A Comprehensive Optimization of Aluminum ... - Springer Link

Electron. Mater. Lett., 11, No. 6 (2015), pp. 931-937 DOI: 10.1007/s13391-015-5075-0

Review Paper

A Comprehensive Optimization of Aluminum Concentration in ZnO Nanocrystals by Novel Simple Methods Rozita Rouzbahani, Mohammad Hossein Majles Ara,* Babak Efafi, and Seyedeh Soraya Mousavi Nanophotonics Laboratory, Physics Department, Kharazmi University, Tehran, 15719-14911, Iran (received date: 14 February 2015 / accepted date: 16 June 2015 / published date: 10 November 2015)

This study investigated the effect of sol (with respect to zinc acetate) and Al dopant concentrations on ZnO structure. The electrical current of pure ZnO and AZO (ZnO:Al) nanofluids indicated that, with the increase in concentration of sol and Al dopant up to 1 molar and 2 at. %, carrier concentration increased continuously, and then after that, it decreased. The lattice stress and crystal size of powders showed that Al+3 ions were better replaced in ZnO structure with 1 molar concentration at 2 at. % Al. Also, a new second phase (ZnAl2O4) was observed in AZO powder at 2.5 at. % Al by X-ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) analyses. The blue shift in the optical band gap of sols could be explained by the BursteinMoss effect and Brus equation. It was concluded that there were more Al+3 ions in the structure of ZnO in AZO powder with 1 molar concentration at 2 at. % Al dopant. Keywords: ZnO, Al concentration, sol-gel, dopant optimization

1. INTRODUCTION Because of its diverse attractions from the fundamental nature to the practical applications, ZnO has attracted exceptional attention among numerous semiconducting oxide materials. ZnO is a wide direct band gap (3.37 eV) IIVI compound semiconductor with large exciton binding energy (60 meV) at room temperature which is significantly larger than other commonly used materials such as ZnSe (~22 meV) and GaN (~25 meV) for blue-green light emitting devices.[1,2] It usually adopts a hexagonal wurtzite crystal structure.[3] Undoped ZnO has n-type property due to native defects, such as oxygen vacancies and zinc interstitials.[4] It demonstrates many outstanding characteristics due to its good optical quality, abundance in nature, non-toxicity, thermal and chemical stability, low material costs, high transparency in the visible and near infrared spectral region, excellent piezoelectric and semiconducting properties.[3,5] We can control the electrical, optical, magnetic, and

*Corresponding author: [email protected] ©KIM and Springer

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chemical-sensing properties by doping.[3,6] Various dopants have been used so far to increase the free charge carrier concentration and hence the conductivity.[7] For n-type, doping with group-III elements (B, Al, Ga, In), as substitute elements for Zn, has been attempted by many groups. On the other hand, n-type semiconductors may also be synthesized by substituting O-atoms by group-VII elements (F, Cl, I).[3] Among these various dopants, Al is one of the best candidates for doping, because its ionic radius is on par with that of Zn and the Al-O bond length (0.27 nm) is close to ZnO bond length (0.197 nm).[8] It is also highly suitable because of its abundance, easy availability, low cost, high transparency, stability, and high conductivity.[5,9] Un-doped ZnO and doped ZnO have been made by a variety of methods, among which are electro-deposition, aqueous solutions, chemical bath deposition (CBD), chemical vapor deposition (CVD), metal organic CVD (MOCVD), pulsed laser deposition (PLD), molecular beam epitaxy (MBE), RF or DC magnetron sputtering, spray pyrolysis, and atomic layer deposition (ALD).[3] The conventional physical techniques are generally safe (no toxic gas emissions) and have deposition rate at room temperature. However, they are

