Optische und Magneto-optische Untersuchungen an zweidimensionalen Übergangsmetalldichalkogeniden und deren Heterostrukturen
Loading...
Files
Date
Authors
Publisher
Philipps-Universität Marburg
Supervisors
Abstract
It was the aim of this thesis to investigate the influence of the substrate
material on the optical properties of TMD monolayers and bilayers. Furthermore measurements
in high external magnetic fields were conducted in order to get a deeper
insight into the spin physics and to determine the g-factors of TMDs.
When comparing the two CVD-grown WS2 samples on sapphire and SiO2 substrate,
small differences in the emission properties are seen. WS2 on sapphire shows a slightly
red shifted emission but exhibits a stronger coupling to the phonon system compared
to SiO2 substrate. This is due to different doping caused by the substrate material.
The phonon coupling was evaluated by measuring the temperature dependence of the
energetic position for each emission band. Further analysis showed that trions in general
interact less with the underlying phonon system than excitons do.
With the help of strong magnetic fields the g-factors for the individual excitonic transitions
were measured. For both CVD-grown WS2 samples, as well as for a CVD-grown
MoSe2 sample, the excitonic and trionic g-factors were found to be 4 in conjunction
with earlier studies. For MoSe2 no excitonic g-factor could be measured as
the exciton vanishes at low temperatures. Furthermore, for the first time ever, it was
possible to measure the g-factors for bound excitonic states in TMDs. Surprisingly, the
measured values differ clearly from the value 4 obtained for free excitons and trions,
revealing deviations in their effective masses. For the g-factors of the bound excitonic
states in WS2 on sapphire substrate 4.33, 3.98, and 3.46 were found. On SiO2 substrate
4.33, 3.98, and 2.42 were found for the three bound excitonic states. MoSe2 on sapphire
substrate produced 3.21, 2.87, and 2.42 as g-factors. The results strongly indicate a
tendency. With larger binding energy and therefor stronger localization of the excitons
the g-factor becomes smaller. Corrections beyond the two band model have to be taken
into account, leading to different effective masses and different valley and orbital
contributions.
In the last part of this thesis measurements of the valley coherence on a CVD-grown
WSe2 sample on MgF2 substrate were carried out. After pumping with linearly-polarized
laser-light the emission also showed a certain degree (17 %) of polarization. Here, for
the first time, this coherent superposition of left-handed and right-handed circularly
polarized light was measured in a transmission geometry. By applying an external
magnetic field, the emission polarization turns against the polarization of the pump
laser with a rate of 1,43 ◦/T. In literature both higher and smaller values are reported.
In Literature the rotation of the polarization with increasing magnetic field is assigned
to the rotation of the in-plane exciton-pseudospin. In steady-state experiments the sum
of all intermediate states is measured, leading to a net rotation.
Here, a new explanation is given. Due to the breaking of the degeneracy between the
two emitting states, they emit at different energies by applying an external magnetic
field. As the index of refraction for different energies can be different, this could introduce
an ellipticity in the emission and therefor explain the observed shift with applied
magnetic field when measuring in a linear polarized basis. For final clarification more
measurements are needed. Also the possible influence of the Faraday effect of substrate
and TMD has to be carried out.
Review
Metadata
Contributors
Supervisor:
Dates
Created: 2018Issued: 2018-11-14Updated: 2018-11-14
Faculty
Fachbereich Physik
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
Keywords
excitong-factorWS2MoSe2Spektroskopie, Exziton, Trion, Photolumineszenz, Zeitauflösungspectroscopytime-resolutionphotoluminescenceMoS2Magnetfeld, g-Faktor, Substrateinfluss, WS2, MoS2, MoSe2, WSe2trionWSe2
DFG-subjects
ZeitauflösungOptische Spektroskopie, ExzitonPhotolumineszenzMagnetfeld
DDC-Numbers
530
show more
Kuhnert, Jan: Optische und Magneto-optische Untersuchungen an zweidimensionalen Übergangsmetalldichalkogeniden und deren Heterostrukturen. : Philipps-Universität Marburg 2018-11-14. DOI: https://doi.org/10.17192/z2018.0509.
License
This item has been published with the following license: In Copyright