Binary chalcogenide semiconductors represent a critical class of materials with diverse structural, electronic, and optical properties. These compounds consist of group VI elements (chalcogens: sulfur, selenium, tellurium) paired with metals or other cations, forming binary systems such as CdS, PbSe, or ZnTe. Their tunable bandgaps, high absorption coefficients, and efficient charge transport make them indispensable in optoelectronics, photovoltaics, and other advanced applications. This article explores their crystal structures, electronic properties, synthesis methods, and technological applications while emphasizing key distinctions between sulfides, selenides, and tellurides.

Crystal structures of binary chalcogenides vary depending on composition and bonding. Many adopt tetrahedral coordination, forming zinc blende (cubic) or wurtzite (hexagonal) structures. For example, CdS and ZnS crystallize in both zinc blende and wurtzite phases, with the latter being more stable at room temperature. PbSe and PbTe, however, favor the rock salt (NaCl) structure due to the larger ionic radii of Pb and heavier chalcogens. The structural stability is influenced by the balance between ionic and covalent bonding, with sulfides being more covalent and tellurides more ionic. This structural diversity directly impacts their electronic and optical behavior.

Electronic properties of binary chalcogenides are primarily governed by their band structures. Sulfides typically exhibit wider bandgaps, ranging from 2.0 eV (CdS) to 3.7 eV (ZnS), making them suitable for UV and visible-light applications. Selenides, such as CdSe (1.7 eV) and PbSe (0.27 eV), cover the visible to near-infrared spectrum, while tellurides like CdTe (1.5 eV) and PbTe (0.31 eV) extend further into the infrared. The bandgap trend follows the chalcogen atomic size: heavier chalcogens reduce bandgaps due to increased orbital overlap and decreased electronegativity. Effective mass and carrier mobility also vary, with tellurides generally exhibiting higher mobility due to reduced carrier scattering.

Bandgap engineering in binary chalcogenides is achieved through composition control, strain, or quantum confinement. For instance, CdSe nanoparticles exhibit size-dependent bandgaps, tunable from 1.7 eV (bulk) to 2.5 eV (2 nm diameter). Similarly, alloying is not discussed here as it pertains to ternary systems, but intrinsic tuning via stoichiometry or defect engineering remains relevant. Vacancies or dopants can introduce mid-gap states, altering conductivity and optical absorption. For example, sulfur vacancies in MoS2 create n-type doping, while selenium-rich PbSe shows p-type behavior.

Synthesis methods for binary chalcogenides include chemical vapor deposition (CVD) and solution-based techniques. CVD offers high-purity, large-area growth with precise control over thickness and crystallinity. For example, MoS2 monolayers are synthesized via sulfurization of MoO3 precursors in a CVD reactor at 800°C. The process parameters—temperature, pressure, and precursor flow rates—dictate phase purity and morphology. In contrast, solution-based methods like hot-injection synthesis enable colloidal nanocrystals with narrow size distributions. CdSe quantum dots are prepared by injecting selenium precursors into a hot cadmium-containing surfactant solution, yielding tunable sizes (3–10 nm) and emission wavelengths (500–700 nm).

Applications in optoelectronics leverage the high absorption coefficients and tunable bandgaps of binary chalcogenides. CdS and CdSe serve as window layers in thin-film solar cells, while PbS and PbSe quantum dots enable infrared photodetectors with spectral selectivity. Light-emitting diodes (LEDs) based on ZnSe emit blue light, and CdTe is a dominant material in commercial thin-film photovoltaics due to its near-ideal bandgap and high efficiency. The direct bandgap of most binary chalcogenides ensures efficient photon absorption and emission, critical for lasers and displays.

Photovoltaic applications highlight the efficiency and cost-effectiveness of binary chalcogenides. CdTe solar cells achieve laboratory efficiencies exceeding 22%, benefiting from high carrier lifetimes and low recombination rates. The material’s robustness under irradiation makes it suitable for space applications. Similarly, Cu2S and Sb2S3 are explored for low-cost, solution-processed solar cells, though stability remains a challenge. The intrinsic doping and defect tolerance of some chalcogenides, like PbSe, further enhance their photovoltaic performance.

Key differences between sulfides, selenides, and tellurides arise from their electronic and structural properties. Sulfides, with wider bandgaps, are optimal for high-energy photonics but require doping for conductivity modulation. Selenides balance bandgap and carrier mobility, making them versatile for visible and near-IR applications. Tellurides excel in infrared detection and thermoelectrics due to narrow bandgaps and high mobility but face challenges with stability and toxicity. The choice of chalcogen thus depends on the target application’s spectral and operational requirements.

Challenges in binary chalcogenide semiconductors include toxicity (Cd, Pb), environmental stability (oxidation of selenides and tellurides), and scalable synthesis. Advances in encapsulation and non-toxic alternatives (e.g., ZnSe replacing CdSe) are ongoing research areas. Additionally, defect control during growth is critical for reproducible device performance.

In summary, binary chalcogenide semiconductors offer a rich platform for optoelectronic and photovoltaic technologies. Their structural versatility, tunable electronic properties, and compatibility with multiple synthesis routes underscore their importance in modern materials science. Continued research into defect engineering, scalable fabrication, and environmentally benign compositions will further expand their applications in energy conversion, sensing, and beyond.