Crystalline personality disorder – Chiportal

The surface of the strontium titanium crystal generates electricity when heated. This surprising feature may also be discovered in other materials and used to create new electronic applications

Glossy crystals, once used to forge diamonds, have recently been discovered to have a split personality. Scientists at the Weizmann Institute of Science have discovered that the surface of a strontium titanate crystal is fundamentally different from the crystal in a manner not previously known. “It has been discovered that the surface of the crystal behaves as a separate substance,” read the discovery in the news and opinion section of the scientific journal Nature.

Strontium titanium crystals are now mainly used as a substrate for growing a variety of advanced electronic materials. These crystals are known to be non-polar, but the institute’s scientists have discovered that a very thin layer on their surface is pyroelectric – that is, it generates electricity when heated – which means that this layer is polar. Because polarity affects the properties of a material, the “split personality” of the crystal must be recognized when producing devices containing strontium titanium. Furthermore, the finding has a broader meaning: polarity may also be found on the surface of other inorganic crystals.

“The polarity of the surface in inorganic material is a new concept,” says the head of the research group, Prof. Igor Lubomirsky from the Department of Materials and Surface. “No one has searched for her so far because she is seemingly contrary to intuition.”
A few years ago, Prof. Lubomirski and his colleague in the department, Prof. Meir Lahav, showed that warming-dependent polarity can form on the surface of non-polar organic crystals, such as certain amino acids. But inorganic crystals are characterized by stiffer symmetry than organic crystals, so it seems that the chances of finding polarity in them – which requires breaking symmetry, for example, through distortions in the structure of the material – are not high. However, Prof. Lubomirski and Prof. Lahav hypothesized that even in inorganic crystals symmetry is broken – not on the face of the crystal though, but on the surface – so perhaps, against all odds, polarity also exists in non-polar inorganic crystals.

To test this hypothesis, scientists had to show that if and when they discover polarity in an inorganic crystal, it is indeed covered inside the surface of the crystal, and not inside the crystal – a task that poses a huge technological challenge. Dr. Elena Mirzada, then a research student, and other group members used unique equipment developed in Prof. Lubomirski’s laboratory to measure very weak electric currents over a period of a few microseconds (millionths of a second). The scientists heated the surface of strontium titanium crystals using a rapidly changing laser beam and measured the resulting electric current. As recently reported in the scientific journal Advanced Materials, they discovered that a current did form, but it decays rapidly – a fact that indicates that the current appears only in the top layer consisting of individual atoms, about 1 nanometer thick (one billionths of a part) thick.

The experimental system that enabled the detection of pyroelectricity on the surface of strontium titanium crystals

To rule out the possibility that the current formed inside the crystal, the scientists conducted additional experiments. Among other things, they coated the crystals with a very thin layer of silica and found that the pyroelectricity had disappeared – a finding that supports that this is only a phenomenon of the surface.

Prof. Lubomirski and colleagues conducted a theoretical analysis of the findings and concluded that the pyroelectricity was created as a result of a common distortion called “surface relaxation”. Because this relaxation exists in many crystals, scientists believe that surface electroelectricity may be manifested in many and varied materials. They propose to take advantage of this phenomenon to design multi-layered devices that will allow the different polarity of the layers to be used to create desirable features for new applications in the electronics, nanotechnology and energy industries.

Moreover, the discovery may hold additional surprises: An opinion piece in Nature that analyzed the study hypothesized that although strontium titanate is an insulating material, its surface may actually be polar metal – an exotic material that is expected to have special electronic properties. Many research groups are looking for polar metals, but according to the journal, it is possible that “they have been under our noses all along and that they are, paradoxically, on the surface of non-polar insulators.”

Yevgeny McGagon and Dr. David Ahara from the Institute’s Department of Materials and Surfaces participated in the study; Dr. Hagai Cohen and Dr. Irit Goldian from the Institute’s Department of Chemical Research Infrastructure; Dr. Dennis Christensen, Dr. Argia Baumik, Dr. Juan Maria Lastra and Prof. Nini Frieds of the Technical University of Denmark; Prof. Ari Morels of Stone Hall University; And Prof. Andrew Rap of the University of Pennsylvania.