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zinc oxide hybrid nanowires
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Journal of Luminescence 128 (2008) 1629–1634 www.elsevier.com/locate/jlumin
Growth, morphology and optical properties of tris(8-hydroxyquinoline)aluminum/zinc oxide hybrid nanowires Periyayya Uthirakumar, Eun-Kyung Suh, Chang-Hee Hong School of Semiconductor and Chemical Technology, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, South Korea Received 14 September 2007; received in revised form 4 March 2008; accepted 11 March 2008 Available online 26 March 2008
Abstract Hybrid tris(8-hydroxyquinoline)aluminum/zinc oxide (Alq3/ZnO) nanowires were successfully grown from a one-step solution method at very low temperature. The transformation of amorphous Alq3 into a-phase crystalline nanowires was achieved by incorporating a certain weight fraction of crystalline ZnO nanomaterials. A growth mechanism was proposed to validate the formation of crystalline Alq3–ZnO hybrid nanowires with the help of nucleation initiated by the ZnO nanomaterials, followed by Alq3 molecular aggregation. Effects of temperature on the evolution of morphologies of hybrid nanowires were examined by the field-emission scanning electron microscopy (FESEM). The photoluminescence (PL) spectra of hybrid nanowires showed a significant threefold enhancement in PL intensity, along with a slight blue-shift emission, when compared to the pure Alq3 molecules, which were attributed due to the incorporation of crystalline ZnO nanomaterials and also the shielding effect of ZnO nanomaterials to avoid the excimer formation between the Alq3 molecules in the excited state. r 2008 Elsevier B.V. All rights reserved. Keywords: Solution method; Photoluminescence; Alq3; Morphology
1. Introduction Recently, extensive investigation on the synthesis of 1D nanomaterials including wires, tubes, rods, and belts has attracted much attention, owing to their unique properties and prospective applications in nanometer-scale devices [1–3]. So far, however, most of the research has focused on inorganic compounds [1–3] and organic polymers [4–6]. Nowadays, low molecular-weight organic nanomaterials have attracted increasing attention [7–10], because their electronic and optical properties are fundamentally different from those of inorganic ones [11,12]. In 1987, Tang and VanSlyke [13] fabricated highluminance low-voltage-driven devices, using efficient organic light-emitting diodes (OLEDs). Hence, tris(8-hydroxyquinoline)aluminum (Alq3) has become one of the most successful electron transport and emitting materials in the Corresponding authors. Tel.: +82 63 270 3928; fax: +82 63 270 3585.
E-mail addresses: [email protected] (P. Uthirakumar), [email protected] (C.-H. Hong). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.03.012
field of OLEDs. It has been studied more extensively from various aspects, such as doping electroluminescent materials into Alq3 [14], or by embedding Alq3 into mesoporous materials [15]. In parallel, different types of Alq3 with variety of field-emission properties were investigated and reported as nanoparticle [16], wirelike [17], and blue luminescent d crystal phase Alq3 [18–20]. Therefore, the crystallization characteristics of Alq3 deserve better understanding, and the possible optoelectronic applications of Alq3 nanostructures need further exploration. In order to achieve this, various methods were explored to synthesize different types of 1D Alq3 nanomaterials. Recent researches are mainly focusing on the synthesis of Alq3 nanowires using different techniques such as physical vapor deposition [21,22] and adsorbent-assisted physical vapor deposition [23] techniques. Cho et al. [24] reported the crystallization of amorphous Alq3 nanoparticles and transformation into nanowires from the vapor condensation system. Technically, most of these methods are carried out at relatively higher temperatures. However, a new and simple solution method was employed to crystallize Alq3
