Seminars
Seminars occur on Wednesdays from 12:00-1:00 PM Pacific in ISB 130 with a hybrid option over Zoom unless otherwise posted. To join the mailing list, please contact Prof. Ehsan Khatami at ehsan.khatami@sjsu.edu with the words "Seminars and Events" included in the subject heading.
This Week
Probing photo-induced interfacial reactions in monolayer TMDs with time-domain THz
emission spectroscopy
Claudia Gollner, SLAC National Accelerator Laboratory
Wednesday, 3/5/2025, 12:00-1:00pm Pacific
Abstract: Low-dimensional transition metal dichalcogenides (TMDs) are at the front line of fields
including opto-electronic devices, valleytronics, and quantum emitters due to their
extraordinary optical and mechanical properties, such as a sizeable bandgap, large
exciton binding energies, and strong spin-orbit coupling. However, many 2D materials
suffer from oxidation and degradation effects under ambient conditions, which hampers
their practical applications. For MoS₂ and WS₂, oxidation occurs along grain boundaries
and sulfur vacancies over time, with H₂O acting as a catalyst. More recently, photo-induced
oxidation has been identified as a key degradation mechanism in WS₂ when exposed to
photon energies above the band edge. Instead of encapsulating WS₂ monolayers with
a commonly used preservative hBN coating, which is limited in its scalability, in
this work, we exploit gold (Au) substrates to prevent photo-induced degradation in
the 2D TMD material and probe the surface properties as well as charge carrier dynamics
with time-domain THz emission spectroscopy (TES), providing a direct method to probe
a transient photo-current by recording the emitted THz electric field, following ultrafast
laser excitation. In this talk, I want to provide a fundamental understanding of TES,
followed by our recent results on photo-induced degradation in WS₂ monolayers on either
fused silica (SiO2) or gold substrates. The latter remains stable under photo-excitation in ambient
conditions, while WS₂ on SiO2 degrades within minutes. TES reveals an interfacial charge transfer from the TMD
monolayer to the metallic substrate, indicating that the enhanced stability of the
Au substrate is due to quenching of the photoexcited carriers through nonradiative
recombination in the metall substrate and thus, the number of excited carriers available
for chemical reaction is greatly reduced. This simple method of using a gold substrate
to prevent photo-induced oxidation could pave the way for future electro-optic devices
based on 2D TMD materials.
Bio: Dr. Claudia Gollner earned her Master of Science in Technical Physics from JKU Linz (Austria), followed by a Master of Research in Controlled Quantum Dynamics at Imperial College London and a PhD in Engineering Sciences from TU Vienna, which she completed with highest honors. During her master's studies, she focused on random lasing in colloidal quantum dots and received several awards for her thesis, including the Erwin Wenzl Prize, the OePG Students Award, and the Wilhelm Macke Master Thesis Prize. In her PhD research, she advanced strong, mid-IR driven THz generation for nonlinear spectroscopy, earning multiple invited talks, an Outstanding Runner-up Student Oral Presentation Award, and the Award of Excellence from the Austrian Federal Ministry of Education, Science, and Research for her dissertation. She successfully secured a Schrödinger Fellowship from the Austrian Science Fund (FWF) and is now working in the research group of Tony Heinz and Aaron Lindenberg at Stanford Institute of Material and Energy Science at SLAC National Accelerator Laboratory to tackle fundamental problems in charge transfer dynamics in two- dimensional transition metal dichalcogenides for opto-electronic device applications.
