Synthesis of highly dispersible TiO₂ nanoparticles and their application in quantum dot light emitting diodes

Abstract

Metal oxide nanoparticles are commonly used as electron transport layers (ETLs) in quantum dot light emitting diodes (QLEDs) because of their wide band gap, high electron mobility, and appropriate conduction and valence band positions. Currently, nanoparticulate ZnO is the most successful electron transportation material in high-performance QLEDs. However, the positive aging effect is widely observed for ZnO-based QLEDs, as the instability of amphiprotic ZnO nanoparticles under acidic, basic, and moist conditions limits their applications. In this study, highly dispersible and alcohol-soluble TiO₂ nanoparticles are synthesized by using a non-hydrolytic sol–gel method, followed by a dimethyl sulfoxide post-treatment. The use of colloidal TiO₂ nanoparticles as an ETL yields optimal QLEDs, with a maximum external quantum efficiency of 12.03%, a highest luminance value of 103 420 cd m⁻², and a current efficiency of 18.06 cd A⁻¹. These results reveal that TiO₂ nanoparticles hold great potential as ETLs in future QLEDs.

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1. Introduction

Colloidal quantum dots (QDs) are widely used in optoelectronic devices due to their desirable features, including narrow emission bandwidth, tunable emission color, high quantum yield, excellent photostability, and solution processability.^1–9 Quantum dot light-emitting diodes (QLEDs) are considered to be a highly promising technology in next-generation lighting and displays.^10–12 The performance of QLEDs has been significantly improved since 1994.¹³ Conventional QLED devices are composed of a sandwich structure: cathode, electron transport layer (ETL), QD emitting layer, hole transport layer (HTL), and anode. The ETL is one of the most crucial components of the QLED device. The electron mobility, conduction and valence band positions, and band gap of the ETL significantly influence the performance and stability of the device.^14–19 Wide band-gap metal oxides, such as ZnO, TiO₂, and SnO₂ nanoparticles, have been widely used as the ETLs in QLED devices. In 2011,²⁰ inorganic ZnO nanoparticles were first used as ETLs in QLEDs due to their high electron mobility, high transmittance and tunable energy levels.^21,22 Currently, high-performance QLEDs with an EQE exceeding 20% are being successfully fabricated based on ZnO-based ETLs. However, the positive aging effect is widely observed for ZnO-based QLEDs, as the instability of amphiprotic ZnO nanoparticles under acidic, basic, and moist conditions limits their applications.^23–25 Additionally, the high electron mobility of ZnO nanoparticles will result in the injection of excess electrons and a charge transportation imbalance, thereby reducing the performance and stability of QLEDs.²⁵ Recently, thermally stable and moisture-insensitive TiO₂ nanoparticles have been considered as one of the most promising alternatives to ZnO nanoparticles.^26–29 Furthermore, the wide band gap of TiO₂ nanoparticles and the energy level of the conduction band bottom facilitate the injection of electrons, making TiO₂ nanoparticles promising candidates for ETL materials.^30–33

TiO₂ is an important indirect and wide band gap semiconductor that has been widely applied in various optoelectronic devices, including QLEDs,^30–33 thin film solar cells,^34,35 and GaN-based LEDs.^36–38 In 2021, Wei et al. synthesized oleic acid-capped TiO₂ nanocrystals by a two-phase approach.³⁰ Oil-soluble TiO₂ nanocrystals were turned into alcohol-soluble ones by post-treatment with thionyl chloride (SOCl₂), and were used as the ETL to fabricate QLEDs having long-term air-stability. Kim et al. used Li-doped TiO₂ nanoparticles as the ETL to prepare highly efficient and green QLEDs with a maximum brightness of 169 790 cd m⁻² and an EQE of 10.27%.³¹

