Precision-manipulation of two-dimensional materials by defect/phase/interface engineering
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이관형 교수 서울대학교 Atomically thin 2D materials, such as graphene and transition metal dichalcogenides (TMDs), consist of two surfaces with negligible influence of a bulk part so that they are very sensitive to environment. Therefore, properties of 2D materials can be altered easily by surface modification. When the van der Waals (vdW) heterostructures are produced by stacking 2D layers or epitaxial growth, the heterointerface conditions, such as cleanness, flatness, and stacking angle, greatly affect the properties of vdW heterostructures. In this talk, I will show novel approaches to fabricate high performance 2D electronic devices by utilizing various surface treatments, such as fluorination and hydrogenation of graphene and layer-by-layer oxidation of MoS2. When different types of defects, such as sp3 bonds and vacancies, are induced on the surface of graphene, the electrical properties of graphene can be tuned. With mild plasma treatment, MoS2 can be oxidized layer-by-layer and monolayer MoS2 can be fabricated from the multilayer MoS2. We also developed novel contact engineering strategies for vdW heterostructure devices by using graphene as etch masks and etch stops in XeF2 etching process. Secondly, I will present the vdW epitaxial growth of α-MoO3 on various 2D growth templates. Monolayer and multilayer α-MoO3 nanosheets are successfully grown on a 2D substrate by simply evaporating amorphous molybdenum oxide thin film in ambient conditions. A single-crystal α-MoO3 nanosheet without grain boundary is epitaxially grown on various 2D substrates despite a large lattice mismatch. The epitaxially grown MoO3 shows the thickness-insensitive electrical properties of high dielectric constant and tunnel resistance by weak interlayer coupling, which are beneficial for nanodevice applications. Finally, I will introduce our recent result on thickness-dependent phase transition of ultrathin MoTe2. The single crystalline 2H-MoTe2 is transformed to high quality polycrystalline Td-MoTe2 by combinational effect of thermal energy, pressure, and doping. Interestingly, large interface energy between layers results in increase of phase transition temperature with decreasing thickness. Our works deepen understanding of scientific phenomena occurring in 2D nanoscale and our engineering approaches for surface and heterointerface in vdW heterostructures provide a novel way to fabricate the high performance 2D devices. |