Atomically MoS2 and WS2 Thin Semiconductors for Nanophotonics

Atomically thin layers of semiconductors, such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), are promising materials for nanoscale photonic devices. These nearly two-dimensional semiconductor support so-called excitons or bound electron-hole pairs, which can be aligned vertically along the thin planes of the material.

Excitons are bound electron-hole pairs that can interact with charge, spin, and phonons. The range of such interactions suggests that excitons may herald a new wave of devices based on nanoscale photonics and optoelectronics.

(Credit: David L. Chandler/MIT News Office)

In his PhD thesis, Rasmus Godiksen from the Eindhoven University of Technology investigated exciton behavior in atomically thin semiconductors such as molybdenum disulfide and tungsten disulfide by exploring the potential of excitons, with a focus on emitted light. These semiconductors are so thin that they can be approximated as two-dimensional materials. Thus, in effect, Godiksen studied excitons in two-dimensional materials.

First, Godiksen and his co-workers showed that two-dimensional excitons are very sensitive to their nano-environment. Using photoluminescence (PL) imaging techniques, they measured fluorescence fluctuations due to charge transfer to the semiconductor. Such fluctuations are spatially correlated in the range of a few tens of microns for a WS₂ monolayer on a thin metal film.

Due to the charge fluctuations from the trap states (trap states are states that trap excited carriers such as electrons, holes, and excitons), they follow power-law statistics with simultaneous changes in emission intensity, lifetime, and exciton-to-trion ratio. The power-law statistics are indicators of trapped and de-trapping excitons, so this provides evidence of the trapped state.

The exciton in WS₂ also has a degree of freedom in terms of the valley, which couples the spin polarization to the momentum direction. The valleys in the band structure can be explored with circularly polarized light. Excitation or detection of an exciton in a valley can be used for information technology.

(Credit: Rasmus Godriksen/Eindhoven University of Technology)

To explain the contrasting spin valley polarization in several layers of WS₂ and WSe₂, Godiksen used layer- and temperature-dependent circular polarization PL measurements. This relates their contrasting polarizations to the different momentum of their conduction band minima.

By varying the distance of the WS₂ bilayer, the enhancement of excitation increases the annihilation of excitons with excitons, leading to a higher polarization. Godiksen's investigated the use of silicon nanoantennas to further enhance the interaction of circularly polarized light with valley-polarized excitons. The results show that the crystalline silicon nano-disk preserves the circular polarization of light in the near field, which is needed for additional enhancement of the valley-polarized emission.

Godiksen's results advance the understanding of exciton interactions with charge, spin, and photons, with implications for a range of nanophotonic devices using MoS₂ and WS₂ atomically thin semiconductors. Single-photon sources are of interest for quantum computing, molecular sensors can increase sensitivity to the single-molecule level, and valley-light devices can pave the way for a new generation of electronic devices based on valley polarization.

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