Enhanced three-photon absorption excited at near-infrared laser pulses and stability of MAPbBr₃ quantum dots encapsulated in mesoporous single MOF-5 crystals

Abstract

Organometallic halide perovskites have attracted attention due to their excellent optical properties. However, the inherent instability limits their wide application in the field of optoelectronics. In this study, a distribution immersion method was used to embed MAPbBr₃ perovskite quantum dots into mesoporous MOF-5 microcrystals, constructing an effective host–guest structure. Compared with MAPbBr₃ quantum dots, the MAPbBr₃@MOF-5 composite not only exhibits superior optical performance but also significantly enhanced long-term stability and thermal stability. Nonlinear optical analysis shows that under the excitation of near-infrared femtosecond laser pulses, the MAPbBr₃@MOF-5 composite exhibits significantly enhanced multiphoton absorption capability. Notably, its effective three-photon absorption coefficient (α₃η ≈ (27.4 ± 3.1) × 10⁻⁵ cm³ GW⁻²) is an order of magnitude larger than that of MAPbBr₃. These outstanding characteristics indicate that the MAPbBr₃@MOF-5 composite has great potential for applications in photonics and information communication, especially in the manufacture of multifunctional nonlinear optical devices and future micro–nano photonic integration technologies.

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Introduction

Hybrid organic–inorganic halide perovskite materials, such as MAPbX₃ (X = Cl, Br, I), have garnered significant attention due to their potential applications in various fields. These materials exhibit exceptional optoelectronic properties, making them highly promising for use in solar cells, light-emitting diodes (LEDs), lasers, photodetectors, and multifunctional sensors.^1–5 Recently, colloidal perovskite MAPbX₃ nanocrystals (NCs), also known as quantum dots (QDs), have been attracting attention due to their excellent luminescent properties, which include high photoluminescence (PL) quantum yields, tunable emission wavelengths, short-radiative lifetimes, and narrow-band emissions.^6–8 In addition to excellent linear optical properties, MAPbX₃ NCs also exhibit remarkable nonlinear optical (NLO) responses.^9,10 Among them, multi-photon excited photoluminescence (MPEPL) is particularly prominent. MPEPL involves the absorption of multiple photons, causing an electron to jump from the ground state to the excited state while leaving a hole in the ground state. Subsequently, the electron relaxes to the lowest energy level of the excited state and radiatively recombines with the hole in the ground state. MPEPL has a wide range of applications in the fields of optoelectronics, transmission of photonic signals and biomedicine.^11,12 However, although MAPbX₃ exhibits excellent performance, its practical applications are limited by the instability of organic cations, which are prone to rapid degradation under conditions such as moisture, UV radiation, oxygen and high temperature.¹³ Therefore, an improvement in the stability of perovskite quantum dots is a key challenge in realizing their optical applications. One feasible solution is to design and fabricate hybrid composite materials incorporating perovskite QDs.

Metal–organic frameworks (MOFs) are a class of porous crystalline materials with fascinating properties, including tunable pore sizes and high specific surface areas. These characteristics make them promising platforms for host–guest chemistry.^14–16 MOFs can be conveniently designed and self-assembled to provide ideal host environments for various guest species.^17–19 MOF-5 (C₂₄H₁₂O₁₃Zn₄) features a three-dimensional structure and is characterized by its high specific surface area, significant porosity, and exceptional thermal stability.²⁰ In comparison to MOFs with one- or two-dimensional structures, such as MOF-8,²¹ which is defined by its one-dimensional chain structure, MOF-5 offers a more comprehensive platform for exploring the influence of pore size on doped quantum dots and was therefore selected for this study. There have been reports of integrating various QDs into MOFs to develop novel composite materials with superior combined properties compared to their individual components.²² Although there have been studies on loading perovskite MAPbX₃ QDs into different hosts to enhance their stability, a more extensive exploration for improving the stability of these materials is still needed.^15,23,24 The fundamental properties, mechanisms, and nonlinear optical characteristics of MAPbX₃ QDs encapsulated in MOFs remain relatively unexplored.

