In-situ investigation of stacked heteronanostructures

Austrian-side PI: Prof. Jani Kotakoski Korean-side PI: Prof. Sang Wook Lee Similar to sheets of paper, which can be extracted from a paper stack, some materials in Nature exist in layers which form stacked crystals. The best know example of such a material is graphite, which consists of layers of carbon atoms arranged in a honeycomb pattern. Only a decade ago, it was shown that similar to an individual paper sheet such a carbon layer can also exists individually, without the support of the other layers in the stack. This one-atom-thick crystal of carbon atoms is called graphene, and it and other similar materials have been in the focus of an immense research effort ever since its discovery. Interestingly, such atomic sheets can also be rolled up, again similar to paper, to form tubular structures, such as carbon nanotubes. Also other similarly quasi-one-dimensional structures exist and exhibit interesting properties. During the last few years, a large research effort has started to investigate more complicated structures, which can be formed by combining different building blocks such as graphene and nanotubes. In this project, we bring together two research teams to study the properties of such combined structures at atomic level. The Korean research group are experts in manufacturing in very high accuracy these structures and specialized devices for their manipulation, whereas the Austrian group are experts in atomic-level imaging of such materials and their structural defects. This combination of expertise allows us to bring our understanding of this new class of structures up to a completely new level, which may pawe the way for their use in applications for example in nanoelectronics, thermoelectrics and on other fields.

This international project together with a Korean and an Austrian research partner investigated the properties of low-dimensional materials and their heterostructures under external perturbance. The most significant findings were related to the changes in atomic structure due to interface effects and external manipulation, and the ways in which they can be controlled. Since the discovery of graphene, a one-atom-thick material consisting of carbon atoms, in 2004, research of other similarly thin materials has lead to the development of a completely new research field within materials science. Many of the discovered so-called 2D materials exist normally in a stacked form, like a stack of papers, similar to how several graphene layers build graphite, the material well-known as the "lead" of a pencil. The large interest in them arises partially from the fact that many of their properties differ from normal materials, because of quantum-mechanical confinement of their electrons to two dimensions, which leads to many unique properties that can be used in applications. In addition to 2D materials, also structures with two restricted dimensions, such as carbon nanotubes, that can be thought of as rolled-up graphene sheets, have similarly interesting properties. One way to further expand the available properties is to take several different 2D materials, and to place them on top of each other, like building a stack using papers with different colors, each coming originally from a unicolored stack. While such structures have already been created for some time, detailed studies on the influence of the different materials on each other have not been yet carried out. Additionally, in many cases the response of both the individual materials and the stacked structures to external stimuli has not yet been studied. This is important, both because such stimuli will be encountered in real-life applications, and because complete understanding of their influence can lead to new ways for tailoring their properties for especially demanding applications. Results from the project have lead to important advances in our understanding of the influence of electron irradiation, ion irradiation, heating and mechanical manipulation in many materials including graphene, molybdenum disulfite, molybdenum ditellurite, indium oxide and a new two-dimensional copper gold structure and their combined structures. During the project, it was also discovered that graphene can be used as a high-temperature support material for atomic-resolution microscopy studies with other materials. Together these results are expected to lead to new ways for creating materials that allow building devices with capabilities that are not currently available.