The crystallization and physical properties of Al-doped ZnO nanoparticles

Applied Surface Science 254 (2008) 5791–5795

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The crystallization and physical properties of Al-doped ZnO nanoparticles K.J. Chen a, T.H. Fang b, F.Y. Hung c,*, L.W. Ji b, S.J. Chang a, S.J. Young a, Y.J. Hsiao d a

Institute of Microelectronics & Department of Electrical Engineering, Center for Micro/Nano Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan b Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan c Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan d Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 January 2008 Received in revised form 25 February 2008 Accepted 14 March 2008 Available online 26 March 2008

Un-doped Al (0–9 at.%) nanoparticles and doped ZnO powders were prepared by the sol–gel method. The nanoparticles were heated at 700–800 8C for 1 h in air and then analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectra and photoluminescence (PL). The results of undoped (ZnO) and Al-doped ZnO (AZO) nanoparticles were also compared to investigate the structural characteristics and physical properties. XRD patterns of AZO powders were similar to those of ZnO powders, indicating that micro-Al ions were substituted for Zn atoms and there were no variations in the structure of the ZnO nanoparticles. From the XRD and SEM data, the grain size of the AZO nanoparticles increased from 34.41 to 40.14 nm when the annealing temperature was increased. The Raman intensity of the AZO nanoparticles (Al = 5 at.%) increased when the annealing temperature was increased. Increasing the degree of crystalline not only reduced the residual stress, but also improved the physical properties of the nanoparticles. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Sol–gel ZnO Crystallization

1. Introduction The applications of ZnO have attracted much attention in recent years. With a wide direct bandgap energy (3.37 eV) and a larger binding energy (60 meV), ZnO is potentially useful in various optoelectronic applications such as optical sensors and light emitters [1,2], etc. In addition, ZnO is also potentially useful in surface acoustic wave (SAW) devices, gas sensing devices and piezoelectric devices [3–7]. In fact, devices containing bulk ZnO, ZnO films, ZnO nanowires and ZnO nanoparticles have all been demonstrated [1–7]. Recently, ZnO have been prepared by sputtering [8], chemical vapor deposition (CVD) [9], molecular beam epitaxy (MBE) [10], spry pyrolysis [11], pulse laser deposition [12] and the sol–gel process [13,14]. Among these methods, the sol–gel process is particularly attractive because of its simplicity and acceptable costs, however the crystalline quality of the ZnO prepared by the sol–gel process might be inferior to the other methods. Notably, the sol–gel processes with an annealing treatment are intimately affect the crystallization and physical properties. Previous literature, Kuo et al. [14] have investigated the optical and electrical properties of sol–gel derived ZnO thin films with a low

* Corresponding author. E-mail address: [email protected] (F.Y. Hung). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.080

annealing temperature. Also, Zhou et al. [15] have studied the effect of annealing temperature on the microstructure, electrical and optical properties of Al-doped ZnO films. Notably, a low annealing temperature cannot improve the crystallization, and the effect of Al-doped concentration is worthy of further investigation. In addition, relevant reports for ZnO doped with metals (Al3+, 3+ In , Ga3+, etc.) indicate that the doping effect increased the optical and electrical properties of the ZnO [16–18]. However, the crystallization at high temperatures has still not been investigated. Furthermore, the sols concentration also affects the crystalline, optical and electrical properties of ZnO. Schuler and Aegerter [19] have investigated the effects of sols concentration on the optical, electrical and structural properties of ZnO: Al coatings. However, higher concentration of sols has not been studied. To understand the effect of high concentration sols with metal dopant and heat treatment on the structural characteristics and physical properties of ZnO nanoparticles, this study doped Al (0– 9 at.%) into ZnO (2 M) nanoparticles by the sol–gel process to investigate the microstructural variations and used different heat treatment conditions to analyze the physical properties of AZO nanoparticles. 2. Experiments In addition to zinc acetate, it is possible to fabricate sol–gel ZnO samples using zinc nitrate [20]. ZnO prepared with zinc acetate

