Yanmei Liu, Min Li, Qingqing Fang, Qingrong Lv, Mingzai Wu, and Shuai Cao

CHINESE JOURNAL OF PHYSICS VOL. 48, NO. 4 AUGUST 2010 Structural and Photoluminescence Properties of Polyethylene Glycol (PEG)-Assisted Growth Co-Doped ZnO Nanorod Arrays Compared with Pure ZnO Nanorod Arrays Yanmei Liu, Min Li, Qingqing Fang, Qingrong Lv, Mingzai Wu, and Shuai Cao School of Physics and Materials Science; Anhui Provincial Key Laboratory of Information Materials and Devices, Anhui University, Hefei 230039, Peoples Republic of China (Received December 11, 2009) Pure ZnO nanorod arrays and well-aligned ZnCoO nanorod arrays were fabricated on glass coated with ZnO films via the hydrothermal method. The structures and morphology of the ZnO arrays were studied by scanning electron microscope, X-ray diffraction, and photoluminescence spectroscopy. Photoluminescence spectra are composed of broad ultraviolet emission and slightly visible light for pure ZnO nanorods and ZnCoO nanorods. Polyethylene glycol (PEG) can adjust the photoluminescence intensity; adequate PEG contributes to good crystallinity of ZnCoO. Room-temperature Photoluminescence spectra of the ZnCoO nanorod arrays show that the ultraviolet emission blueshifts markedly, and the intensity of the visible light emissions increases compared to that of pure ZnO nanorod array. The origins of the light emissions have been discussed according to the relationship of the emission intensity and excitation power density of the emission peak, which indicates that the well-aligned ZnCoO nanorod arrays have better stability of exciton states than ZnO nanorod arrays. PACS numbers: 81.07. b, 78.55.Et, 61.05.cp I. INTRODUCTION One-dimensional (1D) nanostructured ZnO has attracted attention due to its unique properties and great potential applications in the nanodevice field [1 3]. It possesses a wide direct band gap of 3.37 ev at room temperature, high mechanical and thermal stabilities, and large free exciton binding energy (60 mev), which ensures an efficient emission up to room temperature and makes it suitable for short wavelength optoelectronic applications [4]. For pure ZnO, its properties are unstable and cannot meet the increasing needs for the applications. Therefore, it is crucial in improving the efficiency of quantum confinement structures, widening the useable wavelength range, and selectively tuning different radiative recombination processes for the optimum design and ultimate success in nanowire-based optical devices [5]. Doping of transition metals is an effective method for adjusting the energy levels and surface states of ZnO, which can further lead to making changes in the electrical, magnetic, and especially optical properties of 1D nanostructural ZnO. The functional design of nanomaterials in a highly oriented and ordered array is crucial for the next generation device design [6]. Cobalt is a typical element in the transition metals, which has an abundant structure of electron energy states. Co-doping is known as one of the most efficient methods used http://psroc.phys.ntu.edu.tw/cjp 523 c 2010 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA