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usually very costly and are difﬁcult to expand to large scale.[5] For these reasons, the sol-gel process reflects distinct advantages due to its excellent compositional control, homogeneity on the molecular level, simplicity, low cost, good-performance in atmospheric pressure without the need for expensive vacuum equipment, low growth temperature, and the fact that it can be used to deposit films over a large area with a very uniform thickness.[3] Various parameters, including temperature, medium pH, aging period, amount and type of modifier and hydrolysis agent and sol concentration, can change the morphology and crystal structure of final products as well as their band structures.[10] Sol concentration is one of the most important parameters for changing the structural and optical properties, thus we can improve these properties by adjusting sol concentration. There is much research on effect of zinc acetate concentration on ZnO thin films.[11-13] However, these optimal concentrations which were done are still challenging and need to be studied further, so that, our effort has optimized the dopant and molar concentration simultaneously. This optimization will lead to improvement in the performance of devices such as transparent conductive oxide (TCO), light emitting diodes (LED), gas sensors, solar cells and photodiodes.[3] In this study, we used sol-gel method for finding the most suitable atomic content of dopant and sol concentration for AZO structure. We fabricated Al doped ZnO nanoparticles with different molar concentrations (0.5, 1.0, and 1.5 M) at various concentrations of Al dopant (0 - 2.5 at. %). We have investigated the structural, optical, and electrical properties of these AZO nanoparticles.

2. EXPRIMENTAL PROCEDURE 2.1 Synthesis method Pure ZnO and AZO sols were prepared with Zn(CH3COO)2· 2H2O (99.5% Merck) as a precursor, Al(NO3)3·9H2O (99% Merck) as a source of dopant, absolute ethanol (99.9% Merck) as a solvent and triethanolamin (TEA) (99% Merck) as a stabilizer. The concentration of metal ions in the solution was 0.5 - 1.5 mol/L. First, for preparing undoped ZnO, Zinc acetate dihydrate was dissolved in a mixture of ethanol and TEA under magnetic stirring. The molar ratio of TEA to Zinc acetate dihydrate was adjusted to 3:5. After stirring and heating for about 30 minutes, we achieved a colorless, homogeneous and transparent solution. For doping, aluminum nitrate nonahydrate was added to the mixture with atomic percentages of 0 - 3 at. % Al. To prepare the powder, the resulting transparent sols were kept for a few months to complete the gelation and hydrolysis process. During this period of time, white ZnO precipitates were slowly crystallized and settled down in the bottom of the flask. The white precipitates dried at 120°C for 1 h in an oven to vaporize the solvent. After that, they were heated in a furnace at 850°C for 2 h to remove the organic residuals. 2.2 Characterization he electrical current of sols was measured by the setup which is shown in Fig. 1(f). X-ray diffraction analysis (PANalytical PW3050/60 diffractometer) was used to investigate the crystal structure, lattice stress and crystal size of powders. The UV-visible spectra were measured by a UV-

Fig. 1. (a) Representation of ZnO wurtzite crystal structure substitutional doping (b) interstitial doping of Al+3. (c) The top view of crystal structure of pure ZnO, (d) ZnO with substitutional doping, and (e) interstitial doping of Al+3. (f) Schematic illustration of measuring current electrical.

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visible spectrophotometer (Array/Electron Pishro) to study the optical treatment of sols. Chemical compound of the obtained powders were inspected by a Fourier Transform Infrared spectroscope (Perkin-Elmer/FT-IR). Scanning electron microscopy (Mira Tescan) was employed to study the morphology and size of nanoparticles.

3. RESULTS AND DISCUSSION By adding Al+3 ions to the ZnO structure, they can be placed in two positions. In the first position, they can occupy interstitial octahedrally coordinated sites and act like defects; but ideally, in the second position, they should substitute Zn+2 ions in order to provide a free electron (charge carrier) which increases the electronic properties of the ZnO host material. The interstitial Al defects are expected to appear when the solubility limit is overcome and excessive Al+3 ions form nonconductive Al2O3 or ZnAl2O4 phases, acting as traps for charge carriers. These two conditions are shown in Fig. 1(a) and (b). Also, the presence of lattice strain and change in grain size in two anticipated conditions (substitution and interstitial Al) are shown in Fig. 1(c), (d) and (e). The causes of these changes will be discussed later. To investigate the position of Al+3 ion in the structure of ZnO, the electrical current of sols with different molar concentrations (0.5 - 1.5 M) at different Al dopant concentrations (0 - 3 at. %) was measured. In this measurement, 3 mL nanofluid was injected between the two electrodes which had fixed spacing. When a constant voltage (20 V) was applied to the electrodes, there was a current (I) flowing through the nanofluid, which could depend on the free electrons generated by substituting Al+3 for Zn+2 ions in the structure of ZnO.[14,15] The schematic setup is shown in Fig. 1(f). Figure 2 shows the electrical current of the nanofluid with different molar concentrations (0.5 - 1.5 M) at different Al dopant concentrations (0 - 3 at. %). Electrical current of the base fluid (ethanol + TEA) used in the present study varied from 1.1 to 2.1 mA. It can be observed that the electrical current of the base fluid with different molar concentrations was much less than that of ZnO nanofluid. Figure 2 shows that, with increasing Al dopant concentration to 2 at. %, electrical current increased. Due to the use of precursor salt as the source of ZnO and Al dopant, it was expected that, with increasing the concentration of precursor and thus generating more ions in sols, the electrical current would be greater. But an unforeseen phenomenon can be observed in Fig. 2. These increased electrical current values might have been the results of an increase in carrier concentration due to the substitution of Al+3 ions at Zn+2 ion sites and generated free electrons. However, with the increase in the Al doping concentration to above 2 at. %, the electrical current started to decrease. In general, the grain boundary produced a large

