Epitaxy of Vertical ZnO Nanorod Arrays on Highly (001)-Oriented

(18) The well-oriented ZnO thin films have been deposited using metal organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), chemic...
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Epitaxy of Vertical ZnO Nanorod Arrays on Highly (001)-Oriented ZnO Seed Monolayer by a Hydrothermal Route Weibing Wu, Guangda Hu,* Shougang Cui, Ying Zhou, and Haitao Wu School of Materials Science and Engineering, UniVersity of Jinan, Jinan 250022, Shandong, China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 4014–4020

ReceiVed February 25, 2008; ReVised Manuscript ReceiVed May 11, 2008

ABSTRACT: A long ZnO nanorod array with good verticality and thin diameter is synthesized from a single solution by the hydrothermal route. Prior to the growth, a ZnO seed layer with the c-axis texturing and the monolayer distribution is deposited on the substrate by a modified sol-gel spin coating process. A molecule adsorption stabilization mechanism is proposed to explain the seed orientation. Factors affecting the ZnO nanorod growth are systematically investigated. The critical conditions for the rod growth are obtained by changing the polyethyleneimine amount and the pH. The results suggest that the verticality of the array depends heavily on the seed orientation. The atomic force microscopy and X-ray diffraction measurements reveal that the crystallinity and the initial strain relaxation determine the growth activation energy and the rod diameter size.

1. Introduction One-dimensional (1D) ZnO nanomaterials, especially nanorod arrays, are promising candidates for future high-performance applications in the fields of ultraviolet lasers,1 photodetectors,2 field-effect transistors,3 solar cells,4 chemical sensors,5 and the nanogenerators.6 Present approaches to synthesize ZnO nanorod arrays are based on vapor-phase reaction7-11 or solution routes.12-14 Among these, the hydrothermal synthesis is considered the most suitable for moderate and large-scale manufacturing. However, the control on the crystallographic orientation and the diameter size remains a challenge for the hydrothermal synthesis. ZnO nanorod arrays with good verticality are generally obtained by the heteroepitaxy and homoepitaxy techniques. The heteroepitaxy is limited to the substrates having a small lattice mismatch with wurtzite ZnO, such as the single crystal substrates (GaN and Al2O3 (001)) or the buffer layers (MgO(111), AlN, TiN, and Al2MgO4).15-17 The homoepitaxy requires the predeposition of a ZnO layer on the substrates.18 The welloriented ZnO thin films have been deposited using metal organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), chemical vapor transport (CVT), atomic layer deposition (ALD), and the combined sputtering and etching methods,19-21 but none of these approaches are low-cost and versatile for future applications. The simple sol-gel seed layers have been widely used in the hydrothermal synthesis of ZnO nanrod arrays, but most of them are disordered polycrystalline films and have resulted in worse verticality.4,13,22 Recently, Yang et al. developed an acetate-derived dip coating process to prepare the texturing seed layers.23 However, the accurate control over the rod density and the rod diameters has not been realized. To obtain thin nanorod arrays, reducing the precursor concentration has been the general method.12,13 However, the lower precursor concentration often complicates the preparation of long rods.4,21 The use of organic additives has been another effective route to modify the morphology of ZnO arrays,24 but only polyethyleneimine (PEI) is reported to inhibit the lateral growth of ZnO nanorod besides the precipitate reagent hexamethylenetetramine (HMT).4 Although the protonization and the coordination mechanisms of PEI for the synthesis of ZnO nanorod arrays have been proposed in our previous study,25 the * To whom correspondence should be addressed. E-mail: mse_hugd@ ujn.edu.cn.

high quality nanorod array has not been achieved because of the lack of a well-oriented seed layer. In this work, the growth conditions for ZnO nanorod arrays were well designed, and highly vertical ZnO nanorod arrays were synthesized by the hydrothermal approach. Prior to the growth, well-oriented ZnO seed layers were predeposited on the substrates by the sol-gel spin coating method. The critical growth parameters for the epitaxy of the ZnO nanorod arrays, such as the PEI amount and the pH of the solution, were systematically investigated. The morphology evolution and the mechanisms of the rod formation were well-monitored and discussed, respectively.

