Surface Instability, Super Hydrophilicity and Lubricity of Two-dimensional van der Waals Layered Materials


Student thesis: Doctoral Thesis

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Award date2 Sep 2022


In the past decades, the two-dimensional (2D) van der Waals (vdW) layered materials like graphene and transition metal dichalcogenides (TMDs) have been drawing enormous attention in multidiscipline fields. Empowered by their atomic thickness and ultra-flexibility, the layered materials are promising for a variety of miniaturized and flexible devices in applications. Unlike bulk materials, 2D vdW layered materials possess ultra-high surface-to-volume ratio and weak vdW bonding nature at the interface. These lead to notable instability and various physical/chemical properties governed by the interfaces between surface of layered materials and surrounding mediums. Therefore, in-depth understanding in the surface morphologies and properties of layered materials is essential for the structural/morphological control and physical/chemical properties tailoring, as well as for developing future applications.

In this thesis, we study the unique surface characteristics of 2D vdW layered materials, including graphene, carbon nitride and TMDs such as molybdenum disulfides (MoS2), rhenium disulfides (ReS2), and surfaces of 2D materials coupled with different soft/rigid surfaces including liquid/solid interfaces. We have dynamically investigated the oxidation, wrinkling, and sliding processes occurred at the interfaces involving 2D vdW layered materials. Their structural morphology, as well as optical and mechanical properties have been controlled by various environmental conditions and external forces.

Surface characteristics dominate the behavior of low-dimensional materials. i.e., the one-dimensional texture led by functional groups embedded on surface of carbon nitrides thin film exhibited exceptional enhancement in surface wettability. In case of 2D vdW layered materials, the instability at the surface and interface may cause the internal stresses or interfacial stresses inevitably released through mechanical delamination. The resulting puckers/wrinkles on 2D materials could undermine the ideal properties and create barriers in various applications. For instance, the mechanical delamination in soft coating on 2D MoS2 formed the corrosive cavities at the interface. The corrosion under insulation (CUI) effect accelerated the oxidation rate of MoS2 under soft coating. On the other hand, the wrinkles can be used for architecting surface texture and modulating physical/chemical properties. Our study revealed that a minimum critical length is required for stabilizing the wrinkles in graphene, MoS2 and ReS2. Their wrinkling and wrinkle elimination processes were manipulated by thermal annealing and mechanical force. Further, the stable wrinkles on 2D surface can be rendered to 1D and 2D ordered wrinkle patterns via thermal straining and vertical spatial confinement. The various hierarchical patterns in 2D materials generated by our method are highly periodic and favored to crystal symmetry. Nevertheless, surface puckering can be reduced by the 2D ice layer under 2D MoS2, hence the stick-slip motions on 2D surface were enhanced. 

Our findings highlighted the importance to protect the 2D vdW layered materials from damages and oxidations, developed new transferring/printing/processing strategies for various applications. Moreover, new approaches have been developed to achieve hierarchical ordered wrinkle patterns and the super-lubricity on the surface of 2D vdW layered materials.