Introductory Chapter: Piezoelectric Energy Harvesting Based on Two-Dimensional Materials

1. Introduction

Energy harvesting (EH) is the technique of capturing and converting energy from many sources, such as mechanical load, vibrations, temperature gradients, and light, to produce relatively low levels of power in the nW-mW range [1]. The effectiveness and particular material qualities have a significant impact on the potential and performance of energy-harvesting devices. The efficiency of mechanical oscillators at tiny sizes, in addition to the materials required in the energy transduction mechanism itself, is a crucial aspect of motion energy harvesting. The crucial characteristics and uses of piezoelectric two-dimensional materials for energy-harvesting devices are covered in this chapter. Their broader applications—from the military to the aerospace industry, from cars to electronic clothing—indicate the necessity for further research on this subject.

2. Basic concepts

Energy harvesting technology is founded on the notion that devices may instantly use the energy that is present in their ambient surroundings in real time and never need to be temporarily stored [2]. This would make it possible for devices to have theoretically limitless lifespans that are only constrained by the lifespans of their constituent parts. Real-time systems must strictly adhere to predetermined reaction times to operate; hence, it must be demonstrated that this new technology is appropriate for them. The development of discrete devices that can produce electrical energy from kinetic energy (i.e., vibrations or movement), temperature gradients, or incoming light has been the main emphasis of energy harvesting up to this point [3]. The quantity and type of source energy existing in the environment have a fundamental impact on how well energy harvesting works. The limitations of the application and the specifics of the energy source must be known before building an energy-collecting solution. For each use in conventional kinetic energy harvesting, bespoke devices are frequently created, and it is uncommon to find a straightforward solution that covers a wide range of uses.

The phrase “energy harvesting” is typically used to provide electricity to small, low-power electronic components [2]. The most significant areas of use for energy harvesting are connected items, such as wireless sensors and wearable electronic devices. The use of these new technologies has noticeably led to a change in how electronic systems are designed. It poses additional difficulties for system designers, who must now try to maximize the use of ambient power to achieve energy independence in each device. There is a good chance that as time goes on, this problem will get simpler. Electronic circuitry and wireless links do use less power with time. As a result, energy harvesting technology has grown exponentially in every application area, including home automation, healthcare, the military, transportation, and others. The number of devices using ambient energy is predicted to reach 2.6 billion by 2024 [2]. Energy collecting, however, poses a unique set of difficulties, the majority of which can be attributed to the unpredictability and uncontrollability of most ambient energy sources.

The basic operating principle of a two-dimensional piezoelectric material based on hexagonal boron nitride (h-BN) is shown in Figure 1. The material is sensitive to mechanical stress applied by movement or vibration and is reflected in the change of interatomic distance of the chemical bonds between boron and nitrogen.

Figure 1.

The basic principle of piezoelectricity in two-dimensional materials.

3. Novel piezoelectric materials

Piezoelectricity has historically been used in a wide range of sensing and energy-harvesting applications [4]. Nanogenerators (NGs), a new technology product that was created as a possible method for turning random mechanical energy (like human movement) into electric power, are booming in the market [5]. A prototype piezoelectric nanogenerator built of zinc oxide (ZnO) nanowires was suggested in 2006, marking an important development in the understanding and use of piezoelectricity at the nanoscale. Since then, the scientific world has become increasingly interested in piezoelectric nanomaterials, particularly piezoelectric nanowires manufactured from wurtzite-structured materials, such as ZnO and gallium nitride (GaN). The analysis and creation of NGs were then the focus of intense research employing a variety of piezoelectric materials, including ZnO, GaN, lead zirconate titanate (PTZ), and BaTiO₃ (BTO). Due to its advantageous optical, semiconducting, and piezoelectric properties, which make it a qualified candidate for NG fabrication, ZnO is a significant functional material [5, 6, 7]. Due to their distinctive characteristics, such as a high surface-to-volume ratio and outstanding mechanical endurance, two-dimensional (2D) ZnO nanosheets (NSs) are particularly emerging as significant nanostructures in the construction process of high-performance NGs. Recently, a study was conducted to compare the effects of employing curved (CNSs) and straight nanosheets (SNSs). It was found that the piezopotentials produced by the two nanosheets in the event of vertical compression differed slightly in magnitude and distribution, however, in the case of lateral bending, the piezopotential produced by CNSs was consistently significantly bigger than that produced by SNSs [7]. On applying 4 kg of force for more than 4000 cycles, the optimized NGs generated voltages, current densities, and power densities of up to 0.9 V, 16.5 A/cm², and 600 nW/cm², respectively [6].

In recent years, two-dimensional (2D) materials, including hexagonal boron nitride (h-BN), and many transition-metal dichalcogenides (TMDCs), transition-metal dioxides (TMDOs), II-VI semiconductors, III-V semiconductors, III-monochalcogenides and IV-monochalcogenides, and others, have also been found to exhibit piezoelectricity and the piezotronic effect [8, 9]. Two-dimensional materials are discovered to have numerous distinct piezoelectric capabilities in comparison to traditional piezoelectric nanowires. For instance, some 2D materials, including monolayer group IV monochalcogenides, have enormous piezoelectric coefficients that are two orders of magnitude greater than those of traditional ZnO and GaN nanowires. Simulations and experiments also demonstrate that by altering the number of layers of 2D materials, it is possible to effectively change their piezoelectric performance. Since their development, 2D materials have attracted a lot of attention for piezotronic applications because of their special benefits in terms of their piezoelectricity and their capacity to endure extreme strain.

