Electronic and Optical Properties of Lanthanide-Doped MoS₂: Impact of Ionic Size and Orbital Configuration Mismatch

Single-photon emitters (SPEs) are vital for quantum technologies such as simulation, secure communication, and precision sensing. Two-dimensional transition-metal dichalcogenides (TMDCs) are promising SPE platforms owing to their atomic thickness, high extraction efficiency, and integration with photonic devices. However, most TMDC-based SPEs emit in the visible range, limiting telecommunication applications that require infrared wavelengths. Lanthanide (Ln) doping in TMDCs offers a potential solution by introducing sharp, f-orbital-derived infrared emission, though its feasibility and impacts remain unclear due to the large ionic radii of Ln atoms. Here, we employ DFT calculations to explore the structural, electronic, and optical effects of Ln-doped MoS₂ monolayers (Ln=Ce, Er). Formation energy analysis shows that S vacancies adjacent to Ln sites mitigate lattice strain, stabilizing Ln incorporation thermodynamically. Charge-state and band structure analyses reveal f-orbital-derived and host-related defect states near the band gap, originating from orbital configuration mismatch between dopant and host. Optical absorption spectra further indicate in-gap transitions from these states. Notably, Er_Mo exhibits sharp, weak f-f optical transitions at 0.9-1.1 eV, suggesting potential for telecommunication-based SPE, while Ce_Mo shows only defect-related absorption due to its empty f shell.