Optische und Magneto-optische Untersuchungen an zweidimensionalen Übergangsmetalldichalkogeniden und deren Heterostrukturen
| Autor: | Kuhnert, Jan, Heimbrodt, Wolfram (Prof. Dr.) |
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| Jazyk: | němčina |
| Rok vydání: | 2018 |
| Předmět: | |
| Popis: | 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. Übergangsmetalldichalkogenide (TMDs) weisen eine sehr große Licht-Materie-Wechselwirkung auf. So absorbiert eine Lage bereits bis zu 20% des einfallenden Lichts. Verantwortlich für die starke Absorption sind die besonders stark gebundenen Exzitonen in diesen Systemen. Die besonderen Eigenschaften, welche im Monolagenfall den Spin der Elektronen mit dem Tal, in dem sie sich befinden, verknüpft, machen diese Materialien zu potentiellen „Spintronik“- Kandidaten. Dieses Kunstwort ist zusammengesetzt aus „Spin“ und „Elektronik“ und beschreibt Bauteile, bei welchen der Informationsfluss nicht über elektrische Ladung, sondern über deren Spineigenschaften erfolgt. Aufgrund der starken Exzitonenbindungsenergie und des zweidimensionalen Charakters der Übergangsmetalldichalkogenide sollte die unmittelbare Umgebung der Materialien einen bedeutenden Einfluss auf die optischen Eigenschaften der Lagen haben. Erste Untersuchungen konnten bereits substratspezifische Effekte feststellen, es fehlt jedoch bisher an systematischen Untersuchungen. Ziel dieser Arbeit war es im ersten Teil, solch eine systematische Untersuchung durchzuführen und durch Anwendung zahlreicher optischer Spektroskopiemethoden (Ramanspektroskopie, Photolumineszenzspektroskopie, Reflektionsspektroskopie) den Einfluss unterschiedlicher Substrate auf die optischen Eigenschaften der Lagen herauszufinden. Aufgrund der besonderen Spin-Tal-Eigenschaften der Übergangsmetalldichalkogenidmonolagen sind Messungen in externen Magnetfeldern gut geeignet, um ein tieferes fundamentales Verständnis der Vorgänge in diesen Materialien zu erlangen. Im zweiten Teil der Arbeit wurden deshalb Untersuchungen in starken externen Magnetfeldern durchgeführt. |
| Databáze: | OpenAIRE |
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