524 STRUCTURAL AND PHOTOLUMINESCENCE PROPERTIES... VOL. 48 to improve the magnetic property of ZnO nanowires, which extends its applications as a diluted magnetic semiconductor (DMS) in the new emerging spintronic field [7 8]. Moreover, modifying the composition of the source materials can also drastically change the morphology and growth direction of the nanostructures, and thus the tuning of its rather unpleasantly colored emission and conductivity by modifying the energy band structure through doping would be worth exploring. The dopants, defects, and electronic states changes are expected to have an obvious influence on the optical properties of the 1D ZnO nanorods [9 10]. The effects of Co-doping on the optical performance of 1D ZnO nanorods have been reported in the literature [9]. However there exists an inconsistency about the change of the emission peak position in doped ZnO, redshift or blueshift. Also, a report of the role of PEG on the optical properties of ZnCoO nanorods, to our best knowledge, is still lacking. In this paper, the effects of Co dopant and PEG on the optical behavior of ZnO nanorod arrays have been studied. II. SYNTHESIS OF ZNO AND DOPED ZNO NANOROD ARRAYS For the synthesis of ZnCoO nanorod arrays, an appropriate quantity of ammonia (25) was added to a 0.1 M zinc nitrate and cobalt nitrate solution (9:1) to adjust the ph value to 10. The mixture was stirred vigorously for ten minutes to form a homogeneous solution. The resulting solution was then transferred into a Teflon-lined autoclave, which was sealed and maintained at 70 C for 10 h, then left to cool to room temperature naturally. A light green layer of precipitate was deposited on the glass covered with ZnO film which was made by spin coating [11]. After being washed with deionized water several times, the as-obtained sample was finally dried at 115 C for 2 h for further characterization; it was labeled as S1. The method of ZnO buffer layers has been reported [12 13]. For the synthesis of PEG-assisted ZnCoO nanorod arrays, first, 0.32 g PEG2000 was added to about 23 ml of 0.1M zinc nitrate and cobalt nitrate (9:1) mixture solution. The mixture was stirred vigorously for ten minutes to form a homogeneous solution. Then, appropriate ammonia was added to the homogeneous solution to adjust the ph value to 10, and then followed the procedures for the synthesis of ZnCoO nanorod arrays. 0.32 g PEG 2000 assisted growth nanorod arrays were labeled as S2. A 0.42 g PEG2000 assisted nanorod array, which was prepared just as S2 except for the amount of PEG2000, was labeled as S3. S4 was prepared just as S3 but with ph=9. For the synthesis of ZnO nanorod arrays, an appropriate quantity of ammonia (25) was added to 0.1 M zinc nitrate solution to adjust the PH value to 11. Then followed the procedure as in the synthesis of ZnCoO nanorods arrays, thus the white ZnO nanorod arrays were obtained. More ammonia is needed for ZnO nanorod arrays compared with ZnCoO nanorod arrays, which indicates that Co is advantageous to the growth of nanorods arrays. The ZnO nanorod array was labeled as S5. A scanning electron microscope (SEM Sirion200) was employed to examine the morphology of the ZnCoO nanorod arrays. The crystals structures of the samples were char-

VOL. 48 YANMEI LIU, MIN LI, QINGQING FANG, et al. 525 FIG. 1: The plane view SEM image of a PEG-assisted ZnCoO nanorod array (sample S3). acterized by X-ray diffraction (XRD MAP18AHF) using the Cu Ka line at the excitation voltage of 40 kv and a tube current of 100 ma. The photoluminescences (PL LABRA-HR) were measured under the excitation of a He-Cd laser (325 nm) at room temperature. The excitation power density was I 0 =41 kw/cm 2. III. RESULTS AND DISCUSSION Fig. 1 shows the plane view SEM image of S3, which indicates that the highly ordered ZnCoO nanorod array has a flat end and a narrow size distribution with average diameter of 120 nm. All diffraction peaks in Fig. 2(a) are consistent with that of wurtzite ZnO; they were indexed to the ZnO standard spectrum of JCPDS (36-1451), implying that hydrothermal doping and PEG do not change the crystal structure of ZnO too much. For all the samples from S1 to S5 grown on the ZnO films coated on the glass substrates, the (002) peak intensity is much lager than other peaks dominating the XRD spectra, which might originate from the fact that all these nanorods were grown vertically on the film coated glass substrate along the [0001] direction. No secondary phase is detected in the XRD patterns, which shows that Co is doped into the ZnO crystal lattice. The reason why the variation of the position of the (002) diffraction peak centered at 34.38 with Co concentration is negligible is that the effective ionic radius (0.058 nm) of Co 2+ in the tetrahedral configuration is close to that of Zn 2+ (0.060nm) [13] and Co 2+ substitutes into a Zn 2+ position incorporating into ZnO nanorods. The formation mechanism of ZnCoO nanorods has been reported [14 16]. X-ray Diffraction intensity is related to the crystal quality and radiation volume. The larger the nanorods length, the larger the volume of nanorod arrays radiated by X-rays. Under the similar growth condition, the diffraction intensity depends on the nanorods

