ids-Vgs) of a field-effect transistor (FET), and the Fermi levelshift from before to after S-treatment of MoS2 was also evaluated byKelvin probe force microscopy (KPFM) and X-ray photoelectron spec-troscopy (XPS). The work function of S-treated MoS2 obviously in-creased compared to pristine MoS2, and this means that the Fermi levelof S-treated MoS2 shifted towards the valence level. XPS also shows thesame trend. After S-treatment, the peak shifts to lower binding energy,and this indicates a decrease in the carrier density of S-treated MoS2.Also, the carrier densities of MoS2 before and after the sulfur treatmentprocess were compared through the fabrication of FET devices. As aresult, the carrier density of the pristine MoS2 was found to be2.19×1012 cm−2, and it decreased to 6.11×1011 cm−2 (Δn=1.57×1012 cm−2) after the sulfur treatment process. This can be further ex-plained by the free carrier (electron) trap represented by the sulfurpassivation of sulfur atoms, as mentioned above. Based on these results,it is expected that the piezoelectric properties of MoS2 after sulfurtreatment will be improved. Then, the output characteristics of thepiezoelectric generator were analyzed after manufacturing a PENG bytransferring MoS2 to PET as a flexible substrate. Fig. 7f shows the vol-tage and current output performance of pristine MoS2 and S-treatedMoS2. The pink line is the pristine MoS2, and the blue line shows theresults for S-treated MoS2. The current of the S-treated MoS2 versus thepristine MoS2 increased approximately 3-fold from 30 pA to 100 pA,and the voltage increased 2-fold from 10mV to 20mV. Also, the max-imum power of the S-treated MoS2 was enhanced by almost 10 times.This result suggests that sulfur treatment can reduce the density of freecharge carriers by passivation and effectively prevent the screeningeffect. That is to say, the degradation of the piezoelectric properties oflarge-area 2D materials can be improved through defect control. Inaddition, it is suggested that research on 2D materials is worthy ofvarious studies in the future involving characteristic modification.

5. Triboelectric nanogenerators based on 2D layered structurematerials

2D materials are being studied for application in body insertable

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and wearable electronic devices that can be driven by triboelectricity.Among the generators that convert physical force into electrical force,the triboelectric nanogenerator (TENG) is the most popular system.TENG related studies using 2D materials such as graphene have beenreported, but the triboelectrification behavior of 2D materials is still notthoroughly understood as yet. Recently, Seol et al. reported the tri-boelectrification behavior of various 2D materials such as MoS2, WS2,MoSe2, WSe2, graphene, and graphene oxide (GO) using simple TENGfabrication and measurements [121]. Based on these results, they pro-posed a triboelectric series of various 2D materials. Each of the 2Dmaterials was prepared by chemical stripping of bulk flake in the liquidphase. In order to explore the positions of various 2D materials in theexisting triboelectric series, the relative charge polarity of each 2Dmaterial with respect to a representative material of the triboelectricseries was measured by preparing a simple pushing type TENG. For thispurpose, copper (Cu) was used as an electrode, and nylon, which is themost positive materials, was used on one side as a representative fric-tion material to investigate the basic triboelectric output. After com-paring the output characteristics of MoS2 with TENGs using PTFE,PDMS, polycarbonate (PC), PET, mica, and nylon with a thickness of200 nm, the polarity of MoS2 was found to be intermediate betweenthose of PTFE and PDMS. This process was performed not only on MoS2,but also on MoSe2, WS2, WSe2, graphene, and GO. MoS2 and MoSe2exhibited a relatively negative triboelectric charge among the 2D ma-terials investigated in this study and are located between PTFE andPDMS. Other 2D materials, including WS2, WSe2, graphene, and GO,exhibited a relatively positive triboelectric charge and are located be-tween PDMS and PC.

Fig. 8a and b shows the output voltage and current results of various2D materials against nylon in a TENG with a force of 0.3 kgf at a fre-quency of 1 Hz. MoS2 has maximum output voltage and current valuesof 7.48 V and 0.82 μA, respectively, indicating that MoS2 is the mostnegative of the 2D materials investigated in the triboelectrical studies.

