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J. Phys. Chem. C 2007, 111, 596-600
Effect of Seed Layer on the Growth of ZnO Nanorods Jaejin Song and Sangwoo Lim* Department of Chemical Engineering, Yonsei UniVersity, 134 Shinchon-dong, Seodaemoon-gu, Seoul 120-749, Korea ReceiVed: August 25, 2006; In Final Form: NoVember 6, 2006
To investigate the effect of seed layer on the growth of ZnO nanorods during hydrothermal synthesis, different types of sputter-deposited ZnO films were used. The changes in growth rate, diameter, density, and surface area of highly oriented ZnO nanorods on each seed layer were examined. The growth rate of ZnO nanorods has a strong relationship with the intensity of (0001) orientation. The density of nanorods per unit area is larger if the diameter of the nanorods is smaller. The total surface area of ZnO nanorods is determined by the growth rate and density per unit area. It was found that the morphology of the ZnO nanorods is strongly influenced by the thickness of the seed layer and the corresponding crystal size. A thinner ZnO seed layer provides a higher surface area of ZnO nanorods because of the smaller crystal size of the seed layer. The orientation of the ZnO seed layer significantly affects the crystallinity of the nanorods.
Introduction One-dimensional (1D) nanostructures have been extensively studied because of their potential applications in nanoelectronic devices such as field-effect transistors, 1 single-electron transistors,2 photodiodes,3,4 and chemical sensors.5,6 It is well-known that zinc oxide (ZnO) has a wide band gap (3.37 eV) and a high excitation binding energy at room temperature (60 meV).7 Therefore, 1D ZnO nanorods can be used for many applications, including ultraviolet light-emitting devices,8,9 field-effect transistors,10 solar cells,11,12 and chemical sensors.13,14 To grow ZnO nanorods, various synthesis methods have been untilized, such as vapor-liquid-solid (VLS) growth,15,16 chemical vapor deposition,17 and electrochemical deposition.18,19 However, those methods require severe conditions or a catalyst for nanorod growth. The hydrothermal solution method has many merits that can make ZnO nanorods grow at low temperatures at low cost. To fabricate ZnO nanorods in liquid solution, the effects of zinc salt, concentration of zinc salt, pH, growth temperature, and growth time need to be investigated, because the morphology of the ZnO nanorods is affected by those process parameters. When the growth temperature decreases from 90 to 60 °C, the average diameter of the ZnO nanorods decreases.20 When the concentration of the zinc source increases, the length and diameter of the ZnO nanorods increases.21 By controlling the pH of the aqueous solution, the ZnO nanostructure morphologies and growth rate can be changed.22 The length and diameter of the ZnO nanorods are affected by the growing time.23 By growing highly oriented ZnO nanorods, the surface area per unit area can be increased, which will improve the performance of the nanodevices. The surface area of the nanorods is determined by the density and size of the nanorods. Therefore, the surface area of ZnO nanorods can be controlled by modifying the above-mentioned growing environments. In addition, the seed material is also important for the growth of high-quality ZnO nanorods.24 The size of the nanoparticle * Corresponding author. E-mail:
[email protected]. Fax: +82-2-3126401.
on the substrate determines the size of the growing ZnO nanorods, and the growth of the nanorods is enhanced by the presence of the seed materials.24 Various substrates such as silicon, sapphire, polyethylene terephthalate, polystyrene, and polyethylene have been used to grow ZnO nanorods by hydrothermal synthesis methods.24,25 To effectively grow ZnO nanorods on those various substrates, the ZnO seed layer has to be coated. Nevertheless, it is not well understood how the seed layer affects the growth of ZnO nanorods. In this study, the seed material was confined to differentially sputter-deposited ZnO films. Because a metallic dopant such as Al or Ga in the ZnO seed layer can influence the growth and the morphology of ZnO nanorods, an Al-doped ZnO film (AZO) and a Ga-doped ZnO film (GZO) were also used. The effects of different ZnO seed layers on the growth of the ZnO nanorods in terms of density, size, and growth rate was investigated in this study. Experimental Section ZnO nanorods have been grown on p-type silicon (F ) 0.01 Ω‚cm). Before nanorod synthesis, various ZnO seed layers, two different types of ZnO films (ZnO-a and ZnO-b), AZO, and GZO, were deposited on silicon wafers using sputter deposition. The sputtering was carried out at room temperature using ZnO (99.99% purity), AZO (99.99% purity), and GZO (99.99% purity) targets in an Ar (99.999% purity) gas atmosphere at a radio frequency (RF) of 13.56 MHz. Details of the sputtering conditions are summarized in Table 1. The thicknesses of the sputter-deposited ZnO films were 100-1000 nm. In a typical synthesis process, 30 mM of zinc nitrate hexahydrate [Zn(NO3)2‚6H2O, 98%, Aldrich] was dissolved in 80 mL of deionized water prepared in a three-stage Millipore Milli-Q Plus purification system having a resistivity higher than 18.2 MΩ‚cm. Then, ammonium hydroxide (28 wt % NH3 in water, 99.99%, Aldrich) was added to make the pH of the solution 10.3. Hydrothermal ZnO nanorod fabrication was carried out on wafers having different types of ZnO seed layers by suspending the wafers upside down in the solution. The growth temperature and time were set at 60 °C and 6 h,
10.1021/jp0655017 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006
Effect of Seed Layer on the Growth of ZnO Nanorods
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TABLE 1: RF Sputtering Conditions Used to Grow Different ZnO Seed Layers on Si Substrates and Composition of ZnO Thin Filmsa ZnO film
RF power (W)
pressure (mTorr)
Time (min)
Ar flow rate (sccm)
Zn/O atomic ratio (Al or Ga content)
ZnO-a ZnO-b AZO GZO
100 80 80 100
8 1 1 11
15 30 30 30
50 20 20 35
1:0.97 1:0.93 1:0.71 (0.10) 1:0.85 (0.04)
a
Sputtering process performed at room temperature.
