2D Materials as Effective Cantilever Piezoelectric Nano Energy Harvesters

Abstract

Two-dimensional (2D) layered piezoelectric nanomaterials are attractive for application in mechanical energy-harvesting devices. In this study, layered 2D nanosheets, h-BN and MX2 (M = Mo or W; X = S, Se, or Te), are deposited on a silicon substrate to form cantilever energy harvesters. Using density functional theory (DFT) and molecular dynamics (MD) calculations, the effect of the substrate for energy harvesting is studied. The substrate provides 2D layers with stable support while maintaining effective energy harvesting. MD results indicate that the electromechanical conversion energy has contributions from piezoelectricity and flexoelectricity, but the latter is negligible due to small strain gradients for bendingoperated nanoharvesters. It is shown that the out-of-plane piezoelectric constants are substantially larger than their in-plane piezoelectric counterparts. The output power is calculated for a substrate-supported nanogenerator. This work provides an atomistic study of piezoelectricity and a new strategy to implement 2D materials in nano energy harvesters. Piezoelectricity is an energy effective mechanism to establish mechanical and electrical dynamic control in nanodevices. Owing to its high transformation capability and response sensitivity, piezoelectricity is widely used in sensors, transducers, etc. Inspired by a milestone work contributed by Wang et al. about piezoelectric nanogenerators based on zinc oxide (ZnO) nanowire arrays, researchers are focusing more attention on the study of piezoelectric materials at the nanoscale. Among these materials, 2D piezoelectric nanomaterials (PNMs) have many advantages over their bulk constituents. For example, 2D materials are more flexible and can sustain large strain. The ability to withstand high deformation extends the applications of devices working in complex and large strain environments like human activities. In effect, the piezoelectric properties of PNMs can be dramatically modified. A main reason for this is that, increasing with downscaling, surfaces play a relatively larger role for material properties. Therefore, surface atom modifications can drastically modify properties of single-layered materials which are one atom or one molecular layer thick. While graphene is non-piezoelectric because of its centrosymmetry, surface modifications can make graphene-based structures piezoelectric. In addition to transforming non-piezoelectric materials into piezoelectric media, surface atom modifications can also lead to enhancement of piezoelectric coefficients of piezoelectric structures. Despite their numerous advantages, the synthesis of 2D PNM smart structures is difficult to control in a way that avoids introducing unwanted vacancies or impurities. Furthermore, the light weight and small thickness limit the applications of suspended 2D PNMs due to their mechanical fragility. In order to overcome these restrictions so as to utilize 2D PNMs, mechanical support through the substrate is important. Most of the syntheses and applications of 2D materials are carried out on substrates. There are many advantages in synthesizing 2D materials on substrates, as this ensures high quality, large scale, and few defects. For example, the first successful attempt in realizing freestanding graphene was accomplished by mechanical exfoliation. Despite excellent quality samples, the method suffers from poor scalability. In contrast, Hwang et al. proposed a method to grow highquality graphene through van der Waals epitaxy on a c-plane sapphire substrate. The presence of the substrate ensures stability and good quality during epitaxy of graphene. Further, substrates provide mechanical support for 2D materials to perform strain, vibration, bending, and other operations on a macroscale. Baek et al. synthesized Pb(Mg1/3Nb2/3)O3PbTiO3 (PMN-PT) thin films on vicinal (001) silicon and incorporated these heterostructures into microcantilevers to harvest energy. The Si substrate enables structures with precisely specified passive-layer thicknesses to control stiffness Received: April 30, 2021 Accepted: May 24, 2021 Leter http://pubs.acs.org/journal/aelccp © XXXX American Chemical Society 2313 https://doi.org/10.1021/acsenergylett.1c00901 ACS Energy Lett. 2021, 6, 2313−2319 D ow nl oa de d vi a G E O R G IA I N ST O F T E C H N O L O G Y o n Ju ne 2 , 2 02 1 at 2 0: 45 :1 6 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. and displacement. The strain induced in the 2D materials can be precisely and homogeneously controlled by the substrate. Zeng et al. developed a spherical diameter engineering process to tune the bandgap of monolayer MoS2. Furthermore, the precise control of strain is important for energy harvesting. Wu et al. used a polyethylene terephthalate (PET) flexible substrate to control the strain of monolayer MoS2 to generate piezoelectric voltages and then form a