Structures and photoluminescence properties of Alq₃ 1D materials prepared by an extremely facile solution method

Abstract

Ultra-long crystalline Alq₃ 1 dimensional (1D) materials were prepared by using an extremely facile solution approach without any surfactant, SDS, anti-solvent, or other reagents. The Alq₃ 1D materials have smooth surfaces and pentagonal or hexagonal cross-sections. The length of the microrods have α-phase crystalline structures. The prepared Alq₃ samples exhibit excellent green-light photoluminescence (PL) performance. The growth mechanism of the Alq₃ 1D structures were discussed. The limitation conditions of the Alq₃ microrods were also studied. In addition, the influence of the volume of CHCl₃ on the microrods was discussed. This controllable growth method can potentially be extended to other functional organic nanomaterials.

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Introduction

Since the first efficient organic light-emitting diode (OLED) based on tris(8-hydroxyquinoline) aluminum (Alq₃) was reported by Tang and Van Slyke in 1987,¹ Alq₃ has been widely employed as an active material in OLEDs owing to its excellent electron-transporting ability and thermal stability.^2,3 After decades of intensive research and development, Alq₃ continues to be the workhorse in low-molecular weight materials for OLEDs, spin valves (SVs), organic field effect transistors (OFETs) and panel displays.^4–10 Very recently, Alq₃-based bluish green fluorescent composite nano-spheres were successfully synthesized, which can be used as paper sensors for nitroaromatic explosive detection.¹¹

In view of the potential applications in nano-optoelectronic devices and their novel properties such as quantum size effect, macroscopic quantum tunneling effect, surface effect and spin-related effects, the synthesis of Alq₃ micro/nanostructures, particularly one-dimensional (1D) crystals, attracted increasing research interesting in the last decades.^12–16 So far, the Alq₃ nanomaterials were mainly synthesized by vapor deposition-based or solution-based method.^17–24 Compared with the high temperature synthesis process of the vapor phase methods, the solution methods are facile and simple energy-saving route for the fabrication of high-quality Alq₃ nanostructures.²⁵ Moreover, the solution methods allow people to explore the optical and electronic properties of individual Alq₃ nanorods or nanowires.^18,25 In 2007, Hu et al. firstly reported a facile self-assemble growth route assisted by surfactant in solution to synthesize Alq₃ nanorods. The prepared nanorods had regular hexagonal geometry, good crystallinity, and good field-emission performance.¹⁸ Wei et al. prepared Alq₃ nanorods by a facile microemulsion route assisted by surfactants such as cetyltrimethylammonium bromide (CTBA) and sodium dodecyl sulfate (SDS) and investigated the photoluminescence of isolated nanorods.²⁵ In 2010, Wong's group reported the fabrication of Alq₃ sub-microwires by a facile anti-solvent diffusion method and studied the field emission and waveguide properties of the sub-microwires.²⁰ In these experiments, surfactants, anti-solvents, or other reagents were added as assistants. It was believed to be impossible to obtain Alq₃ nanostructures by evaporating Alq₃ solution in air directly.²⁰

In this paper, we adopt for the first time an extremely facile solution process to fabricate high quality Alq₃ 1D crystals without using any surfactant, SDS, anti-solvent, H₂O or other reagents assistant. The Alq₃ 1D crystals were prepared by volatilizing Alq₃ solution at room temperature directly. The prepared Alq₃ samples exhibit excellent green-light PL performance. Our method can potentially be extended to other functional organic nanomaterials.

