Progress in blue colloidal quantum dots for display applications (2025)

KEYWORDS:

• Quantum dots
• blue emission
• QLED
• electroluminescence
• display

2.1.1. CdSe

By adjusting the size of CdSe QDs, the emission spectrum can be tuned to cover the blue, green, and red regions. Typically, blue-emissive CdSe QDs require a size of less than 2 nm (Figure(a)) [Citation10]. However, a study by Qu etal. showed that CdSe seeds with a size of 1.75 nm forms within 4 ms after injection, and rapidly grows to over 2 nm (with absorption over 460 nm) within the following 2 s [Citation11]. As a result, blue-emissive CdSe QDs are generally more difficult to synthesize, with poor mono-dispersity and lower efficiency [Citation11,Citation12].

Several methods have been developed for synthesizing blue-emissive CdSe QDs. For instance, Pan etal. reported an autoclave-assisted synthesis method to apply high temperature and pressure, resulting in the formation of a 1.5 nm CdSe core with a CdS shell of varying thickness. While blue emission was successfully achieved from the small-sized CdSe QDs, the CdS shell enhanced the photoluminescence quantum yield (PLQY) but also caused a significant redshift of the emission wavelength, with the final range spanning 445–517 nm [Citation13]. Such redshift has also been reported for green–red CdSe/CdS QDs due to the epitaxial growth of CdS shell rather than alloying [Citation14]. Liu etal. utilized an oxidative etching method and achieved a blueshift of the emission peak from 546 to 466 nm, although the PLQY remained low at 10% [Citation15]. Overall, achieving blue emission from small CdSe QDs poses a significant challenge, particularly during the nucleation process and shell coating. In addition, the properties of blue-emissive CdSe QDs, such as their mono-dispersity, emission linewidth, and efficiency, are non-superior when compared to other methods capable of producing blue emission.

CdSe-based low-dimensional nanostructures provide another feasible route to achieve blue emission. The emission properties of CdSe nanoplatelets (NPLs) are closely related to their layer count, specifically, 3- to 3.5-layer CdSe NPLs exhibit blue PL emission peaks at ∼462 nm (Figure(b)) [Citation16]. Delikanli etal. reported 3-monolayers CdS/CdSe core/crown NPLs emitting at 456–462 nm, with a full width at half maximum (FWHM) of 10–14 nm [Citation17]. Additionally, CdSe nanorods can also achieve blue emission. Mallem etal. reported CdSe QDs within CdZnS/ZnS rods, exhibiting blue emission with a peak at 472 nm, a FWHM of 39 nm, and an EQE of 1.3% of the QLED [Citation18].

2.1.2. CdS

CdS QDs also exhibit size-dependent wavelength tunability [Citation10]. Yu etal. demonstrated that as the CdS QD size increased from 2.0 to 5.3 nm, the PL peak shifted from 375 to 460 nm [Citation19]. CdS QDs typically show defect-related emission at lower energies, which is believed to arise from the presence of S vacancies on the surface of CdS, acting as electronic defects. Such defect-related emission becomes more pronounced as the size of CdS QDs increases [Citation20], and it can be suppressed by coating an outer shell such as ZnS [Citation21]. Steckel etal. reported CdS/ZnS QDs exhibiting blue emissions ranging from 460 to 480 nm with PLQYs of 20–30%. These QDs featured CdS cores with sizes between 4.7 and 5.2 nm. In addition, they demonstrated EL device with an EQE of 0.1% [Citation22]. Chen etal. synthesized CdS/ZnS QDs with an emission range of 375–475 nm using a thermal-cycling coupled single precursor method. Notably, CdS-based core–shell QDs exhibit relatively low PLQY (∼50%) in the blue region, which may be attributed to defect-induced non-radiative recombination. Furthermore, although the PL peak redshifts to the pure blue region as the size of CdS QDs increases, the quantum confinement effect weakens, resulting in significant broadening of the FWHM (>30 nm), which hinders their further EL application of CdS QDs [Citation21].

2.1.3. CdZnSe

CdZnSe alloyed QDs exhibit excellent spectral tuning capability with high PL and EL efficiency, and high device stability (Figure(c)) [Citation7,Citation23–26]. Blue QLEDs based on the CdZnSe QDs have demonstrated EQE exceeding 20%, along with a record T₉₅ lifetime of 227 h [Citation9].

