Throwing light on nanomaterials

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An international team of scientists has used a novel technique to measure, for the first time, the precise conditions at which certain ultrathin materials spontaneously become electrically polarised. The research provides the fundamental scientific basis for understanding this ‘ferroelectric’ state in materials needed for next-generation ‘smart card’ memory chips and other devices.

‘We provide a complete picture of how the ferroelectric transition temperature changes when the electrical and mechanical conditions change within nanoscale ferroelectric materials,’ said Xiaoxing Xi, professor of physics and materials science and engineering at Penn State University, who led the research effort.

The team is the first to use a technique known as ultraviolet Raman spectroscopy to reveal a range of temperatures, thicknesses and structural configurations at which nanoscale barium titanate can store a switchable electric field. The scientists also performed theoretical calculations to predict the point at which materials transition into this ferroelectric state. The results of these calculations closely match the results of the team's experiments.

‘We found that a film of barium titanate (BaTiO3) whose thickness is just four-tenths of a nanometre can retain its ferroelectric properties when it is layered in thin sandwiches with non-ferroelectric layers of strontium titanate (SrTiO3),’ said Darrell Schlom, professor of materials science and engineering at Penn State and a member of the research team. ‘This layer is just one molecule of barium titanate thick, the thinnest imaginable, but we have shown that it is ferroelectric at room temperature.’

Xi said: ‘The ferroelectric layer can induce ferroelectric properties in neighbouring layers that normally are not ferroelectric, especially in materials that are easily polarised. For example, we found that even one layer of ferroelectric barium titanate is capable of polarising 13 adjacent layers of strontium titanate.’

The scientists found that they could manipulate ferroelectricity by imposing different kinds of electrical and mechanical boundary conditions. The electrical conditions include the degree of resistance to polarisation of the non-ferroelectric material. The mechanical conditions included sandwiching ferroelectric layers between different layers of other materials, which mechanically restricts the movement of the atoms. By varying the thickness and composition of the nanoscale thin films, the researchers were able to change the phase-transition temperature by almost 500 Kelvin, obtaining ferroelectric properties more than 350 Kelvin above room temperature.

‘Our research shows that, under favourable conditions, room-temperature ferroelectricity can be strong and stable in nanoscale systems,’ Xi said.