Luminescent metal-organic frameworks (LMOFs) are a class of interesting and well-investigated MOF materials, which have shown remarkable prospects in the past and have been widely applied in different fields. However, due to their organic hybrid aspect, micro-/nano-patterning LMOFs in devices via a conventional semiconductor process is very challenging. In this work, we have introduced an elegant technique via nonlinear photon-chemical effect to induce the synthesis and growth of LMOFs. A facile technique for local synthesis and micro-pattering Tb-based luminescent metal organic frameworks (Tb(BTC)·G) from a solution of precursors is achieved. A single step approach micro-patterning for device integration with simultaneous chemical synthesis was proposed. Micro-devices with excellent fluorescence performance based on Tb(BTC)·G have been demonstrated. This work first suggested a high resolution bottom-up micro-patterning technique for MOF device fabrication using femtosecond laser direct writing, showing great potential on MOF based micro/nano-devices integration, especially promising for patterning high resolution luminescent MOF devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Metal-organic frameworks (MOFs) are compounds comprising metal ions coordinated to organic ligands that have attracted much attention in recent years due to their high surface area [1,2], well controlled porosity [3,4] and excellent electrochemical properties [5,6] in a wide variety of fields such as light emitting, biomedical sensing and catalysts [7–10]. Solvothermal synthesis is the most popular technique for producing MOFs. It is carried out conventionally in a Teflon reactor with a convention oven technique, which generally takes several hours to generate MOFs . Even with the help of a microwave oven, it still requires several minutes to generate high quality MOFs . The MOFs synthesized by these methods are in the form of crystallized powders. In order to integrate these crystallized powders into devices, many interesting patterning techniques, such as inkjet printing , micro-contact printing , photolithography , and electron-beam lithography , have been introduced to this area. However, these pattering techniques consist of complicated multistep procedures and require a harsh environment. These time consuming and complicated procedures are not suitable for patterning MOFs in many applications. It is inevitable to bring structural damage into materials based on these patterning techniques, which usually involves organic solvent in the process , and reduces the porosity and the surface area leading to performance degradation.
In this work, we have demonstrated the feasibility of femtosecond laser induced in-situ crystallization of Tb-based luminescent metal organic framework (Tb(BTC)·G, BTC = 1,3,5-Benzenetricarboxylic acid, G = guest solvent). To the best of our knowledge, this is the first time we introduce this technique into directly patterning MOFs. By dissolving metal salt and organic ligand into a suitable solvent forming a precursor solution and irradiated by multiple femtosecond laser pulses, the Tb(BTC)·G has been generated at the interface of the substrate and the precursor solution. This method combines the chemical synthesis and device integration into a single step and avoids taking in extra organic plasticizers. The fast injection of laser energy at the laser focal spot accelerates the generation of MOFs. Under the irradiation of multiple femtosecond pulses, the MOFs generate within less than one second. This method reduces the generation time from hours to the order of seconds or even milliseconds. By adjusting the exposure time, different sizes of Tb(BTC)·G micro-discs can be fabricated. This work evidences the great potential that femtosecond laser techniques bring to the fast fabricating and integrating of MOFs.
Materials. All chemicals were used as received without further purification. Terbium(III) nitrate hexahydrate (Tb(NO3)3·6H2O, 99%, Aladdin), 1,3,5-Benzenetricarboxylic acid (H3BTC, 99%, Aladdin), N,N-dimethylformamide (DMF, 99.8%, Aladdin), N-Methyl pyrrolidone (NMP, 99.5%, Aladdin).
Precursor preparation. A mixture of 0.126 g Tb(NO3)3·6H2O, 0.02 g H3BTC and 1 mL NMP (or 1 mL DMF) was degassed in a three-neck flask under vacuum and heated to 60 °C under vigorous stirring for 5 min. Then the mixture was filtered with a polytetrafluoroethylene membrane filter (0.22 µm pore size).