However, the performance of QLEDs based on TiO₂ nanoparticles is still significantly poorer than that of state-of-the-art ZnO nanoparticle-based QLEDs.⁴ A large number of –OH groups and high concentration of oxygen vacancies on the surface of TiO₂ nanocrystals result in severe fluorescence quenching of the quantum dots and deterioration of device performance.^39,40 Most traditionally synthesized TiO₂ nanoparticles are produced using the hydrolysis sol–gel process or the hydrothermal method.^41–43 These methods usually require high temperatures to induce crystallization. In this work, we employed a typical non-hydrolytic sol–gel method to prepare TiO₂ nanoparticles.⁴⁴ By controlling the reaction conditions, TiO₂ nanoparticles with controllable particle sizes can be synthesized. Note that these as-prepared TiO₂ nanoparticles cannot be dispersed in ethanol, which means uniform and flat TiO₂-nanoparticle thin films cannot be fabricated. Therefore, dimethyl sulfoxide (DMSO) was used to improve the solubility of TiO₂ nanoparticles in ethanol by a post-treatment process. As a result, DMSO-capped TiO₂ nanoparticles not only reduce hydroxyl group content on the surface of TiO₂ nanoparticles, but also inhibit fluorescence quenching of quantum dots, thereby enhancing device efficiency and brightness. The use of DMSO-capped TiO₂ nanoparticles as the electron transport layer yields a maximum brightness of 106 789 cd m⁻², a current efficiency of 18.01 cd A⁻¹, and an EQE of 12.08% for red-inverted QLED devices. These results suggest that TiO₂ nanoparticles have a significant potential to replace ZnO nanoparticles as the electron transport layer in QLEDs.

2. Experimental section

2.1 Chemicals

Molybdenum trioxide (MoO₃, 99.95%) and anhydrous ethanol (GR, 99.8%) were purchased from Aladdin Inc. 4,4′-N,N′-Dicarbazole-biphenyl (CBP, 99.5%) was bought from PolyLumTec Inc. Titanium tetrachloride (TiCl₄, 99.9%) and dimethyl sulfoxide (DMSO, 99%) were purchased from Macklin Inc. Red-emitting CdSe/ZnSe/ZnS core/shell/shell quantum dots (10 mg mL⁻¹ in hexane) was provided by Pujiafu OptoElectronics Co. Ltd. All chemicals and solvents were used directly without any further purification.

2.2 Synthesis of TiO₂ nanoparticles

TiO₂ nanoparticles were synthesized using a non-hydrolytic sol–gel method with some modifications.⁴⁴ First, 50 mL of benzyl alcohol was added to a conical flask. Then, 0.54 mL of titanium tetrachloride was slowly dropped into the benzyl alcohol solution under magnetic stirring. The mixture was stirred for 15 minutes, and then placed in an oil bath at 130 °C for different reaction times. After the reaction, the conical flask was cooled to room temperature. The white suspension was then transferred to a centrifuge tube. The mixture was centrifuged, and the supernatant was decanted to collect TiO₂ nanoparticles.

2.3 Preparation of DMSO-capped TiO₂ nanoparticle solutions

TiO₂ nanoparticles were dispersed in 30 mL of anhydrous ethanol, and 3 mL of DMSO was added. The solution was then treated in an ultrasonic bath until a clear and transparent solution was formed. Ethyl acetate was added to precipitate the DMSO-capped TiO₂ nanoparticles, followed by a centrifuge process. Finally, the DMSO-capped TiO₂ nanoparticles were dispersed in anhydrous ethanol to obtain a solution of approximately 25 mg mL⁻¹ for QLED device fabrication.

2.4 Fabrication of QLEDs

An inverted QLED device consists of ITO (150 nm), TiO₂ (35 nm), QDs (20 nm), CBP (40 nm), MoO₃ (7 nm), and Al (100 nm), respectively. First, ITO was ultrasonicated in deionized water and ethanol, respectively. Afterwards, the ITO was dried under a nitrogen flow. Then, the TiO₂ nanoparticle solution was spin-coated onto the ITO substrate at 2500 rpm for 20 s, followed by an annealing process at 150 °C for 15 minutes. Next, the QD solution was spin-coated onto the TiO₂ nanoparticle thin film at 3000 rpm for 20 s and was annealed at 100 °C for 10 minutes. CBP (40 nm), MoO₃ (7 nm), and Al electrodes (100 nm) were subsequently thermally evaporated.

2.5 Characterization studies

X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Advance diffractometer. The UV-vis absorption spectrum of TiO₂ nanoparticles was obtained on a Metash 5200 spectrophotometer. Transmission electron microscopy (TEM) images of TiO₂ nanoparticles were acquired using an FEI Tecnai G2 F20 instrument. Scanning electron microscopy (SEM) images were acquired using a high-resolution field emission scanning electron microscope (Hitachi, SU820). Fourier transform infrared (FT-IR) spectra were recorded using a Nicolet 50 model. The Fermi level and valence band maximum (VBM) of the TiO₂ nanoparticle thin film were measured using an ESCALAB 250XI+ UPS with a He I (21.21 eV) photon source. The current–voltage–luminance curves and current efficiency–voltage–external quantum efficiency curves of the QLED devices were simultaneously recorded using a commercial QLED measurement system (XPQY-EQE-Adv, Guangzhou Xi Pu Optoelectronics Technology Co., Ltd). It should be noted that the QLEDs were manufactured and characterized in air without encapsulation.

