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Abstract
We present an LED, which emits linearly polarized warm-yellow light on a surface with arrayed metal-organic framework (MOF) crystals. We first synthesize porous MOFs based on zinc nitrate and mixed ligands, which is comprised of the acid 1,4-benzenedicarboxylic (H2BDC) and the alkaline triethylenediamine (TED). During the synthesis, the direction of the MOF pore channel is the growth direction of the TED. By adjusting the growth environment to be appropriately acidic, we can have a high concentration of the TED and a large growth rate along the TED ligand, and finally obtain the rod-like MOF-BDC crystal. The long-axis of the rod-like crystal is parallel to the direction of the pore channel. Then, via in situ reaction, we introduce the guest rhodamine-B (RhB) dye molecules into the MOF channel, where the RhB molecules are aligned along the direction of the pore channel due to the steric hindrance effect. The obtained MOF⊃RhB crystals exhibit a polarization ratio of 3.67 at the peak fluorescence wavelength and a fluorescence lifetime of 5.8ns. Indeed, by comparing the dichroic ratio and the polarization ratio, we find that the emission bands of RhB molecules within this rod-like crystal have a much preferred orientation along the long axis of the crystal than the absorption bands. Furthermore, we introduce the rod-like MOF⊃RhB crystals within grooves parallel arrayed in a quartz plate; where, by utilizing the gravitational force associated with the anisotropic geometry of rod-like MOF⊃RhB crystals, the long axes of these crystals and also the direction of the RhB molecules are parallel to the orientation of the grooves. Thus, the MOF⊃RhB crystal arrays on the surface emit warm-yellow light at the same linear state of polarization.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Within visible-light communication (VLC) technology, the low modulation bandwidth of the light-emitting diode (LED) is always the limiting factor for increasing the speed of the data transmission with required illumination. The most commercially used are white LEDs comprising a blue LED chip and a yellow-light phosphor. Their modulation bandwidth is only a few MHz [1,2], due to the large emission lifetime of around 200 ns of the yellow-light phosphor. Thus, although warm-yellow LEDs have been used in lots of indoor lighting environment, there is not yet any VLC system based on warm-yellow LEDs. Hence, a blue bandpass filter is typically added at the receiver to eliminate the slow yellow component and reduce the inter-symbol interference (ISI) [3]. However, from the information theory point of view, the channel capacity and the light intensity reaching the photodetector are reduced. Alternatively, white LEDs with respective red-green-blue but mixed light emissions are used in the VLC transmitter, where these three LED elements can be modulated separately. For these LEDs, wavelength division multiplexing (WDM) and advanced signal processing techniques such as signal equalization can also be applied, and the data rate of RGB-LED based VLC system has been improved to a few Gb/s over some cm distance [4]. However, to balance the luminous efficacy and color-rendering for RGB-LEDs, more complex electronic driving circuits have to be used, which is not cost-efficient. Indeed, the intrinsic bandwidth of RGB-LEDs is still limited (typically some 15 MHz for each element) [2]. Therefore, increasing the modulation bandwidth of LEDs is always of great importance to develop the VLC system.
On the other hand, while the polarization-multiplexing technique has been widely used in present fiber-optic communication systems, there are only few polarization-multiplexed VLC system demonstrated [21], where the polarizers had to be used to obtain respective polarized light components. Because the general LEDs emit unpolarized-light, most part of the light will be filtered out when they are used in the polarization-multiplexed VLC system, which is very inefficient in terms of energy consumption. Therefore, if LEDs directly emitting polarized-light are available, the energy-efficient polarization-multiplexed VLC system is feasible. There have been some studies to tackle this weak point, especially by using the OLED, where the polarized light is directly emitted from uniaxially oriented emissive layers. These methods include the Langmuir-Blodgett technique [5], rubbing/shearing/stretching of the film [6], electrospinning [7], nanoimprinting [8], orientation on pre-aligned substrates [9], usage of orientated conjugated polymer composed of highly crystalline and amorphous parts [10, 11], etc.. However, there is still lacking of method at the molecular level to realize highly orientated material to achieve highly polarized light emission.
