22 June 2016 Engineering 2D materials

Superlattice with Ge-Te atoms sandwiched in between Sb2Te3 layers

A team of researchers has been able to engineer the strain in 2D materials to manipulate their function as well as their electronic and optical properties. Two dimensional materials, such as graphene or molybdenum disulphide, are at the core in the research and development of new technologies for electronic and photonic materials, since they offer unique and combined properties, such as strength, flexibility, lightness and thinness that no other material can offer.

In a recent study published in Nature Communications, ICFO researchers Eric Dilcher and ICFO Prof. Simon Wall, in collaboration with researchers from SUTD (Singapore), MIT (Boston) have been able to demonstrate that, by stacking van der Waals heterostructures of 2D-crystals into a 3D superlattice, it is possible to use strain to control atomic diffusion, thus altering the mobility of charge carriers and subsequently, increasing the switching speed of the material.

In their study, the team of scientists took a Sb2Te3 – GeTe van der Waals superlattice and used strain to increase the diffusivity of Ge and Te atoms within the sample. Sb2Te3 is a two dimensional crystal that has a thickness of 5 atoms. Under certain conditions, Ge-Te atoms can be grown as a two dimensional crystal, arranged in a honeycomb structure. By combining both crystal structures, the researchers were able stack these crystals to create an artificial 3D crystal with properties that can be switched by disordering the GeTe two dimensional crystal.

They found that the energy needed to disorder the GeTe crystal layers can be lowered by specifically designing the GeTe—Sb2Te3 stacking sequence. In addition, the disordering of the GeTe two dimensional crystal happens in a similar way to how ice melts beneath an ice skate. The pressure of the skate’s blade helps melt the surface atoms of the ice, thus creating a slippery surface on which the ice skate slides. In the same manner, the researchers engineered the interfacial layers of GeTe and sandwiched it in between the Sb2Te3 layers, to create disorder due to the stress and pressure imposed by the Sb2Te3 layers.

These results in this study have emphasized the ability to tune the strain profile of vdW heterostructures, opening a new pathway to manipulate their function as well as their electronic and optical properties. In particular materials based on Sb2Te3 and GeTe are now being employed in next generation memories for smart phones, mainly concerning the design of efficient, high speed memory materials based on these stacks of two-dimensional crystals.