526 STRUCTURAL AND PHOTOLUMINESCENCE PROPERTIES... VOL. 48 FIG. 2: XRD patterns of S1, S2, S3, S4, and S5: (a) 2θ/θ scan and (b) θ-rocking curves. length. In Fig. 2(a), S2 grown with a small amount of PEG has the strongest intensity relative to S1 without PEG and S3 with more PEG, which reveals that appropriate amounts of PEG can contribute to the growth of nanorods in the case of the same amount of ammonia. Comparing S3 with S4 of Fig. 2(a) shows that the increase of the amount of PEG can promote the growth of nanorod arrays while the amount of ammonia decreases, meaning that PEG may play the same role as ammonia in guiding the growth of ZnCoO nanorod arrays. Fig. 2(b) shows the rocking curves of S1, S2, S3, S4, and S5, from which it can be

VOL. 48 YANMEI LIU, MIN LI, QINGQING FANG, et al. 527 FIG. 3: PL spectra of S1, S2, S3, S4, and S5 at room temperature. Inset: enlarged visible light emission pattern. clearly seen that S4 has the least full width at half maximum (FWHM) of 6.8, indicating that an appropriate amount of PEG contributes to the nanorods aligning to a high degree and possessing better preferred orientation along the c-axis. PEG with a uniform and ordered chain structure is easily adsorbed at the surface of a metal oxide colloid, which has been investigated in previous reports [17]. When the surface of the colloid adsorbs this type of polymer, the activities of the colloid will greatly decrease. From the view of the kinetics of colloid growth, it can be inferred that ZnCoO crystal nuclei adsorb PEG polymer on its low energy surface such as (100), (010), etc., namely, the side face of the hexagonal column, confining this face s growth; thus leading to preferential growth in [001], parallel to the above side faces. But a too low concentration of PEG cannot confine much area of the nuclei or nanorods, while a too high concentration of PEG confines all aspects of them. Therefore, the optimal addition of PEG into the reactor will modify the anisotropic growth of nanorod arrays. In order to enhance the growth of nanorod arrays, the total quantity of ammonia and PEG must be appropriate and invariable, i.e., an increase of the amount of ammonia must decrease that of PEG, too much ammonia or PEG inhibits the growth of nanorods. In Fig. 3, the PL spectra consist of two main parts: one is the near band edge transition region (NBE) usually lying at the ultraviolet transition region (UV); the other

528 STRUCTURAL AND PHOTOLUMINESCENCE PROPERTIES... VOL. 48 FIG. 4: Excitation intensity dependence of PL spectra for the NBE emission: (a) pure ZnO nanorod array (S5), and (b) PEG-assisted ZnCoO nanorod array (S4). Inset: Logarithmic function of excitation power dependence of the integrated emission intensity: (a) S5 and (b) S4. is the visible light (VL) region related to the deep level defect transitions. The undoped sample (S5) exhibits a stronger UV band peaking at 395 nm and a weaker green emission band peaking at 530 550 nm, which should be attributed to its high purity and perfect crystallinity. The UV emission peaks of Co-doped ZnO nanorod arrays (S1, S2, S3, S4) exhibit obvious blueshifts of about 11 18 nm compared with that of S5. It is generally agreed that the blueshift of the UV band is closely connected with the widening of the band gap of ZnO resulting from Co doping [18]. The large blueshift and origin of the