3. Results and discussion

TiO₂ nanoparticles, as an important metal oxide material, have been extensively studied since their discovery. There are various methods for preparing TiO₂ nanocrystals. In this study, we employed a previously reported non-hydrolytic sol–gel method with some improvements.⁴⁴ The TiO₂ nanocrystals synthesized using this method offer several significant advantages, including their large-scale synthesis, air stability, extremely small size, and narrower size distribution. We experimentally demonstrated that TiO₂ nanoparticles synthesized by the non-hydrolytic sol–gel method can be highly dispersed in ethanol by a DSMO post-treatment process and the hydroxyl group content on the TiO₂ nanoparticle surface is reduced, thereby inhibiting quantum dot PL quenching. These improved properties enable high-performance QLEDs based on a TiO₂ nanoparticle ETL.

All TiO₂ nanoparticles in this study were synthesized at low temperature without high-temperature post-annealing treatment. Fig. 1a and b show low-resolution TEM (LR-TEM) and high-resolution TEM (HR-TEM) images of DMSO-capped TiO₂ nanoparticles synthesized at 130 °C. A low-magnification overview image reveals that the product consists of discrete TiO₂ nanoparticles, with no larger agglomerates formed (see Fig. 1a). The high-resolution TEM image of the TiO₂ nanoparticle displays clear lattice streaks (see Fig. 1b), indicating that TiO₂ nanoparticles are highly crystalline. Based on the TEM images, the particle boundaries are clearly visible, and the particles exhibit a fairly uniform size and shape. The corresponding histogram of particle size distribution was generated (see Fig. 1c). The average particle size of TiO₂ nanocrystals was found to be 5.1 nm. This demonstrates that our final TiO₂ nanoparticles have extremely small size, narrow size distribution, and uniform shape. The X-ray diffraction (XRD) pattern of TiO₂ nanoparticles synthesized at 130 °C is shown in Fig. 1d. Specifically, the characteristic XRD peaks at 25.3, 37.8, 48.1, 55.6, and 62.7° correspond to diffraction from the (101), (004), (200), (211) and (204) crystal planes, respectively. These XRD peaks match those of the standard anatase structure of TiO₂ (JCPDS no. 21-1272). All peaks are attributed to the anatase phase, with no other peaks of impurities. Moreover, the broad diffraction peaks verify the small particle size of the as-synthesized TiO₂ nanoparticles. The particle size of the TiO₂ nanoparticles was determined by using the Scherrer diffraction formula, and the crystal size was calculated to be 4.9 nm, slightly smaller than the 5.1 nm size measured from the TEM image. These results suggest that the prepared TiO₂ nanoparticles have significant potential for use as the electron transport layer in QLEDs. Furthermore, we also investigated the effect of different reaction times on the particle size of the TiO₂ nanocrystals and found that TiO₂ nanocrystals can be produced in about 25 minutes at 130 °C in an oil bath. By controlling the reaction time, the particle size of TiO₂ nanoparticles can be adjusted from 4.4 to 5.9 nm, as shown in Fig. S1.† Raman spectra of as-prepared and DMSO-capped TiO₂ nanoparticles were recorded and are shown in Fig. S2.† The Raman characteristic peaks at 150, 397, 517, and 640 cm⁻¹ are observed, corresponding to the anatase phase of TiO₂. This result is consistent with that of the XRD analysis, confirming that the TiO₂ nanoparticles have pure anatase structure.

Fig. 1 (a) LR-TEM and (b) HR-TEM images; (c) the size distribution, and (d) XRD patterns of TiO₂ nanoparticles synthesized at 130 °C for 90 min.

