Ultraviolet-Emitting ZnO Nanostructures on Steel Alloy Substrates

Jul 17, 2008 - Ahmad Umar† and Yoon-Bong Hahn*. School of Semiconductor and Chemical Engineering and BK21 Centre for Future Energy Materials...
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Ultraviolet-Emitting ZnO Nanostructures on Steel Alloy Substrates: Growth and Properties Ahmad Umar† and Yoon-Bong Hahn* School of Semiconductor and Chemical Engineering and BK21 Centre for Future Energy Materials and DeVices, Chonbuk National UniVersity, 664-14 Duckjin-Dong 1-Ga, Chonju 561-756, Republic of Korea

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2741–2747

ReceiVed September 15, 2007; ReVised Manuscript ReceiVed March 6, 2008

ABSTRACT: ZnO nanostructures with a variety of morphologies and sizes, such as urchinlike structures composed of small-head nanonails, vertically aligned nanonails and nanocones, comblike nanostructures, hierarchical nanostructures, etc., have been synthesized via the simple thermal evaporation process onto the steel alloy substrate without the use of any metal catalyst or additives. It was observed from the detailed morphological and structural studies that substrate temperature, distances between source material and substrates, concentration of zinc vapors, zinc partial pressure, concentrations of reactant gases, and choices of substrates have serious impact on the morphologies and structural properties of as-grown products. Therefore, within certain reaction parameters, specific morphologies can be obtained. Detailed structural observations confirmed that the as-grown products are single-crystalline with the wurtzite hexagonal phase and grown along the (0001) directions. Raman-scattering and room-temperature photoluminescence (PL) studies revealed that the as-grown products have good crystallinity with excellent optical properties. The low-temperature PL (LTPL) studies, measured in the temperature range from 11-300 K, of the vertically aligned ZnO nanonails grown at position “B” revealed the presence of excitonic emissions and their multiple-phonon replicas in the ultraviolet region. Additionally, it has also been found that the temperature dependence of free exciton peak position can be successfully described by the standard Varshini’s expression. 1. Introduction Nanostructures of ZnO acquired a special place because of their diversity in properties, such as direct wide band gap (3.37 eV) at room temperature, large saturation velocity (3.2 × 107cm/ s), high breakdown voltage, and large exciton binding energy (60 meV). These versatile properties of ZnO provide an opportunity to recognize itself as one of the most multifunctional materials; therefore, they can be used as ultraviolet (UV) lasers, light emitting diodes, photodetectors, piezoelectric transducers and actuators, hydrogen storage, chemical and biosensors, surface acoustic wave guides, solar cells, photo catalysts, etc.1–4 Because of its noncentrosymmetric structure, ZnO exhibits a piezoelectric nature and by exploring these properties, Z. L. Wang and co-workers recently fabricated ZnO nanowire-based nanogenerators that were able to convert mechanical energy into electric power.5 Hitherto, variety of ZnO nanostructures were synthesized by numerous fabrication techniques and reported in literature.1–6 However, the controllable synthesis of 1D ZnO nanostructures in terms of size, shape, and composition is very important to realize the grown nanostructures for practical device applications.1–5 Including different 1D ZnO nanostructures, the synthesis of novel and complicated ZnO nanonails have particular interest in optoelectronics because of their special architecture.7 There are very few reports on the synthesis and characterization of ZnO nanonails in the literature. Lao and Ren et al. demonstrated the synthesis of ZnO nanonails via vapor transport and condensation process using the ZnO, In2O3 and graphite powders at 1000 °C.8 Shen et al. grew the ZnO nanonails on silicon substrate using Zn and In powders by a two-zone thermal evaporation system at 300-800 °C.9 Using the mixtures of zinc, indium, and In2S3 powders in the two-heating-zone tube furnace, Shen et al. again synthesized ZnO nanonails at 550-900 °C on a silicon substrate.10 The growth of aligned ZnO nanonails and * E-mail: [email protected]. † Contact e-mail: [email protected].

