Q&A with Sergei Kalinin
This schematic illustrates coupling between physical (magnetic, mechanical, and ferroelectric) and chemical functionalities. In materials with ionic motion frozen, physical effects dominate. However, on nanometer-scale surfaces and interfaces, electrochemically mediated mechanisms can manifest even at low temperatures and give rise to giant functional responses. Figure courtesy of Stephen Jesse, Oak Ridge National Laboratory.
The Aug. 23 issue of Science magazine includes a perspective titled “Functional Ion Defects in Transition Metal Oxides” by Oak Ridge National Laboratory’s Sergei Kalinin and ETH Zurich’s Nicola Spaldin. The following is an interview with Kalinin about the article, which discusses new directions in condensed matter physics and materials science research.
Q: Your perspective deals with the varying ways that scientists from different disciplines study a material. What are some of the differences between physics and chemistry that you address in the article?
Kalinin: Physics is generally a science of what happens to matter when the amount of atoms stays constant. By comparison, chemistry is a science where the number of atoms does change, but it is very rare that people analyze how these phenomena can be connected. For example, what will happen if the number and positions of atoms change during physical measurements?
We started to read a lot of papers about the electrochemistry of batteries and fuel cells, and we realized that materials they are made of are exactly the same as those the physics community studies for their unique physical properties. But these communities rarely talk to each other.
Q: What are transition metal oxides? How are they used in applications today?
Kalinin: Transition metal oxides contain metals that can change their oxidation states, which means they can accommodate a varying number of oxygen atoms in the system. When you change the number of oxygens in the metal, it affects the optical, electronic, transport, and magnetic transport properties. Exploiting these intrinsic physical properties has led to the widespread use of transition metal oxides in applications such as sensors that can detect magnetic and electric fields, convert mechanical energy into electrical energy, or store information in ferroelectric or resistive states.
Q: What are ionic defects? How do they affect the properties of transition metal oxides?
Kalinin: Ionic defects are atomic-level anomalies in the material. This means either ions (usually oxygen) have been removed from their proper lattice sites, creating vacancies, or that a different type of ion has been substituted into the material. These small changes can cause profound changes in the material’s properties -- creating magnetism in a system that was previously non-magnetic, for instance.
What is more important is that ionic defects, mainly the oxygen vacancies, can move into and out of a system, as well as within it. This can cause, for example, a material to transition from a conductive metal to an insulator if you change the oxygen pressure during an experiment. This vacancy motion can happen during “classical” physical measurement, even at low temperatures.
Q: Give us an example of a chemical versus a physical approach to an experiment.
For instance, the usual assumption is when a solid oxide fuel cell operates, vacancies flow through the system; when you do physical measurements, the vacancies do not flow. It makes sense: Fuel cells work at 600 or 700 Celsius, whereas physical measurements are made at room temperature. But now people are exploring fuel cells at progressively lower temperatures, and they’re seeing that ionic dynamics happen at 300 Celsius, which is getting closer to room temperature. Furthermore, once we go to the nanometer scale and apply large biases -- which happens when you use contacts, tunneling barriers and other nanoscale devices -- ion flow can occur at low temperatures. As in many areas, the boundary between physics and electrochemistry is blurred or non-existent on the nanoscale.
Q: What are the implications of taking ionic defects into consideration?
Kalinin: If you combine all these studies in this field, you realize that oxides can no longer be considered to be static objects. In other words, electrochemistry in oxides happens one way or another -- whether we want it or not. If we embrace it and use it, it’s a better way to get improved materials and devices. If we can control electrochemistry at the interfaces between oxides and metals, that may be a much more effective way to control electronic devices than just exploring the physical couplings.
Q: In your perspective, you compare today’s research in oxides to the early studies of semiconductors. What lessons do you draw from the history of science?
Kalinin: When I was reading a few books about the classical history of science, the biggest surprise for me was learning about the discovery of the transistor. I did not know at that time that the original Bell labs team that made the discovery included electrochemists. Very early on, they realized that one reason transistors did not work was that the surface got dirty. There is an anecdotal story about someone figuring out that transistors don’t work because someone had touched the copper door handle, and when even this small amount of material got transferred from the skin to the device, it killed the transistor.
It is very interesting to compare this history with the oxide community, because now we are starting to realize that ionic dynamics can be important to the physics we want to study. The difference is that in the semiconductor field, they figured out that there was a problem and got rid of it. In our case, we realize that: a.) we can’t get rid of it and b.) it’s probably more of an opportunity rather than a problem. If you want to make functional oxides and develop systems with very strong responses, electrochemistry is a way to get there. And of course, we need to understand it in order to control it. In this, I was delighted to realize that we are on the same page with Nicola Spaldin, as well as a lot of scientists at ORNL, Argonne, and many other places, leading to the present perspective.
Q: How does ORNL’s Center for Nanophase Materials Sciences enable research in the areas you’ve discussed?
Kalinin: The science at CNMS is organized in three research themes that allow for interaction and synergistic scientific research among groups with very different expertise. One of the themes is “Electronic and Ionic Functionality on the Nanoscale,” which targets phenomena on the boundary between physics and electrochemistry. This research is ideally positioned at ORNL, since these studies demand an integrated approach that requires scanning probe and electron microscopy, as well as access to neutron facilities and theory, all of which are ORNL strengths.
Sergei Kalinin is the Theme Leader for Electronic and Ionic Functionality on the Nanoscale at ORNL’s Center for Nanophase Materials Sciences.
August 23, 2013