| Author: | Yan, Shanru |
| Title: | Understanding the structural origins of room-temperature 2D ferroic properties and instability of vdW layered selenides |
| Advisors: | Zhu, Ye (AP) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Two-dimensional materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | 137 pages : color illustrations |
| Language: | English |
| Abstract: | Two-dimensional van der Waals (vdW) materials have attracted intensive interest in both fundamental and industrial applications due to their extremely exotic physical, chemical, and biological properties, such as electronic, optoelectronic, mechanical, magnetic, thermal, optical, and catalytic. Recently, several novel characteristics have been discovered in 2D selenides and wide applications. For instance, the typical group III-VI compound In2Se3 shows 2D ferroelectricity and antiferroelectricity; another representative Transition Metal Dichalcogenides (TMDs), the single-layered WSe2-based Field-Effect Transistor (FET) exhibits high carrier mobility and ON/OFF ratio; the MoSe2 as an alternative of MoS2 also shows huge potential in electrochemical, photocatalytic, and optoelectronic systems. Hence, a systematic study of the crystal structure of 2D selenides will enable us to find more unconventional phenomena. In this thesis, we first investigate the phase transition of In2Se3, intermediate β'-In2Se3 is obtained through heating α-In2Se3 to above 200 ℃ and then annealing to room temperature, this thermal-induced phase transition only occurs on the flakes with a thickness below 400nm. Besides, the reversible phase transition between α and β' has also been achieved through ionic liquid gating. The supercell in β'-In2Se3 is mainly due to the freezing phonon modes of the parent β phase which is accompanied by obvious lattice distortion due to the collective Se atom displacement, thus, ferroelasticity has also been observed both in mechanical exfoliated and Chemical Vapor Deposition (CVD)-prepared a few layered flakes through in-situ tensile experiment under polarized optical microscopy. α-In2Se3 and β'-In2Se3 suffer from serious degradation in ambient conditions, because of the non-symmetric structure, indium atoms in α-In2Se3 locate in an octahedral or a tetragonal coordination structure, Crystal Orbital Hamilton Population (COHP) calculation indicates that In-Se bond strength inside octahedron is much weaker than that in tetragon, thus, the surface with octahedral termination is more easily to be degraded than the surface with tetragonal coordination structure. Lastly, we characterized the crystal structure of 1T'- WSe2 by Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), and Raman, anisotropic light absorption is observed in exfoliated specimens through angle-resolved polarized Raman measurement and group theory calculation as a complementary part of the thesis. We present multiple cutting-edge crystal structure characterization techniques, such as HRTEM, 4D Scanning Transmission Electron Microscopy (4D-STEM), Focused Ion Beam (FIB), Energy Dispersive X-ray Spectroscopy (EDS), in-situ Selected Area Electron Diffraction (SAED), XRD, Raman, and Scanning Electron Microscopy (SEM), besides, electric and optical property investigations have been implemented by polarized optical microscopy, angle-resolved polarized Raman, Piezoresponse Force Microscopy (PFM) and Kelvin Probe Force Microscopy (KPFM). In addition to crystal characterization, we also introduce serval 2D material preparation methods, like mechanical exfoliation, bottom-up growth, and ultrasonication. Simple 2D material-based device fabrication is also presented. Our study concerns almost all fields of 2D materials investigation including sample preparation, structure characterization, related property exploitation, and possible device applications, this may provide a practical and efficient process to study novel 2D materials. |
| Rights: | All rights reserved |
| Access: | open access |
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