Femtosecond laser direct writing setup. The experiments were carried out on a home-made direct writing system with a femtosecond laser at the wavelength of 532 nm with pulse duration 170 fs (Fianium). The repetition rate that the laser operates at is 80 MHz. The laser beam was expanded and collimated before focusing on the sample. An objective with a high numerical aperture (100×, NA = 1.4, Olympus) was used to focus the beam. The spot size is 463 nm according to the Abbe diffraction limit. A mechanical shutter (Zhichu Optics) was used to control laser pulses on and off. By moving the sample with a piezo XYZ stage (563.3 CD, Physik Instrumente) while keeping the laser beam steady, arrays of Tb(BTC)·G micro-discs were fabricated.
Characterization. The morphologies of Tb(BTC)·G micro-disc arrays were observed by a high-resolution field-emission scanning electron microscope (NOVA Nano SEM 450, FEI). The high resolution transmission electron microscopy (HRTEM) images, energy dispersive spectrum (EDS), selected area electron diffraction (SAED) patterns and elemental mapping analysis were recorded by a Titan G2 60-300 transmission electron microscope with an acceleration voltage of 300 kV. The photoluminescence images were taken by a home-made wide field fluorescence microscope with an excitation laser wavelength of 405 nm (camera: C4742-95-12EGR, Hamamatsu). The photoluminescence spectra were collected on the same home-made microscope system with an Ocean Optics spectrometer (USB 2000 + UV-VIS). The absorption spectra of the NMP solution and the precursor were recorded with an ultraviolet-visible-near infrared spectrophotometer (UV–1800).
3. Result and discussion
The solvothermal method is the most used approach to synthesize MOFs [18–20]. The synthesizing process is normally as follows: a mixture of metal salt, an organic ligand is dissolved in a single or a mixed solution as the precursor. Then the precursor is heated to a certain temperature and hold for several hours. Finally, MOF crystals are slowly grown from the hot solution. High temperature of the solution provides sufficient energy for nucleation and growth of the MOF crystals. In our work, we choose a femtosecond laser as the energy source to induce in-situ crystallization of the MOFs. By focusing the femtosecond laser beam with a high numerical aperture objective lens, high laser energy is confined in the laser focal spot with a volume less than 1 µm3. This causes a highly confined photon synthesis reaction region.
In our experiments, the precursor has been prepared by following the same procedure described above and a reported work . The detailed processes are described in the experiment section. Figure 1 shows the schematic of the femtosecond laser induced in-situ crystallization of Tb(BTC)·G. Figure 1(a) shows the schematic of the laser direct writing process. To prepare a sample for laser writing, the precursor is sandwiched between two coverslips with a thickness of about 60 µm. The precursor is clear and transparent before laser irradiation (subfigure in Fig. 1(a)). It takes two steps to generate the Tb(BTC)·G. First, a high intensity laser beam is focused at the interface of one coverslip and the precursor, the NMP molecules polymerize at the focal spot within a short exposure time (subfigure II in Fig. 1(a)). Then, adjusting the femtosecond laser to a lower laser beam intensity and longer exposure time, the Tb(BTC)·G generates at the same position (subfigure III in Fig. 1(a)). Figure 1(b) shows the absorption spectra of the NMP solvent and the precursor, it is found that both the NMP solvent and the precursor have nonlinear absorption under the excitation wavelength of 532 nm. An ultrashort pulse laser is required to excite a nonlinear absorption process in the precursor, causing the energy transfer from the laser to the precursor.