respectively. The samples were removed from the solution after the growth process, rinsed with deionized water, and dried. The morphology of the ZnO nanorods was examined using a field-emission scanning electron microscope (FESEM, Hitachi S-4200) after Pt sputtering to avoid charging effects. The crystallinity of the seed layers and nanorods was measured by X-ray diffraction (XRD, Rigaku D/MAX-2500H). The X-ray generator, a monochromatic Cu KR radiation source, was run at 40 kV and 50 mA. A 2°/min scanning speed was used to measure the crystallinity. The composition of the seed layers was measured by X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific Sigma Probe) with a monochromatic Al KR radiation source at room temperature. The energy spectra of the electrons were analyzed with a hemispherical mirror analyzer with an energy resolution of 0.1 eV. No presputter step was carried out prior to acquisition of the spectra. The spectrometer was calibrated using a C 1s peak of 285 eV. The atomic ratios of various ZnO seed layers are summarized in Table 1. Results and Discussion A. Morphologies and Growth Rates of ZnO Nanorods Grown on Various ZnO Seed Layers. ZnO nanorods were grown on the differentially sputter-deposited ZnO thin films. The length, morphology, and number of as-prepared nanorods were examined using SEM. Top-down and cross-sectional SEM images of nanorods grown on different seed layers are shown in Figure 1. All samples were grown simultaneously for 6 h. ZnO nanorods were grown vertically oriented with respect to the surface as hexagonal pillars with a flat facet surface. It is also clear that the size and number of the nanorods are strongly influenced by the properties of ZnO seed layers. The findings in Figure 1 are consistent with previous reports that the orientation, morphology, growth density, diameter, and distribution of the ZnO nanorods can be effectively controlled by using suitable preparation conditions.26 Figure 2 shows the dependence of the ZnO nanorod growth rate on the ZnO seed layer. ZnO nanorods grown on two types of ZnO films having different thicknesses had almost the same thicknesses. Surprisingly, the ZnO nanorod grown on a GZO film showed a 14% increased growth rate compared to that grown on ZnO films, whereas the ZnO nanorod grown on an AZO film showed a 22% decreased growth rate. The results in Figure 2 suggest that the growth rate of the ZnO nanorod during hydrothermal synthesis is strongly dependent on the structure of the seed layer. The average diameter and density per unit area of ZnO nanorods obtained from the corresponding SEM images are plotted in Figure 3. Although the growth rates of nanorods on two different ZnO thin films were identical, as shown in Figure 2, the density of the nanorods was 5.6 times larger, and the average diameter was 50% smaller for the ZnO-a thin film as compared to the ZnO-b film. It has been reported that the typical
Figure 1. SEM images of ZnO nanorods grown on Si substrates with different seed layers: (a) ZnO-a, (b) ZnO-b, (c) AZO, and (d) GZO. The growth time was 6 h, and the temperature was 60 °C.
Figure 2. Growth rates of ZnO nanorods on different seed layers.
nucleation densities of ZnO nanorods on a clean polished Si surface and ZnO nanocrystals are about 106 and 109 cm-2, respectively, when an electrochemical deposition is used.18 In our study, the density of ZnO nanorods on the ZnO-a film was found to be 3 × 1010 cm-2, which is 30 times larger than previously reported. The nanorods grown on the AZO and GZO thin films exhibited 1.5 and 2.5 times larger average diameters and 4.2 and 6.2 times lower densities of nanorods, respectively, compared to those on the ZnO-a thin film. At lower nucleation densities, the lateral growth is not as effectively suppressed, resulting in larger-diameter rods.24 The differences in the ZnO nanorod growth on different seed layers are explained by the competition between neighboring nanorods in addition to the
598 J. Phys. Chem. C, Vol. 111, No. 2, 2007
Figure 3. Changes in density per 1 µm2 and average diameter of ZnO nanorods grown on different seed layers.
