An atomic model of the Schottky interface between a gold layer and a crystal of strontium titanate. Image: University of Warwick.In a display of modern-day alchemy, researchers at the University of Warwick in the UK have shown that a touch of gold – or another noble metal – can change the structure of a crystal and thus alter its intrinsic properties. As the researchers report in a paper in Nature, they have found a way to induce completely novel electric effects in crystals, such as converting movement or heat into electricity, simply by adding a piece of metal to their surface.
The key to the process is breaking the symmetry of the crystal's structure. A crystal can be made from any number of different atoms, but these atoms always form a structure with a symmetric pattern.
"In physics, those materials are rather boring," said Marin Alexe from the Department of Physics at the University of Warwick, who is co-lead author of the paper. "From the point-of-view of functionality, symmetry is not the greatest thing you want to have. You want to break the symmetry in such a way that you would get new effects."
Alexe and his colleagues utilized crystals that can function as a semiconductor, allowing an electrical current to flow through them. By adding a small piece of metal to the crystal surface, the researchers created a junction known as a Schottky junction. This induces an electric field into the semiconductor that excites the semiconductor structure underneath the metal, breaking its symmetry and inducing new effects that were not previously possible.
These included a piezoelectric effect, where movement is converted to electrical energy or vice versa, and a pyroelectric effect, where heat is converted to electrical energy. They are known as interface effects and were confined to a very shallow region of the crystal, underneath the metals.
"Generally, the properties of these crystals are determined by two factors: the intrinsic properties of the elements that the crystal consists of, and how those elements are arranged to form that crystal, which we call its symmetry," explained Mingmin Yang, who conducted the work at the University of Warwick and has since moved to the RIKEN Institute in Japan.
"Our research is demonstrating that how those elements are arranged is not just determined by their own nature, they can also be tuned by external influence. Once we use that influence to change their arrangement, they can exhibit properties that were previously prohibited to them."
The researchers used the noble metals gold and platinum to create their junction due to their high thermodynamic work function, but copper, silver or iridium would also be good options. For the crystals, the researchers utilized strontium titanate, titanium dioxide and silicon. None of these materials would normally show a piezoelectric or pyroelectric effect.
Once the materials display the piezoelectric or pyroelectric effect, they can output electricity when they experience force (in the case of the piezoelectric effect) or a temperature change (in the case of the pyroelectric effect). By detecting any electricity generated in the materials, the researchers were able to confirm the existence of these effects.
These novel effects could allow the crystals to find use as sensors, which require high sensitivity, or in technologies relying on energy conversion. By taking advantage of the piezoelectric effect, the crystals could harvest energy, or work as an actuator or transducer. By taking advantage of the pyroelectric effect, they could work as a sensor or in infrared imaging. In addition, the small scale that this effect is seen on and its high efficiency would make it ideal for use in mobile technologies.
In previous work, the researchers examined mechanical means for breaking crystal symmetry, whereas this work looked at the possibility of breaking symmetry using an electric field. "Materials with broken symmetry are rich in functionalities," said Alexe. "To improve these functionalities, you usually need to tweak the material structure. This requires deploying complicated solid-state chemistry followed by detailed investigations.
"You now have a completely different path to tweak these materials and the ability to tune the effect, something that we have not been able to do before. That opens the field to many other possibilities with these materials and we might not know where those lead."
This story is adapted from material from the University of Warwick, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.