Colloidal synthesis methods have become a cornerstone for producing high-quality II-VI quantum dots (QDs), such as CdSe, CdS, and ZnSe, due to their precise control over size, shape, and optical properties. Among the most widely used techniques are hot-injection and solvothermal synthesis, which enable the formation of monodisperse nanocrystals with tunable electronic and optical characteristics. These methods exploit the principles of nucleation and growth in colloidal solutions, where precursors are decomposed in the presence of organic ligands that stabilize the resulting nanoparticles.

Hot-injection synthesis is a well-established approach for producing II-VI QDs with narrow size distributions. In this method, a rapid injection of precursor solutions into a high-temperature solvent triggers instantaneous nucleation, followed by controlled growth at elevated temperatures. For CdSe QDs, a typical procedure involves injecting a solution of cadmium oleate and trioctylphosphine selenide (TOP-Se) into a hot (300–350°C) coordinating solvent, such as trioctylphosphine oxide (TOPO) or oleylamine. The sudden temperature drop after injection ensures a burst of nucleation, while subsequent heating allows for gradual growth. The size of the QDs is determined by the reaction duration and temperature, with longer growth times yielding larger particles.

Solvothermal synthesis, in contrast, relies on heating precursor solutions in a sealed autoclave at elevated pressures, facilitating the decomposition of precursors and the formation of nanocrystals. This method is particularly useful for ZnSe QDs, where zinc stearate and selenium powder can be dissolved in a high-boiling-point solvent like octadecene and heated to 250–300°C under inert conditions. The confined environment of the autoclave promotes uniform heat distribution, leading to homogeneous nucleation and growth. Solvothermal synthesis is advantageous for scaling up production while maintaining control over particle size and crystallinity.

A defining feature of II-VI QDs is the quantum confinement effect, which arises when the physical dimensions of the nanocrystals become smaller than the exciton Bohr radius. For CdSe, this radius is approximately 5.6 nm, meaning that particles below this size exhibit discrete energy levels rather than continuous bands. As the diameter of CdSe QDs decreases from 6 nm to 2 nm, the bandgap increases from 1.7 eV to 2.8 eV, shifting the photoluminescence (PL) emission from red to blue. Similarly, ZnSe QDs show a bandgap tunability from 3.0 eV (bulk) to 4.5 eV for sub-3 nm particles. This size-dependent bandgap tuning is a direct consequence of the spatial confinement of charge carriers, which enhances the energy separation between electronic states.

Surface ligand chemistry plays a critical role in stabilizing colloidal QDs and passivating surface defects that can quench luminescence. Long-chain alkyl ligands like oleic acid and trioctylphosphine (TOP) bind to surface metal atoms, preventing aggregation and oxidation. However, these ligands also introduce insulating barriers that can hinder charge transport in optoelectronic applications. Exchange with shorter or more conductive ligands, such as thiols or halides, can improve electronic coupling between QDs but may also introduce new trap states. The balance between stability and functionality is a key challenge in ligand engineering.

Photoluminescence quantum yield (PLQY) is a critical metric for assessing the optical quality of QDs, defined as the ratio of emitted to absorbed photons. High-quality CdSe QDs can achieve PLQY values exceeding 80% when synthesized with optimal surface passivation. However, defects such as selenium vacancies or unpassivated cadmium sites can act as non-radiative recombination centers, reducing PLQY to below 10%. Post-synthetic treatments, including shell growth (e.g., ZnS coating) or Lewis acid-base passivation (e.g., with trioctylphosphine sulfide), are often employed to suppress these defects.

Stability remains a persistent challenge for II-VI QDs, particularly under ambient or operational conditions. Oxidation of surface chalcogenide atoms (Se or S) can lead to the formation of trap states, while photo-oxidation under prolonged illumination can degrade PL intensity. Encapsulation with inorganic shells (e.g., ZnS) or embedding in polymer matrices can mitigate these effects, but such strategies often require trade-offs in terms of process complexity or charge transport efficiency. Thermal stability is another concern, as high temperatures can induce Ostwald ripening or ligand desorption, broadening the size distribution and degrading optical properties.

In summary, colloidal synthesis methods like hot-injection and solvothermal techniques provide powerful tools for tailoring the properties of II-VI quantum dots. Quantum confinement enables precise bandgap engineering, while surface ligand chemistry dictates both stability and functionality. Achieving high PLQY and long-term stability requires careful optimization of synthesis conditions and post-processing treatments. These challenges underscore the need for continued research into defect passivation and material design to fully exploit the potential of II-VI QDs in advanced technologies.