Atomic layer deposition (ALD) is a highly precise thin-film growth technique that enables conformal, pinhole-free encapsulation of quantum dots (QDs) with atomic-scale control. This method is particularly advantageous for stabilizing sensitive QD materials such as CdSe and perovskite nanocrystals, which suffer from environmental degradation and surface defects that impair their optoelectronic performance. By applying ALD coatings, researchers can significantly enhance QD stability while maintaining or even improving their optical and electronic properties.

The ALD process relies on sequential, self-limiting surface reactions between gaseous precursors and the substrate. Each cycle typically involves two half-reactions, where the first precursor chemisorbs onto the surface, followed by purging to remove excess reactants. The second precursor then reacts with the adsorbed species to form a monolayer, completing the cycle. This cyclic approach ensures precise thickness control and excellent conformality, even on high-aspect-ratio or irregular surfaces such as QD films.

For CdSe QDs, ALD encapsulation with materials like Al2O3, TiO2, or HfO2 has proven effective in preventing oxidation and ligand desorption, which are primary degradation mechanisms. Studies have shown that a 10 nm Al2O3 ALD coating can extend the photoluminescence (PL) lifetime of CdSe QDs by over an order of magnitude under ambient conditions. The Al2O3 layer acts as a diffusion barrier against oxygen and moisture while passivating surface traps that cause non-radiative recombination. Similarly, TiO2 coatings have been used to enhance charge transport in QD-based solar cells, with reported improvements in power conversion efficiency due to reduced interfacial defects.

Perovskite QDs, such as CsPbX3 (X = Cl, Br, I), are even more sensitive to environmental factors, including humidity, heat, and light-induced halide migration. ALD offers a solution by providing hermetic encapsulation without damaging the delicate perovskite crystal structure. For instance, Al2O3 and SiO2 ALD layers have been shown to stabilize CsPbBr3 QDs against moisture-induced degradation, retaining over 90% of their initial PL intensity after 30 days in humid environments. The low processing temperatures of ALD (typically below 150°C) are critical for avoiding thermal decomposition of perovskite QDs during coating.

Beyond passive protection, ALD coatings can actively enhance optoelectronic performance by modifying the QD surface chemistry. For example, metal oxide ALD layers can introduce beneficial doping or create energy level alignments that improve charge injection in light-emitting diodes (LEDs) or photodetectors. In some cases, the ALD process itself can repair surface defects by filling vacancies or terminating dangling bonds. This is particularly relevant for perovskite QDs, where halide vacancies lead to ion migration and phase segregation.

The choice of ALD material depends on the specific application and QD system. Al2O3 is widely used for its excellent barrier properties and low defect density, while TiO2 and ZnO are preferred for optoelectronic devices due to their favorable band alignment with many QDs. For perovskite QDs, hybrid approaches combining ALD with solution-processed layers have been explored to balance encapsulation quality with processing compatibility.

One challenge in ALD encapsulation is ensuring uniform coating without inducing aggregation or sintering of QDs. This requires careful optimization of precursor exposure times, purge durations, and temperature to minimize particle movement during deposition. Additionally, the initial ALD nucleation on QD surfaces can be non-trivial due to the organic ligands typically present on QDs. Pretreatments such as plasma oxidation or ligand exchange may be necessary to promote uniform film growth.

Recent advances in ALD techniques have expanded the possibilities for QD encapsulation. Spatial ALD, for instance, enables faster deposition rates suitable for roll-to-roll manufacturing, while plasma-enhanced ALD allows for lower temperature processing. Multi-component ALD, where different materials are deposited in alternating cycles, can tailor the mechanical, optical, and electrical properties of the encapsulation layer.

In optoelectronic applications, ALD-encapsulated QDs have demonstrated superior performance compared to unprotected counterparts. In QD-LEDs, ALD layers reduce current leakage and improve charge balance, leading to higher external quantum efficiency. For solar cells, ALD coatings enhance stability under operational conditions while minimizing parasitic absorption losses. Photodetectors benefit from the passivation of surface traps, which lowers dark current and increases detectivity.

Looking ahead, the integration of ALD with other deposition techniques and the development of novel ALD chemistries will further advance QD encapsulation strategies. The ability to precisely engineer interfaces at the atomic scale makes ALD a powerful tool for unlocking the full potential of quantum dots in next-generation optoelectronic devices.

The quantitative improvements observed in ALD-encapsulated QD systems underscore the importance of this technique for both fundamental studies and industrial applications. As device requirements become more stringent, the demand for robust, high-performance encapsulation methods will continue to grow, with ALD positioned as a key enabling technology.