- Email: [email protected]
Preparation and properties of Ni-doped ZnO rod arrays from aqueous solution
Journal of Colloid and Interface Science 330 (2009) 380–385
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation and properties of Ni-doped ZnO rod arrays from aqueous solution Dianwu Wu a , Mei Yang a , Zhongbing Huang a,∗ , Guangfu Yin a , Xiaoming Liao a , Yunqing Kang a , Xianfu Chen a , Hui Wang b a b
College of Materials Sciences and Engineering, Sichuan University, No. 24, South 1st Section, 1st Ring Road, Chengdu, Sichuan 610065, PR China Analytical and Testing Center, Sichuan University, Chengdu 610064, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 17 July 2008 Accepted 25 October 2008 Available online 30 October 2008
Ni-doped ZnO rod arrays were successfully prepared on glass substrate from the aqueous solution at a temperature of 80 ◦ C. The densities, diameters, and lengths of the rods can easily be well controlled through the concentrations of dopants, and the dopant Ni ions were incorporated into the wurtzitestructure of ZnO crystal. Room temperature photoluminescence spectrum of rod arrays show a strong emission band at 410 nm, and the oxygen deficiencies in ZnO structures were significantly reduced with Ni doped in ZnO rod arrays. The field dependence of magnetization measured at room temperature exhibited the obvious ferromagnetic properties. © 2008 Elsevier Inc. All rights reserved.
Keywords: ZnO rod arrays Nickel Doping Ferromagnetism
1. Introduction Owing to a direct wide band gap (3.37 eV), large exciton binding energy (60 meV), and superior conducting properties based on oxygen vacancies, excellent electro and optical properties [1–3], the wurtzite-structured ZnO has become one of the most promising materials for the fabrication of high-technology applications such as photonic crystals, light-emitting diodes, sensors, electroand photoluminescent materials [4,5]. The key requirements for these applications are that ZnO materials have the good electrical, optical, and magnetic properties and that the control of the shape and crystal structure is important. It is well known that the addition of impurities into a wide gap semiconductor can often induce dramatic changes in the optical, electrical, and magnetic properties [6,7]. Therefore, doping a selective element into ZnO has become an important route for enhancing and controlling its optical, electrical, and magnetic performance, which is crucial for their practical applications. Theoretical calculations have showed that transition-metal (TM)-doped ZnO would be a good candidate to achieve Curie temperature above room temperature [8–10]. Since then, great efforts have been devoted to the investigation of magnetic-metal/ZnO one-dimensional (1D) structure materials hetero-structures due to its new functionality in memory devices, detectors and light-emitting sources [11,12]. Of the many magnetic-metal, Ni is an important dopant to achieve Curie tem-
*
Corresponding author. Fax: +86 28 85413003. E-mail address: [email protected] (Z. Huang).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.10.067
© 2008
Elsevier Inc. All rights reserved.
perature above room temperature. Recently, 1D nanostructures of ZnO, such as nanowires, nanorods, nanobelts, and nanotubes, have been reported [13,14], including epitaxial and nonepitaxial structures. Up to now, conventional routes, such as ultrasonic irradiation [15], thermal chemical vapor deposition [16], ion implantation [17], electrochemical deposition [18], and hydrothermal method [19], have been developed to fabricate transition-metal doped 1D ZnO structured materials. There are also a number of reports about the preparation of ZnO nanoarrays on different substrates, such as silicon substrate [20], zinc substrate [21], plastic substrate [22], aluminum substrate [23], and different substrates can gain ZnO of different morphology and different properties. However, only a few papers reported the preparation of doped ZnO rod arrays [18, 24–26]. Nevertheless, these methods usually required special experimental conditions such as electric fields, ultrasound treatment, long reaction time, high temperature or high pressure. On the other hand, a major practical challenge is the formation of good Ni2+ –ZnO junctions. Many previous studies have demonstrated the tendency for dopant ions to be excluded during crystal synthesis [27–29]. In this paper, we successfully synthesized Ni-doped ZnO rod arrays on ZnO seeds substrate using a simple aqueous solution method with controlling Ni contents. This method may be used for preparation of many new device materials in microscale electronics and photonics such as novel memory and optical device, and it will be applied in biological detecting and treatment. ZnO rod arrays with various microstructures parameters can be controlled by changing the concentrations of the dopants.
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
2. Materials and method 2.1. Materials The zinc precursor, zinc nitrate (Zn(NO3 )2 ·6H2 O), hexamethylenetetramine (C6 H12 N4 , HMT) and nickel acetate (Ni(CH3 COO)2 · 4H2 O) was used for the preparation of ZnO rod arrays. 2.2. Preparation of Ni-doped ZnO rod arrays The experiment was conducted in the following procedure. Firstly, ZnO crystal seeds particles were coated on the substrate. The glass substrate was cleaned ultrasonically in the mixed solution (the volume ratio of 98% H2 SO4 :30% H2 O2 is 3:1) and deionized water for 10 min, respectively, and the well cleaned substrates were immersed into the aqueous solution of 0.025 mol/l C6 H12 N4 (HMT) and 0.025 mol/l Zn(NO3 )2 ·6H2 O for 5 min at room temperature. Then, the substrates were dried and subsequently sintered at 500 ◦ C for 10 min in a muffle furnace. The coating process above was repeated 6 times in order to form uniform coverage of ZnO seeds on the entire substrate. Secondly, Ni-doped ZnO rod arrays were grown vertically from the nanocrystal seeds by immersing seed-coated substrate in the aqueous solution of equimolar of C6 H12 N4 (HMT), Zn(NO3 )2 ·6H2 O and different concentrations of Ni(CH3 COO)2 ·4H2 O. The concentration of the dopant in reactive solution can be tuned according to Ni/Zn atomic ratio of 0, 0.1, 0.2, 0.3, and 0.4, and the as-prepared samples in subsequent discussions of this paper are designated as C0, C1, C2, C3 and C4, respectively. The growth time and temperature were 12 h and 80 ◦ C, respectively. Finally, the as-prepared samples were took out the solution, rinsed thoroughly with deionized water for 5 times to eliminate residual salts, and dried for 24 h at room temperature. 