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postgraduate thesis: Growth of transition metal dichalcogenide thin films by molecular beam epitaxy
Title | Growth of transition metal dichalcogenide thin films by molecular beam epitaxy |
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Authors | |
Issue Date | 2015 |
Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
Citation | Jiao, L. [焦璐]. (2015). Growth of transition metal dichalcogenide thin films by molecular beam epitaxy. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_b5576771 |
Abstract | Atomically thin transition metal dichalcogenides (TMD) have attracted intensive research interests due to their extraordinary properties and potential applications in electronics and optoelectronics. In this thesis, epitaxial growths of two-dimensional (2D) MoSe2 and WSe2 thin films were carried out in Molecular Beam Epitaxy (MBE). Multiple characterization techniques were employed to investigate thin films’ structural, morphological, electronic and optical properties.
A series of submonolayer MoSe2 coverage samples have been grown on highly ordered pyrolytic graphite (HOPG) substrate. Growth temperature and post-growth annealing temperature were seen to have obvious impacts on film’s morphology and crystal quality. Layer-by-layer growth mode has been identified for the Van der Waals epitaxy of MoSe2 on HOPG. Dense networks of inversion domain boundaries (IDBs) have been observed in as-grown MoSe2 epifilms by scanning tunneling microscopy (STM) and transmission electron microscopy (TEM), and their density can be tuned by changing the MBE conditions. Scanning tunneling spectroscopy (STS) measurements reveal mid-gap electronic states associated with the IDB defects. STS measurements also reveal energy bandgaps of monolayer (ML) and bilayer (BL) MoSe2.
ML WSe2 thin films were also grown at varying conditions on HOPG substrates through the Van der Waals epitaxy process and the growth characteristics were found similar to that of MoSe2. However, differences are also noted, particularly about the IDB defects. Contrary to MoSe2, as-grown WSe2 films do not contain the line defects. The reason behind such differences will be discussed.
Finally, besides the STM/S studies about the morphological and electronic properties of MBE MoSe2 and WSe2 films, high quality samples have been synthesized on graphene-on-SiC substrate with reduced defect density and well-controlled thicknesses for some ex situ characterizations by photoluminescence and Raman spectroscopy methods. The results will be summarized and discussed in this thesis. |
Degree | Doctor of Philosophy |
Subject | Molecular beam epitaxy Metallic films |
Dept/Program | Physics |
Persistent Identifier | http://hdl.handle.net/10722/221085 |
HKU Library Item ID | b5576771 |
DC Field | Value | Language |
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dc.contributor.author | Jiao, Lu | - |
dc.contributor.author | 焦璐 | - |
dc.date.accessioned | 2015-10-26T23:11:56Z | - |
dc.date.available | 2015-10-26T23:11:56Z | - |
dc.date.issued | 2015 | - |
dc.identifier.citation | Jiao, L. [焦璐]. (2015). Growth of transition metal dichalcogenide thin films by molecular beam epitaxy. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_b5576771 | - |
dc.identifier.uri | http://hdl.handle.net/10722/221085 | - |
dc.description.abstract | Atomically thin transition metal dichalcogenides (TMD) have attracted intensive research interests due to their extraordinary properties and potential applications in electronics and optoelectronics. In this thesis, epitaxial growths of two-dimensional (2D) MoSe2 and WSe2 thin films were carried out in Molecular Beam Epitaxy (MBE). Multiple characterization techniques were employed to investigate thin films’ structural, morphological, electronic and optical properties. A series of submonolayer MoSe2 coverage samples have been grown on highly ordered pyrolytic graphite (HOPG) substrate. Growth temperature and post-growth annealing temperature were seen to have obvious impacts on film’s morphology and crystal quality. Layer-by-layer growth mode has been identified for the Van der Waals epitaxy of MoSe2 on HOPG. Dense networks of inversion domain boundaries (IDBs) have been observed in as-grown MoSe2 epifilms by scanning tunneling microscopy (STM) and transmission electron microscopy (TEM), and their density can be tuned by changing the MBE conditions. Scanning tunneling spectroscopy (STS) measurements reveal mid-gap electronic states associated with the IDB defects. STS measurements also reveal energy bandgaps of monolayer (ML) and bilayer (BL) MoSe2. ML WSe2 thin films were also grown at varying conditions on HOPG substrates through the Van der Waals epitaxy process and the growth characteristics were found similar to that of MoSe2. However, differences are also noted, particularly about the IDB defects. Contrary to MoSe2, as-grown WSe2 films do not contain the line defects. The reason behind such differences will be discussed. Finally, besides the STM/S studies about the morphological and electronic properties of MBE MoSe2 and WSe2 films, high quality samples have been synthesized on graphene-on-SiC substrate with reduced defect density and well-controlled thicknesses for some ex situ characterizations by photoluminescence and Raman spectroscopy methods. The results will be summarized and discussed in this thesis. | - |
dc.language | eng | - |
dc.publisher | The University of Hong Kong (Pokfulam, Hong Kong) | - |
dc.relation.ispartof | HKU Theses Online (HKUTO) | - |
dc.rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works. | - |
dc.rights | This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. | - |
dc.subject.lcsh | Molecular beam epitaxy | - |
dc.subject.lcsh | Metallic films | - |
dc.title | Growth of transition metal dichalcogenide thin films by molecular beam epitaxy | - |
dc.type | PG_Thesis | - |
dc.identifier.hkul | b5576771 | - |
dc.description.thesisname | Doctor of Philosophy | - |
dc.description.thesislevel | Doctoral | - |
dc.description.thesisdiscipline | Physics | - |
dc.description.nature | published_or_final_version | - |
dc.identifier.doi | 10.5353/th_b5576771 | - |
dc.identifier.mmsid | 991011255759703414 | - |