For centuries, the task of materials science has been to describe and understand the laws of nature that determine the properties of materials. Why is copper a good electrical conductor? Why is glass transparent? Why is magnetite… magnetic?

Nowadays that we can already answer these questions, materials scientists face an even more ambitious task: creating materials by design, that is, engineering materials with tailor-made properties. Materials with new properties are especially desired, since they can be useful for some specific applications that we cannot even imagine.

A way to achieve this goal is creating superlattices: periodic structures made of an ordered sequence of building blocks or layers of different materials. The fascinating thing about these structures is that they behave like metamaterials, meaning that they have their own properties that can be tuned by controlling the stacking of the building blocks (composition, structure, width…).

In the framework of a collaborative research project led by Ilaria Zardo, from the University of Basel (Switzerland), counting with the collaboration of the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) (Spain), the Universitat Autònoma de Barcelona (UAB) (Spain) and the Technical University of Eindhoven (The Netherlands), the tuning of the vibrational properties of a crystal phase superlattice has been demonstrated for the first time. This superlattice is different from the conventional ones, since its basic LEGO^® bricks used as building blocks, rather than made of different materials, are made of different crystal phases of the same material. This finding has been published in the journal Nano Letters.

“This discovery paves the way to exciting future developments, because the interfaces between different materials are often rough and defective, while those between different crystal structures of the same material are remarkably sharp and clean, a crucial feature for many applications”, affirms Riccardo Rurali, researcher of the study based at the ICMAB-CSIC. “This is because the atomic species and the nature of chemical bonding stay the same while only the atomic arrangement changes”.

Until today, however, it was under debate if these novel systems could behave as conventional superlattices. Now we see that it is possible, and that they can go even further. These materials can have applications in the field of condensed matter and nanoscience, such as engineering efficient thermoelectric materials and nanoelectronics, or envisaging devices that can use heat for information processing.  

Figure: TEM image of the atomic arrangement of the superlattices. The change in the stacking of the different layers can be appreciated.

Article:

Marta De Luca, Claudia Fasolato, Marcel A. Verheijen, Yizhen Ren, Milo Y. Swinkels, Sebastian Kölling, Erik P. A. M. Bakkers, Riccardo Rurali*, Xavier Cartoixà, Ilaria Zardo*. Phonon Engineering in Twinning Superlattice Nanowires. NanoLetters. DOI: 10.1021/acs.nanolett.9b01775

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