2.3. Characterization The crystal structures of the as-prepared samples were analyzed using X-ray diffractometer (XRD, X’Pert, Holand) with CuK α radiation. The morphology of Ni-doped ZnO rod arrays was characterized by scanning electron microscopy (SEM, JSM-5900LV, Japan). The analysis of elements was conducted the energy-dispersive X-ray spectroscopy (EDX) attached to the scanning electron microscopy (HITACHI-4300S). The UV–visible absorption was measured on Hitachi U-3010 spectrophotometer. The photoluminescence (PL) measurements were performed on a Hitachi F-7000 Fluorometry with a Xe lamp as the excitation light source at room temperature. The excitation wavelength was 330 nm. The ferromagnetic behaviors of the Ni-doped ZnO rod arrays at room temperature were investigated by Vibrating Sample Magnetometer (VSM, Lake shore-7400, USA). 3. Results and discussions 3.1. Morphological and structural characterization Fig. 1 shows SEM images of undoped and Ni-doped ZnO rod arrays grown on substrates. In SEM images, hexagonal ZnO rod arrays were successfully prepared, revealing that Ni-doped ZnO rod arrays keep their vertical alignment, and they are different in the rods diameters and array densities. As-grown Ni-doped ZnO rods have the average diameter of about 250, 200, 250, 300, and 400 nm for C0, C1, C2, C3, and C4, respectively. The heights of these arrays can be clearly measured from Fig. 2, and they are about 0.8, 1.0, 1.5 and 1.2 um for C1, C2, C3 and C4, respectively. From Fig. 2, ZnO crystal seeds particles (the diameter is not more than 100 nm)
381
can also be found on the substrates. It is obvious that the orientation of the rod arrays prepared with the concentrations of dopant of 0.0025 mol/l in Fig. 1b is not completely uniform. However, when the concentration of dopant was increased to 0.005 mol/l in aqueous solution, the orientation of rod arrays (Fig. 1c) became more regular than that shown in Fig. 1b. The concentration of dopant was increased to 0.0075 mol/l, and the diameter of the rods as shown in Fig. 1d became larger but the quantity of the rods became fewer. When the concentration of dopant was up to 0.01 mol/l, the nucleation quantity was remarkably lower, and there were some silk-like impurities (marked in Fig. 1e with the arrow 1) and some rod defects (marked in Fig. 1e with the arrow 2) observed in Fig. 1e. There was a mass of aqua deposition in the bottom of the reactive bottle with the beginning of C3, which maybe considered as NiO particles. From Fig. 1, we also found that one large ZnO rod could be composed by several small ZnO rods clinging to each other (such as the arrows marked in Figs. 1c and 1d). It is well known from the synthesis of particles that the dopant atoms have a strong influence on the amount and size of the resulting rods [30,31]. It was a kinetic equilibrium process of thermodynamic equilibrium and dynamic equilibrium. The incorporation of dopant into the host lattice is hindered through an increase of surface energy and lattice distortion. In particular, the thermodynamically unfavored, purely kinetically driven growth of one dimensional structure is often restrained. The composition of C3 is determined by the EDX, and the result shows the existence of Ni, Zn and O in the rods, suggesting that in this area (the inset of Fig. 3) Ni has entered into the ZnO lattice. Moreover, the EDX taken at a rectangle location throughout the specimens reveal a solid solution of Ni dissolved in ZnO crystal. There are 0.98% atomic ratios of nickel ions doped into the ZnO rods (C3), corresponding with the nominal nickel acetate of 0.0075 M in the reactant. It is obvious that the amount of Ni incorporated into the ZnO matrix is much less than the actual amount of Ni provided in the precursor during the synthesis. This maybe result from the formation of a mass of fine NiO clusters in reactive solution, which precipitated out on the bottom of the container and were discarded in the purification steps. Most importantly, the incorporation of nickel ions into the ZnO host lattice is hindered during the synthesis of Ni-doped ZnO rod arrays. In order to more confirm that Ni was incorporated into the crystal structure, XRD was performed. Fig. 4 displays X-ray diffraction patterns of the as-prepared undoped and doped ZnO samples. The XRD spectra of Ni-doped ZnO rod arrays consist of (002), (100), (101), (102), and (103) peaks, and all the observed diffraction peaks can be indexed to ZnO wurtzite structure. The strong (002) peak proves that ZnO rods with wurtzite structure were obtained in both undoped and Ni-doped ZnO samples. No diffraction peaks of other structures were detected in these samples, indicating that the Ni ion successfully occupied ZnO lattice site and there were no secondary phases or precipitates in the samples. The prepared undoped and Ni-doped ZnO were demonstrated to be preferential orientation along the (002) peak direction. The (002) diffraction peaks of C2 and C3 were obviously higher than that of C0. However, C4 are has the same height of (002) diffraction peak as C0, which might be correlated to the actual quantity of Ni2+ doped into the ZnO crystal lattice. Although C4 has a higher dopant concentration in reactive solution, the actual concentration of Ni2+ doped into the ZnO crystal lattice is lower. Due to the higher bond energy of Ni2+ –O2− compared to that of Zn2+ –O2− , the more energy is required to make Ni2+ ions enter into lattice and form the bond of Ni2+ –O2− . Therefore, Ni2+ substituting for Zn2+ has higher stability relative to the Zn–O structure, and more energy is required for the substituted samples to complete crystallization. In addition, because of the larger radius of Zn2+ (0.74 Å) [32] compared to Ni2+ (0.69 Å), the replacement of Zn2+ by Ni2+ will
382
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
Fig. 1. SEM images of ZnO rod arrays grown with different dopants concentration in reactive solutions. (a) 0 M. (b) 0.0025 M. (c) 0.005 M. (d) 0.0075 M. (e) 0.01 M.
