Vertically Aligned 1D ZnO Nanostructures on Bulk Alloy Substrates

Mar 13, 2007 - We report a highly effective growth of vertically aligned ZnO one-dimensional (1D) nanostructures on conducting alloy substrate (Fe−C...
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J. Phys. Chem. C 2007, 111, 4990-4997

Vertically Aligned 1D ZnO Nanostructures on Bulk Alloy Substrates: Direct Solution Synthesis, Photoluminescence, and Field Emission Jinping Liu,*,† Xintang Huang,*,† Yuanyuan Li,† Xiaoxu Ji,† Zikun Li,† Xiang He,‡ and Fenglou Sun‡ Department of Physics, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, and Plasma Institute, South-Central UniVersity for Nationalities, Wuhan 430074, People’s Republic of China ReceiVed: NoVember 22, 2006; In Final Form: February 5, 2007

We report a highly effective growth of vertically aligned ZnO one-dimensional (1D) nanostructures on conducting alloy substrate (Fe-Co-Ni) in mild solutions (T e 70 °C) in the absence of any seeds, catalysts, and surfactants. The growth conditions such as NH3‚H2O concentration, temperature, and nature of the substrate are correlated to affect the nanostructure formation. Different ZnO single-crystal nanostructures including nanoneedles, hexagonal nanorods, and nanopencils oriented normal to the substrate can be selectively formed in high quantity. The ordered ZnO nanostructures show strong UV excitonic emissions and good field emission (FE) properties. Other metal substrates such as Ti and Ni are also proven to be effective for ZnO nanoarray growth. Since metal substrates are much more economical and scalable than Si, sapphire/Al2O3, GaN, etc., we believe that our approach presents a general economical route toward mass production of controllable ZnO arrays and will facilitate flexible design of device architectures for nanoelectronics.

Introduction Large-scale and high-density semiconductor arrays with onedimensional (1D) nanostructures have been extensively studied for their potential application in future electrooptical devices.1 Among them, ZnO, as a nontoxic n-type semiconductor (Eg ) 3.37 eV), is even more attractive for high-efficiency shortwavelength optoelectronic nanodevices because of its large exciton binding energy of 60 meV and high mechanical and thermal stabilities.1a On the other hand, ZnO nanorod/nanowire arrays have also been demonstrated to be quite effective in piezoelectric nanogenerators,2 dye-sensitized solar cells,3 photonic crystals,4 superhydrophobic surfaces,5 and even biodevices due to their biocompatibility.6 High-temperature techniques7a including chemical vapor deposition (CVD) and thermal evaporation have been widely employed over the past years to synthesize ZnO 1D nanostructure arrays.1a,2,7,8 These methods are energy-consuming and expensive, although they can produce high-quality ZnO nanostructures. In contrast, wet chemical methods which are appealing for their low temperature, facile manipulation, and potential for scale-up have recently been developed for the production of aligned ZnO nanostructures (e.g., nanorods, nanowires, nanoneedles).9-11 Among these reported routes, the most successful one is seeded growth on ZnO-nanoparticle-coated substrates.11 Oriented ZnO nanorod/ nanowire/nanotube arrays can be obtained via these two-step processes. However, the coating of the substrate for the formation of a nucleation layer remains complex and difficult/ irreproducible.12 Therefore, large-scale low-cost controllable growth of well-aligned ZnO 1D nanostructures on properly fitting substrates via a one-step synthetic approach is still crucially expected for novel applications. * To whom correspondence should be addressed. Fax +86-02767861185. E-mail: [email protected] (X.H.); ljpphyccnu@ mails.ccnu. edu.cn (J.L.). † Central China Normal University. ‡ South-Central University for Nationalities.

