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Mechanical Properties of Crystals on the Atomic Scale

Research Group Succeeded in Mechanical Characterization and Structure of Salt Nanocrystallites

№ 133/2010 from May 07, 2010

Researchers around the experimental physicist Leonhard Grill of Freie Universität Berlin, in collaboration with theoretical physicists around Christian Joachim at Centre d’Elaboration de Matériaux et d’Etudes Structurales (CEMES) in Toulouse, in the European research project “Pico-inside” were the first to succeed in structuring crystalline salt films only a few atomic layers thick. This is significant for future practice in materials science. Salt structures are of particular interest because in the future they could be used as electrical insulators within a molecular electronics circuits. The results were published in the latest edition of the prestigious journal Physical Review Letters.

Using this procedure, controlled breaking and cutting of nanocrystallites is possible, as well as specific positioning. In addition, the researchers were able to show that Hooke’s Law, known in macroscopic physics for over 300 years, is also valid on the atomic scale in regard to bending nanostructures. According to Hooke’s Law a macroscopic body stretches in proportion to the force acting on it.

The mechanical behavior of crystals under stress ranges from elastic deformation to permanent change. While materials with macroscopic dimensions are well researched in terms of their mechanical behavior, because of the experimental challenges there are no studies on the scale of a few nanometers – one nm is a billionth of a meter – close to the field of atomic distances within the crystal. The propagation of elastic deformation in crystalline nanostructures is of interest, as is the transition to breaking. Second, the ability to provide for controlled deformation offers the possibility of building specific structures on the atomic level.

After letting the salt crystallites with dimensions in the range of a few nanometers grow on a copper crystal in ultrahigh vacuum, the scientists used the tip of a scanning tunneling microscope, either to move the crystals on the surface like cutting with a knife – whereby channels with the width of a few atoms are formed – or to break them. When breaking the crystallites, at first an elastic bending occurs, followed by a break along a crystalline preferred direction, or plastic deformation. Calculations reveal the distribution of elastic strain in the crystallite and show that the elastic deformation follows Hooke’s Law, as known since the 17th century.

An understanding of mechanical properties on the atomic scale is of great significance for material properties under stress. The reaction ranges from elastic deformation to the formation of dislocation lines and permanent change. The approach successfully implemented in this research opens a possibility to study the mechanical behavior of materials in the smallest dimensions. In addition, according to the researchers, the targeted structuring of nanocrystallites offers the potential to create tailored structures on the atomic level.

Further Information

Leonhard Grill, Institute of Experimental Physics, Freie Universität Berlin
Tel.: +49 (0)30 / 838-52226; Email: grill@physik.fu-berlin.de

Internet:

http://users.physik.fu-berlin.de/~grill

Publication:

Ch. Bombis, F. Ample, J. Mielke, M. Mannsberger, C. J. Villagómez, Ch. Roth, C. Joachim, and L. Grill: Mechanical behavior of nanocrystalline NaCl islands on Cu(111). Physical Review Letters (May 7, 2010)
Internet: http://prl.aps.org