To enable in-situ crystallization of Tb(BTC)·G by the femtosecond laser, it is necessary to choose a suitable solvent to dissolve the metal salt and the organic ligand. First, it has to be a high boiling point solvent. Under high intensity irradiation, the local temperature of the solvent at the focal spot can be significantly higher due to the photon-thermal effect. Choosing a higher boiling point solvent and suitable laser power can avoid forming bubbles and prevent drastic flow dynamics, which is key to generate high resolution controllable LMOF structures. In the meantime, the solvent should be optically transparent to the exciting wavelength, to avoid strong linear absorption. Water, ethanol, N, N-dimethylformamide and N-methyl pyrrolidone are the most commonly used solvents in solvothermal method MOF synthesis, and they are transparent in the visible range. The latter two have a higher boiling point of 152.8 and 203°C respectively. Under the intense irradiation of the focused femtosecond laser, Tb(BTC)·G generates at the focal spot. When using DMF as the solvent, Tb(BTC)·G suspends in the liquid precursor and flow with the moving beam. But for the precursor using NMP as the solvent, Tb(BTC)·G sticks to the substrate. By comparing the pure DMF and NMP solutions, it is found that the NMP can be polymerized triggered by the two-photon absorption process under the irradiation of the femtosecond laser pulse (figure (d)). When the laser beam intensity focused on the substrate is larger than 5.89×107 W/cm2, larger than the polymerization threshold (3.56×107 W/cm2), the NMP polymerization process can be triggered. The polymerized NMP acts as a thin layer of adhesive between the substrate and the Tb(BTC)·G. This process enables a high resolution patterning technique that directly synthesis and grows Tb(BTC)·G on the substrate.
Different Tb(BTC)·G microdisk arrays were synthesized on a glass substrate by this technique in Fig. 2(a), showing the feasibility of directly patterning different sizes of micro Tb(BTC)·G. structures. Tuning the laser power, namely the pulse energy, and the exposure time, different size of microdisks of Tb(BTC)·G can be fabricated. While if the exposure time is shorter than 100 ms, the Tb(BTC)·G microdisks have not had enough time to grow (Fig. 1(c)). With longer exposure time, Tb(BTC)·G microdisks can be synthesized (Fig. 1(d)). In Fig. 2(a), the laser intensity is tuned to 1.93×107 W/cm2, by tuning the exposure time from 200 to 2400 ms with an increment of 440 ms. The laser energy is absorbed by the precursor and accumulates at the focal spot, causing the local temperature of the laser focal spot to increase [21,22]. Tb(BTC)·G generates at the focal spot once the temperature reaches the nucleation temperature. Continuous injection of laser energy facilitates crystal growth. High resolution TEM (HRTEM) (Fig. 2(d)) and selected area electron diffraction (SAED) (Fig. 2(e)) further certify the generation of Tb(BTC)·G with a crystallized structure. The elemental maps (Fig. 2(g)) show the uniform spatial distributions of Tb, O, C and N in the Tb(BTC)·G.
We now analyze the influence of the laser exposure time on the feature size of Tb(BTC)·G. Figure 3(a) shows the SEM image of the Tb(BTC)·G dot fabricated with different exposure time. The laser beam intensity and the exposure time in the first step are fixed at 5.89×107 W/cm2 and 100 ms. In the second step, the laser beam intensity is fixed at 1.93×107 W/cm2 and the exposure time changes from 200 ms to 2400 ms. The minimum diameter of Tb(BTC)·G dot in our experiments is about 370 nm. The diameter change of the Tb(BTC)·G dot with increased exposure time is divided into three stages: slowly increase with the exposure time ranging from 200 ms to 1400 ms, rapidly increase with the exposure time ranging from 1400 ms to 2200 ms and keep steady with the exposure time excessing 2200 ms. The Tb(BTC)·G dot arrays are highly uniform (figure S1) and present similar behavior to the radial rates of growth (Fig. 3(b)).
