English

• 1. Introduction

  Quantum dots (QDs) are semiconductor nanocrystals (NCs) consisting of hundreds to thousands of atoms, with sizes ranging from a few to tens of nanometers. Their size is typically less than twice the Bohr radius of the corresponding exciton, exhibiting size-dependent quantum confinement effects [1,2]. This property allows the modification of QDs' energy band structure, resulting in distinct physicochemical characteristics and emission spectra covering the entire visible light spectrum. Based on chemical composition, QDs are categorized into four groups: Group Ⅱ-Ⅵ (e.g., CdSe) [3,4], Group Ⅳ-Ⅵ (e.g., PbSe, PbS) [5,6], Group Ⅲ-Ⅴ (e.g., InP) [7,8], and perovskite QDs (ABX₃ structure, where A and B are cations, X is a halide anion) [9,10]. QDs offer advantages like tunable photoemission, narrow spectra, and high photoluminescence quantum yields (PLQY), finding applications in photovoltaics [11-13], light-emitting diodes (LEDs) [14], photodetectors [15], transistors [16,17], biological monitoring [18], medical imaging [19], drug delivery [20], diagnosis [21], and treatment [22].

  Despite their promising potential, QDs face challenges in practical applications due to chemical reactivity. Exposure to oxygen and moisture can cause surface degradation, leading to defect accumulation, which reduces luminescence and affects performance and stability [23]. Aggregation due to surface ligand mismatches and high concentrations further reduces quantum efficiency, increases non-radiative recombination, and weakens radiative recombination, causing color shifts. Additionally, aggregation creates large domains between QDs, impairing heat dissipation, and compromising stability. Addressing these issues is crucial for realizing the full potential of QDs in practical applications.

  Here are common strategies to improve the surface instability of QDs. (ⅰ) Ion doping [24]: Introducing impurity ions like Mn²⁺, Br^-, or I^- to modify the luminescence peak and enhance stability and efficiency. (ⅱ) Organic ligand modification [25]: Replacing long-chain ligands such as oleic acid (OA) with alternatives (e.g., lecithin, dodecanethiol, N′-(2-aminoethyl)-N′-hexadecylethane-1,2-diamine, vinyl phosphonic acid, octylphosphonic acid, dimethyldidodecylammonium bromide) [26-31] to reduce non-radiative decay and improve QD performance and stability. (ⅲ) Polymer encapsulation [32-34] Using polymers (e.g., polystyrene (PS), polyvinyl alcohol, polydimethylsiloxane) to shield QDs from oxygen and moisture, increasing their stability. (ⅳ) Inorganic encapsulation [35]: Coating QDs with inorganic compounds like ZnS, ZnO, or SiO₂ to provide a rigid protective layer, with silica being particularly effective due to its optical stability, chemical resistance, and biocompatibility. Among the inorganic encapsulation compounds, silica stands out as an ideal material for coating QDs, due to the exceptional optical, chemical stability, and biocompatibility. Apart from the enhanced performance of QDs, the moderate refractive index and excellent light transparency of silica make it suitable for adjusting the performance of QDs-based optical devices. in-situ hydrolysis of siliconizing reagent on the QDs surfaces and encapsulation of QDs in silica template are two mainstream approaches to achieve effective silica coating on QDs (Fig. 1, inner circle). By adjusting the coating method and the operating conditions, the thickness and uniformity of the silicon dioxide coating layer on the surface of QDs can be precisely controlled to meet different application scenarios (Fig. 1, outer circle).

  Figure 1

  

  Figure 1.  The approaches of silica coating on QD surfaces (inner circle) and their applications in optoelectronic fields (outer circle).

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  In this paper, we review the silica coating methods on QD surfaces and their applications in optoelectronic fields. Firstly, we introduced the silica coating approaches, which can be classified into in-situ hydrolysis of silylating reagents on QD surfaces and encapsulation of QD in silica template, according to the coating mechanism. Next, we discuss the applications of silica-coated QDs in the optoelectronic field, focusing on their use in LEDs, solar cells, and photodetectors. Finally, we provide a comprehensive overview of the review and offer a forward-looking analysis of emerging trends, potential challenges, and future prospects in this domain.

  2. Coating methods of silica on QDs

  Researchers developed various technical approaches to produce the silica-coated QDs. Based on the coating mechanism, these approaches can be classified into in-situ hydrolysis of silylating reagents on QD surfaces and encapsulation of QD in silica template. The first method involves the hydrolysis of silane coupling agent in QD dispensed solvent for in-situ formation of silica coating on the surfaces of the pre-synthesized QDs. The second method refers to the synthesis of QDs in a porous matrix primarily composed of silica and encapsulating them within it. In the subsequent sections, these two approaches will be comprehensively introduced.

  2.1 in-situ hydrolysis of silylating reagents on QD surfaces

  Hydrolysis is a common chemical synthesis technique used for creating thin films and nanomaterials, often involving silicon precursors like ethyl orthosilicate (TEOS), methyl orthosilicate (TMOS) and aminopropyl triethoxysilane (APTES), which form silanols and polymerize into siloxane networks [36]. Taking TEOS as an example, it reacts with water to form a silica coating on QDs. Hydrolysis involves hydroxyl groups (OH^-) displacing ethoxy groups (OC₂H₅) from TEOS, producing silanol and ethanol (Eq. 1). The silanol further reacts with water to generate more silanol (Eqs. 2-4), which then condenses into a silica network through silicon-oxygen bonds (Si-O-Si) (Eq. 5) [37]. The choice of silicifiers depends on material properties required for specific applications. For example, APTES provides more surface functionalization due to its amino groups. Currently, silica-coated QDs are prepared mainly through sol-gel and reverse microemulsion techniques. Below, we will discuss the advancements in these methods for preparing silica-coated QDs.

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  2.1.1   Sol-gel method

  The sol-gel method is a material synthesis methodology that is commonly used to create coatings or films of metal oxides and other inorganic substances [38]. First, QDs are synthesized using methods such as thermal injection and ligand-assisted reprecipitation. Then, silicification reagents, such as TEOS or TMOS, are introduced into organic solvents containing QDs. An appropriate amount of water is added to promote the hydrolysis and condensation reactions of the silanization reagents on the surface of the QDs, ultimately forming a silica matrix.

  Liu et al. encapsulated CsPbBr₃@glass with a hydrophobic silica layer using the sol-gel method (Fig. 2A(ⅰ)) [39]. This was achieved through the ozonolysis reaction between tri-(dimethylamine)silane and O₃, which formed a dense silica layer (CsPbBr₃@Glass@A). Dibutyltin diformate was then used to catalyze the polymerization of TEOS and polydimethylsiloxane, creating a hydrophobic silica network (CsPbBr₃@glass@ASG). The dense silica layer had an average thickness of 29.0 ± 3.4 nm, while the hydrophobic layer was 40.1 ± 7.7 nm thick. This structure improved water resistance, thermal reversibility of PL, and light stability. After 7 weeks, CsPbBr₃@glass@ASG retained 100% of its PL intensity and 88% of its light intensity, while CsPbBr₃@Glass@A kept 90% PL and 81% light intensity. In contrast, uncoated CsPbBr₃@glass retained only 60% of its PL intensity and 50% of its light intensity (Fig. 2A(ii, ⅲ)). Thus, CsPbBr₃@Glass@ASG balances optical properties and external stability.

