Bench Talk

Scientists are always finding new ways of producing and harvesting energy. This will only increase as the demand for energy increases, which is going to happen with the current population and technology growth around the world. While most scientists are concerned with the creation of large amounts of energy and increasing the efficiency of existing devices so that more energy can be harvested from nature, there is a growing movement among the nanotechnology community in creating nanoscale devices that harvest small amounts of energy. Many may ask what’s the point of such small energy harvesting devices, but they have significant potential for performing self-powering operations in small and remotely located devices, such as those used in health monitoring, environmental monitoring, wireless transmissions, sensor, and Internet of Things (IoT) network applications.

The short answer is that they are devices that convert either mechanical or thermal energy into electricity via a physical change within the nanogenerator. There are three main types of nanogenerators:

• Piezoelectric
• Triboelectric
• Pyroelectric

The internal structure and energy harvesting mechanisms vary from nanogenerator to nanogenerator. So, to understand how they work, we need to look at each one individually.

Piezoelectric Nanogenerators

Piezoelectric nanogenerators convert kinetic energy into electrical energy using materials that can exhibit the piezoelectric effect, i.e., the generation of an electrical charge under mechanical stress and/or deformation. The structural configuration of these devices can be very diverse depending on the materials used. Many different types of materials can be used from solid-state materials that adopt a wurtzite or a blende lattice confirmation to perovskite materials and certain polymers. These materials are fabricated in nanowire forms and are then implemented vertically or laterally on top of a metal layer to create a Schottky contact. Some nanowires can also be embedded within a polymer matrix. Regardless of the structural construct, all the nanowires within the nanogenerators are connected to electrodes at either end.

Because there are many ways that the active piezoelectric components can be arranged, there is more than one working mechanism. However, a piezoelectric nanogenerator will usually rely on one of two common mechanisms:

• The first is created by exerting a force perpendicular to each nanowire. In this method involving a perpendicular exertion of force, the piezoelectric material experiences an applied force from a moving tip. This tip causes a deformation of the piezoelectric nanomaterial that generates an electric field. The stretched and compressed parts of the material then exhibit a positive and negative electrical potential, respectively, due to the displacement of ions within the nanowires. Because both cations and anions are displaced, a charge separation within the nanowire(s) occurs which generates an electrical potential at the surface next to the tip, while the opposite surface acts as the ground. Because a metal layer is used next to the nanowires as a Schottky contact, this helps to facilitate the movement of electrons and generate an electrical current.

• The second mechanism is uniaxial compression. Uniaxial compression is used for devices where the nanowires are connected at one end to a Schottky contact and an Ohmic contact at the other end. As a compressive force is applied to one end of the nanowires, it generates a negative piezoelectric potential at the Schottky end of the nanowire. This also causes the Fermi level of the nanowires to increase and electrons flow from the top of the nanowire to the bottom via an external electrical circuit, which generates a current.

Triboelectric Nanogenerators

Triboelectric nanogenerators are devices that convert external mechanical energy into electrical energy via two main principles:

• The first principle is the triboelectric effect. The triboelectric effect is a form of contact electrification, where the material becomes electrically charged under frictional forces.  

• The second principle is electrostatic induction. Electrostatic induction is the distribution of electrical charges in a material due to the other surrounding charged species.

There are multiple mechanisms by which these nanogenerators work. These include:

• Mechanisms involving mechanical stress deforming the nanomaterials—and subsequently releasing—so that the charges will separate across two different materials and generate an electric field that causes electrons to flow.

• Mechanisms where two nanomaterials slide over each other to induce friction and the migration of like charges to separate nanomaterials—again inducing an electric field and electron migration to form a current.

• Mechanisms that use an electrode to induce a certain charge in one nanomaterial. This generates opposite charges on a second nanomaterial, which causes the migration of electrons from one material to the other to induce a current.

Overall, regardless of the specific mechanism, two nanosized films—which can be organic or inorganic in nature—that have opposite tribo-polarities induce a charge transfer mechanism that ultimately leads to the generation of an electric field, the migration of electrons between the nanomaterials, and an electric current.

Pyroelectric Nanogenerators

A pyroelectric generator uses pyroelectric materials—those that generate an electrical current under a change in temperature—to convert external thermal energy such as heat into an electrical current. The types of materials that can be used in these nanogenerators are narrower than other nanogenerators, and they often use either ferroelectric materials or solid-state materials with a wurtzite crystal structure. These materials are again connected at either end by electrodes.

There are two different mechanisms by which pyroelectric nanogenerators self-generate electricity: 

• The first mechanism doesn’t involve a physical change of the nanomaterials inside the nanogenerator; instead it involves the movement of their electric dipoles. The orientation of these dipoles naturally fluctuates at room temperature, but when a higher external temperature (above room temperature) is present around the nanogenerator, these dipoles move and oscillate at a much larger rate. This, in turn, causes the number of induced charges in the electrode to drop—i.e., the charges which are induced by the dipole in the nanomaterial—and this reduction of charges causes electrons to flow and a current to be generated.

• The second mechanism involves a physical change due to a change in temperature. In this mechanism, when there is an increase in localized temperature, it causes the nanomaterial to thermally expand and deform. This deformation creates a piezoelectric potential difference in the material, which causes the electrons to migrate to an external circuit and enables the electric current to flow.

Conclusion

Overall, there are many different types of nanogenerators that rely on different materials and different operational mechanisms to generate electricity from physical movements or thermal energy. While the generated electricity in many of these devices is small, it is enough to power a small remote device that can be self-sufficient in terms of power. This has great potential for future devices that will be used for remote monitoring applications and those that are part of IoT networks.