The synthesis of MOFs generally takes several minutes even several days in solvothermal methods [11,12]. However, there are rare theoretical and experimental investigations of generating Tb(BTC)·G during the orders of milliseconds [23,24]. Our experiment demonstrates the possibility to generate and pattern Tb(BTC)·G in only 200 ms. At the current stage, we still lack an appropriate theory based on crystal growth kinetics to understand this phenomenon. However, there are several issues that are critical for this fast MOF generation. First, the femtosecond laser facilitated fast energy injection within milliseconds. The solvothermal method based Tb(BTC)·G generation normally relies on slowly heating for heat accumulation. Then laser focusing enables energy concentration within a scale of 1 µm3. At the laser focal spot, the precursor absorbs the laser energy and converts it to heat . When the temperature of the precursor at the laser focal spot is high enough to trigger Tb(BTC)·G nucleation, the nucleation starts. After quick nucleation, Tb(BTC)·G grows at the focal spot with energy supply from the laser beam. The growth time is controlled by the laser exposure time. When the laser exposure is off, continuous energy supplied to drive Tb(BTC)·G generation is stopped. Heat diffusion from the focal spot to its surroundings cools down the focal region and Tb(BTC)·G growth stops. Additionally, the diameter of the Tb(BTC)·G stops growing when the exposure time longer than 2200 ms, this is because of the depletion of the metal ions or the organic ligands at the focal spot.
Fluorescence is one of the most attractive characters for rare earth MOFs. Rare earth MOFs, which combine rare earth metal ions and organic ligands together, enable a wide range of photon emission phenomena including linker-based luminescence, metal-based emission, and other more complicated emissions [25,26]. The Tb(BTC)·G synthesized with a femtosecond laser also demonstrates excellent fluorescence properties (Fig. 4(a)). Tb(BTC)·G induced by the femtosecond laser in this work emits a broad spectrum with sharp peaks in specific wavelengths (red spectrum in Fig. 4(b)). The spectrum demonstrates two possible luminescence modes. The first one is the metal ions within frameworks. These sharp peaks are corresponding to the spectrum emitted by Tb ions (black spectrum Fig. 4(b)). The Tb ions within the framework can produce an antenna effect and a pronounced increase in the luminescence intensity [25,27]. Another possible luminescence mode is the linkers. The organic groups in the Tb(BTC)·G absorb the photons of the exciting laser. Emission can be directly from the linker or a charge transfer between the linker and the coordinated metal ions .
In summary, we have demonstrated a femtosecond laser induced in-situ crystallization of Tb-based luminescence metal organic frameworks. The Tb(BTC)·G is synthesized and integrated at the target positions within milliseconds, which is much faster than the normal solvothermal method as well as avoids complicated integration processes. The laser energy injected into the precursor is controlled by changing the exposure time, leading to a regular change of the laser fabricated feature size. Although we still lack appropriate theory to understand the fluorescence mechanism of the Tb(BTC)·G, the fluorescence of the Tb(BTC)·G shows great potential in many applications such as displays and sensors. This approach provides a path for synthesizing and integrating MOFs in one step, promoting the development of miniature devices based on MOF materials.
National Natural Science Foundation of China (51802239, 61775068); Creative Research Group Project of Natural Science Foundation of China (61821003); Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20180507184503128); Advanced Energy Science and Technology Guangdong Laboratory (XHT2020-003, XHT2020-005); Fundamental Research Funds for the Central Universities (2020IVA068).
This work was supported by the Natural Science Foundation of China (No. 61775068, No. 51802239), the Creative Research Group Project of Natural Science Foundation of China (No. 61821003). This work was supported with fund from Science, Technology and Innovation Commission of Shenzhen Municipality (No. JCYJ20180507184503128); Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory (No. XHT2020-003, No. XHT2020-005); the Fundamental Research Funds for the Central Universities (No. 2020IVA068).
Zongsong Gan, Xuewen Wang and Liqiang Mai were in charge of this scientific research project, and Z. G. proposed the ideas for the experiments. Yanan Liu and Nianyao Chai conducted most of the experiments and the data analyses together as well as wrote the manuscript. Zhijun Luo and Junjie Zhao performed the HRTEM analysis and helped discuss and revise the manuscript. Changsheng Xie contributed to revising the manuscript. The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.