In the literature, insulating/semiconducting substrates such as sapphire (Al2O3), GaN, silicon (Si) wafers, and amorphous glass have been mainly utilized to realize the 1D oriented growth of ZnO.11j Preparation of ZnO nanowires on transparent conducting ITO-coated glass has also been accelerated since the first nanowire dye-sensitized solar cell was reported,3 whereas there are also several advantages of growing semiconducting nanostructures (e.g., CNTs, R-Fe2O3) directly on bulk metals, for example, in the formation of robust electrical contacts during growth.13 This electrical contact can enable the straightforward integration of semiconductors into nanoelectronic devices, such as field emission displays and micro/nanosensors. Considering the high oxidizing ability/photocatalytic property of semiconductors (TiO2, ZnO), utilizing metal as the substrate for semiconducting film/array growth has potential applications including self-cleaning surfaces14a for use in architecture and construction and antimicrobial coatings14b for inclusion in airconditioning units, etc. However, very few studies have been reported for fabricating ZnO aligned nanostructures on conducting metal substrates, especially in solution with temperatures lower than 90 °C. In recent years, the direct growth of ZnO arrays on zinc substrates was realized by the surface oxidation of zinc foil in solution at room temperature or under hydrothermal conditions.12,15 In this regard, Zn was consumed during growth and the bottom plane of the oxide-metal interface traveled down. For the development of nanodevices with high performance, a huge challenge remains of creating aligned ZnO directly on Zn-free metal substrates with persistent integrality during the growth. It is important to identify suitable metals on which high-quality ZnO nanostructures can be grown easily with the available technologies. Herein, we present for the first time the direct solution growth of vertically aligned 1D ZnO nanostructures on bulk alloy substrate (Fe-Co-Ni). The growth kinetics, photoluminescence (PL) relating to NH3‚H2O concentration and posttreatment, and field emission properties are discussed. The ZnO growth on Fe-

10.1021/jp067782o CCC: $37.00 © 2007 American Chemical Society Published on Web 03/13/2007

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Figure 1. (a) Low-magnification SEM image of the ZnO nanoneedle array. Inserts are the EDS pattern and the photographs of an alloy substrate (∼100 × 100 × 0.15 mm3) after and before ZnO growth. (b) Enlarged image of the ZnO array. (c) A typical cross-sectional image of the array. (d) The corresponding XRD pattern, indicating the ZnO needles are highly crystallized and growing along the c-axis direction.

Co-Ni provides at least three advantages. First, the growth is not spatially restricted by the presence of coated catalysts or nucleation layers, thus providing a way to fabricate aligned ZnO arrays on metals in one step. Second, the metal substrate can be easily designed with various sizes and shapes, and we thus have the ability to attain ZnO on any shape or size of substrate, providing tremendous flexibility for developing applications where the morphology of the conducting substrates is critical. Third, it implies that other metal substrates with similar texture to Fe-Co-Ni (e.g., Ti, Ni) might also be effective for ZnO synthesis. Experimental Section In a typical synthesis, a piece of an ethanol/water-washed polycrystalline Fe-Co-Ni alloy (or Ti, Ni) substrate (30 × 30 × 0.15 mm3, purity >99.5%, from Shanghai Chemical Reagent Co., Ltd., pretreated by sonication in absolute ethanol and distilled water successively and dried in air at 40 °C) was suspended in 200 mL of aqueous solution containing 0.035 M Zn(NO3)2 and 0.65 M NH3‚H2O in a sealed beaker followed by heating at a constant temperature of 70 °C for 24 h. Changing the concentration of NH3‚H2O resulted in the formation of 1D ZnO with different tip morphologies. Slight stirring was maintained throughout the entire process. After the reaction, the substrate covered tightly with ZnO nanostructures was rinsed with ethanol and dried in air for further characterization. It is worth pointing out here that the alloy substrates with different tailored sizes (for example, 100 × 100 × 0.15 mm3) and shapes were also employed in our experiment. The result showed that alignment of the ZnO nanostructures occurred on flat metal surfaces regardless of their size or shape. The nanostructured products grown on the substrate were directly subjected to powder X-ray diffraction (XRD, Cu KR radiation; λ ) 1.5418 Å) measurement and scanning electron microscopy (SEM, JSM-6700F; 5 kV) characterization. For the transmission electron microscopy (TEM and HRTEM, JEM2010FEF; 200 kV) observations, the ZnO was scraped from

