The morphology and electrooptical properties of ZnO nanorods synthesized on monoethanolamine-based seed layer and KOH-based seed layer were compared. The seed solutions were prepared in monoethanolamine in 2-methoxyethanol and potassium hydroxide in methanol, respectively. Zinc acetate dihydrate was as a common precursor in both solutions. The nanorod-ZnOs were synthesized via the spin coating of two different seed solutions on silicon substrates followed by their hydrothermal growth. The scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL), and Raman studies revealed that the ZnO nanorods obtained from monoethanolamine-based seed layer had fewer defects, better crystals, and better alignment than those realized via KOH-based seed layer. However, the current-voltage (I-V) characteristics demonstrated better conductivity of the ZnO nanorods obtained via KOH-based seed layer. The current measured in forward bias was 4 mA and 40
Zinc oxide (ZnO) which belongs to II–IV group of semiconducting materials is increasingly getting more and more research interests because of its attractive and fascinating properties such as approximately 3.37 eV of direct wide bandgap and about 60 meV of exciton binding energy. In fact, these properties are suitable for numerous applications in optoelectronic and biomedical devices. For examples micro- and nanostructures ZnO were successfully used in various sensing appliances such as UV sensors, biosensors (protein, DNA, and cancer cell detection) as well as gas sensors [
In this study, nanorod-ZnOs were synthesized on two different seeded substrates and their morphological, crystalline, and optoelectrical properties were systematically studied using SEM, XRD, Raman, and photoluminescence spectroscopies as well as current-voltage profiler. The study reflected that the seed solution prepared through 2-methoxyethanol route is superior to KOH one to synthesize nanorod-ZnO with less defects, better crystal, and better alignment. On the other hand, the applicability of KOH route was realized to obtain nanostructured ZnO with superior conductivity.
All chemicals used in this experiment were of the highest available purity and were purchased from Sigma-Aldrich, USA. ZnO seed solutions were prepared in two different solvents, namely, methanol and 2-methoxyethanol. The first solution (0.01 M) was prepared by dissolving 274 mg of zinc acetate dihydrate in 125 mL of methanol at 60°C. A potassium hydroxide solution of 0.03 M was prepared by dissolving 109 mg of KOH in 65 mL of methanol in a separate beaker. To a 25 mL portion of 0.01 M zinc acetate dihydrate solution heated at 60°C, 13 mL of 0.03 M KOH was added drop-wise under vigorous stirring. The solution was incubated at 60°C for 2 h with continual stirring. In the second solution, the concentration of zinc acetate dihydrate was 0.35 M in 2-methoxyetanol. To the second seed solution which was heated to 60°C, monoethanolamine (MEA) was added drop-wise under vigorous stirring until the molar ratio of MEA to zinc acetate dihydrate was reached to 1 : 1. The second seed solution was also incubated at 60°C for 2 h with continual stirring. The measured pH values for monoethanolamine-based seed solution and KOH-based seed solution were 7.69 and 8.90, respectively.
The ZnO-nanorods were synthesized on silicon substrates through a spin-coating method. Prior to coating, the silicon substrates were nicely cleaned with RCA1 and RCA2 [
The morphological characterization of nanostructured ZnO was performed using a scanning electron microscope (SEM, JSM JEOL 6460 LA) whereas the cross-sectional images were collected from Hitachi S3400n SEM instrument. The crystallinity of the ZnO-nanorods was studied using an X-ray diffraction (XRD) with a CuK
The SEM images of ZnO nanorods obtained on sample 1 and sample 2 are shown in Figures
SEM images showing the surface morphologies of ZnO nanorods synthesized on KOH-based seed layer (a) and monoethanolamine-based seed layer (b). The cross-sectional view of the above nanostructured ZnO grown on KOH- and monoethanolamine-based seed layers is demonstrated in (c) and (d), respectively.
The various morphologies for the sol-gel using KOH-based seed layer and monoethanolamine-based seed layer-derived ZnO nanorods can be attributed (i) to the different physiochemical properties of the monoethanolamine-based and KOH-based seed solutions of zinc acetate dihydrate in 2-methoxyethanol and methanol, respectively. The monoethanolamine-based seed solution was thicker than the KOH-based seed solution. MEA and KOH components probably provided the required basicity to the seed solutions. The monoethanolamine-based seed solution was more homogeneous and stable than that of KOH-based seed solutions when left for long time. This might be due to the limited solubility of both zinc acetate dihydrate and KOH in methanol. The longer stability and homogeneity of the 2-methoxyethanol solution was probably due to the stabilizing property of monoethanolamine. The different thickness of the two seed solutions produced different type of crystal and thus different ZnO nanorods on silicon substrates. (ii) When the quantity of alkali was small, fewer particles were appeared as reported in the literature [
Figures
The values of interplane distance
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Sample 1 | 2.99 | 5.185 | 2.593 | 483 |
Sample 2 | 3.001 | 5.198 | 2.599 | 667 |
XRD spectra demonstrating crystal qualities of ZnO nanorods synthesized on KOH-based seed layer (a) and monoethanolamine-based seed layer (b).
The strongest peak (0 0 2) at
No other peaks of impurities such as zinc nitrate hexahydrate and hexamethyltetramine were observed in the spectra. The possible enhancement in the crystal quality of nanorod ZnOs using monoethanolamine-based seeded layers might be due to strain relaxation. It was found that first 10 to 40 min was the deciding factor for the ultimate diameter of nanorod ZnOs. The interface free energy of the seed grains and growth solution was less than the crystal-vapour interface which might occur during the growth of rods. The high-interface free energy of the crystal-vapour interface could be due to the residual strains which are correlated to the heating history of growth process [
Figure
Raman spectra reflecting crystalline, structural, and orientation profiles of ZnO nanorods synthesized on KOH-based seed layer (a) and monoethanolamine-based seed layer (b).
One sharp peak at 520 cm−1 and two weak peaks at 302 cm−1 and 620 cm−1 can be conferred to the TO phonon mode originating from silicon substrate [
To study the optical characteristics of nanorod ZnOs, photoluminescence spectroscopy was carried out on both the samples at room temperature. The typical spectra for the photoluminescence for both the samples are shown in Figure
Photoluminescence spectra demonstrating optical properties of ZnO nanorods synthesized on KOH-based seed layer (a) and monoethanolamine-based seed layer (b).
Usually, the visible emission from ZnO is attributed to different defects such as oxygen vacancies (VO), zinc vacancies (VZn) or a complex defect involving interstitial zinc (Zni), and interstitial oxygen (Oi) [
Figure
I-V curves showing the electrical properties of ZnO nanorods synthesized on KOH-based seed layer (a) and monoethanolamine-based seed layer (b).
Nanorod ZnOs of two different morphologies and opto-electric properties were successfully synthesized on two different seeded layered silicon substrates. While the better crystal quality, fewer structural defect and better aligned ZnO nanorods were obtained through monoethanolamine-based sol-gel method, better electrical conductivity was found in nanorod ZnOs derived through KOH-based sol-gel methods. Thus KOH-based sol-gel method should be selected for synthesizing nanorod-ZnOs for power-saving optoelectrical devices. On the other hand, better aligned, better crystalline, and fewer defects in monoethanolamine-based sol-gel derived nanorod-ZnOs suggest their potential applications in optoelectronic devices of higher longitivity and stability.
The author would like to acknowledge the financial support for the FRGS Grant number (9003-00276) from Ministry of higher education (MOHE). The author would also like to thank the technical staff of Institute of Nano Electronic Engineering and School of Microelectronic Engineering, Universiti Malaysia Perlis for their kind support to smoothly perform the research.