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nanowires successfully by incorporating few weight percentage of zinc oxide (ZnO) at a relatively very low temperature for the first time. To our best knowledge, this could be the facile, mild, and widely applied simplest method with low-cost route to synthesize 1D Alq3–ZnO hybrid nanowires. 2. Experimental Zinc acetate dihydrate (Zn(Ac)2 2H2O), polymethylmethacrylate (PMMA) and Alq3 were chemical-grade reagents, which were used as received without further purification. In the typical procedure, 0.086 g of Zn(Ac)2 2H2O (equivalent to 0.03 g of ZnO) was dissolved completely in 25 ml of methanol, using an ultrasonication water bath at room temperature for about a few minutes. The clear solution was filtered through a filter paper to remove any unknown particles. The resulting transparent filtrate was mixed with Alq3 solution (0.17 g of Alq3 dissolved in 25 ml of chloroform), which was stirred for an hour at room temperature for homogeneous mixing of both components. The resulting mixture was dispersed in the 1 wt% solution of PMMA in chloroform and casting over the glass substrates, to obtain the respective films for further characterization. Similarly, pure Alq3 and ZnO nanoparticles were also synthesized in the same technique for comparison. All the characterizations were made ex situ and at room temperature. The morphology, surface topography and structural features of the samples were analyzed using the field-emission scanning electron microscopy (FESEM, Hitachi S-4700). Transmission electron microscopy (TEM) was examined by TEM-2010 (JEOL, Japan), with an acceleration voltage of 200 kV. X-ray powder diffraction of the samples was obtained using a Rigaku X-ray diffractometer. X-ray diffractograms (XRD) were obtained using a CuKa incident beam (l ¼ 0.1546 nm), monochromated by a nickel filter, operated at 30 kV and 30 mA. Photoluminescence (PL) measurements were carried out at room temperature at an excitation wavelength of 325 nm. 3. Results and discussion Fig. 1 shows typical FESEM images of amorphous Alq3, Alq3–ZnO hybrid nanowires and pure ZnO nanoparticles treated at 60 1C for about 24 h. The amorphous Alq3 does not contain any noticeable shape of grain in an entire region of the samples. However, pure ZnO nanoparticles were primarily covered totally with nanoparticle aggregates ranging from 200 to 400 nm, which were composed of very fine (420 nm) ZnO nanoparticles, as like our previous report [25]. Interestingly, the FESEM image of Alq3–ZnO hybrid materials showed 1D nanowire generation throughout the whole region of the samples. Uniform nanowires growth was observed with an average diameter of 100–300 nm and few micrometers in length, as shown in Fig. 1b. This result indicates that an incorporation of a
Fig. 1. FESEM images of (a) amorphous Alq3, (b) Alq3–ZnO hybrid, and (c) pure ZnO nanomaterials treated at 60 1C for 24 h.
certain weight percent of ZnO nanomaterials resulted in uniform Alq3–ZnO nanowire growth with the phase transformation from amorphous to a-crystalline Alq3–ZnO hybrid materials. To investigate the uniformity of Alq3–ZnO hybrid nanowire formation, low-and high-magnified FESEM images
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Fig. 2. FESEM images of Alq3–ZnO hybrid nanowires prepared at 60 1C: (a) low and (b) higher magnification.