Here, we report a stepwise impregnation method to embed perovskite MAPbBr₃ QDs into mesoporous MOF-5 single crystals. Compared to MAPbBr₃ QDs, the resulting MAPbBr₃@MOF-5 composite materials exhibited improved long-term storage stability and high thermal stability. Moreover, we found that the three-photon absorption coefficient of MAPbBr₃@MOF-5 was significantly enhanced compared to MAPbBr₃ microcrystals.^25,26 This enhancement is attributed to the strong confinement and surface passivation provided by the MOF-5 framework.

Experimental section

The preparation process of the MAPbBr₃@MOF-5 composite is shown in Scheme 1, which includes three steps: (a) synthesis of mesoporous MOF-5 microcrystals by a solvothermal method, (b) introduction of Pb²⁺ into the pores of MOF-5 by an immersion method, and (c) addition of Pb²⁺@MOF-5 to MABr solution to facilitate the formation of perovskite quantum dots within the pores of MOF-5.

Scheme 1 Synthesis process of MAPbBr₃@MOF-5.

The structure of MAPbBr₃@MOF-5 was characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and transmission electron microscopy (TEM). To test the thermal stability, thermogravimetric analysis (TGA) was performed on MAPbBr₃@MOF-5. Additionally, PL, time-resolved PL, and absorption spectra were measured.

Detailed information on the materials and chemicals used, synthesis of MOF-5, MAPbBr₃, and the MAPbBr₃@MOF-5 composite, detection methodology, and instrumental techniques can be found in the ESI.†

Results and discussion

MOF-5 is an excellent metal–organic framework material composed of PTA and tetrahedral ZnO₄ units, featuring high porosity and good stability. The microporous MOF-5 is synthesized by dissolving Zn(NO₃)₂·6H₂O and PTA in a DMF solution and heating at 135 °C for 24 hours. Its micropore sizes are about 1.85 nm,²⁷ which limits the encapsulation of MAPbBr₃ into MOF-5. Therefore, in order to obtain mesoporous MOF-5 crystals, CTAB and TMB were used as templating agents in this work to expand the porosity of the MOF-5 microcrystals. CTAB occupies the pore channels of the MOF-5 microcrystals, and TMB further expands the pore channel sizes by swelling CTAB micelles, thereby increasing the pore sizes of the MOF-5 microcrystals.^20,28 By changing the molar ratios of the reactants, we synthesized a series of MOF-5 microcrystals and studied the changes in PL intensity of MAPbBr₃ encapsulated in MOF-5 microcrystals synthesized with different reactant molar ratios (Fig. S1, ESI†). When the molar ratio is 1 : 2 for CTAB to TMB, the PL intensity of the hybrid material is the strongest compared to other ratios after encapsulating MAPbBr₃ QDs. This may be attributed to the fact that MOF-5 synthesized using this molar ratio has the highest mesoporous porosity and the most suitable mesoporous pore size. The introduction of CTAB can expand the mesoporous volume of MOF-5, but this expansion is limited. Then, using TMB to swell CTAB micelles can significantly increase the pore volume. However, excessive addition of TMB may damage the porous structure of the MOF-5 crystals.²⁰ Therefore, there is an optimal molar ratio to achieve the best encapsulation of MAPbBr₃ QDs. Further confirmation of the effect of CTAB and TMB addition on the pore expansion of the MOF-5 samples was achieved through characterization by XRD and SEM. As shown in Fig. S2 (ESI†), the XRD pattern of the treated MOF-5 sample deviates from that of the theoretical MOF-5 and some impurity peaks can be observed. This may be attributed to the increase in pore size of the treated MOF-5, which consequently leads to changes in its crystal structure, including the introduction of defects (Fig. S2(a), ESI†).²⁰ After CTAB is embedded, the MOF particles maintain their cubic morphology, but larger cavities are observed within the particles (Fig. S2(b), ESI†), indicating effective expansion of the internal pore structure.