VOL. 48 YANMEI LIU, MIN LI, QINGQING FANG, et al. 529 UV will be discussed later. S1, S2, S3, and S4 possess a higher intensity of VL emission with the same emission position compared with S5, which indicates that the Co-doping can increase the number of defects and oxygen vacancies in ZnO nanorods. The type of defects is proposed to be crystal lattice distortion due to the difference between the radius of the Co 2+ and Zn 2+ donors or acceptors introduced by the Co dopant. Compared with the PL of S1 and S3, sample S2 possesses the strongest emission intensity, implying that the addition of an appropriate amount of PEG during the ZnCoO nanorods growth can enhance their UV luminescence intensity. Similarly, a comparison of PL of S4 with that of S3 shows that the ammonia is favorable to the enhancement of UV luminescence while keeping the dosage of PEG unchanged. In order to illuminate the large blueshift of UV of the ZnCoO arrays compared to that of the ZnO array and its origin, the excitation power-dependent PL was measured at room temperature, as shown in the Fig. 4. The excitation power density ranges from about 4.1 to 41 kw/cm 2. As can be seen from the PL spectra in Fig. 4(a), the UV emission peak position of S5 is variable and shifts from 377 nm to 395 nm with increasing excitation density from 4.1 to 41 kw/cm 2, and its integrated intensity increases first then decreases with the excitation power density, as shown in the inset of figure 4(a). For the sample S4, the UV emission peak shows less redshift from 373 nm to 384 nm and a continual increase of the intensity with the increase of excitation density, as indicated in Fig. 4(b). For the NBE emission of semiconductors, the decay of free and bound excitons is the main radiative recombination channel at low excitation density. In the intermediate excitation density, excitonic molecules (biexcitons) and exciton-exciton inelastic scattering are predominant. At high excitation density, the radiative decay of free carriers in a dense electron-hole plasma (EHP) emerges due to exciton ionization from the screen of the Coulomb interaction [19]. For NBE emission, either the exciton or band-to-band (free-electron to free-hole) transition is expected to be the dominant optical transition, depending on their radiative recombination rates and the exciton binding energy. On the basis of the radiative recombination rate model, the luminescence intensity I PL can be expressed as [20, 21] I PL = ηi a laser. (1) In this relation, I laser is the power of the exciting laser radiation, the exponent a represents the radiative recombination mechanism, and the coefficient is a comprehensive characterization of the PL efficiency. The model predicts that for free carriers to band and donor to acceptor pair recombination a < 1, for free-exciton recombination a 1, for bandage recombination, i.e., free carrier electron-hole bimolecular recombination a 2, and for the intermediate case involving free excitons and free carriers 1 < a < 2. The exponents a of undoped and doped samples have been obtained according to Figs. 4 and formula (1). For the undoped sample (S5), the value of a is 1.4 when the excitation density increases from 0.1I 0 to 0.5I 0, indicating that the UV emission originates from the free excitons and free carriers. When the excitation power continues to increase, the UV peak intensity decreases due to high energy destroying the exciton and making the saturation concentration of excitons decrease. The first line exponent a of the doped sample (S4) is 1.1. Thus, the above

530 STRUCTURAL AND PHOTOLUMINESCENCE PROPERTIES... VOL. 48 analysis indicates that the large shift of the UV peak position for the ZnCoO nanorods compared to the ZnO nanorods in Fig. 3 originates from Co dopant inhibiting exciton ionization and enhancing the saturation concentration of excitons. The coefficient of the doped sample is about 0.5 times that of the undoped sample, implying that Co-doping induces a lower PL efficiency which can also be known from the PL intensity. In the energy band structure of the semiconductor, there is a located tail states band. The width of the located tail band is related to the number of defects of the crystal. The fewer defects there are, the narrower is the width of the located tail band. In contrast, the more defects, the wider the located tail band. As mentioned above, when the excitation power continues to increase, materials will be destroyed because of ionization, so the defects will increase, the located tail band will become wide. Due to the widened localized tail band and renormalization of the band gap, the band gap of the ZnO nanorod array becomes narrower, leading to a larger redshift of the UV emission peak position compared to that of the ZnCoO nanorod arrays. IV. CONCLUSIONS In this paper, we have reported on a novel wet chemical method for the simultaneous synthesis and film formation of ZnO and Co-doping ZnO nanorods in an aqueous medium with c axis preferred oriented growth of nanorods with good crystallinity. The Zn 2+ /OH /PEG ratio in the solution has been optimized to obtain a uniform and oriented growth of nanorods over a large substrate area. Their optical properties have been studied at room temperature, showing that the oxygen or defects related VL emission intensity of the ZnO nanorods increases with Co doping, while the UV intensity decreases. The PEG assisted growth can enhance the alignment and UV intensity of the ZnCoO nanorods. The excitation power dependent PL spectra are used to probe the origin of PL in the nanostructures. With increasing excitation power, the localized excitons are ionized into free carriers, resulting in band gap renormalization but not for the Co doped ZnO nanorods. So the radiative recombination processes of ZnO nanorods can be selectively modulated by using proper dopants and surfactants such as PEG and different excitation power densities, and thus, it makes them attractive candidates for nanophotonic device applications. Acknowledgement The work was supported by the Anhui Provincial Key Laboratory of Information materials and devices, the Anhui Provincial Natural Science Fund (090414177), the Fund for Young Teachers (05010210), and the second of Fifteen Fund for Middle-aged and young outstanding teachers in the province of colleges and universities 211(03).

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