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Abstract
All-inorganic halide perovskite CsPbX3(X = Br/Cl/I)quantum dots have gained a considerable attention in the optoelectronic fields. However, the high cost and poor stability of the prepared CsPbX3 quantum dots (QDs) are inevitable challenges for their future practical applications. And the high-performance CsPbX3 QDs are always needed. Herein, a facile and low-cost synthesis scheme was adopted to prepare the CsPbBr3 QDs modified by lead bromide (PbBr2) and tetraoctylammonium bromide (TOAB) ligands at room temperature in open air. The prepared CsPbBr3 QDs exhibited a high photoluminescence quantum yield (PLQY) of 96.6% and a low amplified spontaneous emission (ASE) threshold of 12.6 µJ/cm2. Stable ASE intensity with little degradation was also realized from the CsPbBr3 QDs doped with PMMA. Furthermore, the enhanced ASE properties of the CsPbBr3 QDs-doped PMMA based on distributed feedback (DFB) substrate was achieved with a lower threshold of 3.6 µJ/cm2, which is 28.6% of that of the (PbBr2 + TOAB)-treated CsPbBr3 QDs without PMMA. This work exhibits a promising potential in the on-chip light source.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
All-inorganic halide perovskite CsPbX3 (X = Br/Cl/I) quantum dots have achieved a remarkable progress in the fields of the optoelectronic devices such as solar cells, photodetectors [1] and light-emitting diodes [2–6]. In parallel to their great progress in these areas, the perovskite quantum dots (QDs) also reveal a significant potential in the laser field due to their excellent properties, included spectral tunability, high gain, high photoluminescence quantum yield (PLQY), high defect tolerance and so on [7–9]. For example, in 2017, Huang et al. reported a vertical cavity surface emitting laser (VCSEL) consisting of CsPbBr3 QD thin film and two distributed brag reflectors (DBRs), with the lasing threshold of 98 µJ/cm2 [10]. In 2019, Yan et al. developed the CsPbBr3 QDs with 2-hexyldecanoic acid (DA) modification, the amplified spontaneous emission (ASE) threshold of the CsPbBr3 QDs film reached 89.76 µJ/cm2 [11]. And Pourdavoud et al. achieved a distributed feedback (DFB) laser based on the CsPbBr3 QDs thin film with a threshold of 7.2 µJ/cm2 [12]. In 2020, Yan et al. reported a CsPbBr3 QDs whispering-gallery-mode (WGM) laser, the PLQY reached 96% and the lasing threshold achieved 5.47 µJ/cm2 [13]. In 2022, Chen et al. reported the highly ordered superlattice consisting of well-separated individual CsPbBr3 QDs and showed an ASE threshold of 30 µJ/cm2 [14]. From the studies above, we can find that the ASE or lasing behaviors based on the CsPbBr3 QDs has been investigated. However, it is necessary to improve the CsPbBr3 QDs performance to further reduce the pump threshold.