Note that the as-prepared TiO₂ nanoparticles cannot be dispersed in ethanol. Therefore, DMSO was used to improve the solubility of TiO₂ nanoparticles in ethanol by a post-treatment process. DMSO-capped TiO₂ nanoparticles can be highly dispersed in ethanol, and a transparent and clear TiO₂ nanoparticle solution is formed, as shown in Fig. S3.† DMSO on the surface of TiO₂ nanoparticles has been confirmed by the FT-IR spectrum of TiO₂ nanoparticles (see Fig. S4†). Fig. 2a displays the UV-vis absorption spectrum of the TiO₂ nanocrystal solution, which exhibits an absorption peak at 292 nm and an absorption band edge at 375 nm. The optical band gap of TiO₂ nanocrystals was determined to be 3.93 eV from the UV-vis absorption curve, as shown in Fig. 2b. This value is significantly larger than the band gap value of bulk anatase TiO₂, which is 3.2 eV. Fig. S5 and S6† present a SEM image and an AFM image of the TiO₂ nanoparticle thin film, respectively. The TiO₂ nanoparticle thin film exhibits an average roughness (R_a) of 3.4 nm, which enables the deposition of smooth and flat TiO₂ thin film by a spin-coating process.

Fig. 2 (a) UV-vis absorption spectrum and (b) (αhv)²–hv plot (converted from the absorption spectrum) of DMSO-capped TiO₂ nanoparticles.

The Fermi level and valence band maximum (VBM) position were measured using ultraviolet photoelectron spectroscopy (UPS), as shown in Fig. 3. The VBM was calculated using the formula VBM = 21.21 − (E_cutoff − E_onset), where E_cutoff is the cut-off binding energy, and E_onset is the onset binding energy. The VBM of TiO₂ nanocrystals is calculated to be 7.87 eV below the vacuum energy level. Using the band gap value E_g = 3.93 eV taken from Fig. 2b, the conduction band minimum (CBM) energy level of the DMSO-capped TiO₂ nanoparticles is estimated to be 3.94 eV below the vacuum energy level. The CBM energy level (−3.94 eV) of TiO₂ nanoparticles matches the ITO substrate, thereby facilitating smooth electron transport from ITO to the TiO₂ nanoparticle ETL. Furthermore, its deep VBM energy level (−7.87 eV) can effectively block holes at the interface between the EML and the ETL, confining the electron and hole to the QD emitting layer.²⁹

Fig. 3 UPS spectrum of the secondary-electron cutoff and valence-band edge regions of TiO₂ nanoparticles.

The electron mobilities of TiO₂ and ZnO nanoparticles were determined using Child's law by fitting the space charge-limiting current (SCLC) region (J ∝ V²).⁴⁵Fig. 4a and b show the current density–voltage (J–V) characteristic curves of TiO₂-based and ZnO-based nanoparticle films (ITO/TiO₂/Al and ITO/ZnO/Al). Using the equation from the literature,⁴⁵, and assuming ε_r(TiO₂) = 85 and ε_r(ZnO) = 4, the electron mobilities of TiO₂-based and ZnO-based nanoparticles were calculated as being μ = 3.77 × 10⁻⁵ cm² V⁻¹ s⁻¹ and μ = 9.15 × 10⁻³ cm² V⁻¹ s⁻¹. We also tested the band gap, conductivity, and hole mobility of CBP, as shown in Table 1. The mobility and conductivity of CBP are 5.78 × 10⁻⁵ cm² V⁻¹ s⁻¹ and 1.22 × 10⁻⁹ S cm⁻¹, respectively. Note that the electron mobility of ZnO nanoparticles is higher than the hole mobility of CBP by two orders of magnitude, which indicates a charge transportation imbalance in the ITO/ZnO/QD/CBP/MoO₃/Al device. It was found that the electron mobility of TiO₂ nanoparticles is of the same order of magnitude as the hole mobility of CBP. Therefore, TiO₂ nanoparticles are more suitable as an electronic transport layer than ZnO nanoparticles. This facilitates a better balance of charge carrier transport when TiO₂ nanoparticles are used in electron transport layers.

Fig. 4 Current density–voltage curves of (a) ITO/TiO₂/Al and (b) ITO/ZnO/Al devices.

Table 1 Comparison of the band gap, mobility and conductivity of ZnO and TiO₂ nanocrystal thin films, and CBP

	Band gap (eV)	Mobility (cm^2 V^−1 s^−1)	Conductivity (S cm^−1)
ZnO	3.65	9.15 × 10^−3	1.35 × 10^−5
TiO2	3.93	3.77 × 10^−5	4.52 × 10^−6
CBP	3.10	5.78 × 10^−5	1.22 × 10^−9