To improve the modulation bandwidth of LEDs and achieve highly polarized light emission, one fundamental method is using organic phosphor materials. For example, in a VLC system with a GaN μLED [12], a yellow fluorescent conjugated copolymer was used, which resulted in a short relaxation lifetime of 100 ps and therefore a data rate of 1 Gbit/s. Very recently, a warm-white LED based on a dye-loaded metal-organic framework (MOF) was demonstrated [13], where the yellow component shows an emission lifetime of only 5 ns, which is ~38 times shorter than that of the inorganic phosphor of commercial LEDs. In that work, the MOF is the key component. MOFs are built from metal ions and organic ligands connecting these metal ions [14,15], therefore by choosing proper ligands the MOF can feature an intrinsic luminescence [16]. Also, this kind of molecular porous materials show a crystalline solid frameworks which can effectively encapsulate organic luminescent dye molecules [17,18], where the crystalline structure of MOFs can prevent dye molecules from aggregation-induced quenching [19]. Another very interesting feature of dye molecules encapsulated in MOFs is that, when the dye molecules are tightly confined within the MOF pore, the dye molecules can be highly oriented and ordered. Recently [20], a MOF⊃dye single crystal has exhibited polarized fluorescence or even lasing light. However, the bulk MOF⊃dye material is made of individual single crystals and macroscopically disordered. Therefore, this material usually has very weak preference for any states of polarization.
In this work, MOF crystals are fabricated and arrayed on a surface, where the Rhodamine-B (RhB) dye molecules are in situ encapsulated in structurally ordered MOF crystals, Fig. 1(a), to realize surface-emitting and linearly polarized warm-yellow fluorescence. In the past, the bulk dye@MOF material appears mostly in the form of powders or a single crystal, and therefore this bulk material cannot emit linearly polarized light on a surface in general. In our design, the RhB molecules encapsulated in MOF channels are orientated along the direction of the MOF channel, due to the steric hindrance effect [22]. Also, the MOF⊃RhB crystals are rod-like, Fig. 1(b), and the long axis of the rod-like crystal is inclined to the direction of the MOF channel. Therefore, these crystals are optically anisotropic. Further, these rod-like crystals are laid within grooves parallel arrayed in a quartz plate, by utilizing the gravitational force associated with the anisotropic geometry of rod-like MOF⊃RhB crystals. This arrangement of the crystal arrays ensures uniformly and linearly polarized light-emitting.
Fig. 1 (a) Illustration of synthesis of the MOF⊃RhB crystal, where the dye molecule with the conjugate axial direction is represented by a double arrow line; (b) Strategy of laying rod-like MOF⊃RhB crystal arrays on a surface with parallel grooves, to ensure a linearly polarized light-emitting.
2. Synthesis and characterization of single MOF⊃RhB crystal
We first synthesize the MOF-BDC crystals, namely Zn(BDC)(TED)0.5, by the hydrothermal method. The preparation of the MOF is as follows: a mixture of Zn(NO3)2·6(H2O) (2.8 mmol), H2BDC (2.9 mmol), TED (1.95 mmol) is added in the 30ml N,N-Dimethylformamide (DMF) solution; an ultrasonic treatment is applied to dissolve all chemicals until the solution is clear; afterwards, a certain amount of the nitric acid solution is added in the solution to adjust the pH value <7, whose exact value will be given in the context. The solution is then transferred into a 50ml Teflon-lined autoclave and heated in an oven at 100°C for 48 hours. Finally, after being cooled naturally to the room temperature, the millimeter sized crystals of Zn(BDC)(TED)0.5 are obtained. The single Zn(BDC)(TED)0.5 crystal is colorless and transparent.