the substrate and sonicated in ethanol, and the suspension was further dropped onto a Cu grid, followed by evaporation of the solvent in the ambient environment. Room temperature photoluminescence (PL) spectra were recorded on a JY-Labram spectrometer with a continuous wave He-Cd laser focused at ∼2 µm as the exciting source at 325 nm. The field emission measurements were performed in a vacuum chamber at room temperature in a two-parallel-plate configuration (see the next section for details). Results and Discussion The composition of the alloy substrate was first identified. The X-ray energy dispersive spectroscopy (EDS) result (see the Supporting Information) demonstrates that the substrate contains Fe, Co, and Ni with the atomic ratio of 52.23:18.07:29.70. The XRD pattern further confirms that the substrate is the pure cubic phase of the Fe-Co-Ni alloy, in accordance with literature results.16 Figure 1a illustrates the low-magnification SEM image of nanostructures grown on Fe-Co-Ni substrate with use of 0.65 M NH3‚H2O at 70 °C (Sample 1). It can be seen that large-area structures with uniform growth density are obtained. EDS result (insert of Figure 1a, top-right) confirms the formation of ZnO. The optical image of the array is illustrated in the down-left insert of Figure 1a (left: after ZnO growth; right: before ZnO growth). It shows that the ZnO array can be grown on a square substrate with a side length as large as ∼10 cm. The enlarged image in Figure 1b demonstrates that the vertically grown ZnO nanostructures are needle-like with sharp tips. The tip sizes are typically smaller than 50 nm, as shown in the inserted picture. The cross-sectional image of the nanoneedle array is shown in Figure 1c, which clearly displays that the average length of the needles is ∼5.5 µm. Figure 1d shows the XRD pattern of the as-prepared ZnO nanoneedles. All the peaks can be readily indexed to hexagonal wurtzite ZnO (JCPDS Card No. 36-1451; space group P63mc, a ) 3.24982 Å, c ) 5.20661 Å) except for those arising from the Fe-Co-Ni alloy substrate (see the

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Figure 2. (a, b) SEM images of ZnO hexagonal nanorod arrays. (c, d) SEM images of ZnO nanopencil arrays. (e, f) SEM images of ZnO nanorod arrays prepared at 60 °C with 0.42 M NH3‚H2O.

enlarged image of the pattern in the insert). In the XRD pattern, the (002) peak is dominant, and its intensity is much higher than that of other peaks, revealing the high c-axis growth orientation of the product. It was found that the morphology of the synthesized ZnO arrays was dependent on the concentration of NH3‚H2O. We can control the tips of ZnO 1D nanostructures by simply varying the basicity in the solution. Parts a and b of Figure 2 show that ZnO nanorods obtained with use of 0.55 M NH3‚H2O (Sample 2) are grown vertically in high density over the entire surface of the alloy substrate. The regular prismatic hexagon characteristic of anisotropic ZnO crystal can be observed, as shown in the insert of Figure 2b. The hexagonal ZnO nanostructures have an average diameter of ∼90 nm along the length direction. The average length (see the TEM image in the later section) can be comparable to that of the needles illustrated in Figure 1. A further decrease in the NH3‚H2O concentration leads to the formation of ZnO nanorods with short lengths and pencil-like shapes (0.42 M NH3‚H2O, 70 °C). As shown in Figure 2c,d, these ZnO nanopencils (Sample 3) have a mean diameter and a length of ∼120 nm and 500 nm, respectively. In addition to the solution basicity, temperature also plays a crucial role in the growth process. For example, when the experiment was conducted with 0.42 M NH3‚H2O at 60 °C, an aligned array of ZnO nanorods with an irregular tip morphology was realized, as illustrated in Figure 2e,f (Sample 4). XRD patterns of the above-mentioned ZnO arrays are summarized in Figure 3. The

Figure 3. XRD patterns of Samples 2, 3, and 4, showing that these arrays are also c-axis oriented.

strongest peaks are (002), indicating that individual ZnO nanostructures, crystallized along the c-axis direction, are vertically aligned on the substrate. In our experiment, the chemical reaction is commonly known17 and the formation of ZnO arrays can be classified as heterogeneous nucleation and subsequent crystal growth. ZnO is a polar crystal; it can be described as a number of tetrahedrally coordinated O2- and Zn2+ ions stacking alternatively along the c-axis, exhibiting a positive polar plane that is rich in Zn and a negative polar plane that is rich in O. The growth rates of different planes are reported to be V(0001) > V{10-11} > V{10-10} > V(000-1) when a solution-phase route is employed.18 Thus,