During the synthesis of perovskite QDs, the ligands possess a high diffusion coefficient in solution up to 166 µm2s-1, they easily dissociate from the QDs surface, leading to a large number of the defects aggregating and emerging on the non-stoichiometric QDs surface [15,16]. It needs to further passivate the halide vacancy defects on the surface to achieve the high performance QDs. The surface ligand engineering has been proved to be a viable strategy by introducing a series of ligands to modify crystal surface defects [17]. For instance, Zhu et al. optimized the optical properties of the CsPbBr3 QDs via in situ crystallization with the synergistic effect of 4-bromo-butyric acid (BBA) and oleylamine (OLA) in polar solvents [18]. Zhang et al. prepared the CsPbBr3 nanoclusters by selecting ligands (benzoic acid together with oleylamine) to passivate the surface defects [19]. Zeng et al. synthesized the colloidal CsPbX3 nanocrystals (NCs) with a multi-amine chelating ligand, N′-(2-aminoethyl)-N′-hexadecylethane-1,2-diamine (AHDA), which can anchor on the perovskite surface lattice, leading to an improve of the optical performance [20]. Liu et al. used a molecular superacid of bis(trifluoromethane)sulfonimide (TFSI), which could enhance the photoluminescence (PL) of the CsPbBr3 QDs [21]. Zhang et al. enhanced the optical properties of the CsPbBr3 QDs modified with three different hydrophobic fluorinated aromatic ammonium bromide ligands (oFPEABr, mFPEA-Br and pFPEABr) [22]. However, the above ligand passivation processes were carried out under high temperature and inert gas environment. The stringent environmental condition and high cost for synthesis process have been an inevitable impediment to the commercialization of the perovskite QDs. Therefore, the preparation processes of the CsPbBr3 QDs at room temperature in air have been explored. Such as in 2020, Guan et al. synthesized CsPbBr3@SiO2 QDs via a one-step in situ method under room temperature in air, showing a PLQY of 75% [5]. In 2021, Yin et al. developed the CsPbBr3 nanoplatelets in room temperature with polyethylenimine (PEI) modification, the Br- ion vacancy defect density was reduced and PLQY achieved 40% [23]. Mo et al. prepared the CsPbBr3@ZrO2 NCs at room temperature, presenting a PLQY of 80% and an enhanced stability [24]. In 2022, Luo et al. used the ionic liquids (ILs) 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (NTf2) to modify the CsPbBr3 QDs, which achieved a PLQY of 51% [25]. And Li et al. proposed a synthesis strategy for CsPbBr3@Cs4PbBr6 NCs via tetraoctylammonium bromide ligand induction at room temperature, the PLQY was 94% [6]. According to the these studies, although lots of the preparation methods of CsPbBr3 QDs at room temperature in air have been investigated, it is still desired to further enhance the CsPbBr3 QDs performance to develop the potential of perovskite in lasing field, which is crucial for the development of perovskite optoelectronic devices.
In this work, we synthesized the high performance CsPbBr3 QDs via a simple and rapid ligand-assisted reprecipitation (LARP) method at room temperature in open air. The lead bromide (PbBr2) and tetrabutylammonium bromide (TOAB) were applied as the post-treatment passivation material. Compared to the untreated CsPbBr3 QDs, the (PbBr2 + TOAB)-treated CsPbBr3 QDs showed a higher PLQY of 96.6% and a low ASE threshold of 12.6 µJ/cm2. Then the CsPbBr3 QDs were introduced into the PMMA solution, and the CsPbBr3 QDs-doped PMMA film based on the distributed feedback (DFB) substrate was prepared, exhibiting a lower threshold of 3.6 µJ/cm2, which is 28.6% of that of the (PbBr2 + TOAB)-treated CsPbBr3 QDs without PMMA. This study could further promote the practical applications of the perovskite QDs.
2. Experimental section
2.1 Chemicals
Cesium carbonate (Cs2CO3, 99.99%) was purchased from Macklin. Lead bromide (PbBr2, 99.999%) and Tetraoctylammonium bromide (TOAB, 98%) were obtained from Aladdin. Propionic acid (PA, ≥ 99.5%), Oleylamine (OAM, 80-90%), N, N-Dimethylformamide (DMF, anhydrous, 99.9%), Hexane (HEX, 99.5%), Isopropanol (IPrOH, 99.5%) were purchased from Sigma-Aldrich. Poly (methyl methacrylate) (PMMA, 99%) was obtained from MERYER. All chemicals were used without any further purification.