nanopencils on a silicon substrate by modified thermal evaporation process at 600 and 700 °C, respectively, was recently reported by Shen et al., in which an adiabatic layer was used.11 In all the reported results discussed above, the obtained products were single-crystalline but exhibiting a strong deep level emission, which is related to the structural defects and impurities of the corresponding structures. Nevertheless, it is important to have good optical properties for the synthesized nanostructures to utilize them for the fabrication of efficient optoelectronic devices. Moreover, in all the reported results, high temperature, modified thermal evaporation process and additives were used to synthesize ZnO nanonails. Therefore, it is still needed to explore a simple, low-temperature, and catalyst-free approach to synthesize high-quality ZnO nanonails. In this paper, we present detailed structural and optical properties of urchinlike structures composed of small-head nanonails, vertically aligned nanonails, and nanocones grown on a steel alloy substrate via a simple thermal evaporation method by using only metallic zinc powder in the presence of oxygen at modest temperature ranges between 550 and 440 °C, without the use of any metal catalyst or additives. Importantly, our synthesized ZnO nanostructures are exhibiting only a sharp and strong UV emission in the room-temperature and lowtemperature photoluminescence (PL) spectra, confirming that the as-grown nanostructures have excellent optical properties with good crystallinity. Moreover, compared to other previously reported results, our approach to fabricate ZnO nanonails and nanocones are quite simple, easy, efficient, and cost-effective; all the grown ZnO nanostructures reported here have good reproducibility under the same experimental setup and conditions employed for this work. 2. Experimental Section In a typical reaction process, the source material, high-purity metallic zinc powder (99.999%), was put into a quartz boat and placed at the left-hand side of center of quartz tube. Three pieces of steel alloy substrates (1 × 1.5 cm), mentioned as “A”, “B”, and “C”, were placed

10.1021/cg700887z CCC: $40.75  2008 American Chemical Society Published on Web 07/17/2008

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Figure 1. Typical XRD patterns of different ZnO nanostructures grown on positions (a) A, (b) B, and (c) C placed in the different temperature regions, and (d) bare-steel alloy substrate. inside the quartz tube as depicted in Scheme 1a in the Supporting Information). After this arrangement, the chamber was evacuated to 1 × 10-1 Torr using a rotary vacuum pump. The tube furnace was purged with the high-purity argon gas for 30 min before starting the reaction. After purging, the substrates were pretreated by the mixture of H2 and N2 gases (100 sccm (standard cubic centimeters per minute), each) for 20 min at 450 °C. After pretreatment for 20 min, the furnace was heated to 550 °C with continuous introduction of high-purity oxygen (300 sccm) and nitrogen (200 sccm) gases to facilitate the growth of various kinds of ZnO nanostructures. After reacting for 60 min in mixed gases of oxygen and nitrogen, the substrates were cooled to room temperature under the continuous flow of high-purity argon gas (100 sccm). Prior to the experiment, a moveable thermocouple was inserted into the furnace to determine the temperature distribution along the quartz tube. The substrates were placed into three different temperature zones: zone A (550-525 °C), zone B (525-490 °C), and zone C (490-440 °C). Moreover, the temperature distribution inside the tube during the reaction was simulated by using a computational fluid dynamics (CFD) code and compared with experimental values. It was observed that the simulated values agree well with the measured values (see Scheme 1b in the Supporting Information).12 The as-synthesized ZnO nanostructures were characterized using the field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) equipped with selected area electron diffraction (SAED) and X-ray diffraction (XRD) pattern measured with Cu KR radiation. Ramanscattering spectra were measured at room temperature with the Ar+ laser line (λ ) 513.4 nm) as excitation source. The photoluminescence (PL) measurements excited by the 325 nm line from the He-Cd laser were done at room temperature and low temperature in the range of 11-300 K.