During the synthesis, we optimize the synthesis condition and thus the growth rates of different directions of the MOF cell, to obtain rod-like Zn(BDC)(TED)0.5 crystals with a large length-diameter ratio. This optimization is done by adjust the pH value of the solution. The two ligands of the Zn(BDC)(TED)0.5 are 1,4-benzenedicarboxylic (H2BDC) and triethylenediamine (TED), where the former one is acid and the latter is alkaline. By adjusting the growth environment to be acid, the solubility of the TED in the solution increases and the solubility of BDC decreases. Thus, we can have a high concentration of the TED and a large growth rate along the TED ligand. This results in the rod-like MOF-BDC crystal, where the long-axis of the rod-like crystal is parallel to the direction of the MOF pore channel. Figure 2(a), (b), (c) and (d) show the micrographs of single MOF-BDC crystal, when the amount of the nitric acid is 100 μL, 200 μL, 300 μL or 400 μL respectively, while other synthesis conditions stay same. It can be seen that the length-diameter ratio of the single MOF-BDC crystal first increases and then decreases, along with the increase of the acidity of solution. In the first phase, when the amount of the nitric acid increases, from Fig. 2(a) to Fig. 2(c), the deprotonation ability and the coordination ability of the carboxylic group of the BDC ligand decreases; meanwhile, the protonation ability and the coordination ability of the amino group of the TED ligand increases. Correspondingly, the growth rate of the BDC ligand decreases and the growth rate of the TED ligand increases, which leads to the increase of the length-diameter ratio of the single MOF-BDC crystal. The pH value of the solution in the case of Fig. 2(c) is 1.53. In the second phase, with a further increase in the amount of the nitric acid from Fig. 2(c) to Fig. 2(d), the excess nitric acid might react with the protonated amino group of the TED and form e.g. triethylenediamine dinitrate. This reaction effectively reduces the concentration of TED coordinating with the zinc ions. Therefore, the length-diameter ratio of the single MOF-BDC crystal becomes smaller for too much nitric acid.
Fig. 2 Micrographs of single MOF-BDC crystal, when the amount of the nitric acid is (a) 100 μL, (b) 200 μL, (c) 300 μL or (d) 400 μL; (e) the micrograph of the MOF⊃RhB crystal, with the nitric acid of 300 μL.
Figure 3 shows the molecular structure diagrams of the MOF-BDC crystal, obtained from single crystal X-ray diffraction (XRD) experiments (Bruker D8 QUEST). It can be seen in Fig. 3(a) that the MOF crystal has a periodical pore channel structure along the c-axis, and the direction of the MOF pore channel is the growth direction of the TED ligand, and in fact consistent to the long-axis of the rod-like MOF crystal. Figure 3(b) shows that this MOF-BDC crystal has a six-prismatic shape, which belongs to the hexagon system. The diameter of the largest MOF-BDC pore is 12.9 Å.
Fig. 3 Molecular structure diagrams of the MOF-BDC crystal, obtained from single crystal X-ray diffraction experiments.
Via in situ reaction, we encapsulate the guest RhB dye molecules into the MOF-BDC. For this reaction, instead of the DMF solution, the RhB/DMF solution with a RhB concentration of 0.75mg/ml is used, where the amount of the nitric acid is 300 uL and other solutions are unchanged. Figure 2(e) shows the synthesis product of MOF⊃RhB, collected in a quartz capillary with an inner diameter of 0.5mm. The length and width of the single MOF⊃RhB crystal are about 1 mm and 0.3mm respectively. The morphology of the single MOF⊃RhB crystal is basically the same as that of the single MOF crystal, while the color of the former one becomes pink. The X-ray powder diffraction (XRPD) experiments (Bruker D8 ADVANCE) are also performed for the MOF-BDC and MOF⊃RhB, and their XRPD patterns are shown in Fig. 4. It can be seen that the diffraction peaks of these two crystals are basically same, which means that the spatial framework structures of them should be same, i.e. the MOF structure is not affected by the presence of the RhB molecules and the RhB molecules are not appreciably aggregated.
Further, the polarizability of the fluorescence spectra of the MOF⊃RhB crystal is characterized via the measurement scheme shown in Fig. 5. The measurement scheme consists of five parts: the excitation light source, the polarizer to adjust the state of polarization (SOP) of the excitation light, the MOF⊃RhB crystal sample, the polarizer to select the light at the definite SOP emitted from the MOF⊃RhB crystal, and the detector (Shimadzu RF-5301PC) to measure the fluorescence spectra. In this work, the light source is at a center wavelength of 365 nm and with a linewidth of 1 nm, selected from the light out of a xenon lamp. In the measurement, the long axis of the rod-like MOF⊃RhB crystal is parallel to the y-direction of the local coordinate systems, as defined in Fig. 5. As mentioned earlier, the long-axis of the rod-like MOF⊃RhB crystal is consistent to its channel direction. Therefore, by adjusting the excitation polarizer, the SOP of the incident light can be parallel or perpendicular to the channel direction of the MOF⊃RhB crystal. We denote the unpolarized incident light and the one at the parallel or perpendicular SOP as Pin, Pin,//, Pin,⊥, respectively. Also, by adjusting the emission polarizer, the state of polarization of the fluorescent light can be distinguished. We denote the unpolarized fluorescent light and the one at the parallel or perpendicular SOP as Pout, Pout,//, Pout, ⊥, respectively.
