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Influences of ZnO sol-gel thin film characteristics on ZnO nanowire arrays

prepared at low temperature using all solution-based processing

Jing-Shun Huang

Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 10617 Taiwan, Republic of China

Ching-Fuh Lina兲

Institute of Photonics and Optoelectronics, Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, 10617 Taiwan, Republic of China and Department

of Electrical Engineering, National Taiwan University, Taipei, 10617 Taiwan, Republic of China 共Received 10 September 2007; accepted 2 November 2007; published online 3 January 2008兲 Zinc oxide 共ZnO兲 nanowire arrays with controlled nanowire diameter, crystal orientation, and optical property were prepared on sol-gel ZnO-seed-coated substrates with different pretreatment conditions by a hydrothermal method. The vertical alignment, crystallinity, and defect density of ZnO nanowire arrays are found to be strongly dependent on the characteristics of the ZnO thin films. Field-emission scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction, and room temperature photoluminescence were applied to analyze the quality of the ZnO nanowire arrays. The annealing temperature of the ZnO thin film plays an important role on the microstructure of the ZnO grains and then the growth of the ZnO nanowire arrays. The x-ray diffraction results indicate that the thin film annealed at the low temperature of 130 ° C is amorphous, but the thereon nanowire arrays are high-quality single crystals growing along the c-axis direction with a high consistent orientation perpendicular to the substrates. The as-synthesized ZnO nanowire arrays via all solution-based processing enable the fabrication of next-generation nanodevices at low temperature. © 2008 American Institute of Physics.关DOI:10.1063/1.2828172兴

I. INTRODUCTION

One-dimensional共1D兲 nanowires have been extensively studied in recent years. Among these materials, zinc oxide 共ZnO兲 nanowires have attracted great interest for promising applications in optoelectronics devices such as room tem-perature lasers,1 light emitting diodes,2–5 ultraviolet 共UV兲 detectors,6 field-emission displays,7–9 photonic crystals,10 and solar cells.11–14ZnO is a wide band gap共3.37 eV兲 semi-conductor with a large exciton binding energy 共60 meV兲, exhibiting near-UV light emission, transparent conductivity, and piezoelectricity.15 Several methods have been demon-strated to fabricate 1D ZnO nanostructures, such as vapor-liquid-solid epitaxy 共VLSE兲, chemical vapor deposition 共CVD兲, and pulse laser deposition 共PLD兲, but these gas phase techniques still have some limitations for substrate size and the need for high temperature operation 关above 800 ° C for VLSE共Ref.16兲 and 500 °C for CVD method17兴.

Recently, the growth of ZnO nanowires and microrods in aqueous solutions at low temperature was reported by using the hydrothermal process.18 Hydrothermal process has shown the possibility for applications in light emitting diodes and solar cells with their growth temperature below 100 ° C and easy scale up. This aqueous-based technique has also been used successfully to demonstrate the fabrication of large arrays of vertical ZnO nanowires on glass, 4 in.

diam-eter Si wafers,19 and plastic substrates.7This stimulated the study of using ZnO nanowire arrays on plastic substrates for application in flexible electronic devices.

However, these device applications might be reinforced if the position, orientation, and shape of nanostructures can be controlled to a high degree of precision. The selective growth of ZnO nanostructures on desired areas of Si sub-strates by using lithography process has been reported.7,10,20. Bekeny et al.21 reported that the size of ZnO nanowires var-ied with the molar composition of the chemical precursors. Li et al.22reported that the growth of different morphologies of ZnO nanowires was dependent on substrate temperature in the PLD process. Guo et al.23reported that the diameter and length of ZnO nanowire arrays were controlled at different growth temperatures under hydrothermal conditions. Al-though Ma et al.24reported that an annealing treatment of the substrate can influence the density of the ZnO nanowire ar-rays on indium tin oxide共ITO兲, there is no study on crystal-linity and photoluminescence of ZnO nanowire arrays. Moreover, Kang et al.25 reported that the shape of the ZnO nanowires was sensitive to the orientation of Si substrate via the use of ZnO nanoparticles as a seed layer. However, sys-tematic research on the influence of quality characteristics of ZnO sol-gel thin films on the growth of ZnO nanowire arrays via hydrothermal method has rarely been reported.

In this work, we systematically study the feature-controlled ZnO nanowire arrays via the hydrothermal method and ZnO sol-gel thin films were used as the seed layers with different pretreatment conditions. Our investiga-tion shows that the vertical alignment, the crystallinity, and

a兲Author to whom correspondence should be addressed. Tel: 886-2-3366 3540. FAX: 886-2-2364 2603. Electronic mail: cflin@cc.ee.ntu.edu.tw.

