Center for Materials Engineering

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Browsing Center for Materials Engineering by Subject "bandgap"

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    Electronic Properties of 2D Vander Waals Heterostructures of Janus Transition Metal Dichalcogenides with WS2 Monolayer for Photovoltaic Devices: A First Principle Study
    (Addis Ababa University, 2020-06) Tsigie, Getie; Sintayehu, Nibret (PhD)
    Building two-dimensional (2D) heterostructure emerges novel properties, with promising applications in photovoltaic (PV) cells. By performing density functional theory (DFT) based firstprinciples calculations, electronic properties of WS2 and Janus transition-metal dichalcogenides (JTMDs) monolayers were calculated and depending on the lattice mismatch, layered 2D JTMDs/WS2 heterostructures were formed. The formation of the JTMDs/WS2 van der Waals (vdW) heterostructures have shown great potential for the design of novel electronic devices. In this study, Janus MoSSe/WS2, WSSe/WS2, and MoSTe/WS2 heterostructures were developed and their structural and electronic properties were evaluated using first principles calculations based on DFT calculations using Quantum ESPRESSO and VASP codes. It was found that the heterostructures bandgap is smaller than the Janus TMDs and WS2 monolayer. Structural relaxations were performed using generalized-gradient approximation (GGA) approaches for both the monolayers and heterostructures. Structural stability and electronic properties of JTMDs/WS2 vdW heterostructures with AC and AD stacking were investigated which are the most stable configuration compared with other configurations based on the binding energy and the interlayer distance. Results show that the Janus MoSTe/WS2, MoSSe/WS2, and AD-configuration of WSSe/WS2 vdW heterostructures are indirect bandgap semiconductor, but WSSe/WS2 with ACconfiguration is a direct bandgap. The JTMDs/WS2 vdW heterostructures exhibited a bandgap in the range of 1.54 to 0.54eV. In addition, MoSSe/WS2 and MoSTe/WS2 heterostructures displayed a type-II band alignment which is important to improve the photoelectric conversion efficiency. However, the band alignment of WSSe/WS2 heterostructure is difficult to identify and need additional calculations. First principles study shows that the investigated 2D heterostructures have a suitable bandgap for photovoltaic applications. Among the JTMDs/WS2 vdW heterostructures, MoSSe/WS2 and MoSTe/WS2 manifest type-II band alignment, making them promising candidates for photovoltaic (PV) applications.
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    First Principles Investigation of Van Der Waals Heterostructures of Mos2 and Janus Transition Metal Dichalcogenides for Energy Applications
    (Addis Ababa University, 2021-09) Birhan, Tesfaye; Georgies, Alene (PhD)
    Recent research on the Janus transition metal dichalcogenide (JTMD) with an asymmetric structure has revealed that this material possesses interesting unique properties, notably in solar cells. This work is based on cutting-edge density functional theory (DFT) computations utilizing Generalized Gradient Approximation- Perdew–Burke–Ernzerhof functional (GGA-PBE) as implemented in the Quantum ESPRESSO and VASP codes. To find the most stable optimized heterostructures, eight basic stacking patterns were designed. Then, for MoSSe/MoS2, WSSe/MoS2, and MoSTe/MoS2 heterobilayer, the AAII-S stacking mode was more stable than the other stacking types. According to the findings, the band alignment was type-I for MoSSe/MoS2, MoSTe/MoS2, and type-II for WSSe/MoS2, within, 1.03, 0.30 and 0.84 eV are estimated bandgap, respectively. The electrical band structure, as well as band edge placements, was investigated. When the water redox and oxidation potentials of heterostructures were compared, it was discovered that MoSSe/MoS2, MoSTe/MoS2, and WSSe/MoS2 were not applicable for photocatalytic materials for full water splitting. On the other hand, MoSSe/MoS2 and MoSTe/MoS2 heterostructures were placed lower than the oxidation potential of O2/H2O, making them applicable for oxygen evolution reaction (OER). This work reveals that JTMDs/MoS2 heterostructures are often subsequent material that promotes the development of photovoltaic devices, specially MoSSe/MoS2, and WSSe/MoS2 vdWH. The power conversion efficiency (PCE) of the heterostructures is calculated, and the results show that MoSSe/MoS2 and WSSe/MoS2 show very good efficiency with values of 19.41% and 16.25%, respectively. The result is good when compared to other similar studies: GaTe-InSe (9.1%), MoS2/p-Si (5.23%), organic solar cells (11.7%), and PN-WSe2 (13.8 % ). Since the results are encouraging, we believe it is a good idea to do additional experiments on the heterostructures and adapt them to solar cell applications.