In terms of the maximum output voltage and current, the order offriction between the 2D materials can be predicted from the negative tothe positive direction as follows: (-) MoS2, MoSe2, graphene, GO, andWS2 (+). In order to verify this, the output characteristics were con-firmed with TENGs based on PTFE, which is known to be the mostnegative material. In the case of a TENG using a 2D material and PTFE,the output voltage shows the opposite characteristic compared with thecase of the nylon TENG with the 2D material. This is also confirmed bythe work function comparison using KPFM and the first-principles abinitio simulations. Analysis of the electrostatic properties when theelectrical characteristics have been changed by chemical doping, whichis the simplest method to change the electrical characteristics of a 2Dmaterial, was also conducted. Gold chloride (AuCl3) and benzylbio-logen (BV) are typical p-type and n-type chemical dopants capable ofcontrolling the work function of 2D materials [122–124]. When AuCl3was doped into MoS2, the AuCl3-doped MoS2 was still more positivethan PTFE, and doping with BV showed that the triboelectrificationproperties of MoS2 were changed between PDMS and PC. Based onthese results, the triboelectric series of MoS2, MoSe2, WS2, WSe2, gra-phene, GO, AuCl3 doped MoS2, and BV doped MoS2 is shown in Fig. 8c.Through this study, the triboelectric properties of various 2D materialswere evaluated. Moreover, chemical doping can effectively modify thetriboelectric characteristics of 2D materials. This can result in wideningtheir applications in the study of triboelectric devices using two-di-mensional materials in the future.

Kim et al. first reported a TENG using graphene as a friction ma-terial in 2014 [125]. They reported very simple graphene-based TENGs(G-TENGs) using large graphene sheets grown by CVD on copper (Cu)and nickel (Ni) foils and their output performance. Also, flexible G-TENGs were designed using monolayer, bilayer, trilayer, and quad-layer graphene, and Bernal stacked (rhombohedral stacking) graphenegrown on Ni foil. The change in power output of the G-TENG was ex-plained in terms of the work function, and power was supplied to the

Fig. 7. (a) Stacking structure of bilayer WSe2: AA, AB, AA′, AB′, and A′B. (b) Piezo response and coefficient results for α-quartz, monolayer, and transferred bilayerWSe2. (c) Voltage output performance of monolayer WSe2 and transferred bilayer WSe2 under 0.95% strain. (d) Schematic illustrations showing the filling of thesulfur vacancy after the sulfur treatment process. (e) Piezo response and coefficient results for α-quartz, pristine, and S-treated MoS2. (f) Current and voltage outputperformance of pristine and S-treated MoS2.

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liquid crystal display (LCD), a light emitting diode (LED), and anelectroluminescence (EL) display using the power output of the G-TENGwithout an external energy source. This study presents a simple andnovel method for harvesting mechanical energy by using a flexible andtransparent G-TENG to supply power to low-power portable devices andself-powered electronic systems. After that, various studies on electro-static generators using two-dimensional materials were reported[126–132]. Shankaregowda et al. reported large output performance byincreasing the charge transfer between graphene electrode and indiumtin oxide (ITO) by continuously bringing into contact/separating thegraphene film and PDMS [126]. The TENG fabricated in this workshowed maximum voltage and current outputs of 650 V and 12 μA. Inaddition to research using flat graphene, a crumpled graphene structurewas created to increase the friction surface area and improve the output

performance of the TENG [127]. In addition, an ultra-thin TENG hasbeen reported which can be strongly affixed to human skin by im-proving the triboelectric performance through surface texturing andplasma treatment. Also, they successfully demonstrated a poweredauxiliary communication system that can convert finger contact intolanguage using conformal TENGs on the skin [128].

Recently, flexible and wearable TENGs have been reported based ongraphene multilayers assembled layer by layer (LbL) [129]. LbL as-semblies enable precise, customized graphene multi-layers on thenanometer level to be fabricated on flat, undulating fiber substrates.The performance of G-TENGs and undulated G-TENGs (UG-TENGs)were investigated according to the number of graphene layers. Theirbehavior was analyzed using the electrical and morphological proper-ties of graphene multilayers. This LbL based G-TENG demonstrated its

Fig. 8. Triboelectrification properties of various 2-dimensional layered structure materials. (a) Voltage and (b) current output performance of various 2D materialsagainst nylon in a TENG. (c) Triboelectric series of 2D materials showing the molecular structure of each material.