2.2 Synthesis and passivation of the CsPbBr3 QDs
The CsPbBr3 QDs were synthesized via the ligand-assisted reprecipitation method. The details were as follows: firstly, 3 mL hexane, 3 mL isopropanol and 10 µL Cs precursor solution (0.18 mmol Cs2CO3 was added in 100 µL PA) were thoroughly mixed. Secondly, 200 µL Pb precursor solution (0.42 mmol PbBr2 dissolved in the solution (750 µL, VPA: VDMF: VOAM, 1:1:1)) was quickly added under vigorous stirring for about 15 s. The CsPbBr3 QDs suspension liquid was centrifuged at 6000 rpm for 4 min to remove unreacted precursors, and the precipitates were dispersed in the 100 µL toluene solution. Then, the dual-ligand passivation solution (0.11 mmol PbBr2 and 0.2 mmol TOAB were dissolved in the 100 µL toluene solution) was prepared. Finally, the original CsPbBr3 QDs toluene solution was added with different volume passivation solutions, respectively. The resultant solutions of the (PbBr2 + TOAB)-treated CsPbBr3 QDs were collected for further characterization and device fabrication.
2.3 Fabrication of the CsPbBr3 QDs-doped PMMA films
The 100 µL PMMA solutions with different concentrations were added into the (PbBr2 + TOAB)-treated CsPbBr3 QDs solution. Then, the CsPbBr3 QDs-doped PMMA solutions were spin-coated on the glass substrates. The speed of spin-coating was 1000 rpm, followed by annealing at 100 °C for 10 min.
2.4 Fabrication of the CsPbBr3-doped PMMA film based on the DFB
The CsPbBr3 QDs-doped PMMA solution was spin-coated onto the DFB substrate with the spin-coating speed of 1000 rpm. The DFB substrate was fabricated by the UV-nanoimprint, [26] as follows: firstly, the UV curing adhesive was dropped onto the DFB SiO2 template, and it was squeezed between the glass cover plate and DFB SiO2 temple to make the UV curing adhesive into the DFB master. After the curing with UV lamp for 30 s, the glass cover plate was removed first and the UV curing adhesive was separated from the DFB SiO2 template to obtain the DFB substrate.
2.5 Measurement
Absorption spectra were measured using a U-3010 UV-Vis absorption spectrometer (Hitachi, Japan). PL spectra were obtained by a fluorescence spectrometer (Fluoromax-4 spectrofluometer, Japan). PLQY and photoluminescence lifetime were measured using a FLS9 fluorescence spectrometer (Edinburgh, England). Scanning electron microscopy (SEM) was conducted on a Quanta 250 field emission SEM (FEI, USA). Transmission electron microscopy (TEM) was tested using a JEM-2100Plus TEM (JEOL, Japan). X-ray diffraction spectra were carried out with a X-ray diffractometer (D8 Advance, Bruker). Proton nuclear magnetic resonance spectroscopies were collected by a 400 MHz NMR (JNM-ECZ400S/L1, Japan). XPS spectra were carried out with a Thermo Fisher ESCALAB Xi+ EDS. The devices were excited with a pulsed Nd:YAG laser (355 nm/10 Hz/5.5 ns), and the fiber spectrometer (USB2000) was used to record ASE spectra.
3. Results and discussion
3.1 Characterization of the CsPbBr3 perovskite QDs
Figure 1(a) shows the synthesis scheme of the CsPbBr3 QDs and the photograph of the (PbBr2 + TOAB)-treated CsPbBr3 QDs solution under daylight. Figures 1(b) and 1(c) illustrate the absorption and PL spectra of the untreated and (PbBr2 + TOAB)-treated CsPbBr3 QDs. It can be seen that both samples exhibit the same absorption peaks at 501 nm and the PL peaks centered at around 513 nm. At the same time, the PLQYs were measured (as shown in Figure S1). Compared with the untreated QDs, the PLQY of the (PbBr2 + TOAB)-treated CsPbBr3 QDs was increased from 52.5% to 96.6%. In addition, Fig. 1(d) displays the time resolved photoluminescence lifetimes (TRPL) of the untreated and (PbBr2 + TOAB)-treated CsPbBr3 QDs. The average lifetimes were fitted by ${\tau _{average}} = \frac{{{A_1}\tau _1^2\textrm{ + }{A_2}\tau _2^2}}{{{A_1}\tau _1^{}\textrm{ + }{A_2}\tau _2^{}}}$, where A1 and A2 are amplitudes component, the fast decay component τ1 is ascribed to the trap-assisted recombination at the surface of QDs, and the slow decay component τ2 is associated with the radiative recombination inside the nanocrystal and photoluminescence quantum yield [27]. Then we can know that the average lifetimes of the untreated QDs and (PbBr2 + TOAB)-treated CsPbBr3 QDs are 59.38 and 77.47 ns, respectively (as shown in the inset of Fig. 1(d)). According to the Fig. 1, it demonstrates that the passivation scheme with TOAB-assisted PbBr2 can effectively enhance the performances of the CsPbBr3 QDs, and reduce bromine vacancy traps as well as non-radiative centers on the nanocrystal surface [28].
