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Effect of Aluminum Doping on the Growth and Optical and Electrical Properties of ZnO Nanorods [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Aluminum-doped zinc oxide (AZO) nanorods were successfully prepared by a convenient solvothermal route. The crystal structure and morphology of AZO were characterized by XRD, SEM, and high-resolution TEM. The length and diameter of AZO nanorods decreased with increasing Al content. The opti- cal and electrical properties of AZO were studied by UV/Vis spectroscopy and a four-point probe. The optical band gap of AZO increased initially because of the Burstein-Moss effect and then decreased as the Al content increased owing to the defects of AZO. The electrical resistivity of AZO nanorods varied conversely because of the change of electron and defect concentration (native and impurity defects). The native defect types, which were singly charged zinc and oxygen va- cancies, were confirmed by photoluminescence spectroscopy. Moreover, not only the properties but also the growth mecha- nisms of AZO nanorods were affected by the defect concentra- tions of singly charged zinc vacancies and substituted Al, which were caused by increasing Al content. Finally, the AZO exhibited the smallest electrical resistivity with 1.5 at. % Al doping content, which was four orders of magnitude smaller than that of ZnO. Keywords : conducting materials · defects · doping · growth mechanism · nanostructures (ProQuest: ... denotes formulae omitted.) Introduction One-dimensional (1D) nanostructures have attracted considera- ble attention owing to their unique properties and potential applications in photonic and electronic devices.[1] 1D nanos- tructures are two-dimensional confinement structures, and the one unconstrained dimension can direct the conduction of electrons and photons.[2] They are thus ideal materials to man- ufacture advanced solid-state devices, such as gas sensors, polymer-based solar cells, smart windows, photodetectors, bio- sensors, supercapacitors, light-emitting diodes, field emitters, and field-effect transistors.[3] Among the 1D nanostructure systems, zinc oxide is consid- ered as a multifunctional material owing to its direct band gap of 3.3 eV and a large exciton binding energy of 60 meV at room temperature.[4] 1D ZnO nanostructures have triggered a lot of excitement and allowed breakthrough achievements in all areas of electronics.[5] For instance, Wang et al. converted the nanoscale mechanical energy into electrical energy by means of piezoelectric ZnO nanowire arrays.[6] They also report- ed an integrated system of a self-powered, ultrasensitive, single-nanowire-based multicolor photodetector driven by a high-efficiency microscale microbial fuel cell that was fabri- cated using a carbon fiber-ZnO nanowire hybridized struc- ture.[7] Yu et al. reported a self-powered, ultrafast, visible-blind UV detection and optical logic operation based on ZnO/GaN nanoscale p-n junctions.[8] However, 1D ZnO nanostructures still have drawbacks. For example, the optical and electrical properties of pure ZnO are unstable in air owing to corrosion by oxygen, carbon dioxide, hydrocarbons, sulfur compounds, and water.[9] In addition, the resistivity of pure ZnO is too high for some electronic devices. Hence, ZnO has been doped with different ions to enhance its mechanical, electrical, and optical properties.[4,10] Aluminum-doped zinc oxide (AZO) is a particularly attractive material in transparent conducting oxide applications. This is because of its excellent properties, such as high thermal stabili- ty, good resistance against damage by reducing atmosphere, and low cost of fabrication (better than indium tin oxide).[11] Nevertheless, most works have focused on the preparation of AZO films by different methods, including the sol-gel tech- nique, magnetron sputtering, pulsed laser deposition, chemical vapor deposition, spray pyrolysis, and thermal evaporation.[12] For example, Hagendorfer et al. used metallic Al as a continu- ous and controllable doping source to grow transparent and conductive AZO films by using a low-temperature aqueous so- lution approach.[13] There are a few reports concerning the syn- thesis of AZO powders with different nanostructures.[9, 14] Wang et al. synthesized AZO nanofibers by an electrospinning method and investigated the ethanol sensing properties of the sensor based on these nanofibers.[15] Milliron et al. grew AZO nanocrystals from zinc stearate and aluminum acetylacetonate in organic solvent under an argon atmosphere.[16] Generally, 2- methoxyethanol was usually used as the organic solvent and ethanolamine was used as sol-gel stabilizer.[12a,b] They are toxic organic additives, which are identified as a known or anticipat- ed carcinogen by the National Toxicology Program.[17] The syn- thesis of AZO nanostructures by a facile route without surfac- tant or toxic organic additive still remains of great significance. ZnO has a very rich defect chemistry that has been studied for more than 40 years. Defects associated with charge carriers are of paramount importance in applications for doped ZnO.[18] There are many reports on undoped and doped ZnO defects generated during growth and post-growth conditions (under different partial pressures of oxygen and zinc).[10b, 19] Also, ZnO crystals are rather sensitive to the dopant in growth. The dopant could change the growth kinetics, habit, color, and var- ious physical properties of the crystals over a wide range.[20] Therefore, the defects caused by dopant play a stronger role in controlling the crystal growth and properties. It is important to understand the defect type and effect in doped ZnO. There are few reports, however, on the relationship between the defects, growth mechanism, and properties of AZO nanorods synthe- sized by solvothermal methods. Herein, we demonstrate the synthesis of AZO nanorods by a convenient solvothermal route, in which zinc acetate dihy- drate and aluminum nitrate nonahydrate are selected as Zn source and Al source, respectively, and no capping agents are required. Modifications of the crystal structure, chemical makeup, optical properties, and electrical conductivity brought by doping of these nanorods have been studied systematically. The relationship between the defects, growth mechanism, and properties of ZnO and AZO nanorods are presented and illus- trated. Results and Discussion X-ray diffraction study The X-ray diffraction (XRD) patterns of ZnO and AZO nanorods are shown in Figure 1 a. The XRD pattern clearly shows the single-phase formation of ZnO without any observable impuri- ty phase (such as Al2O3). All the peaks in the XRD patterns can be indexed to wurtzite ZnO (JCPDS card No. 36-1451). Also, the sharp shape and narrow line widths of the diffraction peaks indicate that the samples are highly crystalline. The Rietveld method was used to analyze the XRD patterns for AZOs with initial Al content varying from 0 to 4.0 at. %. The calculated a and c lattice constants are summarized in Table 1. For the ZnO nanorods, the lattice parameters are 3.2497 and 5.2068 ^, which agree well with the reference values of 3.2498 and 5.2066 ^ (JCPDS card No. 36-1451). As shown in Figure 1 b (open squares), the c lattice constant of AZO decreases with in- creasing Al doping concentration. This lattice shrinkage may be caused by the smaller Al3 + (radius 0.53 ^) replacing the larger Zn2 + (radius 0.74 ^),[10] which indicates that Al atoms in the AZO are substituted into the Zn sites. Because the (001) planes only consist of zinc atoms, whereas the (001- ) surfaces only consist of oxygen atoms in the wurtzite structure,[5b] the c lattice constant of ZnO will be changed by another substitu- tion atom. The c/a lattice ratio for AZO versus different initial Al con- centrations is plotted in Figure 1 b (filled squares). The c/a lat- tice ratio decreases when the Al content is varied from 0 to 2.0 at. %, and is almost constant with Al contents higher than 2.0 at. %. Based on previous research, ZnO crystals have a top tetrahedron corner-exposed polar zinc (001) face with positive charge, and a basal polar oxygen (001- ) face with negative charge.[6] ZnO crystals grow preferentially along the [001] direc- tion to form rodlike structures because of the different polar growth of (001- ) and (001) faces.[21] When Al dopes the Zn site, it forms a singly charged zinc vacancy (VZ^n) defect (which will be discussed below). This defect reduces the polar growth of the (001) plane. The anisotropy of ZnO decreases with increas- ing Al content. This is the reason that the c/a lattice ratio de- creases initially. It is impossible that the (001) plane has nega- tive charge. It has an equilibrium value of the two polar planes. This is why the c/a lattice ratio is almost constant as the Al content varies from 2.0 to 4.0 at. %. If this hypothesis is true, the length/diameter ratio of nanorods will decrease with increasing Al content, and the photoluminescence (PL) count ratio will have a similar change rule. The scanning electron mi- croscopy (SEM) and PL results agree well with those predic- tions (see the detailed experimental results). When