Color tunable emission from Eu³⁺ and Tm³⁺ co-doped CsPbBr₃ quantum dot glass nanocomposites†

Erdinç Erol ^ab, Orhan Kıbrıslı ^a, Miray Çelikbilek Ersundu *^a and Ali Erçin Ersundu *^a
aFaculty of Chemical and Metallurgical Engineering, Department of Metallurgical and Materials Engineering, Glass Research and Development Laboratory, Yildiz Technical University, Istanbul, 34220, Turkey. E-mail: miray@yildiz.edu.tr; ersundu@yildiz.edu.tr
^bDepartment of Metallurgical and Materials Engineering, Manisa Celal Bayar University, Muradiye, Manisa, Turkey

First published on 6th December 2021

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1. Introduction

All-inorganic CsPbX₃ (X = Br⁻, Cl⁻, and I⁻) perovskite quantum dots (QDs) have significantly contributed to the development of cutting-edge products in many fields including optoelectronics,^1,2 photovoltaics,^3,4 sensing,^5,6 and photocatalysis^7,8 owing to their superior properties such as high photoluminescence quantum yield (PLQY), excellent color purity (narrow full-width at half maximum) and tunable emission color (with a tunable band gap).^9–12 Among different perovskite QDs, brilliant green-emitting bromide-based perovskites (CsPbBr₃ QDs) step forward especially for backlit displays¹³ and green-emitting LEDs^14,15 because of the hypersensitivity of the human eye to green light.¹³ Despite the unique optical and electronic properties of solution-processable perovskite QDs, their notorious chemical, thermal and photo stability causing them to degrade when exposed to humidity, light or elevated temperatures near 150 °C pose a serious challenge to their commercialization.^16–18 Despite the fact that many methods have been explored for improving the long-term stability of perovskite QDs, e.g., surface passivation,¹⁸ integration of various polymeric or inorganic host materials^19,20 and ion co-doping,²¹ none of them have provided adequate protection. Furthermore, the synthesis of high-quality colloidal perovskite QDs necessitates a sophisticated reaction setup such as an inert environment as well as time-consuming preparation operations that ultimately results in high cost and difficulty of commercial application.²² In this regard, perovskite QD glass nanocomposites (GNCs) shine as the most effective approach to further improve long-term stability since the high rigidity and stability of the glass structure ensures that the QDs are effectively protected under environmental conditions.^23–28

The lack of red and blue emitting components in CsPbBr₃ QD GNCs, however, limits the color tunability of their emission and makes it difficult to incorporate them into novel applications. The doping of lanthanide ions (Ln³⁺) together with CsPbBr₃ QDs allows further control of optical and electronic properties of QDs such as broadening their emission color into the desired spectral region and improving their PLQY values.^29,30 Among different Ln³⁺ ions, Eu³⁺ ions – a well-known ideal red-emitting center with narrow linewidth ascribed to ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions – have demonstrated promising improvements in performance and color tunability of CsPbBr₃ QD GNCs.³¹ Until now, the strong red emission of Eu³⁺ coupled with green-emitting CsPbBr₃ QD GNCs demonstrated promising potential in a variety of applications, including multicolor LEDs,³² white-LEDs,^29,33 and X-ray imaging.³⁴ However, in most of the existing studies emission color of Ln³⁺-doped QDs is controlled by adjusting the lanthanide concentration. For instance, Yuan et al. achieved multicolor LED fabrication by combining the Eu³⁺-doped CsPbBr₃ QD GNCs with a UV chip. When the Eu³⁺ concentration is increased from 0 to 1.44, and 3.18 (mol%) multi-color emission from green to orange, and red is achieved under 365 nm illumination.³² Hence, a further study is urgently required to acquire a better understanding of how to improve the emission properties and produce multiple colors from Ln³⁺-doped CsPbBr₃ QD GNCs at fixed lanthanide concentration.

