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Abstract
Rare-earth (RE) ions doping is currently a promising approach for adjusting the optical properties of CsPbX3 (X=Cl, Br, I) nanocrystals (NCs) in glasses. In this work, we achieved the preparation and property regulation of RE-doped CsPbBr3 NCs using a conventional melt-quenching and subsequent heat treatment technique. The lattice structures and optical properties of the RE-doped CsPbBr3 NCs in glasses were comprehensively analyzed by X-ray diffraction (XRD), transmission electron microscope (TEM), photoluminescence (PL), and transmittance spectra. The results suggested that the entry of dopant ions into the lattice structure showed a strong correlation with the optical property change of the CsPbX3 NCs in glasses. This work provides novel ideas for research on halide perovskite NCs embedded in amorphous materials.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, the considerable potential of all-inorganic halide perovskite ABX3 (A = Cs+, B = Pb2+, X = Cl/Br/I-) nanocrystals (NCs) has been recognized for applications to light emitting diode/laser diode lighting, lasers, and photodetectors. [1–7]. The band gap energy of CsPbX3 (Cl, Br, I) NCs is related to their composition and size. These materials have a wide absorption range in the ultraviolet (UV) and visible regions and a high defect tolerance. Thus, investigation of these materials has become an active research area in optoelectronics [8,9]. However, CsPbX3 (Cl, Br, I) NCs have a low formation energy and therefore exhibit poor thermal and chemical stability in practical applications. Therefore, improving the stability of CsPbX3 (Cl, Br, I) NCs in the natural environment has become an important research direction [10–12].
Glass-coated CsPbX3 (Cl, Br, I) NCs are inert, amorphous materials that can largely evade the impact of harsh environments. In addition, as NCs are isolated from each other by glass matrices, PL quenching induced by the coalescence of CsPbX3(Cl, Br, I) NCs is reduced [13]. CsPbX3 (Cl, Br, I) NCs have been successfully precipitated in glass matrices, such as phosphate [14], borosilicate [15], borogermanium [13], and tellurite [16], using conventional melt-quenching followed by heat treatment. However, defects in the interface between the CsPbX3 (Cl, Br, I) NCs and the glass, imperfect lattice structures, and significant volatilization of the halogen during the melting process have a non-negligible impact on the optical properties of these NCs.
The optical properties of CsPbX3 (Cl, Br, I) NCs, including the optical band gap, photoluminescence (PL) peak position, PL peak width, and PL lifetime, vary for different glass matrices. The prevailing view is that these differences in optical properties mainly result from surface modification and cation doping of CsPbX3 (Cl, Br, I) NCs in different glass matrices, as well as changes in lattice kinetics during NC growth [17,18]. Replacing Pb2+ with rare-earth (RE) ions will cause the existence of defect states due to the generation of vacancies for charge compensation, resulting in lattice shrinkage and broadening of the photoluminescence spectra [19,20]. Besides, changes in the glass network structure affect the diffusion kinetics of CsPbX3 (Cl, Br, I) NC precursor ions during heat treatment, which in turn leads to changes in optical properties [21].
Fluorophosphate (FP) glasses have a low melting point. Compared to other low-melting glass matrices, fluorophosphate glasses combine the benefits of phosphate and fluoride glasses and have a higher thermal conductivity [22]. Compared to tellurate and phosphate glasses, fluorophosphate glasses have a lower hydroxyl concentration, resulting in higher stability, as well as a higher halogen solubility, which increases the concentration of NC precursors and phosphor luminescent cores in the glass [22,23].
In this study, CsPbBr3 NCs-embedded fluorophosphate glasses were prepared by melt-quenching followed by heat treatment. The glass composition was modified by doping B-site cations into the CsPbBr3 NCs in the matrix. CsPbBr3:Y3+, CsPbBr3:Lu3+, and CsPbBr3:La3+ NCs-embedded fluorophosphate glasses were thus prepared. Analysis of X-ray diffraction (XRD), transmission electron microscope (TEM), transmission, and PL excitation spectra confirmed that the dopant ions entered the lattice structure of CsPbBr3 NCs during heat treatment and induced a series of changes in the optical properties of CsPbBr3 NCs-embedded fluorophosphate glasses.