  Figure 2

  

  Figure 2.  Synthesis of silicon-coated QDs by sol-gel method. (A): (ⅰ) The mechanism of synthesizing SiO₂ coating by sol-gel method (on top), and high-resolution transmission electron microscope (HRTEM) of CsPbBr₃@glass@ASG (on bottom), (ⅱ) The PL intensity and (ⅲ) the optical density of different samples during waterproof testing. Reproduced with permission [39]. Copyright 2023, John Wiley & Sons. (B): (ⅰ) The formation of CsPbBr₃@SiO₂ (on top), and transmission electron microscope (TEM) images of QD with different reaction times (on bottom); (ⅱ) Water stability test for CsPbBr₃ NCs and CsPbBr₃@SiO₂; (ⅲ) The PL intensity of QDs in aqueous solutions following various ultrasonication durations. Reproduced with permission [41]. Copyright 2018, American Chemical Society. (C): (ⅰ) The structure (left), and HRTEM of BPNC (right); (ⅱ) PL spectra of different samples; (ⅲ) Light absorption of BPNC and pristine γ-CsPbI₃ PQDs under continuous excitation with blue LEDs. Reproduced with permission [44]. Copyright 2023, John Wiley & Sons.

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  Ding et al. synthesized silica-coated ZnO QDs (ZnO@SiO₂) using an advanced sol-gel method and prepared a QD-composite ink by combining ZnO@SiO₂ with polyvinyl alcohol for UV-protective coatings on PET films [40]. The silica coating significantly reduced the photocatalytic activity of QDs, preventing the degradation of Rhodamine B and avoiding ZnO QD aggregation in water. UV–vis transmission spectra of the QD-composite ink coatings showed excellent UV-blocking capabilities below 355 nm while maintaining high transparency in the visible range. After 100 h of exposure to 30 W UV light, the coatings' UV–vis spectra remained largely unchanged, demonstrating the long-term durability of the QD-based composite materials.

  Though the sol-gel method can produce silica coating on QDs by hydrolysis of siliconizing reagent, most of the silica-coated particles contain several QD nanoparticles and possess a relatively large dimension, which makes these silica-coated QDs unsuitable for applications that require high precision, such as QD-based color conversion micro-LED displays. Therefore, obtaining single QD particles coated with silicon dioxide is crucial for the construction of some advanced QD-based devices.

  Zhong et al. synthesized monodispersed core-shell CsPbBr₃@SiO₂ by controlling the hydrolysis reaction time [41]. CsPbBr₃ QDs were formed by rapidly injecting toluene into a DMF solution of CsBr, PbBr₂, and ligands OA and Oleylamine (OAm). The pH was adjusted with ammonia, and rapid injection into toluene precipitated CsPbBr₃ QDs. The mixture was then added to super-dried toluene with TMOS, where hydrolysis produced silica oligomers that adsorbed onto the QD surfaces. Continued hydrolysis and condensation encapsulated CsPbBr₃ QDs in a silica shell (Fig. 2B(ⅰ)). After 2 min, the NCs retained their cubic structure, but extending hydrolysis to 60 min increased silica thickness to about 14 nm, rising to 17 nm at 120 min. Uncoated CsPbBr₃ QDs lost PL after 16 min of ultrasonication, while CsPbBr₃@SiO₂ QDs maintained bright emission for 40 min, showing enhanced stability (Fig. 2B(ⅱ)). The PL intensity of uncoated QDs dropped to 0% after 16 min, whereas CsPbBr₃@SiO₂ QDs showed higher water dispersibility and PL intensity (177%) after ultrasonication, with a slight decrease to 112% after extended exposure. The silica shell significantly improved the stability of CsPbBr₃@SiO₂ QDs by protecting the core from water-induced degradation (Fig. 2B(ⅲ)).

  Meng et al. used APTES as a bifunctional surface ligand to passivate CsPbBr₃ QDs' surface defects and form Si-O-Si covalent bonds via -SiOCH₃ and -SiOH groups [42]. This dual functionality enhanced the surface chemistry of the perovskite QDs. By carefully controlling the silane hydrolysis rate, the authors synthesized CsPbBr₃@SiO₂ QDs with a uniform silica shell thickness ranging from 1 nm to 2.7 nm, preserving their original cubic shape and optical properties. Complete silica encapsulation significantly boosted the QDs' robustness, maintaining 90% of their initial PL intensity after 6 h in high-humidity conditions.

  Compared to green-emissive CsPbBr₃ QDs, red-emissive CsPbI_3-xBr_x perovskite QDs exhibit poorer PL and stability due to higher trap density and ion migration in harsh environments. Chao et al. successfully coated both CsPbBr₃ and CsPbI_3-xBr_x perovskite QDs (CsPbBr₃@SiO₂ and CsPbI_3-xBr_x@SiO₂) using a sol-gel method [43]. After synthesizing the QDs, silane coupling agents were added to the solution, initiating a silanization reaction at room temperature. This protective silica layer significantly improved the QDs' stability, resulting in CsPbX₃@SiO₂ QDs with narrow emission peaks in their PL spectra, indicating enhanced optical properties.

  Fan et al. developed a bicomponent perovskite nanocomposite (BPNC) material, which consists of CsPbBr₃ QDs (green light) at the core, γ-CsPbI₃ QDs (red light) at the shell, encapsulated by a silica layer both in the interlayer and as the outermost layer (Fig. 2C(ⅰ)) [44]. This structure facilitates energy transfer from the core to the shell via a non-radiative fluorescence resonance energy transfer process, enhancing red luminescence intensity (Fig. 2C(ⅱ)). Under blue light excitation, the PL intensity increases over threefold, with the PLQY nearing 100%. The silica layer is crucial, and it acts as a barrier to prevent diffusion between the core and shell, preserving their structural integrity. Additionally, the outer silica coating protects the QDs and enhances BPNC stability. Even under continuous blue light excitation (5 V, 10 mA), BPNCs exhibit significantly improved blue light tolerance, with a half-life of about 2 h, outperforming original γ-CsPbI₃ in stability (Fig. 2C(ⅲ)). These results demonstrate that the coating structure provides good stability for BPNC.

  2.1.2   Reverse microemulsion method

  The reverse microemulsion method synthesizes QD samples with specific sizes and shapes using tiny oil-in-water (O/W) or water-in-oil (W/O) type emulsion droplets as reaction vessels [45]. Initially, QDs are synthesized via heat injection or ligand-assisted reprecipitation and serve as the core for subsequent coating. The QDs are then dispersed in an inverse microemulsion containing a surfactant (e.g., IGEPAL CO-520), a hydrophobic solvent (like n-hexane or octane), and water, ensuring each QD is surrounded by microemulsion droplets. Introducing silicon dioxide precursors (e.g., TEOS, TMOS) leads to hydrolysis and condensation within these droplets, forming a silicon dioxide coating on the QDs' surfaces.

  An et al. employed the microemulsion method to coat silica on CdSe/ZnS QDs [46]. By adding ammonia water to a cyclohexane solution of IGEPAL CO-520 and TEOS, a stable interface between water and oil is created, forming a microemulsion. Ammonia water catalyzes TEOS reaction, leading to silica growth on the QDs. By varying TEOS amounts and reaction times, they produced QDs@SiO₂ with shell thicknesses from 11 nm to 56 nm. The CdSe/ZnS QDs exhibited an absorption peak at 612 nm and an emission peak at 626 nm, with a FWHM of 24 nm. Notably, increasing the silica shell thickness did not change the emission peak, though the FWHM of QDs@SiO₂ nanoparticles slightly widened to 26 nm.