In this work, highly stable and tunable Eu³⁺ and Tm³⁺ co-doped CsPbBr₃ QD GNCs are successfully synthesized for the first time through a conventional melt-quenching method followed by controlled heat-treatment. Tm³⁺ ions are chosen to shift the main emission peak of CsPbBr₃ QDs to shorter wavelengths as well as to be used as a blue light source.³⁵ Eu³⁺ ions, on the other hand, are selected for compensating the red deficiency to obtain a broader emission spectrum. The incorporation of Eu³⁺ and Tm³⁺ ions to CsPbBr₃ QD GNCs not only enriches the luminescence characteristics of QDs but also allows them to crystallize easily in the glass matrix. By rigorously controlling the heat-treatment temperature or time, tunable emission colors (from blue to cyan, violet, red, green, and eventually white) are achieved under UV excitation at a fixed lanthanide concentration. Ultimately, these findings suggest that Eu³⁺/Tm³⁺ co-doped CsPbBr₃ QD GNCs – that show high thermal, chemical, and photo-stability features together with outstanding and widely tunable emission colors – are suitable candidates for new type warm light sources or multicolor LEDs.

2. Experimental studies

2.1. Sample preparation

Eu³⁺ and Tm³⁺ co-doped CsPbBr₃ QD GNC samples with a nominal composition of 5CaO–12ZnO–2Al₂O₃–30B₂O₃–(35−x−y)SiO₂–(6NaBr–7Cs₂O–3PbBr₂) + xEu₂O₃, yTm₂O₃ (x = 0–1.25; y = 0–0.75 in mol%) are synthesized via the conventional melt-quenching route and subsequent heat-treatment (for the crystallization of CsPbBr₃ QDs). High-purity raw materials of SiO₂, H₃BO₃, ZnO, CaCO₃, Al₂O₃, NaBr, Cs₂CO₃, PbBr₂, Eu₂O₃, and Tm₂O₃ purchased from Sigma-Aldrich and Alfa Aesar are mixed and ground to fine powder by using an agate mortar and pestle to form a homogeneous mixture. The well-mixed raw materials are put into corundum crucibles and melted in a furnace at 1100 °C for 30 min under an air atmosphere. Afterwards, the obtained melt is poured into a pre-heated stainless-steel mold and pressed by another steel plate to obtain as-cast samples. Subsequently, the as-cast samples are annealed at 400 °C (lower than the crystallization temperature of CsPbBr₃ QDs and the glass matrix itself) for 5 h in a muffle furnace to eliminate internal stresses resulting from rapid cooling, and then naturally cooled down to room temperature. Finally, Eu³⁺ and Tm³⁺ co-doped CsPbBr₃ GNC samples are obtained through controlled heat-treatment (crystallization). After controlled crystallization, the GNC samples are cut and well-polished into small specimens with a thickness of ∼2 mm to obtain a high quality surface for optical measurements.

Here we present a series of experiments describing how to determine ideal lanthanide doping concentrations for a tunable emission color from current GNC samples. Briefly, when the doping concentrations of Eu³⁺ and Tm³⁺ are selected to be 1 and 0.75 mol%, respectively, undesirable self-crystallization of QDs occurs before applying any heat-treatment (see Fig. S1, ESI†). Although the self-crystallization of CsPbBr₃ offers some favorable properties in terms of energy efficiency,³⁶ it is not convenient in this study as it does not allow size control of CsPbBr₃ QDs. Furthermore, when Eu³⁺ and Tm³⁺ doping concentrations are selected as 0.5 and 0.5 mol%, respectively, a red emission deficit arises from Eu³⁺ ions, resulting in failure of wide color tuning and ultimately white light emission. Eventually, optimum doping concentrations of Eu³⁺ and Tm³⁺ are determined to be 0.75 and 0.5 mol%, respectively and used throughout this study. After finding the ideal doping concentrations for Eu³⁺ and Tm³⁺, a series of GNC samples is prepared by heat-treating the as-cast sample at a fixed temperature of 500 °C for different times (2, 3, 4, 5 and 15 h). Based on these results, the as-cast sample is further heat-treated at 510 °C for 3 h (to achieve white light emission). Consequently, synthesized samples (undoped and Eu³⁺/Tm³⁺ co-doped QD GNCs) are coded according to their heat-treatment temperatures (i.e., 500 °C and 510 °C) and times (i.e., 2, 3, 4, 5, and 15 h), e.g., 500 °C/3 h (see Table 1).

