Researchers of the ELKH Centre for Energy Research (EK-CER), the MTA-EK-CER Topology in Nanostructures Momentum Research Group operating at EK-CER and the ELKH Wigner Research Centre for Physics (Wigner RCP) under the leadership of Péter Nemes-Incze have been investigating the molecules making up the surface contamination layer on two-dimensional materials. The researchers demonstrated that the surface contamination layer on what are known as van der Waals (vdW) materials universally developed an ordered crystal structure within a few days, a process that is based on the self-organization of normal alkanes comprising 20 to 26 carbon atoms. An understanding of the molecules making up the contamination layer may be important in a number of areas as the organized molecule layer may also dominate the interaction between the surface and its environment. A paper presenting the results has been published in the prestigious scientific journal Nature Communications.
Low-temperature STM image of the self-organizing alkane monolayer on the graphite surface (above), and the simulation of the structure of the molecule single layer (below)
The surface coming into contact with ambient air becomes ‘dirty’: various ambient contaminants ‒ water, oxygen, hydrocarbons, etc. ‒ become attached to the surface, leading to a deterioration in or complete loss of the physical and chemical properties of the surface. In the course of this project the researchers used a low-temperature (9K, or around -264 °C) scanning tunneling microscope (STM) imaging to observe the individual molecules making up the contamination layer, as well as their size, internal structure and formation. By using scanning tunneling spectroscopy (STS), X-ray photoelectron spectroscopy (XPS) and infra-red (IR) spectroscopy measurements clear chemical identification of the molecules that make up the self-organizing single layer became possible. An understanding of the molecules making up the surface contamination layer on 2D materials may have a significant influence both on the basic research and on many of the areas the research is applied to, as the self-organizing molecule layer may even dominate the interaction between the surface and its environment.
Anisotropic friction domains created intentionally on an hBN surface. One zigzag plane of the crystal is indicated by a blue perforated line, while the corresponding three equivalent zigzag directions of the AFM scan are shown by pink arrows.
In order to demonstrate this, the researchers showed that the self-organizing single-layer alkane is also clearly responsible for the development of anisotropic friction domains ‒ something that has been contentious until now ‒ universally observable on the vdW materials, e.g. graphene, graphite, hexagonal boron nitride (hBN) and molybdenum disulfide (MoS2). To show the transformation of these friction domains in a planned manner and allow observation of their ‘redrawing’, they rotated the long molecules making up the single layer along their long axes in the selected direction of the needle of an atomic force microscope.
It was possible to gain a deeper understanding of the structure of the self-organizing alkane layer using density function theory (DFT) calculations, while observation of the dynamics of its development in ambient conditions occurred using ellipsometric measurements.