3. Results and Discussion 3.1. Detailed Structural Properties of As-Grown ZnO Nanostructures. Figure 1a-c show the typical XRD pattern for the ZnO nanostructures deposited on steel alloy substrates placed at position “A”, “B”, and “C”, respectively. The obtained peaks in all the patterns are well matched with the wurtzite hexagonal-phase of pure ZnO, with the cell dimensions comparable to already reported values (JCPDS Card 75-1526). Moreover, a small peak at 43.7° is also observed from the substrates placed at position “B” and “C”, which was originated from the steel alloy substrate as confirmed by examining the XRD for bare steel alloy substrate (Figure 1d). In all the patterns, the ZnO (0002) peak at 34.2° is dominant over all other peaks and hence reveals that the nanostructures are preferentially grown along the [0001] direction. In addition to this, even though the peak positions obtained in all pattern are almost same, as all the nanostructures are of the same materials, i.e., ZnO, the intensities of the peaks in all the patterns are different. For

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Figure 2. (a) Low- and (b) high-magnification FESEM images of urchinlike structures composed of small head nanonails grown on substrate A. (c) Typical TEM images of small head ZnO nanonails grown in urchin-like structures. (insets of c) Typical SAED pattern and schematic growth habit of as-synthesized small-head ZnO nanonails.

comparison, we compared the (0002) peak of all the patterns, and the intensities of the patterns were found to be 20:5:1 for the nanostructures grown at position A, B, and C, respectively. From the relative intensities, one can see that there is a huge difference in the peak intensities, which is purely due to the crystallinity of the as-grown products. Hence, higher growth temperature leads the growth of high-crystalline material in our synthesis system (position “A” products). Images a and b in Figure 2 show the FESEM images and exhibit the general morphologies of the nanostructures grown onto the substrate “A”. It is clearly seen from the lowmagnification image that the urchinlike ZnO morphologies are grown in very high density over the whole substrate surface (Figure 2a). The average diameter of a single urchinlike structure is in the range of 15-20 µm. The high-resolution FESEM images exhibit more clear features of urchinlike morphologies, which confirmed that these structures are made by the accumulation of numerous one-dimensional small-head nanonails (Figure 2b). Interestingly, it is seen that all the small-head nanonails of urchinlike morphologies are radially originated from the center in such a special manner that they form spherical like morphologies. The diameters of the nanonails are not smooth throughout their lengths and the head and root diameters of the nanonails are in the range of 80-100 nm and 40-50 nm, respectively. The lengths of the grown products are in the range of 10-12 µm, which exhibit the high-aspect ratio of the as-grown nanonails. Figure 2c shows the low-resolution TEM images of the small-head nanonails, which revealed the consistency with the FESEM observations, discussed above. The inset of Figure 2c exhibits the typical growth behavior of the small-head nanonails grown in urchin-like morphologies. Moreover, the SAED of the corresponding structures shown in Figure 2c exhibits that the as-grown small-head nanonails are single crystalline with the wurtzite hexagonal phase and grown along the [0001] direction. Figure 3 gives a typical view of the nanostructures grown onto the substrate “B” placed in the temperature range of 525-490 °C. It is seen from the low magnification FESEM image that vertically aligned nail-shaped nanostructures are grown in very high density over the whole substrate surface (Figure 3a). The high-magnification image reveals that the diameters of the as-grown nanonails are varying from tip to the root and gradually decreasing from ∼150-200 nm at the top to about 80-100 nm at the root (Figure 3b). Interestingly,

UV-Emitting ZnO Nanostructures on Steel Alloy Substrate

Figure 3. Typical FESEM images of (a) the ZnO nanostructures grown on substrate “B” and (b) vertically aligned ZnO nanonails; (c) corallike and (d) small urchinlike; (e, f) comblike ZnO nanostructures.

