Technology is constantly evolving, as do the technologies the engine behind it. In the past many energy storage, energy creation and energy harvesting equipment have been created to supply electricity to huge electronic systems as well as personal electronic gadgets in a variety of ways. As the world moves toward Industry 4.0 and the Internet of Things (IoT), there is a chance to develop various kinds of ultra-small gadgets which can be used for remote monitoring and automation and telemedicine.
Powering small-scale devices–especially in remote applications–requires unconventional self-powering mechanisms to be self-sufficient. In recent times there has been an array of energy harvesters that are small in size, referred to as nanogenerators have garnered attention for their ability to power small-scale devices used in remote monitoring, medical as well as IoT applications. The tiny size of these devices ensures that they aren’t too large for the devices they power. Despite their tiny size, they still offer enough energy to allow a variety of devices to self-charge in their own operating environment. In certain instances it may be possible to utilize nanogenerators for large-scale harvesting if many different devices are integrated into a single harvesting system. However this hasn’t been studied in a comprehensive research standpoint.
Although there are a variety of nanogenerators available and applications, they’re all utilized in different environments due to the fact that the production of electric charges is usually controlled by environmental stimuli. One of the most promising and widely discussed and extensively researched Nanogenerators that is being studied extensively is the piezoelectric one, which is often called in shorthand the PENG.
Using the Piezoelectric Effect
PENGs make use of the piezoelectric effect in order to create electricity. The piezoelectric effect occurs caused when an electrical charge is produced when there is a load or stress on a piece of material. It is a reverse phenomenon and, after the stress has been removed, the charge ceases. This means that the piezoelectric effects could be working in the reverse direction in which an electrical voltage is transferred to the materials creating the atomic structure the material to change and then become stressed-induced.
Regarding the mechanisms that are involved it’s the rearrangement of the ions at the atomic scale–within the solid state lattice–that creates the piezoelectricity. Most piezoelectrics are inorganic at the atomic level, and even when they’re not however, they do have some kind in crystal structures (inorganic materials are also crystal-like). This implies it is (mostly) this type of material is an array of regular repeating patterns of well-ordered anions and cations in its lattice of atoms. It’s the deformation of the ions in this patterning lattice that creates an electric charge. The material is still able to hold neutral charges, the overall charges of the substance does not change, but only the distribution of charges at the atomic level change.
In other words, when the load/stress is applied to the piezoelectric materials the ions charged in the opposite direction shift from their initial places within the lattice until they are closer to each the other. This alters the balance of charge within the lattice. It also creates an electric field that is external to the material. The effects of the imbalanced charge can also be felt throughout the entire crystal. This results in appearing as a net charge, either negative or positive on one of the outside sides that the crystal. The result is an electrical charge on the opposite facing crystal. The piezoelectric energy can be attained, but once the stress stimuli are removed and the crystal’s lattice is returned to its original state, and the voltage decreases.
In some scenarios, for example, the movement of limbs in wearable electronics as well as the movement of internal organs within implantsable electronic devices, and the motion of the surrounding environment in remote sensing and monitoring applications for example, movements can cause stress on the piezoelectric materials at an micro-scale that could later be utilized.
In the majority of cases when PENG is employed to harness the stress that is induced and the electrical charge that results is then used to power a tiny device it is connected to. However, in certain situations–mainly sensing–the nanogenerator can act as both the powering device and the sensing device, as the generation of an electrical charge can act 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 are promising to be used in PENG Energy Harvesters for a variety of reasons. First, the tiny size and thinness of 2D materials allow the development of ultra-small harvesting systems that can be small enough to power tiny nodes that are used in IoT systems, as well as power small sensors for remote monitoring applications as well as charge wearable and small-scale implantable medical devices. Contrarily, larger materials can create power and harvesting systems that are too big and incompatible with these kinds of applications. This is the reason you frequently find nanomaterials marketed for wearable/implantable electronic devices, IoT, and remote sensing applications.
Another factor is the durability and flexibility of a variety of 2D materials. Because the piezoelectric effect created by some degree of mechanical deformation, materials that generate the electric current have to be durable and capable of enduring many bend cycles. The very thin nature of 2D material means they possess a very flexible nature. Although graphene is the most flexible material, it also has the greatest flexibility, inorganic substances have an incredibly high degree of flexibility when compared to their more bulky counterparts as well as other inorganic substances generally. If this flexibility is combined with a high strength mechanical which means that 2D materials are able to withstand an enormous amount of mechanical strain, which can lead to PENGs that can endure many twisting cycles, and consequently create electricity for extended durations than other materials.
Additionally, there’s the possibility of exhibiting piezoelectric characteristics. The typical piezoelectric properties can be observed in a variety of inorganic materials. These include synthetic and natural crystal materials such as synthetic ceramics, II-VI and group III-V semiconductors as well as various metallic oxide compounds. Numerous 2D substances are also thought for their piezoelectric characteristics that include semiconducting substances. As for the kinds of PENGs’ materials of interest currently, they include hexagonal boron nutride (h-BN) and various semiconducting transition metals, dichalcogenides of the transition metal, monochalcogenides of group III and IV and graphene that has been chemically modified so that it’s semiconducting in nature , rather than fully conducting because it is naturally inaccessible to electronic bandgap are the most popular selections.
Important Factors to Consider with PENGs for 2D Materials
While the potential to create PENGs with 2D materials is there however, as with any material, must be used in a correct manner. In most cases, piezoelectricity only occurs in the form of a single layer or a handful of layers 2D materials. When you go beyond this, the amount of piezoelectricity produced is not enough to provide power to devices. With each addition of 2D layers get added to the system, this decreasing effect is believed to be due to the distortion of the lattice caused by strain and consequent charge polarization of the crystal. The more layers added, the more rigid the 2D material becomes, which means less is the amount of strain, and consequently it is less likely to be a case of crystal polarization as well as the generated electrical charge.
There is also an additional interesting phenomena discovered in certain 2D materials, referred to for its layer dependent effect. Although it’s not applicable to all 2D materials It’s not just the quantity of layers that affect the piezoelectric properties of a 2D material as well as whether there’s odd or even amount of layers. This is because in certain situations the odd number of layers exhibits piezoelectric properties. However, once the layers become even, another layer gets counterbalanced, leading to piezoresistive properties. Then, it reverts back to piezoelectric properties when the layer gets added and so on until the layers are too numerous to display piezoelectric properties .
Yet, despite the need to ensure that 2D materials are being used correctly there are many kinds of 2D materials that are able to be utilized, such as several materials that their heavier 3D inorganic counterparts do not exhibit piezoelectric properties. There are a variety of ways to develop two-dimensional materials at a commercial scale and, therefore, these types of problems aren’t as important as they be in the past. There’s an opportunity to depart from the conventional piezoelectric materials in the process of creating nanogenerators that are small in size.
It is an atypical phenomenon that occurs in a variety of bulk organic materials however, it can also be seen in a variety of two-dimensional materials. 2D materials can produce the piezoelectric charge, which can be utilized in a variety of PENGs to power small-scale devices. There are numerous advantages of making use of 2D materials in PENGs such as high flexibility and mechanical strength and an inherent thinness. PENGs have a great opportunities for small-scale energy harvesting remote applications, be it IoT monitoring, monitoring medical or monitoring.