Prevention of Photo-Induced Degradation in Monolayer TMDs Probed by THz Emission Spectroscopy

Low dimensional transition metal dichalcogenides (TMDs) are at the front line for opto-electronic devices, valleytronics, or quantum emitters due to their extraordinary optical and mechanical properties, such as a sizeable bandgap, large exciton binding energies or strong spin-orbit coupling. However, many 2D materials with atomic-scale thickness suffer from oxidation and degradation effects under ambient conditions, which hampers their practical applications. In the case of MoS₂ and WS₂, it is understood that gradual oxidation appears along grain boundaries or sulfur vacancies when the monolayers are exposed to the environment for several months. It is further suggested that H₂O acts as a ‘catalyst’ and greatly speeds up the oxidation process. Recently, photo- induced oxidation in WS₂monolayers was identified as the physical mechanism of oxidation in ambient air when the sample is excited with photon energies above the band edge. Thus, in order to realize robust TMD opto-electronic devices, it is necessary to further investigate in protection strategies for large scale 2D materials.<br/>Instead of encapsulating WS₂ monolayers (ML) with a preservative hBN coating, which is limited in its scalability, in this work, we exploit Au substrates to prevent photo-induced degradation in the 2D TMD material and probe the surface properties as well as charge carrier dynamics with THz emission spectroscopy.<br/>We thereby excite the WS₂ monolayer above the band edge energy with 400 nm (3.09 eV) pulses and record the emitted THz transient with electro-optic sampling. Sampling the time variation and polarization of the radiated electric fields provide a contact free, quantitative method for determining the magnitude, polarity, directionality, phase and temporal variation of the source currents, which can reflect the structure symmetry, carrier diffusion, carrier drift, surface depletion or non-linear susceptibility.<br/>The following samples are under investigation: (1) WS₂ monolayers grown by chemical vapor deposition (CVD) on Sapphire transferred to either a fused silica or gold substrate and (2) large-area gold-tape exfoliated WS₂ monolayers which are either transferred to a fused silica substrate or kept on the gold layer. The set of samples is chosen such that the study is independent of the defect state densities, caused by different fabrication methods. We report for the first time on THz emission from single layer TMDs pumped above resonance. We observe a surprisingly high THz signal from the ML, which is only five times lower than the signal from a reference bulk InSb sample for which the THz signal is ascribed to the Photo-Dember effect. The large THz signal from WS₂ on fused silica is attributed to resonant optical rectification as well as to a surface field-induced photocurrent. However, while the emitted THz field is stable when the environment is purged with N₂, resulting in a relative humidity of 0%, the THz signal vanishes under ambient environment within minutes and obvious optical damage of the sample is visible under the microscope. In contrast, in the case of WS₂ on a gold substrate, the THz signal remains stable under ambient conditions, and no optical damage can be observed under the microscope, although higher fluences are applied as compared to the fused silica samples. Moreover, pump polarization studies indicate that THz radiation is due to interfacial charge transfer from the TMD monolayer to the metallic substrate. We thus hypothesize that the enhanced stability of the gold substrate is due to quenching of the photoexcited carriers through nonradiative recombination in the gold substrate and thus, the number of excited carriers available for chemical reaction is greatly reduced.<br/>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.