Experimental

The Alq₃ nanostructures were fabricated by using an extremely facile solution method. Firstly, a certain amount of Alq₃ (purity: 99.5%) powder and chloroform (CHCl₃) solvent were mixed to prepare Alq₃ solution. The CHCl₃ have many advantages compare to other organic solvents such as ethanol, DMF, CH₃CN, CH₂Cl₂ and toluene. It is a very good solvent for Alq₃ and polarity index is 4.4. Moreover, its boiling point is 61 °C, which is suitable for the room temperature self-assembly of organic nanostructures. In this experiment, 200 mg Alq₃ powder and 5 ml CHCl₃ were mixed to prepare Alq₃ solution. The Alq₃ solution was agitated by magnetic stirrer for 12 h till Alq₃ powder was fully dissolved (see Fig. 1A and B). Then, a little of the Alq₃ stock solution (about 2 ml) was dropped onto the tilted polished Si substrates in a beaker by a long capillary needle. The Si substrates were previously cleaned by sequential ultrasonic rinses in acetone, ethanol, and deionized water. The beaker was sealed with parafilm to prevent the CHCl₃ from volatilizing quickly due to its relatively low boiling point. As the chloroform solvent volatilized off thoroughly, crystalline Alq₃ nanostructures were obtained. The whole processes were finished in a cabinet with exhaust device to avoid volatilizing Alq₃ solution in air directly, finally the volatized CHCl₃ and byproduct (COCl₂) can be dissolved in ethanol as possible by this method.

Fig. 1 A schematic view of the preparation process of the Alq₃ solution and crystalline Alq₃ nanostructures. (A) The Alq₃ solution preparation process, and (B) the growth process of Alq₃ microrods.

The microstructure analysis was carried out on a powder X-ray diffractometer (PXRD, Bruker AXS, D8 Advance). The surface morphology and element analysis of the samples were characterized by a high resolution scanning electron microscope (SEM, S-4800) and a transmission electron microscope (TEM, Tecnai G² F20) in combination with energy dispersive X-ray (EDX). Fourier transform infrared spectroscopy (FTIR) pattern was studied by using FT-IR spectrophotometer (Thermo Nicolet, NEXUS 670). The PL signal from the sample was dispersed by a Jobin-Yvon iHR320 monochromator excited at 325 nm and detected by a thermoelectrical cooled Synapse CCD detector at room temperature.

Results and discussion

Fig. 2 shows the SEM images of the as-prepared Alq₃ samples. Randomly distributed Alq₃ 1D materials with smooth surfaces along with their entire length can be clearly seen when the concentration of Alq₃ solution is 40 mg ml⁻¹. Three kinds of Alq₃ 1D materials, rod-shaped microstructures with irregular pentagonal (Fig. 2A), quadrilateral (Fig. 2A), or hexagonal (Fig. 2B) cross-section ones could be observed. The length of the longest rod can reach ∼100 μm with an aspect ratio of length/diameter (L/D ratio) of about 12. It is necessary to point out that the density of the Alq₃ microrods can be possibly controlled by growth environment, such as the concentration of Alq₃ solution and the volatilization speed of the CHCl₃.²⁵ In particular, other nanostructures instead of microrods will be obtained as the concentration of the Alq₃ solution insufficient. For example, rice-like nanoparticles will be fabricated when the concentration of Alq₃ solution is about 7–13 mg ml⁻¹.²⁶ A main reason is that the concentration of the Alq₃ solution directly affects the growth time of Alq₃ crystals (nuclei). The different growth time of the Alq₃ nuclei leads to different shapes and sizes of the particles.

Fig. 2 The SEM photographs of the as-prepared Alq₃ 1D structures. (A) Rod-shaped microstructures with irregular pentagonal or quadrilateral cross-sections. (B) Microrods with regular or irregular hexagonal cross-sections. (C) and (D) Nanorods with regular quadrilateral and hexagonal cross-sections, respectively.

Fig. 3 shows a TEM image and the corresponding energy dispersive X-ray (EDX) of the prepared microrods. The TEM image confirms further that the prepared samples are solid rods. Few defects can be observed from the Alq₃ microrods (Fig. 3A). The result of EDX microanalysis proves the C, N, O and Al chemical composition of the prepared Alq₃ microrods (Fig. 3B). Because the samples for TEM and EDX were scraped from the Si substrates and transferred onto Cu grids coated with carbon films, peaks of Cu and Si elements were also observed.