were presented in Fig. 2. Smooth and uniform Alq3–ZnO hybrid nanowires were observed and distributed throughout the samples; it covers more than 250 mm ranges, as shown in Fig. 2a. The average diameter and length of an individual nanowire were noted to be 100–300 nm and 3–5 mm, respectively. This observation substantiates the homogeneous mixing between the ZnO nanomaterials and Alq3 molecules, which ultimately induce a smooth and uniform Alq3–ZnO hybrid nanowire growth in the whole region. The thermal impact on Alq3–ZnO hybrid nanowires was also studied by annealing at three different temperatures, such as 60, 100, and 200 1C for 24 h along with their pure counterparts. The FESEM images of the representative annealed samples are given in Fig. 3. It can be seen that the population density of Alq3–ZnO hybrid nanowires growth remains the same, but the diameter and length of
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nanowires are different from each other. For example, the diameters of 60 1C-treated nanowires were about 100–300 nm, whereas the 200 1C-treated samples consist of newly formed nanowires having even less than 50 nm, as noticed in Fig. 3c. To confirm the diameter of an individual hybrid nanowire, the 200 1C-annealed sample was subjected to undergo the TEM, a representative image was presented in Fig. 3d. The TEM image clearly depicted that the diameter of a single nanowire was less than 100 nm, which ultimately confirms the formation of very small nanowires after annealing at high temperature. Therefore, based on these observations, new and smaller Alq3–ZnO hybrid nanowires start to grow from the existing nanowire, when it was annealed at high temperature. Hence, one can easily control the diameter and length of Alq3–ZnO hybrid nanowires by optimizing the suitable temperature range and also the weight percentage of ZnO nanomaterials incorporation. Fig. 4 illustrates the XRD patterns of Alq3–ZnO hybrid nanowires and their pure counterparts, annealed at 200 1C. Being pure Alq3 materials, they are perfectly amorphous in nature; it does not diffract any X-ray, leading to no such sharp XRD peaks, even after being annealed at 200 1C. This observation is in contrast to vapor condensation systems, where crystalline Alq3 nanowires were obtained, at above 150 1C, under high vacuum with an inert atmosphere [24]. However, they could not generate such transformations below 120 1C, these observations attributed that sufficient thermal energy is required to transform amorphous Alq3 into crystalline nanowires. At the same time, the pure ZnO nanomaterials displayed its own characteristic very sharp XRD peaks, which correspond to the JCPDS file of ZnO (JCPDS 36–1451; wurtzite-type nature, space group P63mc) [25,26]. Similarly, the XRD spectrum of Alq3–ZnO hybrid nanowires reveals the presence of characteristic sharp peaks of ZnO along with few crystalline Alq3 peaks in the lower 2y region, which ultimately confirms the presence of both components in the hybrid nanowires. The newly appeared X-ray diffraction peaks in the lower 2y region belong to the transformation of amorphous Alq3 into the a-phase crystalline nature materials [18,24]. Hence, the incorporation of few percentages of ZnO nanomaterials facilitates to transform amorphous Alq3 into a-phase crystalline hybrid nanowires. A schematic diagram represents a growth model proposed for the nucleation of ZnO nanomaterials followed by the Alq3–ZnO hybrids nanowires formation (Fig. 5). In order to grow the Alq3–ZnO hybrid nanowires, Alq3 molecules have to be crystallized by the help of any unknown nucleation. But, pure Alq3 molecules does not get any definite shape (Fig. 1a), it means even at 60 1C, pure Alq3 powder does not crystallize to appear any short of grains. Therefore, it should be stimulated by supplying either a sufficient external energy or any foreign materials (acting as a seed to initiate the nucleation) to initiate the hybrid nanomaterial formation, as like a normal crystallization concept. Here, very fine ZnO nanoparticles were
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(010)
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Fig. 3. FESEM images of Alq3–ZnO hybrid nanowires prepared at various temperatures: (a) 60 1C, (b) 100 1C, (c) 200 1C and (d) TEM image of Alq3–ZnO hybrid annealed at 200 1C.
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easily generated at this temperature in this particular reaction condition, as reported elsewhere [25]. Once it formed, the resulting ZnO nanoparticles might act as the seeds, leading to the formation of a definite shape of Alq3
molecules to become a primary nucleation unit from the nearby region. Finally, it starts to grow as a smooth and uniform Alq3–ZnO hybrid nanowire. Besides, very small hybrid nanowires were also formed from the existing nanowires, when it was subjected to anneal at high temperatures, as discussed in Fig. 3c. Fig. 6 reveals the PL spectra of Alq3–ZnO hybrid nanowires, pure Alq3 and pure ZnO samples. The PL spectrum of pure ZnO nanomaterials showed a characteristic blue band edge emission at around 380 nm with a weak and broad deep-level emission in the visible region. The oxygen deficiency and structural defects are responsible for the deep-level emission [25,27]. The enhanced PL intensity with a blue-shift emission of Alq3–ZnO hybrid nanowires is in good agreement with earlier reports [19,28]. The enhancement of PL intensity was almost more than a threefold improvement over the pure Alq3 molecules, which may be attributed to the shielding effect of ZnO materials on the Alq3 molecules, will suppress the interactions among the Alq3 molecules leading to the lower selfquenching in the excited state [28,29]. Furthermore, larger specific surface area and higher surface energy of the smaller nanowires increase the optical absorption, leading to a stronger PL emission [16]. In addition, the PL maximum of an organic semiconductor such as an Alq3 nanostructures does not shift into the high-energy region,
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At 60°C Amorphous Alq3
At 200°C
At 60°C ZnO seed Alq3-ZnO hybrid nanowires
Fig. 5. Schematic representation of the growth model proposed for the Alq3–ZnO hybrid nanowires growth.