Figure 4. Effects of different seed layers on the total surface area per 1 µm2 of ZnO nanorods.
preferential growth axis of the crystal.24 This study observed a 62-nm average diameter for the ZnO nanorods grown on the AZO seed layer, which is slightly smaller than the 80-nm average diameter obtained for nanorods prepared at 70 °C for 5 h in the presence of hexamethylenetetramine (HMT) in an aqueous solution.27 The total surface area per unit area of the ZnO nanorods was calculated from the growth rate, density, and size of the nanorods and is summarized in Figure 4. If the nanorods are highly packed, a crystal plane of hexagonal ZnO nanorods might be attached to one another. However, the SEM images show that only a few of the nanorods are attached to each other. The nanorods grown on the ZnO-a thin film had a surface area of 7.0 µm2/µm2‚h, which is 7 times higher than the 2D thin film per unit time. The surface area of the ZnO nanorods grown on GZO, ZnO-b, and AZO thin films followed next. Nanorods on the ZnO-a thin film exhibited a 2.8-times larger surface area than those on the ZnO-b thin film, which can be attributed to the higher density of nanorods per unit area. The higher density of nanorods might be caused by the nanorods’ smaller diameter. The 3.7-times lower surface area of the nanorods on the AZO film compared to those on the ZnO-a film is attributed to the lower growth rate (by a factor of 0.77) and density (by a factor of 0.24). The results shown in Figure 4 imply that the surface area of ZnO nanorods is strongly determined by the growth rate and density per unit area. B. Effect of Seed Layer Thickness on the Growth of ZnO Nanorods. To investigate the effect of ZnO seed layer thickness on ZnO nanorod growth by hydrothermal synthesis, ZnO nanorods were grown on sputter-deposited ZnO thin films
Song and Lim
Figure 5. Growth rates of ZnO nanorods with different thicknesses of ZnO film.
Figure 6. Changes in density per 1 µm2 and average diameter of ZnO nanorods grown on different thicknesses of ZnO film.
having different thicknesses. Figure 5 shows the dependence of the ZnO nanorod growth rate on the ZnO seed layer thickness, which was varied from 200 to 950 nm by controlling the sputtering time. A significant change in the ZnO nanorod growth rate was not found for ZnO thin film thicknesses between 330 and 950 nm, whereas the growth rate was slightly higher when the thickness was 200 nm. The change in the density and average diameter of the ZnO nanorods with increasing thickness of ZnO film is shown in Figure 6. The density of nanorods per unit area increases as the thickness of the ZnO seed layer decreases, which is consistent with the previous observation seen in Figure 3. However, the average diameter of the nanorods decreased for thinner ZnO seed layers. Thus, the results clearly show that the density of the ZnO nanorods increased as the average diameter decreased. The surface areas of nanorods grown on ZnO seed layers of different thicknesses are shown in Figure 7. When the thickness of the ZnO seed layer was 200 nm, the surface area was the highest because the nanorods had the highest density and smallest diameter. Therefore, it is suggested that a thinner seed layer can provide a higher surface area of ZnO nanorods per unit area, which can be explained by the change in crystal size depending on the seed layer thickness. The XRD patterns of three ZnO films having different thicknesses are shown in Figure 8. When ZnO with a tetrahedral coordination formed by the sp3-hybridized orbital has the wurtzite structure, the direction of each apex is parallel to the c axis, and the ZnO films tend to grow toward the (0001) direction.28 The strong self-texture of the ZnO film is favorable for obtaining a highly oriented or excellent single-crystalline epitaxial film.28 It is observed that the intensity of the signal
Effect of Seed Layer on the Growth of ZnO Nanorods
Figure 7. Effects of ZnO film thickness on the total surface area per 1 µm2 of ZnO nanorods.
Figure 8. X-ray diffraction spectra of ZnO seed layers having different thicknesses: (a) 950, (b) 630, (c) 330, and (d) 200 nm.