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Figure 4. TEM and HRTEM images of (a, b) ZnO nanoneedles and (c, d) hexagonal nanorods. Inserts in parts a and c are the SAED patterns.

the most stable crystal structure should be a regular prismatic hexagon elongated along the c-axis and surrounded by six equivalent {101h0}, facets: (101h0), (11h00), (01h10), (1h010), (1h100), and (011h0), as observed in Figure 2b. On the other hand, the more rapid the growth rate, the quicker the disappearance of the basal plane.17bAccordingly, the relative growth rates of the crystal facets mainly including (0001), {101h1}, and {101h0} will determine the aspect ratio and the final shape of the ZnO nanostructures. The sharp tips appear due to the most rapid growth rate along the c-axis. The growth rates of the {101h0} facets are relative small and they remain to form the hexagonal prisms, while some of the {101h1} facets remain to form pyramidic/needlelike tips. As demonstrated here, the NH3‚H2O concentration and growth temperature are correlated to affect the relative growth rate of different facets, resulting in the selective growth of ZnO nanoneedles, hexagonal nanorods, and nanopencils. Further structural characterization of the ZnO nanoneedles and hexagonal nanorods was performed by TEM. Figure 4a displays a TEM image of several individual ZnO nanoneedles. The dimensions of the needles under TEM observation are in good agreement with the SEM results. The SAED pattern taken from one of the needles is shown in the insert, and it confirms the single-crystal nature of the ZnO needles grown along the c-axis. The high-resolution TEM (HRTEM) image shown in Figure 4b gives a lattice fringe of about 1.92 Å, corresponding to the (101h2) plane of wurtzite ZnO, which further indicates the [0001] growth direction of the nanoneedle.17c The TEM and HRTEM images and SAED pattern of the ZnO hexagonal nanorods are shown in Figure 4c,d and the results also confirm the formation of single-crystal nanostructures with the growth orientation along the [0001] direction. To determine the growth kinetics in ZnO nanosynthesis, a tedious, time-dependent synthetic study was conducted in the

Figure 5. Plot of the average length of ZnO nanoneedles as a function of growth time.

reaction system with 0.65 M NH3‚H2O at 70 °C. A plot of the average length of ZnO nanoneedles as a function of growth time and the related kinetics is shown in Figure 5. It shows that the growth of nanoneedles is fast in the first 5 h, forming a linear correlation between the needle length and the reaction time: Y ) -0.12417 + 0.92567X. The slope, 0.92567 µm/h, represents an apparent growth rate for the needle growth along the [0001] orientation within the first 5 h at 70 °C. Another important piece of data, the apparent nucleation time of 8 min, could also be determined from this equation by extrapolating to the length to zero. This set of kinetic data has rarely been reported before although it can give some exciting information involved in the first few minutes of solution synthesis of ZnO nanoarrays. It should be pointed out that the nucleation time can be lowered or prolonged by changing the growth temperature, and the detailed temperature-dependent nucleation will be discussed quantitively elsewhere by using the approach proposed above. Parts a and b of Figure 6 illustrate the surface images of the Fe-Co-Ni alloy substrate after 0 and 8 min of reaction,

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Figure 6. (a) SEM image of the surface of the Fe-Co-Ni alloy substrate, showing the nanoscale roughness. (b) ZnO nanoparticles formed on the substrate after 8 min of reaction. The insert is the corresponding XRD pattern. (c, d) SEM images of underdeveloped arrays of ZnO needles and hexagonal rods after 30 min growth, respectively. The dashed lines show the coalescence growth.

