|Title:||Fabrication and characterization of two-dimensional layered materials for electronic and optoelectronic applications|
Graphene -- Electric properties.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Physics|
|Pages:||xxi, 109 leaves : illustrations ; 30 cm|
|Abstract:||Graphene has been attracting great interest because of its distinctive band structure and physical properties. Single crystalline graphene is limited to small sizes as it is produced mostly by exfoliating graphite. Large-area graphene films of the order of centimeters on copper substrates are fabricated by chemical vapor deposition (CVD) technique but they are in polycrystalline structure. The films are predominantly single-layer graphene, with a small percentage of the area having few layers, and are continuous and homogenous. The realization of n-and p-type graphene field-effect transistors (GFETs) by controlling merely the thickness of a zinc oxide (ZnO) nanomesh deposited on the graphene was demonstrated. This nanopatterning technique could open up new opportunities for developing electronic and optoelectronic devices that are based on graphene. The absence of a bandgap for graphene has limited its application in nanoelectronics. The well-studied semi-conducting two-dimensional (2D) material is the layered metal chalcogenides (LMDCs), the most common being molybdenum disulfide (MoS₂). Large-area MoS₂ films of the order of centimeters on sapphire substrates were prepared by CVD. The films can be single-layer or multi-layer. Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic, and spintronic performances. We developed an electromechanical device that can apply biaxial compressive strain to tri-layer MoS2 supported by a piezoelectric substrate and covered by a transparent graphene electrode. Photoluminescence (PL) and Raman characterizations show that the direct bandgap can be blue shifted for ~ 300 meV per 1% strain. First principle investigations confirm that the blue-shift of the direct bandgap and reveal a higher tunability of the indirect bandgap than the direct one. The exceptionally high strain tunability of the electronic structure in MoS2 promises a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for 2D semiconductors. Although strain engineering in 2D materials is possible nowadays, most of the strain engineering techniques require external agent to apply strain onto the 2D materials. We developed a novel approach to apply continuous strain to any 2D materials on arbitrary substrates. Monolayer MoS₂ was transferred onto patterned SiO₂/Si substrates with inclined trenches of different sizes, 5×5, 10×10 and 20×20 μm². One side of the MoS₂ layer was in contact with the inclined plane of the trench and the other side was free standing within the trench. This structure created continuous strain from tensile to compressive. An exceptional total Raman and PL shift of 12 cm⁻¹ and 14 nm were recorded respectively from the 20×20 μm² sample. It is found that the amount of induced strain depends on the size of the trenches. The approach provides a platform to study the strain induced properties in 2D semiconductors.|
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