In Fig. 1(a), the XRD pattern of MAPbBr₃@MOF-5 is shown. Both the XRD peaks of MOF-5 and MAPbBr₃ in the XRD pattern of MAPbBr₃@MOF-5 were observed. After embedding QDs, MAPbBr₃@MOF-5 demonstrated peaks of 14.82°, 30.00°, 33.64° and 43.06° corresponding to the planes of (010), (200), (210) and (202) of MAPbBr₃, respectively. The TEM image (Fig. 1(b)) shows that the PeQDs are embedded in the holes of MOF-5. Based on the size distribution histogram and Gauss fitting of MAPbBr₃ QDs (Fig. 1(c)), the average size of the QD particles is 12 nm. The average size of the QDs in other regions, as determined from the TEM results, is shown in Fig. S2(c–f) (ESI†). The HRTEM images are shown in Fig. 1(d and e). The interplanar distance from the fringes is 2.96 Å, which could be attributed to the (200) planes of the MAPbBr₃. The SEM mapping images, as shown in Fig. 1(f), show the uniform distribution of the four elements, indicating the coexistence of the MAPbBr₃ QDs and MOF-5. The inset of Fig. S2(b) (ESI†) displays an image of the MAPbBr₃@MOF-5 sample. Fig. S3 (ESI†) depicts the TGA analysis of MAPbBr₃@MOF-5, showing that when heated to 400 °C in a nitrogen atmosphere, the composite material experiences a mass loss of only 12.1%. The result indicates that MAPbBr₃@MOF-5 demonstrates good thermal stability and possesses a high decomposition temperature.

Fig. 1 Structure characterization. (a) XRD patterns of MAPbBr₃@MOF-5. (b) TEM (scale bar: 100 nm) and (c) histogram of MAPbBr₃ particle size distribution with Gauss fitting. (d) and (e) HRTEM of MAPbBr₃@MOF-5 (scale bar, 20 nm). (f) SEM mapping of elements (Zn, N, Pb and Br) in MAPbBr₃@MOF-5 (scale bar, 20 μm).

The fluorescence properties of the synthesized MAPbBr₃@MOF-5 composite were subsequently analyzed. The PL spectra of MAPbBr₃ and MAPbBr₃@MOF-5 are shown in Fig. 2(a). After embedding QDs into MOF-5, the MAPbBr₃@MOF-5 composite exhibits a narrow green peak centered at 533 nm under UV excitation (375 nm). The narrow green peak of MAPbBr₃@MOF-5 can be attributed to the emission of MAPbBr₃ QDs. Compared to MAPbBr₃, the PL peak wavelength of MAPbBr₃ QDs in the MAPbBr₃@MOF-5 composite has a blue shift of 20 nm. The absorption spectra in Fig. 2(b) also show a blue shift in the absorption edges of MAPbBr₃@MOF-5 compared to MAPbBr₃ (more details can be found in the section titled ‘Characterization’ in ESI†). These properties can be attributed to the strong quantum confinement effect of perovskite QDs. When the size of metal halide perovskite QDs is reduced to the nanometer scale, the electronic band structure of the perovskite undergoes changes. Due to the quantum confinement effect, the emission peak shifts from green to blue.^29,30 The PL decay curves of MAPbBr₃@MOF-5 and MAPbBr₃ monitored at 533 nm under UV light (375 nm) excitation are shown in Fig. 2(c). The PL decay of MAPbBr₃@MOF-5 can be described by a bi-exponential fit.³¹ The average fluorescence lifetime of MAPbBr₃ increased from 2.4 ns to 28.5 ns for MAPbBr₃@MOF-5. Fig. S4(a) (ESI†) illustrates the variation of the photoluminescence (PL) lifetime within the temperature range from 300 to 430 K and the derived lifetime decreases from 43.6 ns to 5.5 ns, as shown in Fig. S4(b) (ESI†).^32,33 As the temperature increases, the thermal excitation drives the lattice structure to undergo distortion. Simultaneously, the generation of thermal defects initiates a pronounced non-radiative recombination process.³⁴ The non-radiative decay rate of MAPbBr₃@MOF-5 rises with increasing temperature. This elevated non-radiative decay rate results in a significant reduction in the PL lifetime. It is well-known that the PL decay of perovskite materials is often attributed to radiative and non-radiative recombination processes of excitons, with non-radiative recombination typically associated with short lifetimes in the lifetime fitting.³⁵ The extended lifetime may be attributed to surface passivation, which reduces the nonradiative relaxation of surface traps in encapsulated MAPbBr₃ QDs.^36–38 Additionally, there is significant spectral overlap between the emission of MOF-5 crystals and the absorption of MAPbBr₃ QDs. Therefore, energy transfer from MOF-5 crystals to MAPbBr₃ QDs is expected to occur in MAPbBr₃@MOF-5.³⁹ Moreover, by adjusting the concentration of MABr, we can manipulate the emission of MAPbBr₃@MOF-5. Fig. 2(d) displays a series of PL spectra for MAPbBr₃@MOF-5, showing that as the concentration of MABr increases (0.5, 1, 1.5, 2, 2.5 mg mL⁻¹), the PL intensity at first increases and then decreases, reaching a maximum at a concentration of 1.5 mg mL⁻¹. As the concentration of MABr increases, the amount of MAPbBr₃ QDs in the MAPbBr₃@MOF-5 composite rises, leading to a proportional enhancement of the composite's photoluminescence (PL) intensity. However, if the amount of MABr added exceeds a certain threshold, quantum dots aggregation may occur, resulting in a reduction in PL intensity.

