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2D Materials in Piezoelectric Nanogenerators (PENGs)

As technology advances, the same is true we've got the technology that is powering it. Over the years, numerous energy creation, energy storage, and harvesting devices have been developed to power up large electronic systems and individual electronic devices inside a range of ways. As society moves towards Industry 4.0 and the Internet of Things (IoT), likely to chance to create various kinds of ultra-small devices you can use for automation and remote monitoring, and telemedicine applications.

Powering small-scale devices-especially in remote applications-requires unconventional self-powering mechanisms to become self-sufficient. Recently, a variety of small-scale energy harvesters, referred to as nanogenerators, have gathered interest for powering small-scale devices in medical, remote monitoring, and IoT applications. The small size of these units ensures they are not too bulky for that small devices they're powering. Despite their small size, they can still provide enough electricity for many devices to self-charge utilizing their natural operating environment. In some cases, it could be also easy to use nanogenerators for large-scale harvesting-if many individual devices are integrated into just one harvesting system-however, this is something which is not checked out yet from the widespread research perspective.

While there are a number of various nanogenerators, they are all used in different operating environments since the generation of an electrical charge is often governed by the stimuli within the surrounding environment. One of the more promising, widely talked about, and widely researched nanogenerators is the piezoelectric nanogenerator-often known in shorthand as a PENG.

Using the Piezoelectric Effect

PENGs make use of the piezoelectric effect to generate an electric charge. The piezoelectric effect happens when an electrical charge is generated under an applied stress/load on the material. The piezoelectric effect is really a reversible effect, so once the stress has been removed, the electrical charge stops. This means that the piezoelectric effect can be employed in the other direction where an electrical voltage can be applied to the material, causing the atomic structure from the material to deform and be stress-induced.

In the specific mechanisms, it's the rearrangement of ions in the atomic level-within the solid-state lattice-that generates the piezoelectricity. Most piezoelectric materials are inorganic anyway, so when they aren't, they've some type of crystal structure (inorganic materials have this also). Which means that (mostly) the piezoelectric has a regular and repeating array of well-ordered cations and anions within its atomic lattice. It's the deformation of the ions in this particular regularly patterned lattice that generates an electric charge. As the material retains an overall neutral charge-the overall charge of the material doesn't change, only the localized distribution of charges in the atomic level changes.

So, when the stress/load is used to the piezoelectric material, the oppositely charged ions change from their original positions inside the lattice to some extent where they lie nearer to one another. This rearrangement alters the charge balance within the lattice and induces another electric field. The effects of the charge imbalance also permeate through the material. The result is the look of a net charge-either positive or negative-on one of the outer faces from the crystal. This subsequently results in a voltage over the oppositely charged crystal face. The piezoelectric charge can be harnessed, but when the strain stimulus is removed, the crystal lattice returns to the natural state and the voltage ceases.

In certain scenarios-such because the movement of the limb in wearable electronics, the movement of internal organs in implantable electronics, or the movement from the local surrounding environment in remote sensing/monitoring applications, to name a few-movements can make stresses over the piezoelectric material in the atomic scale that may then be harnessed.

In many cases where the PENG is used, the harnessing from the induced stress and the resulting electrical charge can then be employed to power a small device that it is attached to. However, in a few situations-mainly sensing-the nanogenerator can act as both powering device and the sensing device, because the generation of the electrical charge can behave as a usable and detectable output for the sensor in load-bearing/stress-sensing situations.

Why 2D Materials Are Showing Promise for PENG Energy Harvesters

2D materials show promise for PENG Energy Harvesters for a number of reasons. First, the inherent thinness and small size 2D materials enable the development of ultra-small harvesting devices which are small enough to power the very small nodes in IoT systems, power really small sensors in remote monitoring applications, and charge small-scale implantable or wearable medical devices. By comparison, bulkier materials would create harvesting/power systems which are too large and unfeasible of these kinds of applications. For this reason it's easy to see nanomaterials touted for wearable/implantable electronics, IoT, and remote sensing applications.

Another aspect may be the mechanical strength and flexibility of many 2D materials. Because the piezoelectric effect is induced by a few degree of mechanical deformation, the types of materials generating the electrical current need to be robust and be able to withstand many bending cycles. The inherent thinness of 2D materials implies that there is a very high amount of flexibility. While graphene has got the highest flexibility, inorganic materials have relatively high flexibility when compared with their bulkier counterparts, and other inorganic materials generally. When this flexibility is along with high mechanical strength, this means the 2D materials can withstand a lot of mechanical stress, leading to PENGs that can withstand many bending cycles, and as a result, be able to provide an electrical charge for extended time periods than when utilizing other materials.

Then, additionally, there are the opportunity to exhibit piezoelectric properties. Traditionally, piezoelectric properties are noticed in a range of inorganic materials, including natural and artificial crystal materials, synthetic ceramics, group III-V and II-VI semiconductors, as well as other metal oxide complexes. A variety of 2D materials are also known to exhibit piezoelectric properties, some of which are semiconducting materials. With regards to the materials of interest for PENGs, currently, hexagonal boron nitride (h-BN), various semiconducting transition metal dichalcogenides, group III and IV monochalcogenides, and chemically modified graphene-so that it is more semiconducting anyway rather than fully conducting because it naturally has no electronic bandgap-are the go-to choices.

Factors to Be Aware of with 2D Material PENGs

While the potential for creating PENGs using 2D materials exists, they, like every material, need to be utilized in the right way. In many cases, the piezoelectricity is only observed in single and few layered 2D materials. Once you get beyond this, the amount of piezoelectricity generated is insufficient to power devices. As more 2D layers are added, this diminishing effect continues to be related to the lattice distortion brought on by strain and the consequent charge polarization in the crystal. The greater layers, the stiffer the 2D material is, so the lower the induced quantity of strain, and for that reason, the lower the degree of crystal polarization and generated electrical charge.

There have also been some other interesting phenomenon discovered with some 2D materials referred to as layer dependence effect. While it isn't applicable to any or all 2D materials, it is not only the amount of layers that can influence the piezoelectric properties of the 2D material, but additionally whether it comes with an odd or even quantity of layers. It is because, in some cases, an odd quantity of layers has piezoelectric properties, but when the number of layers becomes even, the other layer becomes counterbalanced resulting in piezoresistive properties. This then reverts to piezoelectric properties once another layer is added, and so forth until the layers become too great to exhibit piezoelectric properties anyway

Nevertheless, despite needing to ensure that the 2D materials are used in the correct manner, there are several 2D materials that may be harnessed, including a few materials where their bulkier 3D inorganic counterparts show no piezoelectric properties. Nowadays there are also many ways to produce single and few layered 2D materials on the commercial level, so these kinds of challenges are not as crucial as they would happen to be even just not too long ago. So, there is a opportunity to escape in the traditional piezoelectric materials with regards to creating these small-scale nanogenerators.

Conclusion

The piezoelectric effect is a common phenomenon in a selection of bulk inorganic materials, but it's also observed in a range of 2D materials. 2D materials that may produce a piezoelectric charge that can be used in a selection of PENGs for powering small-scale devices. There are plenty of advantages of utilizing 2D materials in PENGs, including high flexibility and mechanical strength, as well as an inherent thinness, and PENGs offer a large amount of potential for small-scale energy harvesting in remote applications-be it IoT, monitoring, or medical applications.

Liam Critchley, Mouser Electronics