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2.2. Characterization studies

The final chemical composition of the selected as-cast sample is investigated through the energy dispersive X-ray spectroscopy (EDS) technique. A Zeiss EVO LS 10 SEM linked with the EDAX Element SDD system is used for EDS analysis. The electron beam is accelerated to 15 kV for EDS measurements. The K lines are used for characterization of B, Na, Al, Si, Ca, and Zn, the L lines for analysis of Cs, Eu, and Tm, and M lines for detection of Pb. The final chemical composition of the selected as-cast sample is calculated by taking seven different EDS spectra from different spots on the sample and the average of these measurements are given in wt% (see Table S1, ESI†). Differential thermal analysis (DTA) measurements are carried out using a PerkinElmer™ Diamond TG/DTA (with an error estimate of ±1 °C) instrument to investigate the thermal behavior of the as-cast sample. Accordingly, 25 ± 1 mg of the powdered sample is heated in a platinum pan at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere to determine the glass transition (T_g) temperature. The amorphous nature of the as-cast sample and phase characteristics of the CsPbBr₃ embedded GNC samples are identified via XRD measurements using a Philips/PANalytical X'pert Pro diffractometer with an average wavelength of Cu Kα₁ and Cu Kα₂ radiation (λ = 0.15418 nm, voltage: 45 kV, current: 40 mA) in the 2θ range from 10 to 80° with a scanning step size of 0.02°. The XRD peak positions and intensities of crystallized phases are compared with the PDF card files. The optical absorption spectra of samples are measured using PG T80+ UV-Vis spectrophotometer operated at a resolution of 1 nm in the wavelength range from 300 to 900 nm.

Subsequently, the optical bandgap (E_g) values are calculated using the Tauc relationship³⁷ by extrapolating the linear part of the plot of (αhν)²versus hν – considering a direct transition between the valence and conduction band – where α is the optical absorption coefficient, and hν is the energy of the incident photon. Using the determined band gap values, the effective mass approximation (EMA) method is then utilized to calculate the crystallite sizes of CsPbBr₃ QDs. EMA proposed by L. E. Brus is a well-known approach to calculate radii of small nanoparticles with respect to their measured band gap values.^24,25,38

Hence, crystallite sizes of CsPbBr₃ QDs is calculated through the EMA method according to the following relation:^24,39

	(1)

where E_g,bulk (CsPbBr₃: 2.3 eV) is the bulk band gap, r is the nanoparticle radius, (CsPbBr₃: 0.15m_e) and (CsPbBr₃: 0.14m_e) are effective mass of electron and hole, respectively.⁴⁰ε₀ is the vacuum permittivity, ε (CsPbBr₃: 4.797) is the relative permittivity, ℏ is Planck's constant, and e is the charge of the electron.⁴¹

Photoluminescence measurements are recorded on an Edinburgh Instruments FS5 fluorescence spectrophotometer equipped with a 150W xenon lamp as the excitation source having a high spectral resolution (signal to noise ratio of water Raman signal >6000:1). PLQY measurements are conducted with a signal to noise ratio of the water Raman signal greater than 6000:1 and 0.5 nm spectral resolution using same PL setup in an integrated sphere (150 mm internal spherical cavity, polytetrafluoroethylene coated) under a 365 nm excitation wavelength and evaluated by the following equation with an estimated error rate of ±5%²⁴

	(2)

where E_reference and E_sample are the scattering intensities of excitation light not absorbed by the reference and the sample, respectively. L_sample is emission intensity determined by integrating the area under the calibrated emission spectra. The difference in scattering integrated areas between the reference and the sample determines the number of absorbed photons.

