Influence of resonant plasmonic nanoparticles on optically accessing the valley degree of freedom in 2D semiconductors

The valley degree of freedom is one of the most intriguing properties of atomically thin transition metal dichalcogenides. Together with the possibility to address this degree of freedom by valley-contrasting optical selection rules, it has the potential to enable a completely new class of future electronic and optoelectronic devices. Resonant optical nanostructures emerge as promising tools for controlling the valley degree of freedom at the nanoscale. However, a critical understanding gap remains in how nanostructures and their nearfields affect the polarization properties of valley-selective chiral emission hindering further developments in this field. In order to address this issue, our study delves into the experimental investigation of a hybrid model system where valley-specific chiral emission from monolayer molybdenum disulfide is interacting with a resonant plasmonic nanosphere. Contrary to the intuition suggesting that a centrosymmetric nanoresonator preserves the degree of circular polarization in the farfield, our cryogenic photoluminescence microscopy reveals almost complete depolarization. We rigorously study the nature of this phenomenon numerically considering the monolayer-nanoparticle interaction at different levels including excitation and emission. We find that the farfield degree of polarization strongly reduces in the hybrid system when including excitons emitting from outside of the system's symmetry point, which in combination with depolarisation at the excitation level causes the observed effect. Our results highlight the importance of considering spatially distributed chiral emitters for precise predictions of polarization responses in these hybrid systems. This finding advances our fundamental knowledge of the light-valley interactions at the nanoscale but also unveils a serious impediment of the practical fabrication of resonant valleytronic nanostructures

Exciton Dynamics in MoS₂‑Pentacene and WSe₂‑Pentacene Heterojunctions

We measured the exciton dynamics in van der Waals heterojunctions of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSs). TMDCs and OSs are semiconducting materials with rich and highly diverse optical and electronic properties. Their heterostructures, exhibiting van der Waals bonding at their interfaces, can be utilized in the field of optoelectronics and photovoltaics. Two types of heterojunctions, MoS2-pentacene and WSe2-pentacene, were prepared by layer transfer of 20 nm pentacene thin films as well as MoS2 and WSe2 monolayer crystals onto Au surfaces. The samples were studied by means of transient absorption spectroscopy in the reflectance mode. We found that A-exciton decay by hole transfer from MoS2 to pentacene occurs with a characteristic time of 21 ± 3 ps. This is slow compared to previously reported hole transfer times of 6.7 ps in MoS2-pentacene junctions formed by vapor deposition of pentacene molecules onto MoS2 on SiO2. The B-exciton decay in WSe2 shows faster hole transfer rates for WSe2-pentacene heterojunctions, with a characteristic time of 7 ± 1 ps. The A-exciton in WSe2 also decays faster due to the presence of a pentacene overlayer; however, fitting the decay traces did not allow for the unambiguous assignment of the associated decay time. Our work provides important insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions