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Different ZnO Nanostructures Fabricated by a Seed-Layer Assisted Electrochemical Route and Their Photoluminescence and Field Emission Properties Bingqiang Cao,*,†,‡ Xuemei Teng,‡ Sung Hwan Heo,§ Yue Li,‡,§ Sung Oh Cho,§ Guanghai Li,‡ and Weiping Cai*,‡ Kay Lab of Materials Physics and Anhui Kay Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, Anhui, China, and Department of Nuclear and Quantum Engineering, Korea AdVanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon, Korea ReceiVed: October 10, 2006; In Final Form: December 14, 2006
A simple seed-layer assisted electrochemical deposition (ECD) route has been successfully developed for preparation of different ZnO nanostructures, and their optical and field emission properties are also studied. ZnO films, nanowires, and nanosheets could be prepared in a rational way by just controlling the ECD current density. The corresponding growth mechanisms are also discussed on the basis of the characteristics of the ZnO crystal structure and the influences of the seed-layer and ECD current density. Except for ZnO nanosheets, both the room-temperature and low-temperature photoluminescence measurements of the ZnO films and nanowire arrays show strong ultraviolet excitonic emission, which proves their good crystal quality. Detailed analysis of the field emission (FE) properties indicates that the hierarchical ZnO nanowire array shows good FE property due to their high aspect ratio, small radius curvature, and proper density.
1. Introduction Zinc oxide (ZnO), with a direct band gap of 3.37 eV and a relatively high exciton binding energy (60 meV) at room temperature, is a promising candidate for blue or ultraviolet (UV) emission, and its applications in optical and electrical industries, such as diodes and laser devices, have the potential to revolutionize the display, illumination, and information storage systems in near future.1 Since the 1960s, preparations of ZnO films have been an active field because of their applications in sensors, actuators, resonators, and transparent electrodes for solar cells.2 High quality ZnO films for photoelectric device applications could be acquired by many kinds of vapor-phase methods. Nowadays increasing research interests have also been paid on ZnO nanostructures due to their novel properties and wide utilities in nanodevices. Many kinds of ZnO nanostructures, such as one-dimensional (1D) nanowire, nanorod, nanoring, nanohelix, and two-dimensional (2D) nanosheet/ nanoplate with different crystal morphologies and physical properties, have been synthesized.3 Among them, 1D ZnO nanostructures in highly oriented and ordered arrays have been demonstrated to be of crucial importance for the development of novel devices, such as room-temperature UV lasers,4 solar cells,5 gas sensors,6 and varistors.7 Many vapor-phase methods have been developed to obtain ZnO nanowire arrays, such as physical/chemical vapor deposition,8,9 and pulsed laser deposition.10,11 For example, heteroepitaxy growth of ZnO nanowire arrays usually needs expensive substrates and metal catalysts, together with complex facilities.11 Moreover, these methods * To whom all correspondence should be addressed. E-mail:
[email protected] (B.C.);
[email protected] (W.C.). † Now at Universita ¨ t Leipzig, Fakulta¨t fu¨r Physik and Geowissenschaften, Institut fu¨r Experimentelle Physik II, Linne´strasse 5, 04103 Leipzig, Germany. ‡ Institute of Solid State Physics. § Korea Advanced Institute of Science and Technology (KAIST).