PL emission spectra of samples are used to determine the colorimetric properties, such as Commission Internationale d'Eclairage (CIE) 1931 color coordinates, color rendering index (CRI), and correlated color temperature (CCT) by using OSRAM ColorCalculator through the equations explained in our previous study.^42–44

Thermal, chemical and photo stability tests are carried out on a selected GNC sample heat-treated at 500 °C for 15 h. Accordingly, temperature-dependent PL measurements (thermal stability tests) are recorded up to 8 heating–cooling cycles by placing a Pike Technologies heated solid transmission attachment in the fluorescence spectrophotometer and by increasing the temperature from 25 °C to 250 °C and then cooling down to room temperature. The photostability is assessed by stimulating the sample under a 442 nm laser with power density of 5 W cm⁻² up to 10 h and recording the PL emission at various time intervals. PL emission spectra are measured with a ±3% error rate. Chemical stability is investigated by immersing the selected GNC sample in deionized water for up to 45 days at room temperature and recording the corresponding PLQY value with an ±5% error rate to interpret the chemical effect of moisture on emission intensities at different time intervals.

3. Results and discussion

3.1. Thermal and structural analysis

The DTA result of the as-cast sample is used to determine the appropriate heat-treatment temperatures to form CsPbBr₃ QDs in the glass matrix. As can be clearly seen in Fig. 1aT_g is noted to be ∼485 °C. Accordingly, CsPbBr₃ QDs are grown by heat-treating the as-cast sample to slightly above T_g at 500 °C (for 2, 3, 4, 5, and 15 h) and 510 °C (for 3 h). Fig. 1b reveals five diffraction peaks at 2θ values of 15.21°, 21.55°, 30.47°, 37.54°, and 43.66° attributed to the (100), (110), (200), (211) and (220) crystalline planes of cubic CsPbBr₃, respectively (superimposed on the broad halo peak) for the samples heat-treated at 500 °C for 15 h (for both Eu³⁺ and Tm³⁺ co-doped and undoped GNCs) – evidencing that the QDs are successfully prepared in the glass matrix via heat-treatment.^26,27 Importantly, the XRD diffraction peaks of the CsPbBr₃ QD GNC sample heat-treated at 500 °C for 15 h become slightly more intense and sharper upon the doping of Eu³⁺ and Tm³⁺ ions. This is probably because the incorporation of Eu³⁺ and Tm³⁺ acts as a nucleating agent and facilitates the crystallization and growth of CsPbBr₃ QDs in the glass matrix.³⁴ The XRD pattern of the as-cast sample, on the other hand, exhibits diffuse scattering halo without any diffraction peaks due to its amorphous character.²⁴ Meanwhile, GNC samples heat-treated at 500 °C for 2, 3, 4, 5 h and at 510 °C for 3 h also display the same broad halo peaks indicating that heat-treatment time is insufficient to induce complete crystallization of CsPbBr₃ QDs. Hence, it can be concluded that the size of CsPbBr₃ QDs in glasses increases with the increasing heat-treatment time.

3.2. Optical absorption properties

The digital images of the as-cast sample and Eu³⁺/Tm³⁺ co-doped GNCs under daylight and their corresponding optical absorption spectra are illustrated in Fig. 2. As displayed in Fig. 2a, the as-cast sample is colorless and transparent. On the other hand, colors of GNCs change from very pale (except for the 500 °C/2 h sample) to vivid yellow with increasing heat-treatment time demonstrating the successful growth of CsPbBr₃ QDs in the glass network. However, the absence of color change in the 500 °C/2 h sample indicates that the applied heat-treatment temperature and time is not enough to form CsPbBr₃ QDs. The very pale-yellow colors observed in the 500 °C/3 h and 510 °C/3 h samples are probably due to the partial crystallization (or relatively small average size) of the CsPbBr₃ QDs. This is consistent with the XRD patterns of the related samples (see Fig. 1b) in which diffraction peaks are not observed unless the heat-treatment is increased to sufficient temperatures or times. As represented in Fig. 2b, the absorbance spectra of samples display five absorption bands located at 358 nm (³H₆ → ¹D₂ transition of Tm³⁺), 393 nm (⁷F₀ → ⁵L₆ transition of Eu³⁺), 465 nm (⁷F₀ → ²D₂ transition of Eu³⁺), 686 nm (³H₆ → ³F_2,3 transition of Tm³⁺), and 792 nm (³H₆ → ³H₄ transition of Tm³⁺).^35,45–47 With increasing heat-treatment time (from 2 h to 3 4, 5, and 15 h) or temperature (from 500 °C to 510 °C) intensities of the absorption bands at 686 and 792 nm remain unchanged, while intensities of the absorption bands at 358, 393 and 465 nm disappear gradually. This is attributed to the crystallization of QDs causing the samples to change their color towards the red end of the visible spectrum. In addition, the exciton absorption peak of the CsPbBr₃ QDs becomes observable in the 500 °C/3 h sample (indicated as number 4 on the graph given in Fig. 2b) and shifts towards the long wavelength region with increasing heat-treatment time or temperature. The redshift in exciton absorption proves the growth and increment in the mean size of CsPbBr₃ QDs in the glass network – ultimately revealing the size and band gap modulation of QDs that is attributed to the quantum confinement effect.^24,26,48 Accordingly, the band gap values (in the range of 2.45–3.34 eV) and estimated crystallite sizes (in the range of 2.05–4.40 nm) of the as-cast sample and CsPbBr₃ QD GNCs are detailed in Table 1 and Fig. S2 (ESI†).