The authors declare no competing financial interest.
See Supplement 1 for supporting content.
1. H. K. Chae, D. Y. Siberio-Perez, J. Kim, Y. Go, M. Eddaoudi, A. J. Matzger, M. O’Keeffe, and O. M. Yaghi, “A route to high surface area, porosity and inclusion of large molecules in crystals,” Nature 427(6974), 523–527 (2004). [CrossRef]
2. T.-F. Liu, D. Feng, Y.-P. Chen, L. Zou, M. Bosch, S. Yuan, Z. Wei, S. Fordham, K. Wang, and H.-C. Zhou, “Topology-guided design and syntheses of highly stable mesoporous porphyrinic zirconium metal–organic frameworks with high surface area,” J. Am. Chem. Soc. 137(1), 413–419 (2015). [CrossRef]
3. I. M. Honicke, I. Senkovska, V. Bon, I. A. Baburin, N. Bonisch, S. Raschke, J. Evans, and S. Kaskel, “Balancing Mechanical Stability and Ultrahigh Porosity in Crystalline Framework Materials,” Angew. Chem., Int. Ed. 57(42), 13780–13783 (2018). [CrossRef]
4. H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, AÖ Yazaydin, R. Q. Snurr, M. O’Keeffe, and J. Kim, “Ultrahigh porosity in metal-organic frameworks,” Science 329(5990), 424–428 (2010). [CrossRef]
5. P.-Q. Liao, J.-Q. Shen, and J.-P. Zhang, “Metal-organic frameworks for electrocatalysis,” Coord. Chem. Rev. 373, 22–48 (2018). [CrossRef]
6. Z. X. Cai, Z. L. Wang, J. Kim, and Y. Yamauchi, “Hollow Functional Materials Derived from Metal-Organic Frameworks: Synthetic Strategies, Conversion Mechanisms, and Electrochemical Applications,” Adv. Mater. 31, 1804903 (2019). [CrossRef]
7. D. F. Sava, L. E. S. Rohwer, M. A. Rodriguez, and T. M. Nenoff, “Intrinsic Broad-Band White-Light Emission by a Tuned, Corrugated Metal-Organic Framework,” J. Am. Chem. Soc. 134(9), 3983–3986 (2012). [CrossRef]
8. C.-Y. Sun, X.-L. Wang, X. Zhang, C. Qin, P. Li, Z.-M. Su, D.-X. Zhu, G.-G. Shan, K.-Z. Shao, and H. Wu, “Efficient and tunable white-light emission of metal–organic frameworks by iridium-complex encapsulation,” Nat. Commun. 4(1), 2717 (2013). [CrossRef]
9. L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne, and J. T. Hupp, “Metal–organic framework materials as chemical sensors,” Chem. Rev. 112(2), 1105–1125 (2012). [CrossRef]
10. J. Meng, X. Liu, C. Niu, Q. Pang, J. Li, F. Liu, Z. Liu, and L. Mai, “Advances in metal-organic framework coatings: versatile synthesis and broad applications,” Chem. Soc. Rev. 49(10), 3142–3186 (2020). [CrossRef]
11. N. L. Rosi, J. Kim, M. Eddaoudi, B. Chen, M. O’Keeffe, and O. M. Yaghi, “Rod packings and metal-organic frameworks constructed from rod-shaped secondary building units,” J. Am. Chem. Soc. 127(5), 1504–1518 (2005). [CrossRef]
12. N. A. Khan and S. H. Jhung, “Synthesis of metal-organic frameworks (MOFs) with microwave or ultrasound: Rapid reaction, phase-selectivity, and size reduction,” Coord. Chem. Rev. 285, 11–23 (2015). [CrossRef]
