RSM2013 Proc. 2013, Langkawi, Malaysia

Investigating The Annealing Effect on The Conventional Growth of ZnO Nanorod Through

Electrical Characterization R. Haarindra Prasad, U. Hashim, K.L. Foo, and Mohd Shafiq

Biomedical Nano Diagnostics Research Group Institute of Nanoelectronic Engineering (INEE), Universiti Malaysia Perlis (UniMAP)

01000 Kangar Perlis Malaysia. Email: haarindraprasad@yahoo.com

Abstract— Zinc oxide nanorods was synthesized by using hydrothermal growth due to simplicity and involve low temperature processing that is 930C. Low temperature processing is very essential for ZnO nanorod synthesis because defect on developing nano-device can be avoided. Development of nano-device with minimal defect is essential to ensure that the performances of the nano device is optimum for sensing bio-molecular substances. Zinc oxide has become the most remarkable choice among other metal oxides semiconductor due to many criteria such as economical cost, unique physical and electrical properties and biocompatible. Initially, ZnO thin films was prepared by using sol gel method. The ZnO seed solution was prepared using conventional sol-gel route. Zinc oxide solution was prepared in two different solvents which are isopropanol (IPA) and methanol (MeOH) in order to investigate the influence of solvent to the quality of ZnO nanorods. MEA, the sol stabilizer was added to the solution for the following 2 hours. Aluminum IDE electrode was deposited on the silicon wafer sample <100> using traditional wet etching method. Positive photoresist (PR) was coated on the silicon wafer and followed with soft back for 90 seconds. IDE pattern transfer was done by exposing UV light (365nm) onto the PR for 10 seconds. After that, developing and etching process occurred for pattern transfer the IDE electrode onto the silicon wafer. The prepared seed solution was coated on silicon wafer by using speed coating method. Some of the coated samples underwent annealing process at temperature 2000C for 2 hours. The annealed and non-annealed sample undergoes hydrothermal growth method to synthesize ZnO nanorods. The synthesized nanorods underwent I-V test and capacitances to investigate the electrical behavior of ZnO nanorods. The annealed ZnO nanorods provided higher current, which was 900μA, as compared the non-annealed ZnO nanorods which was only 55 μA.

I. INTRODUCTION Development of nanostructures by utilizing metal oxide structures has become more emerging in nanotechnology fields. Advancement of nanotechnology has provide tools and equipments which are convention for the researchers to develop desired size and shape of variable types of nanostructures such as nanowires, nanorods, nanorings, nanobelts [1-5] and etc. Instead of using lithography method, bottom up approach or growth method is fully utilized to develop nanostructure from metal oxide substrate due to high oxygen vacancy from atmosphere with metal oxides at operating temperature environment [2].

Zinc oxide has become the most remarkable choice among other metal oxides semiconductor due to many criteria such as economical cost, unique physical and electrical properties and biocompatible [6-9]. Zinc oxide is consider cheap and has large band gap (3.37 eV) which is very suitable for developing electronic device, piezoelectric sensor, solar application, optical sensing, SAW application, and bio-molecular detection [10-14]. At present state, many researchers are interested in fabricating and developing biosensor which has elevated sensitivity and selectivity characteristic of biosensor device. Therefore, synthesizing quasi 1 dimensional (1D) nanostructure such as nanowires and nanorods has become more emerging. Sensitivity and selectivity of synthesized nanowires is always conformed due to large volume to surface ratio and the shape curvature of the nanowires tip which is convenient to conduct surface modification in order to attach target bio-molecular substance [15]. Zinc oxide nanorods has been synthesized using hydrothermal method at operating temperature 930 C. Developing zinc oxide nanorods is not critical step but optimization the growing process is a matter that need to be considered. Various parameters are involved in growing zinc oxide nanorods by using hydrothermal method, such as operating temperature, annealing process, pre annealing temperature, post annealing temperature, surrounding humidity, and thickness of ZnO thin films. In this study, the effect of annealing on the zinc oxide nanorods will be observed by conducting Current versus Voltage (I-V) test before and after the device undergoes annealing process.