Figure 9. Crystal sizes of ZnO films of different thicknesses.
for the (0001) orientation becomes weaker with increasing ZnO film thickness. The crystal sizes of ZnO films with different thicknesses were calculated using the Scherrer formula29,30
D ) 0.9 λ/(B cos θ) where D is the crystal size, λ is 1.54 Å, and B is the observed full with at half-maximum (fwhm). Figure 9 shows the change in crystal size depending on the thickness of the ZnO thin film obtained from the XRD spectra shown in Figure 8. The size of the crystal decreases as the thickness of the ZnO film decreases. Therefore, it can be concluded that the decrease in the diameter of the ZnO nanorods with decreasing seed layer thickness is due to the smaller crystal size of the ZnO seed layer, which leads to an increase in the total surface area of the nanorods. It
J. Phys. Chem. C, Vol. 111, No. 2, 2007 599
Figure 10. X-ray diffraction spectra of four types of ZnO seed layers: (a) ZnO-a, (b) ZnO-b, (c) AZO, and (d) GZO. The GZO seed layer shows the strongest (0001) direction, whereas the AZO seed layer shows the weakest (0001) direction.
Figure 11. X-ray diffraction pattern of ZnO nanorods grown on different ZnO seed layers: (a) ZnO-a, (b) ZnO-b, (c) AZO, and (d) GZO.
was also found that the crystal size of the ZnO seed layer increases linearly with the thickness of the seed layer. C. Effect of the (0001) Orientation on the Growth of ZnO Nanorods. Figure 10 shows the XRD patterns for the four types of ZnO films used in this study. A typical hexagonal c axis (002) was found in most cases. However, the AZO thin film has a slightly different crystallinity, which has a strong (100) orientation and a very weak (0001) direction. The low growth rate of ZnO nanorods on the AZO thin film shown in Figure 2 might be due to the weak c axis (0001) direction, which is effective in obtaining ZnO nanorod growth. A GZO film has the strongest (0001) direction, which might be the reason for the highest growth rate of ZnO nanorods, as shown in Figure 2. This observation suggests that the vertical growth rate of the ZnO nanorods has a strong relationship with the (0001) orientation of the seed layer. The broader (0001) orientation peak of the ZnO-a film is evidence of its smaller crystal size compared to the ZnO-b film, which is the main reason for the higher density of nanorods. The nanorods on the ZnO-b thin film have a growth rate identical to those obtained on the ZnO-a thin film, even though the ZnO-b film is 8.5 times thicker (Figure 2). It is suggested that the vertical and lateral growth rates are independent. Finally, Figure 11 shows the XRD patterns of nanorods grown on different ZnO seed layers. It is observed that the nanorods grown on the ZnO-a and GZO seed layers have a highly oriented (0001) direction, which is evidence of the epitaxial growth of ZnO. Nanorods grown on ZnO-b and AZO show a relatively
600 J. Phys. Chem. C, Vol. 111, No. 2, 2007 weak (0001) direction. Therefore, it is concluded that the crystallinity of the ZnO nanorods is also influenced by the orientation of the ZnO seed layer. The findings of this study are somewhat consistent with those of a previous report that the orientations of the nanorods prepared on non-c-axis-oriented ZnO thin films were poor compared to those on c-axis-oriented films.31 Conclusions By using different types of ZnO films, the effects of the seed layer on the growth of ZnO nanorods during hydrothermal synthesis was investigated. The changes in growth rate, diameter, density, and surface area of highly oriented ZnO nanorods on different seed layers were examined and analyzed. The growth rate of ZnO nanorods has a strong relationship with the intensity of the (0001) orientation. A GZO layer exhibited the highest growth rate of ZnO nanorods, which is attributed to its strong (0001) direction. As the diameter of the nanorods decreases, the density of the nanorods per unit area increases. The total surface area of the ZnO nanorods is mainly determined by the growth rate and the density per unit area. It was found that the morphology of the ZnO nanorods is strongly influenced by the thickness of the seed layer and the corresponding crystal size. A thinner ZnO seed layer provides a higher surface area of ZnO nanorods because of the smaller crystal size of the seed layer. Therefore, for the growth of highly oriented ZnO nanorods with larger surface areas, it is critically important to use a thinner ZnO seed layer that has a strong (0001) orientation. In addition, the crystallinity of the ZnO nanorods was also strongly related to the orientation of the ZnO seed layer. Acknowledgment. This work was supported by Grant R012006-000-10230-0 from the Basic Research Program of the Korea Science & Engineering Foundation. This work was also supported in part by the Yonsei University Research Fund of 2005. The authors thank Dr. Jeon Kook Lee at Korea Institute of Science and Technology for providing various ZnO sputtered films. References and Notes (1) Chung, S.-W.; Yu, J.-Y.; Heath, J. R. Appl. Phys. Lett. 2000, 76, 2068-2070. (2) Notargiacomo, A.; Di Gaspare, L.; Scappucci, G.; Mariottini, G.; Giovine, E.; Leoni, R.; Evangelisti, F. Mater. Sci. Eng. C 2003, 23, 671673.
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