  Lv et al. successfully engineered hydrophobic CdSe/ZnS QDs with a biocompatible silica coating using a novel reverse microemulsion method [47]. The resulting QDs have a narrow size distribution, averaging about 14 nm in diameter, and the silica-coated QDs maintain a uniform size distribution with an average diameter of 29 nm. Remarkably, the PL intensity of these QDs remains stable across varying temperatures, with almost 100% intensity from 20 ℃ to 50 ℃ and 80% at 90 ℃. Additionally, after 12 h under a 365 nm UV lamp, the PL intensity of the silica-encapsulated QD solution remains at 85% of its initial value.

  Tang et al. prepared ultra-thin, core-shell structured silica-coated Mn²⁺-doped CsPbX₃ (X = Br, Cl) QDs using a simple reverse microemulsion method [48]. The process started with a DMF precursor solution of PbBr₂, PbCl₂, MnBr₂, and CsBr introduced into toluene at room temperature, followed by the addition of TEOS as a silane source. After TEOS hydrolysis, a silica layer was deposited on the QDs, forming orthorhombic CsPbMnX₃ QDs. To reduce the hydrolysis reaction of TEOS, trioctylphosphine oxide (TOPO) is selected as a surfactant to modify the surface of QDs, which can effectively prevent the cleavage of CsPbMnX₃ core QDs caused by silane hydrolysis (Fig. 3A). Each QD was encased in a silica shell, indicating isotropic growth and a smooth surface (Fig. 3B). During water resistance testing, uncoated CsPbMnX₃ QDs lost luminescence after 6 days, while the silica-coated QDs maintained approximately 90% of their initial efficiency in aqueous media (Figs. 3C and D).

  Figure 3

  

  Figure 3.  Synthesis of silica-coated QDs by reverse microemulsion method. (A) Schematic diagram of the process of synthesizing CsPbMnX₃@SiO₂ core-shell QDs. (B) TEM and HRTEM images of CsPbMnX₃ QDs and CsPbMnX₃@SiO₂. (C) Images of CsPbMnX₃ and CsPbMnX₃@SiO₂ QDs in aqueous solution under ultraviolet light. (D) PLQY of CsPbMnX₃ and CsPbMnX₃@SiO₂ QDs. Reproduced with permission [48]. Copyright 2019, John Wiley .& Sons.

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  Huang et al. combined reverse microemulsion with surface modification to synthesize ZnO@SiO₂ QDs with a core-shell structure and transparent nanodispersions [49]. These ZnO@SiO₂ QDs, with a thick silica shell and an average diameter of 12 nm, exhibited excellent monodispersity in toluene, high optical transparency, and significant UV absorbance, making them suitable for UV protection and optoelectronic applications. The highly transparent ethyl cellulose/ZnO@SiO₂ composite film remained optically stable after 100 h of UV aging due to the dispersion's properties.

  2.2 Template method for encapsulation QDs

  The template method is a versatile and effective technique for fabricating nanomaterials with customized shapes and sizes, particularly in the synthesis of silica-coated QDs [50]. Typically, mesoporous silica materials like molecular sieves are used as templates. These templates are immersed in a solution containing QD precursors, allowing the precursor molecules to penetrate the pores, where they undergo chemical reactions to form QDs. To enhance the crystallization and growth of QDs within the silica matrix, heat treatment is often necessary. This synthesis can be divided into two strategies based on temperature: the low-temperature synthesis method and the high-temperature solid-state method, each designed to achieve precise control and optimize the performance of the QDs.

  2.2.1   Low-temperature synthesis

  The low-temperature synthesis method utilizes silica molecular sieves as templates to grow QDs inside them at low temperatures. The growth speed of QDs in the template is relatively slow, which makes the size and shape of the QDs accurately controllable, and a uniform thickness of silica coating.

  Fan et al. encapsulated CsPbBr₃ in mesoporous silica SBA-15, which has a large surface area, uniform pores, high refractive index, and is stable up to 120 ℃ [51]. Perovskite QDs were synthesized in situ within the pores by mixing PbO, phenacyl bromide, and SBA-15 in 1-octadecene under nitrogen. After adding a cesium source, QDs formed and diffused into the SBA-15 (Fig. 4A(ⅰ)). The 10.5 nm pore size of SBA-15 accommodated 8.6 nm CsPbBr₃ QDs, enhancing their PLQY from 24.84% to 63.96% and luminescence lifetime from 32.6 ns to 104.97 ns (Fig. 4A(ⅱ, ⅲ)). The SBA-15′s ordered structure reduces ion diffusion, improves crystallinity, and passivates surface defects, preserving 95% of the QDs' initial PL after six months, compared to 55% for pure CsPbBr_3, showing the material's superior stability against water and oxygen (Fig. 4A(ⅳ)).

  Figure 4

  

  Figure 4.  Low-temperature synthesis of silica-coated QDs. (A): (ⅰ) Synthesis diagram of CsPbBr₃/SBA-15; (ⅱ) TEM image of SBA-15 and CsPbBr₃/SBA-15; (ⅲ) Time-resolved PL decays of CsPbBr₃ and CsPbBr₃/SBA-15; (ⅳ) PL stability of CsPbBr₃ and CsPbBr₃/SBA-15 at room temperature. Reproduced with permission [51]. Copyright 2023, John Wiley & Sons. (B): (ⅰ) Synthesis diagram of CsPbBr₃@SiO₂ OH and CsPbBr₃@SiO₂ TMe; (ⅱ) TEM images of CsPbBr₃@SiO₂ TMe and HTEM images of CsPbBr₃@h-SiO₂ OH; (ⅲ) Moisture stability of CsPbBr₃@SiO₂ OH and CsPbBr₃@SiO₂ TMe; (ⅳ) Thermal stability of CsPbBr₃@SiO₂ OH and CsPbBr₃@SiO₂ TMe. Reproduced with permission [52]. Copyright 2023, John Wiley & Sons.

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  Huang et al. synthesized mesoporous silica (m-SiO₂) using Pluronic F108, hydrochloric acid (HCl), trimethylphenol (TMB), and TEOS [52]. CsPbBr₃ QDs were then produced at 110–160 ℃ within the hollow mesoporous structure. The silica structure contained silanol sites that adsorbed cesium and bromine ions, forming CsPb₂Br₅ impurities detrimental to QD performance (CsPbBr₃@SiO₂ OH). To address this, the authors treated the structure with trimethylchlorosilane (TMCS) to eliminate silanol groups, preventing impurity formation and yielding a purer CsPbBr₃@SiO₂ TMe (Fig. 4B(ⅰ)). After synthesis, perovskite QDs were dispersed in the hollow mesoporous silica. The crystal plane spacing of CsPbBr₃@SiO₂ TMe is 0.341 nm, corresponding to the (111) plane of orthorhombic CsPbBr₃, whereas CsPbBr₃@SiO₂ OH showed spacings of 0.410 nm and 0.217 nm, related to CsPb₂Br₅ (Fig. 4B(ⅱ)). Under 80% humidity at 28 ℃, CsPbBr₃@SiO₂ TMe retained 87% of its PL intensity after 720 h, while CsPbBr₃@SiO₂ OH dropped to 10% after 288 h. Similarly, after 360 h, CsPbBr₃@SiO₂ TMe maintained 68% of its PL intensity, whereas CsPbBr₃@SiO₂ OH retained only 7% after 192 h, indicating superior stability of CsPbBr₃@SiO₂ TMe (Fig. 4B(ⅲ, ⅳ)).