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exhibits a smoother texture, however nanostructured fine grains are often observed when zinc nitrate is used. Bahadur et al. [21] found that ZnO prepared using zinc nitrate shows a rapid and random crystallization compared to that prepared using zinc acetate. So, zinc nitrate was used as the precursor material in the present study. To prepare the un-doped ZnO (ZnO) nanoparticles, 0.1 M zinc nitrate hexahydrate (N2O6Zn6H2O) was synthesized with the stabilizer ethylene glycol (HOCH2CH2OH) in de-ionized water. For the Al-doped ZnO (AZO) nanoparticles, aluminum nitrate 9hydrate (Al(NO3)39H2O) was added into the solution to serve as the aluminum source. The amount of Al(NO3)39H2O was controlled precisely and kept at the concentration of aluminum. Three Al atomic contents of 5 at.%, 7 at.% and 9 at.% were chosen, then stirred in clear solution at 80 8C for 1 h. Meanwhile, citric acid (C6H8O7) was dropped into these solutions slowly until the solutions had a uniform distribution and became transparent. After that, the solutions were placed in an oven at 120 8C until the solvent evaporated and the remainder was pulverized into nanoparticles. Finally, the nanoparticles were put into an oxidizing furnace to perform two heat treatments to understand the effect of annealing temperature (including 700 8C—1 h and 800 8C—1 h). In addition, a differential scanning calorimeter (DSC) and a thermo gravimetric analyzer (TGA) were used to determine thermal properties of the fabricated ZnO and AZO. The structural characteristics were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectra. Also, PL was used to understand the relation between the Al concentration and the optical properties.

3.2. X-ray diffraction and SEM observation Fig. 2(a) shows the XRD patterns of ZnO nanoparticles annealed at 700 and 800 8C for 1 h. Both of the ZnO samples were polycrystalline and correspond to hexagonal structure. With increasing the annealing temperature, the intensity of the major diffraction peaks increased indicating that the crystallization of ZnO nanoparticle was improved at higher temperatures. Notably, this result was similar to that of AZO nanoparticles. After 5 at.%-Al doping, AZO nanoparticles also appeared in the polycrystalline structure and no diffraction peaks of Al2O3 or other impurities were observed (Fig. 2(b)). However, the full-width at halfmaximum (FWHM) on the (0 0 2) peak had a tendency to decrease with increasing the annealing temperature from 700 8C to 800 8C (compare Fig. 2(a) with (b)). In other words, the crystalline degree of AZO was larger than ZnO when the temperature was raised from 700 to 800 8C. In addition, our results and previous reports [22,23] also show that increasing the heat treatment temperature improves the crystallization of ZnO nanoparticles. To understand the crystalline mechanism of AZO, the grain size of the AZO nanoparticles has been estimated from the FWHM of (0 0 2) the diffraction peak using the Scherrer formula [24] d¼

0:9l b cos uB

where l is the X-ray wavelength of 1.54 A˚, u is the Bragg diffraction angle of the (0 0 2) peak, and b is the FWHM of u. The

3. Results and discussion 3.1. TGA and DSC analysis To determine the crystalline conditions, differential scanning colorimetry (DSC) and TGA of ZnO gel nanoparticles were carried out. The specimens were heated from room temperature to 900 8C with an increment of 10 8C/min in air. Fig. 1 shows a combined plot of DSC and TGA. Notably, the TGA data plots the weight loss of the nanoparticles which is found to take place till 520 8C. For the DSC curve (room temperature 400 8C), an exothermic peak and two endothermic peaks are found at 270, 147 and 317 8C, respectively. These peaks are attributed to the evaporation of water and organics. A large exothermic peak is exhibited at 490 8C, due to the crystallization of ZnO. Therefore, the crystallization of ZnO nanoparticles occurred at temperatures over 520 8C. For this reason, this study selected 700 and 800 8C to estimate the crystallization of AZO nanoparticles.

Fig. 1. DTA-TGA curve of ZnO.

Fig. 2. (a) XRD patterns of un-doped ZnO powders sintered at 700 and 800 8C. (b) XRD patterns of 5 at.% AZO powders sintered at 700 and 800 8C.