usually require processing temperatures in excess of 450 °C, which significantly limits the choice of possible substrates and device integration processes. As one of the notable superiority over other wide band gap semiconductors, cost-effective and environment benign wetchemical routes could be applied to prepare ZnO films and nanostructures.12 Electrochemical deposition (ECD) is a typical soft chemical method, which could be conducted at atmospheric pressure and at near room temperature. Moreover, it is also attractive due to its low experimental cost and potential for scaleup. ECD of a ZnO film was primarily demonstrated by Izaki13 and Peulon14 10 years ago. Li et al.15 first illustrated a template combined ECD method to prepare ZnO nanowire arrays using an anodic aluminum membrane. But the ordered nanowire arrays will be destroyed when the template is removed. Xu et al.16 applied the ECD method to prepare ZnO nanorods and nanosheets by adding several capping agents. Given the effect of impurities on semiconductor physical properties, the unintentional impurities from capping agents or catalysts are not preferred for the realization of reliable nanodevices. Moreover, the physical properties of the as-prepared ZnO films or nanostructures are not satisfactory for device performance. For example, room-temperature ZnO UV photoluminescence (PL) has not been observed in the ZnO films prepared by solution methods.17,18 ZnO nanowire arrays grown in the channels of dielectric anodic aluminum membrane could hardly be integrated with other devices. To date, fabrication of different ZnO nanostructures through the soft chemical route in a rational way is still a valuable challenge, which would benefit in the electronic device integrations due to the mild growth conditions.19 In this Article, we develop a seed-layer assisted templatefree ECD method to prepare ZnO films, nanowire arrays, and nanosheets at temperature as low as 70 °C by an easily controllable way on silicon substrates. Their corresponding
10.1021/jp066661l CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007
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growth mechanisms are discussed in detail. The absence of alien material deposition between ZnO nanostructures and the substrate improves the interface qualities and thus the physical properties, such as strong UV light and efficient field electron emissions, which are all important for ZnO-based photoelectrical device applications. 2. Experimental Section First, a 50 nm thick layer of ZnO film (seed layer) was deposited on single-crystal silicon (100) substrates with low resistivity, smaller than 10 Ω•cm, by ordinary radio frequency sputtering methods at a base pressure better than 1 × 10-5 Pa before the high purity gas mixture (O2/Ar + O2 flow ratio was 60%) was introduced. The sputtering pressure was maintained at 1 Pa.20 Second, galvanostatic cathodic deposition was employed on the ZnO seed-layer coated silicon substrates at a current range from 0.8 to 1.5 mA. Note that the deposition area on cathode is all the same in our experiments. So, when we change the ECD currents, the corresponding current density changes in the same way. To be concise, we use the deposition current instead of the deposition current density in this paper. Zinc sheets (99.99% purity) act as the anode electrode, and the electrolyte solution was zinc nitrate aqueous solution (0.05 M). The pH value of the solution was about 6. The deposition temperature was fixed at 70 °C by a water bath and the deposition time was 3 h. The samples were characterized by a field emission scanning electron microscope (FE-SEM, FEI Sirion 200), X-ray diffraction (XRD, Philips X’Pert, Cu KR line: 0.15419 nm), and transmission electron microscope (HRTEM, JEOL-2010), respectively. Room-temperature photoluminescence (PL) spectra were measured using 267 nm light of a 10 mW quasi-continuous wave Ti:sapphire laser as excitation. Low temperature (4.2 K) PL tests were also conducted for ZnO film and nanowire array under the excitation of a He-Cd laser (325 nm) in the UV region. For field emission measurements, the different samples on Si substrates were first attached to an end of stainless-steel rod using conducting glue as cathode with the other as anode. The distance between the electrodes was 200 µm, 300 µm, 400 µm, respectively. A voltage with a sweep step of 100 V was applied between the anode and cathode to supply an electric field. In addition, the seeded ZnO layer on the silicon substrate was also characterized by atomic force microscopy (AFM, AUTOprobe CP) and XRD to study its surface, roughness, and orientation. 3. Results and Discussion 3.1. Morphology Evolution. Figure 1 shows the XRD spectra of the samples electrodeposited under different ECD currents. At currents of 0.8, 1.0, and 1.25 mA, only strong (002) peaks in addition to very weak (101) peaks corresponding to the ZnO wurtzite structure (JCPDS, #36-1451) are observed, which indicates that these three ZnO samples are all of highly c-axis orientation. But, for the sample deposited under a current of 1.5 mA, the diffraction peak intensity ratio of the planes (100), (002), and (101) is similar to that of ZnO powders, indicating its nearly random orientation. Macroscopically, all the cathodic substrates were covered with a layer of white and homogeneous ZnO film or particle film in squared centimeter order. But their microstructures show a notable evolution with the increasing deposition currents. Figure 2 shows the SEM image of ZnO electrodeposited under a current of 0.8 mA. The film shows flat and compact surface morphology and seems to be of many hexagonal nanosheets from the top
Figure 1. XRD spectra comparison of the samples electrodeposited under different currents: (a) 0.8 mA; (b) 1.0 mA; (c) 1.25 mA; (d) 1.5 mA.