Fig. 2. Electrical current of ZnO nanofluid with different molar concentration (0.5-1.5M) at different Al dopant concentration (0 3 at. %).

effect on the electrical properties. The grain boundary increased with decreasing crystallization, and the electrical current reduced due to the decrease in crystallization, which decreased mobility.[16] This drastic reduction in electrical current beyond the doping concentration of 2 at. % Al was because of the segregation of dopant atoms in noncrystalline regions, producing disturbances in the network. The decreased electrical current could be attributed to the present interstitial Al defects which might have been due to solubility limitations. It means that the extra aluminum atoms might not occupy the correct places inside the ZnO lattice and distort the crystal structure (Fig. 1(b)), which may negatively affect the electronic mobility.[17] Figure 2 shows that AZO (2 at. %) nanofluid with 1 molar concentration had the highest electrical current among the other molar concentrations. The XRD patterns of pure ZnO and AZO powders with 1 molar concentration at different Al doping concentrations are shown in Fig. 3. All of the un-doped and Al-doped ZnO powders have a hexagonal wurtzite crystal structure.[18] The intensity of major peaks (100), (002), and (101) begins to decrease with increasing Al doping concentration to above 2 at. % Al, indicating that an excess increase in doping concentration deteriorates the crystallinity of powders, which may be due to the formation of stresses.[18] To confirm the relationship between crystallinity and Al doping concentration, we investigated all the changes in the position of diffraction angles. Figure 4(a) shows the peak position of pure ZnO and AZO powders with 1 molar concentration at different Al doping concentrations. First, by adding Al dopant to ZnO, the diffraction angles were shifted to the higher values, which in turn meant that the crystal structure had to be under stress;[19] then, with increasing Al


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Fig. 3. (a) XRD patterns of pure ZnO and AZO powders with 1 molar concentration at different Al doping concentration. (b) and (c) Inset show zoomed-in version of data indicating peaks from ZnAl2O4.

dopant from 2 to 2.5 at. %, the diffraction angles were shifted to lower values than 2 at. % Al, indicating that, with increasing Al dopant concentration to above 2 at. %, the number of Al+3 ions which could substitute Zn+2 ions was lower; therefore, most of the Al+3 ions occupied interstitial sites.[20] To study the effect of molar concentration, the peak position of AZO (2 at. %) powders with 1 and 1.5 molar concentrations was compared, as shown in Fig. 4(b). We can see that AZO (2 at. %) powder with 1 molar concentration was under more stress than AZO (2 at. %) powder with 1.5 molar concentration, which could be caused by substitution of more Al+3 ions in the crystal structure.[19] On the other hand, more aluminum atoms were replaced at Zn vacancies (VZn) in the structure of ZnO with 1 molar concentration; as a result, there were more VZn in this structure than the structure of ZnO with 1.5 molar concentration. Although VZn are native defects in the structure of pure ZnO, they are useful for AZO structures, because they can improve the chance of Al+3 ions for substitution in the structures of ZnO. Using Scherrer equation,[21] we can see in Table 1, increasing Al dopant concentration led to decrease of the crystal size of AZO powder with 1 molar concentration up to 2 at. % Al. This result was expected due to the difference in