each nanonail consists of perfectly atomically flat with well defined symmetry and smooth edge facets hexagonal top surfaces which suggest that the direction of the as-grown nanonails is along the c-axis. In addition to the aligned ZnO nanonails, some interesting morphologies have also been observed at the edge of the substrate near to the source material, which are shown in Figure 3c-f. Interestingly, it was seen that many nanonails grow together in tightly confined region and form partially aligned ZnO coral type structures. Actually, it seems that these coral-like structures are made by the accumulation of two to three small urchinlike morphologies composed of ZnO nanonails as shown in Figure 3d. The basic architecture and dimensions of the grown nanonails in these structures, such as perfectly hexagonal caps and decreased diameter stems, are same as the vertically aligned ZnO nanonails grown onto the substrate. Moreover, some more interesting morphologies of ZnO, i.e., nanocombs, are also observed from the same region (images e and f in Figure 3). The nanocombs are composed of 1D ZnO nanonails grown perpendicularly on one side of the nanobelts. Interestingly, it is also seen that, in some cases, the caps of adjacent nanonails grown in comblike structures, are connected each other due to their large cap diameters (Figure 3f inset). The dimensions of the grown nanonails are similar to the structures discussed in Figure 3a-d. Some nanocombs are grown randomly onto the upper portion of the aligned nanonails, whereas some seem to originate from the substrate itself. The detailed structural observations of the as-grown nanonails and nanocombs were analyzed by TEM. Figure 4a shows the low-magnification TEM image of a single ZnO nanonail and exhibits the full consistency with the FESEM observations, in terms of their morphology and dimensionality, shown in Figure 3. The nanonail shows clean and smooth surface throughout their lengths with the flat top surfaces. The inset of Figure 4a shows the typical growth behavior of the as-grown nanonails, which shows that the nanonails have perfectly hexagonal (0001) top surfaces. The atomically resolved HRTEM image of the circled portion of corresponding nanonail shown in Figure 4a

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Figure 4. Typical TEM and HRTEM images of (a, b) ZnO nanonail and (c, d) comblike structures grown on substrate “B”. Insets of b and d exhibit the corresponding SAED pattern of the circled portion of the analogous nanostructures shown in a and c, respectively. Schematic growth habit of as-synthesized ZnO nanonails (inset a).

demonstrates the lattice distances, measured from lattice fringes, of about 0.52 nm, which corresponds to the (0001) planes of ZnO. Inset of Figure 4b shows the subsequent SAED pattern, which confirms the single-crystallinity with the [0001] growth direction for the as-grown nanonail. Images c and d in Figure 4 show the low- and high-resolution TEM images of the asgrown ZnO nanocombs. The shape and dimensions of comblike structure shown in TEM image is similar as discussed in images e and f of Figure 3. The high-resolution TEM image (Figure 4d) combined with corresponding SAED pattern (inset Figure 4d) unambiguously confirmed that the ZnO nanonails grown in comblike structures are single-crystalline and grown along the [0001], c-axis direction. The typical morphologies of ZnO nanostructures grown onto the substrate “C” placed in the temperature range of 490-440 °C have been shown in Figure 5. The low- and highmagnification FESEM images reveal that the deposited products have conelike morphologies and are grown onto the substrate surface in high density in a partially aligned manner (Figure 5 a and its inset). The diameters of nanocones are not uniform throughout their lengths, and it gradually increases from root to top. The diameters at their roots and tops are in the range of 50-80 and 100-120 nm, respectively. Interestingly, it is seen that the tops of the nanocones exhibit atomically flat hexagonalshaped morphologies (inset of Figure 5a). In addition to the partially aligned nanocones, other morphologies, i.e., comblike structures and hierarchical structures, are also observed from the edge of the sample and shown in Figure 5b. The FESEM images of as-grown comblike structures exhibit that these structures are formed by the growth of 1D ZnO nanowires at one side of the wide nanobelt stem. The arrangements of the nanowires on the comblike structures are not regular and uniform, which is most probably because of the lower supply of zinc source, as the substrate “C” was far from the source material. Additionally, it is seen that some hierarchical structures are also mixedly grown with the comblike morphologies. In these structures, ZnO nanocones are grown onto the outer surfaces of core ZnO nanowires and give it a shape of

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Figure 5. Typical FESEM images of the ZnO nanostructures grown on substrate “C”: (a) aligned ZnO nanocones, (b) comblike, and (c) hierarchical ZnO nanostructures. (d) Comblike structures grown on substrate “C” and (insets of d) their corresponding SAED pattern; (e, f) typical TEM and HRTEM images of ZnO nanocones grown on substrate “C” and (inset of f) their SAED pattern. (inset e) Schematic growth habit of as-synthesized ZnO nanocones.

