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Study of the Charge-Transfer Dynamics at the Interface of Two-Dimensional Materials
In this thesis the charge-transfer dynamics in two-dimensional transition metal dichalco- genide (TMD) heterostructures (HS) were studied. In order to enable efficient study of TMD heterostructures a re-imagined experimental technique was employed: optical second-harmonic (SH) imaging microscopy....
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| Format: | Dissertation |
| Sprache: | Englisch |
| Veröffentlicht: |
Philipps-Universität Marburg
2021
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| Online Zugang: | PDF-Volltext |
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| Zusammenfassung: | In this thesis the charge-transfer dynamics in two-dimensional transition metal dichalco-
genide (TMD) heterostructures (HS) were studied. In order to enable efficient study
of TMD heterostructures a re-imagined experimental technique was employed: optical
second-harmonic (SH) imaging microscopy. The newly developed setup allows for the
study of µm small samples, giving access to the crystal orientation as well as enabling
time-resolved measurements with a time-resolution of about 10fs. After designing
und building the experiment it was deployed to measure and determine the lifetimes
of excitons in mono- and multilayers of MoS_2 . This proof-of-principle experiment
found lifetimes for the monolayer of τ_1 = 1.9 ± 0.7ps and τ_2 = 48.5 ± 2.1ps as well
as τ_1 = 3.2 ± 0.9ps for the multilayer, well in accordance with current literature.
Furthermore it allowed for association of the pump-induced decrease in the nonlinear
susceptibility with the generation of intralayer excitons, giving direct access to their
lifetimes.
After validation of the viability of the experiment for the further study of hetero-
structure charge-carrier dynamics we used the setup to measure a MoSe_2 /WSe_2 hetero-
structure, which was misaligned under a stacking angle of 32.3°. We discovered that it
was possible to measure selectively the time-resolved changes of the individual mono-
layers inside the heterostructure via probe polarization dependent measurements.
This was achieved by carefully tuning the probe polarization to suppress either of
the monolayers and extract the lifetime from other. This unique characteristic of the
setup in tandem with energy dependent measurements revealed a bi-directional charge
transfer. Following the resonant excitation of one layer a delayed decrease in the op-
tical second-harmonic signal becomes visible when sensitive to the other layer of the
heterostructure. This delayed filling is absent when the probe polarization is tuned
to the pumped material further corroborating the identification as a charge-transfer
process in which interlayer excitons are formed. The hole transfer times found were
τ_MoSe_2 →WSe_2 =210±60fs from MoSe_2 into WSe_2 following excitation at 1.80eV, as
well as τ_WSe_2 →MoSe_2 =610±150fs following excitation at 2.09eV.
The next step to deepen our understanding of the charge-transfer was a system-
atic study of samples consisting of MoS_2 and WSe_2 with different stacking angles. By
applying the insight gained from the previous study we identified an electron trans-
fer from WSe_2 into MoS_2 after selectively exciting the WSe_2 . Overall three stacking
configuration were measured and clear differences were identified. The transfer times
were found to vary significantly for different stacking angles in a range from 12 ± 4fs
up to 85 ± 9fs. These disparities were accredited to an enhanced spatial wavefunc-
tion overlap between states in the interfacial plane. Furthermore, the importance of
hybridized states for the charge-recombination dynamics was illustrated. Therefore
the pump-photon energy was tuned to 1.85eV. The larger excitation energy opened
previously non-active radiative recombination channels via states around the Γ-point,
which led to an equalization of the decay times for all samples to ≈ 200ps.
Lastly in an exploratory study the aptitude of a new material class in the form
of chiral metal halides for application in more complex heterostructures was inves-
tigated. The results consolidated the interest in the material showing promise for
future applications in more complex Van der Waals coupled heterostructures. |
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| Umfang: | 123 Seiten |
| DOI: | 10.17192/z2021.0390 |