  Chen et al. utilized ordered mesoporous silica (m-SiO₂) as a microreactor for in-situ growth of ultra-small CsPbBr₃ QDs at environmental factors. The PLQY of the resulting CsPbBr₃/m-SiO₂ (CPB/MS) powder reached 68%, significantly higher than the 180 ℃ [53]. This method suppressed excessive QD growth, light, the luminescence decay of CPB/MS nanocomposites was value, compared to 16% for discrete CsPbBr₃ powder. Thus, the CPB/MS material showed remarkable chemical stability, enhanced resistance to luminescence decay, and superior thermal quenching performance.

  2.2.2   High-temperature solid-state synthesis

  Compared with low-temperature synthesis, high-temperature conditions improve the control of QD size and morphology due to accelerated reaction rates, allowing precursor molecules to react and deposit uniformly in silica microspheres. This enhances production efficiency and is suitable for large-scale production. Crucially, the mesoscopic structure of porous SiO₂ is compromised when merged in melted precursor salt at high temperatures, resulting in a solidified structure that encapsulates the QDs, enhancing their robustness. Selecting appropriate silica microspheres as templates allows for precursor molecule introduction into their pores, transforming into QDs during subsequent heat treatment. The silicon source material deposits on the QDs' surfaces at high temperatures, forming a silicon coating layer.

  Zhang et al. encapsulated CsPbBr₃ in a SiO₂ porous matrix using high-temperature solid-state synthesis [54]. Porous all-silicon molecular sieve templates were immersed in a precursor salt solution (CsBr and PbBr₂) and dried at 80 ℃. The composites were then heated to 600–900 ℃, leading to the formation of CsPbBr₃ QDs within the molecular sieve's closed pores. As the pore structure collapsed, the molecular sieve encapsulated the QDs, resulting in dense CsPbBr₃-SiO₂ composites (Fig. 5A(ⅰ)). At 400 ℃ and 500 ℃, the pores remained stable, preventing collapse and delaying QD formation. However, at 600 ℃, the pore structure collapsed, creating small CsPbBr₃ QDs. As the calcination temperature rose, the molecular sieve's pores softened, reducing the template's confinement effect, causing QD size to increase from 6.7 nm to 30.1 nm (Fig. 5A(ⅱ)). The highest PLQY of 63% was achieved in the CsPbBr₃-SiO₂–700 sample (CsPbBr₃ to molecular sieve's mass ratio of 1:3, calcination temperature of 700 ℃), while other conditions yielded PLQY below 45%. The silica-coated QDs demonstrated excellent stability; after 50 days in water, the CsPbBr₃-SiO₂–600 sample retained 88% of its initial PLQY. In contrast, CsPbBr₃-SiO₂ samples synthesized at higher temperatures (700 ℃, 800 ℃, and 900 ℃) maintained their PLQY (Fig. 5A(ⅲ)). The stability of the CsPbBr₃-SiO₂ samples in water highlighted the effectiveness of silica coating in protecting QDs from degradation.

  Figure 5

  

  Figure 5.  High-temperature solid-state synthesis of silica-coated QD. (A): (ⅰ) The schematic diagram for the synthesis of CsPbBr₃-SiO₂; (ⅱ) TEM images of CsPbBr₃-SiO₂ at different calcination temperatures; (ⅲ) The PLQY of CsPbBr₃-SiO₂ synthesized at different calcination temperatures in water immersion. Reproduced with permission [54]. Copyright 2020, Springer Nature. (B): (ⅰ) The schematic diagram of synthesis CsPbBr₃-SiO₂; (ⅱ) TEM images of CsPbI₃-SiO₂ with different magnifications; (ⅲ) The phase transition routes of CsPbI₃-SiO₂. Reproduced with permission [55]. Copyright 2021, John Wiley & Sons. (C): (ⅰ) Image of the PeNCs-SiO₂ family exposed to UV light at 365 nm and daylight; (ⅱ) PL spectra of the PeNCs-SiO₂ family; (ⅲ) PL intensity in UV light conditions changes. Reproduced with permission [56]. Copyright 2021, Royal Society of Chemistry. (D): (ⅰ) TEM images of CsPbBr₃-SiO₂ NCs at different calcination temperatures; (ⅱ) Schematic and TEM images of CsPbBr₃-SiO₂ NCs obtained by internal sintering and external sintering of K₂CO₃ at 600 ℃, respectively; (ⅲ) PLQY of ultrasound in water; (ⅳ) PL decay traces of the samples; (ⅴ) Normalized PL strength of internal K₂CO₃ sintered CsPbBr₃-SiO₂ NCs collected during 25-100 ℃ heat treatment (top) and 25-300 ℃ annealing (bottom) in air. Reproduced with permission [58]. Copyright 2022, American Chemical Society.

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  Lin et al. successfully synthesized γ-CsPbI₃ NCs encapsulated with SiO₂/AlO_x using high-temperature sintering and atomic layer deposition (ALD) techniques (Fig. 5B(ⅰ)) [55]. Initially, CsI and PbI₂ were mixed with MCM-41 to form a precursor solution. The solution was sintered at 700 ℃ under nitrogen, collapsing the MCM-41 structure into amorphous SiO₂, which trapped CsPbI₃ NCs. To enhance stability, an AlO_x layer was deposited using ALD, preventing oxygen penetration and oxidation. As shown in Fig. 5B(ⅱ), CsPbI₃ are uniformly distributed in the SiO₂ matrix, with a spherical structure of about 20 nm and a lattice spacing of 0.42 nm corresponding to the (200) plane of CsPbI₃. The SiO₂/AlO_x encapsulated CsPbI₃ nanocrystals undergo a direct phase transition from γ to α via β upon heating, and reversely from α to γ through β when cooling, bypassing the δ phase (Fig. 5B(ⅲ)). This novel phase transition is facilitated by the protective SiO₂/AlO_x encapsulation, which significantly enhances the phase stability of the CsPbI₃ NCs. Moreover, after 3000 h of water immersion and UV irradiation tests, minimal PL strength loss was observed, indicating excellent water and UV resistance.

  Lin et al. extended their method to perovskite QDs with various halide components, achieving PL covering the entire visible spectrum (Fig. 5C(i, ii)) [56]. After 1560 h in ambient conditions, silica-coated QDs showed superior PL stability under UV exposure compared to polymer-coated controls (Fig. 5C(ⅲ)). Additionally, these silica-coated QDs demonstrated improved stability in water. Yang et al. enhanced the stability of CsPbBr₃ QDs against water, light, and heat by embedding them in mesoporous silica microspheres (CsPbBr₃ PQDs/MSs) through high-temperature synthesis [57]. The CsPbBr₃ PQDs/MSs retained over 80% of their initial PL intensity after 2 weeks in water and about 90% after 6 h at 150 ℃.

