Optical Phenomena of Si@MoS₂ Core@Shell Architecture at the Nanoscale

Transition metal dichalcogenides (TMDs) are attractive for next-generation photonics due to their large exciton binding energy and exciton transition dipole moment. However, because of their few-atom-layered nature, observing nanoscale light-matter interaction through absorption and emission of the electromagnetic field in TMDs has been challenging. This unfortunately makes isolated TMDs inappropriate for use in optical-to-electrical conversion applications.<br/> <br/>Recently, TMD-encapsulated nanospheres, or core@shells, have presented a potential avenue for strong light-matter interactions by leveraging the functionality of the nanoparticle core material. In a core@shell architecture, the dielectric nanosphere core acts as an optical cavity in the Mie regime while the encapsulating 2D material shell acts as a quantum emitter. When the two constituents couple, light-matter interaction is significantly improved and optical-to-electrical conversions such as lasers and quantum information processing that had been otherwise limited in 2D materials can be realized. This geometry has potential applications in nanophotonic devices such as all-optical switches, exciton−polariton lasers, and quantum information processing, as well as various optoelectronic applications such as ultrasensitive sensors, light-emitting devices, and solar cells. However, it is challenging to grow 2D materials on curved surfaces. Moreover, there is an urgent need to develop experimental techniques to probe complex systems that cannot be easily modelled computationally. In our work, we fabricate Si@MoS₂ and use valence electron energy loss spectroscopy (VEELS) to achieve enhanced light-matter interaction and understand the principles underlying this phenomenon.<br/> <br/>We encapsulate silicon nanospheres with multilayers of MoS₂ via chemical vapor deposition. Using transmission electron microscopy (TEM) and single-particle scattering spectroscopy, we demonstrate energy coupling in this system with the silicon magnetic dipole mode undergoing Rabi splitting at the MoS₂ A-exciton wavelength in its scattering spectrum. The coupling constant is measured to be significantly higher than that of nanoparticle-on-TMD film geometries, and is an important validation of the Si@MoS₂ system for adoption in photodetection-based nanotechnology. Photoluminescence enhancement is also demonstrated by Si@MoS₂, effectively affirming that the core acts as an antenna to excite the TMD shell emitter.<br/> <br/>We then extract the local dielectric functions of Si@MoS₂ with high spatial resolution via VEELS, which offers information on the valence electron excitations, to understand how the electronic structure of this heterostructure evolves from its two constituents. Using a cross-sectioned Si@MoS₂, the evolution of the electronic structure from uncovered silicon to the core-shell interface is evaluated. Notably, the resulting dielectric function of Si@MoS₂ demonstrates features attributed to heterostructuring semiconductors with different band gaps. As dielectric functions embed valence electron excitation, plasmonic excitation, and intra/inter-band transitions, they directly relate to optical parameters such as absorption, reflectance, and transmissivity of the material. The presentation will explain how optical properties of core@shells can be understood and provide a platform to improve their property-performance relationship.