The sol-gel synthesis method has emerged as a versatile and scalable approach for producing metal halide perovskite nanocrystals (NCs), particularly cesium lead bromide (CsPbBr3). This wet-chemical technique offers precise control over composition, morphology, and optical properties, making it suitable for applications requiring tunable luminescence and high quantum yields. The process involves the formation of a colloidal solution (sol) that transitions into a gel-like network, followed by controlled crystallization to yield perovskite NCs. Key factors influencing the synthesis include precursor reactivity, ligand chemistry, and stability mechanisms, all of which dictate the final properties of the nanocrystals.

Precursor reactivity plays a critical role in determining the nucleation and growth kinetics of CsPbBr3 NCs. In sol-gel synthesis, cesium precursors such as cesium oleate or cesium carbonate are combined with lead halide salts (e.g., PbBr2) in a coordinating solvent. The reactivity of these precursors is influenced by their solubility and dissociation rates. For instance, cesium oleate exhibits higher solubility in nonpolar solvents compared to cesium carbonate, leading to faster nucleation. Similarly, the choice of lead precursor affects the reaction dynamics; PbBr2 dissolved in dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) tends to form smaller NCs due to rapid nucleation, whereas less reactive precursors yield larger crystals. The molar ratio of Cs:Pb:Br is also crucial, as deviations from stoichiometry can introduce defects or secondary phases. A typical ratio of 1:1:3 ensures the formation of phase-pure CsPbBr3, while excess lead or halide can alter the optical properties by creating halide vacancies or interstitial defects.

Ligands are essential for stabilizing the perovskite NCs and preventing aggregation during synthesis. Oleic acid (OA) and oleylamine (OAm) are commonly used as surface ligands due to their ability to bind to lead and halide ions, passivating surface traps and enhancing photoluminescence quantum yield (PLQY). The ligand concentration must be carefully optimized; excessive ligands can hinder crystal growth, while insufficient amounts lead to poor colloidal stability. For example, a molar ratio of 2:1 (OAm:OA) is often employed to balance surface passivation and growth kinetics. Additionally, the chain length and functional groups of ligands influence the NCs' solubility and stability. Shorter-chain ligands like butylamine improve charge transport but reduce colloidal stability, whereas longer-chain ligands like octadecylamine enhance stability at the cost of increased interparticle distance. Mixed-ligand systems, such as OA/OAm with phosphonic acids, have been shown to improve both stability and PLQY by forming a more robust surface coating.

Stability challenges remain a significant hurdle for CsPbBr3 NCs synthesized via sol-gel methods. Perovskite NCs are prone to degradation under moisture, light, and heat due to their ionic nature and low formation energy. Surface ligand desorption is a primary degradation pathway, leading to aggregation and loss of optical properties. To mitigate this, post-synthetic treatments such as ligand exchange or shell encapsulation are employed. For instance, treating NCs with didodecyldimethylammonium bromide (DDAB) can replace labile OA/OAm ligands, improving stability in polar environments. Another strategy involves incorporating hydrophobic polymers or inorganic shells (e.g., SiO2) to shield the NCs from environmental factors. Thermal stability is also a concern, as CsPbBr3 NCs typically degrade above 80°C. Incorporating higher-valence cations (e.g., Sn2+ or Sr2+) into the lattice can enhance thermal stability by strengthening the ionic bonds.

Optical property tuning is achieved through precise composition control, particularly via halide exchange and cation doping. The bandgap of CsPbBr3 NCs can be adjusted by substituting Br with Cl or I, enabling emission tuning from 400 nm (CsPbCl3) to 700 nm (CsPbI3). Halide exchange is typically performed by reacting CsPbBr3 NCs with halide salts (e.g., PbCl2 or PbI2) in solution, with the reaction kinetics governed by the halide's electronegativity and ionic radius. For example, Cl- exchanges rapidly due to its small size, while I- requires longer reaction times. Cation doping, such as partial substitution of Pb2+ with Mn2+ or Sn2+, introduces new emission pathways or modifies the band structure. Mn-doped CsPbBr3 NCs exhibit dual emission at 520 nm (host emission) and 600 nm (Mn2+ d-d transition), while Sn doping can reduce toxicity and alter the bandgap for near-infrared applications. The doping efficiency depends on the precursor's redox potential and ionic radius compatibility with the host lattice.

The sol-gel method also allows for morphology control, enabling the synthesis of nanocubes, nanoplates, and nanowires. The shape is influenced by the reaction temperature, precursor concentration, and ligand dynamics. For example, higher temperatures (160–200°C) favor nanocube formation due to faster growth along all crystallographic directions, while lower temperatures (120–140°C) yield nanoplates via anisotropic growth. The addition of zwitterionic ligands like betaine can further direct morphology by selectively binding to specific crystal facets. The aspect ratio of nanowires can be tuned by adjusting the Pb:Cs ratio, with excess Pb promoting one-dimensional growth.

Despite its advantages, sol-gel synthesis of CsPbBr3 NCs faces challenges in reproducibility and scalability. Batch-to-batch variations can arise from subtle changes in precursor purity, solvent quality, or reaction conditions. Automated synthesis platforms and machine learning-assisted optimization are being explored to address these issues. Additionally, the use of greener solvents like ethanol or water instead of traditional organic solvents is an area of active research to improve sustainability.

In summary, sol-gel synthesis offers a flexible route to producing CsPbBr3 NCs with tailored optical and structural properties. By understanding and controlling precursor reactivity, ligand interactions, and stability mechanisms, researchers can optimize the synthesis for specific applications. Continued advancements in ligand engineering, doping strategies, and environmental stability will further enhance the utility of these materials in fields such as lighting, displays, and sensing.