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XPS was conducted to investigate the impact of the DMSO ligand on the electrical properties of TiO₂ nanocrystals. The as-prepared and unmodified TiO₂ nanocrystals, which cannot be dispersed in ethanol, form a turbid solution as shown in Fig. S7.† Compared with unmodified TiO₂ nanocrystals, DMSO-capped TiO₂ nanocrystals can be highly dispersed in ethanol. Each XPS O 1s spectrum shown in Fig. 5a or b reveals an asymmetric peak that can be fitted with two symmetric Gaussian peaks, and these two O 1s peaks are ascribed to the lattice oxygen (O–Ti) and surface hydroxyl (–OH) oxygen, respectively. It was found that DMSO post-treatment can lead to a notable reduction in surface hydroxyl (–OH) oxygen content from 37.36% to 30.08%, thereby inhibiting quantum dot PL quenching and improving the performance of QLEDs. According to the literature reports,^39,40 the quantum dot PL quenching mainly results from surface hydroxyl (–OH) oxygen.

Fig. 5 XPS O 1s spectra of (a) unmodified TiO₂ nanocrystals and (b) DMSO-capped TiO₂ nanocrystals.

The schematic structure of an inverted QLED device is presented in Fig. 6a. The QLED device consists of ITO/TiO₂ nanoparticles (35 nm), QDs (20 nm), CBP (40 nm), MoO₃ (7 nm), and Al (100 nm). Inverted QLED devices exhibit superior electroluminescent properties compared to conventionally structured QLED devices.^46,47 These properties include low turn-on voltage, high brightness, and high external quantum efficiency. As shown in the energy band diagram in Fig. 6b, there is a small injection barrier between the Fermi level of the ITO electrode and the CBM position of the TiO₂ nanoparticles, allowing efficient electron injection. The small potential barrier between the VBMs of CBP and MoO₃ allows for easy hole injection from the Al electrode into the quantum dot layer.

Fig. 6 The schematic structure of (a) the TiO₂ nanoparticle-based QLED device and (b) the corresponding energy band diagram of the inverted QLED.

Fig. 7a and b show the voltage-dependent current density, brightness, current efficiency, and EQE curves of a DMSO-capped TiO₂ nanocrystal-based QLED device. The experimental results reveal that when TiO₂ nanocrystals are used as the ETL, the device achieves a maximum brightness of 103 420 cd m⁻², a current efficiency of 18.06 cd A⁻¹, and an EQE of 12.03%. Notably, this is the highest efficiency achieved for the QLED device with a TiO₂ electron transport layer. In order to verify the reproducibility between batches, we fabricated 55 QLED devices in 13 batches. The EQE histogram in Fig. 7c shows an average EQE of 10.48%, with the highest EQE of 12.03%. It should be noted that our QLED devices were fabricated and tested directly in air (60% humidity, 25 °C) without any encapsulation. For comparison, ZnO nanoparticle-based QLEDs were also fabricated and characterized under the same conditions. No significant difference in current efficiency or EQE was observed between the two types of QLEDs. The current efficiency of a TiO₂-based QLED (18.01 cd A⁻¹) is slightly lower than that of a ZnO-based QLED (20.01 cd A⁻¹), while the maximum brightness of 103 420 cd m⁻² for the TiO₂-based QLED is also only slightly lower than that of ZnO-based QLEDs (113 206 cd m⁻²). Moreover, the EQEs for both QLEDs are quite similar. Transient electroluminescence spectra of TiO₂- and ZnO-based QLED devices were recorded and shown in Fig. 7d. It was found that the ZnO-based QLED demonstrates a faster rise in electroluminescence compared to the TiO₂-based device, indicating that electron transportation and injection in the TiO₂-based QLED device is weaker than that in the ZnO-based device, which is consistent with the results of the mobility experiments. Although the performance of our TiO₂-based QLED is inferior to that of state-of-the-art ZnO-based devices,⁴ the EQE value of our TiO₂-based QLED is still the highest when compared with previously reported TiO₂-based QLEDs (see Table S1†). These results demonstrate the significant application potential of our TiO₂-based QLEDs.

Fig. 7 (a) Current density–voltage–luminance (J–V–L) and (b) current efficiency–voltage–external quantum efficiency curves of the TiO₂-based QLEDs. (c) Histogram of the EQEs measured for 55 QLED devices. (d) The transient electroluminescence spectra of TiO₂- and ZnO-based QLED devices.