the Al con- tent is lower than 2.0 at. %, the length/diameter ratio of the nanorods is decreased. Then the length/diameter ratio is almost constant with Al content. Morphology analysis Figure 2 shows the SEM images of ZnO and AZO with different Al contents synthesized by the solvothermal method. It is found that the morphologies of all the samples are nanorods and rodlike nanoparticles. As shown in Figure 2 a, the ZnO nanorods are 400 nm in length and 40 nm in diameter. The length and diameter of the AZO nanorods decrease rapidly with the increase of Al content. The length decreases from 300 to 100 nm and the diameter decreases from 30 to 18 nm as the Al content increases from 0.5 to 2.0 at. % (Figure 2b-e). As the Al content increases further, the length and diameter of the AZO nanorods decrease slightly. The length changes from 100 to 80 nm and the diameter changes from 18 to 15 nm (Figure 2 e-g). All the lengths and diameters of the nanorods are summarized in Table S2 in the Supporting Information. Not only the size but also the shape anisotropy of AZO decrease initially then keep almost constant with increasing Al content. These changes can be partly attributed to the effect of the Al3 + dopant ion. On the one hand, the lattice shrinks when the Al3 + ion replaces the Zn2 + ion, and the c/a lattice ratio de- creases (as described above and shown in Figure 1 b). The shape anisotropy of AZO reduces.[16] On the other hand, one free electron is released when an Al3 + ion replaces the Zn2 + ion (described below).[12a] The free electron increases the elec- tron charge density of the crystal surface, which makes it more difficult for the negatively charge ZnðOHÞ42^ ions to diffuse to the surface because of charge repulsion. This will be discussed later in detail. Hence, the growth rate of AZO is slow, and leads to the reduction of AZO nanocrystal size.[22] To evaluate dopant incorporation, the Al contents of AZO were characterized by energy-dispersive X-ray (EDX) spectros- copy. Figure 2 h shows the EDX result of the AZO at Al/Zn = 1.5 at. % in the original material: the Al content of the AZO is about 1.28 at. %. The nominal doping concentration and the doping concentration measured with EDX spectroscopy are listed in Table 2. The Al contents of AZO are quite close to that of the original material, thus indicating that Al is mostly incor- porated into the ZnO matrix during the synthesis process. As shown in Figure 3, we further studied the crystal struc- ture of pure ZnO and AZO with 1.5 at. % Al content by using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Figure 3 a shows that pure ZnO presents a nano- rod structure approximately 400 nm in length and about 40 nm in diameter. AZO nanorods are approximately 140 nm in length and about 23 nm in diameter (Figure 3 c). The results are consistent with the size observed by SEM. The growth fea- ture was revealed by HRTEM. As shown in Figure 3 b, the inter- planar spacing of 0.521 nm in ZnO nanorods corresponds to the (001) lattice plane of wurtzite ZnO, which agrees with the XRD data (0.52068 nm) in Table 1. This indicates that the growth direction of ZnO nanorods is along the [001] direction (arrow in Figure 3 b), which is also corroborated by the fast Fourier transform (FFT) pattern (Figure 3 b, inset). The HRTEM analysis also implies that the nanorods are hexagonal phase ZnO (JCPDS card No. 36-1451). In Figure 3 d, the interplanar spacings of AZO nanorods are 0.279 and 0.239 nm, corre- sponding to the separation between (100) and (101) lattice planes of wurtzite ZnO, respectively. The angle between the two planes is 1508. Combination of the [100] and [101] direc- tions in the corresponding FFT pattern (Figure 3 d, inset) yields a growth axis along the [001] direction, which agrees with the growth direction indicated in the HRTEM image in which the interplanar spacing of the (001) face is 0.518 nm. This interpla- nar spacing is also consistent with the XRD data (5.2056 ^). Clearly, not only the ZnO nanorods but also the AZO nanorods grow along c axes (i.e. , [001]), and the c lattice constant of AZO is smaller than that of ZnO. This further confirms that Al atoms are substituted into the Zn site, which causes lattice shrinkage. The rodlike nanoparticles existing in the samples are also monocrystalline ZnO or AZO and possess the same crystal structures, and thus there is no phase separation be- tween them (Figure S1). Growth mechanism The growth kinetics and crystallization of different nanostruc- tured ZnO materials have been widely studied and many growth mechanisms, such as Ostwald ripening, growth modifi- cation, and oriented attachment, have been proposed.