Figure 6. Typical Raman-scattering spectra of the as-grown ZnO nanostructures synthesized on different substrates; (a) position A, (b) position B, and (c) position C.

hierarchical nanostructures (Figure 5c). The dimensions and morphologies of the nanocones grown in these hierarchical morphologies (inset of Figure 5c) are almost similar to those grown on the substrate, as discussed in figure 5 (a). Further structural characterizations of these structures were done by the TEM and HRTEM. Figure 6d shows the low-resolution TEM image of the comblike structure and confirmed that the dimensionality and shape of the observed structure is fully matched with the observed comblike structures demonstrated in Figure 5b. It can be seen from TEM image that the nanowires are grown from one side of the wide nanobelt stem (Figure 5b). The corresponding SAED pattern of the circled portion of

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comblike structures exhibits that the as-grown ZnO nanowires in comblike structures are single-crystalline and grown along the [0001] direction. Figure 5e demonstrates the typical lowresolution TEM image of the nanocones grown at position “C” in partially aligned manner. It is clearly seen from the image that the diameters of the grown nanocones are not uniform throughout their lengths and ranges from 50 nm in the bottom to about 100 nm in the top. The TEM observations are fully consistent with the observed FESEM results shown in Figure 5a. The inset of Figure 5e shows the typical growth behavior of the as-grown nanocones. The HRTEM image and SAED pattern of the circled portion of corresponding nanocone shown in Figure 5e confirmed that the grown nanocones are singlecrystalline, wurtzite hexagonal phase pure ZnO and grown along the [0001] direction. 3.2. Raman-Scattering and Photoluminescence (PL) Properties of As-Grown ZnO Nanostructures. To investigate the vibrational properties of as-grown nanostructures, room-temperature Raman-scattering studies were performed. Raman spectra are sensitive to crystallization, structural disorder and defects in micro- and nanostructures. It is well-known that with a wurtzite hexagonal phase ZnO belongs to the space group C46v with two formula unit primitive cell where all the atoms occupying the C3V sites. At point of the Brillouin zone, singlecrystalline ZnO have eight sets of optical phonons in which A1, E1, and E2 modes are Raman active. Among these, the E2 modes are Raman active only, whereas the A1 and E1 are also infrared active and therefore split into two components: longitudinal (LO) and transverse (TO) optical components.13a Figure 6 shows the typical Raman-scattering spectra of ZnO nanostructures grown at different temperature zones (A-C) in a single reactor furnace. It can be seen from the spectra that a single, dominant, and high-intensity peak at 437 cm-1, in all the cases, is seen that is attributed to be as the Raman active optical-phonon E2 (high) mode of ZnO crystal. A much suppressed peak in the range of 580-583 cm-1, assigned as E1L mode, originated because of the formation of defects such as zinc interstitials and oxygen vacancies, etc.,13b have also been observed in all cases. In addition to these, two very suppressed peaks at 331 and 379 cm-1 attributed to be as E2H - E2L (multi phonon process) and A1T modes, respectively, were also seen.14 It has been observed that a peak at 331 cm-1, assigned to be as E2H - E2L (multiphonon process), can be founded only when the ZnO is single crystal.14 Hence, because of the presence of single and dominant Raman-active E2 mode with a much suppressed E1L mode, in all the cases, confirmed that the asgrown ZnO nanostructures have wurtzite hexagonal phase and possess very good crystal quality. As all the structures were from the same material, i.e., ZnO, hence one can expect the similar Raman-scattering results from all the samples. However, the intensities of the principle peak of ZnO, E2 (high), obtained at 437 cm-1, was different in all the cases. The relative intensities of the as-grown products were found to be 2:1.5:1 for the nanostructures grown at positions A, B, and C, respectively, which confirmed that the structures grown at higher temperature possess higher crystallinity. The Raman-scattering are fully consistent with the XRD observations. The optical properties of as-grown ZnO nanostructures were performed by the room-temperature photoluminescence (PL) spectroscopy. Figure 7 shows the typical room-temperature photoluminescence spectra for the as-grown ZnO nanostructures. It has been observed that ZnO generally exhibits two emission peaks, i.e., ultraviolet (UV) emission or near-band-edge emission (NBE) and green emission or deep-level emission bands.14 The