In order to further evaluate the potential application of the DMSO-capped TiO₂ nanocrystal ETL for cadmium-based QLEDs, we performed photoluminescence (PL) quenching experiments of CdSe/ZnSe/ZnS core/shell/shell quantum dot layers applied to spin-coated TiO₂ nanoparticle thin film and ZnO nanoparticle thin film. The results are as shown in Fig. 8. The pristine quantum dot film on the glass substrate exhibits the highest PL intensity, whereas a slight decrease in PL intensity was observed when the quantum dots were deposited on the ZnO nanoparticle thin film owing to a PL quenching phenomenon. This result is mainly attributed to the transfer of excitons from the QDs to ZnO nanoparticles. When the QDs were deposited on the film of DMSO-capped TiO₂ nanocrystals, the PL quenching observed was greater when compared with that of the ZnO nanoparticles, indicating that TiO₂ nanocrystals have a higher concentration of surface defects. The photoluminescence quenching efficiencies of ZnO and TiO₂ nanoparticles are 13.0% and 20.2%, respectively. This is the main reason why the performance of TiO₂ nanoparticle-based QLEDs is still significantly lower than that of state-of-the-art ZnO nanoparticle-based QLEDs.

Fig. 8 Photoluminescence quenching experiments of CdSe QDs on glass, TiO₂ nanoparticle thin film, and ZnO nanoparticle thin film.

Additionally, we investigated the stability of the TiO₂-based QLED device. For comparison, the ZnO-based QLED device was also fabricated and characterized under the same conditions (60% humidity, 25 °C). As shown in Fig. 9, the half-life of the unencapsulated ZnO-based QLED, with an initial brightness of 1006 cd m⁻², is 0.565 h. In contrast, the TiO₂-based QLED has a half-life of 2.56 h, which is five times longer than that of the ZnO-based QLED under the same conditions. This result indicates that the highly dispersible DMSO-capped TiO₂ nanoparticles can not only be used in high-performance QLED devices but they also meet the requirements of longer lifetime under air-aging conditions without encapsulation. Furthermore, the effect of encapsulation on the stability of the device was investigated. ZnO- and TiO₂-based QLEDs were simply encapsulated with ultraviolet-curing epoxy and a cover glass in air. Although the device stabilities of ZnO- and TiO₂-based QLEDs can be significantly improved by encapsulation (see Fig. S8†), the stabilities of TiO₂-based QLEDs are still significantly lower than those of state-of-the-art ZnO nanoparticle-based QLEDs. This is probably because our QLED devices are fabricated and encapsulated in air. Although the effect of water vapor on the stability of the device cannot be completely eliminated, our TiO₂-based QLED still exhibits greater stability than a ZnO-based device after encapsulation. These results show that DMSO-capped TiO₂ nanoparticles provide an effective means for achieving high-performance and air-stable QLEDs.

Fig. 9 The operational lifetime measurements of QLEDs based on TiO₂ and ZnO nanoparticles as the electron transportation layers. The devices were fabricated and monitored under ambient conditions of 60% relative humidity and 25 °C without encapsulation.

4. Conclusions

In summary, highly crystalline TiO₂ nanoparticles with a narrow size distribution were synthesized by a conventional non-hydrolytic sol–gel method. Post-treatment with DMSO resulted in the TiO₂ nanoparticles being highly dispersible in ethanol. QLEDs using TiO₂ nanoparticles as the electron transport layer achieved a maximum external quantum efficiency of 12.03%, a maximum luminance of 103 420 cd m⁻², and a current efficiency of 18.06 cd A⁻¹. Most importantly, these devices exhibited significantly improved lifetime and stability performances compared to ZnO-based QLED devices. TiO₂ nanoparticles, which are thermally stable, less sensitive to oxygen and moisture, and abundant in terms of their resources, are expected to replace ZnO nanoparticles as a more stable and efficient electron transport layer material in QLEDs.

Author contributions

B. T. Hu synthesized the nanocrystals and fabricated the QLEDs. X. A. Shi and M. Liu analyzed the experimental results. D. C. Pan designed the project. D. C. Pan and B.T. Hu wrote the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Special Fund for Science and Technology Development of Guangxi (grant no. AD25069078), the Guangxi Science and Technology Major Project (AA23073018), the Guangxi Natural Science Foundation (2023GXNSFDA026056), and the Guangxi Science and Technology Base and Talent Special Project (grant no. AD23026210).

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns of TiO₂ nanoparticles with different reaction times. Photographs of DMSO-capped TiO₂ nanoparticle solutions with different concentrations. FT-IR spectra of TiO₂ nanoparticles with and without DMSO post-treatment. SEM and AFM images of DMSO-capped TiO₂ nanoparticle thin films. See DOI: https://doi.org/10.1039/d5dt00410a

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