Detailed Study on Photoluminescence Property ... - ACS Publications

Jul 6, 2010 - Yanjun Fang , Yewu Wang , Yuting Wan , Zongli Wang , and Jian Sha. The Journal of Physical Chemistry C 2010 114 (41), 18022-18022...
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J. Phys. Chem. C 2010, 114, 12469–12476

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Detailed Study on Photoluminescence Property and Growth Mechanism of ZnO Nanowire Arrays Grown by Thermal Evaporation Yanjun Fang, Yewu Wang,* Yuting Wan, Zongli Wang, and Jian Sha* Department of Physics, Zhejiang UniVersity, Hangzhou 310027, P. R. China ReceiVed: April 25, 2010; ReVised Manuscript ReceiVed: June 17, 2010

The photoluminescence (PL) property and growth mechanism of ZnO nanowires, which are essential to both fundamental and applied studies, are not yet well understood. Here extensive investigations have been carried out to deeply understand the two issues. At first, ZnO nanowire arrays are fabricated on bare glass substrate via a noncatalytic thermal evaporation method. Afterward, the PL measurements reveal that the emission peaks located at 405 and 616 nm are related to the Zn vacancy defect, and the red peak located in the region between 750 and 800 nm has relations with the interaction between the Zn vacancy and Zn interstitial defects. Detailed experiments show that the oxygen flux plays a very important role in the morphology evolution of the as-grown products, which change from nanoflakes to nanowires and finally to nanonails with the increase of oxygen flow rate. The competition between the axial growth and the radial growth results in the morphology evolution. Finally, a two-stage vapor-solid (VS) growth model is proposed to interpret the growth behavior of the ZnO nanowires. Our results have made a positive progress toward the PL property and growth mechanism of ZnO nanowires. 1. Introduction ZnO, as a wide band gap semiconductor (Eg ) 3.37 eV) with a large exciton binding energy (60 meV),1 has attracted considerable attention in recent years due to its excellent optoelectronic properties. Up to now, abundant ZnO nanostructures have been fabricated, such as nanowires,1 nanotubes,2 nanobelts,3 nanotetrapods,4 and so on. Among them, the onedimensional ZnO nanostructures, especially the well-aligned ZnO nanowire arrays, are important because they possess high surface to volume ratio and provide a direct path for charge transport,5 which are potential in the application of various kinds of high-efficiency devices like light-emitting diodes,6 dyesensitized solar cells,5 field emission devices,7 etc. Huang et al.8 first reported on the synthesis of well-aligned ZnO nanowires via a Au-catalyzed vapor-liquid-solid (VLS) process, after which a number of methods have been utilized to fabricate ZnO nanowire arrays.9–11 The catalyst-free thermal evaporation method is one of the most attractive routes for growing ZnO nanowires, which avoids the contamination induced by metal catalyst that will be detrimental to the device performance.12 However, the exact growth mechanism is still under debate.13–15 Meanwhile, although the extensive studies on the photoluminescence (PL) property of ZnO nanowire arrays have been reported,16 the origin of the defect-related PL peaks also remains controversial.17 It is therefore necessary to carry out much work to figure out the growth mechanism as well as the relationship between the defects and the optical performance of ZnO nanowires. In this work, we present a cost-effective method to fabricate well-aligned ZnO nanowire arrays on glass substrate without the assistance of any catalysts and/or ZnO seed layer via a thermal evaporation technology. It is found that the oxygen flow rate has a substantial effect on the morphology of ZnO nanowire * To whom correspondence should be addressed: e-mail: yewuwang@ zju.edu.cn (Y.W.), [email protected] (J.S.); Fax +86-571-87951328; Tel +86-571-87953746.

arrays. The PL property of the ZnO nanowires is studied in detail to explore the origin of the emission bands. Finally, a two-stage vapor-solid (VS) growth model is developed to interpret the growth behavior of the ZnO nanowires based on the detailed investigations. The mechanism of the morphology evolution of ZnO nanowires with different oxygen flow rate is also discussed at length. This study provides a better understanding of the influence of Zn supersaturation ratio and oxygen partial pressure on morphology and PL property of the ZnO nanowire arrays, which should be essential to their fundamental and applied studies. 2. Experimental Methods The growth of well-aligned ZnO nanowire arrays was carried out in a horizontal quartz tube inserted into a tube furnace. A commercially available Zn powder and Ar/O2 forming gas (99.999%, the concentration of O2 is 1% and it is denoted as oxygen in the following part) were employed as Zn and O source materials, respectively. High-purity Ar gas (99.999%) was used as carrier gas during the whole growth process. The soda lime glass slice used as substrate was cut into squares and ultrasonic cleaned sequentially in acetone and ethanol for 10 min. In a typical procedure, 0.5 g of Zn powder was put into the center of a quartz boat, and the substrate was covered on top of the Zn powder with a vertical distance of 1 cm or so. Then the quartz boat was pushed into the center of the quartz tube, and the outlet of the quartz tube was sealed with a gas washing bottle to isolate the inside space of the quartz tube from the atmosphere. After loading the sample, the Ar gas and oxygen were introduced into the quartz tube with the flow rate of 1200 sccm (standard cubic centimeters per minute) and 0-75 sccm, respectively. At the same time, the temperature of the furnace was elevated from room temperature to 630 °C within 30 min, after which the temperature was kept constant. 30 min later, the quartz boat was pulled out immediately, and a uniform layer of dark-yellow products was presented on the top of the

10.1021/jp103711m  2010 American Chemical Society Published on Web 07/06/2010

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Figure 1. Typical SEM images of the well-aligned ZnO nanowire arrays (growth time: 60 min; oxygen flow rate: 25 sccm): (a) 45° tilted view image, (b) top view image, (c) low-magnification cross-section view image, and (d) high-magnification cross-section view image.

substrate. The deposition of Zn on the as-grown nanowires was implemented by radio-frequency magnetron sputtering method in an Ar atmosphere with the sputtering power of 20 W and the background pressure of 0.8 Pa at room temperature. The deposition time was 90 s, and the estimated thickness of the Zn film on the nanowires was about 20 nm. The as-grown products were characterized by scanning electron microscope (SEM, KYKY 3200). The X-ray diffraction (XRD, PANalytical X’Pert PRO) with Cu KR radiation was used to investigate the crystal structure, and the energy dispersive X-ray (EDX) attached to the SEM was utilized to study the composition of the products. The PL measurements were performed on the luminescence spectrometer (Edinburgh Instruments FLS 920) with Xe lamp emitting at 325 nm as excitation source. A cycle refrigerator was used to lower the temperature of the sample to 22 K during the PL measurement. 3. Results and Discussion 3.1. Morphologies and Crystal Structure. Representative SEM images of the as-grown products are shown in Figure 1 (growth time: 60 min; oxygen flow rate: 25 sccm). As can be seen in Figure 1a, the ZnO nanowires are well-aligned and uniform on a large scale. Figure 1b is a top-view image of the

Fang et al. ZnO nanowires, which shows that the as-grown nanowires have a hexagonal cylindrical geometry with flat heads. The crosssection view of the nanowire arrays is shown in Figure 1c, which reveals that the nanowires are predominantly grown perpendicular to the substrate along their c-axis directions. The height of the nanowires is more than 14 µm, and the mean diameter is about 1200 nm. Figure 1d is an enlarged view of Figure 1c, revealing that the nanowire arrays are converged at their roots and forming a continuous film with the thickness of 2 µm or so. Since no film has been predeposited onto the substrate, thus this film must be formed during the growth process of the nanowires, which will be discussed later. It is interesting to find that the change of oxygen flow rate has a significant impact on the morphology of the as-grown products. If oxygen flow rate is 0 sccm during the fabrication process, only a 3 µm thick ZnO film composed of small flakes is obtained on the substrate, as shown in Figure 2a. However, when a tiny amount of oxygen (15 sccm) is introduced into the system, the ZnO nanowire arrays appear on the substrate as shown in Figure 2b. It is estimated that the mean diameter and height of nanowire arrays are about 328 nm and more than 5 µm, respectively. With the increase of oxygen flow rate to 25 sccm, the diameter and height of the ZnO nanowire arrays are dramatically increased to about 1180 nm and 14 µm (Figure 2c), respectively. When the oxygen flow rate is further increased to 45 sccm, the height of the nanowire arrays is increased to 33 µm. However, the mean diameter of the nanowire arrays does not go on to increase but decreases to 445 nm, and the diameters become much more uneven (Figure 2d). With the oxygen flow rate increasing to 75 sccm, the height of the nanowire arrays does not change much, but the tips of them increase abruptly, forming nanonail structures (Figure 2e). The change of the height and diameter of the nanowires with the variation of the oxygen flow rate is summarized in Figure 5a and will be analyzed in the final part. XRD analysis was utilized to characterize the crystallographic properties of the ZnO nanowire arrays. Parts a and b of Figure 3 show the XRD patterns of the ZnO nanowires fabricated without the introduction of oxygen and with the oxygen flow rate of 15 sccm, respectively. All the diffraction peaks can be indexed to the hexagonal wurtzite phase of ZnO (JCPDS card:

Figure 2. Evolution of the morphology of the as-grown ZnO nanostructures with the change of the oxygen flow rate (growth time: 60 min): top view SEM images with the oxygen flow rate of (a) 0, (b) 15, (c) 25, (d) 45, and (e) 75 sccm. Corresponding cross-section view SEM images are shown in the insets.

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Figure 3. XRD patterns of ZnO grown (a) without introducing the oxygen and (b) with the oxygen flow rate of 15 sccm.

36-1451), and no peaks related to the impurities are seen in the patterns, which confirms that the growth of the ZnO nanowires arrays is a catalyst-free process and corresponds well with the EDX result (inset of Figure 7a, the element Al comes from the SEM sample holder as well as the conductive tape). It is found that when the oxygen flow rate is 0 sccm, the intensity of the (002) peak is relatively lower than the other peaks, especially the (100) and (101) peaks, which means that the in-plane growth is dominating while the c-axis growth is suppressed. However, when oxygen is introduced into the growth tube, the preferred

growth direction changes immediately to the (002) direction, which illustrates that the growth mode turns from twodimensional (2D) growth to one-dimensional (1D) growth. The strong intensity and the narrow width of the (002) peak indicates the high crystal quality as well as the highly oriented nature of the as-grown ZnO nanowire arrays. In order to fully understand the growth process of the ZnO nanowire arrays, a series of experiments were conducted with the oxygen flow rate of 35 sccm for a deposition time not exceeding 60 min. The representative SEM images are presented

Figure 4. Evolution of the morphology of the as-grown ZnO nanowire arrays with the increase of growth time (oxygen flow rate: 35 sccm): 45° tilted view SEM images for the growth time of (a) 25, (b) 30, (c) 40, (d) 50, and (e) 60 min. Corresponding cross-section view SEM images are shown in the insets.

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Figure 5. (a) Change of the height and the diameters of the as-grown ZnO nanowires with the change of the oxygen flow rate. The straight line is a linear fit to the experimental data (R stands for the correlation coefficient). (b) Change of the height and the diameters of the as-grown ZnO nanowires with the increase of growth time (oxygen flow rate: 35 sccm). The straight line is a linear fit to the experimental data (R stands for the correlation coefficient). (c) Variation of the buffer layer thickness and (d) the change of areal density of the ZnO nanowires with the increase of growth time (oxygen flow rate: 35 sccm).

in Figure 4. In the early stage of the growth process (the growth time was 25 min, when the temperature is about 525 °C, 106 °C higher than the melting point of Zn), the ZnO vapor is deposited onto the glass substrate forming a layer of ZnO nanoclusters, which is mainly the nucleation process. When the preset temperature 630 °C is reached (i.e., the growth time is 30 min), the ZnO nanowire arrays start to emerge with the average diameter and height of about 475 nm and 4.6 µm, respectively. In the following growth process, the height of the ZnO nanowires increases monotonically with the increase of time, yet the diameters do not follow the same trend but decrease first and then increase. The change of diameters and height of ZnO nanowires at different growth time are summarized in Figure 5b. The continuous increase of the height of the ZnO nanowires is easy to understand due to the unidirectional growth of the nanowires. We think that the decrease of the diameters of the ZnO nanowires in the initial stage is related to their tapered morphology. With the increase of time, the tapered ZnO nanowires gradually change to the cylindrical ones; thus, their diameters begin to increase. Besides the variation of diameters and height, there is another noteworthy phenomenon, i.e., the change of the film at the root of the ZnO nanowires. This film, which was usually referred as buffer layer or seed layer, has already been studied by some researchers,14,15 but all of them held the opinion that this layer was deposited on the substrate first and afterward the nucleation and growth of the ZnO nanowires began. The thickness of the