optimizing the process temperature and quantity of incorporated ZnO nanomaterials. A growth mechanism was proposed to explain the formation of Alq3–ZnO hybrid nanowires from the nucleation initiated by the ZnO nanomaterials, followed by the molecular aggregation of Alq3. A significant threefold enhancement in PL intensity was observed in Alq3–ZnO hybrid nanowires when compared to the pure Alq3 molecules, which was due to the incorporation of crystalline ZnO nanomaterials and also due to the shielding effect of ZnO nanomaterials.
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This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-005-J07501).
Fig. 6. PL spectra of pure Alq3, Alq3–ZnO and pure ZnO annealed at 60 1C.
References because of the relatively weak Van der Waals forces among the nearest-neighbor molecules [30]. However, nanosized inorganic semiconductors often observed a blue-shift PL emission, due to the quantum-confinement effect. Here, ZnO nanomaterials playing an important role to blue shift the PL emission from 530 to 504 nm, very close to that of the a-phase reported by Colle et al. and other researchers [19,21], as well as more than threefold enhancement in PL intensity. 4. Conclusions New types of Alq3–ZnO hybrid nanowires were successfully synthesized from a simple solution method at very low temperature. Crystalline Alq3–ZnO hybrid nanowires were obtained by the incorporation of few weight fractions of crystalline ZnO nanomaterials. The length and diameters of Alq3–ZnO hybrid nanowires can be controlled by
[1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [2] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [3] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nat. 409 (2001) 66. [4] C.R. Martin, Acc. Chem. Res. 28 (1995) 61. [5] Y. Ma, J. Zhang, G. Zhang, H. He, J. Am. Chem. Soc. 126 (2004) 7097. [6] H. Liu, J. Kameoka, D.A. Czaplewsky, H.G. Craighead, Nano Lett. 4 (2004) 671. [7] H.B. Fu, J.N. Yao, J. Am. Chem. Soc. 123 (2001) 1434. [8] F. Debuigne, L. Jeunieau, M. Wiame, J.B. Nagy, Langmuir 16 (2000) 7605. [9] J. Jang, J.H. Oh, Adv. Mater. 15 (2003) 977. [10] Y. Tamaki, T. Asahi, H. Masuhara, J. Phys. Chem. A 106 (2002) 2135. [11] D.B. Xiao, L. Xi, W.S. Yang, H.B. Fu, Z.G. Shuai, Y. Fang, J.N. Yao, J. Am. Chem. Soc. 125 (2003) 6740. [12] Z.Y. Tian, Y. Chen, W.S. Yang, J.N. Yao, L.Y. Zhu, Z.G. Shuai, Angew. Chem. 116 (2004) 4152. [13] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913.
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[14] C.H. Chen, C.W. Tang, J. Shi, K.P. Klubek, Thin Solid Films 363 (2000) 327. [15] G.S. Huang, X.L. Wu, Y. Xie, F. Kong, Z.Y. Zhang, G.G. Siu, P.K. Chu, Appl. Phys. Lett. 87 (2005) 151910. [16] J.J. Chiu, W.S. Wang, C.C. Kei, T.P. Perng, App. Phys. Lett. 83 (2003) 347. [17] J.J. Chiu, C.C. Kei, T.P. Perng, W.S. Wang, Adv. Mater. 15 (2003) 1361. [18] M. Colle, R.E. Dinnebier, W. Brutting, Chem. Commun. 23 (2002) 2908. [19] M. Colle, J. Gmeiner, W. Milius, H. Hillebrecht, W. Brutting, Adv. Funct. Mater. 13 (2003) 108. [20] M. Muccini, M.A. Loi, K. Kenevey, R. Zamboni, N. Masciocchi, A. Sironi, Adv. Mater. 16 (2004) 861. [21] X. Tian, J. Fei, Z. Pi, C. Yang, D. Luo, F. Pei, L. Zhang, Solid State Commun. 138 (2006) 530.
