Phonon-assisted inter-valley scattering determines ultrafast exciton dynamics in MoSe2_2 bilayers

While valleys (energy extrema) are present in all band structures of solids, their preeminent role in determining exciton resonances and dynamics in atomically thin transition metal dichalcogenides (TMDC) is unique. Using two-dimensional coherent electronic spectroscopy, we find that exciton decoherence occurs on a much faster time scale in MoSe$_2$ bilayers than that in the monolayers. We further identify two population relaxation channels in the bilayer, a coherent and an incoherent one. Our microscopic model reveals that phonon-emission processes facilitate scattering events from the $K$ valley to other lower energy $\Gamma$ and $\Lambda$ valleys in the bilayer. Our combined experimental and theoretical studies unequivocally establish different microscopic mechanisms that determine exciton quantum dynamics in TMDC monolayers and bilayers. Understanding exciton quantum dynamics provides critical guidance to manipulation of spin/valley degrees of freedom in TMDC bilayers.Comment: 6 pages, 4 figure

„Systematische Überwachung von SARS-CoV-2 im Abwasser“ – Start eines nationalen Pilotprojekts

Etablierte Surveillancesysteme zur Überwachung der Verbreitung von SARS-CoV-2 können durch eine abwasserbasierte Surveillance ergänzt werden, um Informationen zu relevanten Krankheitserregern und zum Trend der Infektionsdynamik zu gewinnen. Vorgestellt wird ein durch die EU gefördertes Pilotprojekt zur Evaluierung von Umsetzbarkeit und Nutzen der SARS-CoV-2-Abwassersurveillance in Deutschland.Peer Reviewe

Biexciton fine structure in monolayer transition metal dichalcogenides

The optical properties of atomically thin transition metal dichalcogenide (TMDC) semiconductors are shaped by the emergence of correlated many-body complexes due to strong Coulomb interaction. Exceptionalelectron-holeexchangepredestinesTMDCstostudyfundamentalandappliedproperties of Coulomb complexes such as valley depolarization of excitons and fine-structure splitting of trions. Biexcitons in these materials are less understood and it has been established only recently that they are spectrally located between exciton and trion. Here we show that biexcitons in monolayer TMDCs exhibit a distinct fine structure on the order of meV due to electron-hole exchange. Ultrafast pump-probe experiments on monolayer WSe2 reveal decisive biexciton signatures and a fine structure in excellent agreement with a microscopic theory. We provide a pathway to access biexciton spectra with unprecedented accuracy, which is valuable beyond the class of TMDCs, and to understand even higher Coulomb complexes under the influence of electron-hole exchange.This work was supported by Deutsche Forschungsge- meinschaft (DFG) within CRC 1558 and RTG 2247. A. W. A. acknowledges funding from DFG via grant No. AC290-2/1. The spectroscopic experiments were jointly supported by NSF DMR1306878 (A.S.) and NSF MR- SEC program DMR-1720595 (K.T.). X.L. gratefully ac- knowledges support from the Welch foundation (F-1662).Center for Dynamics and Control of Material

Phonon-assisted inter-valley scattering determines ultrafast exciton dynamics in MoSe2 bilayers

While valleys (energy extrema) are present in all band structures of solids, their preeminent role in determining exciton resonances and dynamics in atomically thin transition metal dichalcogenides (TMDC) is unique. Using two-dimensional coherent electronic spectroscopy, we find that exciton decoherence occurs on a much faster time scale in MoSe2 bilayers than that in the monolayers. We further identify two population relaxation channels in the bilayer, a coherent and an incoherent one. Our microscopic model reveals that phonon-emission processes facilitate scattering events from the K valley to other lower energy Γ and Λ valleys in the bilayer. Our combined experimental and theoretical studies unequivocally establish different microscopic mechanisms that determine exciton quantum dynamics in TMDC monolayers and bilayers. Understanding exciton quantum dynamics provides critical guidance to manipulation of spin/valley degrees of freedom in TMDC bilayers.Center for Dynamics and Control of Material