buffer layer, however, increases with time as shown in Figure 5c. This phenomenon is contradictory to the conclusion mentioned above. Meanwhile, the density of the nanowires declines with the growth time (Figure 5d). We propose that the growth of some nanowires impinges the other nanowires, which blocks the axial growth of some nanowires in the initial stage.18 But the radial growth does not cease, which leads to the convergence of the adjacent nanowires at the roots and the formation of a continuous film.13 Thus, more and more short nanowires merge into this layer, which gives rise to the increase of the thickness of the buffer layer and the decline of the density. 3.2. PL Property. The room-temperature PL measurements were carried out to characterize the optical property of the ZnO nanowire arrays prepared with different oxygen flow rate. Typical PL spectra are shown in Figure 6a. Generally, these spectra can be divided into four parts, that is, the ultraviolet region below 420 nm, the green region located at about 490 nm, the yellow region at around 616 nm, and the red peak falling into the region between 750 and 800 nm. The ultraviolet peak, which has frequently been seen in the previous reports, can be attributed to the near band gap emission (NBE) commonly related to the free exciton recombination through exciton-exciton collision.8 As to the origin of the green peak which was also commonly seen in the literature, most people ascribed it to the deep-level emission related to the recombination of photon generated hole with intrinsic defect of singly ionized oxygen vacancy.19 Besides these two peaks, there are another two peaks

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Figure 6. PL spectra of the ZnO nanowire arrays: (a) Room-temperature PL spectra of ZnO nanowires with various oxygen flow rate (the spectra have been normalized to the ultraviolet peak and shifted along the Y-axis direction for visual clarity). (b) PL spectrum of the ZnO nanowires taken at the temperature of 22 K (oxygen flow rate: 35 sccm). (c) PL spectra in the ultraviolet part of (b) taken with the increase of the exciting power. (d) Room-temperature PL spectra of bare ZnO nanowire arrays (dashed line) and Zn/ZnO with 90 s sputtering (solid line). (The wavelength between 640 and 660 nm is omitted in order to avoid the second-order diffraction peak centered at 650 nm.)

that were seldom been reported in the previous literature about the ZnO nanostructures prepared by thermal evaporation method, i.e., the yellow peak and the red peak. The yellow peak has often been seen on the ZnO nanostructures prepared by hydrothermal growth and was attributed to the oxygen interstitial defect,20 but Djurisic et al.21 attributed this peak to donor-acceptor transitions involving zinc vacancy complexes. Moreover, Dong et al.22 recently have suggested that the peak located at about 620 nm are relevant to the large Zn vacancy clusters based on the results of positron-annihilation spectroscopy (PAS) and depth-resolved cathodoluminescence spectroscopy (DRCLS) measurements on ion-implanted ZnO crystal. As to the red peak, it has rarely been mentioned in the literature. Although the majority of the studies attributed it to the excess oxygen,17 Cross et al.23 observed that the annealing treatment of ZnO nanowires under vacuum or in Ar/H2 mixture led to the enhancement of the red peak located at about 750 nm and correlated it to the interstitial Zn defect. More recently, Dong et al.22 attributed the red peak at around 775 nm to the small Zn vacancy clusters or isolated Zn vacancy defect according to the PAS and DRCLS measurements. Since these two peaks are full of controversies, further experiments were carried out to explore the origins of them, which will be shown later. After a careful investigation of the PL spectra depicted in Figure 6a, a shoulder located at 405 nm is observed, and its