Fig. 1. (a) Schematic illustration of the CsPbBr3 QDs synthesis process and the photograph of the (PbBr2 + TOAB)-treated CsPbBr3 QDs solution under daylight, (b) absorption, (c) PL spectra and (d) time resolved photoluminescence lifetimes (TRPL) of the untreated and (PbBr2 + TOAB)-treated CsPbBr3 QDs.
To further evaluate the effect of the dual-ligands on the CsPbBr3 QDs, we analyzed the transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) spectra, and proton nuclear magnetic resonance spectroscopy (1HNMR) of the untreated and (PbBr2 + TOAB)-treated CsPbBr3 QDs as shown in Fig. 2. As shown in the Figs. 2(a) and 2(b), both the untreated and (PbBr2 + TOAB)-treated CsPbBr3 QDs samples all possess cube perovskite morphologies with the crystal interplanar distance of 0.41 nm.
Fig. 2. TEM images of the (a) untreated, and (b) (PbBr2 + TOAB)-treated CsPbBr3 QDs. The insets in (a) and (b) show the crystal interplanar distance and selected-area electron diffraction (SAED) images of the QDs, (c) XRD spectra, XPS spectra of (d) Pb 4f, (e) Br 3d, and (f) 1HNMR spectra of the untreated and (PbBr2 + TOAB)-treated CsPbBr3 QDs.
However, we can note that compared to the (PbBr2 + TOAB)-treated CsPbBr3 QDs, there are more “black dots” on the surface of the untreated QDs, this is because of the PbBr2 products formation by the decomposition of the untreated CsPbBr3 QDs, which is easily affected by external environment [29]. After the CsPbBr3 QDs were treated with PbBr2 + TOAB, the passivation solution can effectively reduce bromine vacancy traps as well as non-radiative centers on the CsPbBr3 QDs surface. The crystal structure stability of the ligand treated CsPbBr3 QDs will be improved, then the crystal structure is not easy to be decomposed [30]. Figure 2(c) exhibits the XRDs of both samples, the diffraction peaks of them are located at 15.081°, 21.498°, 30.378°, 34.195°, 37.603°, corresponding to (001), (110), (002), (120), (121) crystalline planes of the monoclinic phase (PDF #18-0364) [31]. This proves that the crystalline phase of the (PbBr2 + TOAB)-treated QDs is consist with the CsPbBr3 material, and there is no recrystallization phenomenon. Furthermore, we can find that the diffraction peak intensities of the (PbBr2 + TOAB)-treated QDs are much stronger than that of the untreated QDs. Therefore, compared to the untreated CsPbBr3 QDs, the (PbBr2 + TOAB)-treated CsPbBr3 QDs exhibited a higher crystallinity. Figures 2(d) and 2(e) represent the XPS spectra of the untreated and (PbBr2 + TOAB)-treated QDs. In Fig. 2(d), as for the untreated QDs, two peaks appear at 138.1 eV (4f7/2) and 142.9 eV (4f5/2) [32], corresponding to the Pb2+ ions. And the Br 3d5/2 and 3d3/2 peaks are located at 68 and 68.9 eV (as shown in Fig. 2(e)). After the QDs were treated with PbBr2 + TOAB, the Pb 4f as well as Br 3d XPS spectra shift to a lower binding energy (see Figs. 2(d) and 2(e)). This is probably because that the chemical environment around the QDs after post-treatment was changed, and the Pb-Br-TOAB was formed on the QDs surface. The interaction of the Pb-Br-TOAB is much stronger than that of the Pb-Br, leading to a decrease of the Pb-Br bond energy after passivation [22,33]. The 1HNMR result also supports this reason. It can be seen from Fig. 2(f) that the characteristic peak of the TOAB appears in that of the (PbBr2 + TOAB)-treated QDs, which means that ligands were anchored on the surface of the CsPbBr3 QDs.