The synthesis of CdZnSe alloyed QDs typically involves two main methods: direct nucleation and ion exchange. In the direct nucleation approach, Se precursors are injected into a mixed precursor of Zn and Cd, and the precise control over the reactivity of Zn and Cd is crucial for achieving blue emission. Alternatively, a phosphine-free method can be used, where mixed Zn and Cd precursors are injected into a Se precursor solution [Citation27]. Ion exchange typically initiates with the formation of ZnSe nuclei, followed by the injection of Cd precursors to facilitate the exchange and alloying processes between Cd and Zn [Citation28]. Alloying of CdSe/ZnSe QDs through cation exchange has also been reported [Citation23].

Core–shell alloying is a key research focus in CdZnSe-based QDs. Since Cd is more reactive than Zn, the direct injection of Se precursor into a mixed Cd and Zn precursor usually leads to a preferential reaction between Cd and Se, resulting in a gradient structure of CdSe/Cd_xZn_1–xSe/ZnSe [Citation29]. Recently, Gao etal. demonstrated a one-step method for fabricating large-size red emissive Cd_xZn_1–xSe QDs with a full-gradient structure [Citation7]. Although the relationship between gradient alloying and PL or EL properties has not been reported for blue emission, this study provides valuable insights into achieving a fully gradient alloyed structure by leveraging the difference in reactivity between Cd and Zn, offering a promising approach to enhance device performance.

The size and shell design of CdZnSe-based blue QDs are also crucial factors influencing their PL and performance. Research by TCL reveals that surface-bulk coupling critically influences blue QLED performance. Their findings demonstrate that enlarging the CdZnSe core size not only optimizes defect passivation but also spatially separates the exciton wavefunction from surface states, effectively suppressing surface-bulk interactions. For shell engineering, while conventional gradient shells with high energy barriers enhance exciton confinement and compensate for core-size-induced redshift, they often compromise charge injection efficiency. To address this, the team proposed a multi-shell architecture incorporating intermediate CdZnSeS/ZnSeS subshells to protect excitons from surface charges, resulting in emission blue-shift. Furthermore, the strategic addition of an outer CdZnS shell reduces charge injection barriers while maintaining lattice integrity and minimizing non-radiative recombination. By utilizing a large core with a non-monotonic gradient shell, CdZnSe-based blue QDs achieve a high QY exceeding 90%, while the corresponding QLEDs demonstrate an EQE of over 20% and record T₉₅ lifetimes ranging from 75 to 227 h [Citation9].

2.1.4. CdZnS

CdZnS is a material class known for its narrow emission linewidths and high PL and EL efficiencies in the deep blue region (Figure(d)) [Citation30]. High-efficiency CdZnS-based QLEDs include both wurtzite- and sphalerite-phase CdZnS QDs [Citation31]. The PL and EL properties of these QDs exhibit an initial increase followed by a decrease with spectral redshift. Therefore, the optimal PL and EL performance typically achieved when the emission peaks are positioned below 455 nm [Citation31–34]. A recent study on high-performance CdZnS-based QLEDs have demonstrated a pure-blue EL at 458 nm by using ∼10 nm sphalerite-phase CdZnS/ZnS QDs as the emission layer. These devices achieve a maximum EQE of 23% with a T₉₅ lifetime of 87 h [Citation35].

Similar to CdS QDs, CdZnS QDs typically exhibit defect-related emission at lower energies, which can be suppressed through shell coating [Citation31,Citation33]. However, the selection of suitable shell materials for CdZnS QDs remains limited. Since the commonly employed ZnSe shell will form a type-II heterostructure with CdZnS and lead to altered PL characteristics, ZnS remains the preferred shell material for CdZnS QDs [Citation36,Citation37]. According to a study by Wang etal., the incorporation of Cd_xZn_1–xS interlayers between the CdZnS core and an ultrathin ZnS shell effectively reduces lattice mismatch, thereby improving charge injection efficiency and providing effective exciton confinement. This approach enables the achievement of high PL and EL efficiencies, with a PLQY approaching 100% and an EQE of 18% [Citation31].