Fig. 3 (A) TEM image and (B) EDX element analysis of Alq₃ microrods.

The FTIR and PXRD spectra were recorded to reveal the composition and microstructure of the obtained Alq₃ microrods. The FTIR spectrum was measured in the range of 400–1000 cm⁻¹ (see Fig. 4A). The peaks at 522.62, 548.09, and 749.20 cm⁻¹ belong to the stretching vibrations of Al–O, and the peak at 419.53 cm⁻¹ is assigned to the Al–N stretching vibrations of α-phase Alq₃.¹⁸ The PXRD pattern in Fig. 4B matches well with that of the α-phase Alq₃ 1D structures,^{14,17–19,22} confirming further that the prepared microrods are α-phase Alq₃ with a preferred orientation of [001] direction, that is, the crystallographic c-axis direction.¹⁹ For reference, the FTIR and PXRD of Alq₃ powder were also measured as shown in Fig. 4C and D, respectively.

Fig. 4 (A) The FTIR pattern of the α-Alq₃ microrods. (B) PXRD pattern of sample of Alq₃ microrods. (C) The FTIR spectrum of Alq₃ powder. (D) PXRD pattern of Alq₃ powder.

According to the above analysis, we tentatively give a possible growth mechanism of Alq₃ microrods fabricated by volatilizing Alq₃ solution and the sketch map was illustrated in the Fig. 5. At the first step, Alq₃ solution was injected onto the Si substrates and form one large droplet, then the chloroform (CHCl₃) begin to volatilize from the Alq₃ stock solution at room temperature attributing to its relatively low boiling point. Gradually, the concentration of the droplet increases slowly and supersaturates that leads to nucleation.²⁵ Solvent evaporation during crystal seeds formation results in extensive phase separation. Simultaneously, the position of many Alq₃ molecules can transfer toward to the nuclear seeds then formed nanoparticles. This produces relatively large Alq₃ molecules rich domains between nanoparticle aggregates within the nanorods. Eventually, theses Alq₃ nanoparticles grow into crystalline rods along the crystallographic c-axis direction under the forces of van der Waals, Coulomb interaction of the anionic π electrons of the quinoline rings of Alq₃ molecules.^17,25,27 In addition, the hydrogen bonding in organic solvent (CHCl₃) play an very important role in the formation of the Alq₃ microrods in liquid phase.²⁸ For example, Whitesides's group reported that the self-assembly of bisCA (1,2,3) and bisM (5,6) forms nanorod materials [(bisCA)n(bisM)n] in organic solvents such as CHCl₃ and that these rods aggregate into mesoscopic aggregates of rod.²⁹ In fact, the migration of Alq₃ molecules are involved in the whole self-assembly process during the formation of the Alq₃ rods. However, the detailed kinetic process of Alq₃ molecules need further investigation.

Fig. 5 Sketch of the growth mechanism of Alq₃ microrods.

Fig. 6 shows the temperature-dependent (50 K, 100 K, 150 K, 200 K, 250 K and 300 K) PL spectra of the Alq₃ microrods. The peaks show a good symmetry in the range of 400 nm to 700 nm, which has an obvious difference with that of the ε-phase Alq₃ fabricated by the physical vapor deposition (PVD) method.³⁰ One possible reason might be the difference in crystalline phase and the crystal size. Also, all spectra are similar in shape with one broad band. As a molecule is excited from S₀, the electron remains correlated with the hole left behind.³¹ The electrons and holes will be located either in a molecule or different neighboring molecules. The abundant intrinsic molecular electronic states, together with the interaction of intermolecules, result in a large number of energy levels in the Alq₃ rods, which give birth to broad emission bands.^30,31

Fig. 6 (A) Temperature-dependent PL spectra of Alq₃ microrods excited at 325 nm. (B) The calculated CIE chromaticity coordinates (0.271, 0.543) at 50 K and (0.262, 0.528) at 300 K of the Alq₃ microrods.