3.2 ASE properties of the CsPbBr3 QDs films
To explore the ASE properties of the (PbBr2 + TOAB)-treated CsPbBr3 QDs, the passivation solutions with different volumes were introduced to treat crystal surface defects of the CsPbBr3 QDs. Figures 3(a)-(e) show the ASE spectra of the CsPbBr3 QDs with different volume passivation solutions (100, 200, 300, 400 and 500 µL). We found that all the (PbBr2 + TOAB)-treated CsPbBr3 films could exhibit the ASE phenomena. The QD films under low excitation fluences exhibit a broad spontaneous emission peak. With the increase of the pump intensity, a peak centered at 527 nm emerges and rapidly increases with an extremely narrow full width half maximum (FWHM). The ASE thresholds are 22, 14.5, 12.6, 16.6 and 19.1 µJ/cm2, with the increase of the passivation solution volume from 100 to 500 µL. We can find that the ASE threshold of the QDs decreases firstly and then increases. A lowest ASE threshold can be obtained when the 300 µL passivation solution was added. This phenomenon is because that when the passivation solution is too little, the QDs surface defects can’t be completely passivated, which fails to suppress the Auger recombination well [34]. And when the excessive passivation solution is added, a bromine-rich environment is formed while the concentration of the CsPbBr3 QDs is reduced, resulting in an increase of the ASE threshold. For comparison, the device based on the untreated CsPbBr3 QDs was prepared, and no ASE behavior was found (see Figure S2 in Supplement 1). It proves that the passivation solution can improve the optical performance of the CsPbBr3 QDs.
Fig. 3. The emission spectra of the CsPbBr3 QDs treated with passivation solution of (a) 100µL, (b) 200µL, (c) 300µL, (d) 400µL, (e) 500µL, at different pumping intensities, the insets in (a)-(e) show the dependence of the peak intensity and the FWHM of the emission spectra on the pump energy intensity. (f) Dependences of the emission intensities on the pump energy intensity for the devices. The inset in (f) shows the threshold of the (PbBr2 + TOAB)-treated CsPbBr3 QDs films as a function of the passivation solution volume.
In addition, to enhance the stability of the CsPbBr3 QDs, the PMMA was merged with the (PbBr2 + TOAB)-treated CsPbBr3 solution. Figure 4 shows the ASE spectra of the CsPbBr3 QDs-doped PMMA films with different PMMA concentrations. We can see that compared to the CsPbBr3 QDs without PMMA, the CsPbBr3 QDs-doped PMMA films exhibit lower ASE thresholds. As PMMA concentration increases from 20 to 80 mg/mL, the ASE threshold firstly decreases, and then increases (see Fig. 4(f)). When the PMMA concentrate is 40 mg/mL, the lowest ASE threshold of 7.2 µJ/cm2 can be reached. This is due to that when the concentration of PMMA is too low, the thickness of the prepared film can’t provide sufficient optical feedback. When the concentration is too high, the CsPbBr3 QDs will exhibit a poor dispersion, leading to a poor-quality film and QDs quenching. Therefore, there is an optimal PMMA concentration to balance the film thickness and quality.