2. Experiment
2.1 Sample preparation
Conventional melt-quenching followed by heat treatment was used to prepare fluorophosphate glasses embedded with Y3+, Lu3+, and La3+-doped NCs. Table 1 shows the respective precursor compositions and molar ratios. The components were weighed, fully mixed, and placed in a 50-mL alumina crucible. The crucible was placed in a high-temperature (870 °C) melting furnace. The glass was stirred during the melting process to ensure complete mixing. After 20 min, the molten glass was poured onto a copper plate and cast into a glass precursor. The glass precursor was annealed, cut into 15 mm × 15 mm squares, ground into thin sheets with a thickness of approximately 2 mm, and polished using a nanoscale diamond suspension with granulometry of 200 nm or less on a polishing disc. The processed samples had uniform sizes and thicknesses and a flat surface to facilitate subsequent heat treatment. The annealing process consisted of two stages: holding and cooling. The samples were held at 280 °C for 2 h and then cooled to room temperature at a rate of 5°C/h. Four glass sheets from each of the four sample groups were placed in an annealing furnace at 360°C and subjected to different heat-treatment durations of 4, 12, 20, and 30 h.
Table 1. Compositions and molar ratios of glass precursors
2.2 Testing
The transmittance of the prepared samples was recorded in the visible light range of 250 nm to 750 nm using a spectrophotometer (Lambda 950, PerkinElmer, USA). The phases of the prepared samples were characterized by powder XRD (D8 Advance A25, Bruker, Germany) between 20° and 40° at a scan rate of 0.6° 2θ/min. The test samples were mechanically ground, passed through a 400-mesh sieve, heat treated, and formed into uniform sizes using a laboratory mold. The TEM images were recorded using an FEI TECNAI F20 microscope operating at 200 kV. PL spectra were obtained by transient/steady-state fluorescence spectrometer (FLS 920P, Edinburgh, UK).
3. Results and discussion
3.1 Influence of the glass composition on the lattice structure of CsPbBr3 NCs
Figure 1 shows images of samples with different heat treatment durations at 360 °C under daylight and UV light excitation. All the samples appeared yellowish-green in color in daylight and green emission under 365 nm light irradiation. The PL intensity was not uniformly distributed in FP-3 and FP-La-3 heat treated for 4 h, which can be attributed to the insufficient heat treatment time induced inhomogeneous precipitation of NCs in glasses. By comparison, the samples doped with Y3+ and Lu3+ under the same heat treatment conditions exhibited a uniform color in daylight. It is worth noting that the PL intensity under UV light excitation shows anomalous weakening after the 12 h heat treatment process. One possible reason could be ascribed to defect-induced fluorescence quenching which is related to the rapid growth of NCs with 12 h heat treatment. The detailed mechanism will be further discussed using PL spectra.
Fig. 1. Images obtained under daylight and 365 nm UV irradiation of FP-3, FP-Y-3, FP-Lu-3, and FP-La-3 that were heat treated at 360 °C for 4, 12, 20, and 30 h.