Fig. 2 (a) The normalized PL spectra, (b) the UV-Vis absorption, (c) the PL decay plots of MAPbBr₃@MOF-5 and MAPbBr₃ and (d) the PL spectra of MAPbBr₃@MOF-5 synthesized with varying concentrations of MABr.

Notably, we have observed a substantial improvement in the stability of the MAPbBr₃@MOF-5 composite material. We performed thermal stability tests and long-term stability tests on the composite material and compared the results with those of MAPbBr₃ powder. As shown in Fig. 3(a), MAPbBr₃@MOF-5 maintained 49.3% of its original photoluminescence (PL) intensity at 55 °C, and even still emitted light at 125 °C. After 90 days of exposure to air at room temperature, MAPbBr₃@MOF-5 retained 63.9% of its initial PL intensity, whereas the MAPbBr₃ QDs showed complete quenching after 14 days (Fig. 3(b)). Based on these results, we can conclude that the MAPbBr₃@MOF-5 composite is significantly more stable in air and at high temperatures compared to MAPbBr₃ PeQDs.

Fig. 3 Thermal stability test (a) and long-term storage stability test (b) of MAPbBr₃@MOF-5. Insets: Normalized PL intensity as a function of experimental temperature (a) and storage time (b) for MAPbBr₃@MOF-5 (red line) and MAPbBr₃ (black line).

Under excitation by near-infrared femtosecond laser pulses, MAPbBr₃@MOF-5 demonstrates excellent MPEPL (more experimental details can be found in the section titled ‘Multi-photon excited photoluminescence’ in the ESI†). Fig. S5 (ESI†) illustrates the schematic diagram of the multi-photon excitation photoluminescence measurements. Fig. 4(a and b) shows the MPEPL spectra of the MAPbBr₃@MOF-5 sample under excitation at 800 nm for 2-photon absorption (2PA) and 1200 nm for 3-photon absorption (3PA), respectively. The relationship between PL intensity and pump intensity exhibits the characteristics of multiphoton absorption. The insets of Fig. 4(a and b) show that the slopes are 2.02 at 800 nm and 3.04 at 1200 nm, indicating 2PA and 3PA, respectively. Fig. S6 (ESI†) presents the MPEPL spectra of the MAPbBr₃ sample under excitation at 800 nm for 2PA and 1200 nm for 3PA, respectively. The insets of Fig. S6(a) and (b) (ESI†) display that the slopes are 1.99 at 800 nm and 2.96 at 1200 nm, indicating 2PA and 3PA, respectively.

Fig. 4 Multiphoton-excited photoluminescence (MPEPL) of MAPbBr₃@MOF-5. (a) 2PPL spectra at 800 nm, and (b) 3PPL spectra at 1200 nm.