cause only slight decrease in lattice parameters. Although doping does not alter the crystal structure, it causes the lattice constant to change as evidence of the (002) peak position shift. The c-axis constant d(002) of the Ni-doped ZnO rod arrays are decreased by 0.68% for C1 relative to undoped sample, and increased by 0.57, 0.94 and 0.23% for C2, C3, and C4, respectively, suggesting that doping Ni ions will slightly change the c-axis constant. Although the change is very little, the concentration of dopant plays a role in the c-axis constant. In addition, the small quantities of silk-like impurities in Fig. 1d may be slightly existed (less than 1%) and probably not induce a detectable peak. 3.2. Optical properties Fig. 5 shows the absorption of corresponding UV–visible spectra of undoped and Ni-doped ZnO rod arrays. In order to obtain a precise measurement of the shifts in the wavelength of the spectra, we use the point of inflection obtained from the derivative curve of the absorption spectra (marked in Fig. 5 with the arrows). The spectra become broader with blue-shifted of 2.88, 3.07, 7.05,
and 1.91 nm for C1, C2, C3, and C4 compared to C0, respectively, suggesting an indirect evidence for the decrease of band gap (E g ) and the energy broadening of valence band states attributable to the doping [33]. When the doped Ni2+ enters into the ZnO crystal structure, the localized band edge states form at the doped sites with a reduction of E g due to the crystal lattice confinement effect. The PL spectra (Fig. 6) consist of two UV emission peaks which centered at 398 nm and 410 nm, respectively, a distinct shoulder peak centered at 465 nm, and a weak green emission band centered at 530 nm. With nickle doped into the ZnO crystal lattice, the intensity of the peaks centered at 410 nm gradually increased. And the purple luminescence band (398 nm) belongs to the exciton recombination corresponding to near-band-edge emission of ZnO rod arrays. The strong peak centered at 410 nm most likely originates from the coordinative, unsaturated Zn sites in the ZnO rod arrays or the formation of band tailing in the band gap, which is often induced by the introduction of impurity into the semiconductor [34–36]. Different shallows levels are formed in the band gap, due to the presence of interstitial zinc atoms, which give
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
383
Fig. 4. XRD patterns of the prepared undoped and Ni-doped ZnO rod arrays.
Fig. 2. SEM images of the side views of doped ZnO rod arrays grown with different dopants concentration in reactive solutions. (a) 0.0025 M. (b) 0.005 M. (c) 0.0075 M. (d) 0.01 M.
Fig. 5. UV–visible absorption spectra of undoped and Ni-doped ZnO nanorod arrays measured at room temperature.
Fig. 3. EDX spectra of Ni-doped ZnO rods shown in Fig. 1c. The inset is the area where EDX was operated.
rise to red-shifted peaks [37]. With the increase of Ni concentration, the peaks of UV emission are getting strong. The result is well agreed with the UV–visible spectra, showing that the Nidoped ZnO rod arrays have a high optical property. However, the
visible luminescences from oxygen ion defect states were usually observed at about 530 nm, presenting an interesting result. Compared to the undoped sample, the green emission peak intensity of doped ZnO rod arrays reduced obviously, and with the increase of the concentrations of dopant, the intensity of green emission peaks became weaker, and then turned stronger. This might be ascribed to the oxygen vacancy and the transition of a photogenerated electron from a dark level below the conduction band to a deeply trapped hole [38]. There are several reasons: because of good crystallinity, there are only a few defect sites in the wurtzite lattice, which would act as electron traps shifting the photoluminescence into the visible range. Besides, the dopant atoms also play a role in the photoluminescence behavior of the ZnO rods, since Ni2+ acts as deep traps in materials, which open up non-radiative relaxation pathways [39–41]. According to the SEM images, C2 has the perfect crystallinity and compact, orderly arrays. The morphology of C1 sample is also orderly but not enough dense. There was a mass of deposition at the bottom of the bottle with the beginning of C3,
384
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
the formation of some Ni-related secondary phase, such as NiO phase. However, NiO phase can be easily ruled out, since bulk NiO is antiferromagnetism with a Neel temperature of 520 K [43], and XRD result clearly proves no NiO crystal structure in the doped arrays. The origin of ferromagnetizatic behavior of Ni-doped ZnO arrays could be considered as a result of the exchange interaction between free delocalized carriers (hole or electron from the valence band) and the localized d-spins on the Ni ions [44,45]. 4. Conclusions In summary, well aligned Ni-doped ZnO rod arrays have been successfully synthesized by a simple aqueous solution route. The as-prepared products are single-crystalline wurtzite structure. Photoluminescence spectra measurement demonstrated that Ni-doped ZnO rod arrays exhibit a near band edge UV emission peak centered at about 410 nm, and a green emission peak centered at about 540 nm. The M–H curve of Ni-doped ZnO rod arrays measured by VSM at room temperature indicated that they have obvious ferromagnetic characteristic. Fig. 6. PL spectrum of different Ni-doped samples measured at room temperature. The excitation wavelength of PL is the 310 nm.
Acknowledgments This work has been supported by the National Natural Science Foundation of China (project No. 60871062). The supports of Sichuan Province through a Science Fund for Distinguished Young Scholars of Sichuan Province (08ZQ026-007) and Key Technologies Research and Development Program of Sichuan Province (2008SZ0021 and 2006Z08-001-1) are also acknowledged with gratitude. This work was also supported by the Research Fund for the Doctoral Program of Higher Education from Ministry of Education of China (No. 20070610131). We thank Analytical & Testing Center, Sichuan University for the assistance with the microscopy work. References
Fig. 7. Magnetic hysteresis loops of Ni-doped ZnO rod arrays at room temperature, the inset (a) shows its enlargement view at the origin, and the inset (b) shows the magnetization curve of zero field.
