We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements determined the room-temperature exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron−phonon interaction by the built-in dipole moment. By combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial separation of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.

Two‐dimensional (2D) transition metal dichalcogenides (TMDs) exhibit exciting properties and versatile material chemistry that are promising for device miniaturization, energy, quantum information science, and optoelectronics. Their outstanding structural stability permits the introduction of various foreign dopants that can modulate their optical and electronic properties and induce phase transitions, thereby adding new functionalities such as magnetism, ferroelectricity, and quantum states. To accelerate their technological readiness, it is essential to develop controllable synthesis and processing techniques to precisely engineer the compositions and phases of 2D TMDs. While most reviews emphasize properties and applications of doped TMDs, here, recent progress on thin‐film synthesis and processing techniques that show excellent controllability for substitutional doping of 2D TMDs are reported. These techniques are categorized into bottom–up methods that grow doped samples on substrates directly and top–down methods that use energetic sources to implant dopants into existing 2D crystals. The doped and alloyed variants from Group VI TMDs will be at the center of technical discussions, as they are expected to play essential roles in next‐generation optoelectronic applications. Theoretical backgrounds based on first principles calculations will precede the technical discussions to help the reader understand each element's likelihood of substitutional doping and the expected impact on the material properties.

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The refractive index for pristine monolayer MoS 2 in the near-infrared is predicted to be ≥4. However, experimental literature reports n can decrease by ∼2 at energies below the band edge. In this work, we show variability in the optical response can correspond to changes in oscillator amplitude and dielectric polarizability of all measured excitonssuggesting exemplary charge transfer and local dielectric media effects discussed here are significant contributors in quantum nanophotonic interactions. Monolayer metal organic chemical vapor deposited MoS 2 films are evaluated herein using spectroscopic ellipsometry to assess changes in the refractive index (n) and extinction coefficient (k) due to dopant-induced screening effects from chemical adsorbates and mild film degradation. Notably, large reversible changes in the refractive index (Δn ≈ 2.2) are observed by varying n-and p-type chemical adsorbates. The extent of tailorable dopant-induced screening of MoS 2 optical constants illustrated in this work is also shown to be highly dependent on film quality. This suggests a dominant role of pre-existing structural defects on the optical properties of reported films to date. The tailoring of semiconducting transition metal dichalcogenide optical constants in a reversible manner is expected to have broad implications in the development of quantum optical and optoelectronic devices (e.g., high-precision engineered excitonic effects, electroabsorption modulators, and high-efficiency semiconductor nanophotonic technologies).