The MPEPL signal was collected and processed according to previous reports.^25,31 The multiphoton absorption cross section (σ_n) and multiphoton absorption coefficient (α_n) can be converted:⁴⁰

In the above equation, σ_n is the n-photon-absorption cross section; ρ is the sample molar concentration; ℏω is the photon energy of the incident laser beam; α_n is the n-photon-absorption coefficient; N_A is Avogadro's constant.

Using the above formulas, the effective multiphoton absorption coefficients (α₃η) of MAPbBr₃@MOF-5 are calculated to be α₂η ≈ (1.23 ± 0.05) cm GW⁻¹ for 2PA (800 nm), and α₃η ≈ (27.4 ± 3.1) × 10⁻⁵ cm³ GW⁻² for 3PA (1200 nm). Table 1 shows that the α₂η of MAPbBr₃@MOF-5 remains at the same level as perovskites, and the α₃η of MAPbBr₃@MOF-5 is an order of magnitude higher than that of perovskites. Moreover, the consistency of α_nη for MAPbBr₃@MOF-5 across different excitation intensities highlights its outstanding performance, especially in three-photon frequency conversion.

Table 1 Comparison of effective multiphoton absorption coefficients, α_nη, of MAPbBr₃, MAPbI₃ and MAPbBr₃@MOF-5

Multiphoton excitation	α n η (cm GW^−1) (at 800 nm)	α 3 η (10^−5 cm^3 GW^−2) (at 1200 nm)
MAPbBr3@MOF-5	1.23 ± 0.05	27.4 ± 3.1	This work
MAPbBr3 single crystal	7.35 ± 0.21		Ref. 9,10,25 and 41
MAPbBr3 micro crystal		1.90	Ref. 25 and 26
MAPbI3 single crystal	9.2	1.33	Ref. 9,42 and 43

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This result indicated the influence of the MOF-5 framework in confining MAPbBr₃ QDs and providing protection, significantly enhancing their nonlinear optical properties. Recent studies have shown that due to variations in surface free energy, PeQDs tend to nucleate and grow within the pores of MOFs, as this process can reduce the energy barrier and critical radius.^31,44 Without the confinement of pores, PeQDs would aggregate and cause fluorescence quenching. When MAPbBr₃ is encapsulated into MOF-5, its growth is restricted by the pores of MOF-5, leading to more effective surface passivation. The quantum confinement effect will significantly impact the band structure and dielectric properties of PeQDs, leading to substantial changes in the multiphoton absorption coefficient.⁴⁵ This surface passivation can reduce nonradiative recombination at the surface traps and increase the probability of multiphoton transitions.^46–48 Additionally, the presence of local field effects may enhance the nonlinear optical properties of the composite.⁴⁶ Above all, interfacial energy transfer from transition-metal ions (Zn²⁺) may enhance their optical performance.⁴⁹ Therefore, MAPbBr₃ QDs encapsulated in MOF-5 exhibit a significantly enhanced multiphoton absorption coefficient.

Conclusions

In summary, we have successfully embedded MAPbBr₃ QDs into the pores of mesoporous MOF-5 through a stepwise immersion strategy. The resulting MAPbBr₃@MOF-5 composites not only exhibit excellent optical performance but also demonstrate outstanding long-term stability and thermal stability. Furthermore, the confinement and surface passivation offered by the MOF-5 framework significantly enhance the nonlinear optical properties of the composites. Specifically, the two-photon absorption coefficient of MAPbBr₃@MOF-5 is comparable to that of pure MAPbBr₃ perovskite, while its three-photon absorption coefficient is an order of magnitude greater than that of the pure perovskite. Given its exceptional optical characteristics and high stability, the MAPbBr₃@MOF-5 composites present great potential for applications in biomedicine, optoelectronic devices, and future photonic integration technologies.

Data availability

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

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is supported by the STI 2030-Major Project (2022ZD0209900), the National Natural Science Foundation of China (No. 11904206), the Instrument Improvement Funds of Shandong University Public Technology Platform (ts20230103), the Shenzhen Science and Technology Program (JCYJ20220531103016036), and the Shenzhen Key Laboratory of 2D Metamaterials for Information Technology (ZDSYS201707271014468).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc05087h

‡ Both Wenqing Wang and Renhua Liu contributed equally to this manuscript.

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