Figure 2. SEM image of ZnO film electrodeposited under a current of 0.8 mA.
view, which are the (001) planes of a wurtzite ZnO crystal. So such a film should have a c-orientation consistent with its XRD result in Figure 1a. When the ECD current increases to 1.0 mA, large-area well-aligned ZnO nanowire arrays in high density are acquired, as shown in the SEM image of Figure 3a . From the high magnification SEM image (Figure 3b), we can see that such ZnO nanowires are all straight, smooth, and relatively vertical to the substrate with uniform diameters of about 100 nm. Their lengths could be easily controlled by the growth time. From its XRD spectrum (Figure 2b), we could conclude that all the ZnO nanowires grow along the c-axis. When the current further increases to 1.25 mA, the electrodeposited film seems to be composed of white ZnO particles watched with our naked eyes. But low SEM images of Figure 4a,b show that the ZnO particles are somewhat like connective microcalabashes, and their shells are made up of densely arrayed ZnO nanowires with sharp ends. Such hierarchical ZnO nanostructured arrays are growing nearly vertical to the substrate. One of the broken microcalabashes shown in Figure 4c indicates that there are ZnO nanoparticles filled inside. The TEM images and selected-area electron diffraction pattern shown in Figure 4d,e prove that single-crystal ZnO nanowire grows along the c-axis. More interestingly, when the ECD current is as high as 1.5 mA, two-
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Figure 3. SEM images of a large-area ZnO nanowire array (a, 30° tiled-view) electrodeposited under a current of 1.0 mA and its corresponding enlarged image (b).
Figure 4. SEM images of ZnO microcalabashes (a, 30° tiled-view), a single ZnO microcalabash composed of densely arrayed ZnO nanowires electrodeposited under a current of 1.25 mA (b), and a split microcalabash (c). TEM image of a single ZnO nanowire (inset is the corresponding selected-area electron diffraction pattern) (d) and its lattice fringe image (e).
dimensional ZnO platelets with tens of nanometers in thickness and several microns in dimension, which we call nanosheets, are acquired, as shown in Figure 5a. Many nanosheets are hexagons or polygons that have regular edge sides with an angle of 120° between adjacent sides, as demonstrated in Figure 5b,c. The corresponding electron diffraction pattern of Figure 5d, taken under the electron beam perpendicular to the surface of a nanosheet, can be indexed as a hexagonal ZnO along the [0001] axis. Thus we could suggest that the nanosheets grow along the 〈011h0〉 crystallographic direction within the {0001} planes,21 which are different from those of ZnO nanowires. 3.2. Growth Mechanism. From the chemical view of point, a one-step cathodic electrochemical deposition mechanism has been proposed for the formation of ZnO.22,23 The reduction of nitrate ions from NO3- to NO2- and appearance of more hydroxyl ions (OH-) in the electrolyte lead to the combination of Zn(OH)2 and formation of ZnO on the cathode electrode by
a simultaneous dehydration process at the temperature of the water bath (70 °C). The introduction of ZnO films to synthesize 1D ZnO nanowire arrays has been reported formerly in other preparation methods. For example, Tseng24 and Wang25 have illustrated CVD methods to prepare ZnO nanowire arrays on Ga-doped and undoped ZnO films, respectively. Cross26 has reported a hydrothermal method to fabricate ZnO nanowire arrays on ZnO films but the as-prepared samples only show defect-related visible emissions. In addition, no ZnO nanostructures other than nanowire arrays were reported. To elucidate the growth mechanism of different ZnO nanostructures in this electrochemical route, we first check properties of the ZnO seed layer. From its AFM image in Figure 6a, we can see that such a film is composed of smooth and featureless grains about 20 nm in size. But the XRD measurements shown in Figure 6b indicate that the ZnO seed layer has a strong preferential orientation along
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Figure 5. SEM images of ZnO nanosheets on the substrate (a) and their enlarged image (b) electrodeposited under a current of 1.5 mA. TEM image of ZnO nanosheets (c) and its selected-area electron diffraction pattern (d), indicating the up-down planes are (000 ( 1).
Figure 6. AFM image (a) and XRD spectrum (b) of the ZnO seed layers on silicon substrate with an obvious (001) orientation and mean roughness about 3 nm.
