|Author:||Tan, Choon Kiat|
|Title:||Optical and electrical properties of two-dimensional materials|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Transition metal compounds
|Pages:||xvii, 81 pages : color illustrations|
|Abstract:||Two-dimensional (2D) materials provide an ideal platform for the study of fundamental properties of system with atomic-level thickness. These studies motivate a large number of device application in various fields, for example, electronics and display technology. After the first discovery of remarkable properties such as high impermeability, large heat and electrical conductivity etc., of 2D carbon sheet known as graphene, other layered materials including transition metal dichalcogenides (TMD) have been widely investigated. These TMD possess a sizable bandgap that is very promising for future optical and electronic devices. However, other 2D family groups such as 2D layered superconductor and p-n junctions formed by 2D materials still remain relatively unexplored. In this thesis, firstly, we characterized the basic properties of 2D iron-based superconductor. This study shows that the thickness of exfoliated FeTe is correlated to its number of layers, similar to other TMD materials. The effect of the layer thickness on the Raman spectra have been measured. As the number of layers goes down to few layers, the separation of the Raman peaks A₁g and E1/2g decreases. Furthermore, the superconductivity in the exfoliated FeTe and FeSe has been studied. The temperature dependent resistances were measured and structural transition sometimes could be observed.|
Secondly, Raman spectra of encapsulated bulk and few layers MoS₂ in multiple uniaxial strain direction have been measured. A modified bending technique has been employed to apply precise uniaxial tensile strain along different directions of MoS₂ which was encapsulated in between octadecagon polyethylene terephthalate (PET) and a layer of polymethlmethacrylate (PMMA). For bulk MoS₂, the Raman shift rates of the E1 2g and A1g mode (the change of the Raman peak position versus strain level) are almost the same for strains applied along different directions. The Raman shift rates of few layers MoS₂, on the other hand, are larger than those in bulk MoS₂. In addition, they also exhibit distinctive anisotropic strain responses. We believe that the Armchair and Zigzag directions of the exfoliated MoS₂ might be determined by monitoring the variation of Raman Shift of the E1 2g and A1g modes in different strained direction of MoS₂. Lastly, the Raman spectroscopic study of the effect of electrode preparation on the structural properties of As₂S₃ has been carried out. As₂S₃ is an intrinsic p-type semiconductor with a melting point around 310 °C. Electrode preparation involving E-beam lithography and E-beam evaporation, might result heating on the samples. Raman spectra have been measured before and after the two above mentioned processes. Raman spectra of As2S3 before and after the processes are indicating that the electrode preparation does not influence the AssS3. Furthermore, p-type As₂S₃ and n-type MoS₂ have been used to study the p-n junction behaviour in 2D materials. Current-Voltage (I-V) curves were measured under various circumstances such as low temperature and illuminating under white light. IV curves of As₂S₃ and PN junction showed high resistance which might due to the low mobility in As₂S₃ (10⁻¹⁰cm²/Vs).
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