UV-Emitting ZnO Nanostructures on Steel Alloy Substrate

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Figure 7. Typical room-temperature PL spectra of the as-grown ZnO nanostructures synthesized on different substrates; (a) position A, (b) position B, and (c) position C.

origination of NBE is due to the direct recombination of freeexcitons through an exciton-exciton collision process15 while regarding the appearance of green emission in room-temperature PL spectra is controversial and many hypotheses are presented to explain the defect emission.16–18 Even many hypotheses are reported in the literature regarding the origination of green emission but the most cited reason is that the green emission originates because of the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancies.19a In our as-grown ZnO nanostructures, sharp and strong UV emission centered at 383 nm for nanostructures grown at position “A”, 379 nm for nanostructures grown at position “B”, and 383 nm for the structures grown at position “C” has been observed, whereas no distinctive green emission is seen. Djurisic et al. reported that the variation in the positions of the band edge emission in various ZnO nanostructures may occur due to their different sizes because of their different surface to volume ratios.19b In our case, the ZnO nanonails grown at position B (nanonails grown at position “B” contain larger diameter than the structures grown at position “A” and C) exhibit a shift of only ∼4 nm, which is most probably due to the size effect of the grown nanostructures. However, more studies are needed to obtain more conclusive evidence. From the room-temperature PL studies, it can be concluded that because of the presence of strong and sharp NBE with no green emission, the as-grown ZnO nanostructures possess very good crystallinity and excellent optical properties. Detailed optical properties of as-grown nanostructures were done by low-temperature PL (LTPL) studies. For LTPL studies, the vertically aligned ZnO nanonails grown at position “B” have been chosen. Figure 8a shows the PL spectra as a function of temperature. The observation of only one strong UV emission without other luminescence-related deep level defects even at room temperature is quite remarkable. Figure 8b shows the highresolution PL spectra of near band edge emission. To exactly identify the origins of the peaks in near band edge emission at 11 K, the spectrum is studied by means of semilogarithmic scale and shown in Figure 9a. To determine the accurate peak positions, experimental data were fitted using a Gaussian line shape function that exhibits seven different peaks (Figure 9 a). The peak at 3.372 eV is attributed to the free exciton (FX) recombination while peak at 3.3601 is assigned to be as bound exciton (BX) recombination based on the consistency of these values with the reported literatures.20–24 In addition to these two recombination peaks, we observed a series of peaks at 3.31502, 3.24114, 3.17811, 3.09822, and 3.02463 eV. Among these, the dominant peak at 3.31502 eV is assigned to the first-order transverse optical (TO) phonon replica of free exciton recom-

Figure 8. (a) Low and (b) high-magnification temperature-dependent PL spectra of the as-grown ZnO nanonails, grown at position “B”, measured from 11 to 300 K.

Figure 9. (a) Typical photoluminescence spectrum measured at 11 K and (b) PL peak energies as a function of temperature.

bination; the reason for the assignment is the energy spacing between this peak and the free exciton peak is close to the transverse phonon energy (56.98 meV).19,25 The evidence for this assignment is further supported by the temperature dependence of the peaks as discussed below. The strong emission