[22] S.C. Lyu, Y. Zhang, C.J. Lee, Chem. Mater. 15 (2003) 3294. [23] Y.S. Zhao, C. Di, W. Yang, G. Yu, Y. Liu, J. Yao, Adv. Funct. Mater. 16 (2006) 1985. [24] C.P. Cho, C.A. Wu, T.P. Perng, Adv. Funct. Mater. 16 (2006) 819. [25] P. Uthirakumar, Y.S. Lee, E.K. Suh, C.H. Hong, Phys. Lett. A 359 (2006) 223. [26] D. Jezequel, J. Guenot, N. Jouini, F. Fievet, J. Mater. Res. 10 (1995) 77. [27] K. Vanheusden, W.L. Waren, C.H. Seager, D.R. Tallant, J.A. Voigt, J. Appl. Phys. 79 (1996) 7983. [28] P. Uthirakumar, C.H. Hong, E.K. Suh, Y.S. Lee, Chem. Mater. 18 (2006) 4990. [29] P. Uthirakumar, Y.S. Lee, E.K. Suh, C.H. Hong, J. Lumin. 128 (2008) 287. [30] A.P. Alivistos, Science 271 (1996) 933.
Journal of Luminescence 128 (2008) 1629–1634 www.elsevier.com/locate/jlumin
Growth, morphology and optical properties of tris(8-hydroxyquinoline)aluminum/zinc oxide hybrid nanowires Periyayya Uthirakumar, Eun-Kyung Suh, Chang-Hee Hong School of Semiconductor and Chemical Technology, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, South Korea Received 14 September 2007; received in revised form 4 March 2008; accepted 11 March 2008 Available online 26 March 2008
Abstract Hybrid tris(8-hydroxyquinoline)aluminum/zinc oxide (Alq3/ZnO) nanowires were successfully grown from a one-step solution method at very low temperature. The transformation of amorphous Alq3 into a-phase crystalline nanowires was achieved by incorporating a certain weight fraction of crystalline ZnO nanomaterials. A growth mechanism was proposed to validate the formation of crystalline Alq3–ZnO hybrid nanowires with the help of nucleation initiated by the ZnO nanomaterials, followed by Alq3 molecular aggregation. Effects of temperature on the evolution of morphologies of hybrid nanowires were examined by the field-emission scanning electron microscopy (FESEM). The photoluminescence (PL) spectra of hybrid nanowires showed a significant threefold enhancement in PL intensity, along with a slight blue-shift emission, when compared to the pure Alq3 molecules, which were attributed due to the incorporation of crystalline ZnO nanomaterials and also the shielding effect of ZnO nanomaterials to avoid the excimer formation between the Alq3 molecules in the excited state. r 2008 Elsevier B.V. All rights reserved. Keywords: Solution method; Photoluminescence; Alq3; Morphology
1. Introduction Recently, extensive investigation on the synthesis of 1D nanomaterials including wires, tubes, rods, and belts has attracted much attention, owing to their unique properties and prospective applications in nanometer-scale devices [1–3]. So far, however, most of the research has focused on inorganic compounds [1–3] and organic polymers [4–6]. Nowadays, low molecular-weight organic nanomaterials have attracted increasing attention [7–10], because their electronic and optical properties are fundamentally different from those of inorganic ones [11,12]. In 1987, Tang and VanSlyke [13] fabricated highluminance low-voltage-driven devices, using efficient organic light-emitting diodes (OLEDs). Hence, tris(8-hydroxyquinoline)aluminum (Alq3) has become one of the most successful electron transport and emitting materials in the Corresponding authors. Tel.: +82 63 270 3928; fax: +82 63 270 3585.