2.1.5 CdSeS

CdSeS QDs, a ternary alloy of CdSe and CdS, are another promising material for achieving blue emission. In 2013, Aubert etal. introduced the homogeneously alloyed CdSe_1–xS_x QDs covering a spectral range from blue to green (Figure(e)) [Citation38]. As the S ratio increases, the spectrum of the CdSe_xS_1–x QDs undergoes a blueshift. Similar to CdS QDs, CdSeS QDs exhibit defect-related emission at lower energies.

Notably, while there are few reports on the synthesis of CdSeS, it has demonstrated excellent performance in QLEDs. In 2020, Pu etal. reported blue QLEDs based on CdSeS/ZnSeS/ZnS QDs, achieving an EQE of ∼10% and exhibiting impressive stability, with a T₅₀ lifetime of over 10000 h [Citation39]. In 2023, Gao etal. reported large-sized CdSeS/ZnSeS QDs with a PLQY of over 70%. The resulting QLEDs achieved an EQE of 10% and a T₉₅ lifetime of 65 h [Citation7].

2.1.6. CdZnSeS

Quaternary CdZnSeS QDs enable full-spectrum emission tuning through precise control of the Cd/Zn and Se/S compositional ratios (Figure(f)) [Citation29,Citation40]. The synthesis of blue-emissive CdZnSeS QDs is typically carried out through phosphine-free methods, where Se and S precursors are injected into Cd and Zn precursors [Citation41]. Due to the varying reactivity of the four elements (Cd > Zn, Se > S), recent reports suggest that this type of QDs exhibit a gradient structure, where the core is enriched with Cd and Se while the intermediate shell consists of an alloyed Cd_1–yZn_xSe_1–yS_y composition. The gradient composition can be precisely adjusted by modifying the precursor ratios, forming either CdS-rich or ZnSe-rich regions. The outermost shell is ZnS-rich, and its growth is regulated through subsequent reactions. This gradual transition from the core to the outer shell effectively reduces strain induced by lattice mismatch, ultimately enhancing the PLQY [Citation29]. The varying reactivity of the four elements presents significant challenges in achieving precise alloying and size control of quaternary QDs. Consequently, there are relatively few reports on high-efficiency and stable devices based on quaternary QDs, and most current research remains focused on ternary QDs.

2.2.1. ZnSe

To comply with RoHS standards for environmentally friendly materials, it is essential to develop the synthesis of Cd-free QDs. ZnSe, with a large band gap (2.7 eV) and a narrow emission linewidth, is considered one of the promising Cd-free materials for blue emission. However, due to the quantum confinement effect, ZnSe QDs typically exhibit deep blue emission (<440 nm) [Citation42,Citation43]. While a high PLQY of over 70% can be achieved, their PL stability is relatively poor [Citation44], and the EL efficiency remains low (<8%) [Citation45,Citation46]. These limitations hinder the broader application of ZnSe-based QDs.

To achieve pure blue emission, it is essential to synthesize large-size ZnSe QDs with weak confinement. Jang etal. successfully synthesized a 12.2-nm ZnSe core through alternating injection of Zn and Se precursors, which resulted in a PL emission of 443 nm with a narrow FWHM of only 13 nm [Citation47]. Similarly, through optimized alternating precursor injection conditions, Gao etal. synthesized 10-nm ZnSe QDs with a QY of 95% and a FWHM of only 9.6 nm after coating a ZnS shell. The resulting QLEDs achieved a record EQE of 12.2% at 445 nm [Citation48].

To achieve pure blue emission with the wavelength exceeding 450 nm, the synthesis of substantially larger ZnSe QDs is required. However, this approach faces inherent limitations imposed by the fundamental constraints of QD growth. A study by Li etal. revealed that the growth of ZnSe QDs is constrained by precursor reactivity. Their study identified that alkylamine additives significantly enhance the activation of Zn precursors, thereby promoting ZnSe QD growth and improving PLQY [Citation49]. However, despite these advancements, the attainable size of ZnSe QDs remained constrained to ∼10 nm.