[20, 21, 23] These theories do not apply to the growth of AZO nanorods. For example, no oriented attachment of nanoparticles has been found.[23b] Taking into account all the aforementioned ele- ments, the growth mechanism of AZO nanorods is related to both its intrinsic crystal structure and defects caused by Al doping. It is generally recognized that ZnO is a polar crystal that has a (001) face with positive charge and (001- ) face with negative charge. The overall shape and aspect ratio of crystals depended on the relative rates of growth of the various faces. The growth rates of the faces of wurtzite ZnO are in the fol- lowing order:[23a] Vð001Þ> Vð011^Þ> Vð010Þ> Vð011Þ> Vð001^Þ. The growth rate is very likely the dominant factor for the nanorod growth along [001] . If zinc acetate and sodium hy- droxide are dissolved in absolute alcohol under solvothermal conditions [Eq. (1)]: ... (1) the ZnðOHÞ24^ ion is responsible for the polar growth of ZnO crystals. On the (001) face with supplies as planar Zn2 + nets, the reaction will occur as [Eq. (2)]: ... (2) On the (001- ) face with supplies as planar O2^ nets, the reaction will occur as [Eq. (3)]: ... (3) From Equation (2), the charge can influence the growth rate of the (001) face. For Al-doped ZnO, the electron and VZ^n are oppositely charged to the substitutional defect AlZþn (this will be discussed below) [Eq. (4)]: ... (4) The electron and zinc vacancy with one negative charge result in an increase of the negative charge of the (001) face and, therefore, lower the growth rate as compared to other faces. The closer the growth rate of the different faces, the lower the anisotropy of AZO. Therefore, the morphology of AZO changes from 1D nanorod to rodlike particle with increas- ing Al doping. Nevertheless, the defects cannot increase infin- itely as Al content increases. They should have an equilibrium value. So, the size aspect ratio and axial ratio will decrease to a constant with increasing Al content. These phenomena have been demonstrated by our experiments. The growth mecha- nisms for the ZnO and AZO nanorods are illustrated schemati- cally in Figure 4. Optical properties The band gap (Eg) is an important parameter for optoelectronic applications. The efficacy of doping toward the generation of free carriers can be characterized by the optical properties of AZO.[24] The band gap can also be calculated by using UV/Vis- near-infrared (NIR) spectrophotometry. Figure 5 a shows the UV/Vis spectra of ZnO and AZO thin films prepared by drop- ping a powder/ethanol dispersion onto glass slides. The ZnO and AZO nanorods reveal a consistent blueshift as the doping concentration increases from 0 to 1.5 at. % and a redshift from 2.0 to 4.0 at.% relative to that of 1.5 at. % AZO. The optical band gap calculated from the absorption data (Table S1) agreed with the result of the Tauc's plot (Ahu)2 as a function of photon energy (hu) (Figure 5 b). The optical band gap does not increase monotonically with the increase of Al content, but achieves a maximum value of 3.528 eV at 1.5 at. % and then a decrease from 2.0 to 4.0 at. % (Figure 5 c). This result is in line with the variation of DETotal upon doping (Figure 5 d). Based on the previous research, not only the quantum confinement (QC) effect[25] but also the Burstein-Moss (BM) effect[24,26] could induce the increase of optical band gap. Now the question is whether this blueshift is attributable to a BM-type shift in AZO nanorods, or is it partially due to the QC effect caused by the incorporation of the Al3+ ion ? On the basis of quantum confinement theory, the optical band gap of semiconductors is determined by the crystal size, that is, it increases as the crystal size decreases.[27] The blueshift caused by the QC effect increases from 0.002 to 0.022 eV when the effective diameter of the nanorods decreases from 48.04 to 14.61 nm (Table S2), whereas the broadening of the optical band gap to the nanorods increases from 0.087 to 0.199 eV and then decreases from 0.199 to 0.059 eV. In Figure 5 d, the QC-type shifts increase monotonically with the Al content, but do not fit to the rule of blueshift (DETotal). The DETotal has a higher value than that of the QC-type shift. So its change rule can not only be explained by a confinement effect. The result suggests that the Al doping affects the optical band gap, in what is believed to contain a BM-type shift as a result of Al doping.