Ultrafast spatiotemporal dynamics of photoexcitations in two-dimensional semiconductors

This thesis focuses on two families of Van der Waals materials: the transition metal dichalcogenides and the group IV monochalcogenides. First, we explore the spatio-temporal dynamics of photo-excited species in monolayer transition metal dichalcogenides. Using pump-probe spectroscopy, and a range of steady state characterization techniques (ellipsometry, reflectance, photoluminescence), we study the formation and decay of excitons in these materials at low excitation densities. We find that the photodynamics are largely dominated by the screening of the Coulomb interaction by excited states. At high excitation densities, when the screening of the Coulomb interaction is such that the excitons are no longer stable, we observe their complete ionization into a dense plasma of unbound electrons and holes. This interaction driven transition from an insulator a metallic state is a typical Mott transition. Combining pump-probe techniques with microscopic imaging allows us to observe these processes in space with 10 fs temporal resolution and 10 nm spatial localisation. Above the Mott transition, we observe an ultrafast spatial propagation of the excitations in the first few hundred femtoseconds after excitation. We hypothesise that this new regime of ultrafast transport is driven by the Fermi pressure of the dense electron-hole gas. Because atomically thin materials are very sensitive to their environment, it is also possible to tune the energies of electronic states through changes in the surroundings. Here, we engineer a potential landscape for excitons in the monolayer by introducing spatial variations in the dielectric environment. We can then watch excitations evolve in this landscape on an ultrafast time scale. Specifically, we observe the funnelling of excitons in a lateral homojunction. Finally we demonstrate the exfoliation down to the monolayer of two representative member of the group-IV monochalcogenides family.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2022

Author dc:creator

• Gauriot, Nicolas

Advisor dc:contributor.advisor

• Rao, Akshay

• 2d materials
• optics
• spectroscopy

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/fc3700a8-ebcd-4302-b233-6efc868cee47/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.100643

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/355796