E-mail addresses: [email protected] (P. Uthirakumar), [email protected] (C.-H. Hong). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.03.012
field of OLEDs. It has been studied more extensively from various aspects, such as doping electroluminescent materials into Alq3 [14], or by embedding Alq3 into mesoporous materials [15]. In parallel, different types of Alq3 with variety of field-emission properties were investigated and reported as nanoparticle [16], wirelike [17], and blue luminescent d crystal phase Alq3 [18–20]. Therefore, the crystallization characteristics of Alq3 deserve better understanding, and the possible optoelectronic applications of Alq3 nanostructures need further exploration. In order to achieve this, various methods were explored to synthesize different types of 1D Alq3 nanomaterials. Recent researches are mainly focusing on the synthesis of Alq3 nanowires using different techniques such as physical vapor deposition [21,22] and adsorbent-assisted physical vapor deposition [23] techniques. Cho et al. [24] reported the crystallization of amorphous Alq3 nanoparticles and transformation into nanowires from the vapor condensation system. Technically, most of these methods are carried out at relatively higher temperatures. However, a new and simple solution method was employed to crystallize Alq3
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nanowires successfully by incorporating few weight percentage of zinc oxide (ZnO) at a relatively very low temperature for the first time. To our best knowledge, this could be the facile, mild, and widely applied simplest method with low-cost route to synthesize 1D Alq3–ZnO hybrid nanowires. 2. Experimental Zinc acetate dihydrate (Zn(Ac)2 2H2O), polymethylmethacrylate (PMMA) and Alq3 were chemical-grade reagents, which were used as received without further purification. In the typical procedure, 0.086 g of Zn(Ac)2 2H2O (equivalent to 0.03 g of ZnO) was dissolved completely in 25 ml of methanol, using an ultrasonication water bath at room temperature for about a few minutes. The clear solution was filtered through a filter paper to remove any unknown particles. The resulting transparent filtrate was mixed with Alq3 solution (0.17 g of Alq3 dissolved in 25 ml of chloroform), which was stirred for an hour at room temperature for homogeneous mixing of both components. The resulting mixture was dispersed in the 1 wt% solution of PMMA in chloroform and casting over the glass substrates, to obtain the respective films for further characterization. Similarly, pure Alq3 and ZnO nanoparticles were also synthesized in the same technique for comparison. All the characterizations were made ex situ and at room temperature. The morphology, surface topography and structural features of the samples were analyzed using the field-emission scanning electron microscopy (FESEM, Hitachi S-4700). Transmission electron microscopy (TEM) was examined by TEM-2010 (JEOL, Japan), with an acceleration voltage of 200 kV. X-ray powder diffraction of the samples was obtained using a Rigaku X-ray diffractometer. X-ray diffractograms (XRD) were obtained using a CuKa incident beam (l ¼ 0.1546 nm), monochromated by a nickel filter, operated at 30 kV and 30 mA. Photoluminescence (PL) measurements were carried out at room temperature at an excitation wavelength of 325 nm. 3. Results and discussion Fig. 1 shows typical FESEM images of amorphous Alq3, Alq3–ZnO hybrid nanowires and pure ZnO nanoparticles treated at 60 1C for about 24 h. The amorphous Alq3 does not contain any noticeable shape of grain in an entire region of the samples. However, pure ZnO nanoparticles were primarily covered totally with nanoparticle aggregates ranging from 200 to 400 nm, which were composed of very fine (420 nm) ZnO nanoparticles, as like our previous report [25]. Interestingly, the FESEM image of Alq3–ZnO hybrid materials showed 1D nanowire generation throughout the whole region of the samples. Uniform nanowires growth was observed with an average diameter of 100–300 nm and few micrometers in length, as shown in Fig. 1b. This result indicates that an incorporation of a
Fig. 1. FESEM images of (a) amorphous Alq3, (b) Alq3–ZnO hybrid, and (c) pure ZnO nanomaterials treated at 60 1C for 24 h.