Ni2+ could not thoroughly enter into the ZnO crystal lattice, and C4 is uneven and has some impurity at the surface of ZnO rods. 3.3. Magnetic properties Fig. 7 presents the magnetization (M) vs magnetic field (H) of Ni-doped ZnO rod arrays. Hysteresis curves are obtained at room temperature, showing all Ni-doped samples have obvious ferromagnetic characteristic. Their coercivities (H c ) are 134, 71, 48, and 123 Oe for C1, C2, C3, and C4, respectively (shown in the inset of Fig. 7). The saturation magnetizations (M S ) of Ni-doped ZnO arrays are up to 0.28, 0.49, 0.99, and 0.09 emu/g, respectively. From C1 to C3, ferromagnetism is enhanced significantly due to the higher Ni2+ concentrations. However, when the Ni2+ concentration in reactive solution is up to 0.01 M (C4), ferromagnetism is decreased rapidly. This is because of only a few Ni2+ doped into the ZnO lattice, and the results are consistent with the conclusion from UV–visible absorption spectra. The saturation magnetizations we measured are larger than the previous reports on Ni-doped ZnO nanoparticles [6], this might be due to the more Ni2+ doped into the ZnO lattice. There are a few of controversial explanations as to whether the ferromagnetism arises from the homogeneous magnetic doping or from magnetic precipitation [42], one of which is
[1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russor, P.D. Yang, Science 292 (2001) 1897. [2] Z.L. Wang, J.H. Song, Science 312 (2006) 242. [3] X.L. Zhang, Y.S. Kang, Inorg. Chem. 45 (2006) 4186. [4] T.F. Chung, L.B. Luo, Y.H. He, I. Leung, Appl. Phys. Lett. 91 (2007) 233112. [5] H.J. Choi, H.K. Seong, J.Y. Chang, K. Lee, Y.J. Park, J.J. Kim, S.K. lee, R.R. He, T. Kuykendall, Adv. Mater. 17 (2005) 14479. [6] B.B. Lia, X.Q. Xiua, R. Zhanga, Z.K. Taoa, L. Chena, Z.L. Xie, Y.D. Zheng, Mater. Sci. Semicond. Proc. 9 (2006) 141. [7] G. Sanon, R. Rup, A. Mansingh, A. Phys. Rev. B 44 (1991) 5672. [8] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39 (2000) L555. [9] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 40 (2001) L334. [10] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [11] Y. Wu, R. Fan, P. Yang, Nano Lett. 2 (2002) 83. [12] M.S. Gudiksen, L.J. Lauhon, J. Wang, D.C. Smith, C.M. Lieber, Nature 415 (2002) 617. [13] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Webber, R. Russo, P. Yang, Science 292 (2001) 1897. [14] M.T. Bjork, B.J. Ohlsson, C. Thelander, A.I. Perssom, K. Deppert, L.R. Wellenberg, L. Samuelson, Appl. Phys. Lett. 81 (2002) 4458. [15] J.J. Wu, S.C. Liu, J. Phys. Chem. B 106 (2002) 9546. [16] A. Ishizumi, Y. Kanemitsu, Appl. Phys. Lett. 86 (2005) 253106. [17] L.W. Yang, W.L. Wu, T. Qiu, G.G. Siu, P.K. Chu, J. Appl. Phys. 99 (2006) 074303. [18] G.R. Li, D.L. Qu, W.X. Zhao, Y.X. Tong, Electrochem. Commun. 9 (2007) 1661. [19] E. Oh, S.H. Jung, K.H. lee, S.H. Jeong, Mater. Lett. (2008). [20] C.Y. Geng, Y. Jiang, Y. Yao, X.M. Meng, J.A. Zapien, C.S. Lee, Y. Lifshitz, S.T. Lee, Adv. Funct. Mater. 14 (2004) 6. [21] Y. Liu, Y. Chu, L.L. Li, L.H. Dong, Y.J. Zhuo, Chem. Eur. J. 13 (2007) 6667. [22] P.X. Gao, J.H. Song, J. Liu, Z.L. Wang, Adv. Mater. 19 (2007) 67. [23] T.F. Chung, J.A. Zapien, S.T. Lee, J. Phys. Chem. C 112 (2008) 820. [24] J.H. He, C.S. Lao, L.J. Chen, D. Davidovic, Z.L. Wang, J. Am. Chem. Soc. 47 (2005) 16377. [25] Y.J. Tak, K.J. Yong, J. Phys. Chem. C 112 (2008) 74. [26] J.B. Cui, U.J. Gibson, Appl. Phys. Lett. 87 (2005) 133108. [27] P.V. Radovanovic, D.R. Gamelin, J. Am. Chem. Soc. 123 (2001) 12207. [28] F.V. Mikulec, M. Kuno, M. Bennati, D.A. Hall, R.G. Griffin, M.G. Bawendi, J. Am. Chem. Soc. 122 (2000) 2532.
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
[29] D.A. Schwartz, N.S. Norberg, Q.P. Nguyen, J.M. Parker, D.R. Gamelin, J. Am. Chem. Soc. 125 (2003) 13205. [30] S.J. Pearton, W.H. Heo, M. Ivill, D.P. Norton, T. Steiner, Semicond. Sci. Technol. 19 (2004) R59. [31] S.A. Chambers, T.C. Pronbay, C.M. Wang, K.M. Rossa, S.M. Heald, D.A. Schwartz, K.R. Kittilstved, D.R. Gamelin, Mater. Today 9 (2006) 18. [32] A. Ohtomo, et al., Appl. Phys. Lett. 72 (1998) 2466. [33] R. Viswanatha, S. Sapra, S.S. Gupta, B. Satpati, J. Phys. Chem. B 108 (2004) 6303. [34] J. Jie, G. Wang, X. Han, Q. Yu, Y. Liao, G. Li, J. Hou, Chem. Phys. Lett. 387 (2004) 466. [35] S.M. Zhou, X.H. Zhang, X. Meng, K. Zou, X. Fan, S. Wu, S.T. Lee, Nanotechnology 15 (2004) 1152. [36] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Phan, R. He, H. Choi, Adv. Funct. Mater. 12 (2002) 323.
385
[37] V. Ischenko, S. Polarz, D. Stavarache, K. Fink, M. Driess, Adv. Funct. Mater. 15 (2005) 1945. [38] C.G. Kim, K. Sung, T.M. Chung, D.Y. Jung, Y. Kim, Chem. Commun. (2003) 2068. [39] T. Busgen, M. Hilendorff, S. Irsen, F. Wilhelm, A. Rogalev, J. Phys. Chem. C 112 (2008) 2412. [40] N.S. Norberg, K.R. Kittilstved, J.E. Amonette, R.K. Kukkadapu, D.A. Schwartz, J. Am. Chem. Soc. 125 (2004) 9387. [41] D.A. Schwartz, N.S. Norberg, Q.P. Nguyen, J.M. Parker, D.R. Gaelin, J. Am. Chem. Soc. 125 (2003) 13205. [42] J.H. Park, M.G. Jang, S. Ryu, Appl. Phys. Lett. 84 (2004) 1338. [43] D.A. Schwartz, K.R. Kittilstved, D.R. Gamelin, Appl. Phys. Lett. 85 (2004) 1395. [44] K.R. Kittilstved, W.K. Liu, D.R. Gamelin, Nat. Mater. 5 (2006) 291. [45] C. Cheng, G.Y. Xu, H.Q. Zhang, Y. Luo, Mater. Lett. 62 (2008) 1617.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation and properties of Ni-doped ZnO rod arrays from aqueous solution Dianwu Wu a , Mei Yang a , Zhongbing Huang a,∗ , Guangfu Yin a , Xiaoming Liao a , Yunqing Kang a , Xianfu Chen a , Hui Wang b a b
College of Materials Sciences and Engineering, Sichuan University, No. 24, South 1st Section, 1st Ring Road, Chengdu, Sichuan 610065, PR China Analytical and Testing Center, Sichuan University, Chengdu 610064, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 17 July 2008 Accepted 25 October 2008 Available online 30 October 2008
Ni-doped ZnO rod arrays were successfully prepared on glass substrate from the aqueous solution at a temperature of 80 ◦ C. The densities, diameters, and lengths of the rods can easily be well controlled through the concentrations of dopants, and the dopant Ni ions were incorporated into the wurtzitestructure of ZnO crystal. Room temperature photoluminescence spectrum of rod arrays show a strong emission band at 410 nm, and the oxygen deficiencies in ZnO structures were significantly reduced with Ni doped in ZnO rod arrays. The field dependence of magnetization measured at room temperature exhibited the obvious ferromagnetic properties. © 2008 Elsevier Inc. All rights reserved.