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intensity of the transverse optical-phonon peaks suggests a strong coupling of transverse optical phonon and free exciton. The peak at 3.24114 eV is attributed to the free excitonic emission including a TO and the first-order longitudinal optical (LO) phonons (73.88 meV) because the energy difference between this line and the FX is close to the sum of the energies of a TO and LO phonons. Similarly, the peaks at 3.17811, 3.09822, and 3.024638 eV are assigned to the FX-TO-2LO, FX-TO-3LO, and FX-TO-4LO emissions. It is generally accepted that the exciton-phonon coupling is governed by two mechanisms, deformation potential and Frohlich potential. The TO scattering is mainly determined by deformation potential related to shortrange interaction. On the other hand, the LO scattering included contributions from both Frohlich potential and deformation potential. In nanocrystallite, the deformation potential would influence on short-range interaction.22 Thus, there are many related TO replicas. Figures 8b and 9b show the temperature dependence of the NBE peak positions. Both the BX and FX emissions shift to lower energy (red-shift) with increasing temperature. The BX and FX emissions appear to be thermally quenched above 150 K, consistent with the results for bulk ZnO,26 nanocrystalline ZnO,22 and nanorods27 reported in the literature. Moreover, for the semiconductors with a direct-band gap, if the exciton binding energy is independent of the temperature, the band-edge exciton emission peak shifts to lower energy (red-shift) as the temperature of the ZnO nanonails is increased, with the peak energy following the Varshini’s empirical relation.27 The Varshini’s empirical formula successfully describes the temperature dependence of FX peak positions.27

E(T) ) E(0) - RT2/(β + T)

(1)

where R and β are constants. Given the binding energy of the FX in ZnO being nearly independent of temperature, E(0), R, and β are 3.3723 eV, 5 × 10-3 eV/K, and 2869 ( 60 K, respectively. The dashed line in Figure 10b represents the fitting curve for the data. It is clear from the figure that Varshini’s equation can give sufficient prediction of the position of the free exciton peak as a function of temperature. The fitting value of E(0) is in good agreement with the FX peak positions obtained in high-quality ZnO samples.27 3.3. Formation Process of As-Grown ZnO Nanostructures. Because no metal catalyst was used to synthesize ZnO nanostructures and after the growth no metal particles or any other type of impurities were found on the grown products, as seen from the TEM and SEM images, hence, one can predict that the conventional vapor-liquid-solid (VLS) model, in which metal catalyst particles are always present at one end of the nanostructures to act as the energetically favored site for adsorption of gas phase reactants,28,29 does not work for the growth of these nanostructures. In a typical reaction process, as the source material, metallic zinc powder was heated at 450 °C under the mixed flow of nitrogen and hydrogen, as a pretreatment step; metallic was zinc melted (the melting point of zinc ) 419.5 °C), evaporated, and transported by the flowed nitrogen and deposited onto the substrates in the form of zinc clusters, which again melt and convert into zinc vapors at the temperature more above the melting point of zinc. After pretreatment, when the furnace temperature raises to 550 °C with the introduction of oxygen and nitrogen gases, the deposited zinc clusters and zinc vapors reacted with the injected oxygen and started to be oxidized, arranged in proper cation-anion coordination and forming the ZnO vapors via a simple chemical reaction: Zn(g) + O2(g) f ZnO(g). These formed ZnO vapors

Figure 10. Distribution of zinc vapor inside the tube furnace: (a) simulated result for zinc distribution by using a computational fluid dynamics (CFD) code and (b) schematic graph for the distribution of zinc.

then condensed and nucleated in the form of ZnO nanocrystals on to the whole substrates surfaces. The newly coming molecules (zinc atoms and oxygen) from the continuous supply of the reactants may start to deposit onto the previously formed ZnO nanocrystals, which leads to the growth of various nanostructure. Moreover, the zinc and oxygen molecules arranged in such a manner that the lattice fringes of the structures become continuous without any grain boundary and crystal defects, which was confirmed by the high-resolution TEM images of the as-synthesized structures. According to the crystal habits of ZnO, it shows a faster growth rate in the [0001] direction as compared to other growth facets,14 and interestingly, it was found that all the nanostructures were grown along the [0001] direction in preference. It was interesting to note that all the as-synthesized nanostructures have larger cap diameters which are due to the fact that the shaft top has longer to absorb the incoming reactants vapor species as compared to the bottom parts, even though the incoming reactant vapor species deposit epitaxially on the shaft and cap of the nanostructures sprout.30 Moreover, the incoming reactant vapor species absorbed onto the bottom pushed up the nanostructures for increasing their lengths.31 In addition to the basic mechanism, it is observed that the experimental parameters such as substrate temperature, distances between source material and substrates, concentration of zinc vapors, zinc partial pressure, concentrations of reactant gases, and choices of substrates also play an important role for the growth of different kinds of nanostructures in a single reactor. Our experimental results show that particular type of nanostructures can be obtained in a specific temperature zone and it may be possible to tailor a specific type of ZnO nanostructure by the tuning of the reaction temperature.32–34 Regarding the distance between the substrates and source material, it was observed that if the distance between the source materials and substrates increases the vapor pressure and the concentration of the zinc decreases.35 To verify this, we performed simulation studies by using a computational fluid

