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Identifier |
000434232 |
Title |
Spin-Valley polarization in transition metal dichalcogenides : CW vs pulsed excitation |
Alternative Title |
Σπιν-Valley πόλωση στα διχαλκογενίδια μετάλλων μετάπτωσης |
Author
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Βασιλειάδης, Χρήστος
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Thesis advisor
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Κιοσέογλου, Γεώργιος
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Reviewer
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Κοπιδάκης, Γεώργιος
Στρατάκης, Εμμανουήλ
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Abstract |
After the discovery of graphene, new horizons opened up in the scientific community
for two-dimensional materials. Their exceptional physical and optical properties, as
they become thinner and thinner, have attracted an extensive scientific interest. As
graphene is not so suitable for optoelectronic applications due to the zero bandgap,
there is another category of materials more suitable for such applications. This
category of materials is called Transition Metal Dichalcogenides (TMDS) which are
semiconductors and therefore are presented as alternatives to graphene. Thin layers
of these materials have a direct-gap at the K-point of the Brillouin zone with a range
of bandgaps in the visible spectrum making them suitable for optoelectronic
applications. A strong photoluminescence (PL) emission is created, as an indirect-todirect
transition occurs, when they are thinned down to a monolayer level.
In this thesis, temperature-dependent spin-valley polarization studies on monolayer
WS2 samples with two different laser excitation sources, continuous wave (CW) and
pulsed laser are presented. The spin-valley polarization of the monolayer WS2 with
CW laser excitation is well-known and it has been practiced by many groups. The basic
purpose was to compare the data of the same experiment but with different excitation
sources. The spin-valley polarization with pulsed laser is a little more difficult to carry
out, because it was necessary to have the same wavelength in both lasers. With
various optical devices and filters, this was finally achieved.
At the same time some temperature dependence photoluminescence (PL)
experiments took place in order to characterize the behavior of the excitonic
complexes involved in the emission process. Α series of two photon
photoluminescence (2PPL) experiments were also performed simultaneously; in
addition to the A-exciton emission of WS2, the second harmonic generation (SHG)
signal was also recorded. During this study, the behavior of SHG signal was
investigated while changing the repetition rate in pulsed laser settings.
Finally, Transient Absorption Spectroscopy (TAS) experiments were performed to
probe the exciton lifetime, in order to shed light in the behavior of the spin-valley
polarization under different excitation laser sources.
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Language |
English |
Issue date |
2020-11-27 |
Collection
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School/Department--School of Sciences and Engineering--Department of Materials Science and Technology--Post-graduate theses
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Type of Work--Post-graduate theses
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Permanent Link |
https://elocus.lib.uoc.gr//dlib/9/b/d/metadata-dlib-1606309252-52470-4581.tkl
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Views |
425 |