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the defect density of ZnO nanowire arrays are strongly de-pendent on the characteristics of the thin films. Field-emission scanning electron microscopy 共FESEM兲, energy dispersive spectroscopy共EDS兲, x-ray diffraction 共XRD兲 pat-tern, and room temperature photoluminescence 共PL兲 spec-trum were applied to analyze the quality of so produced ZnO nanowire arrays.

II. EXPERIMENTAL DETAILS

The ZnO thin films served as the seed layers were deposited on silicon substrates by a sol-gel method.26 A coating solution contained zinc acetate dihydrate 关Zn共CH3COO兲22H2O, Merck, 99.5% purity兴 and equivalent

molar monoethanolamine共MEA兲 共NH2CH2CH2OH, Merck, 99.5% purity兲 dissolved in 2-methoxyethanol 共2MOE兲 共CH3OCH2CH2OH, Merck, 99.5% purity兲. The

concentra-tion of zinc acetate was chosen to be 0.5 mol. The resulting solution was then stirred at 60 ° C for 2 h to yield a homo-geneous and stable colloid solution, which served as the coating solution after being cooled to room temperature. Then the solution was coated onto p-type silicon共100兲 sub-strates by a spin coater at the rate of 1000 rpm for 20 s and then 3000 rpm for 30 s at room temperature. Subsequently, the gel films were preheated for 10 min to remove the re-sidual solvent. The procedures from coating to preheating were repeated ten times. Then the ten-layer films were an-nealed in a furnace at different temperatures ranging from 130 to 900 ° C for 1 h.

After uniformly coating the silicon substrates with ZnO thin films, hydrothermal growth of ZnO nanowire arrays was achieved by suspending these ZnO seed-coated substrates upside down in a glass beaker filled with aqueous solution of 50 mM zinc nitrate hexahydrate关Zn共NO3兲26H2O, Sigma

Al-drich, 98% purity兴 and 50 mM hexamethylenetetramine 共HMT兲 共C6H12N4, Sigma Aldrich, 99.5% purity兲. During the

growth, the glass beaker was heated with a laboratory oven and maintained at 90 ° C for 4 h. At the end of the growth period, the substrates were removed from the solution, then immediately rinsed with de-ionized water to remove any re-sidual salt from the surface, and dried in air at room tem-perature. The general morphologies of the ZnO thin films and thereon ZnO nanowire arrays were examined by FESEM. An EDS spectrum of the ZnO nanowires was mea-sured with the same SEM system. The crystal phase and crystallinity were analyzed at room temperature by XRD us-ing Cu K␣ radiation. The room temperature PL, measured using a Nd:yttrium aluminum garnet共YAG兲 laser at 266 nm as the exciting source, was used to characterize the optical properties of the ZnO thin films and thereon ZnO nanowire arrays.

III. RESULTS AND DISCUSSION

Figure 1 shows the top view FESEM images for the surface morphologies of ZnO sol-gel thin films and thereon ZnO nanowire arrays at different annealing temperatures of the thin films. Figures1共a兲,1共c兲,1共e兲, and1共g兲 indicate that the grains of thin films reveal a noticeable transformation with increasing annealing temperatures from 130 to 900 ° C.

At the annealing temperature of 130 ° C, no grain forms and the surface is smooth. At the annealing temperature of 300 ° C, the film contains fine grains and the particle size is about 80 nm. Once the annealing temperature increases, the grains become larger and densely packed. This result con-cerned with the annealing temperature is consistent with the result of the research of Wang et al.27It demonstrates that the quality of ZnO thin film was improved due to the redistribu-tion of crystalline grain by supplying sufficient thermal en-ergy and the small grain has been joined into great crystalline surface.

Figures1共b兲,1共d兲,1共f兲, and1共h兲show the ZnO nanowire arrays corresponding to Figs. 1共a兲, 1共c兲, 1共e兲, and 1共g兲, re-spectively. They were grown at a fixed temperature共90 °C兲, while the thin films were annealed at 130, 300, 600, and 900 ° C, respectively. They show that the obtained ZnO nanowire arrays are typically hexagonal-shaped. As the an-nealing temperatures of the ZnO thin films increase from 130 to 900 ° C, the diameters of the ZnO nanowire arrays increase from 60 to 260 nm. The reason may be that the high annealing temperature evidently enhances the interaction among the grains and leads the grains to merge together to form bigger ZnO seeds, and thus increases the diameter of the ZnO nanowires thereon. Therefore, the size of the grains is a key factor that influences the nucleation of ZnO nano-wire arrays. Furthermore, it is notable that the ZnO nanonano-wire arrays on the ZnO thin films annealed at 130 ° C are well-aligned vertically and uniformly 关Fig. 1共b兲兴, and the well-defined crystallographic planes of the hexagonal single-crystalline nanowires can be clearly identified, providing a strong evidence that the nanowire arrays orientate along the

c-axis.