2. Experimental Procedures 2.1. Deposition of ZnO Seed Layers on the Substrates. Seed layers were deposited on the substrates by the conventional sol-gel spin coating and the rapid thermal treatment. Prior to the deposition, the substrates, including the insulating substrates (Si, SiO2/Si, and glass) and the conductive substrates (ITO and Pt), were thoroughly cleaned. The precursor concentration of zinc acetate dihydrate (Zn(CH3COO)2 · 2H2O, Sinopharm China) in the ethanol solution was assigned to 0.3 M. For the oriented seeds (denoted as P1), monoethanolamine (C2H7NO, MEA, Sinopharm China) was used as the stabilization reagent, and an appropriate amount of water was added to adjust the hydrolysis of zinc acetate. The precursor solution was spin coated onto the substrates at a speed of 4500 rpm; then, the wet films were rapid heated at 250 °C, 300 °C, and 500 °C for 10 min, 10 min, and 5 min, respectively. In comparison with the oriented seed layer, another seeding process was used to obtain the randomly oriented seed layers (denoted as P2). Besides the same zinc acetate concentration and the spin coating process, dielethanolamine (C4H11NO2, DEA, Sinopharm China) instead of MEA was used to stabilize the sol solution, and especially no water was added. The heat treatment was conducted at 150 and 550 °C for 5 min, respectively. 2.2. Hydrothermal Growth of ZnO Nanorod Arrays. The aqueous growth solution was prepared with equimolar (0.05 M) zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 98%) and HMT ((CH2)6N4, Aldrich). PEI (molecule weight 4000, 50% in water, Aldrich) and nitrate acid (HNO3, 98%) were added to adjust the supersaturation and the pH of the growth solution. Linear PEI is a polymeric amine with a high charge density that allows it to absorb tightly on negatively charged substrates. The seed layers with varied orientations were used for the hydrothermal synthesis. The seed layers were put into the growth solution upside down and kept at 95 °C for a certain period. The resultant films were dried using the recently reported supercritical drying

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Figure 1. XRD patterns of the seed layers deposited on Si substrates by the seeding process P1 and P2. technique,26 through which the morphology disorder caused by the bundling effect due to the capillary stress was considered to have been avoided. 2.3. Characterization. The crystallinity and the orientation of the seed layer and the nanorod arrays were analyzed by an X-ray diffractometer (XRD, D8-Advanced, Bruker, Cu KR). To examine the thickness, the surface morphology, and the morphology evolution of the seed layer during the growth, the as-deposited seed layer was etched by a conventional photolithography process (Supporting Information), and an atomic force microscope (AFM, S-2500, Hitachi) was used. A scanning electron microscope (SEM, JSM-6380LA, JEOL) and a field emission scanning electron microscope (FESEM, JSM-6700F, JEOL) were used to examine the morphologies of the arrays. The ultraviolet and visible photoluminescence (PL) of the nanorods was measured at room temperature using a Xe lamp with an excitation wavelength of 325 nm (FLS920, Edinburgh).

3. Results and Discussion 3.1. Size, Orientation, and Distribution of ZnO Seeds. The seed layers with good and random orientations were deposited by the seeding processes P1 and P2, respectively. In Figure 1, the XRD pattern of the seed layer P1 displays only the (002) diffraction peak of the wurtzite ZnO, indicating the good orientation in the c-axis direction. The Scherrer line width analysis gives a seed diameter of around 30 nm. However, the XRD pattern of the seed layer P2 shows the typical polycrystalline characteristics with a relatively comparable intensity for the (100), (002), and (101) peaks. The polycrystalline seed layers have been used in the hydrothermal synthesis of ZnO nanorod arrays. The thickness has been estimated to be in the range of 50-100 nm.7,12 However, the AFM image of the patterned seed layer (Figure 2a) shows that the thickness, corresponding to the projecting part, is about 33 nm and very uniform. The amplified AFM image reveals a smooth surface morphology with a roughness less than 1 nm and average diameters of about 40 nm (Figure 2b). Comparing the diameter size with the thickness, we affirm that the oriented seeds should be a monolayer distributed on the substrate. The seed size is somewhat larger than from the calculation and the thickness, which may originate from (1) the probe tip convolution effect and (2) the tabular shape with the thickness smaller than the diameter size. The small fluctuation (∼1 nm) of the wide (002) facets (40 nm) provides further proof for the tabular shape. Note that, although only the seed layer on the Si substrate is observed for ease of AFM imaging, similar seed layers are

Figure 2. AFM images of the photolithographic ZnO seed layers prepared by the seeding process P1. (a) The patterned seed layer and (b) the high-magnification images of the projecting part.