nanogenerator to harvest energy. Different from the piezoelectric voltage generated along the armchair direction, Kim et al. put forward an electrical output along the armchair and zigzag directions of a triangular-shaped MoS2 monolayer. The output power of a nanogenerator based on movement along the armchair direction is twice as large compared to that along the zigzag direction. The synthesis process and nanogenerator operations are all carried out on a substrate. In contrast to the pioneering studies on piezoelectricity in materials containing an odd number of layers, Lee et al. verified that a piezoelectric response can be found in bilayer WSe2 through turbostratic stacking. In their study, monolayered WSe2 is first synthesized on a sapphire substrate and then transferred to a flexible PET substrate. On this basis, another monolayered WSe2 is synthesized and transformed onto the former monolayered WSe2 to form a turbostratic stacking structure. The turbostratic stacking bilayer WSe2 retains the piezoelectric effect and is tested with high output power. Lee et al. used a mechanochemical exfoliation method to obtain 2D piezoelectric hexagonal boron nitride (h-BN) nanoflakes and transfer them onto an electrode line-patterned plastic substrate to characterize their ability for harvesting energy. The utilization of a substrate allows precise characterization of energy generation from 2D piezoelectric BN nanoflakes. Recently, Kuang et al. incorporated BN nanosheets of a few percent in weight ratio into polydimethylsiloxane (PDMS) to form composites. With the support of PDMS, BN nanosheets are able to produce piezoelectric voltages up to ∼5.4 V with d33 ≈ 12 pC/N. Besides, lead(II) iodide (PbI2) nanosheets and α-In2Se3 18 are used simultaneously with substrates to construct flexible 2D piezoelectric devices to harvest energy. In this study, several 2D PNMs are deposited on a Si (111) surface to form cantilever beams to study the substrate effect on energy harvesting of 2D PNMs using density functional theory (DFT) and molecular dynamics (MD) methods. The DFT method has been widely used as an accurate approach to describe electronic structures of crystals as well as charge distributions in energy-harvesting devices like piezoelectric and triboelectric nanogenerators. The DFT results here allow determining charge differences between the substrate and 2D PNMs. To limit the combined computational efforts, we determine the separation of the substrate and the 2D PNMs during bending using MD instead of DFT. Then, by combining the DFT and MD results, the effects of the substrate on the application of 2D PNMs are obtained and discussed. MD results allow us to determine the polarization change and the piezoelectric characteristics of the 2D PNMs. The differences between bent and corrugated structures are also discussed. After that, a cantilever beam model using 2D PNMs is proposed as an energy-harvesting structure. Formal Si has a diamond structure with lattice constant parameters of a = b = c = 5.4 Å (shown in Figure 1a), while the lattice constants are a = b = 3.8 Å (Figure 1b) when Si is cut to expose the (111) surface. Because the hexagonal close-packed 2D layers have the same configuration, we only give the sketch of h-BN (Figure 1c). The details of heterostructures of BN and MX2 with Si (111) and simulations of DFT and MD can be found in the Supporting Information. As mentioned earlier, substrates are important for the synthesis and application of 2D PNMs with elastic and transparent features. However, the effects of the substrate on 2D PNMs’ properties are not well known. The substrate is essential to maintain a robust and sustainable device. Further, it is evident from eq S14 that the induction of charges by the substrate is important to determine the polarization. Before studying the charge transfer at the interface, we first optimize the heterostructures listed in Table S1. The side views of the optimized heterostructures are shown in Figure S2. Table S4 gives the obtained equilibrium distance between the 2D PNMs and the Si substrate. A Bader charge analysis is carried out to quantify the amounts of transferred charge for each heterostructure, and the calculated values of transferred charges for each unit cell are tabulated in Table S5. In order to analyze the charge transfer quantitatively, the amount of transferred charge δ, defined as δ = (ηc/ηtot) × 100%, is determined where ηc is the transferred charge and ηtot is the total charge for an isolated 2D layer. The calculated values are listed in Table S5. Our DFT calculations reveal that the charge transfer between the 2D layers and the Si (111) surface is small. Thus, the transferred charges do not affect the piezoelectric properties of heterostructures in MD. We have carried out calculations of charge transfer under ideal conditions where the total energy, force, and distance between Si and 2D PNMs are in equilibrium. It has been confirmed that the maximum