Al-doped ZnO nanorod arrays and nanostructures were fabricated on seed coated glass substrates via CoSP continuous Spray Pyrolysis reactor. The as-synthesized aluminium doped ZnO nanoparticles and nanorods were analyzed through different characterization techniques. The doping of Al atoms did not result in significant changes in the structure and crystal orientation but the morphology of the film changed to branched nanorods and nanosheets with the change in seed solution and annealing temperature, respectively. Also, the current-voltage curves of the ZnO and AZO nanorod arrays was measured and it was found that the current response of AZO nanorods was higher than that of ZnO nanorods, proving the Al incorporation as a dopant.
Keywords: Nanostructures; Zinc oxide; electron microscopy; Al-doped; I-V
Recently, the investigation of one-dimensional (1D) nanostructures has been one of the most attractive topics in physics, chemistry and material science due to their significance in both fundamental and technological fields such as display panels, solar cells, and photocatalytic devices owing to their high conductivity and transparency in the visible wavelength region. ZnO is one of the metal oxide semiconductors suitable for use as a TCO thin film because of its higher thermal stability, good resistance against hydrogen plasma processing damage, and relatively low cost compared with ITO. However, pure ZnO thin films have poor conductivity and hence it has been reported that the doping of Al can increase the conductivity of n-type ZnO which is favorable for its applications in electronic, optoelectronics, and field emission devices.1 Among the doped ZnO thin films, Al-doped ZnO (AZO) thin films are particularly attractive owing to their good conductivity, transparency and relatively low cost.2
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Much attention has been devoted in the recent past for the development of inverted organic solar cells (IOSCs) based on zinc oxide (ZnO) nanostructures as an electron transport layer (ETL), which is attributed to its excellent chemical and thermal stability, high electron mobility, and easy fabrication.3,4 Owing to the smaller interfacial area between the ZnO NRs and the active layer, the power conversion efficiency (PCE) of ZnO NR-based IOSCs is often lower than that based on nanoparticles.5 If disperse and ultrathin ZnO nanostructures can be achieved, it will facilitate electron transfer of IOSCs by increasing the interfacial area between ZnO nanostructures and the active layer.6
Many efforts have been also made by doping the ZnO nanostructures by various chemical elements such as gallium (Ga),6 indium,7 and aluminum (Al),8 into ZnO nanostructures to improve the conductivity of ZnO nanostructures. Among them, Al - doped ZnO (AZO) nanostructures are capable of reaching the highest conductivity without deterioration in optical transmission.8,9 It is possible to increase the device performance by doping Al in ZnO nanostructures.
ZnO nanorod arrays have proved to be promising candidate for high-bendable and flexible DSSCs. Especially, in order to enhance electrical and optical properties, group II, III, IV, V, and VI elements have been selected to dope into the ZnO structure.10 Al, a group III element, has been regarded as one of the most representative dopants for forming n-type ZnO with good optical quality, low resistivity, high conductance, and high crystal quality.
Several physical or chemical methods have been developed in succession for the preparation of ZnO nanorod array, including vapor-liquid-solid process utilizing Au or Sn as catalyst. In most other techniques, nanoparticles and nanostructures are created separately using different physical and chemical methods. It is therefore not possible to study the role of self assembly process on the properties of nanostructures synthesized. In this work, we report on CoSP technique to synthesize both nanoparticles and nanostructures of Al-doped ZnO at one go.
The details of the seed layer are mentioned in another report.11 Additional study on seed layer has been done by mixing some ZnO nanoparticles (average size ~ 20 nm) in the seed solution and this sol is sonicated for about a min. This ZnO nps mixed solution named as 'Seed 2' is also used as seed layer.
Table 5.1. List of seed solution and annealing temperature.
Zinc Chloride in methanol
Always on Time
Marked to Standard
ZnO nps mixed in 'Seed 1'
The annealing temperature is increased to 180 Â°C to see the effect of annealing temperature on the morphology of Al-ZnO films. In this way, three combinations have been made as tabulated in Table 5.1. Unless otherwise stated the seed solution used is 'Seed 1' and annealing temperature is 110 Â°C. The substrates are washed with detergent and then ultrasonically cleaned in deionized water, acetone and propanol, respectively and dried in air before substrate pre-treatment.
Nanorod preparation through CoSP process
The starting material used is zinc acetate dihydrate in distilled water and aluminium nitrate nonahydrate is used as the dopant material. The spray solution containing zinc acetate dihydrate [(CH3COO)2Zn.2H2O, 0.1M] (Sigma- Aldrich 98% pure) and aluminium nitrate (1-2 wt%) dissolved in DI water (precursor solution) is injected into the reactor, which leads to Al-doped ZnO nanoparticle creation. Rest procedure and process parameters are kept the same as mentioned earlier.11 In this way, both nanoparticles and nanorod array thin films are created in one single step which no other technique offers.
The structural and morphological characterization of Al-doped ZnO nanoparticles/nanorods were carried out by means of SEM and XRD. X-ray diffractometer (Phillips X'PERT PRO), having CuKÎ± incident beam (Î» = 1.54AËš) was used for the XRD studies. TEM and HRTEM studies were done using Phillips CM12 120KV transmission electron microscope and 200kV Technai G20- high-resolution transmission electron microscope, respectively. ZEISS EVO-50 model scanning electron microscope (SEM) was used to study the surface morphology of the nanostructured films. Perkin-Elmer UV-VIS-NIR spectrophotometer was used to measure the absorbance of the films in the visible region. Keithley 2602 source meter was used to measure the I-V curves.
