The composition and structure of the ubiquitous hydrocarbon contamination on van der Waals materials

Observation of the early stages of environmental contamination in graphene by friction force

Exposure to ambient air contaminates the surface of graphene sheets. Contamination may arise from different sources, and its nature alters the frictional behavior of the material. These changes in friction enable the observation of the early stages of contaminants' adsorption in graphene. Using a friction force microscope, we show that molecular adsorption initiates at the edges and mechanical defects in the monolayer. Once the monolayer is covered, the contaminants spread over the additional graphene layers. With this method, we estimate the contamination kinetics. In monolayer graphene, the surface area covered with adsorbed molecules increases with time of air exposure at a rate of 10-14 m2/s, while in bilayer graphene, it is one order of magnitude smaller. Finally, as the contaminants cover the additional graphene layers, friction no longer has a difference concerning the number of graphene layers.

Exploring the Surface Oxidation and Environmental Instability of 2H-/1T'-MoTe2 Using Field Emission-Based Scanning Probe Lithography

An unconventional approach for the resistless nanopatterning 2H- and 1T'-MoTe2 by means of scanning probe lithography is presented. A Fowler-Nordheim tunneling current of low energetic electrons (E = 30-60 eV) emitted from the tip of an atomic force microscopy (AFM) cantilever is utilized to induce a nanoscale oxidation on a MoTe2 nanosheet surface under ambient conditions. Due to the water solubility of the generated oxide, a direct pattern transfer into the MoTe2 surface can be achieved by a simple immersion of the sample in deionized water. The tip-grown oxide is characterized using Auger electron and Raman spectroscopy, revealing it consists of amorphous MoO3 /MoOx as well as TeO2 /TeOx . With the presented technology in combination with subsequent AFM imaging it is possible to demonstrate a strong anisotropic sensitivity of 1T'-/(Td )-MoTe2 to aqueous environments. Finally the discussed approach is used to structure a nanoribbon field effect transistor out of a few-layer 2H-MoTe2 nanosheet.

Filtering Robust Graphite without Incommensurate Interfaces by Electrical Technique

Two-dimensional (2D) van der Waals (vdW) layered materials have attracted considerable attention due to their potential applications in various fields. Among these materials, graphite is widely employed to achieve structural superlubricity (SSL), where the interfacial friction between two solids is almost negligible and the wear is zero. However, the development of integrated SSL systems using graphite flakes still faces a major obstacle stemming from the inherent delamination-induced instability in vdW layered materials. To address this issue, we propose a nondestructive filtering technique that utilizes electrical measurement to identify robust graphite flakes without delamination. Our experimental results confirm that all the filtered graphite flakes exhibit delamination-free behavior after more than 7000 cycles of sliding on a series of 2D and 3D substrates. Besides, we employ three types of characterizing methods to confirm that the filtering process does not impair the graphite flakes. Moreover, with focused ion beam (FIB) assisted slicing characterization and statistical analysis, we have discovered that all of the filtered flakes possess a graphite layer thickness below 100 nm. This is consistent with the thickness of the single crystalline graphite layer of our samples reported in the literature, suggesting the absence of incommensurate interfaces in the filtered graphite flakes. Our work contributes to a deeper understanding of the relationship between graphite conductance and incommensurate interfaces. In addition, we present a possible solution to address the delamination problem in layered materials, and this technique shows the potential to characterize the internal microstructure of grains and the distribution of grain boundaries in vdW materials on a large scale.

Atomic Friction Processes of Two-Dimensional Materials

In this Perspective, we present the recent advances in atomic friction measured of two-dimensional materials obtained by friction force microscopy. Starting with the atomic-scale stick-slip behavior, a beautiful highly nonequilibrium process, we discuss the main factors that contribute to determine sliding friction between single asperity and a two-dimensional sheet including chemical identity of material, thickness, external load, sliding direction, velocity/temperature, and contact size. In particular, we focus on the latest progress of the more complex friction behavior of moiré systems involving 2D layered materials. The underlying mechanisms of these frictional characteristics observed during the sliding process by theoretical and computational studies are also discussed. Finally, a discussion and outlook on the perspective of this field are provided.

Controllable Friction on Graphene via Adjustable Interfacial Contact Quality

Despite the numerous unique properties revealed through tribology research on graphene, the development of applications that utilize its rich tribological properties remains a long-sought goal. In this article, a novel approach for reversible patterning of graphene's frictional properties using out-of-plane mechanical tapping is presented. The friction force between the atomic force microscopy (AFM) tip and the graphene film is increased by up to a factor of two, which can be attributed to variations in the interfacial binding strength between the graphene and substrate through the tapping process. The reversible and repeatable frictional properties of graphene make it a promising material for information storage applications with a high storage capacity of ≈1600 GB inch-2 , allowing for direct writing and erasing of information, akin to a blackboard. These findings highlight the potential for friction tuning in lamellar materials and emphasize the significance of understanding nanoscale friction on graphene surfaces.

Kinetics of Nanobubbles in Tiny-Angle Twisted Bilayer Graphene

Realization of high-quality van der Waals (vdWs) heterostructures by stacking two-dimensional (2D) layers requires atomically clean interfaces. Because of strong adhesion between the constituent layers, the vdWs forces could drive trapped contaminants together into submicron-size "bubbles", which leaves large interfacial areas atomically clean. Here, we study the kinetics of nanobubbles in tiny-angle twisted bilayer graphene (TBG) and our results reveal a substantial influence of the moiré superlattice on the motion of nanoscale interfacial substances. Our experiments indicate that the bubbles will mainly move along the triangular network of domain boundaries in the tiny-angle TBG when the sizes of the bubbles are comparable to that of an AA-stacking region. When the size of the bubble is smaller than that of an AA-stacking region, the bubble becomes motionless and is fixed in the AA-stacking region, because of its large out-of-plane corrugation.

Influence of Atmospheric Contaminants on the Work Function of Graphite

Airborne hydrocarbon contamination occurs rapidly on graphitic surfaces and negatively impact many of their material properties, yet much of the molecular details of the contamination remains unknown. We use Kelvin probe force microscopy (KPFM) to study the time evolution of the surface potential of graphite exposed to ambient. After exfoliation in air, the surface potential of graphite is not homogeneous and contains features that are absent in the topography image. In addition, the heterogeneity of the surface potential images increased in the first few days followed by a decrease at longer exposure times. These observations are strong support of slow conformation change, phase separation, and/or dynamic displacement of the adsorbed airborne contaminants.