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potential as an energy harvesting platform for next-generation, com-mercially viable, self-powered, and wearable electronics. The othermaterial for rubbing against graphene does not only need to be apolymer material such as PET or PDMS, but also can be an insulatingmaterial such as Al2O3 [130] or a water drop [131]. The results of astudy on various 2D based TENGs are presented in Table 1. The sim-plified device structure, friction material, and power are also sum-marized. Research is continuing on 2D material-based electrostaticgenerators, and there are many aspects to be studied, such as the im-provement of output characteristics and analysis of the electrostaticphenomenon.

5.1. Summary and perspectives

We have reviewed piezo/triboelectric energy harvesters that con-vert physical energy to electrical energy and mainly focused on 2Dmaterials as components of harvesters. The unique characteristics of 2Dmaterials are transparency, flexibility, high surface-to-volume ratio,and the possibility of making very thin devices, even if a stackingstructure is made. With these advantages, 2D materials with piezo/triboelectric potential can be applied as body insertable types orwearable electronic devices. For this purpose, herein, theoretical andexperimental studies of various 2D materials as piezo/triboelectricharvesters have been reviewed in detail, and the future possibilities ofpiezo/triboelectric devices have been further discussed. Even thoughthere is still room for further study of the 2D material-based piezo/triboelectric energy harvesters, it is clear that they can pave the way tohybrid devices, including for energy storage [133–136].

Acknowledgements

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) funded bythe Ministry of Education (NRF-2016R1A6A3A11930389).

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Sang A. Han is pursuing her Ph.D. degree under the su-pervision of Prof. Sang-Woo Kim at SKKU AdvancedInstitute of Nanotechnology (SAINT), SungkyunkwanUniversity (SKKU). Her research interests are synthesis andcharacterizations of two-dimensional materials and theirapplications for energy harvesting and storage. Now she isPOST Doc in University of Wollongong (UOW).

Jaewoo Lee received his B.S. degree from the Departmentof Chemical Engineering at Seoul National University ofScience and Technology, Korea (2008). He had worked atthe Advanced Batteries Research Center, the KoreaElectronics Technology Institute (KETI) as an assistant re-searcher (2011–2015). He is currently a PhD student underthe supervision of Prof. Jung Ho Kim at the Institute forSuperconducting and Electronic Materials (ISEM),Australian Institute for Innovative Materials (AIIM),University of Wollongong, Australia (2016 - Present). Hisresearch interest is the development of silicon-based anodematerials for lithium-ion battery

Jianjian Lin is currently a full professor at QingdaoUniversity of Science and Technology. She attained herbachelors and masters degree from Qilu University ofScience and Technology (2008 and 2011, respectively), andPh.D. degree (2015) at the University of Wollongong,Australia. Her research focuses on nanomaterials for energyapplications.

Sang-Woo Kim is a professor in the Department ofAdvanced Materials Science and Engineering atSungkyunkwan University (SKKU). His recent research in-terest is focused on piezoelectric/triboelectric nanogenera-tors, sensors, and photovoltaics using nanomaterials. Hehas published over 200 peer-reviewed papers and holdsover 130 domestic/international patents. Now he is a di-rector of SAMSUNG-SKKU Graphene/2D Research Centerand is leading National Research Laboratory for NextGeneration Hybrid Energy Harvester. He is currently ser-ving as an Associate Editor of Nano Energy and anExecutive Board Member of Advanced Electronic Materials.

Jung Ho Kim is currently tenured Professor at the Institutefor Superconducting and Electronic Materials (ISEM),Australian Institute for Innovative Materials (AIIM),University of Wollongong, Australia. He received hisBachelor's (1998), Master's (2000), and Ph.D. (2005) de-grees from Sungkyunkwan University (SKKU), Korea. He iscurrently acting as an editorial board member for ScientificReports (Nature Publishing Group) and associate editor forScience and Technology of Advanced Materials (Taylor &Francis). His major research is the rational design of ma-terials for energy storage and harvesting applications.

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