Fig. 5 Scheme for characterizing the polarizability of the fluorescence spectra of the MOF⊃RhB crystal. The local coordinate systems are defined with respect to the transmission direction.
The fluorescence spectra of the rod-like MOF⊃RhB crystal are measured for three cases, as shown in Fig. 6. The first case is with the excitation polarizer and without the emission polarizer, Fig. 6(a), where the legends indicate the SOP of the incident light; the second case is without the excitation polarizer and with the emission polarizer, Fig. 6(b), where the legends indicate the SOP of the fluorescent light; the third case is with both polarizers and the incident light is Pin,//, Fig. 6(c), where the legends indicate the SOP of the fluorescent light. All these fluorescence spectra show a peak wavelength at 560 ± 1 nm, which corresponds to the fundamental relaxation of the excited states of RhB molecules. It should be noted that this peak wavelength of 560 ± 1 nm is almost same to the wavelength of the maximum absorption [23]. So, different from most published works for fluorescence of the RhB molecules, the rod-like MOF⊃RhB crystal can ensure the fundamental fluorescence without a shift of the peak wavelength. Further, in the first case shown in Fig. 6(a), it can be seen that this crystal has different maximum fluorescence intensities for the incident lights with different SOPs, namely Pin,// and Pin,⊥. This difference comes from the anisotropic absorption of this crystal, namely dichroism. At the peak wavelength, the dichroic ratio (Poutin,// / Pout in,⊥) is 1.67. This value is very close to that of the RhB molecules aligned in the nematic liquid crystalline [24]. This means that the absorption bands of the RhB molecules could be considered as parallel transitions and have a preferred orientation along the long axis of the rod-like MOF⊃RhB crystal. Besides, in the second and third cases shown in Fig. 6(b) and (c), it can be seen that the fluorescent lights for the unpolarized incident light or the one at the parallel SOP exhibit almost same polarizabilities. Their respective polarization ratios (Pout,// / Pout,⊥) at the peak wavelength are 3.57 or 3.67, practically same but higher than the dichroic ratio. Therefore, when this material is used for practical applications, the excitation polarizer is not needed. Also, this higher polarization ratio reveals that the emission bands of the RhB molecules have a much preferred orientation along the long axis of the rod-like MOF⊃RhB crystal, compared to the absorption bands. It should be noted that this comparison has not been seen in other works, to the best of our knowledge.
Fig. 6 Fluorescence spectra of the MOF ⊃RhB crystal; (a) with the excitation polarizer and w/o the emission polarizer, where the legends indicate the SOP of the incident light; (b) without the excitation polarizer and with the emission polarizer, where the legends indicate the SOP of the fluorescent light; (c) with both polarizers and the incident light is Pin,//, where the legends indicate the SOP of the fluorescent light.
3. Fabrication and characterization of surface LED based on MOF⊃RhB crystal arrays
Based on the synthesized rod-like MOF⊃RhB crystals, we fabricate a surface LED with arrayed MOF⊃RhB crystals. This surface is a quartz plate, within which there are parallel arrayed grooves with a width of ~1mm. First, MOF⊃RhB crystals within the DMF solution are absorbed by a dropper and transferred into a groove. During the transfer process, the rod-like crystals lie down in the groove, due to the gravitational force associated with the anisotropic crystal geometry. Also, in order to align the long axis of the crystal to the direction of the groove, the quartz plate is slightly tilted. This transfer process is repeated for these parallel grooves, one after another. Finally, the crystal arrays are arranged within the grooves with their long axes aligned parallel to the direction of the groove, as shown in Fig. 7(a). The picture of Fig. 7(a) is taken for the surface LED sample with aligned MOF⊃RhB crystal arrays under an ultraviolet lamp (at the wavelength of 365 nm).
Fig. 7 (a) Surface LED sample with aligned MOF⊃RhB crystal arrays under an ultraviolet lamp (at the wavelength of 365 nm). (b) and (c) are the microscope pictures of the MOF⊃RhB crystal arrays under a broad-band unpolarized light with a wavelength range from 460 nm to 490nm, where the fluorescent lights pass first through an optical filter cutting off the light below 590nm and then a polarizer. Pout,// or Pout,⊥ indicate the SOP of the fluorescent lights, which are in parallel or perpendicular to the direction of the groove (namely the long axis of the crystal), respectively.