  During high-temperature silica coating, QDs and silica tend to form large particles (Fig. 5D(ⅰ)). He et al. minimized this aggregation using K₂CO₃, producing silica-coated CsPbBr₃ QDs with a size of 8.6 ± 1.3 nm [58]. By sintering potassium carbonate inside and outside the silica, they prevented particle fusion and increased PLQY from 37% to 87% (Fig. 5D(ii, ⅲ)). Ultrasonic treatment in water significantly reduced the PLQY of non-treated and externally sintered samples, but the internally sintered QDs maintained a PLQY of 85%, confirming the protective effect of collapsed pores. Additionally, K₂CO₃ extended the lifetimes of the sintered particles and improved thermal stability up to 300 ℃ (Fig. 5D(ⅳ, ⅴ)).

  Song et al. fabricated CsPbX₃@SiO₂ composites with stable PLQY values of 11.4%, 80.6%, and 16.2% for red, green, and blue QDs, respectively, over 30 days at 25 ℃ and 80% humidity [59]. However, the PLQY of red QDs from high-temperature synthesis remains low. Deng et al. improved this by incorporating Zn to passivate defects, resulting in red silica-coated CsPbI₃_Cs₄PbI₆ QDs with a PLQY of 43.58% and stable PL after 1100 h in water and ambient storage [60].

  3. Applications of silica-coated QDs

  The silica-coated can improve the performance of QDs, such as increased PL intensity, PLQY, narrowed FWHM, enhanced stability at various conditions. Currently, the applications of silica-coated QDs are mainly focused on optoelectronic applications, specifically LED, solar cells, and photodetector. In this section, we will introduce the applications of silica-coated QDs in these scenarios.

  3.1 Light-emitting diodes

  LEDs, semiconductor light sources that emit light through electron-hole recombination in the PN junction, are crucial in daily lighting, displays, signaling systems, medical instruments, and automotive lighting [61,62]. Silica-coated QDs serve as color-converted layers in white LEDs, backlight layers for liquid crystal displays, and as red/green/blue color pixels in micro-LED displays, usually excited by UV or blue LEDs [63]. These QDs can also function as emitting layers in QD LED (QLED) displays.

  Wang et al. fabricated red, green, and blue CsPbX₃@SiO₂ NCs films for white-LED (WLED) devices, achieving PLQYs of 10.9%, 94.0%, and 33.7%, respectively, after silica coating (Fig. 6A(i, ii)) [64]. These films demonstrated enhanced stability against UV irradiation, thermal shock, and humidity. The resulting WLEDs exhibited a high CRI of 93.4, CIE coordinates of (0.327, 0.343), an NTSC color gamut of 126.4%, and a luminous efficacy of 47.75 lm/W (Fig. 6A(ⅲ)). The spectra remained stable even after 36 h of continuous operation. Notably, CsPbX₃@SiO₂ NC films show scalability and bendability on flexible substrates, broadening their application potential (Fig. 6A(ⅳ, ⅴ)).

  Figure 6

  

  Figure 6.  Application of silica-coated QDs in color conversion-based LED devices. (A): (ⅰ) Structure of WLED; (ⅱ) EL spectrum of the WLED device; (ⅲ) The chromaticity diagram for the blue, green, and red CsPbX₃@SiO₂ NC films; (ⅳ) Working stability of WLED devices; (ⅴ) The scalable and bendable light-emitting CsPbX₃@SiO₂ NC films were deposited on flexible substrates with a larger area or textile fibers. Reproduced with permission [64]. Copyright 2022, Elsevier. (B): (ⅰ) EL spectra of CsPbX₃@AGs@PS composite film; (ⅱ) CIE chromaticity coordinates of the CsPbX₃@AGs@PS composites films prepared LED devices excited by blue backlight at 460 nm; (ⅲ) Commercial CdSe composite film display images; (ⅳ) CsPbX₃@AGs@PS composite film display images; (ⅴ) The stability of CsPbX₃@AG@PS was observed after 48 h of continuous backlight illumination. Reproduced with permission [65]. Copyright 2022, Elsevier. (C): (ⅰ) A schematic diagram of the deposition and patterning of the DBR layer; (ⅱ) The PL intensity of the glass with and without the DBR layer; (ⅲ) The micro-LED display's chromaticity diagram for the red, green, and blue QDPR pixels; (ⅳ) The blue micro-LED display's brightness percentage trends over 500 h at 85 ℃ and 90% relative humidity with and without QD-PR pixels. Reproduced with permission [68]. Copyright 2022, Elsevier.

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  Chen et al. explored silica-coated perovskite QDs (CsPbX₃@AGs) as backlight films [65]. When excited by gallium nitride LED, CsPbCl₂Br@AGs, CsPbBr₃@AGs, and CsPbBrI₂@AGs emitted bright blue, green, and red light, respectively (Fig. 6B(ⅰ)). After dispersing the silica-coated QDs in PS, CsPbBr₃@AGs@PS membranes emitted green light at 518 nm, while CsPbBrI₂@AGs@PS membranes emitted red light at 632 nm. Under blue backlighting, the composite film achieved a white light coordinate of (0.3010, 0.3108) with a red-green-blue ratio of 26.2%:68.3%:5.5%, closely resembling commercial backlight films (Fig. 6B(ⅱ)). The CsPbX₃@AGs films showed performance comparable to or superior to commercial CdSe-based backlights (Fig. 6B(ⅲ, ⅳ)). After 48 h of continuous illumination, the luminous flux remained above 99.5% of the initial value, demonstrating excellent optical stability (Fig. 6B(ⅴ)).

  For QD-based full-color micro-LED displays, QDs need to be patterned, which may weaken their performance due to chemical contact [66,67]. Silica coating protects QDs from damage during patterning, improving micro-LED display performance and stability. Chang et al. synthesized silica-coated CdSe/ZnS QDs and prepared QD photoresist (QDPR) for patterning via photolithography (Fig. 6C(ⅰ)) [68]. A patterned black matrix minimized light crosstalk, and a distributed Bragg reflector (DBR) enhanced color purity (Fig. 6C(ⅱ)). The DBR layer, patterned red and green QDPR color conversion layer, and blue micro-LED chips were bonded to prepare the full-color micro-LED display panels, which showed a color gamut of 88.74% Rec. 2020 standard (Fig. 6C(ⅲ)). The displays showed a brightness decay rate below 15% at 85 ℃ and 90% humidity after 500 h, indicating excellent stability (Fig. 6C(ⅳ)).

  In QLED devices, QDs are excited by injecting electrons and holes that recombine to emit photons. The performance of QLEDs depends on factors such as QD quality and the electron and hole injection rates [69,70]. The ligands and coatings of QDs impact carrier transfer and luminescent properties, and silica's low electric conductivity makes it generally unsuitable for QD coatings despite improving stability. Thus, adjusting the SiO₂ coating thickness is crucial for enhancing silica-coated QDs in QLED devices.