certain weight percent of ZnO nanomaterials resulted in uniform Alq3–ZnO nanowire growth with the phase transformation from amorphous to a-crystalline Alq3–ZnO hybrid materials. To investigate the uniformity of Alq3–ZnO hybrid nanowire formation, low-and high-magnified FESEM images
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Fig. 2. FESEM images of Alq3–ZnO hybrid nanowires prepared at 60 1C: (a) low and (b) higher magnification.
were presented in Fig. 2. Smooth and uniform Alq3–ZnO hybrid nanowires were observed and distributed throughout the samples; it covers more than 250 mm ranges, as shown in Fig. 2a. The average diameter and length of an individual nanowire were noted to be 100–300 nm and 3–5 mm, respectively. This observation substantiates the homogeneous mixing between the ZnO nanomaterials and Alq3 molecules, which ultimately induce a smooth and uniform Alq3–ZnO hybrid nanowire growth in the whole region. The thermal impact on Alq3–ZnO hybrid nanowires was also studied by annealing at three different temperatures, such as 60, 100, and 200 1C for 24 h along with their pure counterparts. The FESEM images of the representative annealed samples are given in Fig. 3. It can be seen that the population density of Alq3–ZnO hybrid nanowires growth remains the same, but the diameter and length of
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nanowires are different from each other. For example, the diameters of 60 1C-treated nanowires were about 100–300 nm, whereas the 200 1C-treated samples consist of newly formed nanowires having even less than 50 nm, as noticed in Fig. 3c. To confirm the diameter of an individual hybrid nanowire, the 200 1C-annealed sample was subjected to undergo the TEM, a representative image was presented in Fig. 3d. The TEM image clearly depicted that the diameter of a single nanowire was less than 100 nm, which ultimately confirms the formation of very small nanowires after annealing at high temperature. Therefore, based on these observations, new and smaller Alq3–ZnO hybrid nanowires start to grow from the existing nanowire, when it was annealed at high temperature. Hence, one can easily control the diameter and length of Alq3–ZnO hybrid nanowires by optimizing the suitable temperature range and also the weight percentage of ZnO nanomaterials incorporation. Fig. 4 illustrates the XRD patterns of Alq3–ZnO hybrid nanowires and their pure counterparts, annealed at 200 1C. Being pure Alq3 materials, they are perfectly amorphous in nature; it does not diffract any X-ray, leading to no such sharp XRD peaks, even after being annealed at 200 1C. This observation is in contrast to vapor condensation systems, where crystalline Alq3 nanowires were obtained, at above 150 1C, under high vacuum with an inert atmosphere [24]. However, they could not generate such transformations below 120 1C, these observations attributed that sufficient thermal energy is required to transform amorphous Alq3 into crystalline nanowires. At the same time, the pure ZnO nanomaterials displayed its own characteristic very sharp XRD peaks, which correspond to the JCPDS file of ZnO (JCPDS 36–1451; wurtzite-type nature, space group P63mc) [25,26]. Similarly, the XRD spectrum of Alq3–ZnO hybrid nanowires reveals the presence of characteristic sharp peaks of ZnO along with few crystalline Alq3 peaks in the lower 2y region, which ultimately confirms the presence of both components in the hybrid nanowires. The newly appeared X-ray diffraction peaks in the lower 2y region belong to the transformation of amorphous Alq3 into the a-phase crystalline nature materials [18,24]. Hence, the incorporation of few percentages of ZnO nanomaterials facilitates to transform amorphous Alq3 into a-phase crystalline hybrid nanowires. A schematic diagram represents a growth model proposed for the nucleation of ZnO nanomaterials followed by the Alq3–ZnO hybrids nanowires formation (Fig. 5). In order to grow the Alq3–ZnO hybrid nanowires, Alq3 molecules have to be crystallized by the help of any unknown nucleation. But, pure Alq3 molecules does not get any definite shape (Fig. 1a), it means even at 60 1C, pure Alq3 powder does not crystallize to appear any short of grains. Therefore, it should be stimulated by supplying either a sufficient external energy or any foreign materials (acting as a seed to initiate the nucleation) to initiate the hybrid nanomaterial formation, as like a normal crystallization concept. Here, very fine ZnO nanoparticles were
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(010)
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Fig. 3. FESEM images of Alq3–ZnO hybrid nanowires prepared at various temperatures: (a) 60 1C, (b) 100 1C, (c) 200 1C and (d) TEM image of Alq3–ZnO hybrid annealed at 200 1C.