Keywords: ZnO rod arrays Nickel Doping Ferromagnetism
1. Introduction Owing to a direct wide band gap (3.37 eV), large exciton binding energy (60 meV), and superior conducting properties based on oxygen vacancies, excellent electro and optical properties [1–3], the wurtzite-structured ZnO has become one of the most promising materials for the fabrication of high-technology applications such as photonic crystals, light-emitting diodes, sensors, electroand photoluminescent materials [4,5]. The key requirements for these applications are that ZnO materials have the good electrical, optical, and magnetic properties and that the control of the shape and crystal structure is important. It is well known that the addition of impurities into a wide gap semiconductor can often induce dramatic changes in the optical, electrical, and magnetic properties [6,7]. Therefore, doping a selective element into ZnO has become an important route for enhancing and controlling its optical, electrical, and magnetic performance, which is crucial for their practical applications. Theoretical calculations have showed that transition-metal (TM)-doped ZnO would be a good candidate to achieve Curie temperature above room temperature [8–10]. Since then, great efforts have been devoted to the investigation of magnetic-metal/ZnO one-dimensional (1D) structure materials hetero-structures due to its new functionality in memory devices, detectors and light-emitting sources [11,12]. Of the many magnetic-metal, Ni is an important dopant to achieve Curie tem-
*
Corresponding author. Fax: +86 28 85413003. E-mail address: [email protected] (Z. Huang).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.10.067
© 2008
Elsevier Inc. All rights reserved.
perature above room temperature. Recently, 1D nanostructures of ZnO, such as nanowires, nanorods, nanobelts, and nanotubes, have been reported [13,14], including epitaxial and nonepitaxial structures. Up to now, conventional routes, such as ultrasonic irradiation [15], thermal chemical vapor deposition [16], ion implantation [17], electrochemical deposition [18], and hydrothermal method [19], have been developed to fabricate transition-metal doped 1D ZnO structured materials. There are also a number of reports about the preparation of ZnO nanoarrays on different substrates, such as silicon substrate [20], zinc substrate [21], plastic substrate [22], aluminum substrate [23], and different substrates can gain ZnO of different morphology and different properties. However, only a few papers reported the preparation of doped ZnO rod arrays [18, 24–26]. Nevertheless, these methods usually required special experimental conditions such as electric fields, ultrasound treatment, long reaction time, high temperature or high pressure. On the other hand, a major practical challenge is the formation of good Ni2+ –ZnO junctions. Many previous studies have demonstrated the tendency for dopant ions to be excluded during crystal synthesis [27–29]. In this paper, we successfully synthesized Ni-doped ZnO rod arrays on ZnO seeds substrate using a simple aqueous solution method with controlling Ni contents. This method may be used for preparation of many new device materials in microscale electronics and photonics such as novel memory and optical device, and it will be applied in biological detecting and treatment. ZnO rod arrays with various microstructures parameters can be controlled by changing the concentrations of the dopants.
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
2. Materials and method 2.1. Materials The zinc precursor, zinc nitrate (Zn(NO3 )2 ·6H2 O), hexamethylenetetramine (C6 H12 N4 , HMT) and nickel acetate (Ni(CH3 COO)2 · 4H2 O) was used for the preparation of ZnO rod arrays. 2.2. Preparation of Ni-doped ZnO rod arrays The experiment was conducted in the following procedure. Firstly, ZnO crystal seeds particles were coated on the substrate. The glass substrate was cleaned ultrasonically in the mixed solution (the volume ratio of 98% H2 SO4 :30% H2 O2 is 3:1) and deionized water for 10 min, respectively, and the well cleaned substrates were immersed into the aqueous solution of 0.025 mol/l C6 H12 N4 (HMT) and 0.025 mol/l Zn(NO3 )2 ·6H2 O for 5 min at room temperature. Then, the substrates were dried and subsequently sintered at 500 ◦ C for 10 min in a muffle furnace. The coating process above was repeated 6 times in order to form uniform coverage of ZnO seeds on the entire substrate. Secondly, Ni-doped ZnO rod arrays were grown vertically from the nanocrystal seeds by immersing seed-coated substrate in the aqueous solution of equimolar of C6 H12 N4 (HMT), Zn(NO3 )2 ·6H2 O and different concentrations of Ni(CH3 COO)2 ·4H2 O. The concentration of the dopant in reactive solution can be tuned according to Ni/Zn atomic ratio of 0, 0.1, 0.2, 0.3, and 0.4, and the as-prepared samples in subsequent discussions of this paper are designated as C0, C1, C2, C3 and C4, respectively. The growth time and temperature were 12 h and 80 ◦ C, respectively. Finally, the as-prepared samples were took out the solution, rinsed thoroughly with deionized water for 5 times to eliminate residual salts, and dried for 24 h at room temperature. 