Fig. 4. Peak position of pure ZnO and AZO powders (a) with 1 molar concentration at different Al doping concentration, and (b) with 1 molar and 1.5 molar concentration at 2 at. % Al. Table 1. Crystal size of pure ZnO and AZO powder with 1 molar concentration at different Al dopant. Al at. %

FWHM°

Crystal Size (nm)

0

0.079

121.6

2

0.137

66.0

2.5

0.088

95.7

radius between Al+3 (0.054 nm) and Zn+2 (0.074 nm) ions.[18] When Al dopant concentration exceeded 2 at. %, the crystal size began to increase, indicating again that there were fewer Al+3 ions in the structure of ZnO in AZO powder with 2.5 at. % Al. Also, interstitial Al defects were expected to form a secondary phase like ZnAl2O4. As can be seen in Fig. 3(b) and (c), when doping exceeded 2 at. % of Al, two new weak peaks were observed around ~31 and 37 deg. of 2θ scale, revealing the formation of ZnAl2O4 phase.[22-25] These results suggested that the solubility was limited to 2 at. % of Al in ZnO with 1 molar concentration. UV-visible absorption spectra of the prepared pure ZnO and AZO sols with 1 molar concentration at different Al dopant concentrations are shown in Fig. 5(a). The absorbance spectra of the sols suggested that they had high UV absorbance properties at the wavelength of below 400 nm. The absorption curve of the samples exhibited a strong absorption peak (λp) around 290 nm. It is clear that the



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Theoretically, the increase of carrier concentration in degenerated semiconductors causes these two opposite effects. The band gap of the AZO sols showed a blue-shift in comparison with the pure ZnO sols. The band gap widening (BGW) was explained by Burstein-Moss effect (BM effect), whereby the conduction band becomes significantly filled at high doping concentration and the lowest energy states in the conduction band are blocked. According to BM effect, the optical band gap widening is:[26-28] 2

Fig. 5. Optical absorption spectra from as-synthesized Al-doped ZnO nanocrystal solutions showing the shift in absorption band edge with (a) Al-doping, and (b) molar concentration.

absorption edge systematically shifted to the different wavelengths with adding Al dopant. These results can be also seen in Fig. 5(b), which shows the characteristic spectra of AZO (2 at. %) sols with 1 and 1.5 molar concentrations. The optical band gap can be determined by the extrapolation of the linear region from α2 versus hν plot near the onset of the absorption edge to the energy axis. Figure 6(a) shows the plot of (αhν)2 versus photon energy of pure ZnO and AZO sols with 1 molar concentration at different Al dopant concentrations. It is well known that band gap widening and narrowing occur in heavily doped semiconductors.

2/3

2

2

h 1 1 1.8e (2) ≈ --------2 ⎛ ------ – ------⎞ – ------------------⎝ ⎠ m m 4πε e h 0 εr R 8R bulk where Eg is the bulk energy band gap (3.3 eV), h is Plank’s constant, e is the charge of electron, R is the radius of quantum dot, εr is the dielectric constant of ZnO, and me and mh are the effective mass of electron and hole, respectively where me ~ 0.26m0, mh ~ 0.59m0 and εr ~ 8.5 for ZnO. The results showed tendency to a decline in the calculated size of nanoparticles with adding Al dopant concentration up to 2 at. % and an increase of Al dopant from 2 to 2.5 at. % resulting in the growth of the calculated size. This issue was caused by substituting more Al+3 at Zn+2 ions in the sol with 2 at. % Al than the one with 2.5 at. % Al. According to the quantum confinement theory, the energy band gap of semiconductor depends on the crystallite size and its value will increase when there is a decrease in the crystallite size.[30] We also made this comparison for AZO sols with different molar concentrations. Figure 6(b) shows the plot of (αhν)2 versus nano

Eg

Fig. 6. The plot of (αhν)2 vs. photon energy of pure ZnO and AZO sols (a) with 1 molar concentration at different Al dopant, and (b) with 1 molar and 1.5 molar concentration at 2 at. % Al.

