Photoluminescence (PL) in two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) and graphene derivatives has emerged as a critical area of study due to their unique optoelectronic properties. Unlike bulk semiconductors, these materials exhibit layer-dependent emission characteristics, strong excitonic effects, and sensitivity to external perturbations like strain and doping. Understanding these phenomena is essential for applications in light-emitting devices, sensors, and quantum technologies.

Layer-dependent photoluminescence is a defining feature of 2D materials. Monolayer TMDCs like MoS2, WS2, and WSe2 exhibit direct bandgaps, leading to strong PL emission, while their bilayer and bulk counterparts have indirect bandgaps, resulting in significantly weaker emission. For instance, monolayer MoS2 shows a prominent PL peak around 1.8–1.9 eV due to direct excitonic transitions, whereas the PL intensity drops by orders of magnitude in bilayer and thicker films. This transition from direct to indirect bandgap is attributed to the quantum confinement effect and changes in the electronic band structure as layer count increases. Graphene derivatives, such as graphene oxide and reduced graphene oxide, exhibit broad PL spectra due to defect-related recombination, with emission energies tunable by varying the oxidation level and chemical functionalization.

Excitonic effects dominate the PL behavior in 2D materials due to reduced dielectric screening and strong Coulomb interactions. Monolayer TMDCs exhibit tightly bound excitons with binding energies of several hundred meV, far exceeding those in conventional semiconductors. These excitons can further interact with excess charge carriers to form trions, or charged excitons, which appear as additional peaks in PL spectra redshifted from the neutral exciton peak. For example, in monolayer WSe2, trion peaks are observed approximately 20–30 meV below the neutral exciton peak, depending on doping levels. The relative intensity of trion and exciton peaks serves as a sensitive probe of carrier concentration, enabling non-invasive doping characterization.

Strain engineering provides another powerful tool to modulate PL in 2D materials. Applied strain alters the band structure, leading to shifts in PL peak positions and changes in emission intensity. Uniaxial strain in monolayer MoS2 can induce a PL redshift of up to 70 meV per percent strain due to bandgap narrowing, while biaxial strain may cause splitting of the excitonic peaks due to reduced crystal symmetry. Strain can also enhance or suppress PL by modifying the radiative recombination pathways. For instance, controlled strain in WS2 monolayers has been shown to increase PL intensity by reducing non-radiative recombination centers, whereas excessive strain introduces defects that quench emission.

Quenching and enhancement mechanisms in 2D materials are influenced by both intrinsic and extrinsic factors. Defects, such as vacancies and grain boundaries, act as non-radiative recombination centers, reducing PL intensity. Chemical doping or adsorption of molecules can also quench PL by introducing additional scattering or trapping sites. On the other hand, plasmonic nanostructures or photonic cavities can enhance PL through Purcell effects, where the local density of optical states increases the radiative recombination rate. For example, coupling monolayer MoS2 with silver nanoparticles has demonstrated PL enhancement factors exceeding 100 due to localized surface plasmon resonance. Similarly, integrating TMDCs into optical cavities can lead to significant emission intensity boosts and directional light output.

Environmental interactions play a crucial role in the PL stability of 2D materials. Exposure to ambient conditions can lead to oxidation or adsorption of contaminants, which may quench PL over time. Encapsulation with hexagonal boron nitride (hBN) or other inert layers has proven effective in preserving PL intensity by shielding the material from environmental degradation. Temperature also affects PL properties, with linewidth broadening and peak shifts observed at elevated temperatures due to enhanced electron-phonon interactions. Cryogenic temperatures, in contrast, reveal sharp excitonic features and allow the observation of fine structure splitting in some TMDCs.

The interplay between excitons, trions, and defects creates a complex landscape for PL in 2D materials. Time-resolved PL measurements reveal recombination dynamics, with exciton lifetimes typically ranging from picoseconds to nanoseconds depending on material quality and environmental factors. Trions generally exhibit shorter lifetimes due to additional scattering channels. Defect-assisted recombination often leads to multi-exponential decay profiles, highlighting the competition between radiative and non-radiative processes.

Applications of 2D material PL span diverse fields. In optoelectronics, TMDCs are promising candidates for ultrathin light-emitting diodes and lasers due to their high quantum yield and tunable emission. Sensors leveraging PL changes in response to molecular adsorption or strain offer high sensitivity for environmental monitoring and biomedical diagnostics. Quantum technologies benefit from the deterministic generation of single photons in defect-engineered 2D materials, enabling advancements in quantum communication and computing.

Future research directions include optimizing PL efficiency through defect passivation, exploring heterostructures for tailored emission properties, and integrating 2D materials with photonic platforms for enhanced light-matter interactions. The continued development of advanced characterization techniques will further elucidate the underlying mechanisms governing PL in these systems, paving the way for novel applications.

In summary, photoluminescence in 2D materials is a rich and multifaceted phenomenon shaped by layer thickness, excitonic effects, strain, and environmental interactions. Mastery of these factors enables precise control over emission properties, driving innovation in next-generation optoelectronic and quantum devices.