2.3. Characterization The crystal structures of the as-prepared samples were analyzed using X-ray diffractometer (XRD, X’Pert, Holand) with CuK α radiation. The morphology of Ni-doped ZnO rod arrays was characterized by scanning electron microscopy (SEM, JSM-5900LV, Japan). The analysis of elements was conducted the energy-dispersive X-ray spectroscopy (EDX) attached to the scanning electron microscopy (HITACHI-4300S). The UV–visible absorption was measured on Hitachi U-3010 spectrophotometer. The photoluminescence (PL) measurements were performed on a Hitachi F-7000 Fluorometry with a Xe lamp as the excitation light source at room temperature. The excitation wavelength was 330 nm. The ferromagnetic behaviors of the Ni-doped ZnO rod arrays at room temperature were investigated by Vibrating Sample Magnetometer (VSM, Lake shore-7400, USA). 3. Results and discussions 3.1. Morphological and structural characterization Fig. 1 shows SEM images of undoped and Ni-doped ZnO rod arrays grown on substrates. In SEM images, hexagonal ZnO rod arrays were successfully prepared, revealing that Ni-doped ZnO rod arrays keep their vertical alignment, and they are different in the rods diameters and array densities. As-grown Ni-doped ZnO rods have the average diameter of about 250, 200, 250, 300, and 400 nm for C0, C1, C2, C3, and C4, respectively. The heights of these arrays can be clearly measured from Fig. 2, and they are about 0.8, 1.0, 1.5 and 1.2 um for C1, C2, C3 and C4, respectively. From Fig. 2, ZnO crystal seeds particles (the diameter is not more than 100 nm)
381
can also be found on the substrates. It is obvious that the orientation of the rod arrays prepared with the concentrations of dopant of 0.0025 mol/l in Fig. 1b is not completely uniform. However, when the concentration of dopant was increased to 0.005 mol/l in aqueous solution, the orientation of rod arrays (Fig. 1c) became more regular than that shown in Fig. 1b. The concentration of dopant was increased to 0.0075 mol/l, and the diameter of the rods as shown in Fig. 1d became larger but the quantity of the rods became fewer. When the concentration of dopant was up to 0.01 mol/l, the nucleation quantity was remarkably lower, and there were some silk-like impurities (marked in Fig. 1e with the arrow 1) and some rod defects (marked in Fig. 1e with the arrow 2) observed in Fig. 1e. There was a mass of aqua deposition in the bottom of the reactive bottle with the beginning of C3, which maybe considered as NiO particles. From Fig. 1, we also found that one large ZnO rod could be composed by several small ZnO rods clinging to each other (such as the arrows marked in Figs. 1c and 1d). It is well known from the synthesis of particles that the dopant atoms have a strong influence on the amount and size of the resulting rods [30,31]. It was a kinetic equilibrium process of thermodynamic equilibrium and dynamic equilibrium. The incorporation of dopant into the host lattice is hindered through an increase of surface energy and lattice distortion. In particular, the thermodynamically unfavored, purely kinetically driven growth of one dimensional structure is often restrained. The composition of C3 is determined by the EDX, and the result shows the existence of Ni, Zn and O in the rods, suggesting that in this area (the inset of Fig. 3) Ni has entered into the ZnO lattice. Moreover, the EDX taken at a rectangle location throughout the specimens reveal a solid solution of Ni dissolved in ZnO crystal. There are 0.98% atomic ratios of nickel ions doped into the ZnO rods (C3), corresponding with the nominal nickel acetate of 0.0075 M in the reactant. It is obvious that the amount of Ni incorporated into the ZnO matrix is much less than the actual amount of Ni provided in the precursor during the synthesis. This maybe result from the formation of a mass of fine NiO clusters in reactive solution, which precipitated out on the bottom of the container and were discarded in the purification steps. Most importantly, the incorporation of nickel ions into the ZnO host lattice is hindered during the synthesis of Ni-doped ZnO rod arrays. In order to more confirm that Ni was incorporated into the crystal structure, XRD was performed. Fig. 4 displays X-ray diffraction patterns of the as-prepared undoped and doped ZnO samples. The XRD spectra of Ni-doped ZnO rod arrays consist of (002), (100), (101), (102), and (103) peaks, and all the observed diffraction peaks can be indexed to ZnO wurtzite structure. The strong (002) peak proves that ZnO rods with wurtzite structure were obtained in both undoped and Ni-doped ZnO samples. No diffraction peaks of other structures were detected in these samples, indicating that the Ni ion successfully occupied ZnO lattice site and there were no secondary phases or precipitates in the samples. The prepared undoped and Ni-doped ZnO were demonstrated to be preferential orientation along the (002) peak direction. The (002) diffraction peaks of C2 and C3 were obviously higher than that of C0. However, C4 are has the same height of (002) diffraction peak as C0, which might be correlated to the actual quantity of Ni2+ doped into the ZnO crystal lattice. Although C4 has a higher dopant concentration in reactive solution, the actual concentration of Ni2+ doped into the ZnO crystal lattice is lower. Due to the higher bond energy of Ni2+ –O2− compared to that of Zn2+ –O2− , the more energy is required to make Ni2+ ions enter into lattice and form the bond of Ni2+ –O2− . Therefore, Ni2+ substituting for Zn2+ has higher stability relative to the Zn–O structure, and more energy is required for the substituted samples to complete crystallization. In addition, because of the larger radius of Zn2+ (0.74 Å) [32] compared to Ni2+ (0.69 Å), the replacement of Zn2+ by Ni2+ will
382
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
Fig. 1. SEM images of ZnO rod arrays grown with different dopants concentration in reactive solutions. (a) 0 M. (b) 0.0025 M. (c) 0.005 M. (d) 0.0075 M. (e) 0.01 M.