Optische Eigenschaften von Quasiteilchen in Mono- und Bilagen von TMDCs

In this thesis, we investigate the optical properties of excitonic quasiparticles in different transition metal dichalcogenides (TMDCs). These novel two-dimensional (2D) semiconductor materials exhibit remarkable mechanical, optical and electronic properties and feature an additional valley degree of freedom. Due to their atomically thin nature, excitons form as quasiparticles with high binding energies in monolayers (ML) and bilayers (BL) of MoSe2 and MoTe2. We combine photoluminescence (PL), pump probe (PP) and 2D coherent spectroscopy (2DCS) to reveal and quantify valley polarization properties and the interaction of phonons with exciton, trion and biexciton. ML TMDCs are direct semiconductors, whereas BLs in most TMDCs are indirect and less studied. We perform 2DCS measurements of the BL MoSe2 and find a dephasing time for electrons of 20 fs and a scattering of holes of 400 fs, forming an indirect exciton in MoSe2. Moreover, we observe an intra-layer biexciton signature. The BL of MoTe2, which is yet little investigated in literature, shows a bright emission like the ML. We perform a comprehensive spectroscopic study of ML and BL MoTe2 and analyze the lineshape of temperature-dependent PL spectra, comparing ML and BL. The small difference in lineshape can be traced to symmetry and well width, confirming a direct excitonic transition in the BL, unlike in other TMDCs. We further disentangle the homogenous and inhomogeneous parts of the PL linewidth. The inhomogeneous linewidth predicts an inter- and intra-layer trion in the BL separated by 2.2 meV. In general, the trion state in the BL is confirmed by the mass-action law. We deduce a connection between the homogenous linewidth broadening and the phonon-limited mobility, considering acoustical as well as optical phonons. This results in a high acoustical phonon limited mobility of 6000 cm2/Vs (2000 cm2/Vs) and 4300 cm2/Vs for the ML exciton (trion) and BL exciton. The optical phonon limited mobility at room temperature is found to be 300 cm2/Vs and 150 cm2/Vs in ML and BL which are the highest values found so far for TMDCs. Exciton and trion resonances in TMDCs allow for optical manipulation of the valley degree of freedom. We find that resonant excitation of the exciton in PP results in a high negative polarization degree of −65% which is lost within 1 ps due to electron-hole exchange interactions and bandgap renormalization in MoTe2. The trion is less influenced by these effects resulting in a polarization of 95 % immediately after excitation, depolarizing within 3 ps. Both depolarization timescales are consistent with the fast decay of excitons and trions due to defect-assisted recombination. During the trion formation time of 1.5-2.5 ps, we measure a polarization of 30 % in ML MoTe2. These are the first observations of valley polarization in MoTe2. Offresonant excitation results in a non-zero polarization of the PL when exciting higher excitonic states. Phonon assisted cascade processes conserve spin and valley, where the electron transfer is dominated by spin- (valley-) conserving scattering at low (high) temperatures.In dieser Arbeit studieren wir die optischen Eigenschaften exzitonischer Quasiteilchen in Übergangsmetall-Dichalcogeniden (TMDCs). Dieser zweidimensionale (2D) Halbleiter hat bemerkenswerte mechanische, optische und elektronische Eigenschaften, sowie einen weiteren Valley-Freiheitsgrad. TMDCs sind nur einige Atomlagen dick, und ihre Quasiteilchen weisen hohe Bindungsenergien in Mono- (ML) und Bilagen (BL) auf. Durch die Kombination von Photolumineszenz (PL), Pump-Probe (PP) und 2D kohärenter Spektroskopie (2DCS) untersuchen und quantifizieren wir die Wechselwirkung von Phononen mit Exzitonen, Trionen und Biexzitonen. ML TMDCs sind direkte Halbleiter, wohingegen BL in den meisten TMDCs indirekt und wenig untersucht sind. Wir führen 2DCS Messungen am MoSe2 BL durch und untersuchen den Transfer von direktem zu indirektem Exziton, wobei das Elektron innerhalb von 20 fs dephasiert und das Loch innerhalb von 400 fs. Außerdem beobachten wir Anzeichen von Biexzitonen. MoTe2 BL wurde bisher wenig untersucht und zeigt eine strake Lumineszenz ähnlich zum ML. Wir führen eine umfassende spektroskopische Studie an ML und BL MoTe2 durch und analysieren temperaturabhängige PL-Linienformen von ML und BL. Wir begründen kleine Veränderungen der BL Linienbreite im Vergleich zum ML durch Veränderungen in der Symmetrie und Schichtdicke. Dies bestätigt eine direkte Bandlücke in MoTe2 BL, welche in anderen TMDCs nicht beobachtet wird. Die PL Linienbreite wird in den homogenen und den inhomogenen Anteil zerlegt. Durch die Analyse des inhomogenen Anteils erwarten wir zwei 2.2 meV energetisch von einander entfernte Trionen: Inter- und Intralagentrionen. Der Trionenzustand an sich wird durch das Massenwirkungsgesetz bestätigt. Homogene Linienbreite und Mobilität hängen zusammen, wobei wir die Einflüsse akustischer und optischer Phononen betrachten. Wir bestimmen hohe, durch akustische Phononen begrenzte, Mobilitäten von 6000 (2000) cm2/Vs und 4300 cm2/Vs für das Exziton (Trion) der ML und BL. Die durch optische Phononen limitierten Mobilitäten bei Raumtemperatur liegen bei 300 cm2/Vs und 150 cm2/Vs für ML und BL. Dies sind die höchsten gemessenen Mobilitäten in TMDCs. Der Valley-Freiheitsgrad der Quasiteilchen kann optisch manipuliert werden. Eine resonante Anregung der Exzitonen in PP erzeugt eine Polarisation von -65 %, welche in 1 ps durch Elektron-Loch-Wechselwirkung und Bandlückenrenormierung depolarisiert. Die Trionen sind weniger von diesen Mechanismen betroffen und zeigen eine Polarisation von 95 % direkt nach der Anregung, welche in 3 ps depolarisiert. Defektassoziierte Rekombination zeigt vergleichbare Zeitskalen. Während der Trionformation messen wir etwa 30 % Polarisation in MoTe2 ML. Dies sind die ersten Polarisationsmessungen an MoTe2. Wir messen ebenfalls eine Polarisation, wenn wir höhere exzitonische Zustände anregen. Deren Valley- und Spinzustände werden durch phononische Kaskadenprozesse bis in den Grundzustand erhalten, wobei Spin (Valley) bei tiefen (hohen) Temperaturen erhalten wird