Figure 2 shows the XRD patterns of FP glass samples after the heat treatment process. As shown in Fig. 2(a), the XRD patterns show several diffraction peaks of NCs and a broad diffraction hump comes from the FP glass matrix. Compared with the standard PDF (JCPDS No. 54-0752), the narrow diffraction peaks can be determined to CsPbBr3 NCs glasses. The result demonstrates the precipitation of CsPbBr3 NCs in the FP glasses after the heat treatment process. Notably, the diffraction peaks at the (110) and (211) planes in the patterns of the heat-treated FP-Y-3 and FP-Lu-3 samples were shifted to larger angles compared to those in the pattern of the heat-treated FP-3 sample. This shift resulted from lattice contraction due to the replacement of Pb2+ with Y3+ and Lu3+ ions with relatively small ionic radii in the CsPbBr3 NCs during heat treatment. No diffraction peaks were observed in the XRD pattern of the FP-3-La sample. The possible reason is the small size or small number of NCs. A reduction in the size of the NCs would make the detection of the corresponding diffraction peaks more difficult. To further clarify the distribution of CsPbBr3 NCs in the FP glass matrix, TEM images were used to observe the microstructure of the FP glass ceramic. As shown in Fig. 2(b) and (c), the average size of the nanocrystals in the undoped sample is about 2.8 nm and the interplanar spacing is 2.07 Å, which further confirms the existence of CsPbBr3 NCs in the FP glass.
Fig. 2. XRD patterns and TEM images of FP glass samples that were heat treated at 360 °C for 40 h. (a) XRD patterns of FP-3, FP-Y-3, FP-Lu-3, and FP-La-3, (b) and (c) TEM images of CsPbBr3 NCs in FP-3. The right inset shows the lattice spacing.
3.2 Analysis of PL characteristics of NCs in glass matrices
Figure 3(a) shows the transmission spectra of the FP-3 sample before and after the heat treatment process. The absorption edge of the annealed FP-3 sample is at about 320 nm, whereas the absorption edge of the sample after 20 h heat treatment process shifts to 520 nm. As shown in Fig. 3(b), the PL peak position is at 519 nm with strong green emission and almost no emission below 500 nm. According to the excitation spectrum, the excitation spectrum covers 300-500 nm, which further confirms that the absorption in the range of 300-500 nm can be attributed to the absorption of CsPbBr3 NCs in the FP glass.
Fig. 3. Spectra of the undoped FP-3 sample. (a) Transmittance spectra of FP-3 samples after annealing and after heat treatment at 360°C for 20 h. (b) PLE and PL spectra of sample FP-3 heat-treated at 360°C for 20 h (the monitoring peak of the PLE spectrum is 525 nm and the excitation of the PL spectrum is 365 nm)
Figure 4 shows the normalized PL spectra of the four groups of samples obtained under 365 nm UV excitation. In the spectra of the samples heat treated for 4, 12, 20, and 30 h, the PL peak positions for FP-Y-3, FP-Lu-3, and FP-La-3 are blue-shifted relative to those of FP-3. This blueshift could be attributed to the lattice contraction induced by Y3+, Lu3+, and La3+ doping. The degree of blueshift of the PL peaks differed among the spectra of CsPbBr3 NCs-embedded FP glasses with different ions doping.
Fig. 4. Normalised PL spectra obtained under 365 nm excitation for the FP-3, FP-Y-3, FP-Lu-3, and FP-La-3 samples heat treated at 360 °C for (a) 4 h, (b) 12 h, (c) 20 h, and (d) 30 h
The wavelength shifts of the FP-Y-3, FP-Lu-3, and FP-La-3 samples in the PL spectra relative to the undoped sample FP-3 and the FWHM of the four groups of samples were combined as shown in Table 2. For the samples subjected to different treatment conditions, the FWHMs for the spectra of CsPbBr3:Y3+, CsPbBr3:Lu3+, and CsPbBr3:La3+ NCs were almost all smaller than that for the spectra of CsPbBr3 NCs. This result indicates that Y3+, Lu3+, and La3+ ions in the lattice structure of CsPbBr3 NCs effectively reduce surface defects during heat treatment and stabilize the phase structure. With increasing heat treatment time, the FWHM of the doped samples becomes maximum at 12 h and then decreases. This indicates that after 12 h of heat treatment, the NCs generate many defects due to the fast growth rate, which affects the luminescence. For the samples treated for 20 and 30 h, the repair of defects in the NCs causes the FWHM of the samples to decrease again. This phenomenon is consistent with the observation in Fig. 1 that after 12 h of heat treatment, the luminescence of the samples under UV radiation dims. Compared to the undoped FP-3 samples, the wavelength shift first increases and then stabilizes with increasing heat treatment time.