deposited on various substrates by the seeding process P1 (see Supporting Information, Figure S1), suggesting that the orientation is not relative to the substrates. Yang’s group has proposed that the orientation is due to the stabilization of the polar (002) facet by the adsorption of molecular groups (hydroxyl or acetate) but has not demonstrated it.23 We assume that the stabilization of the polar facet by adsorption is very similar to the hydrothermal growth, during which the ZnO crystal structure is constructed by dehydration between the OH- ligands of the zinc hydroxyl species adsorbed on the Zn facets of the growing crystals.27 If this assumption is correct, the orientation can be improved if the conditions producing more zinc hydroxyl species are provided. Thus, the addition of a small amount of water and the utilization of MEA instead of DEA in our present experiment will facilitate the orientation of the seed layer according to the assumption. On one hand, the water addition favors the hydrolysis of zinc acetate. On the other hand, the utilization of MEA instead of DEA not only reduces the stabilization of the zinc complex to make the hydrolysis possible28 but also allows the stabilization reagent to volatilize prior to the decomposition of zinc acetate. All these facilitate the hydrolysis of zinc acetate and provide more zinc hydroxyl species for the stabilization of the polar Zn facets. Thus, the orientation becomes an intrinsic thermal dynamics feature, similar to the hydrothermal growth. The detailed experiments and analysis on the specific orientation mechanisms are underway. 3.2. Growth Parameters, Morphology, and Evolution of ZnO Nanorod Arrays. Several parameters, such as seed layer, growth temperature, precursor concentration, PEI amount, and the pH, affect the growth feature of the ZnO nanorods. For

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Figure 3. Top-view and cross-sectional SEM and the FESEM images of the nanorod arrays grown at 95 °C for 4 h in the solution without PEI (a, b), 4 h with 1.7 mL of PEI (c, d), and 24 h with 1.7 mL of PEI (e, f).

ease of the study, the growth temperature, and the precursor concentration are assigned to 95 °C and 0.05 M in all growth procedures, respectively. In present study, we put emphasis on the effects of PEI amount and the pH. Accompanying these studies, the effect of the seed orientation is especially discussed. First, the effect of the PEI amount on the morphologies of the nanorods was investigated. Figure 3a-f gives the top-view and the cross-sectional images of the arrays grown in 60 mL of growth solutions. Because the randomly oriented seed layer P2 is used, the rods are all randomly oriented. As no PEI is added, the average diameters of the rods are as large as 500-800 nm, resulting from the rapid lateral growth at higher supersaturation.4,13 In the same way, the mutually traversing growth occurs for two intercrossing rods (Figure 3a,b). After the traversing growth, the rods grow in their individual directions. Besides the rapid lateral growth, the similar in-of-plane lattice constant for the hexagon wurtzite ZnO is also a responsible factor. As a proper amount of PEI (1.5 mL) is introduced to the solution, the average diameter decreases sharply to 50-80 nm. This should be contributed to both the inhibited lateral growth by the adsorption of the protonized PEI on the lateral plane of the rods and the low supersaturation due to the coordination of side amine groups of PEI to zinc ions.25 There is an upper limit for the PEI amount (1.9 mL) over which even the heteronucleation is suppressed. More seriously, the seeds are etched off from the substrates. In addition, because of the inhibited lateral growth by the PEI adsorption, no traversing growth occurs even if the rods meet together (Figure 3c,d). In contrast, the rods turn their polar facets

toward the solution side to absorb enough precursor species for the continuous growth, and almost all the top facets of the rods point to the solution side. Finally, a nest-like structure results as the growth time is prolonged to 24 h (Figure 3e,f). Although the length is about 10 µm long, the diameter size (80 nm) is the same as those grown for 4 h (Figure 4c-f). Our results suggest that the addition of PEI is of significant importance to obtain long and thin rod arrays because the consumed Zn2+ ions can be complemented through the coordination equilibrium, which makes the preparation of the long rods simple.4,21 The pH of the solution plays an important role on whether or not PEI molecules are adsorbed on the lateral facets of ZnO nanorods. The protonization of PEI produces two results: the positively charged PEI molecules29 and the increase of the solution pH. Only if the pH of the solution is higher than the isoelectric point (IEP) of ZnO (7.2)30 can PEI be adsorbed on the lateral plane of the rods by the electrostatic attraction. To obtain the optimal conditions for the epitaxy of the long and thin rod array, the pH dependence of the morphologies of the arrays is studied in 60 mL of solution with 1.9 mL of PEI. The pH is regulated with nitrate acid. If no nitrate acid is added, the combined effects of the strong coordination and the high pH (10.7) above the dissolution point of ZnO (∼ 10.5) make the growth impossible. As nitrate acid is added, because of the protonization, the growth starts with the H+ ions replacing the Zn2+ ions from the Zn-amino complex. Figure 4a-d gives the cross-section and the top-view SEM images of the arrays grown for 4 h in the solutions with the

