Korean researchers find atomic stacking order can add new functions to materials

The science ministry said Tuesday that a Korean research team has demonstrated a way to add new functions to materials by changing only the order in which atoms are stacked, without introducing new elements. The work, led by Lee Dae-su of Pohang University of Science and Technology, was published in the journal Science on Oct. 2, according to the Ministry of Science and ICT.

The team, which also includes Lee Gil-ho and Choi Si-young of POSTECH and Park Se-young of Soongsil University, designed a method it calls selective atomic gradient engineering. Atoms tied to a material's electronic properties are left in place, while the proportion of differently sized atoms is varied layer by layer through the thickness of the film. The uneven arrangement leaves the atoms slightly displaced in one direction, producing a polarity inside the material, and reversing the stacking order reverses the direction of that polarity. The same atoms mixed evenly showed no such directionality.

Applied to a metal thin film, the approach preserved the metal's electrical conductivity while producing a resistance that differed depending on whether current flowed in one direction or the other. Reversing the atomic order flipped the sign of that resistance difference, showing that a material's electrical direction can be set during fabrication.

In an ultrathin insulating film about 5.1 nanometers thick, the design sharply reduced leakage current while maintaining a high capacity to store electrical energy. Its ability to convert temperature changes into electrical signals was about 100 times higher than that of a comparable thin film of similar thickness, and reversing the atomic stacking order also reversed the direction of the signal generated by a change in temperature. The ministry said the results are expected to be used in developing electronic devices that behave differently depending on the direction of current, ultrathin insulating films for next-generation semiconductors that reduce leakage, and infrared and thermal sensors that detect temperature change.

"We proved that by changing only the order in which atoms are stacked, without adding a new element, it is possible to keep a material's original strengths and add a new function," said Lee Dae-su, the professor at POSTECH who led the research.

Lee said the team plans to extend the method to more diverse materials, including magnets and superconductors, and to carry out follow-up work to realize it in actual devices such as ultrathin insulating films and thermal sensors.