[28] In Figure 5 d, on subtracting out the impact of the QC effect, the broadening of the optical band gap increases gradually when the Al3+ doping concentration is lower than 1.5 at. % (see Table S3 for details). It decreases as the concentration ex- ceeds 1.5 at. %. With lower Al3+ doping concentrations (^ 1.5 at. %), the increase of the DEBM may be caused by the so- called BM effect. If the carrier concentration is larger than 3.68 ^ 1018 cm^3, the conduction band of AZO is partially filled with electrons and the Fermi level is located near the conduc- tion band minimum or even lo- cated inside the conduction band. The filling of electrons in the conduction band with in- creasing carrier concentration shifts the Fermi level to a higher energy state. Thus it blocks tran- sition to lower energies owing to the Pauli Exclusion Principle. As shown in Figure 5 d, the carri- er concentration is larger than the conduction band density of state in ZnO (3.68^1018 cm^3).[29] Studies of carrier concentration using this method have been re- ported in the literature, and the carrier concentration was similar to our result.[24, 30] The shift of the optical band gap to higher energy upon increasing the carri- er concentration can be ex- plained in terms of the BM effect.[26, 31] This is the reason for an initial increase in the band gap. When the Al3+ doping concentration is higher than 1.5 at. %, the optical band gap decreases with the increase of Al content. Some researchers used the many-body effect to explain this phenomenon. If increasing numbers of electrons are doped into the conduction band (carrier concentration above Mott concentration), the Coulomb interactions and exchange energy between electrons cannot be ignored, and the lowest conduction-band states shift down. Meanwhile, the electron- impurity interactions also cannot be ignored, and the highest valence-band states shift up.[28b] The optical band gap shrinks. However, the electron concentration of the samples is smaller than the Mott concentration of AZO (NMott^4.5^1020 cm^3). The many-body effect cannot explain this result. In combina- tion with decreasing electron concentration, the shrinkage of the optical band gap may be caused by the defect trapping electrons. The optical band gap of AZO increased initially ([Al] ^ 1.5 at. %) because of the BM effect and then decreased as the Al content increased ([Al] >1.5 at. %) owing to the defects of AZO. Photoluminescence (PL) spectra were used to characterize the defects of ZnO and AZO nanorods. Figure 6 shows the PL spectra obtained at room temperature for dispersions of AZO with different Al contents (0-4.0 at. %). The PL spectra are found to be dependent on the Al contents. There are six differ- ent emission peaks in the PL spectra. They are probably rela- tively near band-edge emission and the variation of the intrin- sic defects in ZnO. As shown in Figure 6 a, the violet emission at approximately 3.0 eV is attributed to an interstitial zinc Zni when ZnO is doped with Al atoms.[32] Moreover, when the di- ameter of the nanorods was smaller than two times the sur- face depletion region (dZnO= 69 nm), the defect would be thor- oughly depleted.[33] Because the diameters of all the samples were smaller than 40 nm, the Zni was thoroughly depleted. All the Zni centers would be deprived of electrons and converted to Zn2 + and electrons. Therefore, this emission peak did not appear for the pure ZnO sample. Nevertheless, when ZnO was doped with Al, the electron concentration increased, and the Zni centers would be generated again and display the emis- sion peak at approximately 3.0 eV. Also, when the ZnO is doped with Al atoms, the variations of green and blue emis- sions are the most pronounced. In ZnO dispersion, high- strength green and blue emissions are observed, but in AZO dispersion, the green emission is clearly quenched. According to the defect theory, the energy interval for electronic transi- tion from the bottom of the conduction band to the neutral interstitial oxygen Oi level is 2.28 eV, and that of the singly charged zinc vacancy VZ^n is 2.66 eV.[32] In our experiment, the green and blue emissions are 2.32 and 2.59 eV, respectively. To compare the change of relative concentration of these defects in our sample, the PL count ratios of green and blue emission peaks are calculated ; with Al content increasing, the ratio de- creases (Figure 6 b). Based on defect chemistry in ZnO, Oi comes from the Fren- kel reaction. VZ^n and VOþ arise from the Schottky reaction and further ionization reactions.