13. L. L. Da Luz, R. Milani, J. F. Felix, I. R. Ribeiro, M. R. Talhavini, B. A. Neto, J. Chojnacki, M. O. Rodrigues, and S. A. Júnior, “Inkjet printing of lanthanide–organic frameworks for anti-counterfeiting applications,” ACS Appl. Mater. Interfaces 7(49), 27115–27123 (2015). [CrossRef]
14. H. K. Arslan, O. Shekhah, J. Wohlgemuth, M. Franzreb, R. A. Fischer, and C. Wöll, “High-Throughput Fabrication of Uniform and Homogenous MOF Coatings,” Adv. Funct. Mater. 21(22), 4228–4231 (2011). [CrossRef]
15. G. Lu, O. K. Farha, W. Zhang, F. Huo, and J. T. Hupp, “Engineering ZIF-8 Thin Films for Hybrid MOF-Based Devices,” Adv. Mater. 24(29), 3970–3974 (2012). [CrossRef]
16. J. Zhuang, J. Friedel, and A. Terfort, “The oriented and patterned growth of fluorescent metal–organic frameworks onto functionalized surfaces,” Beilstein J. Nanotechnol. 3, 570–578 (2012). [CrossRef]
17. H. Thakkar, S. Eastman, Q. Al-Naddaf, A. A. Rownaghi, and F. Rezaei, “3D-Printed Metal-Organic Framework Monoliths for Gas Adsorption Processes,” ACS Appl. Mater. Interfaces 9(41), 35908–35916 (2017). [CrossRef]
18. M. Huangfu, X. Tian, S. Zhao, P. Wu, H. Chu, X. Zheng, J. Tang, and J. Wang, “Post-synthetic modification of a Tb-based metal-organic framework for highly selective and sensitive detection of metal ions in aqueous solution,” New J. Chem. 43(26), 10232–10236 (2019). [CrossRef]
19. B. Li, H. Wen, Y. Cui, W. Zhou, G. Qian, and B. Chen, “Emerging Multifunctional Metal-Organic Framework Materials,” Adv. Mater. 28(40), 8819–8860 (2016). [CrossRef]
20. Q. Yang, Q. Xu, and H.-L. Jiang, “Metal-organic frameworks meet metal nanoparticles: synergistic effect for enhanced catalysis,” Chem. Soc. Rev. 46(15), 4774–4808 (2017). [CrossRef]
21. D. Von der Linde, K. Sokolowski-Tinten, and J. Bialkowski, “Laser-solid interaction in the femtosecond time regime,” Appl. Surf. Sci. 109-110, 1–10 (1997). [CrossRef]
22. S. M. Eaton, H. Zhang, P. R. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Y. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13(12), 4708–4716 (2005). [CrossRef]
23. Y. Liu, Z. J. Luo, C. Xie, and Z. Gan, “General and Fast Patterning of Semiconductor Nanocrystals by Femtosecond Laser Direct Writing,” J. Phys. D: Appl. Phys. 53(12), 125105 (2019). [CrossRef]
24. Y. Liu, F. Li, L. Qiu, K. Yang, Q. Li, X. Zheng, H. Hu, T. Guo, C. Wu, and T. W. Kim, “Fluorescent Microarrays of in Situ Crystallized Perovskite Nanocomposites Fabricated for Patterned Applications by Using Inkjet Printing,” ACS Nano 13, 2042–2049 (2019). [CrossRef]
25. M. D. Allendorf, C. A. Bauer, R. K. Bhakta, and R. J. Houk, “Luminescent metal-organic frameworks,” Chem. Soc. Rev. 38(5), 1330–1352 (2009). [CrossRef]
26. Y. Cui, Y. Yue, G. Qian, and B. Chen, “Luminescent functional metal-organic frameworks,” Chem. Rev. 112(2), 1126–1162 (2012). [CrossRef]
27. H.-Q. Yin, X.-Y. Wang, and X.-B. Yin, “Rotation Restricted Emission and Antenna Effect in Single Metal-Organic Frameworks,” J. Am. Chem. Soc. 141(38), 15166–15173 (2019). [CrossRef]