In our recent investigation (Figure), we developed a nucleation model based on LaMer theory, demonstrating that higher precursor reactivity results in a larger critical nucleation radius, a smaller number of nucleation events, and consequently a larger final QD size. Building on this model, we developed a reactively-controlled epitaxial growth (RCEG) method. Using highly reactive Se and Zn sources, we successfully synthesized monodisperse ZnSe QDs of ∼5 nm in size. By continuously injecting low-reactivity Zn and Se precursors, ZnSe QDs with an average size of 35 nm were obtained, which exhibited pure blue emission in the range of 455–470 nm [Citation50,Citation51].

Additionally, Peng’s group demonstrated that the entropy ligand strategy is an effective approach to address the solubility reduction associated with the increase in QD size. They successfully applied this strategy in the synthesis of large-size CdSe QDs (∼30 nm) [Citation52,Citation53]. This work provides a new perspective for the synthesis of large-size ZnSe QDs.

During the synthesis of large-size ZnSe QDs using highly reactive Zn and Se precursors, side reactions frequently occur, leading to the formation of Se-rich surfaces and oxygen-related defects. These defects subsequently result in a reduction in PLQY as the size increases. To address this challenge, our group developed an etching strategy by utilizing potassium fluoride and myristic acid, which significantly enhanced the PLQY of ZnSe/ZnS QDs to over 90% with an emission peak at 450 nm. The QLEDs based on these QDs demonstrated a maximum EQE of 4.2% at 456 nm [Citation54].

It is important to note that the luminance, efficiency, and stability of ZnSe-based QLEDs currently lag behind those of Cd-based QLEDs and ZnSeTe-based QLEDs, with a record T₅₀ lifetime of only 237 h [Citation48]. Improving the performance of ZnSe-based QLEDs requires not only controlling the defects and enhancing PL efficiency during the synthesis of ZnSe QDs, but also addressing the underlying issues affecting their EL performance. Further exploration is needed to clarify the underlying mechanism of these limitations.

2.2.2. ZnSeTe

Incorporating Te into ZnSe is another strategy to regulate the emission to the pure blue region [Citation55]. A small amount of Te in ZnSeTe can redshift the PL spectrum, enabling pure blue emission with a Te content below 10% (Figure(a)). However, most ZnSeTe QDs without shell coating exhibit low PLQY. Moreover, as the Te content increases, the PL spectrum becomes asymmetrical, showing a pronounced tailing effect at lower energies. For instance, in the report by Kim etal., as the Te content of ZnSeTe increased from 0% to 3.3%, 6.7%, and 10%, the PL peak gradually shifted from 448 to 452, 457, and 474 nm, while the FWHM broadened from 15 to 22, 36, and 40 nm, respectively [Citation56].

The origin of spectral asymmetry and tailing has been extensively discussed in several reports, with the primary cause attributed to the recombination of localized hole states resulting from the inhomogeneous distribution of Te within the ZnSe lattice [Citation55,Citation57,Citation58]. During the synthesis of ZnSeTe QDs, Te tends to exist in the form of clusters within the ZnSe lattice due to its significantly higher reactivity compared to Se [Citation55]. To address this issue, Hoogland etal. enhanced the homogenization of Te within the ZnSeTe lattice by introducing phenylcarbonyl fluoride into the synthesis. This facilitated the controllable generation of hydrofluoric acid (HF), which promote the homogeneous distribution of Te. As a result, PL emission with a peak of 457 nm and a narrow FWHM of 22 nm was achieved, exhibiting spectrally symmetrical and stable emission [Citation59]. The introduction of HF can also facilitate surface-specific growth, which promotes to repair defects, resulting in a narrower PL emission with a peak of 448 nm, a FWHM of 14 nm, and a PLQY of 97% [Citation60]. Additionally, Yu etal. enhanced the balance of reactivity between Te and Se by using biphenyl-Te, a less reactive Te precursor than commonly used Te-TOP precursor. This strategy, combined with a quasi-type-II structure design (Figure(b)), resulted in a PL emission with a FWHM of only 19 nm at 455 nm [Citation61]. In addition, control of shell growth to reduce exciton-longitudinal optical phonon coupling, along with the elimination of lattice distortion, are also effective strategies for narrowing the emission linewidth [Citation62,Citation63].

Overall, ZnSeTe QDs emerge as one of the best Cd-free materials for blue emission with a PLQY approaching 100%. The ZnSeTe-based bottom-emitting QLEDs achieved a high EQE exceeding 20% and high stability comparable to that of Cd-based QDs, with a T₉₅ lifetime exceeding 200 h [Citation56,Citation64]. Significant progress has also been achieved in ZnSeTe-based top-emitting QLEDs [Citation65].