Table 2. FWHMs for spectra and wavelength (blue) shifts obtained under 365 nm excitation for the four groups of samples (unit: nm)
The PL of CsPbBr3 NCs in glass matrices exhibits different properties at different excitation wavelengths due to the mutual independence of self-trapped excitons [24]. Figure 5 shows the normalized PL spectra of FP-3, FP-Y-3, FP-Lu-3, and FP-La-3 obtained under light excitation at 365 nm, 395 nm, and 450 nm. The PL peaks in the spectra of CsPbBr3, CsPbBr3:Y3+, CsPbBr3:Lu3+, and CsPbBr3:La3+ NCs in the glass matrices obtained under 365 nm excitation appeared at 518, 515, 515, and 514 nm, respectively, and were redshifted to 535, 536, 536, and 529 nm, respectively, in the spectra obtained under 395 nm excitation. The PL peaks in the spectra of the NCs obtained under 450 nm excitation were also slightly redshifted relative to those obtained under 395 nm excitation. Overlapping dips occurred at the absorption cut-off edges of the CsPbBr3 NCs embedding glasses at approximately 525 nm in the PL spectra of FP-3, FP-Y-3, FP-Lu-3, and FP-La-3 heat-treated for 20 h under UV excitation at 395 nm and 450 nm, which could be attributed to self-absorption in the FP glasses. Note that among the four groups of samples, the most pronounced self-absorption and relatively prominent redshifts of the peaks in the spectra obtained under 395 nm and 450 nm excitation were observed for FP-Y-3 and FP-Lu-3.
Fig. 5. Normalised PL spectra obtained under 365 nm, 395 nm, and 450 nm excitation for samples heat treated at 360°C for 20 h: (a) FP-3, (b) FP-Y-3, (c) FP-Lu-3, and (d) FP-La-3.
Figure 6 shows the PL spectra and absorption spectra of FP-Y-3 and FP-Lu-3 under 450 nm excitation after heat treatment at 360 °C for 20 h. The wavelengths of the emission spectra overlap with those of their absorption spectra at 526 nm, confirming that the double PL peaks originate from the reabsorption-PL process.
Fig. 6. PL spectra and absorption spectra of samples (a) FP-Y-3, (b) FP-Lu-3 under 450 nm excitation after heat treatment at 360 °C for 20 h.
3.3 Influence of the glass composition on the optical band gap of CsPbBr3 NCs
In addition to XRD analysis, PL spectral analysis of samples prepared under the same conditions was also used to confirm the formation of CsPbBr3:Y3+, CsPbBr3:Lu3+, and CsPbBr3:La3+ NCs after heat treatment. Differences in the transmittance spectra and optical band gaps of doped and undoped samples were observed. The optical band gap energy (Eg) of CsPbBr3 NC glasses was determined from absorption spectra. The Taus and Davis-Mott equation can be used to accurately determine the band gap energy of perovskite NCs from absorption spectra [25–27]:
where α is the absorption coefficient, k is a constant, hν is the electron energy, and n is a constant for a given optical transition.Figure 7 shows the transmittance of samples subjected to 12 and 30 h of heat treatment, where absorption cut-off edges appear at approximately 525 nm, which is almost flat in the range of 540 nm to 750 nm. The appearance of absorption cut-off edges at 525 nm was mainly due to the absorption of the respective light source by the CsPbBr3, CsPbBr3:Y3+, CsPbBr3:Lu3+, and CsPbBr3:La3+ NCs in the matrix material.
Fig. 7. Optical transmittance measured at room temperature for FP-3, FP-Y-3, FP-Lu-3, and FP-La-3 subjected to heat treatment at 360 °C for (a) 12 h and (b) 30 h (test range: 250-750 nm).
