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Physics Dissertation Defense: Coherent and Incoherent Dynamics of Quasiparticles in Monolayer Molybdenum Diselenide

This is a past event.

Wednesday, March 27 at 2:00pm to 5:00pm

CP - Chemistry & Physics, 220
11200 SW 8th ST 33199, Chemistry & Physics, Miami, Florida 33199

Coherent and Incoherent Dynamics of Quasiparticles in Monolayer Molybdenum Diselenide

By Michael Titze

Major Professor: Dr. Hebin Li

Monolayer Materials have attracted significant research interest since their discovery in 2004. The efforts were rewarded with a nobel prize in 2010 for the discovery of monolayer graphite, the same year in which the first monolayer transition metal dichalcogenide (ML-TMD) was found to have a direct bandgap. In contrast to graphene ML-TMDs have a direct bandgap in the visible or near-infrared spectral range, making them ideally suited for optoelectronic device applications. Explicit inversion symmetry breaking of the unit cell in ML-TMDs furthermore leads to a new interesting property, called valley pseudo-spin. Electrons excited within one valley are restricted to this valley due to momentum trapping. Investigating the valley pseudo-spin dynamics is of importance for both understanding of the fundamental physics as well as device applications since the valley pseudo-spin is a potential information carrier and has potential use for information storage or computing application.

Additionally, the confinement to two dimensions leads to enhanced Coulomb interaction and increased dielectric screening between electron and hole. The screening of the Coulomb interaction in turn leads to a significantly increased binding energy between electron and hole, such that the bound electron-hole state, so-called exciton, is stable up to room temperature and above. The same reasoning leads to an enhanced stability of charged excitons, so-called trions, which are the main focus of this dissertation. The electronic dynamics in ML-TMDs are therefore completely dominated by excitons and trions, requiring an in-depth understanding of these quasiparticles for device performance optimization.

Time-resolved techniques can offer rich information compared to steady-state measurements providing access to transients and revealing the lifetime of unstable or metastable states which may be invisible in steady-state measurements. Measuring the incoherent dynamics of trions with a non- degenerate pump-probe experiment, the trion dynamics in ML MoSe2 are explained through defect trapping. Furthermore, two-dimensional coherent spectroscopy (2DCS), a nonlinear coherent technique, that resolves a nonlinear signal as a function of two time delays is used to extract the coherence time of trions in ML MoSe2. Collinear and non-collinear implementations of 2DCS were used to measure the coherence time of excitons and trions in ML MoSe2.

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