For instance, many 2D materials, such as the thin-layered MoS₂, only exhibit piezoelectric effects in their odd-numbered layers, whereas their even layers only exhibit a modest response [8]. Additionally, the device system is limited in its ability to access out-of-plane activities since the piezoelectricity in many 2D materials is mostly detected within the in-plane piezoelectric polarization. Non-centrosymmetry, functionalization, and doping can be induced in the resulting crystal lattice of the two-dimensional material with a higher degree of anisotropy by reducing the thickness, which also breaks the crystal’s symmetry. Other types of 2D layered materials that may be of interest include post-transition metal dichalcogenides (PTMDs), such as SnS₂, which have been confirmed theoretically but have yet to be fully explored experimentally [8].

New energy-harvesting technologies are based on 2D materials and utilize a variety of mechanisms, including photovoltaic, thermoelectric, piezoelectric, triboelectric, and hydrovoltaic devices. Additionally, advances in the harvesting of osmotic pressure and wireless energy have been made [10].

In contrast to graphene, which has identical atoms and a center of symmetry, monolayers of h-BN, MoS₂, WSe₂, and TMDCs exhibit significant piezoelectricity because their inversion symmetry is broken [11]. To create cantilever energy harvesters, layered 2D nanosheets of h-BN and MX₂ (M = Mo or W; X = S, Se, or Te) have been coated on a silicon substrate [12]. Non-centrosymmetric two-dimensional (2D) layered materials have a lot of potential for nanoscale electromechanical systems and electronic devices [13]. Due to their mechanical flexibility and piezoelectricity, some 2D materials have the potential to be employed in nanodevices that can generate wearable power. By adding strain to the structure, the bandgap can be tuned. The bandgap of trilayer MoS₂ has been found to grow by 300 meV for every 1% biaxial compressive strain. A vertical electric field application has also been proposed to potentially zero out the bandgap in 2D TMDCs, converting the structure from semiconducting to metallic [14].

The Janus monolayers lack the reflection symmetry regarding the core metal atoms, which permits out-of-plane electric polarizations, in contrast to ideal 2D materials. Future atomically thin piezoelectric applications in the fields of transducers, sensors, and energy harvesting devices should favor 2D tin dichalcogenide-derived Janus monolayers [15].

A compilation of two-dimensional materials with piezoelectric properties is illustrated in Figure 2.

Figure 2.

Main two-dimensional piezoelectric materials.

4. Applications

Significant efforts have been made to produce sustainable, mobile, and distributed power sources for the energy of a new era due to the rapid development of portable devices, wearable electronics, and the Internet of Things. The rising energy needs, particularly for portable and wearable devices, may be met by scavenging the otherwise wasted energy from the ambient environment into electrical power [10]. Future wireless nanosystems without power supply, such as environmental monitoring, implanted medical sensors, and personal electronics, will depend on energy-harvesting cells based on 2D piezoelectric materials [13].

Sn-based chalcogenide Janus structures have a potential future for atomically thin applications in mechanical sensors, energy sensors, etc., thanks to their strong piezoelectric characteristics [15]. Despite the advancements made to date, 2D materials energy-harvesting devices still face numerous obstacles and opportunities in the creation of self-powered electronics and wearable technology before widespread use is even conceivable.

It has been demonstrated that flexible PZT piezoelectric energy harvesters based on mica can capture energy from muscle movements. A workable approach to self-powered bio-devices is to scavenge mechanical energy from the motions of people or internal organs [16]. It demonstrated excellent mechanical and electrical durability by being continually bent and unbent at a high strain 40,000 times without obviously degrading output performance.

Numerous materials are theoretically anticipated to be piezoelectric in their 2D forms due to the 2D confinement and spontaneous breaking of inversion symmetry [17]. A new platform for the investigation of novel physics at the atomic scale as well as prospective device applications is created by the combination of piezoelectricity and other fascinating features in 2D materials. Although the basic idea behind piezotronic and piezophototronic phenomena is widely understood, further research is still needed to better understand how piezoelectricity interacts with semiconductor characteristics.

A compilation of the applications of two-dimensional materials with piezoelectric properties is depicted in Figure 3.

Figure 3.

Main applications of piezoelectric two-dimensional materials.

5. Conclusions

Piezoelectricity is a characteristic of macroscopic strain-induced electric polarization that couples mechanical and electrical characteristics to enable effective mechanical-to-electrical energy transfer. The piezoelectric effect, which has benefits like high efficiency, lightweight, and tiny scale, is the most alluring method for mechanical-to-electrical energy conversion. Numerous 2D materials have been anticipated to have intrinsic piezoelectricity, however, due to the enormous challenges in material synthesis, most of them have not yet been empirically examined. Modern semiconductor processes, as well as classic electronic technologies, are simple to integrate with 2D piezoelectric materials. The following aspects of 2D piezoelectric materials need to be studied to achieve their commercial success: the impact of strain on an electronic structure on piezoelectric charges, how carrier concentrations can improve the piezoelectric output, and the search for more chemically stable piezoelectric materials under ambient conditions.

Acknowledgments

The author appreciates the support of the University of Guanajuato to develop this research.

Thanks

The author wants to thank his wife and son for their support and time to edit this book. The author appreciates the support of Tea Jurcic working for IntechOpen as an author service manager.