Moreover, the PL intensity and peak position are slightly dependent on the temperatures. Generally, the PL intensity decreases greatly with the temperature increases from 50 K to 300 K, that is the emission intensity at low temperature is stronger than that of high temperature attributed to the weak coupling of photons and phonons at low temperature.²⁶ The area ratio of PL spectra at room temperature and the maximum at low temperature, which is defined as internal quantum efficiency (IQE) in some paper, are calculated to be 47.7% of the sample.³⁰ The peak positions reveal an evident blue shift (about 6 nm) with the increase of the temperature. For example, the peak position is 509.7 nm at 300 K and 515.6 nm at 50 K, as shown in the Fig. 6A. The blue shift in PL energies with increasing temperature has been observed in other organic semiconductors such as PHP,³² MeLPPP,³² PPV and MEH-PPV,^33,34 which have typically been attributed to lattice fluctuations. The shift in electronic energies reflect on the temperature dependence of the actual relaxation process whereby the exciton remains more localized on smaller chain segments on increasing the temperature.³²

The spectra exhibit a very strong light emission with its maximum peak position at 515.6 nm at 50 K which corresponds to the radiative recombination of singlet excitons S₁ → S₀ in mer-Alq₃.^33,34 The CIE chromaticity coordinates of the PL spectra have been calculated by using software CIE1931. The calculated CIE coordinates of the PL spectra at 50 K and 300 K are (0.271, 0.543) and (0.262, 0.528), respectively (see Fig. 6B), which are typical green emissions.^35,36

Conclusions

In summary, we demonstrate a new one-step solution process for the controlled growth of 1D structured Alq₃. The Alq₃ microrods were fabricated by volatilizing Alq₃ solution directly at room temperature. No surfactant, SDS, anti-solvent, H₂O or other reagents assistant was used in our experiment. PXRD and FTIR spectra suggest that the fabricated Alq₃ 1D microrods have α-phase crystalline microstructures. The PL spectrum of the microrods exhibits a very strong peak at 515 nm with a symmetric curve in the range of 450 nm to 650 nm. Our method also could be extended to other functional organic materials.

Acknowledgements

The authors are grateful for financial support from the Natural Science Foundation of China (61176019, 11374184, 11444007, and 61106083).

