In 1880 Jacques and Pierre (wife of Marie) Curie discovered that pressure generates electrical charges in some insulating crystals such as quartz and tourmaline, a phenomenon the researchers termed the”piezoelectric effect.” Later they noticed that electrical fields could deform piezoelectric materials. This effect is called the “inverse piezoelectric effect.”
Now, in a recent development first reported in the journal Nature, a research team from the Institut Català de Nanociència i Nanotecnologia (ICN2) in Barcelona have discovered that semiconductor materials also generate electricity in response to bending, an effect the scientists termed “flexoelectric.” Such materials, they found can generate more charge than insulators, and, like the reverse piezo effect, electrical fields also deform “flexo” materials.
The researchers have filed a patent and are seeking industrial partners to develop practical technological applications for flexoelectricity. The discovery is significant because semiconductors are common materials primarily used in transistors and photo-voltaic cells. The results indicate that it is possible in principle to use them as pressure sensors and electrical micro-generators.
Researchers show that bending semiconductors generates electricity
Some materials can generate a small voltage when bent and, conversely, can bend in response to a voltage. This phenomenon is called flexoelectricity, and until now, it was thought that the effect only existed in electrical insulators (materials that do not conduct electricity). However, a research team from the Institut Català de Nanociència i Nanotecnologia (ICN2) in Barcelona reports today in an article in Nature that flexoelectric-like effects are more ubiquitous than previously thought. The ICN2 researchers report that semiconductors, which can be thought of as halfway between electrical insulators and actual metals, also generate electricity in response to bending.
This is important because semiconductors are a common family of materials commonly used in transistors (the heart of integrated circuits) and photo-voltaic cells. The new results show that it is possible in principle to use them as pressure sensors and electrical micro-generators.
The authors of the study are Dr Jackeline Narvaez, PhD student Fabián Vásquez-Sancho, and the ICREA Research Professor Gustau Catalan.
Flexoelectricity happens as a consequence of any asymmetry in the deformation of a material. Bending a material forces the atoms closer together inside of the bend, and farther apart on the outside. This redistribution of the atoms forces the redistribution of their electrical charges, which can be exploited to establish an electrical current between the inside and the outside of the bend.
Until now, it was known that all electrical insulators can be flexoelectric. The surprising discovery by the ICN2 scientists is that semiconductors can also be flexoelectric –and in fact, they can generate more charge than insulators. The key ingredient of this new bending-induced electricity is in the surface of the materials.
The surface is the device
The Nature article, published online today, describes how crystals of an initially insulating material are able to generate 1000 times more bending electricity when they are doped to become semiconductors. The reason is that, although the insides of these crystals do become conducting as a result of doping, the surfaces remain insulating. This means that the surfaces not only can still polarize, but in addition, they receive a boost of extra charge from the semiconducting interior.