II. METHODS AND MATERIAL Initially, ZnO seed solution was prepared using conventional sol-gel route. ZnO solution was prepared in two different solvents which are isopropanol (IPA) and methanol (MeOH) in order to investigate the influence of solvent to the quality of ZnO nanorods. In this work, 816g of zinc acetate dihydrate [Zn(CH3COO)2.2H2O] was dissolved in 20ml of the solvents. The zinc acetate powder was stirred vigorously with solvent at the speed rpm 1000 for 20 minutes. Monoethanolamine (MEA), the sol stabilizer was added to the solution for the following 2 hours. The ratio of zinc acetate

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RSM2013 Proc. 2013, Langkawi, Malaysia

solution and the stabilizer, monoethanolamine (MEA) is maintained as 1. The prepared sol gel solution was kept for more than 24 hours for aging process. Aluminum Interdigitated Electrode Array (IDE) electrode was deposited on the silicon wafer sample <100> using traditional wet etching method. Positive photoresist (PR) was coated on the silicon wafer and followed with soft bake for 90 seconds. IDE pattern transfer was done by exposing UV light (365nm) onto the PR for 10 seconds. After that, developing and etching process has done for pattern transfer the IDE electrode onto the silicon wafer. Prepared zinc oxide solution was coated on silicon wafer with the orientation <100> by using spin coating method. According to previous studies, it was proven that spin coating is an appropriate method compares to dip coating due to uniformity coated of ZnO thin film on the surface of 2 X 2 cm of Si substrate sample [3]. The speed of spin coater was adjusted to 3000rpm for 30 seconds. The coated samples were separated into two categories. First category is that the sample does not undergo annealing process, while second category undergo annealing process with temperate 300oC for 2 hours. Zinc oxide seeded layer were then immersed in the mixture solution of Hexamethylenetetramine (HMT) and Zn(NO3)2 for synthesizing ZnO nanorods through hydrothermal growth. Hexamethylenetetramine (HMT) and Zn(NO3) were dissolved in 200 ml of DI water. The concentration of growth solution was fixed at 0.025mol/L. The mixture solution was stirred at speed 1000 rpm for 25 minutes. The as-prepared samples were immersed in the solution for growing process to occur at temperature 930C for 5 hours. The synthesized ZnO nanorods on silicon samples underwent electrical characterization. Current versus voltage test and capacitance test were taken on the grown ZnO nanorods samples in order to observe the different in the behavior of electrical characteristic.

III. RESULTS AND DISCUSSION

Fig. 1(a)-(d) illustrate the current-voltage (I-V) curve of

Zinc oxide nanorods which were characterized before and after annealing process. Fig 1(a) and Fig (b) shows the I-V behavior of ZnO nanorods which the thin films were prepared using IPA solvent. Fig 1(c) and Fig (d) shows the I-V behavior of ZnO nanorods which the thin films were prepared using MeOH solvent. It was reported that the electrical behavior of ZnO nanorod varied substantially after undergoing annealing process Fig 1(b) shows that the annealed ZnO nanorods provided higher current, which was 900μA at 5V, as compared the non-annealed ZnO nanorods which was only 25 μA at 5V. It was illustrated that annealing process had enhanced the electrical conductivity of the ZnO nanorods. The electron mobility rate of annealed nanorods increased due to improvement of crystalline of ZnO nanorod. High crystalline nanorods demonstrate reproducible I-V curve because large number of ZnO nanorods had contributed to the electrical conduction. Furthermore, types of solvents used for synthesizing ZnO thin film also influenced the I-V behavior of the annealed ZnO nanorods. ZnO nanorods which its thin films synthesized with MeOH solvent showed higher current flew as compared to IPA solvent. Besides that, Fig 1(b) shows that ZnO nanorods prepared using IPA resulted schotty-contact behavior while Fig 1(d) shows that ZnO nanorods synthesized through MeOH exhibited ohmic-contact behavior.

Fig 2(a) Capacitance test of ZnO nanorod prepared with IPA solvent

Fig 1(a) ZnO nanorod of IPA solvent before annealing

Fig 1(b) ZnO nanorod of IPA solvent after annealing

Fig 1(c) ZnO nanorod of Meoh solvent before annealing

Fig 1(d) ZnO nanorod of Meoh solvent after annealing

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Fig. 2 represents the capacitance values of ZnO nanorods which the frequency ranging from 1 Hz to 1MHz. From the data obtained, it showed that the capacitance value of the nanorods increased dramatically at low frequency. This phenomenon was occurred due to the dielectric charge was high at low frequency. Therefore, large number of charges were stored in ZnO nanorods which gave rise to its capacitance value. The result also indicated that annealed nanorods exhibited high capacitance value compared to unannealed ZnO nanorod. Fig 2(a) shows that the capacitance values for annealed and non-annealed nanorods were 175 μF and 7.5 μF, respectively.

Fig 3 SEM image of synthesized ZnO nanowire

Fig. 3 shows the synthesized ZnO nanowire by using hydrothermal method. The average diameter size of ZnO nanowire is 490nm and the average length is 4.14 μm. Drastic changes in size dimension of the nanowires after undergoing annealing process does not occurred. Studies revealed that annealing process with constant temperature does not affect the size dimension of the nanowires. Hence, further studies regarding controlling the size of ZnO nanowires through annealing process is required.