the c-axis, which will have an important influence on the initial ECD nucleation process (see the following paragraphs). Now let us discuss the growth mechanism of different nanostructures in such a seed-layer assisted electrochemical process. On the one hand, it is well-known that ZnO is a polar crystal and has positively Zn2+-terminated (0001) and negatively O2--terminated (0001h) polar surfaces, which induce a net dipole moment along the c-axis. Thus the surface energy of the polar {0001} plane is higher than those of nonpolar {011h0} and {21h1h0} planes.27 So preferential growth along the c-axis (〈0001〉direction) is energetically favorable. In other words, {0001}-
oriented ZnO nuclei will grow faster. On the other hand, in the electrochemical process, the ZnO nucleation and growth are also influenced by the ECD parameters, including cathodic potentials,28 electrolyte,29 and cathodic substrates,30 In our cases, all ECD parameters are fixed except the deposition currents. The ZnO seed layer on the cathode has a c-orientation, which means that the (001) planes are parallel to the substrate. So in a proper ECD current range (e.g., 0.8-1.25 mA in our experiments), when the thermodynamically favored (0001)oriented ZnO nuclei deposit on the seed layer, such ZnO nuclei will continue fast growth along the c-axis easily due to the exact
2474 J. Phys. Chem. C, Vol. 111, No. 6, 2007 lattice match. As a result, (0001)-oriented ZnO films or nanowire arrays are prepared as shown in Figures 2 and 3. The reason for these two different morphologies is that different deposition currents will cause different growth velocity ratios along the c-axis to the a-axis.28 Obviously, under higher deposition current, the crystal growth velocity along the c-axis will be much faster than those of other directions and then the growth ratio along the c-axis to the a-axis will be bigger. So, deposition at 0.8 mA/cm2 results in a compact (001)-oriented ZnO film and, when the deposition current is 1.0 mA, the products are wellaligned nanowires. With increasing ECD current, the ZnO growth velocity is faster, resulting in nanowires with lengths that are not quite uniform, as shown in Figure S1a (Supporting information), where some nanowires are obviously longer. When the newly formed ZnO clusters grow on the protruded parts by secondary nucleation, it will lead to accumulation of ZnO nuclei and then nanoparticles, which serve as the cores of microcalabashes, as shown in Figure 4c. Such ZnO nanoparticles continually grow densely and quickly along the c-axis and then lead to the formation of prickly ZnO microspheres (Figure S1b), which are composed of nanorods, as shown in Figure 4b. If such a process is repeated (Figure S1c), microcalabashes are finally formed, as shown in Figure 4a. Although the deposition current is further increased (e.g., 1.5 mA), the corresponding growth velocity of ZnO nuclei from the electrolyte is so fast that they do not have enough time to adopt their crystallographic orientation on the substrate and grow along the a-axis in two-dimensional directions, as shown in Figure S2a. With the growth process continuing, small ZnO nanoplates finally grow into big 2D nanosheets about 20 µm in dimension, as shown in Figure S2bd. Obviously, such nucleation and growth modes of ZnO nanosheets are another form of anisotropy growth, which is different from that of the nanowire array and also energetically unfavorable in view of surface free energy. Illy et al. have also observed ZnO nanosheets growing on polycrystalline zinc substrate and attributed this 2D growth to the close surface energy of 1D/2D crystals under proper experimental conditions.31 But, in our ECD experiment, we think this unique 2D anisotropy growth of the ZnO nanosheets is a fast nonequilibrium growth process caused by the high deposition currents, which is similar to our former ECD results on conducting glass substrates.28 3.3. Photoluminescence Properties. For the room-temperature photoluminescence spectrum of ZnO, it is generally accepted that there are two emission bands. One is in the UV range, which is associated with exciton emission, and another is in the visible range, which originates from the electronhole recombination at a deep level, presumably caused by oxygen vacancy or zinc interstitial defects.32,33 Figure 7a shows the room-temperature PL spectra of all the ZnO nanostructures prepared in such an electrochemical route. The dominant UV emission peaks around 380 nm are observed for ZnO film and nanowire arrays together with the rather weak visible emission at about 530 nm, which indicates both the ZnO film and nanowire arrays are of high crystal quality. The UV emission of the ZnO film shows a 5 nm red shift as compared with those of other ZnO nanostructures, which could be attributed to the emissions from band-tail states or defect-related states.34 Due to their structure homogeneities, the ZnO nanostructures have FWHMs (full widths at half-maximum) smaller than those of the ZnO film. These shift and extension of the UV peak were also observed in the electrochemically prepared ZnO films on gold-coated silicon and conducting glass substrates.35,36 At test
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Figure 7. Room-temperature PL spectra of different ZnO Nanostructures (a) and 4.2 K PL spectra of ZnO film and ZnO nanowire array (b).