[18] The main defect reactions are as follows [Eqs. (5) and (6): ... (5) ... (6) The emission peaks of four defect types can also be found in the PL spectra[19] (Figure 6 a). When Al3+ is doped in ZnO [Eqs. (7)(9)] ... (7) ... (8) ... (9) As shown in Equation (7), if there is no other defect reaction the electron concentration increases with increasing Al con- tent. However, not only the doping reaction but also intrinsic defect reactions [Eqs. (5) and (6)] exist in AZO nanorods. When the electron concentration is in- creased, VOþ traps the electron. It becomes the neutral VO^ [Eq. (8)] . Based on the reaction equilibri- um theory, the concentration of VZ^n ([VZ^n]) will increase because the [VO^ ] decreases, and the in- cremental VO^ will consume the intrinsic Oi^ and decrease [Oi^ ] [Eq. (9)] . The PL count ratio I(Oi^ )/I(VZ^n ) thus decreases with the decrease of [Oi^ ] and in- crease of [VZ^n] . This predicted result is in good agreement with the experimental result (Figure 6 b). Therefore, it is believed that the shrinkage of the optical band gap is caused by the defect trapping electrons. As noted above, the charge compensation included VZ^n and electrons in AZO nanorods. As the Al content increases to a certain extent, the compensating effect of VZ^n will exceed that of the electron. The electron concentration will decrease. The optical band gap will shrink caused by the electron concentration decreasing. Electrical properties Resistivity is another important property for applications in transparent electronics. For instance, a low-resistivity substrate can improve the performance of solar cells. To measure the electrical resistivity, the samples were pressed into pellets under 30 MPa pressure and then measured on a four-point probe instrument. Figure 7 shows the electrical resistivity of ZnO and AZO samples versus Al3 + doping concentration at room temperature, and the color of ZnO/AZO deepened with the increase of Al content. The resistivity of the samples de- creases significantly when the Al3 + doping concentration is be- tween 0 and 1.5 at. %, followed by increases if the Al3+ doping concentration is higher than 1.5 at. %. The minimum value is 820 Wcm at an Al content of 1.5 at. %. XRD results indicate that all the samples are without any other impurity phase, such as Al2O3, and the limit of Al incorporation in ZnO is higher than 1.5 at. %.[16] So the minimum in the conductivity curve is not caused by a segregation of Al contents. This is in good agreement with the change of electron concentration (as shown in Table S3). The resistivity (1) can be estimated by using Equation (10): ... (10) in which N is the carrier concentration, e is the carrier charge, and the m is the mobility. So the major reason for the observed behavior is that the resistivity of the AZO sample varied with electron concentration. On the one hand, when an Al3 + ion replaces the Zn site, one free electron is released to the conduction band [Eq. (7)] . The electron concentration will increase with dopant concen- tration. Hence, the resistivity of AZO decreases with increasing Al content from 0 to 1.5 at. %. Then, defect trapping electrons will be generated on further increasing the Al content, thus leading to the decrease of electron concentration and resulting in increases in the resistivity of the AZO sample. The inset of Figure 7 shows that the color of samples is from white to ma- zarine blue, which indicates that the defect of VZ^n increases with the increase of Al content (as described above).[34] It is fur- ther proof that the Al was substituted in the Zn site. On the other hand, according to Matthiessen's rule [Eq. (11) ; where gb is grain boundary]: ... (11) dopant will produce disorders in the lattice and increase the scatter mechanism by ionized impurities, which will also bring about an increase in the resistivity. In our study, an Al content of 1.5 at. % is the optimum dopant concentration, and the lowest electrical resistivity is 820 W cm, which is four orders of magnitude lower than that of ZnO. Conclusion Aluminum-doped zinc oxide (AZO) nanorods with different Al contents have been successfully prepared by a convenient sol- vothermal route. The effects of Al content on the structure, op- tical and electrical properties, and growth mechanism of the AZO powders were investigated. It was found that the defects VZ^n and AlZþn are the important factors for the AZO nanorod growth and properties. They cause the electron concentration and crystal size to decrease. The optical and electrical proper- ties of AZO are dependent on electron concentration. The opti- cal band gap Eg of AZO nanorods increases with Al content ini- tially, and then decreases. The maximum Eg is 3.528 eV when the dopant concentration