  Trinh et al. created CsPbBr₃@SiO₂ as the emission layer, resulting in improved chemical and electrical stability and enhanced device performance [71]. The device structure comprised ITO/PEDOT:PSS/A-CsPbBr₃@SiO₂/TPBi/LiF/Al, with A-CsPbBr₃@SiO₂ QDs averaging 15–27 nm, larger than pure CsPbBr₃ (11 nm) (Fig. 7A(ⅰ)). They noted higher current density in QLEDs with uncoated CsPbBr₃ QDs due to pinholes, while initial brightness increased slowly, suggesting non-radiative recombination (Fig. 7A(ⅱ)). In contrast, A-CsPbBr₃@SiO₂ QD-based QLEDs reached maximum brightness of 3200 cd/m² at 830 mA/cm², with current efficiency of 5.3 cd/A, indicating more effective use of electrons and holes (Fig. 7A(ⅲ, ⅳ)). This signifies a higher light output per unit current, indicating a more efficient utilization of both electrons and holes. Moreover, QLED employing A-CsPbBr₃@SiO₂ QDs demonstrated exceptional reliability, exhibiting a half-life of 43 min at 100 cd/m² under ambient conditions markedly better than the mere 2 min observed in QLED using original CsPbBr₃ QDs (Fig. 7A(ⅴ)). Furthermore, A-CsPbBr₃@SiO₂ QDs-based QLED maintained approximately 56% of their initial EL value after sustaining operation for 8 days at 3.2 V and approximately 20% humidity (Fig. 7A(vi)), validating the viability of employing silica-coated QDs as light emission layer in QLED devices.

  Figure 7

  

  Figure 7.  Application of silica-coated QDs in EL-based LED. (A): (ⅰ) Device structure of QLED; Current density-voltage-brightness characteristics of QLED with (ⅱ) CsPbBr₃ QLED and (ⅲ) A-CsPbBr₃@SiO₂ QLED; (ⅳ) Current efficiency-voltage characteristics of QLED; (ⅴ) Operational stability of QLED; (vi) Storage test of all-solution-processed QLED. Reproduced with permission [71]. Copyright 2021, American Chemical Society. (B): Current density-brightness-voltage (ⅰ), current efficiency-EQE-brightness (ⅱ), EL spectrum (ⅲ), CIE chromaticity diagram (ⅳ), and lifetime (ⅴ) of CsPbBr₃ and CsPbBr₃/p-SiO₂ composite based QLED; (vi) A photograph of the CsPbBr₃/p-SiO₂ composite QLED. Reproduced with permission [72]. Copyright 2024, American Chemical Society.

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  However, low charge injection efficiency can limit the effectiveness of silica-coated QD-based QLEDs. Kim et al. addressed this by using a porous p-SiO₂ coating on CsPbBr₃ QDs (CsPbBr₃/p-SiO₂), improving charge injection and overall performance [72]. CsPbBr₃/p-SiO₂ composite films exhibited low leakage current densities and a high luminous efficiency of 42,314 cd/m² (Fig. 7B(ⅰ)). Compared with CsPbBr₃ QDs-based QLED devices, CsPbBr₃/p-SiO₂ based devices exhibit superior EQE and current efficiency, with EQE_max and CE_max values of 16.97% and 70.06 cd/A, respectively (Fig. 7B(ⅱ)). The chrominance coordinates (x, y) on the CIE chrominance diagram for CsPbBr₃ and CsPbBr₃/p-SiO₂ based QLED devices (ranging from (0.1677, 0.7401) to (0.1676, 0.7447)) aligned with measured EL and PL spectra (Fig. 7B(ⅲ, ⅳ)). The mechanical strength and thermal stability of p-SiO₂ contributed to enhanced durability, giving CsPbBr₃/p-SiO₂ QLEDs about seven times the stability of CsPbBr₃ QD-based QLEDs, demonstrating the potential for high-performance QLEDs with this coating (Fig. 7B(ⅴ)). The overall device, showcasing these advancements, is visually represented in Fig. 7B(vi).

  3.2 Solar cells

  Solar cells, or photovoltaic cells, transform solar energy into electrical energy, significantly impacting daily life [73-75]. These cells use the photovoltaic effect in semiconductor materials, where photon energy excites electrons, generating electron-hole pairs. QDs innovate solar cell design by enhancing light absorption, improving charge transport, and increasing stability, thereby advancing efficiency and durability [76].

  The high refractive index of silica enhances light capture in solar cells, boosting photoelectric conversion efficiency [77,78]. Rho et al. used silica-coated QD-embedded silica nanoparticles (SiO₂/QD/SiO₂) as a light-trapping layer in dye-sensitized solar cells (DSSC) [79]. While the unmodified DSSC showed a short-circuit current density (J_SC) of 7.38 mA/cm² and 3.92% efficiency, the SiO₂/QD/SiO₂ DSSC improved to 9.28 mA/cm² and 4.82% efficiency. The SiO₂/QD/SiO₂ DSSCs also demonstrated better incident photon-to-current efficiency (IPCE) across a wider wavelength range, confirming the effectiveness of the light-trapping layer. The stability of SiO₂/QD/SiO₂ DSSCs in iodide/triiodide-based electrolytes was validated over 168 h, highlighting the durability of the SiO₂ coating. Notably, SiO₂/QD/SiO₂ acts as a supplementary light-trapping layer, enhancing solar cell performance and stability without serving as a photosensitizer.

  Silica-coated QDs also function as passivation layers, reducing charge recombination and improving stability and efficiency. Liu et al. explored ZnS/SiO₂ coatings as passivation layers for CdS/CdSe co-sensitized solar cells based on mixed metal oxides (MMOs) [80]. Silica acts as an energy barrier, preventing electron migration to the electrolyte and suppressing recombination. Solar cells treated with ZnS/SiO₂ showed improved IPCE by 25% and 18% for ZnS/1h-SiO₂ and ZnS/2h-SiO₂ (where 1 h and 2 h represent the time of hydrolysis of silica), respectively. The ZnS/SiO₂ dual passivation layer further increased impedance, reducing charge recombination. Solar cells with ZnS/1h-SiO₂ achieved a power conversion efficiency (PCE) of 4.91%, a 36% improvement over single-layer passivation and 55% over unpassivated cells.

  Silica-coated QDs can act as light-emitting downshifting (LDS) layers, converting high-energy photons into lower-energy ones that are more easily absorbed by solar cells. Zhao et al. integrated SiO₂-coated Cs₃Bi₂Br₉ QDs as an LDS layer in silicon solar cells (Fig. 8A) [81]. The Cs₃Bi₂Br₉ QDs efficiently converted ultraviolet photons to blue-region photon energy, reducing energy loss and increasing output. The silica shell facilitated chemical bonding with the silicon substrate, enhancing the connection between QDs and the photonic framework. The solar cell with the Cs₃Bi₂Br₉ QD LDS layer achieved a PCE of 15.24%, a J_SC of 26.87 mA/cm², and a V_OC of 0.52 V. With the Cs₃Bi₂Br₉@SiO₂-LDS layer, the cell improved to a PCE of 16.30%, J_SC of 27.33 mA/cm², and V_OC of 0.54 V, demonstrating a 1.82% increase in efficiency (Fig. 8B). The SiO₂ coating enhanced electron transfer and reduced nonradiative recombination by passivating interfacial defects (Fig. 8C).

  Figure 8

  

  Figure 8.  Application of silica-coated QDs in solar cells. (A) Schematic diagram of the QD-LDS layer solar cells structure and working mechanism. (B) The J-V characteristics of solar cells without the QD-LDS layer and with a different QD-LDS layer in daylight. (C) Thermal imaging pictures of solar cells taken both in the dark and after 30 min in the sun. Reproduced with permission [81]. Copyright 2024, Elsevier.

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  3.3 Photodetectors

  A photodetector is a device that can receive and detect optical radiation by converting light signals, such as ultraviolet, visible, and infrared light, into electrical signals. It is widely used in areas like optical detection [82,83], spectral analysis [84], and optical communication [85,86]. QDs are crucial in photodetectors, improving light absorption, carrier separation efficiency, response speed, and sensitivity. Researchers enhance spectral selectivity by adjusting QD size and composition, allowing selective light detection. Silica-coated QDs act as a barrier layer, preventing hole carrier injection from the active region, thereby boosting detector performance and stability.