intensity relative to the NBE peak increases with the increase of oxygen flow rate. Lin et al.24 presented the energy levels of the intrinsic defects in ZnO film based on the full-potential linear muffin-tin orbital calculation, among which the energy level of Zn vacancy defect was 3.06 eV below the bottom of conduction band. The energy interval of 3.06 eV is exactly consistent with the energy of the 405 nm emission peak. Xiu et al.25 found this peak in the Sb-doped p-type ZnO film and assigned it to the Zn vacancy. Wu et al.26 also discovered the 405 nm peak in the cathodoluminescence (CL) spectrum of ZnO film prepared by pulse laser deposition and attributed it to the radiative recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the VZn- center existing in the depletion layer. In addition, Zhang et al.27 performed the ab initio calculation and suggested that the VZn had high formation enthalpy at Zn-rich conditions but had comparatively lower formation enthalpy at O-rich conditions. It is consistent with the fact in our case that the intensity of the 405 nm emission increases with the increase of oxygen flow rate. We therefore ascribe the 405 nm emission to the VZn defect. To further clarify the origin of the peaks in the roomtemperature PL (RT-PL) spectra, the low-temperature PL (LTPL) measurements of the ZnO nanowire arrays grown with the oxygen flow rate of 35 sccm were performed at 22 K. As shown in Figure 6b, the ultraviolet peak clearly splits into four parts.

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The peak centered at 3.308 eV (375 nm) is commonly seen in the literature, but the origin is still under debate. Zhang et al.28 attributed it to the donor-acceptor pairs (DAP) emission, while Look et al.29 suggested it should correspond to the acceptorbound exaction (A0X) in a N-doped p-type ZnO film. The DAP peak position would change if we had increased the excitation power,30 which is not observed in our case as shown in Figure 6c. Moreover, no p-type dopant is used in our fabrication process; therefore, it is not related to p-type doping. Schirra et al.31 revealed that this band originated from a free electron transition to a neutral acceptor (e, A0) according to the CL measurement, and the acceptor was a complex defect related with basal plane stacking faults based on the TEM data. Kurbanov et al.32 further proved that this peak was associated with the surface specklike defects on the basis of spatially resolved CL measurements and denoted it as A-line. Consequently, we assign this A-line to the structural defects located at the surface of the ZnO nanowires considering that the fast growth dynamics will inevitably induce a large amount of defects on the nanowires. Besides, the peaks at 3.237 eV (383 nm) and 3.164 eV (392 nm) can be assigned to the longitudinal optical (LO) phonon replica of A-line because the energy intervals of these three peaks are 71 and 73 meV, respectively, which agree well with the energy of LO phonon in ZnO (72 meV).33 Furthermore, there is a shoulder at around 3.060 eV (405 nm), which can be assigned to the Zn vacancy defect as mentioned before. Owing to the reason that the ultraviolet is composed of four peaks, we infer that the red shift of the ultraviolet peaks in Figure 6a is due to the variation of relative intensities of the four parts with the change of oxygen flow rate during the fabrication process. The red region is also found to split into two parts at low temperature: one peak at 1.650 eV (751 nm) and one shoulder at 1.616 eV (767 nm). Since the red peak showed a continuous red shift from 754 to 780 nm with the increase of oxygen flow rate at room temperature (Figure 6a), we suggest that there exists a competition relationship between the two components of the red peak. Because the Zn vacancy defects will increase while the Zn interstitial defects will decrease with the increase of oxygen flow rate, we assign the peak at 1.650 eV to the Zn interstitial defect and the peak at 1.616 eV to the small Zn vacancy clusters or isolated Zn vacancy defect according to the previous reports.22,23 In order to get more insight into the origin of the peaks in the ultraviolet, yellow, and red regions, the RT-PL measurements were carried out on the ZnO nanowires without and with the deposition of Zn by RF magnetron sputtering, and the spectra are found to change significantly after the deposition of Zn. As evidently shown in Figure 6d, the peaks located at 405 and 616 nm are both quenched after the deposition of Zn, which indicates that the Zn vacancy defects are filled by the deposition of Zn clusters and confirms these two peaks are Zn vacancy related. At the same time, the peak at 768 nm is shifted toward the high energy side to 758 nm, which infers that the Zn interstitial defects increase a lot while the Zn vacancy defects are dramatically suppressed due to the deposition of Zn. Moreover, the intensity of the NBE peak increases more than 1 order of magnitude, which is mediated by the surface plasmons due to the interaction between the metal Zn and the semiconductor ZnO.34 Besides, the intensity of the green peak also increases almost twice, which is opposite to the case of Au deposition. The exact reason is unclear yet, and further investigation is under way. The above analysis makes the assignments of the PL peaks aforementioned much more convincing.