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average grain size of AZO nanoparticles increased from 34.41 to 40.14 nm when the annealing temperature was increased from 700 to 800 8C. Fig. 3 shows the SEM images of 5 at.% Al-doped ZnO nanoparticles at 700 and 800 8C. The grain size of the AZO nanoparticles increased with increasing the temperature. This result is associated not only with the grains growing more easily when the temperature is higher, but also with the Al dopants. To understand how the content of Al-doped ZnO powders affect related properties, three Al atomic contents of 5 at.%, 7 at.% and 9 at.% were used to perform a heat treatment of 800 8C in air for 1 h. In Fig. 4, we see the major diffraction peaks decreased with increasing the Al concentration, which indicates Al-doping resulted in a decrease in the crystalline quality. Comparing the crystallization of ZnO with AZO, a large amount of Al dopants resulted in lattice disorder, which is associated with the stress generated. Besides the stress problem, the grains grew more easily when Al dopants were incorporated with ZnO.

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Fig. 4. XRD patterns of un-doped and Al-doped ZnO powders with various concentrations. The samples were prepared at 800 8C.

3.3. Raman spectra of ZnO powders The structure of ZnO belongs to the C6V symmetry group, which predicts two A1, two E1, two E2 and two B1 modes [25]. Among these, A1 and E1 modes are polar and split into transverse (TO) and longitudinal optical (LO) phonons, all being Raman and infrared active. E2 modes are only Raman active, and B1 modes are infrared and Raman inactive (silent modes). Fig. 5(a) shows the Raman spectra of ZnO nanoparticles with different Al concentration that were heated at 800 8C in air for 1 h (using a solid-state laser (532 nm)). ZnO nanoparticles showed the usual modes that were observed at 331 cm1 (second-order

Fig. 3. SEM images of the 5 at.% Al-doped ZnO powders sintered at (a) 700 8C and (b) 800 8C.

vibration corresponding to the E2 (high)–E2 (low)), 380 cm1 (A1 (TO)), 409 cm1 (E1 (TO)), 438 cm1 (E2 (high)), 537 cm1 TO + TA(M), 581 cm1 (E1 (LO)), and 658 cm1 E2 (low)–B1 (high), respectively. The results were the same with the previous reports [25–29]. The 331 cm1 mode could be observed by enhancement of Raman active and inactive phonons with lattice symmetry due to disorder-activated Raman scattering (DARS) [30,31]. In addition, the Raman band was at around the M point of the Brillouin zone [32].

Fig. 5. (a) Raman spectra of un-doped and Al-doped ZnO powders with various concentrations. The ZnO powders were prepared at 800 8C. (b) Raman spectra of 5 at.% Al-doped ZnO powders with different annealed temperature.