respectively. From Figure 6a, nanoscale sized roughness of the alloy surface can be seen clearly. In accordance with this observation, previous work has also reported that this kind of alloy substrate possesses a mean roughness of less than 10 nm.13a In contrast, the surface of the substrate after 8 min of reaction is covered with numerous nanoparticles which are 5-20 nm in size. It is very interesting to observe that these particles are ZnO with their [0001] crystal orientation roughly perpendicular to the substrate, confirmed by the XRD result shown in the insert of Figure 6b. In combination with the growth kinetics depicted in Figure 5, the formation of these particles can be considered as the nucleation process. Nanosized “dimples” on the substrate surface act as many nucleation sites. The morphology of ZnO nanostructures after 30 min of growth with 0.65 and 0.55 M NH3‚H2O at 70 °C is shown in Figure 6, parts c and d, respectively. ZnO nanostructures obtained at the early growth stage have the same tip morphology as those formed after 24 h of reaction. “Coalescence growth” observed in previous work11e is also detected here, as indicated by dotted lines. The formation of final ZnO nanoneedles and hexagonal nanorods is achieved by the fusion of smaller neighboring counterparts. After extended reaction, these fused needles and rods grown from the roughly oriented ZnO nuclei begin to impinge on other neighboring crystals, giving rise to the preferred orientation of the arrays. In the present work, we have found a direct relationship between the formation of ZnO arrays and the nature (surface, chemical activity, etc.) of the substrates. When a relatively flat substrate such as silicon with a mean roughness of ∼0.08 nm13a was used instead of polycrystalline Fe-Co-Ni alloy, only disordered ZnO nanorods were formed, as indicated in Figure 7a,b. In this case, ZnO crystals were sparsely nucleated on the substrate and had freedom to grow to large sizes in all orientations. It is noteworthy that the lattice mismatch between

ZnO and single-crystal Si is another possible reason for the disordered growth. Smooth amorphous glass substrate was also not favorable for the ordered growth. In addition, pure amphoteric metals such as Al still suffered from difficulty in ZnO growth under current synthetic conditions. We recently reported that ZnO/Zn-Al layered double hydroxide (Zn-Al LDH) hierarchical heterostructures and LDH rather than pure ZnO nanostructures were generated when Al substrate was immersed in Zn2+and OH--contained solutions.19 However, we did find that the Fe-Co-Ni alloy substrate possessed both high durability to basic environment and suitable surface to support the oriented ZnO nuclei, benefiting the subsequent c-axis growth for the formation of well-aligned arrays. As an extension of our method, we have shown in Figure 7c-f that vertically aligned ZnO nanoneedle arrays can also be attained on other metal substrates such as chemically stable Ti and Ni. We believe that metals of Co, Cr, Fe, and their alloys could also support ZnO growth and further experiments are now under investigation. The photoluminescence (PL) emissions of the as-grown ZnO nanoneedle, hexagonal nanorod, and nanopencil arrays on the Fe-Co-Ni substrate were evaluated by using 325 nm UV excitation (He-Cd laser) at room temperature. The spectra are shown in Figure 8a. All these arrays exhibit a strong UV emission around 382 nm and a broad green emission at ∼550 nm. The UV emission is the band-edge emission resulting from the recombination of excitonic centers.8,11h The visible emission is usually considered to be related to various intrinsic defects produced during ZnO preparation. It is generally accepted that the green emission originates from the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy.8,11h,19 The above PL result is similar to that of ZnO/Zn-Al LDH heterostructures reported in our earlier work.19 Nevertheless, the intensity ratios of the

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Figure 7. (a) Disordered ZnO nanorods grown on Si substrate with 0.65 M NH3‚H2O at 70 °C. (b) The corresponding EDS result. (c, d) ZnO nanoneedle array grown on the Ti substrate. Inserts in parts c and d are the XRD and cross-sectional image, respectively. (e, f) ZnO nanoneedle array obtained on the Ni substrate. Inserts are the XRD and cross-sectional image.

