Nanometer-scale chemical modification of 2D materials

In the project "Nanometer-scale chemical modification of 2D materials", we study how to design new two dimensional (2D) materials with specific properties. Designing and developing new materials is an important aspect of materials science. Materials properties can be modified by e.g. altering the original atomic structure of the material. 2D nanomaterials that consist of a single or a few layers of atoms are especially interesting regarding materials design due to their 2D nature. By changing the atomic structure of a 2D material even by a single atom, the properties of the material can change radically. These new materials could then fit better the needs of a new generation of applications with superior performance and capabilities. The applications range form e.g sensors to nanoelectronics and filters. The methods used in this study include electron microscopy and computational simulations. Electron microscopy allows us to see the materials structure down to individual atoms. By using the unique electron microscope set up at the University of Vienna, we will introduce gas molecules onto the sample surface. These molecules then interact with the electron beam that is used to image the sample leading to etching of the sample surface. Conventionally etching is avoided while using electron microscopes, because it introduces uncontrolled and undesired effects in the sample. Our aim is to control the etching and use it in advantage to engineer 2D materials. We will control the etching by introducing specific alterations in the atomic lattice. This will allow us to controllably create nanoscale features such as cuts in predetermined directions leading to formation of nanowires and nanoribbons, and pores of different size. These features can then be used in varying new nanoscale applications such as sensors and filters. To fully understand the mechanisms involved in the process we will use atomistic simulations to model the system further. We will create a new neural network potential to describe the interactions of gas molecules on 2D surface with full chemical description of the system. At present this can not be achieved with the existing methods at the required scale. This part of the project will result in one of the first potentials to model chemical reactions in large scale systems with quantum precision. Our work will help develop and design new 2D materials with controlled properties using a novel approach for a new generation of nanoscale applications.

The two year FWF Lise Meitner project "Nanometer-scale chemical modification of 2D materials" at the University of Vienna reveals new information on the structure and properties of two dimensional materials (2D). These materials consist of a single or few layers of atoms and have interesting properties. The results of the project reveal how the materials behave in different atmospheres when they are studied in an electron microscope. These results provide important implications on their suitability in applications that operate in air and predict structural changes that radically affect their properties. Within the project, a new method to incorporate individual metal atoms into the 2D material is presented. The metal atoms can change the original properties of the material radically. Such nanostructures are significant in developing new materials for catalysis and energy conversion, linking the project's results to the global energy challenges. The studied single layer of molybdenum ditelluride (MoTe2) consists of alternating Mo and Te atoms. Our results show that the material is reactive in an oxygen atmosphere. Significant etching of the surface takes place above the pressure of 1107 torr. The oxygen radicals land on the sample surface and react with the Te atoms. Computational simulations reveal how two oxygen atoms remove a single Te atom from a pristine MoTe2 surface in an energetically favorable reaction that can take place even at room temperature. The process leads to a significant etching and degradation of the surface within several minutes. Hydrocarbon contamination that can typically be found on any nanomaterial's surface, accelerates the etching. We report up to forty times higher etching rate on a highly contaminated surface. Comparatively, a single layer of molybdenum disulfide (MoS2), another 2D material with a similar atomic structure, is found inert in the oxygen rich atmosphere. No indication of the dynamic oxygen mediated etching process can be found and our computational simulations confirm the experimentally measured result. To further study the chemical modification of 2D materials, new atomic species can be incorporated into the structure. Within the project, we used graphene samples consisting of a single layer of carbon atoms. By employing low energy ion irradiation in two consecutive steps, a successful method to add metal atoms into the structure was found. The method overcomes issues commonly seen in the process and leads to a high concentration of structures that are stable in room temperature. The system consists solely of individual metal atoms that are bound into graphene replacing some of the original atoms. The method is applicable to various systems, making it versatile and appealing. The results are at the interface of physics and chemistry and have a significant influence on designing new functional nanosurfaces based on single metal atoms.