Figure 2共a兲 gives the XRD patterns of those ZnO thin films annealed at 130, 300, 600 and 900 ° C, respectively. The XRD patterns reveal that the共002兲 peak intensity varies with annealing temperature. When the ZnO thin films are annealed below 300 ° C, there is no preferred 共002兲 c-axis orientation. At the temperature of 600 ° C, 共100兲, 共002兲, 共101兲, 共102兲, and 共110兲 diffraction peaks corresponding to the ZnO wurtzite structure are observed in the XRD pattern, where the preferred共002兲 c-axis orientation dominates. This reason may be that the use of 2MOE and MEA solvents of higher boiling points has resulted in a strongly preferred 共002兲 orientation.28

When the annealing temperature in-creases to 900 ° C, the polycrystalline structure emerges with the 共100兲 and 共101兲 peaks while the intensity of the 共002兲 peak decreases. This result indicates that the crystalline qual-ity of each grain becomes poor.29 Therefore, we may con-clude that the preferred c-axis orientation initially increases with annealing temperature until it reaches the optimal situ-ation at a certain annealing temperature. Afterwards the

c-axis orientation intensity decreases gradually.

Figure 2共b兲 shows the XRD patterns of the ZnO nano-wire arrays corresponding to those shown in Fig.2共a兲. Please note again that the nanowire arrays are grown at the same temperature of 90 ° C, while the thin films were annealed from 130 to 900 ° C. The peaks in the x-ray diffraction pat-terns are indexed to the hexagonal phase of ZnO. It is found that no other characteristic peaks corresponding to the

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impu-rities of the precursors such as zinc nitrate and zinc hydrox-ide are observed in the XRD patterns. At the temperatures of 130, 300, and 600 ° C, only a very strong 共002兲 diffraction peak and a very weak 共101兲 peak are observed, indicating that the three ZnO samples are all of high c-axis orientation. It is noticeable that for the sample annealed at 130 ° C the XRD pattern shows only the共002兲 diffraction peak. In addi-tion, the intensity of共002兲 diffraction peak is strongest, com-pared to other samples annealed at higher temperatures. This implies its perfect c-axis orientation and this result is in ac-cordance with its SEM image关Fig.1共b兲兴. On the other hand, for the sample annealed at 900 ° C, the共002兲 diffraction peak becomes weak, and at the same time, the 共100兲 and 共101兲 peaks become strong, indicating its tendency toward random orientation. It means that the films annealed at 900 ° C has

worse morphology 关Fig.1共g兲兴 and hence results in the

rela-tively random orientation of nanowire arrays as observed in the Fig.1共h兲.

It is interesting to note that the共002兲 diffraction peak of the seed layer annealed at 130 ° C is smaller than that at 600 ° C, while the 共002兲 diffraction peak of thereon ZnO nanowire arrays at 130 ° C is larger than that at 600 ° C. The previous investigation of the thin films annealed at 130 ° C indicates that it is nearly amorphous. However, the growth of ZnO nanowire arrays on amorphous ZnO thin films along the 共002兲 plane is even more notable than that on polycrystalline thin films. It may be because the polycrystalline ZnO grains with a certain orientation limit the growth along the 共002兲 plane. In comparison, the amorphous ZnO seed layer does not limit the growth along the共002兲 plane. This indicates that the ZnO nanowire arrays prepared by the hydrothermal FIG. 1. FESEM images of ZnO sol-gel thin films with annealing at 共a兲 130 ° C, 共c兲 300 °C, 共e兲 600 °C, and 共g兲 900 °C. 共b兲, 共d兲, 共f兲, and 共h兲 show the ZnO nanowire arrays were grown at a fixed temperature共90 °C兲, while the thin films were annealed at 130, 300, 600, and 900 ° C, respectively. The grain sizes become larger and denser with increasing annealing tem-perature, which results in the increas-ing of the diameter of ZnO nanowire arrays.

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method have preferential orientation along the 共002兲 plane, in particular, on the thin films without a certain orientation.

Figure 3 shows the chemical composition of the ZnO nanowire arrays determined by the typical EDS. The peak at 0.5 keV is from oxygen and peaks at 0.9 and 8.6 keV are due to Zn. The peak at 1.7 keV is from the silicon substrates. Besides silicon, the EDS spectra reveal the presence of Zn and O elements, which confirms that the nanowire arrays are primarily ZnO.