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Figure 7. (a) EDX result of the Zn/O ratio of the ZnO nanowires as a function of the growth time (oxygen flow rate: 35 sccm). The right inset is a typical EDX spectrum of the as-grown ZnO nanowires. (The element Al comes from the SEM sample holder as well as the conductive tape.) (b) Typical SEM image of the ZnO nanowires with the oxygen flow rate of 75 sccm and growth time of 45 min.

3.3. Growth Mechanism. It is widely accepted that growth of ZnO nanowires is dominated by the VS mechanism in a catalyst-free thermal evaporation process.13 But Li et al.15 recently proposed a self-catalyzed VLS mechanism to interpret the growth of the ZnO nanowires without the use of any metal catalyst. They suggested that the Zn/ZnOx liquid droplets were critical to the initial nucleation of ZnO nanowires, which means that the nanowires are rich in Zn at the beginning of the growth. However, as revealed by our EDX data (Figure 7a), the Zn/O ratio increases with time, and in the initial stage of the growth process the nanowires are rich in O. We speculate that the growth of the ZnO nanowires in our fabrication conditions is governed by the VS mechanism and can be divided into two stages, i.e., the nucleation stage and the 1D growth stage, as schematically illustrated in Figure 8a. At the beginning, when the furnace’s temperature just exceeds the melting point of Zn, the Zn vapor pressure is relatively low, while the oxygen is rich due to the constant oxygen flow plus the residual air in the quartz tube. Because the comparatively high oxygen partial pressure facilitates the heterogeneous nucleation process on the substrate,35 the ZnO vapor starts to condense and nucleates on the whole substrate. With the reaction proceeding, the Zn vapor pressure raises a lot due to the elevated temperature, while the oxygen partial pressure is gradually reduced because the residual air is diluted by the carrier gas. In such a Zn-rich condition, the 1D growth of ZnO is favored,36 and the ZnO nanowires will grow on the pre-existing nucleus due to the anisotropic homoepitaxial growth.37 It is depicted in

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Figure 8. Schematic illustrations of (a) the growth mechanism of the well-aligned ZnO nanowires via a VS process and (b) the morphology evolution of the as-grown ZnO nanostructures with the change of oxygen flow rate.

Figure 5b that the height of the nanowires is almost proportional to the growth time and can be linearly fitted well with the equation

H ) V(t - t0) where H is the height of the nanowires, V is the growth rate, and t and t0 are the growth time and the initial nucleation time, respectively. Since V is constant, it is a typical VS growth process which is governed by the surface diffusion and deposition.38 However, the diameters of the nanowires do not evolve in the same way but decrease before the growth time of 40 min and then increase monotonously. Coincidently, the Zn/O ratio also increases slowly first and then increases quickly during the last 20 min (Figure 7a). According to the EDX results, we can interpret the radial growth of the ZnO nanowires as follows. At the beginning of the growth process, with the increase of Zn vapor pressure, the evaporated Zn vapor and the oxygen will react with each other and form a ZnO layer deposited onto the Zn source, which will hinder the evaporation of Zn vapor and result in the slow increase of Zn/O ratio.36 Therefore, the diameters of the nanowires gradually decrease and the nanowires become tapered due to the faster axial growth rate than the radial growth rate. Yet with the reaction going on, the accumulated Zn vapor under the ZnO shell will lead to the increase of Zn vapor pressure there, finally inducing the split of the ZnO shell and resulting in the release of Zn vapor.39 In this way, the Zn/O ratio increase quickly in the last 20 min, and the nanowires come back to be cylindrical because of the comparable radial growth rate and the axial growth rate. On the basis of the above analysis, we tend to infer that the radial growth rate of the ZnO nanowires is mainly determined by the Zn supersaturation ratio while the axial growth is governed by the O partial pressure because there is a significant fluctuation in the Zn vapor pressure whereas

