Monolayer transition metal dichalcogenides (TMDCs) exhibit unique optical properties dominated by strong excitonic effects due to quantum confinement and reduced dielectric screening. Unlike their bulk counterparts, monolayer TMDCs possess a direct bandgap, leading to pronounced photoluminescence (PL) and rich excitonic phenomena. The optical response of these materials is governed by tightly bound excitons, trions, and dark excitons, which can be further modulated by external factors such as strain, doping, and electric fields.

In monolayer TMDCs, the band structure consists of two prominent valleys at the K and K' points in the Brillouin zone, giving rise to spin-orbit split excitonic transitions known as A and B excitons. The A exciton corresponds to the lowest-energy optical transition between the valence band maximum and conduction band minimum, while the B exciton arises from transitions involving the spin-orbit split valence band. The energy separation between A and B excitons typically ranges from 100 to 200 meV, depending on the specific material. For instance, in monolayer MoS2, the A exciton peak appears around 1.9 eV, while the B exciton is observed near 2.1 eV.

Excitons in monolayer TMDCs exhibit exceptionally large binding energies, often exceeding 500 meV, due to reduced dielectric screening in the 2D limit. This strong Coulomb interaction leads to stable excitonic states even at room temperature. In contrast, bulk TMDCs have much smaller exciton binding energies, typically below 100 meV, as a result of enhanced screening from the surrounding atomic layers. The PL spectra of monolayer TMDCs are thus dominated by sharp excitonic peaks, whereas bulk TMDCs show broader emission features due to weaker excitonic effects and indirect bandgap transitions.

Trions, or charged excitons, are another critical feature in the optical response of monolayer TMDCs. These three-particle states form when an exciton binds to an additional electron or hole, resulting in negatively or positively charged trions. Trion peaks typically appear at energies 20 to 50 meV below the neutral A exciton peak in PL spectra. The relative intensity of trion versus exciton peaks is highly sensitive to doping levels. For example, in electron-doped MoSe2, the negatively charged trion becomes the dominant emission feature, while in hole-doped samples, the positively charged trion is more pronounced.

Dark excitons, which are spin-forbidden or momentum-indirect transitions, also play a significant role in the optical properties of monolayer TMDCs. These states are optically inactive but can influence PL spectra through thermal activation or coupling with bright excitons. The energy separation between bright and dark excitons is material-dependent, with values ranging from 30 to 100 meV. In WSe2, for instance, the dark exciton lies approximately 50 meV below the bright A exciton. At low temperatures, dark excitons can trap a significant fraction of the excited population, leading to a reduction in PL intensity.

Strain engineering provides a powerful means to modulate the optical properties of monolayer TMDCs. Applied tensile or compressive strain shifts the bandgap and alters the excitonic energies. Uniaxial strain can break the degeneracy of the A exciton peaks in different crystal directions, leading to polarization-dependent PL. Biaxial strain uniformly tunes the bandgap, with reported shifts of up to 100 meV per percent strain. Strain also affects the spin-orbit coupling, modifying the energy separation between A and B excitons.

Doping is another effective method to control excitonic behavior in monolayer TMDCs. Intentional doping, whether through chemical functionalization, electrostatic gating, or defect engineering, changes the carrier concentration and thus the balance between excitons and trions. Heavy doping can lead to the complete suppression of neutral excitons in favor of trions or even a Fermi-edge singularity in the PL spectrum. Additionally, doping influences the lifetime of excitonic states, with higher doping densities generally leading to faster non-radiative recombination.

Electric fields enable dynamic tuning of excitonic properties in monolayer TMDCs. An out-of-plane electric field induces Stark shifts, moving exciton peaks to lower or higher energies depending on the field direction. In-plane electric fields can dissociate excitons or trions, reducing PL intensity. Electric fields also modify the dielectric environment, altering the exciton binding energy. For example, placing a monolayer TMDC in a dual-gated device structure allows precise control over both carrier density and electric field, enabling fine-tuning of the PL spectrum.

Comparisons between monolayer and bulk TMDCs reveal stark differences in their optical responses. Bulk TMDCs exhibit weaker excitonic effects due to enhanced dielectric screening and often display indirect bandgap behavior, leading to lower PL quantum yields. The exciton binding energy in bulk TMDCs is an order of magnitude smaller than in monolayers, making excitons less stable at elevated temperatures. Additionally, trions are less prevalent in bulk systems due to faster charge screening and higher defect densities.

The temperature dependence of PL spectra further highlights the distinctions between monolayer and bulk TMDCs. In monolayers, the PL intensity remains strong even at room temperature due to robust exciton stability, whereas bulk TMDCs often show significant thermal quenching. The linewidth of excitonic peaks in monolayers is also more sensitive to temperature, broadening significantly as phonon scattering increases.

In summary, monolayer TMDCs exhibit a rich array of optical phenomena driven by strong excitonic effects, including A/B excitons, trions, and dark excitons. These properties are highly tunable through strain, doping, and electric fields, offering versatile control over their optical response. The contrast with bulk TMDCs underscores the impact of dimensionality on excitonic behavior, with monolayers displaying superior exciton stability and sharper PL features. Understanding these optical properties is essential for harnessing the potential of monolayer TMDCs in fundamental studies and future technological applications.