Notes and references

1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913–915.
2. S. J. Fang, Z. Y. Pang, L. Lin, F. G. Wang, W. F. Zheng, Y. H. Li, Y. Dai and S. H. Han, Phys. E, 2011, 43, 1470–1474.
3. M. Muccini, M. A. Loi, K. Kenevey, R. Zamboni, N. Masciocchi and A. Sironi, Adv. Mater., 2004, 16, 861–864.
4. M. Brinkmann, G. Gadret, M. Muccini, C. Taliani, N. Masciocchi and A. Sironi, J. Am. Chem. Soc., 2000, 122, 5147–5157.
5. D. L. Sun, L. F. Yin, C. J. Sun, H. W. Guo, Z. Gai, X. G. Zhang, T. Z. Ward, Z. H. Cheng and J. Shen, Phys. Rev. Lett., 2010, 104, 236602.
6. B. Yu, L. Z. Huang, H. B. Wang and D. H. Yan, Adv. Mater., 2010, 22, 1017–1020.
7. S. J. Zhang, N. J. Rolfe, P. Desai, P. Shakya, A. J. Drew, T. Kreouzis and W. P. Gillin, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, 86, 075206.
8. A. Mishra, N. Periasamy, M. P. Patankar and K. L. Narasimhan, Dyes Pigm., 2005, 66, 89–97.
9. S. N. Al-Busafi, F. O. Suliman and Z. R. Al-Alawi, Dyes Pigm., 2014, 103, 138–144.
10. K. P. Dhakal, H. Lee and J. Y. Kim, Synth. Met., 2014, 190, 44–47.
11. Y. X. Ma, H. Li, S. Peng and L. Y. Wang, Anal. Chem., 2012, 84, 8415–8421.
12. S. J. Zhang, T. Kreouzis and W. P. Gillin, Synth. Met., 2013, 173, 46–50.
13. W. P. Gillin, S. J. Zhang, N. J. Rolfe, P. Desai, P. Shakya, A. J. Drew and T. Kreouzis, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 82, 195208.
14. T. Fukushima and H. Kaji, Org. Electron., 2012, 13, 2985–2990.
15. H. Bi, H. Y. Zhang, Y. Zhang, H. Z. Gao, Z. M. Su and Y. Wang, Adv. Mater., 2010, 22, 1631–1634.
16. C. P. Peter, L. W. Kennet and M. J. Brett, Adv. Mater., 2006, 18, 224–228.
17. C. P. Cho, C. A. Wu and T. P. Perng, Adv. Funct. Mater., 2006, 16, 819–823.
18. J. S. Hu, H. X. Ji, A. M. Cao, Z. X. Huang, Y. Zhang, L. J. Wan, A. D. Xia, D. P. Yu, X. M. Meng and S. T. Lee, Chem. Commun., 2007, 3083–3085.
19. A. M. Collins, S. N. Olof, J. M. Mitchels and S. Mann, J. Mater. Chem., 2009, 19, 3950–3954.
20. G. Xu, Y. B. Tang, C. H. Tsang, J. A. Zapien, C. S. Lee and N. B. Wong, J. Mater. Chem., 2010, 20, 3006–3010.
21. D. J. Jan, S. S. Wang, S. J. Tang, K. Y. Lin, J. J. Yang and J. L. Shen, Thin Solid Films, 2011, 520, 1005–1009.
22. M. Cölle and W. Brütting, Phys. Status Solidi A, 2004, 201, 1095–1114.
23. M. Rajeswaran and T. N. Blanton, J. Chem. Crystallogr., 2005, 35, 71–76.
24. M. Braun, J. Gmeiner, M. Tzolov, M. Coelle, F. D. Meyer, W. Milius, H. Hillebrecht, O. Wendland, J. U. von Schütz and W. Brütting, J. Chem. Phys., 2001, 114, 9625–9632.
25. W. Chen, Q. Peng and Y. D. Li, Adv. Mater., 2008, 20, 2747–2750.
26. W. F. Xie, J. H. Fan, H. Song, F. Jiang, H. M. Yuan, Z. X. Wei, Z. W. Ji, Z. Y. Pang and S. H. Han, Chem. Commun., 2015, 004228.
27. M. Cölle, C. Gärditz and M. Braun, J. Appl. Phys., 2004, 96, 6133–6141.
28. A. Nangia and G. R. Desiraju, Chem. Commun., 1999, 605–606.
29. I. S. Choi, X. H. Li, E. E. Simanek, R. Akaba and G. M. Whitesides, Chem. Mater., 1999, 11, 684–690.
30. W. F. Xie, Z. Y. Pang, Y. Zhao, F. Jiang, H. M. Yuan, H. Song and S. H. Han, J. Cryst. Growth, 2014, 404, 164–167.
31. G. Y. Zhong, X. M. Ding, J. Zhou, N. Jiang, W. Huang and X. Y. Hou, Chem. Phys. Lett., 2006, 420, 347–353.
32. S. Guha, J. D. Rice, Y. T. Yau, C. M. Martin, M. Chandrasekhar, H. R. Chandrasekhar, R. Guentner, P. Scanduicci de Freitas and U. Scherf, Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 67, 125204–125211.
33. S. H. Lim, T. G. Bjorklund and C. J. Bardeen, Chem. Phys. Lett., 2001, 342, 555–562.
34. T. W. Hagler, K. Pakbaz, K. F. Voss and A. J. Heeger, Phys. Rev. B: Condens. Matter Mater. Phys., 1991, 44, 8652–8666.
35. R. J. Curry and W. P. Gillin, J. Appl. Phys., 2000, 88, 781–785.
36. M. Cölle and C. Gärditz, Appl. Phys. Lett., 2004, 84, 3160–3162.

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