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The 380 cm1 mode was lifted the degeneracy of infrared active optical phonons into a transverse (TO) branch, as is well known by the Lyddane–Sachs–Teller relation [23]. The 409 cm1 mode is associated with lattice disorder along the c-axis of the ZnO crystal, the 438 cm1 mode corresponds to E2 mode of wurtzite ZnO and a very sharp feature. The 582 cm1 E1 (LO) corresponds to wellresolved Raman peaks due to multiphonon and resonance processes and are related to oxygen deficiency [25,30]. According to the results of Li et al. [31], the 537 cm1 peak is a Zn–C mode, and the 658 cm1 peak was a Zn–CH2 mode, which are associated with the precursor materials in the sol–gel process. Relevant studies [32–34] also show that the 537 cm1 mode being very close to the 540 cm1 mode, could most likely be assigned to TO + TA(M) second order vibration, since the 2B1 (low) mode is around 510–520 cm1, while the contribution of the first-order B1(high) mode is around 560 cm1. In addition, the 658 cm1 mode is very close to the mode near 645–650 cm1, which is likely due to the 101 cm1 E2 (low) + 521 cm1 B1 (high) [32]. Comparing the AZO with the ZnO nanoparticles (Fig. 5(a)), A1 (TO) and E1 (LO) modes are shifted to the high frequency side. With increasing the Al dopant concentration, the intensity of E2 (high)– E2 (low), E2 (high), A1 (TO) and E1 (LO) modes decreased. The 438 and 582 cm1 modes decreased with an increased Al concentration, which contributed to Al ions in the ZnO lattice substituting for Al in the Zn position. The intensity of the 331, 385, 438 and 582 cm1 modes decreased with increasing the Al concentration. Fig. 5(b) shows the room temperature Raman spectra for 5 at.% Al-doped ZnO powders at 700 and 800 8C for 1 h in air. From the results, all peak intensities increased clearly when the temperature was increased. The same tendency was also observed in Al-doped ZnO samples using spray pyrolysis [35], and shows that the crystalline quality of the films had been improved. Notably, the spray pyrolysis [35] is different from the sol–gel method; our present sol–gel method was able to avoid surface point defects at higher annealing temperatures. According to a relevant study [26], an increase in Raman peak intensity indicates that the residual stress had reduced leading to higher crystalline quality. In addition, the reduction of the relative peak-intensity 385 cm1 A1 (TO) to 414 cm1 E1 (TO) at low temperature is associated with lattice disorder along the c-axis of the ZnO crystal [36]. However, the intensity ratio of 385 cm1 A1 (TO) to 414 cm1 E1 (TO) increased from 0.944 to 0.956 with increasing the annealing temperature. On the other hand, the increase of the second-order modes to the first-order modes decrease with increasing annealing temperature, due to the better crystallization of the thin films and this again supports the secondorder nature of the 537 and 658 cm1 mode. 3.4. Optical properties of ZnO powders Photoluminescence (PL) was used to check the optical quality and the possible effects of Al doping. Fig. 6 shows the PL spectra of the AZO nanoparticles using 325 nm UV light from a He-Cd laser at room temperature. In Fig. 6(a), the ZnO and AZO nanoparticles were heat treated at 800 8C for 1 h in air. In this spectrum, the ZnO powders contain a UV emission band at 375 and 386 nm, and a strong and broad green emission band occurs at about 533 nm (inset in Fig. 6(a)). Hsieh et al. [37] showed that the near band emission of 375 nm was caused by the transition from conduction band to valence band. Another UV emission of 386 nm was attributed to the free exciton recombination in ZnO. On the other hand, the strong green emission (533 nm) resulted primarily from intrinsic defects. The intrinsic defects are associated with deep level emissions such as oxygen vacancies or zinc interstitials [38]. Notably, the oxygen vacancies and interstitials were induced by

Fig. 6. PL spectra of ZnO powders (a) with different Al concentration annealed at 800 8C and (b) with 5 at.% Al concentration under various annealing temperatures.

the thermal treatment process and sol–gel method. After Al doping, the UV emission peak position of AZO nanoparticles exhibited a slight blue-shift from 3.21 to 3.26 eV, and the intensity deceased with increasing the Al concentration, which is attributed to an increase in nonradiative recombination. The PL spectrum of 5 at.% Al-doped ZnO nanoparticles is presented in Fig. 6(b). UV emission peaks are observed at 382 and 380 nm, and the full width at the half maximum (FWHM) decreased slightly from 169 to 167 meV with increasing the temperature. The intensity of the UV emission peak increased when the temperature was raised from 700 to 800 8C. This should be one reason why the increased intensity of UV emission is associated with the grain size and crystal orientation [37]. The results are consistent with the XRD and SEM data (Fig. 2(b) and Fig. 3). According to a report by Choi et al. [39], the Zn atoms not only reacted with O atoms easily but also increased the amount of coupled Zn atoms and O atoms at a higher heat treatment temperature. For Raman measurements in AZO nanoparticles with different annealing temperatures, the AZO nanoparticles at 800 8C show the best crystallize quality suggesting that increasing the annealing temperature not only raised the crystalline quality but also improved the optical properties. 4. Conclusion For AZO powders, the crystallized temperature of ZnO nanoparticles is above 520 8C and the grain size increased with an increase in the heat treatment temperature. With increasing the

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Al concentration, the crystalline quality of ZnO degenerated, which is associated with the stress generated which resulted in lattice disorder. For Raman spectroscopy, the 438 and 582 cm1 modes decreased with increasing Al concentration, which is attributed to Al ions bonding with oxygen or Al ions substituting for oxygen deficiency. In addition, the two peaks at 543 and 658 cm1 were assigned as Zn–C and Zn–CH2 modes, respectively. The PL characteristics show that the optical quality degenerated gradually with increasing the Al concentration. 5 at.% Al-doped ZnO powders had the best characteristics and optical properties in this study.

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Acknowledgement

[24]

The authors are grateful to the Chinese National Science Council for its financial support (Contract: NSC 96–2221-E-006–103-MY2; NSC 97–2218-E-006–011).

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