Crystal Plus Gold: a Powerful Semiconductor Duo?

Researchers at the University of Warwick have found that by adding a small amount of gold—or any other noble metal—to a crystal, they can alter its structure and induce electric effects it wasn’t previously capable of, such as converting heat into electricity.  This finding allows the new substance to effectively function as a semiconductor. 

The team’s method, which was explained in the journal Nature last month, demonstrates that the modified crystal material could be useful in sensing applications, energy conversion, and mobile technologies.


Altering the Crystal’s Structure

According to the research paper, the method relies on altering the symmetry of the crystal’s structure. A crystal can be made from several different atoms, so long as it has an ordered structure of particles that are formed in a symmetrical pattern.


Engineers can yield new semiconductor properties from crystals by altering its symmetry via an external stimulus. Image used courtesy of the University of Warwick


However, from a functional point of view, symmetry is not the goal, says co-lead author Professor Marin Alexe. Instead, researchers try to break this symmetrical structure in a way that yields new effects.

In this study, the crystal was found to function as a semiconductor and allow an electrical current to flow through it. By adding a small piece of metal—in this case, gold—to the crystal’s surface, the researchers created a Schottky junction. 


An atomic model of a gold-strontium titanate Schottky interface

An atomic model of a gold-strontium titanate Schottky interface. Image used courtesy of the University of Warwick


This induces an electric field that excites the semiconductor structure beneath the metal, breaking its symmetry and giving rise to new effects, including a piezoelectric and pyroelectric effect known as “interface effects.” Interface effects are confined in a shallow region of the crystal underneath the metal.


Great Potential for Sensing Applications

In this study, the researchers used the noble metals platinum and gold to create their junction. These were selected due to their high thermodynamic work function; however, other noble metals such as copper, silver, or iridium would be viable options, too. For the crystals themselves, titanium dioxide and silicon were used. 


Device that can characterize direct piezoelectric effect of Schottky junctions

Rendering of a device that can characterize the “direct piezoelectric effect of Schottky junctions.” Image used courtesy of the University of Warwick


Normally, these materials would not show a piezoelectric or pyroelectric effect. Once they undergo the changes stated in this study, however, they can output electricity when they experience a force (piezoelectric effect) or a temperature change (pyroelectric effect). The materials could therefore be used in sensing applications that require high sensitivity, or in other technologies that require energy conversion. 

Wrapping up the research paper, Professor Alexe said that his team’s work provides a completely different path to tweak the materials and tune the resulting effects, opening up many possibilities and applications, some of which may still be unknown.

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