Figure 7(b) and (c) show the microscope pictures of the MOF⊃RhB crystal arrays of the surface LED sample under a broad-band unpolarized light with a wavelength range from 460 nm to 490nm, where the fluorescent lights pass first through an optical filter cutting off the light below 590nm and then a polarizer. Therefore, the observed fluorescent lights are reddish. Pout,// or Pout,⊥ indicate the fluorescent lights with definite SOPs, which are in parallel or perpendicular to the direction of the groove (namely the long axis of the crystal), respectively. These two figures confirm that the surface LED sample based on rod-like MOF⊃RhB crystal arrays has a linearly polarized fluorescence. Figure 8(b) shows that the fluorescence spectrum of our MOF⊃RhB has a CIE coordinate of (0.400, 0.588), where the calculated color temperature is 4200 K. Therefore, the surface LED emits a warm-yellow light indeed.
Fig. 8 (a) Time-resolved fluorescence of MOF⊃RhB powders, the measurement 1/e- lifetime τ of which is 5.8ns, and (b) its CIE coordinates.
We also study the lifetime of the synthesized MOF⊃RhB crystals, via a time-resolved fluorescence measurement (Edinburgh FLS920P). The employed laser is at 357.8 nm. It should be noticed that the MOF⊃RhB crystals are first ground into powder and placed in a fluorescent cuvette for the measurement, because the PXRD result has meant that the grinding process does not affect the aggregation state of RhB. The time-resolved measurement result is shown in Fig. 8(a), from which it can be read that the MOF⊃RhB crystals have a fluorescence lifetime of ~5.8ns. This value is in the same order of those values of RhB in water (1.5 ns), in octanol (3.2 ns) and in the gas phase (5.97 ns) [25], but far less than the fluorescence lifetime of the inorganic phosphors (usually hundreds ns), which proves a feasibility of this material to be used in high-speed light communication system.
4. Summary
In this work, we synthesize rod-like MOF⊃RhB crystals and, by aligning these crystals into ordered arrays, realize a surface LED emitting linearly polarized warm-yellow light. During the synthesis, we optimize the synthesis condition, especially the pH value of the solution and thus the growth rates of different directions of the MOF cell, to obtain rod-like Zn(BDC)(TED)0.5 crystals with a large length-diameter ratio. The long axis of this rod-like MOF crystal is consistent with the direction of the MOF channel. Then, via in situ reaction, we encapsulate the guest RhB dye molecules into the MOF pores. Due to the steric hindrance effect, the RhB molecules are orientated along the direction of the MOF channel. This orientation of the RhB molecules results in polarization-dependent fluorescence, which prefers to absorb and emit the light at the SOP in parallel to the direction of the MOF channel. The measured dichroic ratio and the polarization ratio of the fluorescence of the MOF⊃RhB crystals are 1.67 and 3.67 at the peak fluorescence wavelength, respectively. These values reveal that the emission bands of the RhB molecules have a much preferred orientation along the long axis of the rod-like MOF⊃RhB crystal, compared to the absorption bands. During the fabrication of the surface LED, we utilize the gravitational force associated with the anisotropic geometry of the rod-like MOF⊃RhB crystals, and align the crystals within the groove arrays on the quartz plate. This process also ensures the long axes of the rod-like crystals and the orientation of the RhB molecules to be parallel to the direction of the groove. Thus, the surface LED emits a polarized warm-yellow light. Also, the MOF⊃RhB crystal shows a fluorescence lifetime of 5.8 ns.
The demonstrated warm-yellow light-emitting and its polarization-dependent fluorescence exhibits the feasibility of this MOF⊃RhB material to be used in high-speed polarization-multiplexed light communication system, 3D display, liquid crystal display backlights and other fields. It is worth mentioning that many organic fluorescent dyes have a rod-like molecular structure like RhB, and many MOFs have the similar anisotropic growth behavior and long-range ordered pore structure. Therefore, by changing the guest molecules and selecting proper MOF materials, MOF⊃dye crystals with different polarization characteristics with desired light-emitting could be obtained.
Funding
National Natural Science Foundation of China, China (Grant No. 61575096); Scientific Research Foundation for the Returned Overseas Chinese Scholars (Grant No. 105757).
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