cause only slight decrease in lattice parameters. Although doping does not alter the crystal structure, it causes the lattice constant to change as evidence of the (002) peak position shift. The c-axis constant d(002) of the Ni-doped ZnO rod arrays are decreased by 0.68% for C1 relative to undoped sample, and increased by 0.57, 0.94 and 0.23% for C2, C3, and C4, respectively, suggesting that doping Ni ions will slightly change the c-axis constant. Although the change is very little, the concentration of dopant plays a role in the c-axis constant. In addition, the small quantities of silk-like impurities in Fig. 1d may be slightly existed (less than 1%) and probably not induce a detectable peak. 3.2. Optical properties Fig. 5 shows the absorption of corresponding UV–visible spectra of undoped and Ni-doped ZnO rod arrays. In order to obtain a precise measurement of the shifts in the wavelength of the spectra, we use the point of inflection obtained from the derivative curve of the absorption spectra (marked in Fig. 5 with the arrows). The spectra become broader with blue-shifted of 2.88, 3.07, 7.05,
and 1.91 nm for C1, C2, C3, and C4 compared to C0, respectively, suggesting an indirect evidence for the decrease of band gap (E g ) and the energy broadening of valence band states attributable to the doping [33]. When the doped Ni2+ enters into the ZnO crystal structure, the localized band edge states form at the doped sites with a reduction of E g due to the crystal lattice confinement effect. The PL spectra (Fig. 6) consist of two UV emission peaks which centered at 398 nm and 410 nm, respectively, a distinct shoulder peak centered at 465 nm, and a weak green emission band centered at 530 nm. With nickle doped into the ZnO crystal lattice, the intensity of the peaks centered at 410 nm gradually increased. And the purple luminescence band (398 nm) belongs to the exciton recombination corresponding to near-band-edge emission of ZnO rod arrays. The strong peak centered at 410 nm most likely originates from the coordinative, unsaturated Zn sites in the ZnO rod arrays or the formation of band tailing in the band gap, which is often induced by the introduction of impurity into the semiconductor [34–36]. Different shallows levels are formed in the band gap, due to the presence of interstitial zinc atoms, which give
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
383
Fig. 4. XRD patterns of the prepared undoped and Ni-doped ZnO rod arrays.
Fig. 2. SEM images of the side views of doped ZnO rod arrays grown with different dopants concentration in reactive solutions. (a) 0.0025 M. (b) 0.005 M. (c) 0.0075 M. (d) 0.01 M.
Fig. 5. UV–visible absorption spectra of undoped and Ni-doped ZnO nanorod arrays measured at room temperature.
Fig. 3. EDX spectra of Ni-doped ZnO rods shown in Fig. 1c. The inset is the area where EDX was operated.
rise to red-shifted peaks [37]. With the increase of Ni concentration, the peaks of UV emission are getting strong. The result is well agreed with the UV–visible spectra, showing that the Nidoped ZnO rod arrays have a high optical property. However, the
visible luminescences from oxygen ion defect states were usually observed at about 530 nm, presenting an interesting result. Compared to the undoped sample, the green emission peak intensity of doped ZnO rod arrays reduced obviously, and with the increase of the concentrations of dopant, the intensity of green emission peaks became weaker, and then turned stronger. This might be ascribed to the oxygen vacancy and the transition of a photogenerated electron from a dark level below the conduction band to a deeply trapped hole [38]. There are several reasons: because of good crystallinity, there are only a few defect sites in the wurtzite lattice, which would act as electron traps shifting the photoluminescence into the visible range. Besides, the dopant atoms also play a role in the photoluminescence behavior of the ZnO rods, since Ni2+ acts as deep traps in materials, which open up non-radiative relaxation pathways [39–41]. According to the SEM images, C2 has the perfect crystallinity and compact, orderly arrays. The morphology of C1 sample is also orderly but not enough dense. There was a mass of deposition at the bottom of the bottle with the beginning of C3,
384
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
the formation of some Ni-related secondary phase, such as NiO phase. However, NiO phase can be easily ruled out, since bulk NiO is antiferromagnetism with a Neel temperature of 520 K [43], and XRD result clearly proves no NiO crystal structure in the doped arrays. The origin of ferromagnetizatic behavior of Ni-doped ZnO arrays could be considered as a result of the exchange interaction between free delocalized carriers (hole or electron from the valence band) and the localized d-spins on the Ni ions [44,45]. 4. Conclusions In summary, well aligned Ni-doped ZnO rod arrays have been successfully synthesized by a simple aqueous solution route. The as-prepared products are single-crystalline wurtzite structure. Photoluminescence spectra measurement demonstrated that Ni-doped ZnO rod arrays exhibit a near band edge UV emission peak centered at about 410 nm, and a green emission peak centered at about 540 nm. The M–H curve of Ni-doped ZnO rod arrays measured by VSM at room temperature indicated that they have obvious ferromagnetic characteristic. Fig. 6. PL spectrum of different Ni-doped samples measured at room temperature. The excitation wavelength of PL is the 310 nm.
Acknowledgments This work has been supported by the National Natural Science Foundation of China (project No. 60871062). The supports of Sichuan Province through a Science Fund for Distinguished Young Scholars of Sichuan Province (08ZQ026-007) and Key Technologies Research and Development Program of Sichuan Province (2008SZ0021 and 2006Z08-001-1) are also acknowledged with gratitude. This work was also supported by the Research Fund for the Doctoral Program of Higher Education from Ministry of Education of China (No. 20070610131). We thank Analytical & Testing Center, Sichuan University for the assistance with the microscopy work. References
Fig. 7. Magnetic hysteresis loops of Ni-doped ZnO rod arrays at room temperature, the inset (a) shows its enlargement view at the origin, and the inset (b) shows the magnetization curve of zero field.