2.2.3. InP

InP QDs exhibit size-dependent PL emission similar to CdSe, covering the visible wavelength range of 390–720 nm for sizes between 1 and 8 nm. After the growth of ZnS shells, the spectral range extends to 450–750 nm (Figure(c)) [Citation66,Citation67]. Due to their small size, InP QDs are a promising material for achieving blue emission. However, InP QDs has a larger Bohr exciton radius compared to CdSe (InP ∼9.6 nm; CdSe ∼5.6 nm) [Citation11,Citation68], making PL emission highly sensitive to their size. As a result, blue emission from InP QDs typically requires the size to be limited to ∼2 nm.

However, the high reactivity of the precursors makes it challenging to synthesize small-sized InP QDs with uniform sizes. The narrow linewidth (∼22 nm) of the InP single-dot spectrum demonstrates that the maximum limitation of the emission linewidth in InP QDs is related to the homogeneity of their size [Citation68]. Currently, methods used to control the reaction rate of InP QDs primarily involve the introduction of Zn [Citation69] and halogen ions [Citation70,Citation71]. Additionally, due to their small size and poor dimensional uniformity, Förster resonance energy transfer (FRET) between QDs is preferred to occur, which significantly reduces the PLQY when the QDs are deposited from solution to the film [Citation72]. This degradation in PLQY adversely affects their EL properties. Furthermore, the increased specific surface area of small-sized InP QDs may exacerbate the effects of surface-related defects, such as phosphorus oxide (PO_x) defects, further impacting their performance [Citation73–75].

To increase the size of QDs and passivate surface defects, the main strategy involves coating a thick shell on small InP cores. Although the lattice mismatch between ZnSe and InP is relatively small (3.4%), it cannot provide type-I confinement. ZnS also can be used as a shell material, which offers stronger confinement for InP, but the significant lattice mismatch between ZnS and InP (7.8%) may lead to lattice strain and potential defects [Citation76]. The issue of lattice mismatch can be addressed by manipulating the core size and introducing ZnSeS intermediate shells in red and green InP QDs [Citation74,Citation77,Citation78], however, for the small InP QDs required for blue emission, the redshift of the emission peaks due to electron delocalization becomes prominent. As a result, ZnS is typically the only suitable shell in these cases [Citation79]. The lattice mismatch between ZnS and InP and the defects caused by the strain accumulation may be important factors limiting the PL and EL performance of blue-emissive InP QDs, including low QY and large FWHM [Citation80,Citation81]. In addition, the thick ZnS shell is also a strong barrier to charge injection in QLED [Citation31]. All of these factors contribute to the poor PL and EL properties of InP-based QDs.

To address the challenges of ZnS shell coating on InP cores, Kim etal. and Zhang etal. introduced GaP as a transition layer to reduce the lattice mismatch. This not only mitigates the strain but also serves as an energy level barrier that enhances the PL and EL properties of blue InP-based QDs [Citation82,Citation83]. Doping Ga into InP can lead to a blueshift [Citation84]. Moreover, Zhang etal. utilized a wide-bandgap middle layer ZnMnS to further attenuate the lattice mismatch and to confine the exciton within the InP core, thus improving the performance [Citation70].

Overall, various strategies were developed to address the controllable synthesis of blue-emissive InP QDs including size control, effective shell confinement, and lattice mismatch reduction, and the PLQY of InP QDs has reached over 80% [Citation72,Citation80,Citation83,Citation85]. However, the performance of QLEDs based on InP QDs still lags significantly behind that of ZnSe or ZnSeTe QDs, with the current reported EQE being only 2.6% [Citation86].

2.2.4. GaN

GaN, with its wide bandgap of 3.4 eV, is an ideal material for blue light emission. It is commonly doped with elements such as In to enhance its emission properties. GaN-based LEDs and lasers are widely recognized for their superior electron mobility, high luminance, and remarkable stability, which make them indispensable in optoelectronic applications. GaN’s impact on the field of optoelectronics was recognized with the 2014 Nobel Prize in Physics. However, the synthesis of GaN presents challenges, particularly in the form of QDs. GaN is typically synthesized via vapor deposition at high temperatures (above 800 °C), which complicates the development of GaN QDs with high optical performance under lower temperature conditions. This is due to the lack of suitable N precursors and the stringent reaction conditions required for GaN growth.