2

h ( 3π n) BM Δg = -------------------------(1) 2 * 8π m BM where Δg is the blue shift of optical band gap, h is Plank’s constant, n is the carrier concentration, and m* is electron effective mass in the conduction band. Since Al dopant concentration increased up to 2 at. %, the optical band gap energy and therefore the carrier concentration in the conduction band increased. The increase in the optical band gap up to 2 at. % Al was due to the fact that Al ions tended to occupy ZnO lattice planes, leading to an increase in the transport path of charge carriers into ZnO lattice, as confirmed by electrical parameters above. Furthermore, when Al doped ZnO, donor electrons were formed at the bottom of the conduction band. The doubly occupied states were prevented by Pauli principle; therefore, the valence electrons were excited to higher energy levels in the conduction band with the required extra energy.[29] The decrease of band gap for the sols with Al concentration of higher than 2 at. % was consistent with the decrease of carrier concentration. As a result, we ensure that aluminum was successfully found in the ZnO structure up to 2 at. %. Also, the changes in the value of optical band gap were related to the size of nanoparticles. Using the effective-mass model (Brus equation), the band gap Eg (eV) can be approximately written as:[19] bulk

– Eg


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for all of the nanomaterials that were assigned to the formation of metal aluminates,[31] which might be an indication for the presence of Al-O in the AZO samples. In all of the powders, a strong vibration appearing at 481 - 512 cm−1 indicated the presence of Zn-O.[32] As can be seen in Fig. 7, in AZO powder at 2.5 at. % Al, an additional peak was seen, which was probably related to the formation of the second phase seen in XRD analysis.[33,34] To investigate the morphologies of the synthesized AZO nanostructures, the morphology of the products was observed by scanning electron microscope (SEM). Based on the SEM image (Fig. 8), the average grain size of the powder was around 50 nm and its spherical shape morphology was confirmed.

4. CONCLUSIONS Fig. 7. FTIR spectra of pure ZnO and AZO powders containing varying degrees of Al doping.

Fig. 8. SEM image of AZO powder.

photon energy of AZO (2 at. %) sols with 1 and 1.5 molar concentrations. Using Equation (2), the size of nanoparticles in AZO sol with 1 molar concentration was smaller than AZO sol with 1.5 molar concentration. These results confirmed the results obtained in XRD analysis. In order to investigate the presence of chemical compound in the obtained materials, the powders were analyzed by FTIR analysis. Figure 7 gives the FTIR spectra of pure ZnO and AZO powders with 1 molar concentration at different Al dopant concentrations. All of the samples had a broad band around ~3400 cm−1, which was assigned to the stretching and bending vibrations of surface -OH groups emitted from ethanol. The doped samples showed a peak around ~700 cm−1

In conclusion, pure ZnO and AZO powders with different molar concentrations (0.5, 1, and 1.5 M) at different Al dopant concentrations (0 - 2.5 at. %) were successfully prepared by the cost-effective sol-gel method, and their electrical, structural, optical, and morphological properties were investigated. Measurement of electrical current passing through the sols with different molar concentrations (0.5, 1, and 1.5 M) at different Al dopant concentrations (0 - 3 at. %) showed that the sol with 1 molar concentration at 2 at. % Al had the highest electrical current meaning that there were more free electrons in this sol. This result was according to the use of precursor salt, reduce the electrical current with increasing the concentration of precursor (1.5 M at 2.5 at. % Al) was very interesting. This issue was due to the substitution of more Al+3 ions and generation of more free charges. The XRD spectra confirmed that the powders were hexagonal in structure. By investigating the peak intensity, lattice stress, and crystal size of the powders, it was concluded that there were more Al+3 ions in the structure of ZnO in AZO powder with 1 molar concentration at 2 at. % Al dopant. Also, this result was shown in optical band gap by Burstein-Moss effect and Brus equation. The second phase (ZnAl2O4) was observed in AZO powder at 2.5 at. % Al by XRD and FTIR analyses. The SEM image showed that the grain size of the powders was about 50 nm and their morphologies were spherical. Experimental results also demonstrated that a solid solubility limit of aluminum ions in the prepared zinc oxide nanocrystals was achieved at 2 at. % Al with 1 molar concentration. It seems that this AZO sol is optimal for use in electronic and optoelectronic applications.

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