Figure4共a兲shows the room temperature PL spectra of a set of ZnO thin films annealed at different temperatures. From this figure, an evident ultraviolet near-band edge emis-sion peak at 385 nm is observed, which originates from the excitonic recombination. As the annealing temperature in-creases from 130 to 900 ° C, the PL peak in the UV region is gradually enhanced. It is believed that the higher annealing temperatures facilitate the migration of grain boundaries and promote the coalescence of small crystals, and thus reduce the concentration of nonradiative recombination centers. However, at the temperature of 900 ° C, defects related to deep-level emission around 500– 550 nm from the ZnO nanowire arrays were also observed. This defect-related

green emission is believed to come from oxygen vacancies. At the temperatures of 130 ° C, the ZnO thin films may not form a good crystal phase, and hence the UV emission in-tensity is very low. Therefore, we may conclude that as the thfilm annealing temperature increases the UV peak in-creases but the peak in the green region also inin-creases, indi-cating that the oxygen vacancies abruptly increase at high temperature due to the volatilization of Zn atom.

The room temperature PL characteristics of thereon ZnO nanowire arrays with different annealing temperatures of thin films are shown in Fig. 4共b兲. The UV peak increases with annealing temperature. The UV emission of ZnO nanowire arrays corresponding to the near-band edge emission is due to the recombination of free excitons through an exciton-exciton collision process.

At the temperature of 900 ° C, the defect-related green emission of the nanowire arrays is lower than that of the seed layers. The green emission is also known to be a deep level emission caused by the impurities and structural defects in the crystal such as oxygen vacancies, zinc interstitials, and so on.30Therefore, it is suggested that the nanowire arrays may reduce the defect density and hence lower the defect-related emission caused by the thin films.

At low temperatures of 130 and 300 ° C, the ZnO nano-wire arrays exhibits a larger UV emission than ZnO thin films, indicating that the nanowire arrays do enhance the UV emission. Moreover, at the temperature of 130 ° C, the ZnO nanowire arrays have a highly preferred 共002兲 orientation and vertical alignment.

IV. CONCLUSION

In summary, our work provides a systematic study of feature-controlled ZnO nanowire arrays via the hydrothermal method. Our investigation demonstrates that the sol-gel thin-film pretreatment conditions have strong influences on the features of the ZnO nanowire arrays grown thereon. The an-nealing temperature of the ZnO sol-gel thin film can affect the microstructure of the ZnO grains and then the growth of FIG. 2. 共Color online兲 XRD spectra of 共a兲 ZnO seed layers annealed from

130 to 900 ° C and共b兲 thereon ZnO nanowire arrays.

FIG. 3.共Color online兲 EDS spectra of ZnO nanowire arrays with ZnO seed layers annealed from 130 to 900 ° C.

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the ZnO nanowire arrays. As the annealing temperature in-creases from 130 to 900 ° C, the grain size of the thin films increases, and the diameter of thereon ZnO nanowire arrays increases from 60 to 260 nm. The c-axis orientation of the thin films also increases with the annealing temperature until it reaches the optimal situation at a certain annealing tem-perature and then gradually decreases. However, the 共002兲 diffraction peak of thereon ZnO nanowire arrays decreases with annealing temperature. The thin films influence the nucleation of the ZnO and subsequently affect the diameter and orientation of the thereon nanowire arrays. At the tem-perature of 130 ° C, the ZnO nanowire arrays align very ver-tically with growth along the c-axis direction. The PL mea-surements show a strong and dominant UV emission at 385 nm, indicating that the low-temperature growth results

in low levels of oxygen vacancies in the nanowires. This work provides all solution-based processing route to fabrica-tion of low-cost highly oriented ZnO nanowire arrays at low temperature. These vertical nanowire arrays are highly suit-able for use in ordered nanowire-polymer devices, such as solar cells and light emitting diodes.

ACKNOWLEDGMENTS

This work was supported by the National Science Coun-cil, Taiwan, Republic of China, with Grant Nos. NSC96-2221-E-002-277-MY3 and NSC96-2218-E-002-025.

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FIG. 4.共Color online兲 Room temperature PL spectra of 共a兲 ZnO seed layers and共b兲 thereon ZnO nanowire arrays with annealing temperature of ZnO seed layers from 130 to 900 ° C for 1 h in air 共excitation wavelength: 266 nm兲

數據

FIG. 1. FESEM images of ZnO sol- sol-gel thin films with annealing at 共a兲 130 ° C, 共c兲 300 °C, 共e兲 600 °C, and 共g兲 900 °C
Figure 4 共a兲 shows the room temperature PL spectra of a set of ZnO thin films annealed at different temperatures.
FIG. 4. 共Color online兲 Room temperature PL spectra of 共a兲 ZnO seed layers and 共b兲 thereon ZnO nanowire arrays with annealing temperature of ZnO seed layers from 130 to 900 ° C for 1 h in air 共excitation wavelength:

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