Zinc sulfide (ZnS) and cadmium sulfide (CdS) are well-studied II-VI semiconductor materials that serve as excellent host matrices for phosphors when doped with transition metals such as manganese (Mn) and copper (Cu). These materials exhibit tunable luminescence properties, making them suitable for applications in optoelectronics, sensors, and radiation detection. The luminescent behavior of these doped systems arises from electronic transitions within the dopant ions or defect-related states, influenced by the host lattice and synthesis conditions. This article examines the solid-state synthesis methods, luminescence mechanisms, and key degradation factors of Mn- and Cu-doped ZnS and CdS phosphors.

Solid-state synthesis is a widely used method for preparing Mn- and Cu-doped ZnS and CdS phosphors due to its simplicity and scalability. The process involves mixing stoichiometric amounts of high-purity precursors, such as zinc or cadmium salts, sulfur sources, and dopant salts, followed by heat treatment in a controlled atmosphere. For ZnS:Mn, typical synthesis involves heating a mixture of ZnS and Mn compounds at temperatures between 900°C and 1100°C under an inert or reducing atmosphere to prevent oxidation. Similarly, CdS:Cu is synthesized by annealing CdS and copper salts at temperatures ranging from 500°C to 800°C. The choice of atmosphere is critical; sulfur-rich conditions are often maintained to compensate for sulfur loss at high temperatures. The resulting materials are then ground into powders for further characterization.

Doping concentrations play a crucial role in determining the luminescent properties of these materials. In ZnS:Mn, Mn²⁺ ions typically substitute Zn²⁺ sites at concentrations between 0.1% and 5% atomic percent. Higher Mn concentrations can lead to concentration quenching due to energy transfer between neighboring Mn²⁺ ions. For CdS:Cu, Cu⁺ or Cu²⁺ ions occupy Cd²⁺ sites, with optimal doping levels usually below 2% to avoid non-radiative recombination. The luminescence efficiency is highly dependent on the uniformity of dopant distribution, which is influenced by the synthesis temperature, duration, and precursor reactivity.

The luminescence mechanisms in Mn- and Cu-doped ZnS and CdS are primarily governed by electronic transitions involving the dopant ions and host-related defects. In ZnS:Mn, the characteristic orange emission at around 585 nm arises from the ⁴T₁→⁶A₁ transition of Mn²⁺ ions in a tetrahedral crystal field. This transition is spin-forbidden but gains intensity due to strong spin-orbit coupling in the ZnS lattice. The excitation energy is typically absorbed by the ZnS host and transferred to Mn²⁺ ions through dipole-dipole interactions or defect-mediated pathways.

In CdS:Cu, luminescence is more complex due to the multiple oxidation states of copper and the presence of intrinsic defects. The emission spectrum often includes bands in the green to red region (500–700 nm), attributed to transitions between Cu-related energy levels and sulfur vacancies. The exact emission wavelength depends on the Cu oxidation state (Cu⁺ or Cu²⁺) and local coordination environment. Additionally, surface states and grain boundaries can influence the luminescence by introducing non-radiative recombination centers.

Defect chemistry plays a significant role in the luminescent behavior of these materials. In ZnS, sulfur vacancies (Vₛ) and zinc interstitials (Znᵢ) act as shallow donors, while zinc vacancies (V_Zn) and sulfur interstitials (Sᵢ) serve as acceptors. These defects can form complexes with dopant ions, altering the emission characteristics. For example, Mn²⁺ ions adjacent to sulfur vacancies may exhibit shifted emission peaks due to changes in the crystal field symmetry. In CdS, cadmium vacancies (V_Cd) are common and can form defect complexes with Cu ions, leading to broad emission bands.

Degradation of luminescence in these phosphors is a critical concern for practical applications. Several factors contribute to the loss of emission intensity over time. Thermal degradation occurs due to increased non-radiative recombination at elevated temperatures, particularly in CdS-based systems where ionic mobility is higher. Oxidation of dopant ions is another issue; Mn²⁺ in ZnS can oxidize to Mn³⁺ under prolonged exposure to air or moisture, reducing luminescence efficiency. Similarly, Cu⁺ in CdS may oxidize to Cu²⁺, leading to quenching of emission.

Photodegradation is also a significant factor, especially under high-energy excitation. In ZnS:Mn, prolonged UV exposure can cause sulfur vacancy migration and defect aggregation, leading to decreased Mn²⁺ emission. In CdS:Cu, photo-induced oxidation of Cu⁺ ions or cadmium vacancy formation can degrade luminescence. Environmental humidity accelerates these processes, as water molecules can penetrate grain boundaries and react with the host lattice or dopant ions.

To mitigate degradation, encapsulation or surface passivation techniques are often employed. Coating phosphor particles with inorganic shells such as SiO₂ or Al₂O₃ can reduce exposure to ambient oxygen and moisture. Additionally, post-synthesis annealing in controlled atmospheres can heal surface defects and improve stability. The choice of host material also affects degradation resistance; ZnS generally exhibits better stability than CdS due to its wider bandgap and lower ion mobility.

The luminescence efficiency and stability of these materials are further influenced by particle size and morphology. Nanocrystalline ZnS:Mn and CdS:Cu often exhibit higher luminescence quantum yields compared to bulk counterparts due to increased surface-to-volume ratios and quantum confinement effects. However, nanoparticles are also more susceptible to surface-related degradation mechanisms. Optimizing synthesis parameters to balance crystallinity, particle size, and defect concentration is essential for achieving high-performance phosphors.

In summary, Mn- and Cu-doped ZnS and CdS phosphors exhibit rich luminescence properties governed by dopant-host interactions and defect chemistry. Solid-state synthesis provides a versatile route for their preparation, though careful control of doping levels and processing conditions is necessary to maximize performance. Degradation mechanisms, including thermal, oxidative, and photochemical processes, pose challenges that require mitigation strategies for long-term stability. Understanding these factors is crucial for advancing the application of these materials in optoelectronic and sensing technologies.