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Electron Energy Loss Spectroscopy And Atomic Structure Characterization Of Two-dimensional Materials

Posted on:2017-03-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:J H HongFull Text:PDF
GTID:1221330485979625Subject:Materials Science and Engineering
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As a new two-dimensional (2D) material system, recently atomically thin transition metal dichalcogenides (TMDs) has attracted extensive research interests in condensed matter physics and materials science, owing to their unique electronic, optical, valleytronic and optoelectronic properties stemming from their unique atomic structures. After a brief review on recent advancement of 2D TMDs family, this dissertation will present our research on only one member of 2D crystal system-molybdenum disulfide (MoS2).Firstly, the thesis demonstrates the angle-resolved electron energy loss spectroscopy (EELS) of MoS2 nanolayers to systematically measure the anisotropic valence electron excitation. The low-loss spectra of the monolayer system present the strongest anisotropy-the optical energy gap in the out-of-plane mode is 2.4 eV, which is remarkably different from the well-known gap of 1.8 eV in the in-plane-polarization mode (similar to the optical spectrum at normal incidence). When the thickness of MoS2 increases from monolayer to multilayer, the anisotropy decrease dramatically, which indicates the three-dimensional character of the electronic structure involved. We also analyzed the angle-integrated energy loss spectra from MoS2 nanosheets with different thicknesses by curve fitting and parameter extraction. In combination with previously reported results using angle resolved photoemission spectroscopy (ARPES), we can further conclude that the conduction band valley (Q point) lifts and valence band maximum (Γ point) drops when the thickness decrease, moreover, their movements are asymmetric.Secondly, we describe the effect of sample growth methods on the main atomic defects in monolayer MoS2, revealed by atomically resolved annular dark field scanning transmission electron microscopy (ADF-STEM) imaging. We found experimentally that the sulfur vacancies are the domiant point defects in monolayer MoS2 prepared by mechanical exfoliation and chemical vapor deposition (CVD), while the most common defects in physical vapor deposited (PVD) monolayers are antisite defects with Mo replacing S sublattice. Our DFT calculations further consistently interpret the difference between these methods in the predominant defects from the perspective of energetics. Regarding the common antisite defects, the DFT calculation show ansite Mos has a magnetic moment, indicating PVD monolayer is a two-dimensional dilute magnetic semiconductor. To probe the effect of different point defects on the electron mobility, electric transport measurements are conducted and demonstrate that antisite defects decrease the electron mobility more severely than vacancies, to some extent.Thridly, we report the tracking of adatom and vacancy and their evolution or migration on/in monolayer MoS2 through atomic-scale in-situ observation using aberration corrected transmission electron micsroscopy. The experimental statistical counts and dwell time of adatoms demonstrate that the adsoption on the top of Mo sublattice is the ground state, the hexagon center (hollow site) and top of S sublattice correspond to two different metastable states. In combination with DFT calculation, the transition between gound state and metastable states are 0.6 eV and 1.1 eV. The statistical site distribution of Mo adatom hopping is shown as a scatter polar diagram, semiquantitatively reflecting the adatom’s two-dimensional potential landscape on monolayer MoS2. On the other hand, we also observed the atomic-scale migration of Mo vacancy and tracked its metastable state, to confirm the DFT calculation of the kinetic pathway. The atomic process involved has a large energy barrier of 2.9 eV, which means that the beam effect plays a dominant role in vacancy’s migration and we have to consider the beam-atom interaction. We calculate the knock-on Rutherford cross section, to quantify the dependence of atom displacement cross section on the migration energy barrier.Fourthly, the atomic structure characterization shows the dense inversion domain boundary in MoSe2 monolayer prepared by molecular beam epitaxy (MBE). Both scanning tunneling spectroscopy (STS) and DFT calculation demonstrate these grain boundaries induce the metallic mig-gap states with considerable density of states, which imply that these boundaries will greatly enhance the d-band catalysis of nano-sized MoS2 in applications such as hydrodesulfurization (HDS) and hydrogen evolution reaction (HER). Besides the MoSe2 system, we also observe the formation and disappearance of high-symmetry grain boundaries (along zigzag direction) in the monolayer MoS2 at room temperature. These atomic processes occur without elevated temperature or impurity atom and are always accompanied by sulfur atom’s migration and the creation/accumulation of sulfur vancancies. Our DFT calculation of the energetics shows that sulfur vacancy nearby the high-symmetry grain boundary will decrease the formation energy compared with the case without vacancies. The formation energy decreases more dramatically when the vacancy-boundary distance decreases and the number of vacancies increases. Sulfur vacancies may come partly from the intrinsic defects and partly from the beam irradiation damage, but promote the in-situ formation or evolution of grain boundaries.Finally, we summarize the research on electron energy loss spectroscopy and atomic structure characterization of two-dimensional TMDs, analyze the weakness in our finished work, and uncover many challenging issues which hint considerable prospects to the future research.
Keywords/Search Tags:two-dimensional materials, MoS2, anisotropic, electron energy loss spectroscopy (EELS), point defects, migration, grain boundary
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