For the samples heat treated at 12 and 30 h, all the absorption cut-off edges of the Y3+, Lu3+, and La3+-doped NC glasses were blue-shifted relative to those of the undoped glass because the lattice structure was modified by the entry of Y3+, Lu3+, and La3+ ions into CsPbBr3 NCs during heat treatment. The replacement of Pb2+ ions in CsPbBr3 NCs by ions with slightly smaller radii manifested as a broadening of the optical band gap of CsPbBr3 NCs [19]. Figure 8 shows the relationship between the energy and (αhυ)2 calculated using the Tauc plot method and the transmittances of FP-3, FP-Y-3, FP-Lu-3, and FP-La-3 subjected to different heat treatment conditions and the resulting forbidden bandwidths [21]. For the samples heat treated for 12 and 30 h, the forbidden band widths ranged from 2.35 eV to 2.41 eV, and the optical band gaps of FP-Y-3, FP-Lu-3, and FP-La-3 were broadened compared to that of FP-3. The band gap widths of Y3+, Lu3+, and La3+-doped CsPbBr3 NCs subjected to heat treatment at 360 °C for 12 h were 2.35 eV, 2.37 eV, and 2.40 eV, respectively, whereas that of the undoped CsPbBr3 NCs was 2.35 eV. The forbidden bandwidths of Y3+, Lu3+, and La3+-doped CsPbBr3 NCs subjected to heat treatment at 360 °C for 30 h were broadened to 2.35 eV, 2.35 eV, and 2.41 eV, respectively, whereas that of the undoped CsPbBr3 NCs was 2.34 eV. This result indicated that the replacement of Pb + 2 ions with dopant ions induced changes in the bond length and bond angle of the lead halide octahedral structure, and the structural enhancement increased the forbidden band gap width of the NCs. In particular, the broadening of the forbidden band width was most pronounced for the La3+-doped samples, indicating that La3+ entered the lattice of CsPbBr3 NCs more efficiently than the other dopant ions. This result, together with the change in the diffraction angles in the XRD patterns and PL peaks under 365 nm excitation, indicated that the dopant ions entered the lattice structure of the NCs during heat treatment of the glass samples and induced changes in the lattice structure of the CsPbBr3 NCs.
Fig. 8. Relationship between (αhυ)2 and energy of FP-3, FP-Y-3, FP-Lu-3, and FP-La-3 subjected to heat treatment at 360 °C for (a) 12 h and (b) 30 h (in the figure, each straight line was obtained by extrapolating the linear part of the corresponding curve to the horizontal axis, where the coordinate of the intersection point is marked and has units of eV).
4. Conclusion
Fluorophosphate glasses embedded with CsPbBr3 NCs doped with Y3+, Lu3+, and La3+ were successfully prepared in this study. The effective precipitation of CsPbBr3 NCs in the glass matrices during heat treatment was confirmed by XRD and TEM analyses. The CsPbBr3 NCs precipitates exhibited typical characteristics of broadband absorption and narrowband emission. The shifting of XRD diffraction peaks to large angles, the blueshift of PL peak positions, and the broadening of the optical band gaps of the ion-doped CsPbBr3 NCs relative to the undoped NCs all indicated that Y3+, Lu3+, and La3+ ions entered the host lattice of CsPbBr3 NCs during heat treatment. Differences in the NC peak position and FWHM in the spectra of different samples were attributed to the doping of cations into the CsPbBr3 NCs in the glass. The PL peak positions for the NCs changed with the excitation wavelength of the light source and heat-treatment duration due to the mutual independence of self-trapped excitons and changes in size distribution, respectively. The results of this study are expected to inspire further exploration of glass materials embedded with NCs doped with rare earth ions.
Funding
National Natural Science Foundation of China (Grant 61905119).
Acknowledgements
The XRD test was performed at the Analysis and Test Center of the School of Materials Science and Engineering of Nanjing University of Posts and Telecommunications.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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