conditions the same as for the ZnO nanosheets, the visible emission is rather stronger than the UV emission, which indicates there may be more defects caused by the high electrodeposition current induced fast growth velocity, as proposed in the above discussion. Surface states are another important factor that may seriously influence the PL property in nanomaterials. The low UV emission in ZnO nanosheets may also be attributed to the surface states being nonradiative centers due to their large surface-to-volume ratio.37 Figure 7b shows two typical low-temperature PL spectra for ZnO film and nanowires. As the temperature decreased to 4.2 K, the main UV peaks blue-shifted to 368 nm, caused by the temperaturedependent band gap shrinkage (Varshni relationship)38 which further proves the exciton origin of the UV peaks. For the PL spectrum of a ZnO nanowire, besides the shallow bound exciton, its longitudinal and transversal optical (LO and TO) phonon lines were also observed. 3.4. Field Emission Properties. Besides its great interest for short wavelength light-emitting devices, ZnO is also a good candidate for field emission (FE) application and many groups have reported the good FE performance of ZnO nanowire arrays.39-41 Here the successful synthesis of ZnO nanostructures with different morphologies paves the way to study its FE property contrastively. Figure 8 shows the emission current density versus applied electrical field (J-E) curves for the different nanostructured ZnO corresponding to Figures 2-5. All the ZnO films, ordered nanowire arrays, and hierarchical calabash-like nanowire arrays (H-nanowire arrays) show steady
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Figure 8. J-E plots of field emission from ZnO films (a), nanowire arrays (b), H-nanostructured arrays (c), and nanosheets (d) at measuring distances of 200, 300, and 400 µm.
Figure 9. Field emission comparisons of the different ZnO nanostructures at a measuring distance of 200 µm by a J-E plot (a) and an F-N plot (b).
FE properties at various measuring distances (d) between the electrodes. But the ZnO nanosheet film doses not show a steady emission current, which may be caused by the surface inhomogeneity. For simple comparison, Figure 9a shows the J-E curves measured at the same conditions (d ) 200 µm). The turn-on fields, defined as the applied field to draw an emission current of 10 µA/cm2, are 16.9, 15.5, and 9.5 V/µm, respectively, for ZnO film, dense nanowire array, and H-nanowire array, which are all lower than those values of ZnO needles (∼18 V/µm for 0.01 µA/cm2)39 grown by the vapor-phase method. According to the Fowler-Nordheim (F-N) theory, the relationship between current density (J) and applied electric field (E) can be described as follows:42
J)A
( ) (
)
β2E2 -BΦ3/2 exp Φ βE
where A ) 1.54 × 10-10 (A V-2 eV), B ) 6.83 × 109 (V m-1 eV-3/2), and Φ is the work function, which is about 5.4 eV for ZnO.43 β is the field enhancement factor defined as Elocal ) βE ) βV/d, where Elocal is the local electric field near the emitter tip. According to this equation, the plot of ln(J/E2) vs E -1 (F-N plot) should be a straight line. From data in Figure 9a, corresponding F-N plots for different ZnO nanostructures are shown in Figure 9b. All the F-N plots, except that of the ZnO nanosheet film, show nearly straight lines with different slopes, indicating the field emission process from the ZnO film or nanowire arrays is a barrier tunneling, quantum mechanical process (F-N mechanism). The observed kinks on the F-N plot of the ZnO film located in the “turn-on” field region may be caused by the absorbates-induced emission saturation.44 Due to the instability of the emission current, the emission process of the ZnO nanosheet film could not be well fitted by the F-N
2476 J. Phys. Chem. C, Vol. 111, No. 6, 2007 mechanism. The slopes obtained from the F-N plots can be used to estimate the β values, which have been shown in Figure 9b. In the normal electron emission region, the FE properties of ZnO nanowire arrays, such as turn-on electric field and β factor, are all better than that of the ZnO film due to their high aspect ratio and small curvature radius, which are all efficient for field emission. But the β value of the ordered ZnO nanowire array with high density is much smaller than that of the hierarchical ZnO nanowire array, which is caused by the local electric field screening effect around the tips of nanowires.45,46 Such wafer-scale ZnO nanowire arrays with good FE performance and low preparation cost could promise the industrial application of flat display in the near future. 4. Conclusion In summary, we have illustrated a simple and rational seedlayer assisted electrochemical deposition route to prepare differently nanostructured ZnO and studied their optical and field emission properties. The oriented ZnO seed layer on the electrode induces the formation of (001)-oriented ZnO nuclei when the ECD current density is not very high (