In the early development of colloidal GaN QDs, Mićić etal. synthesized spherical GaN QDs of ∼3 nm in size by pyrolyzing a {Ga(NH)_3/2}_n polymer in trioctylamine at 360 °C. These QDs exhibited emission at 370 nm, but the emission did not show the typical band-edge characteristics of quantum confinement [Citation87]. Similarly, Manz etal. synthesized GaN QDs by pyrolyzing Ga azide, although their emission also lacked the quantum confinement features [Citation88]. A significant breakthrough in the hot-injection synthesis of GaN QDs was not achieved until 2019, when Choi etal. reported their findings. By using lithium bis(trimethylsilyl) (LiHMDS) as the N source and GaCl₃ as the Ga source in octadecene (ODE), they were able to synthesize GaN QDs with tunable PL band-edge emission in the range of 315–355 nm. Furthermore, by doping with Zn, the PL peak was shifted to 460 nm, demonstrating the potential for tuning the emission properties of GaN QDs (Figure(a)) [Citation89]. In addition, Zheng etal. successfully synthesized O-doped GaN QDs (∼3.8 nm in size) with a PL emission at 440 nm and a PLQY of 14.3%. They achieved this synthesis by using lithium bis(trimethylsilyl)amide (LiDHS) as the N precursor and GaCl₃ as the Ga precursor in amines with high boiling points such as octadecylamine (ODA), oleamine (OAm), and dodecadecamine (HDA) (Figure(b, c)) [Citation90].

Recently, Ondry etal. developed a molten salt decomposition method for synthesizing group III–V GaP and GaAs QDs, enabling higher reaction temperatures that address the limitations of conventional low-temperature approaches [Citation91]. This breakthrough offers a promising pathway for future GaN QD synthesis.

2.2.5. I-III-VI QDs

The I–III–VI brass-based compounds, with the general formula of ABX₂ (A: Cu⁺ or Ag⁺; B: Ga³⁺ or In³⁺; X: S²^– or Se²^–), exhibit exceptional compositional flexibility for engineering ternary and quaternary QDs [Citation92,Citation93]. One of the key advantages of these compounds is their adjustable band gap, which can be tuned to suit a variety of optoelectronic applications. However, one significant challenge in using I–III–VI compounds, particularly in QD applications, is their tendency to exhibit over-wide emission peaks. This issue stems from the random distribution of Cu/Ag-related acceptor centers within the QD structure. These acceptor centers lead to significant variation in the Coulombic interaction between the delocalized conduction band (CB) electrons and the localized holes [Citation94]. Additionally, strong electron–phonon coupling exacerbates the problem, which is partly due to the intrinsic presence of deep and localized trap states [Citation94,Citation95].

CuGaS₂-based QDs typically exhibit sky-blue or turquoise emission (Figure(a, b)) [Citation96,Citation97], while CuInS₂ QDs emit in the orange and red region [Citation97,Citation98]. Both types of QDs can achieve high PLQY of over 70% with the addition of Zn doping [Citation99,Citation100]. However, one limitation is that the FWHM for both CuGaS₂ and CuInS₂ QDs tends to exceed 80 nm, which significantly reduces their suitability for high-resolution applications. To address this issue, Niu etal. made a breakthrough by adjusting the nucleation temperature during the synthesis of CuGaS₂ QDs, resulting in a reduced FWHM to only 29 nm with an emission peak at 475 nm. This improvement in spectral narrowing was attributed to band-to-hole recombination [Citation101].

AgGaS₂-based QDs exhibit inherent green emission at 515 nm, which shifts to blue region (∼450 nm) through Zn doping, achieving a PLQY of 58–69% after shell coating (Figure(c, d)) [Citation102]. AgIn_xGa_1–xS₂ QDs, with a high In content, represent an alternative to achieve blue emission. Lee etal. reported nearly unity PLQY via homogeneous nucleus and shell growth, achieving a narrow FWHM of 30 nm, demonstrating excellent spectral purity suitable for display applications [Citation103].