Tuning exciton diffusion, mobility and emission line width in CdSe nanoplatelets via lateral size

We investigate the lateral size tunability of the exciton diffusion coefficient and mobility in colloidal quantum wells by means of line width analysis and theoretical modeling. We show that the exciton diffusion coefficient and mobility in laterally finite 2D systems like CdSe nanoplatelets can be tuned via the lateral size and aspect ratio. The coupling to acoustic and optical phonons can be altered via the lateral size and aspect ratio of the platelets. Subsequently the exciton diffusion and mobility become tunable since these phonon scattering processes determine and limit the mobility. At 4 K the exciton mobility increases from ∼ 4 × 103 cm2 V−1 s−1 to more than 1.4 × 104 cm2 V−1 s−1 for large platelets, while there are weaker changes with size and the mobility is around 8 × 101 cm2 V−1 s−1 for large platelets at room temperature. In turn at 4 K the exciton diffusion coefficient increases with the lateral size from ∼ 1.3 cm2 s−1 to ∼ 5 cm2 s−1, while it is around half the value for large platelets at room temperature. Our experimental results are in good agreement with theoretical modeling, showing a lateral size and aspect ratio dependence. The findings open up the possibility for materials with tunable exciton mobility, diffusion or emission line width, but quasi constant transition energy. High exciton mobility is desirable e.g. for solar cells and allows efficient excitation harvesting and extraction.TU Berlin, Open-Access-Mittel – 202