Capturing exciton wavefunctions by time-resolved photoemission orbital tomography

by Wiebke Bennecke
Doctoral thesis
Date of Examination:2025-03-11
Date of issue:2025-11-05
Advisor:Prof. Dr. Stefan Mathias
Referee:Prof. Dr. Stefan Mathias
Referee:PD Dr. Martin Wenderoth
Referee:Prof. Dr. Ulrich Höfer
Persistent Address: http://dx.doi.org/10.53846/goediss-11615

 

 

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Abstract

English

Excitons are realizations of a correlated many-body wavefunction, consisting of a Coulomb-bound electron and hole pair. They are the dominant excitations in semiconducting organic and low-dimensional quantum materials and thus of great relevance for optoelectronic applications and information technology. To unlock the full optoelectronic potential and to harvest and control exciton-mediated energy conversion pathways, a microscopic understanding of the exciton is crucial. Ultimately, this relies on access to the correlated exciton wavefunction, which has hardly been realized in experiments. This thesis expands on the concepts of photoemission orbital tomography to gain unprecedented insight into the correlated wavefunction of excitons in organic semiconductors, 2D transition metal dichalcogenide heterostructures, and 2D-organic hybrid interfaces. This includes exciton localization, hybridization, charge and energy transfer, and ultrafast exciton formation and relaxation dynamics. Working with the prototypical organic semiconductor buckminsterfullerene C$_{60}$, time-resolved photoemission orbital tomography is employed to unravel the multiorbital electron and hole contributions to the correlated exciton wavefunction thereby elucidating the charge-transfer character of the excitons. The same concepts are then generalized to image the relative electron-hole distribution of the moiré interlayer exciton formed by charge-transfer in a WSe$_2$/MoS$_2$ heterostructure and to identify and visualize a hybrid exciton bridging the PTCDA/WSe$_2$ hybrid interface. Aiming to access the exciton wavefunction with atomic resolution, this thesis presents a lab-based approach to image three-dimensional wavefunctions based on photon energy-dependent measurements, thereby paving the way for femtosecond three-dimensional photoemission orbital tomography. To this end, a highly efficient experimental approach based on an EUV pulse-preserving monochromator is combined with a data-driven algorithmic reconstruction. The power of this approach is demonstrated by the three-dimensional imaging of the frontier orbitals of PTCDA on an Ag(110) surface.
Keywords: momentum microscopy; time-resolved ARPES; TMDs; excitons; photoemission orbital tomography