is 1.5 at. %. Also, the electrical resis- tivity of AZO nanorods decreases with the increase of Al con- tent, and shows a minimum when the Al content is 1.5 at. %. The optimum dopant concentration is 1.5 at. % and the lowest electrical resistivity is 820 W cm. The low-resistivity AZO powder would be the best precursor for low-cost preparation of larger-scale AZO film in transparent and conductive oxides. Experimental Section Materials Anhydrous ethanol, sodium hydroxide, and zinc acetate dihydrate were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Aluminum nitrate nonahydrate was purchased from J&K Chemicals. Synthesis of Al-doped ZnO nanorods ZnO and AZO nanorods were prepared according to the following procedure. At room temperature and under vigorous magnetic stir- ring, zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 2 mmol) was dis- solved in ethanol (20 mL). An appropriate amount (0-0.08 mmol) of aluminum nitrate nonahydrate (Al(NO3)3·9H2O) was added to the solution of zinc acetate dehydrate. Then, this solution contain- ing Zn2 + and Al3 + was added dropwise to sodium hydroxide solu- tion (40 mL, 20 mmol) in ethanol. When the reaction mixture became a clear solution, the mixture was transferred to a Teflon- lined stainless-steel autoclave and heated at 150 8C for 24 h. After the autoclave cooled down, the precipitate was collected by centri- fugation, followed by washing with deionized water and ethanol three times. Finally, the products were dried at 60 8C for 24 h. Seven samples with different Al/Zn molar ratios (0, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 at. %) were prepared by using this solvothermal method. Characterization The powder X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Rigaku D/Max 2500, Japan) with monochro- mated CuKa radiation (l = 1.541874 ^). The morphology was ob- served with a field-emission scanning electron microscope (FEI Nova Nano SEM 450). Transmission electron microscopy (TEM) ob- servations were performed on an FEI Tecnai G2 F20 S-Twin instru- ment. The absorption spectra of the samples were recorded on a UV/Vis-NIR spectrometer (Shimadzu UV-3600, Japan) with a wave- length range of 300-800 nm. Samples were dispersed in ethanol under ultrasonication with a concentration of 8 mg mL^1 and then dropped onto glass slides to form thin films. For electrical property analysis, samples were first pressed into pellets under 30 MPa pres- sure and then measured on a four-point probe instrument (Lucas Labs Pro4). The photoluminescence (PL) spectra of the samples were recorded on a fluorescence spectrophotometer (Edinburgh Instruments FS920) with an emission wavelength range of 340- 900 nm and an excitation wavelength of 325 nm. The samples were dispersed in ethanol under ultrasonication with a concentra- tion of 2 mg mL^1. All the measurements were performed at room temperature. Acknowledgements This study was financially supported by the National Basic Re- search Program of China (973 Program, 2012CB933700-G), Na- tional Natural Science Foundation of China (NSFC 21101165), Guangdong Innovative Research Team Program (Nos. 2011D052 and KYPT20121228160843692), Shenzhen Basic Research Plan (JC201005270372A and JSGG20120615161915279), and Shenzhen Electronic Packaging Materials Engineering Laboratory (2012- 372). We thank Dr. Guohua Zhong and Dr. Fusheng Liu from Shenzhen University for helpful discussions. [1] T. Zhai, L. Li, Y. Ma, M. Liao, X. Wang, X. Fang, J. Yao, Y. Bando, D. Gol- berg, Chem. 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Solids 2000, 61, 1237-1242; b) J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai, ACS Appl. Mater. Interfaces 2012, 4, 4024 - 4030 ; c) H. Matsui, H. Tabata, N. Hasuike, H. Harima, B. Mizobuchi, J. Appl. Phys. 2005, 97, 123511. Received : November 26, 2013 Revised : February 17, 2014 Published online on March 26, 2014 Ruiqiang Chen,[a] Pengli Zhu,*[a, b] Libo Deng,[a] Tao Zhao,[a] Rong Sun,*[a] and Chingping Wong[a,b, c] [a] R. Chen, Dr. P. Zhu, L. Deng, T. Zhao, Prof. R. Sun, Prof. C. Wong Shenzhen Institutes of Advanced Technology, Chinese Academy of Science 1068 Xueyuan Avenue, Shenzhen University Town Shenzhen 518055 (P. R. China) E-mail : [email protected] [email protected] [b] Dr. P. Zhu, Prof. C. Wong School of Materials Science and Engineering Georgia Institute of Technology 771 Ferst Drive, Atlanta, GA 30332 (USA) [c] Prof. C. Wong Department of Electronics Engineering The Chinese University of Hong Kong Shatin NT, Hong Kong SAR (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201300398. (c) 2014 Blackwell Publishing Ltd. |