Ultrafast Charge and Energy transfer Dynamics in Organic-Transition Metal Dichalcogenides Heterostructures

Abstract

In this thesis, the ultrafast optical properties of a less well studied Transition Metal Dichalcogenide, monolayer MoTe2 is characterized. Key excitonic properties such as exciton saturation, lifetime, Exciton-Exciton Annihilation rate and its complex spectral evolution arising from many body effects at both the electronic and optical gap as a function of fluence were extracted and studied. These key properties are crucial to the potential application of TMD in optoelectronic devices. Afterwards the ultrafast energy, charge transfer and spin-valley physics of Organic-TMD Heterostructures, namely Pentacene/MoTe2 and Pentacene/WSe2 were examined. It was found that excitation at the electronic gap of MoTe2 leads to a Type II to type I band alignment cross over in Pentacene/MoTe2 heterostructure, due to a mechanism known as carrier-driven bandgap renormalization. Triplet exciton transfer from MoTe2 to Pentacene via Dexter transfer by selectively exciting MoTe2 was demonstrated. Upon excitation above the Mott threshold of MoTe2, it was found that the triplet excitons can dissociate across the interface, forming a long-lived triplet charge transfer state. The exact mechanism by which this process takes place is unknown, but the long-lived triplet charge transfer state offers unexplored opportunities for high temperature Bose-Einstein Condensation of excitons. Finally, the spin-valley physics of WSe2 in an Pentacene/WSe2 heterostructure were studied using helicity and time resolved ultrafast pump probe spectroscopy. A valley polarization of 0.2 and a lifetime of 19 ps in Pentacene/WSe2 heterostructure was observed. Excitation resonant to and above the optical gap of WSe2 leads to a 4-fold and 2 orders of magnitude difference in the charge transfer rate and lifetime respectively. Surprisingly, helicity resolved measurements showed that the valley polarization magnitude and lifetime of WSe2 does not vary with the excitation wavelength. The results suggest that ground state charge transfer from Pentacene to WSe2 determines the valley polarization and lifetime of WSe2.