  Chun et al. fabricated photodetector devices using QDs capped with 3-aminopropyl(diethoxy)methylsilane (QD@APDEMS) as hole carriers in the barrier layer (Fig. 9A(ⅰ)) [87]. The device current density of QD@APDEMS indicates that APDEMS significantly enhance photodetector performance and mitigate current leakage, improving signal-to-noise ratio and sensitivity (Fig. 9A(ⅱ)). The QD@APDEMS-based photodetector showed a high EQE of 85% in the visible spectrum, converting light into current effectively (Fig. 9A(ⅲ)). At a 0.1 V bias, response rates (R values) for QD@APDEMS, QD, and bare photodetectors are 0.469, 0.459, and 0.359 A/W, respectively (Fig. 9A(ⅳ)). Detectivity (D* values) for QD@APDEMS, QD, and bare devices are 1.77 × 10¹², 2.28 × 10¹¹, and 2.77 × 10¹¹ cm Hz^-1/2 W^-1, respectively (Fig. 9A(ⅴ)), demonstrating superior performance of QD@APDEMS-based photodetectors.

  Figure 9

  

  Figure 9.  Application of silica-coated QDs in photodetector. (A): (ⅰ) QD@APDEMS photodetector device structure; (ⅱ) Dark J-V curves and (ⅲ) EQE spectra of QD@APDEMS photodetector; (ⅳ) QD@APDEMS photodetector device responsivity (at −0.1 V bias); (ⅴ) Detectivity of QD@APDEMS photodetector (at −0.1 V bias). Reproduced with permission [87]. Copyright 2023, John Wiley & Sons. (B): (ⅰ) Illustration of photodetector structure; Responsivity (ⅱ), detectivity (ⅲ), the rise/decay time (ⅳ), and EQE (ⅴ) of photodetector. Reproduced with permission [89]. Copyright 2024, Royal Society of Chemistry.

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  Black et al. reported a unique heterostructured photodetector made of CdSe/ZnS QDs (CdSe/ZnS@SiO₂ QDs) covered in graphene and silica [88]. In these detectors, CdSe/ZnS@SiO₂ QDs enhance photoelectrical response performance through photosensitive defect states and semiconductor cores. The photogating effect captures light-excited holes while releasing electrons into graphene, achieving a charge carrier gain of up to 10⁹, greatly improving sensitivity. Varying illumination power tunes graphene mobility, reducing from 1.14 × 10^–1 m² V^-1 s^-1 to 4.5 × 10^–2 m² V^-1 s^-1 due to Coulomb scattering. This demonstrates the potential of integrating CdSe/ZnS@SiO₂ QDs with graphene heterostructures for high-performance, tunable photodetectors.

  Silica-coated QDs facilitate the creation of an integrated electric field between n-type and p-type layers, promoting carrier separation and enhancing detector response time and efficiency. Ely et al. developed a photodetector using CsPbBr₃@SiO₂ as the p-type layer, gallium oxide as the n-type layer, and Au as the hole collector (Fig. 9B(ⅰ)) [89]. The detector showed a high detection rate of over 9 × 10¹³ Jones across 350–640 nm (Fig. 9B(ⅱ)). Under illumination at 160 µW and 40 µW, photocurrent increased by 4 orders of magnitude, reaching peaks of R values = 44.4 A/W and 32.0 A/W at +5 V for 470 nm and 405 nm, respectively (Fig. 9B(ⅲ, ⅳ)). It achieved a peak EQE of 5000%, a switching ratio of ~10⁴, and rise/decay times of 5.0/4.6 ms (Fig. 9B(ⅴ, vi)). High-voltage bias enabled EQE > 100%, due to efficient charge injection and superior material quality.

  4. Summary and outlook

  This review explores the methodologies for fabricating silica-coated QDs and surveys advancements in their applications across diverse domains. To date, researchers have employed two principal coating techniques: hydrolysis and template method. The hydrolysis method, which includes the sol-gel and reverse microemulsion techniques, allows for precise control over the thickness and structure of the silica shell by adjusting precursor quantities and reaction times, ensuring uniform shell thickness and effective protection of the QDs. Template methods, which are further classified into low-temperature synthesis and high-temperature solid-phase techniques, utilize templates such as molecular sieves and microspheres to guide the growth and coating of QDs. The distinctive physicochemical attributes of QD@SiO₂ materials endow them with broad application prospects in the realms of optoelectronics. In the domain of optoelectronics, their integration into LED, solar cells and photodetectors are recognized for their superior photoelectric conversion efficiency coupled with enhanced stability.

  Though ambient progresses, silica coating techniques still require enhancements to further improve the performance of QDs. The hydrolysis method for preparing silica coatings necessitates a prolonged reaction time and is easily affected by environmental factors like humidity and temperature. Consequently, precise control of reaction conditions is essential for achieving more uniform coating speed. The template method yields silica-coated QDs with large dimensions (typically exceeding 200 nm), necessitating size reduction for applications such as high-resolution micro-displays. A feasible strategy might involve reducing the size of large silica-coated QDs through mechanical grinding methods and effectively screening for well-coated QDs. Efficient surface modifications on silica coatings are often employed in most optoelectronic applications, such as the modification of high-conductance molecules to enhance charge transport efficiency. This requires high uniform and dense silica coatings on QDs, a challenge that can be addressed through more controllable coating processes and the adoption of multiple coating layers.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Siting Cai: Writing – original draft. Xiang Chen: Writing – original draft. Shuli Wang: Writing – review & editing, Project administration. Xinqin Liao: Writing – review & editing. Zhong Chen: Project administration. Yue Lin: Writing – review & editing, Project administration.

  Acknowledgments

  This work is supported by the National Natural Science Foundation of China (Nos. 62374142 and 22005255), Fundamental Research Funds for the Central Universities (Nos. 20720220085 and 20720240064), External Cooperation Program of Fujian (No. 2022I0004), Major Science and Technology Project of Xiamen in China (No. 3502Z20191015), Xiamen Natural Science Foundation Youth Project (No. 3502Z202471002).

  
  
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  Figure 1  The approaches of silica coating on QD surfaces (inner circle) and their applications in optoelectronic fields (outer circle).

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  Figure 2  Synthesis of silicon-coated QDs by sol-gel method. (A): (ⅰ) The mechanism of synthesizing SiO₂ coating by sol-gel method (on top), and high-resolution transmission electron microscope (HRTEM) of CsPbBr₃@glass@ASG (on bottom), (ⅱ) The PL intensity and (ⅲ) the optical density of different samples during waterproof testing. Reproduced with permission [39]. Copyright 2023, John Wiley & Sons. (B): (ⅰ) The formation of CsPbBr₃@SiO₂ (on top), and transmission electron microscope (TEM) images of QD with different reaction times (on bottom); (ⅱ) Water stability test for CsPbBr₃ NCs and CsPbBr₃@SiO₂; (ⅲ) The PL intensity of QDs in aqueous solutions following various ultrasonication durations. Reproduced with permission [41]. Copyright 2018, American Chemical Society. (C): (ⅰ) The structure (left), and HRTEM of BPNC (right); (ⅱ) PL spectra of different samples; (ⅲ) Light absorption of BPNC and pristine γ-CsPbI₃ PQDs under continuous excitation with blue LEDs. Reproduced with permission [44]. Copyright 2023, John Wiley & Sons.