UV to the visible emission (IUV:IVS) are different, indicating there are different defect concentrations in ZnO nanostructures obtained in these two works. For many photoluminescent applications, it is desirable to have the intensity of the visible emission as low as possible. Hence, IUV:IVS is usually employed as an important criterion to evaluate the quality of ZnO. From this point of view, the ZnO nanoneedle array prepared with high NH3‚H2O concentration has superior optical quality over hexagonal nanorod and nanopencil arrays, as evidenced in Figure 8a. It is observed that the IUV:IVS ratio increases with the increase of NH3‚H2O concentration. In our solution synthesis, the growth units are [Zn(OH)4]2- produced by the reaction between Zn2+ and OH-, and OH- is released from reagent NH3‚H2O. ZnO are crystallized by continuously adding the O-contained growth units to the as-formed nuclei. Accordingly, the O atom in ZnO nanostructures comes indirectly from NH3‚H2O. On the basis of the above viewpoint, it is reasonable that more O atoms can be incorporated into ZnO with higher NH3‚H2O concentration, i.e., pH value. When the amount of NH3‚H2O is deficient, the OH- ion concentration is low, which can result in a deficiency of [Zn(OH)4]2- (or O atoms). In the process of crystallization, defects denoted as oxygen vacancies acting as the origin of visible emission can be readily produced in the ZnO arrays. With the increase of NH3‚H2O concentration, the concentration of oxygen vacancy is reduced, leading to the improvement of crystal and optical qualities.

PL measurement was also combined with an annealing treatment to investigate the optical property of the synthesized ZnO arrays. Figure 8b-d shows PL spectra from the same three samples after annealing in air at 400 °C for 5 h. The results show that the green emission of ZnO nanoneedle and hexagonal nanorod arrays has completely disappeared and there is an almost negligible green peak observed for the ZnO nanopencil array. All three UV emission peaks have been enhanced and become sharper substantially. The intensity of the UV emission from the ZnO arrays, and their large IUV:IVS values, suggest that the optical quality of the materials could, after simple and comparatively low-temperature posttreatment, be improved significantly. The direct growth of vertically aligned ZnO 1D nanostructures on conducting metal substrates may facilitate their electrical contact with the external circuit. These arrays will, therefore, have promising applications as field emission displays, gas sensors, biosensors, etc. As a preliminary test, the field emission (FE) performances of ZnO nanoneedles, hexagonal nanorods, and nanopencils (Samples 1-3) have been investigated. The FE measurement was carried out at the pressure of ∼1 × 10-6 Pa. The test anode was an aluminum stick with a diameter of 2 mm. The distance between the sample/cathode and the anode was 170 µm. The current, under increasing applied voltage, was recorded by an electrometer with picoampere sensitivity. More than five positions of the ZnO nanostructure arrays were tested

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Figure 8. (a) Room temperature PL spectra of the arrays of as-prepared ZnO nanoneedles (Sample 1), hexagonal nanorods (Sample 2), and nanopencils (Sample 3). (b-d) PL spectra of Sample 1, Sample 2, and Sample 3 after annealing at 400 °C for 5 h, respectively.

determined to be 4.2, 6.4, and 7.5 V/µm for ZnO nanoneedles (Sample 1), hexagonal nanorods (Sample 2), and nanopencils (Sample 3), respectively. An emission current density of 1 mA/ cm2 (the minimum to produce the luminance of 300 cd/m2 for a video graphics array field-emission display with a typical highvoltage phosphor screen efficacy of 9 lm/W) is achieved at 7.2 V/µm for Sample 1, 8.2 V/µm for Sample 2, and 11.3 V/µm for Sample 3. The low Eto and high FE current of ZnO nanoneedle array are mainly attributed to their sharp tips and high aspect ratios. The high aspect ratio and small radii of curvature of the nanoneedles can generate a high local electric field at the tips, which decreases the FE potential barrier and so increases the FE current. ZnO hexagonal nanorods have flat tips and ZnO nanopencils have low aspect ratios, which is not favorable for good FE performance. The Eto value of the ZnO needle array in this work (4.2 V/µm) is lower than those observed in ZnO nanowires grown on Si wafer (6 V/µm),20 ZnO nanowire-nanocolumn-combined hierarchical structures on amorphous carbon (6.9 V/µm),21 and ZnO nanoneedles on Gadoped conductive ZnO film (∼20 V/µm).22 It is close to those of reported ZnO nanoscrews (3.6 V/µm),23 ZnO pencil-like structures (3.7 V/µm),21 and some other good field emitters such as single-wall carbon nanotubes (1.5-4.5 V/µm)24 and AlN nanotips (4.7 V/µm).25 It should be pointed out that no apparent FE could be observed for the ZnO nanorods grown on Al foil19 reported in our earlier work, because ZnO was separated from the conducting Al substrate by insulating LDH. This result indicates that electrical contact is a prerequisite for FE. The FE characteristics are described by the simplified Fowler-Nordheim (FN) equation:24 Figure 9. Field emissions from ZnO 1D nanostructures. (a) J-E curves of the samples. The insert shows the turn-on electric fields of 4.2, 6.4, and 7.5 V/µm at a current density of 0.01 mA/cm2 (10 µA/cm2) for Sample 1, Sample 2, and Sample 3, respectively. (b) The corresponding Fowler-Nordheim (FN) plots.