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Figure 4. Top-view and cross-sectional SEM images of ZnO nanorod arrays grown in the solution with 0.05 M precursor and 1.9 mL of PEI at 95 °C for 4 h at the pHs (a) 8.41, (b) 7.42, (c) 7.06, and (d) 6.44.

varied pH. Because the well-oriented seed layers are used, all arrays have good verticality. In comparison with Figure 3, it is clear that the verticality of the rods depends heavily on the orientation of the seeds. In Figure 4a, the rods are sparsely distributed on the substrate. Beside of the rods about 1 µm long and 80nm wide, other seeds just burgeon, suggesting that the pH 8.36 is near to the critical growth conditions, in which only the growth of the seeds with proper crystalline are energetically supported. As the pH decreasing to 7.42 and 7.06, more seeds are developed into rods. The rod density, length and diameter size increase correspondingly (Figure 4b and c). On one hand, the decrease of the pH will release more Zn2+ ions from the Zn-amino complex, increasing the supersaturation degree. On the other hand, the pH closest to the IEP (7.2) reduces the adsorption of PEI on the lateral planes. The growth rates both in the lateral and the c-axis directions are enhanced. When the pH is further decreased to 6.44, however, the diameter continues to increase but the length is short (Figure 4d). The relatively lower OH- concentration will be responsible for the short length because the dehydration rate of the zinc hydroxyl species is lowered.27 Also, on every top plane of the rods, one can see a central hole, which is caused by the dissolution of the metastable polar (002) facet.12 In this sense, the aligned ZnO nanotubes are expected if the pH was lower than 6.4 and the growth time is prolonged. Note that, although the supersaturation increases with decreasing the pH, it is not enough for the homonucleation because the coordination ability of PEI to Zn2+ is much stronger than the protonization.

According to the above discussion, the optimal pH for the epitaxy of the rod with proper rod density and diameter sizes should be in the range of 8.4-7.4. To reveal the epitaxy mechanisms, the morphology evolution at the early stages is monitored with the AFM and XRD measurements. The hydrothermal growth of the oriented seed layers was conducted at the assigned pH ∼8.0 for 5 min, 15 min, 30 min, and 45 min, respectively. From the AFM images, it is found that the surface roughness is increased in the first 5 min (Figure 5a), indicating the initiation of the growth. Prolonging the growth time to 15 and 30 min, the seeds continue to grow both in the c-axis direction and the in-of-plane. The decrease in the seed density and the increase in seed size suggest that the large diameter size is mainly caused by the coalescence of the adjacent seeds. After 30-min growth, the seeds show typical rod-like shape (Figure 5c), and the in-of-plane growth is terminated (Figure 7a). The increase in growth time only causes the growth in the c-axis direction, showing the intrinsic thermal dynamics feature. The cross-sectional FESEM images also show that the seeds grow slowly in the first 30 min (see Supporting Information, Figure S2a-c). The growth occurs mainly in the c-axis direction, and the space between the seeds increased in this stage. The larger bottom sizes compared with the top end in the AFM images and the FESEM results originate from the probe tip convolution effect, especially for the rod-like shape. From the XRD patterns (Figure 6), it can be seen that, with the slight enhancement of the peak intensity, the crystallographic quality is abruptly improved, as indicated by the sharp (002)

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Figure 5. AFM images of the seed layers grown in the solution with the precursor concentration of 0.05 M, 1.9 mL of PEI, and pH 8.0 at 95 °C for (a) 5 min, (b) 15 min, (c) 30 min, and (d) 45 min, respectively.

Figure 6. XRD patterns of the seed layers grown in the solution with the precursor concentration of 0.05 M, 1.9 mL of PEI, and pH 8.0 at 95 °C for (a) 5 min, (b) 15 min, and (c) 30 min, respectively.

peak. The full width at half maximum (fwhm) decreases from the initial 0.34 to 0.158, 0.131, and 0.089, respectively. Meanwhile, after 30 min growth, the peak position right shifts from the initial diffraction angle of 34.48 to that of the powder diffraction angle (34.55), suggesting a strain relaxation accompanying the crystallinity improvement. Thus, the first few tens of minutes are crucial for the final diameter size of the rods. The strain relaxation should be responsible for the increase

