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dc.contributor.authorLI, GEN
dc.date.accessioned2019-07-15T12:32:53Z
dc.date.available2019-07-15T12:32:53Z
dc.date.issued2019en
dc.date.submitted2019
dc.identifier.citationLI, GEN, Theoretical and experimental study on electron transport in oxygen-plasma-treated monolayer MoS2, Trinity College Dublin.School of Physics, 2019en
dc.identifier.otherYen
dc.identifier.urihttp://hdl.handle.net/2262/88798
dc.descriptionAPPROVEDen
dc.description.abstractIn recent years, two dimensional (2D) molybdenum disulfide (MoS2) has attracted a wide range of interest due to its interesting physics properties, such as valley electronics and quantum spin Hall effect, and its potential applications for the semiconductor industry such as in field-effect transistors (FETs) and photodetectors. Chapters 1 to 4 contain background knowledge related to this work, including the crystal structures of MoS2, plasma engineering, density functional theory (DFT) and the transfer matrix method (TMM). In Chapter 5, we introduce the experimental instruments used in this work, including mechanical exfoliation, electron beam lithography (EBL), Atomic Force Microscopy (AFM), Raman and photoluminescene (PL) spectroscopy, and an electric measurement system. In Chapter 6, we utilize radio-frequency oxygen plasma to treat 2D MoS2 FET to enhance the performance of the device. We study the surface morphology of the same device before and after two-second rapid plasma treatment. We find that the surface thickness to be doubled after treatment by using AFM. We find that both Raman E and A peaks are attenuated. An A exciton peak is quenched and broadened from PL spectroscopy. We further conduct electrical measurements to evaluate the device performance. We find that photoresponsivity and mobility are enhanced after 2s of plasma exposure. The threshold voltage of the device shifts to a more negative value, indicating the FET becomes more easily switched on. Moreover, we also utilize polymer encapsulation technique to modify the device. We find that polymer protection can improve the device mobility and significantly enhance the device stability. The polymer protection technique can further be utilized to realize site-specific modification on MoS2. This Chapter gives insight into surface modification and mobility engineering of 2D MoS2 nano devices. In Chapter 7, based on the experimental observations in Chapter 6, we apply DFT to study the electronic and magnetic properties of oxygen-plasma-treated monolayer MoS2. We consider three types of unit cells, which are proposed based on our experimental observation in the Chapter 6. We firstly optimize the lattice parameters of the studied unit cells. We further combine the three types of unit cell to make various 2 × 2 super cells. By calculating their band structures, we find that sulphur vacancies can cause significant quenching of band gap and that oxygen adatoms can make the direct band gap of pristine MoS2 to indirect band gap. Moreover, from the spin-dependent DOS, we also find that neither sulphur vacancies nor oxygen adatoms can introduce a ferromagnetic phase in to ML MoS2, which is consistent with previous work. Regarding spin-orbit coupling, our calculated SOC strength of pristine MoS2 is consistent with previous work. Oxygen adatoms can cause the location of band splitting to change, which is attributed to the modification of band structure by oxygen adatoms. This Chapter gives insight into band-structure engineering and valley electronics of 2D materials. In Chapter 8, we firstly show how to fabricate MoS2/MoOx heterostructures using a plasma. Then, we study the electron transport in it by TMM. We analyse the tunneling process, in a double-well structure and step-well structure under the condition of electric field and no electric field. Our work shows that the increasing of transverse momentum will result in the red shift of the resonant peak. We also show that low electric fields (±0.3 V ) can enhance the magnitude of peaks and intensify the coupling between longitudinal and transverse momentums. However, it can’t optimize the resonant tunnelling condition due to the heavier electron effective mass of MoS2/MoOx heterostructures than that in traditional semiconductor superlattices. Thus, a higher bias is applied and ideal resonant tunnelling peaks are obtained, indicating that negative differential resistance (NDR) effect can be observed. Moreover, a step-well structure shows a better performance regarding resonant tunnelling than a double-well structure, due to the absence of well separation which can alter the phase of electrons to affect resonant tunnelling condition. This Chapter gives insights into the physics of resonant tunnelling effect and NDR in 2D-materials nano devices, and also sheds light on the design of quantum electronic devices.en
dc.language.isoenen
dc.publisherTrinity College Dublin. School of Physics. Discipline of Physicsen
dc.rightsYen
dc.subjectMonolayer MoS2, plasma treatment, DFT, resonant tunelling effect, electronic band structuresen
dc.titleTheoretical and experimental study on electron transport in oxygen-plasma-treated monolayer MoS2en
dc.typeThesisen
dc.contributor.sponsorChinese Schorlaship Councilen
dc.contributor.sponsorOtheren
dc.type.supercollectionthesis_dissertationsen
dc.type.supercollectionrefereed_publicationsen
dc.type.qualificationlevelDoctoralen
dc.identifier.peoplefinderurlhttps://tcdlocalportal.tcd.ie/pls/EnterApex/f?p=800:71:0::::P71_USERNAME:LIGEen
dc.identifier.rssinternalid204792en
dc.rights.ecaccessrightsopenAccess


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