UV-Emitting ZnO Nanostructures on Steel Alloy Substrate

dynamics (CFD) code (Figure 10) and found that with increasing the distance between the source material and substrate, the concentration of the zinc vapors decreases, which affect the morphologies of the synthesized products. It is reported that at high-temperature and zinc vapor pressure, the nanostructures with long lengths will be obtained.36 Interestingly, this phenomenon was also observed in our structures, i.e., at position “A”, high-aspect-ratio small-head nanonail-shaped nanowires were formed, whereas with increasing the distance of the substrates from the source material, the lengths of the nanostructures shorten (Figures 2–5). Furthermore, except the abovementioned parameters, i.e., substrate temperatures, distances between substrate and source material, partial pressure, and concentrations of zinc, the choice of substrates also has an impact on the morphologies of deposited structures and specific structures can be obtained on a particular substrate in our experiments. For comparison, by keeping constant all other reaction parameters, we carried out some experiments using different substrates, i.e., silicon, aluminum, and steel alloy, and it was found that at position “B”, nanonails type of structures can be grown only on the steel alloy substrates, whereas hexagonal-shaped nanowires and nanorods are obtained on silicon and aluminum substrates, respectively at position “B” (see the Supporting Information, Figure 2).12 More experiments are underway to understand the formation of different kinds of nanostructures by varying the substrates and will be reported in the next article. 4. Conclusions In summary, we have successfully synthesized a variety of ZnO nanostructures, for instance, urchinlike structures composed of small-head nanonails, vertically aligned nanonails and nanocones, comblike nanostructures, hierarchical nanostructures, etc., without the use of any metal catalyst or additive, simply by thermal evaporation of metallic zinc powder in thr presence of oxygen at the temperature range between 550 and 5440 °C. It was observed by various experiments that a particular kind of nanostructure can be grown at specific reaction conditions. The detailed structural studies confirmed the single-crystallinity with a hexagonal wurtzite phase and preferential growth along the [0001] direction for all the deposited ZnO nanostructures. The appearance of high-intensity, sharp, and dominant E2 mode in the Raman-scattering spectra confirmed that the as-grown ZnO nanostructures are good in crystal quality with the wurtzite hexagonal structure. Strong near band edge emission without any visible emission has been observed from all the as-grown ZnO nanostructures in room-temperature and low-temperature photoluminescence spectra. The exciton emission and multiplephonon replicas in the ultraviolet region have been identified. Moreover, it has been found that the Varshini’s expression can predict adequately the FX peak positions at different temperatures. Therefore, because of simple, cheap, catalyst-free, lowtemperature growth and good crystal and excellent optical properties, the as-synthesized ZnO nanostructures are promising for the fabrication of efficient ultraviolet nano-optoelectronic devices in the near future. Acknowledgment. This work was supported in part by the Brain Korea 21 project in 2008 and the Korea Research Foundation grant (KRF-2005-005-J07502) (MOEHRD). Authors wish to thank Mr. T. S. Bae and J. C. Lim, KBSI, Jeonju branch, and Mr. Jong-Gyun Kang, Centre for University Research Facility (CURF) for taking good quality FESEM and TEM images, respectively.

Crystal Growth & Design, Vol. 8, No. 8, 2008 2747 Supporting Information Available: Additional figures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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