to verify the reproducibility, and the average FE current data were reported. Figure 9a shows the J-E curves of ZnO products. From the inset of Figure 9a, the turn-on fields (Eto), defined as the electric field to obtain a current density of 10 µA/cm2, are

ln(J/E2) ) ln(Aβ2/φ) + (-Bφ3/2/β)(1/E) where J is the current density, E is the applied field, φ is the work function of the emitting material, β is the field enhancement factor, and A and B are constants with values of 1.56 × 10-10 A V-2 eV and 6.83 × 103 V eV-3/2 µm-1, respectively. Figure 9b presents the corresponding FN plots. The three curves show rough linearity at high applied fields, indicating that the

Vertically Aligned 1D ZnO Nanostructures FE current originates from barrier-tunneling electrons extracted by the electric field. On the basis of the reported work function of ZnO (5.3 eV)20-22 and the slopes of FN plots, the field enhancement factors β are calculated to be ∼2350, 792, and 1144 for nanoneedles, hexagonal nanorods, and nanopencils, respectively. The β value of 2350 is high enough for various FE applications and can be attributed to geometrical features of ZnO nanoneedles. The FE performance may be further improved by manipulating the geometry configuration of ZnO.11g In this work, we have demonstrated the straightforward integration of low-temperature grown ZnO on conducting metal substrates into FE displays, and the as-synthesized ZnO shows low Eto, comparable to those of reported ZnO nanostructures fabricated at high temperatures. Conclusions In summary, arrays of aligned ZnO nanoneedles, hexagonal nanorods, and nanopencils have been controllably synthesized on conducting bulk metal substrates by using a low-temperature solution-based route. Fe-Co-Ni, Ti, and Ni have shown their excellence for large-scale growth of ZnO arrays. PL measurement has demonstrated intensive UV exciton luminescence of these ZnO ordered structures. Our result provides an economical route for the mass production of ZnO arrays and also enables the straightforward integration of ZnO into nanoelectronic devices, such as field emission displays and gas sensors/ biosensors. Acknowledgment. We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 50202007). We also thank the Center of Physics and Chemistry at University of Science and Technology of China for the PL measurement. We are appreciative of the valuable suggestions from the reviewers for the revision of this paper. Supporting Information Available: EDS result and XRD pattern of the Fe-Co-Ni alloy substrate. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Huang, M.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (b) Nakamure, S. Science 1998, 281, 956. (c) Duan, X.; Huang, Y.; Agarwai, R.; Lieber, C. M. Nature 2003, 421, 241. (2) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (3) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (4) Wang, X. D.; Neff, C.; Graugnard, E.; Ding, Y.; King, J. S.; Pranger, L. A.; Tannenbaum, R.; Wang, Z. L.; Summers, C. J. AdV. Mater. 2005, 17, 2103. (5) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (6) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P. X.; Hughes, W. L.; Yang, R. S.; Zhang, Y. AdV. Funct. Mater. 2004, 14, 944. (7) (a) Fang, X. S.; Zhang, L. D. J. Mater. Sci. Technol. 2006, 22, 1. (b) Park, W. I.; Yi, G. C.; Kim, M.; Pennycook, S. J. AdV. Mater. 2002, 14, 1841. (c) Wu, J. J.; Liu, S. C. AdV. Mater. 2002, 14, 215. (d) Wang, X. D.; Song, J. H.; Li, P.; Ryou, J. H.; Dupuis, R. D.; Summers, C. J.; Wang,

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