Fig. 4. The emission spectra of the (PbBr2 + TOAB)-treated CsPbBr3 QDs (a) without PMMA, and with (b) 20 mg/mL, (c) 40 mg/mL, (d) 60 mg/mL, (e) 80 mg/mL PMMA, the insets in (a)-(e) show the peak intensity and FWHM of the emission spectra. (f) Dependences of the emission intensities on the pump energy intensity for the devices. The inset in (f) shows the thresholds of the (PbBr2 + TOAB)-treated CsPbBr3 QDs-doped PMMA films as a function of the PMMA concentration.
We further investigated the ASE stability of the (PbBr2 + TOAB)-treated CsPbBr3 QDs doping without and with PMMA, their time-dependent emission intensity tests under pump intensity of 14.5 µJ/cm2 were performed at room temperature in air (see Fig. 5). We can find that the ASE intensity of the (PbBr2 + TOAB)-treated CsPbBr3 QDs without PMMA remains at 84.3% of the initial value in air exposure for 7 days, while the ASE intensity of the CsPbBr3 QDs-doped PMMA film can still remain at 97% of the initial value. This indicates that the PMMA can further protect the CsPbBr3 QDs from degradation in the air [35], it can effectively improve the stability of the CsPbBr3 QDs.
Fig. 5. Normalized emission intensities of the (PbBr2 + TOAB)-treated CsPbBr3 QDs without and with PMMA over 7 days under 14.5 µJ/cm2 pump intensity.
3.3 CsPbBr3 QDs-doped PMMA based on distributed-feedback substrate
Furthermore, to further enhance the ASE properties of the CsPbBr3 QDs, the (PbBr2 + TOAB)-treated CsPbBr3 QDs-doped PMMA solution was spin-coated onto the second-order DFB substrate, which is shown in Fig. 6(a). Figure 6(b) shows the SEM of the DFB substrate. We can observe that the DFB grating period is 257 nm, while maintaining the duty cycle of 60%. Figure 6(c) displays the emission spectra of the CsPbBr3 QDs-doped PMMA based on the DFB substrate. A sudden increase of the peak intensity and narrowing of the emission peak reveal an achievement of the ASE action, and the ASE threshold appears at 3.6 µJ/cm2 (see Fig. 6(d)), which is 28.6% of that of the (PbBr2 + TOAB)-treated CsPbBr3 QDs without PMMA. Table 1 gives a summary of the pump thresholds and wavelengths in other reports. Comparing to the other reports, our study exhibits a lower pump threshold.
Fig. 6. (a) Scheme of the CsPbBr3 QDs-doped PMMA film based on distributed-feedback substrate, (b) SEM diagram of the DFB resonant cavity, (c) the emission spectra of the CsPbBr3 QDs-doped PMMA based on the DFB at different pump intensities, (d) the peak intensity and FWHM of the emission spectra.
Table 1. Summary of the perovskite optical properties
4. Conclusions
In summary, we synthesized the CsPbBr3 QDs via a facile and low-cost method at room temperature, and used dual-ligand (PbBr2 + TOAB) to enhance the performances of the CsPbBr3 QDs. Compared to the untreated CsPbBr3 QDs, the (PbBr2 + TOAB)-treated CsPbBr3 QDs exhibited a higher near-unity PLQY enhancing from 52.5% to 96.6% and a low ASE threshold of 12.6 µJ/cm2. Furthermore, the PMMA was merged with the (PbBr2 + TOAB)-treated CsPbBr3 QDs, resulting in a significantly enhanced stability and ASE performance of the CsPbBr3 QDs. Notably, a lower pump threshold of 3.6 µJ /cm2 was achieved for the CsPbBr3 QDs-doped PMMA based on the DFB, it is 28.6% of that of the (PbBr2 + TOAB)-treated CsPbBr3 QDs without PMMA. Our work illustrates the promising potential in the on-chip light source.
Funding
National Natural Science Foundation of China (52177225, 61605105); Young Talent Support Program of Shaanxi Province University (20200113); China Postdoctoral Science Foundation (2019M653635); Scientific Research Plan Projects of Shaanxi Education Department (20JG003).
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.
Supplemental document
See Supplement 1 for supporting content.
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