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  Figure 3  Synthesis of silica-coated QDs by reverse microemulsion method. (A) Schematic diagram of the process of synthesizing CsPbMnX₃@SiO₂ core-shell QDs. (B) TEM and HRTEM images of CsPbMnX₃ QDs and CsPbMnX₃@SiO₂. (C) Images of CsPbMnX₃ and CsPbMnX₃@SiO₂ QDs in aqueous solution under ultraviolet light. (D) PLQY of CsPbMnX₃ and CsPbMnX₃@SiO₂ QDs. Reproduced with permission [48]. Copyright 2019, John Wiley .& Sons.

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  Figure 4  Low-temperature synthesis of silica-coated QDs. (A): (ⅰ) Synthesis diagram of CsPbBr₃/SBA-15; (ⅱ) TEM image of SBA-15 and CsPbBr₃/SBA-15; (ⅲ) Time-resolved PL decays of CsPbBr₃ and CsPbBr₃/SBA-15; (ⅳ) PL stability of CsPbBr₃ and CsPbBr₃/SBA-15 at room temperature. Reproduced with permission [51]. Copyright 2023, John Wiley & Sons. (B): (ⅰ) Synthesis diagram of CsPbBr₃@SiO₂ OH and CsPbBr₃@SiO₂ TMe; (ⅱ) TEM images of CsPbBr₃@SiO₂ TMe and HTEM images of CsPbBr₃@h-SiO₂ OH; (ⅲ) Moisture stability of CsPbBr₃@SiO₂ OH and CsPbBr₃@SiO₂ TMe; (ⅳ) Thermal stability of CsPbBr₃@SiO₂ OH and CsPbBr₃@SiO₂ TMe. Reproduced with permission [52]. Copyright 2023, John Wiley & Sons.

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  Figure 5  High-temperature solid-state synthesis of silica-coated QD. (A): (ⅰ) The schematic diagram for the synthesis of CsPbBr₃-SiO₂; (ⅱ) TEM images of CsPbBr₃-SiO₂ at different calcination temperatures; (ⅲ) The PLQY of CsPbBr₃-SiO₂ synthesized at different calcination temperatures in water immersion. Reproduced with permission [54]. Copyright 2020, Springer Nature. (B): (ⅰ) The schematic diagram of synthesis CsPbBr₃-SiO₂; (ⅱ) TEM images of CsPbI₃-SiO₂ with different magnifications; (ⅲ) The phase transition routes of CsPbI₃-SiO₂. Reproduced with permission [55]. Copyright 2021, John Wiley & Sons. (C): (ⅰ) Image of the PeNCs-SiO₂ family exposed to UV light at 365 nm and daylight; (ⅱ) PL spectra of the PeNCs-SiO₂ family; (ⅲ) PL intensity in UV light conditions changes. Reproduced with permission [56]. Copyright 2021, Royal Society of Chemistry. (D): (ⅰ) TEM images of CsPbBr₃-SiO₂ NCs at different calcination temperatures; (ⅱ) Schematic and TEM images of CsPbBr₃-SiO₂ NCs obtained by internal sintering and external sintering of K₂CO₃ at 600 ℃, respectively; (ⅲ) PLQY of ultrasound in water; (ⅳ) PL decay traces of the samples; (ⅴ) Normalized PL strength of internal K₂CO₃ sintered CsPbBr₃-SiO₂ NCs collected during 25-100 ℃ heat treatment (top) and 25-300 ℃ annealing (bottom) in air. Reproduced with permission [58]. Copyright 2022, American Chemical Society.

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  Figure 6  Application of silica-coated QDs in color conversion-based LED devices. (A): (ⅰ) Structure of WLED; (ⅱ) EL spectrum of the WLED device; (ⅲ) The chromaticity diagram for the blue, green, and red CsPbX₃@SiO₂ NC films; (ⅳ) Working stability of WLED devices; (ⅴ) The scalable and bendable light-emitting CsPbX₃@SiO₂ NC films were deposited on flexible substrates with a larger area or textile fibers. Reproduced with permission [64]. Copyright 2022, Elsevier. (B): (ⅰ) EL spectra of CsPbX₃@AGs@PS composite film; (ⅱ) CIE chromaticity coordinates of the CsPbX₃@AGs@PS composites films prepared LED devices excited by blue backlight at 460 nm; (ⅲ) Commercial CdSe composite film display images; (ⅳ) CsPbX₃@AGs@PS composite film display images; (ⅴ) The stability of CsPbX₃@AG@PS was observed after 48 h of continuous backlight illumination. Reproduced with permission [65]. Copyright 2022, Elsevier. (C): (ⅰ) A schematic diagram of the deposition and patterning of the DBR layer; (ⅱ) The PL intensity of the glass with and without the DBR layer; (ⅲ) The micro-LED display's chromaticity diagram for the red, green, and blue QDPR pixels; (ⅳ) The blue micro-LED display's brightness percentage trends over 500 h at 85 ℃ and 90% relative humidity with and without QD-PR pixels. Reproduced with permission [68]. Copyright 2022, Elsevier.

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  Figure 7  Application of silica-coated QDs in EL-based LED. (A): (ⅰ) Device structure of QLED; Current density-voltage-brightness characteristics of QLED with (ⅱ) CsPbBr₃ QLED and (ⅲ) A-CsPbBr₃@SiO₂ QLED; (ⅳ) Current efficiency-voltage characteristics of QLED; (ⅴ) Operational stability of QLED; (vi) Storage test of all-solution-processed QLED. Reproduced with permission [71]. Copyright 2021, American Chemical Society. (B): Current density-brightness-voltage (ⅰ), current efficiency-EQE-brightness (ⅱ), EL spectrum (ⅲ), CIE chromaticity diagram (ⅳ), and lifetime (ⅴ) of CsPbBr₃ and CsPbBr₃/p-SiO₂ composite based QLED; (vi) A photograph of the CsPbBr₃/p-SiO₂ composite QLED. Reproduced with permission [72]. Copyright 2024, American Chemical Society.

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  Figure 8  Application of silica-coated QDs in solar cells. (A) Schematic diagram of the QD-LDS layer solar cells structure and working mechanism. (B) The J-V characteristics of solar cells without the QD-LDS layer and with a different QD-LDS layer in daylight. (C) Thermal imaging pictures of solar cells taken both in the dark and after 30 min in the sun. Reproduced with permission [81]. Copyright 2024, Elsevier.

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  Figure 9  Application of silica-coated QDs in photodetector. (A): (ⅰ) QD@APDEMS photodetector device structure; (ⅱ) Dark J-V curves and (ⅲ) EQE spectra of QD@APDEMS photodetector; (ⅳ) QD@APDEMS photodetector device responsivity (at −0.1 V bias); (ⅴ) Detectivity of QD@APDEMS photodetector (at −0.1 V bias). Reproduced with permission [87]. Copyright 2023, John Wiley & Sons. (B): (ⅰ) Illustration of photodetector structure; Responsivity (ⅱ), detectivity (ⅲ), the rise/decay time (ⅳ), and EQE (ⅴ) of photodetector. Reproduced with permission [89]. Copyright 2024, Royal Society of Chemistry.

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