IV. CONCLUSION

According to I-V test and capacitance test, it has proven that the electrical behavior of ZnO naorod has improved after undergoing annealing process. This is also concluded, that annealing process has enhanced the electrical conductivity of ZnO nanorods due to increased of current supply. Annealing process has influenced the electrical behavior of nanorods by increasing the current supply and increased the capacitance value at low frequency. Annealing process has also improved the uniformity of synthesized nanorods. Capacitance performances of ZnO nanorods also improved after undergoing annealing process.

ACKNOWLEDGEMENT

Author Haarindra Prasad gratefully acknowledges collaborators at the Institute of Nano Electronic Engineering (INEE) at University Malaysia Perlis (UniMAP). This project was aided by INEE at (UniMAP), through the Nano Technology project. The views expressed in this publication are those of the authors and do not necessarily reflect the official view of the funding agencies on the subject.

REFERENCES

[1] Muhammad Kashif, Uda Hashim, Md. Eaqub Ali, Syed M. Usman Ali, Mohamad Rusop, Zaffar Hussain Ibupoto and Magnus Willander. Effect of Different Seed Solutions on the Morphology and Electrooptical Properties of ZnO Nanorods. Journal of Nanomaterials, vol. 2012, Article ID 452407, 6 pages, 2012. doi:10.1155/2012/452407.

[2] M. Kashif, U. Hashim, M. E. Ali, S. M. U. Ali, Z. M. Willander. Structural and impedance spectroscopy study of Al-doped ZnO nanorods grown by sol-gel method. Emerald Microelectronics International, Vol. 29 (3), pp.131-135

[3] M. Kashif, Syed M. Usman Ali, M. E. Ali, U. Hashim. Effect of UV on impedance spectroscopy of Sn doped ZnO Nanorods.Journal of Ovonic Research. Vol. 8, No. 3. July-Aug 2012.

[4] X. Chen, C. K. Y. Wong, C. A. Yuan, and G. Zhang, "Nanowire-based gas sensors," Sensors and Actuators B: Chemical, vol. 177, pp. 178-195.

[5] Q. Cui, K. Yu, N. Zhang, and Z. Zhu, "Porous ZnO nanobelts evolved from layered basic zinc acetate nanobelts," Applied Surface Science, vol. 254, pp. 3517-3521, 2008.

[6] Y. Hames, Z. h. Alpaslan, A. Kösemen, S. E. San, and Y. Yerli, "Electrochemically grown ZnO nanorods for hybrid solar cell applications," Solar Energy, vol. 84, pp. 426-431.

[7] Norton, David P.,et al. "ZnO: growth, doping & processing." Materials today 7.6 (2004): 34-40.

[8] He, Jr H., Chang S. Lao, Lih J. Chen, Dragomir Davidovic, and Zhong L. Wang. "Large-scale Ni-doped ZnO nanowire arrays and electrical and

Fig 2(b) Capacitance test of ZnO nanorod prepared with Meoh solvent

306

RSM2013 Proc. 2013, Langkawi, Malaysia optical properties." Journal of the American Chemical Society 127, no. 47 (2005): 16376-16377.

[9] S. Joshi, M. Parmar, and K. Rajanna, "A novel gas flow sensing application using piezoelectric ZnO thin films deposited on Phynox alloy," Sensors and Actuators A: Physical, vol. 187, pp. 194-200.

[10] Y. Hames, Z. h. Alpaslan, A. Kösemen, S. E. San, and Y. Yerli, "Electrochemically grown ZnO nanorods for hybrid solar cell applications," Solar Energy, vol. 84, pp. 426-431.

[11] Lamara, T., et al. "Freestanding CVD diamond elaborated by pulsed-microwave-plasma for ZnO/diamond SAW devices." Diamond and related materials 13.4 (2004): 581-584.

[12] Song, Dengyuan, Armin G. Aberle, and James Xia. "Optimisation of ZnO: Al films by change of sputter gas pressure for solar cell application." Applied Surface Science 195.1 (2002): 291-296.

[13] Lu, Hongbing, et al. "High surface-to-volume ratio ZnO microberets:

Low temperature synthesis, characterization, and photoluminescence." The Journal of Physical Chemistry B 110.46 (2006): 23211-23214.

[14] Tam, K. H., et al. "Defects in ZnO nanorods prepared by a hydrothermal method." The Journal of Physical Chemistry B 110.42 (2006): 20865-20871.

[15] Zhao, Q., et al. "Enhanced field emission from ZnO nanorods via thermal annealing in oxygen." Applied physics letters 88.3 (2006): 033102-033102.

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