3.3. Photoluminescence, color tunability and colorimetric properties

3.4. Chemical, photo, and thermal stability investigations

Chemical, photo and thermal stability features are critically important in the practical applications of QDs for solid state lighting. To demonstrate the benefits of incorporating CsPbBr₃ QDs into a glass matrix chemical and photo stability tests and temperature-dependent PL measurements including heating–cooling cycles are performed with the selected 500 °C/15 h sample. Chemical stability of the selected GNC sample is assessed by storing it in water and monitoring its PLQY value at different intervals up to 45 days (see Fig. 5a). No noticeable decrease in PLQY of the selected GNC sample is observed even after 45 days of storage in water revealing that the rigid nature of glasses inhibits possible chemical reactions between the QDs and the external environment.

Photostability test of the selected GNC is determined by recording the PL emission intensity at various time intervals upon stimulating the sample under a powerful laser excitation (see Fig. 5b). As depicted in Fig. 5b, no substantial change in PL intensity values can be noticed due to the robust protection provided by the rigid glass structure.

Thermal stability analysis of CsPbBr₃ QDs in glass is carried out by recording temperature-dependent PL spectra of the selected sample up to 225 °C (see Fig. 5c and d). The thermal quenching mechanism causes the PL emission intensity to decrease as the temperature increases from 25 to 225 °C. However, after cooling the sample back to 25 °C, it recovers its original intensity value (see Fig. 5c). Additionally, thermal cycling experiments further demonstrate that the emission properties of QDs are highly reversible as evidenced by the absence of a significant reduction in PL intensity of the selected sample after 8 heating–cooling cycles (see Fig. 5d). The above results show that the Eu³⁺ and Tm³⁺ co-doped CsPbBr₃ QD GNCs have excellent chemical, photo and thermal stability, making them a suitable choice for solid-state lighting applications.

4. Conclusions

In conclusion, Eu³⁺ and Tm³⁺ co-doped CsPbBr₃ QD GNCs are successfully synthesized in a glass host via the melt-quenching method followed by a controlled heat-treatment process. The incorporation of Eu³⁺ and Tm³⁺ into CsPbBr₃ QD GNCs not only improves the PLQY but also provides unique features such as efficient tuning of emission colors from blue to cyan, violet, red, green, and white by simply controlling the heat-treatment conditions – that could be a promising way to obtain multi-color LEDs or other optoelectronic devices. The produced GNC samples demonstrate superior thermal, chemical and photo stability features by preserving their original PL intensities even after being subjected to temperatures as high as 225 °C, immersion in water for 45 days, and laser irradiation at 5 W cm⁻² for 10 h, respectively. The encouraging results of this research might provide further insight into new approaches for developing innovative and high-performance solid-state lighting devices for use in a variety of applications.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by Yildiz Technical University Scientific Research Projects Coordination Unit under project number FBA-2021-4543. The authors of this study express their gratitude to Naji Vahedigharehchopogh for his help in digital imaging, and to Ulaş Korkmaz, Beyza Özlem, Utku Ekim and Boğaçhan Ertekin for their help in sample preparation studies. The authors would like to acknowledge that this paper is submitted in partial fulfilment of the requirements for a PhD degree at Yildiz Technical University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1cp05016h

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