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easily generated at this temperature in this particular reaction condition, as reported elsewhere [25]. Once it formed, the resulting ZnO nanoparticles might act as the seeds, leading to the formation of a definite shape of Alq3
molecules to become a primary nucleation unit from the nearby region. Finally, it starts to grow as a smooth and uniform Alq3–ZnO hybrid nanowire. Besides, very small hybrid nanowires were also formed from the existing nanowires, when it was subjected to anneal at high temperatures, as discussed in Fig. 3c. Fig. 6 reveals the PL spectra of Alq3–ZnO hybrid nanowires, pure Alq3 and pure ZnO samples. The PL spectrum of pure ZnO nanomaterials showed a characteristic blue band edge emission at around 380 nm with a weak and broad deep-level emission in the visible region. The oxygen deficiency and structural defects are responsible for the deep-level emission [25,27]. The enhanced PL intensity with a blue-shift emission of Alq3–ZnO hybrid nanowires is in good agreement with earlier reports [19,28]. The enhancement of PL intensity was almost more than a threefold improvement over the pure Alq3 molecules, which may be attributed to the shielding effect of ZnO materials on the Alq3 molecules, will suppress the interactions among the Alq3 molecules leading to the lower selfquenching in the excited state [28,29]. Furthermore, larger specific surface area and higher surface energy of the smaller nanowires increase the optical absorption, leading to a stronger PL emission [16]. In addition, the PL maximum of an organic semiconductor such as an Alq3 nanostructures does not shift into the high-energy region,
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At 60°C Amorphous Alq3
At 200°C
At 60°C ZnO seed Alq3-ZnO hybrid nanowires
Fig. 5. Schematic representation of the growth model proposed for the Alq3–ZnO hybrid nanowires growth.
optimizing the process temperature and quantity of incorporated ZnO nanomaterials. A growth mechanism was proposed to explain the formation of Alq3–ZnO hybrid nanowires from the nucleation initiated by the ZnO nanomaterials, followed by the molecular aggregation of Alq3. A significant threefold enhancement in PL intensity was observed in Alq3–ZnO hybrid nanowires when compared to the pure Alq3 molecules, which was due to the incorporation of crystalline ZnO nanomaterials and also due to the shielding effect of ZnO nanomaterials.
20000 Alq3-ZnO Relative PL intensity
15000
10000
Alq3
5000
Acknowledgment
ZnO 0 350
400
450
500 550 600 650 Wavelength (nm)
700
750
800
This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-005-J07501).
Fig. 6. PL spectra of pure Alq3, Alq3–ZnO and pure ZnO annealed at 60 1C.
References because of the relatively weak Van der Waals forces among the nearest-neighbor molecules [30]. However, nanosized inorganic semiconductors often observed a blue-shift PL emission, due to the quantum-confinement effect. Here, ZnO nanomaterials playing an important role to blue shift the PL emission from 530 to 504 nm, very close to that of the a-phase reported by Colle et al. and other researchers [19,21], as well as more than threefold enhancement in PL intensity. 4. Conclusions New types of Alq3–ZnO hybrid nanowires were successfully synthesized from a simple solution method at very low temperature. Crystalline Alq3–ZnO hybrid nanowires were obtained by the incorporation of few weight fractions of crystalline ZnO nanomaterials. The length and diameters of Alq3–ZnO hybrid nanowires can be controlled by
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