the oxygen flow rate is kept constant. It is in good accordance with the change of the diameters and the height of the ZnO nanowires. As displayed in Figure 4, the diameters and the height of the ZnO nanowires also change with the variation of oxygen flow rate, which can be well explained by the mechanism mentioned above, too. The whole process is schematically summarized in Figure 8b. When no oxygen is introduced into the quartz tube, the oxygen partial pressure in the tube is relatively low, which is hard to induce the 1D axial growth of ZnO. Therefore, the 2D growth is dominating and the ZnO flakes are formed. With the increase of oxygen flow rate, the 1D growth of ZnO starts and the ZnO will grow along the [0001] direction on the nucleus to form ZnO nanowires. It is found that the height of the nanowires increases nearly linearly with the oxygen flow rate when the oxygen flow rate is comparatively low, which can be fitted by the equation

H ) K(Q - Q0) where H is the height of the nanowires, K is the proportional factor, Q is the actual oxygen flow rate, and Q0 is the least oxygen flow rate needed to induce the 1D growth of ZnO nanowires. Soci et al.40 also reported the height of GaAs nanowires was in proportional to the TMGa molar flow, which is analogous to our case. As for the diameters, they increase first and then start to decrease with the increase of oxygen flow rate. The increase of the diameters is probably due to the longer nucleation time and larger sizes of the nucleus formed in a relatively oxygen-rich environment, and the decrease of diameters with the further increase of oxygen flow rate mainly results from the ZnO shell deposited onto the Zn source as mentioned before that hinders the evaporation of Zn vapor and reduces the Zn supersaturation ratio, finally gaving rise to the tapered

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morphology of the ZnO nanowires. When an excessive oxygen flow rate (75 sccm) is used during the fabrication process, the nail-like ZnO nanostructures are formed, owing to the much thicker ZnO shell formed in an oxygen-rich environment on the Zn source that will split at the end of the growth stage and result in a sudden release of Zn vapor which leads to the 2D growth at the tips of the nanowires.39 When we reduced the growth time to 45 min while kept the other parameters unchanged, then no such nanonail structure but nanowire arrays would be obtained (Figure 7b). This phenomenon further confirms that the big tips are formed at the end of the growth process. Generally speaking, the morphology evolution of the ZnO nanostructures is a result of the competition between the axial growth and the radial growth with the change of the Zn supersaturation ratio and the oxygen partial pressure during the growth process. 4. Conclusions In conclusion, well-aligned ZnO nanowire arrays were fabricated on the glass substrate via a low-temperature noncatalytic thermal evaporation process. The oxygen flow rate during the growth process plays an essential role in the morphology evolution of the as-grown products, which results in the change from 2D to 1D growth and finally to 2D growth. Besides, the ZnO buffer layer at the root of the nanowires is seen to grow simultaneously with the nanowires. The RT-PL results reveal that the emission peaks located at 405 and 616 nm are related to the Zn vacancy defect, while the red peak is in connection with the interaction between the Zn vacancy and Zn interstitial defects according to the LT-PL result. The RTPL measurements of the ZnO nanowires with and without the deposition of Zn further confirm the above assignments. At last, the two-stage VS growth model is utilized to interpret the growth behavior of the ZnO nanowire arrays. Meanwhile, the morphology evolution of the ZnO nanowires is explained based on the mechanism of the competition between the axial growth and the radial growth due to the different Zn supersaturation ratio and oxygen partial pressure. The implications of the observations presented here could help us fully understand the PL property and growth mechanism of ZnO nanowires. In the future, further work, especially the theoretical calculation and simulation, is called for to understand quantitatively the two issues. Acknowledgment. This work was supported by National Natural Science Foundation of China (Nos. 10874148 and 60976012), Zhejiang Provincial Natural Science Foundation of China (No. Y4090251), “Qianjiang Talent Project” of Zhejiang Province (No. J20091163), SRF for ROCS (No. J20080457), the Fundamental Research Funds for the Central Universities (No. 2009QNA3023), National Basic Research Program of China (2007CB613403), and PCSIRT. References and Notes (1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (2) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (3) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947.

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