Exotic Layered Materials

2D materials are flat molecules that consist only of surface atoms. Their structure is chemically inert, which is the reason why they do not form bonds with one another. These materials can however be glued together by so called van der Waals forces, forming extremely thin Lego-like layered van der Waals structures. These structures are usually constructed via a process that involves first the separation of individual atom layers from larger crystals by laborious manual means, followed by their manual one by one stacking. In the ELMer project, new 2D materials are synthesized by using a single-step room temperature process, and their structure and properties are studied by means of transmission electron microscopy. To achieve this, 2D materials are grown in between two sheets of individual layers of carbon atoms, also known as graphene. Ultimately, such 2D materials may improve many commonplace technologies that include, for instance, solar cells and batteries for electric vehicles, but also low energy consumption electronics in form of computers, hand held devices, and flat panel displays.

The main goal of this project was to create and understand new atomically thin materials that do not occur naturally as layered crystals. By developing a graphene-based synthesis approach, we succeeded in producing and characterizing approximately a dozen two-dimensional metal iodides with diverse electronic, magnetic, and optical properties. These materials had previously been inaccessible experimentally and were known mainly from theoretical predictions. Using advanced electron microscopy and spectroscopy, we determined their atomic structures and investigated the chemical processes that govern their formation. The results revealed how confinement between graphene layers can stabilize novel crystal structures and provided important insight into the chemistry of low-dimensional materials. The project therefore significantly expanded the library of experimentally accessible two-dimensional materials and established new opportunities for studying their fundamental properties. An unexpected scientific breakthrough emerged during the investigation of one of these materials, atomically thin silver iodide. One objective of the project was to examine how the crystal structure changes at elevated temperatures. However, the experiments revealed far richer behaviour than anticipated. By observing the material at temperatures exceeding 1,000 C with atomic resolution, we directly recorded its melting process and discovered evidence for an intermediate state known as the "hexatic phase". This phase had been predicted theoretically for decades as a possible step between a crystal and a liquid in two dimensions, but direct observations in complex materials had remained extremely rare. The discovery provided new insight into one of the fundamental questions of condensed-matter physics: how order disappears when a crystal melts in two dimensions. The findings demonstrate how newly created materials can open unexpected paths towards answering long-standing questions in basic science.