of the diameter. We believe that the interface free energy of the ZnO crystal and the growth solution is lower than that of the crystal/vapor interface, which also results in the rod growth. The large interface free energy of the crystal/vapor interface, that is, the residue strain, relates to the heating history. The strain relaxation provides the driving force for the increase of the diameter. This may occur by conventional Ostwald ripening mechanism, especially for the well-oriented seeds since the seeds share small lattice mismatch in-of-plane because of the hexagonal shape.31 The Ostwald ripening may also assist the growth in the c-axis direction, resulting in the separation of the rods at the bottom ends (see Supporting Information, Figure S3). It is demonstrated that the initial strain relaxation is general for various seed layers and occurs in all directions, which results in comparable growth rate in various directions (see Supporting Information, Figure S4). Once the strain relaxation is completed, the growth happens only in the c-axis direction (Figure 5d). It is found that the large seeds always grow faster than the small ones. It also contributed to the strain relaxation since the small seeds having large strain have to experience a long period of relaxation, leaving them behind gradually (Figure 4a). After the strain relaxation was completed, the growth rate in the c-axis direction was accelerated (see Supporting Information, Figure S2d). The strain relaxation is also found in the vapor phase transport methods.32,33This implies that the size and the crystallinity of the seed layer is of crucial importance for the rod diameters,33 implying the necessity of improving the seeding process.

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Figure 7. FESEM image (a) and XRD pattern (b) of the nanorod array grown for 12 h in the solution with the precursor concentration of 0.05 M, 1.9 mL of PEI, and pH 8.0 at 95 °C.

3.3. Epitaxial Growth and the Luminescence Characteristics of the Vertical ZnO Nanorod Arrays on Highly (001)-Oriented ZnO Seed Monolayer. On the basis of the above discussion, the high-quality ZnO nanorod array should be synthesized in the following conditions: (1) using the welloriented seed monolayer for good verticality, (2) adding more PEI to provide more adsorbing molecules, and (3) adjusting the pH over the ICP (7.2) to make lateral facets of the rods negatively charged. Finally, the expitaxy of ZnO nanorod arrays on the (002)-oriented seed monolayer was realized in 60 mL of solution with 0.05 M zinc nitrate, 1.9 mL of PEI, pH 8.0 and grown at 95 °C for 12 h. From Figure 7a, it can be seen that the rods have good verticality and are free-standing on the

substrate. The rods are as long as 6 µm with the diameter of about 80 nm. In Figure 8b, it is found that only the (002) peak appears in the XRD pattern, according with the FESEM results. The inset picture in Figure 7a indicates that the rods develop from the individual seeds and no intermediate seeds lie between the rods and the substrate. Epitaxial growth of single crystal rods from such a monolayer is desirable for future high performance since electrons and photons transfer faster in a single crystal phase than across multiple particles.13 However, there are still some rods with worse verticality. We attribute this to the few seeds not fully oriented to the substrates, as is found in the XRD pattern (Figure 1). This can be excluded by further improving the seeding process. The rods with smaller

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References

Figure 8. Room-temperature PL spectra of the as-grown and the annealed ZnO nanorod arrays grown in 60 mL of solution with the precursor concentration of 0.05 M, 1.9 mL of PEI, and pH 8.0 at 95 °C.

size (30 nm) can be obtained at the lower growth temperatue (85 °C) (see Supporting Information, Figure S5). The room-temperature PL spectra of the as-grown and annealed arrays are shown in Figure 8. The PL spectrum of the as-prepared array shows a very strong and broad defect peak centered at 600 nm while a very small band-edge emission appears around 380 nm, relating to the feature of the hydrothermal growth.34 The defect peak is reported to originate from the interstitial oxygen ions.13 The defect concentration is largely decreased after the heat treatment at 350 °C in N2 atmosphere, as indicated by the sharp band-edge transition peak.

4. Conclusions The expitaxial growth of the vertical ZnO nanorod arrays from a well-oriented ZnO seed monolayer was realized. In this study, the seed layer with good orientation and in monolayer distribution was deposited. The orientation mechanisms were elucidated. The verticality of the array is largely improved by using the oriented seed layer. The solution chemistry for the epitaxy was obtained by changing the PEI amount and the pH. The morphology evolution of the rods was monitored by AFM and XRD measurements. The growth mechanisms in the initial stage were discussed. The results illuminate that the crystallinity or residue strain of the seeds play crucial roles in the diameters and the distribution density of the nanorod arrays. Arrays with better morphologies are expected by further modifying the seeding process. Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 90207025 and No. 50502016), and the Doctoral Foundation of University of Jinan, China (B0536). Supporting Information Available: The photolithography processing of the seed layer; XRD patterns of the seed layers deposited on various substrates; FESEM images of the seed layers grown for different time; cross-sectional FESEM image of the array grown for 4 h; XRD patterns of the seed layers deposited on Si substrates by the seeding process P2 and grown for 15 min. This material is available free of charge via the Internet at http://pubs.acs.org.

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