Building a picture of transition metal oxides Novel electronic and magnetic properties can emerge in low-dimensional systems, where electrons are confined to a narrow area. We spoke to Professor Milan Radovic about his research into titanates, and how their properties can be tailored and controlled in lowdimensional systems, which could then open up wider possibilities in oxide electronics. The modern electronics industry uses large quantities of silicon in device production embedded in the current technology. However, transition metal oxides (TMOs) also have a range of interesting properties which are way more versatile than silicon. “Transition metal oxides can be insulating, metallic, magnetic, super-conducting, etc., and can also be used for optoelectronics,” says Professor Milan Radovic, a scientist in the Spectroscopy of Novel Materials Group at the Paul Scherrer Institute (PSI) in Switzerland. The main problem in terms of using transition metal oxides for industrial applications is that they are essentially very expensive as raw materials. “Silicon can be found pretty much everywhere and its big crystal can be grown easily,” points out Professor Radovic. “The transition metal oxides, however, include some rare earth elements which are “rare” and therefore very expensive and it will be hard to imagine that devices based on such elements will be produced on a large industrial scale. They are however very relevant for certain special needs and purposes, perhaps in novel quantum computing technology.”
Stronthium titanate As the Principal Investigator of several research projects based at the PSI, Professor Radovic and his team are now investigating how to control electron behaviour in titanates such as strontium titanate (SrTiO3), which is among the most widely used of the transition metal oxides. Strontium titanate has some very interesting electrical properties, now Professor Radovic aims to gain deeper
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insights into this compound, which will help lay the foundations for further development in oxide electronics. “We want to understand it in order to control and further improve its properties,” he says. The electronic structure is the key consideration in terms of understanding the properties and behaviour of the system. “If the system is metallic than it is usually simple, while if there is a gap between conduction and valence bands, then you may aim to modify system to generate more exotic properties.” This research includes investigation into titanium dioxide (TiO2), which can be thought of as the parent compound of the titanates. Professor Radovic’s research is largely focused on artificial materials in the form of
thin films with a thickness of just a few unit cells. “Strontium is an element that tends to diffuse, segregate or intercalate in the oxide matrix. Therefore we decided to use titanium dioxide, which is a simpler compound, and to investigate the effect of Sr contents, hoping to create a system with novel properties,” outlines Professor Radovic. The focus here is on low-dimensional electron systems, where electrons freely move in the confined area, leading to the emergence of new physics and properties that many scientists are now working to harness. In their research, Professor Radovic and his colleagues are able to transform a 3-dimensional transition metal oxide into a 2-dimensional system. “In this 2-D system the electrons propagate only in plane. This dramatically affects the electronic properties of the system,” he outlines. A pulse laser deposition system is used to make a 2-D system (physically, it is an utra- thin film) in what is essentially a bottom-up process, an approach which gives researchers a high degree of freedom. “We have more parameters to play with,” explains Professor Radovic. “During the growth of the thin film you can incorporate substitutions or defects which consequently affect a crystal lattice and charge carrier concentration.” Researchers are investigating how they can then control and tune electron behaviour. This essentially involves putting two oxides in close proximity to each other, with a 2-D electron gas at the interface between them, the structure of which is
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