Monolayer transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 exhibit unique electronic and optical properties due to quantum confinement effects. Unlike their bulk counterparts, these atomically thin materials possess direct bandgaps, strong spin-orbit coupling, and valley-selective optical transitions, making them promising candidates for valleytronics and quantum photonics. The layer-dependent bandgap transitions in MoS2 and WSe2 monolayers are critical for understanding their behavior in optoelectronic applications, particularly in the context of valley polarization and single-photon emission.
In monolayer MoS2, the bandgap transitions occur at the K and K' points of the Brillouin zone, where the valence band splits due to strong spin-orbit coupling. The direct bandgap of monolayer MoS2 is approximately 1.8 to 1.9 eV, significantly larger than the indirect bandgap of bulk MoS2, which ranges from 1.2 to 1.3 eV. The conduction band minimum and valence band maximum are both located at the K points, enabling efficient light-matter interactions. The spin-split valence band results in two distinct excitonic transitions, labeled A and B, separated by approximately 150 meV due to spin-orbit coupling. The A exciton, corresponding to the lower energy transition, is optically active and dominates the photoluminescence spectrum. The valley-dependent optical selection rules allow for selective excitation of carriers in the K or K' valleys using circularly polarized light, a key feature for valleytronic applications.
Similarly, monolayer WSe2 exhibits a direct bandgap of around 1.6 to 1.7 eV, with the valence band splitting larger than that of MoS2 due to heavier tungsten atoms, resulting in a spin-orbit splitting of approximately 400 meV. The A and B excitons in WSe2 are thus more widely separated, with the A exciton at lower energy. The large spin-orbit coupling in WSe2 enhances the valley polarization lifetime, making it particularly suitable for valleytronic devices. The valley-selective optical transitions are robust at room temperature, enabling practical applications in information encoding and processing based on valley degrees of freedom.
Valleytronics exploits the valley pseudospin as a carrier of information. In monolayer TMDCs, the valleys at K and K' are degenerate in energy but possess opposite Berry curvatures and spin configurations. Circularly polarized light can selectively excite carriers in one valley, creating a population imbalance between K and K'. This valley polarization can be detected through the helicity of emitted light, providing a means to read out the valley state. The valley lifetime, which determines how long the polarization persists, is influenced by factors such as temperature, defect density, and external fields. In high-quality MoS2 and WSe2 monolayers, valley polarization lifetimes can reach several picoseconds at room temperature, sufficient for valley-based logic operations.
Single-photon emitters in monolayer TMDCs are another area of intense research. These emitters arise from localized states, often associated with defects or strain-induced potential fluctuations. In WSe2 monolayers, single-photon emission has been observed with narrow linewidths and high photon purity, making them attractive for quantum communication and computing. The emission energy of these quantum emitters typically lies below the free exciton energy, suggesting involvement of bound excitons or defect-related states. The precise atomic configuration of these emitters remains under investigation, but their reproducibility and tunability via strain or electric fields highlight their potential for integrated quantum photonic devices.
The interplay between strain and electronic properties further enriches the behavior of monolayer TMDCs. Applying uniaxial strain can tune the bandgap and modify the valley energetics, potentially enhancing or suppressing valley polarization. Strain engineering also offers a pathway to create artificial superlattices or quantum dots within the monolayer, enabling tailored optoelectronic responses. For instance, localized strain can trap excitons, leading to spatially confined emission sites that function as single-photon sources.
Dielectric environment plays a significant role in modulating the optical properties of MoS2 and WSe2 monolayers. The exciton binding energy, which can exceed 500 meV due to reduced dielectric screening in the 2D limit, is sensitive to the surrounding materials. Encapsulation with hexagonal boron nitride (hBN) has been shown to reduce inhomogeneous broadening and enhance exciton mobility, improving the performance of valleytronic and quantum light-emitting devices. The dielectric screening also affects the radiative recombination rate, with implications for the efficiency of light-emitting devices.
Temperature-dependent studies reveal additional insights into the exciton dynamics. At low temperatures, the photoluminescence spectrum of monolayer TMDCs exhibits sharp peaks corresponding to neutral excitons, charged trions, and defect-bound excitons. The trion binding energy, typically in the range of 20 to 30 meV, depends on the carrier concentration and dielectric environment. As temperature increases, phonon-assisted processes become more prominent, leading to linewidth broadening and reduced valley polarization. Understanding these thermal effects is crucial for designing devices that operate reliably across different temperature ranges.
Electric field tuning provides another degree of control over the electronic and optical properties. Applying a perpendicular electric field can induce doping, shifting the balance between neutral excitons and trions. In heterostructures of MoS2 and WSe2, interlayer excitons with spatially separated electrons and holes can be formed, exhibiting long lifetimes and tunable emission energies. These interlayer excitons are promising for realizing excitonic condensates or for use in optoelectronic memory devices.
The integration of monolayer TMDCs into functional devices requires precise control over material quality and interface properties. Advances in growth techniques, such as chemical vapor deposition with optimized precursors and substrates, have enabled the production of large-area monolayers with uniform properties. Defect passivation strategies, including chemical treatments and encapsulation, further improve the optical and electronic performance. Scalable fabrication methods are essential for translating laboratory discoveries into commercial applications.
In summary, the layer-dependent bandgap transitions in MoS2 and WSe2 monolayers underpin their unique capabilities in valleytronics and quantum photonics. The direct bandgap, strong spin-orbit coupling, and valley-selective optical transitions enable novel device functionalities not achievable with bulk materials. Single-photon emitters and robust valley polarization offer exciting opportunities for quantum technologies, while strain, dielectric environment, and external fields provide versatile tuning knobs. Continued research into material synthesis, defect engineering, and device integration will be critical for harnessing the full potential of these 2D semiconductors.