Results and discussion:
Figure 1 shows the X-ray diffraction pattern of the as collected Al-doped ZnO nanoparticles and nanorods. The patterns exhibited the strong characteristic peak for the (002) plane of wurtzite-type ZnO (hexagonal) for nanorod film. In addition, no significant peaks for metal aluminum or aluminum oxide were observed. This revealed that the doping of Al atoms did not result in significant changes in the structure and crystal orientation of the wurtzite type ZnO nanorods. Al-doped and undoped ZnO samples exhibited the hexagonal wurtzite structure in correspondence with the JCPDS card number 36-1451. These results show that less Al-doping (like, 1 or 2%) would not change the crystal structure of ZnO. The diffraction intensities of (0 0 2) plane for Al-doped sample becomes prominent for the nanostructured film. The overall particle size calculated from Scherrer's formula was between 21 and 24 nm for 1 & 2% Al doping which was conï¬rmed from the TEM images.
Figure 2 shows the TEM images of the as collected Al-doped ZnO nanoparticles. The image clearly indicates that the average size of the nanoparticles varies from 18 to 24 nm. The average size of undoped ZnO nanoparticles was around 20 nm. Figure 2(c) shows a high resolution transmission electron microscopic (HRTEM) image of the regularly hexagonal nanoparticles showing lattice fringes (spacing of 0.26 nm) which corresponds to the (002) plane. It shows the Al atoms have been successfully doped in the ZnO nanoparticles.
The morphology of the nanostructured films formed by the self assembly of 1% and 2% doped nanoparticles is shown in Fig. 3 (a) & (b), respectively. The dimension of doped nanorods increases with the doping percentage which may be due to increased size of the nanoparticles that self assemble on the seeded substrates.
Figure 4 shows the AFM image (1.5 ÂµmÃ-1.5 Âµm) of the samples deposited for 5 min for both 1 and 2% doped Al-ZnO films. As calculated from the AFM data, the diameter of the nanorods, in case of 1% doping, varies from 80-100 nm and the height is about 50 nm. Whereas, for 2% doping, the diameter of the nanorods is in the range of 100-180 nm and height is about 100 nm. Surface roughness (rms) changes from 14 nm to 40 nm with doping percentage. This increase in surface roughness with doping percentage is due to the increase in the particle size that self-assemble on the seeded substrates.
To investigate the effect of seed layer and annealing temperature, different seed solution and annealing temperature as tabulated in Table 1 have been tested. It is found that the morphology of the nanostructured film changes to branched nanorods instead of vertically aligned nanorods with change in the seed solution to 'Seed 2'. Further, when the annealing temperature of the seed layer is increased to 180 Â°C from 110 Â°C, keeping the seed solution as 'Seed 2', the morphology completely changes to nanosheets instead of nanorods. Fig. 5 (a) & (b) shows variety of nanostructures formed due to different seeding and annealing temperatures of the seed layer. Fig. 5 (a) shows the branched nanostructures for 'Seed 2' which were formed by bunching of nanorods and 5 (b) shows the nanosheets for higher seed annealing temperatures. For 'Seed 2', the nucleation of ZnO nanoparticles vapor (coming from the second zone) on the islands created by nanoparticles at the substrate is in random direction resulting in non-aligned branched nanorods. By preparing ultrathin layer of ZnO seeding one can synthesize small diameter ZnO nanorods. Thus, the technique is capable of synthesizing different kinds of nanostructures at the same time simply by changing the seed solution and annealing temperature. These nanostructures can be used in variety of devices according to the application.
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The optical properties of the samples have been investigated by photoluminescence spectra. Fig. 6 displays the room temperature photoluminescence spectra for the Al-ZnO nanorods for 1 & 2% doping. For the nanorods, there are two emission bands in the spectra; one sharp UV luminescence at approximately 398 nm and one broad red emission peak from 675 - 800 nm. As can be seen that the graph slightly shifts towards higher wavelength for 2% doping, this may be attributed to the rise in particle size for higher doping percentage.
To examine the performance of Al-ZnO nanorod array thin films, the structures with ZnO and Al-ZnO nanorod arrays to be measured were fabricated as illustrated in figure 7 (a). Al dot contacts are deposited on the top of the nanorod arrays. The current-voltage curves of these thin film devices at room temperature are shown in Fig. 7 (b). Al forms Ohmic contacts with Al-doped ZnO nanorod arrays that are shown through a linear current-voltage (I-V) characteristic. It is found that the as-grown AZO nanorod arrays exhibited a slightly higher current response than the as-grown ZnO nanorod arrays. The resistance (as calculated from the slope of this graph) changes from 602 to 406 Î© for 1% doping. In addition, their current responses are similar to that reported in the literature.12 This result shows the potential application of doped and undoped ZnO nanorods for ultra-high density nanodevices.
The Al-doped ZnO nanoparticles and nanostructures have been successfully synthesized by CoSP reactor. The morphology of the samples doped with Al ions have altered noticeably. SEM and AFM images revealed that the dimensions of Al-ZnO nanorods change with the doping percentage in a similar way as the size of the corresponding nanoparticles. XRD pattern indicated that the resultant Al-ZnO nanorods had a structure of wurtzite-type ZnO and the doping of Al atoms did not result in significant changes in the structure and crystal orientation. The seed solution used for the deposition of nanostructures and the annealing temperature has a direct impact on the morphology of nanostructures created. The use of ZnO nanoparticles (created by same technique) in the seed layer leads to branched nanostructures and increased annealing temperature changes the morphology to nanosheets. In addition, the current-voltage curves of the thin film devices with ZnO and Al-ZnO nanorod arrays has been measured and it is found that the current response of AZO nanorods is higher than that of ZnO nanorods.