Ni2+ could not thoroughly enter into the ZnO crystal lattice, and C4 is uneven and has some impurity at the surface of ZnO rods. 3.3. Magnetic properties Fig. 7 presents the magnetization (M) vs magnetic field (H) of Ni-doped ZnO rod arrays. Hysteresis curves are obtained at room temperature, showing all Ni-doped samples have obvious ferromagnetic characteristic. Their coercivities (H c ) are 134, 71, 48, and 123 Oe for C1, C2, C3, and C4, respectively (shown in the inset of Fig. 7). The saturation magnetizations (M S ) of Ni-doped ZnO arrays are up to 0.28, 0.49, 0.99, and 0.09 emu/g, respectively. From C1 to C3, ferromagnetism is enhanced significantly due to the higher Ni2+ concentrations. However, when the Ni2+ concentration in reactive solution is up to 0.01 M (C4), ferromagnetism is decreased rapidly. This is because of only a few Ni2+ doped into the ZnO lattice, and the results are consistent with the conclusion from UV–visible absorption spectra. The saturation magnetizations we measured are larger than the previous reports on Ni-doped ZnO nanoparticles [6], this might be due to the more Ni2+ doped into the ZnO lattice. There are a few of controversial explanations as to whether the ferromagnetism arises from the homogeneous magnetic doping or from magnetic precipitation [42], one of which is
[1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russor, P.D. Yang, Science 292 (2001) 1897. [2] Z.L. Wang, J.H. Song, Science 312 (2006) 242. [3] X.L. Zhang, Y.S. Kang, Inorg. Chem. 45 (2006) 4186. [4] T.F. Chung, L.B. Luo, Y.H. He, I. Leung, Appl. Phys. Lett. 91 (2007) 233112. [5] H.J. Choi, H.K. Seong, J.Y. Chang, K. Lee, Y.J. Park, J.J. Kim, S.K. lee, R.R. He, T. Kuykendall, Adv. Mater. 17 (2005) 14479. [6] B.B. Lia, X.Q. Xiua, R. Zhanga, Z.K. Taoa, L. Chena, Z.L. Xie, Y.D. Zheng, Mater. Sci. Semicond. Proc. 9 (2006) 141. [7] G. Sanon, R. Rup, A. Mansingh, A. Phys. Rev. B 44 (1991) 5672. [8] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39 (2000) L555. [9] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 40 (2001) L334. [10] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [11] Y. Wu, R. Fan, P. Yang, Nano Lett. 2 (2002) 83. [12] M.S. Gudiksen, L.J. Lauhon, J. Wang, D.C. Smith, C.M. Lieber, Nature 415 (2002) 617. [13] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Webber, R. Russo, P. Yang, Science 292 (2001) 1897. [14] M.T. Bjork, B.J. Ohlsson, C. Thelander, A.I. Perssom, K. Deppert, L.R. Wellenberg, L. Samuelson, Appl. Phys. Lett. 81 (2002) 4458. [15] J.J. Wu, S.C. Liu, J. Phys. Chem. B 106 (2002) 9546. [16] A. Ishizumi, Y. Kanemitsu, Appl. Phys. Lett. 86 (2005) 253106. [17] L.W. Yang, W.L. Wu, T. Qiu, G.G. Siu, P.K. Chu, J. Appl. Phys. 99 (2006) 074303. [18] G.R. Li, D.L. Qu, W.X. Zhao, Y.X. Tong, Electrochem. Commun. 9 (2007) 1661. [19] E. Oh, S.H. Jung, K.H. lee, S.H. Jeong, Mater. Lett. (2008). [20] C.Y. Geng, Y. Jiang, Y. Yao, X.M. Meng, J.A. Zapien, C.S. Lee, Y. Lifshitz, S.T. Lee, Adv. Funct. Mater. 14 (2004) 6. [21] Y. Liu, Y. Chu, L.L. Li, L.H. Dong, Y.J. Zhuo, Chem. Eur. J. 13 (2007) 6667. [22] P.X. Gao, J.H. Song, J. Liu, Z.L. Wang, Adv. Mater. 19 (2007) 67. [23] T.F. Chung, J.A. Zapien, S.T. Lee, J. Phys. Chem. C 112 (2008) 820. [24] J.H. He, C.S. Lao, L.J. Chen, D. Davidovic, Z.L. Wang, J. Am. Chem. Soc. 47 (2005) 16377. [25] Y.J. Tak, K.J. Yong, J. Phys. Chem. C 112 (2008) 74. [26] J.B. Cui, U.J. Gibson, Appl. Phys. Lett. 87 (2005) 133108. [27] P.V. Radovanovic, D.R. Gamelin, J. Am. Chem. Soc. 123 (2001) 12207. [28] F.V. Mikulec, M. Kuno, M. Bennati, D.A. Hall, R.G. Griffin, M.G. Bawendi, J. Am. Chem. Soc. 122 (2000) 2532.
D. Wu et al. / Journal of Colloid and Interface Science 330 (2009) 380–385
[29] D.A. Schwartz, N.S. Norberg, Q.P. Nguyen, J.M. Parker, D.R. Gamelin, J. Am. Chem. Soc. 125 (2003) 13205. [30] S.J. Pearton, W.H. Heo, M. Ivill, D.P. Norton, T. Steiner, Semicond. Sci. Technol. 19 (2004) R59. [31] S.A. Chambers, T.C. Pronbay, C.M. Wang, K.M. Rossa, S.M. Heald, D.A. Schwartz, K.R. Kittilstved, D.R. Gamelin, Mater. Today 9 (2006) 18. [32] A. Ohtomo, et al., Appl. Phys. Lett. 72 (1998) 2466. [33] R. Viswanatha, S. Sapra, S.S. Gupta, B. Satpati, J. Phys. Chem. B 108 (2004) 6303. [34] J. Jie, G. Wang, X. Han, Q. Yu, Y. Liao, G. Li, J. Hou, Chem. Phys. Lett. 387 (2004) 466. [35] S.M. Zhou, X.H. Zhang, X. Meng, K. Zou, X. Fan, S. Wu, S.T. Lee, Nanotechnology 15 (2004) 1152. [36] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Phan, R. He, H. Choi, Adv. Funct. Mater. 12 (2002) 323.
385
[37] V. Ischenko, S. Polarz, D. Stavarache, K. Fink, M. Driess, Adv. Funct. Mater. 15 (2005) 1945. [38] C.G. Kim, K. Sung, T.M. Chung, D.Y. Jung, Y. Kim, Chem. Commun. (2003) 2068. [39] T. Busgen, M. Hilendorff, S. Irsen, F. Wilhelm, A. Rogalev, J. Phys. Chem. C 112 (2008) 2412. [40] N.S. Norberg, K.R. Kittilstved, J.E. Amonette, R.K. Kukkadapu, D.A. Schwartz, J. Am. Chem. Soc. 125 (2004) 9387. [41] D.A. Schwartz, N.S. Norberg, Q.P. Nguyen, J.M. Parker, D.R. Gaelin, J. Am. Chem. Soc. 125 (2003) 13205. [42] J.H. Park, M.G. Jang, S. Ryu, Appl. Phys. Lett. 84 (2004) 1338. [43] D.A. Schwartz, K.R. Kittilstved, D.R. Gamelin, Appl. Phys. Lett. 85 (2004) 1395. [44] K.R. Kittilstved, W.K. Liu, D.R. Gamelin, Nat. Mater. 5 (2006) 291. [45] C. Cheng, G.Y. Xu, H.Q. Zhang, Y. Luo, Mater. Lett. 62 (2008) 1617.