Turning a pinch of salt into an electrical switch

A team of scientists from the LCN at UCL, the University of Liverpool in the UK and the Advanced Microscopy Laboratory at the University of Zaragoza in Spain have discovered a way to induce and control a fundamental electrical switching behaviour by interfacing atomically thin layers of two materials that do not exhibit such behaviour by themselves.

Working on a project funded primarily by Specs GmbH in Germany and the Engineering and Physical Sciences Research Council (EPSRC) in the UK, the team reported in the journal Nature Nanotechnology how separating an atomically thin layer of rock salt material – including ordinary table salt – from the surface of metallic copper by including an atomically thin layer of copper nitride in between causes the positively and negatively charged atoms (called ions) in the rock salt to shift vertically with respect to each other. This creates a layer of so-called “electric dipoles”, whose orientation can be switched by applying a large electric field.

If you turn most materials upside-down, they look the same at the atomic level. Because of this symmetry, the electrical charges in the atoms cannot have a preference for orienting along a particular direction. In some materials, however, this symmetry is broken, and positive and negative charges can be spatially separated and line up to form electric dipoles. If these dipoles can be switched between multiple orientations with an electric field and remain in those states after the electric field is removed, the material is commonly referred to as a ferroelectric because it is the electrical analogue of a ferromagnet.

Ferroelectric materials play an important role in various electrical and electromechanical devices, and because of their intrinsic switching behaviour there is great interest in using nanoscale ferroelectrics for a new form of high density data storage. However, the outermost layers of a ferroelectric material often lose their ability to switch when they are incorporated into an electrical circuit. This makes it difficult to scale ferroelectric materials down to the atomically thin level. Furthermore, only a limited class of materials is known to exhibit the dipole switching that is the key property of ferroelectrics.

To overcome these difficulties, the scientists explored whether the new properties that many materials can display when they are made to be only a few atomic layers thick could be exploited to create a different kind of dipolar switching material. Atomically thin materials, often referred to as two-dimensional (2D) materials, can have properties that are dramatically different from those of their thicker counterparts even though they are in essence the same material. There is currently great interest in exploring how these properties can be further enhanced by building structures with layers of different 2D materials.

The team started by forming an atomically thin layer of nitrogen and copper (coper nitride) on the surface of a copper crystal. The copper nitride is an insulating layer, and it has dipoles that point perpendicular to the surface but that cannot be reversed with an electric field. On top of this, they deposited an atomically thin layer of rock salt material, specifically sodium chloride (ordinary table salt) and potassium bromide, which do not have net dipoles.

The interaction between the copper nitride and rock salt layers when they are bonded to each other has two dramatic consequences,” said Jose Martinez Castro, the lead author of the paper. “First, the copper nitride induces electric dipoles in the rock salt layer. Second, because the copper nitride is both insulating and weakly interacting with the rock salt, it allows the dipoles to be reversibly switched by applying a large enough electric field even though the rock salt is only a single atomic layer away from the metallic copper.”

Many of the most promising proposed applications for 2D materials involve incorporating them into electrical circuits, so much attention has been focused on conducting 2D materials. However, 2D insulators are beginning to play an increasingly important role. “This work highlights the exciting opportunities that are possible when different atomically thin materials are brought together,” remarked Cyrus Hirjibehedin, the project’s lead scientist. “By stacking two 2D materials, even those that are insulators, we can create new behaviour that neither material would be able to exhibit individually. This opens a wealth of new possibilities for developing a new generation of 2D material structures.”

Figure: Illustration of an electrical circuit exploiting rock salt crystals. Insets show a ball model of the crystal structure of an ultrathin rock salt layer on a copper nitride layer in the surface of a bulk copper crystal as well as an STM image of this structure. (Image courtesy of David Serrate and Jose Martinez Castro)
    

PublicationElectric polarisation switching in an atomically-thin binary rock salt structure, Nature Nanotechnology (2017); DOI: 10.1038/s41565-017-0001-2

Funding for this project was provided by Specs GmbH in Germany; the Engineering and Physical Sciences Research Council (EPSRC) in the UK; and the Ministry of Economy and Competitiveness (MINECO) in Spain.

Other contributors: 
Jose Martinez Castro (former LCN); Marten Piantek, Sonja Schubert and David Serrate (University of Zaragoza); Mats Persson (University of Liverpool)