Low Temperature Growth and Optical Properties of Zinc Oxide Nanostructure Prepared by Two-step Solution-based Deposition Method

A.N. Afaah1,2,a, N.A.M. Asib1,2,b, A. Aadila1,2,c, M. Rusop 1,3,d, R. Mohamed2,4,e, Z. Khusaimi1,2,f

1NANO-SciTech Centre (NST), Institute of Science, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia

2Faculty of Applied Sciences, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia

3NANO-Electronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia

4Faculty of Applied Sciences, Universiti Teknologi MARA (UiTM) Pahang, 26400 Bandar Pusat Jengka, Pahang, Malaysia

aafaahabdullah@yahoo.com, b amierahasib@yahoo.com, caadilaazizali@gmail.com druzianamohd@pahang.uitm.edu.my, enanouitm@gmail.com, fzurai142@salam.uitm.edu.my

Keywords: ZnO nanostructures, mist-atomization, solution-immersion, Raman, PL

Abstract. ZnO thin films with typical c-axis (0 0 2) orientation were successfully deposited on

glass substrates by two-step deposition method; mist-atomization and solution-immersion. The

samples were annealed at selected temperature range of 350-500 ᵒC. The prepared samples then

analyzed by Raman spectroscopy and photoluminescence (PL) spectroscopy. The optical properties

of the samples were studied. The results of different annealing temperatures are also compared to

investigate the optical and physical properties of each sample. Photoluminescence (PL) spectra

showed low intensity in UV emission and high intensity in the visible emission, which indicates a

good surface morphology of the ZnO nanorod. The Raman intensity changes in all sample were also

tested.

Introduction

Zinc oxide is one of the most studied oxide semiconductors due to chemical and thermal stability

n-type semiconductor with bandgap energy of 3.37 eV and large exciton binding energy of 60 meV

at room temperature. Recently, nanostructured ZnO materials have received extensive interest due

to their distinguished performance in electronics, photonics and optics. It is a versatile functional

material that has a diverse group of growth morphologies, such as nanowires [1], nanorods [2],

nanoflakes [3] etc. Several methods such as hydrothermal [2, 4], thermal evaporation [3], sol-gel

[5], sol-gel spin coating [6], RF magnetron sputtering [7] chemical vapor and condensation [8],

pulsed laser deposition etc have been reported for the fabrication of ZnO nanostructures. However,

these processes involve elevated temperatures of 450-900 °C.

In contrast, solution-based method is more convenient over other methods as it is less expensive

with easier composition control, large deposition area, and the most importantly can be carried out

at relatively lower temperatures. The present work is a challenge to address the problems of

elevated operating temperature and high energy consumption with environmentally friendly

methods. In this paper, we focus on the synthesis of flower-like ZnO from zinc salts in solution. We

concentrate on the two-step deposition method; mist-atomizer and solution immersion method. The

optical properties of as-prepared samples are studied.

Advanced Materials Research Vol. 832 (2014) pp 749-753

Online available since 2013/Nov/21 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.832.749

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 136.186.1.81, Swinburne University, Hawthorn, Australia-04/09/14,12:10:03)

Experimental Details

Materials. The materials used in this work is zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and

stabilizer, hexamethylenetetramine (HMTA, C6H12N4). Gold (Au) target is used as the seeded

catalyst of the substrate.

Synthesis. Six nanometer (6 nm) thick gold (Au) was coated on the glass substrates. 6 nm said to be

the most optimized thickness in synthesizing crystalline ZnO nanostructures [9]. For the preparation

of Zn2+

solution, (Zn(NO3)2.6H2O) was dissolved in deionised water. The solution stirred to make

sure it is homogenous. Then, of HMTA solution is added to the Zn2+

solution. Deionised water was

added to the mixture to ensure the total volume was 250 mL. The mixture was then stirred and

heated at 60 ᵒC for an hour. Then, the solution was aged for another 24 hours.

After ageing process, the solution now ready to be used for the first-deposition method; mist-

atomization method. The Au-seeded glass substrates were placed on hot plate placed in the mist-

atomizer chamber (Fig. 1). The substrates are preheated at 250 ᵒC for 10 minutes. Then, the ZnO

solution was sprayed onto the glass substrates and left for 6 hours to make sure all the ZnO mists

settled onto the substrates.

Fig. 1, Mist-atomizer chamber

Then, the substrates undergo second deposition method; solution-immersion method. The

solution preparation for this method is the same as prepared for the first deposition method. The

substrates were placed in boiling tubes and 30 mL ZnO solution was poured into the boiling tubes.

The boiling tubes needed to be sealed tightly for the formation of good crystalline ZnO

nanostructures. Each sample was immersed in 90 ᵒC water bath for 4 hours. The substrates then

rinsed with deionised water to remove any organic salts and contamination. The glass substrates

were annealed at different annealing temperatures of 350, 400, 450 and 500 ᵒC. The samples were

then compared to an un-annealed substrate.

Characterization. The prepared samples were analyzed by Raman spectroscopy and PL

measurement (Horiba Jobin Yvon).

Results and Discussions

Raman studies. The crystallinity and structure of the deposited ZnO was defined by Raman

spectroscopy. As a fact, ZnO has three acoustic phonon branches. It consists of two transverse and

one longitudinal. The group theory of A1+2B1+E1+2E2 predicted the Raman active zone-centre

optical phonons of wurtzite ZnO, where the A1, E1 and 2E2 modes are Raman active while the B1

modes are forbidden modes. The phonons of A1 and E1 symmetries are polar phonons. Therefore,

they demonstrate different frequencies for transverse optical (TO) and longitudinal optical (LO)

phonons.

ZnO solution inlet Argon gas inlet

750 Nanoscience, Nanotechnology and Nanoengineering

The Raman spectrum of ZnO nanorod is shown in Fig. 2. A dominated and strong intensity peak

at 435 cm-1

corresponds to the E2 (high) mode of the non-polar optical phonons [10]. Generally, the

phonons of E2 symmetry have two frequencies E2 (high) and E2 (low). E2 (high) is related with the

oxygen atom and E2 (low) related with the Zn sub lattice [10, 11].

The peak at 333 cm-1

is attributed to the 2E2 mode. The broad peak at 584 cm-1

may corresponds

to the polar transverse optical (TO) A1 and longitudinal E1 optical (LO) phonon mode [11].

Fig. 2, Raman spectrum of ZnO nanorods annealed at (a) un-annealed, (b) 350 ᵒC, (c) 400 ᵒC, (d)

450 ᵒC, and (e) 500 ᵒC

PL studies. The room temperature PL spectrum of the sample is shown in Fig. 3. The PL spectrum

shows a bandgap emission along with a broad and intense emission in the green-yellow region. The

UV emission peak is around 405 nm and the green-yellow peak centered at 653 nm. The UV

emission is said to be attributed to recombination of free excitons, which is near band-edge

emission [12].

According to B. Mari et. al, these defects are commonly interrelated to singly ionized oxygen

vacancies [13]. They suggested the defect-state luminescence at the visible emission made the peak

broader. This is the proof of the presence of structural defects or electron acceptor defects such as

Zn vacancy or O interstitials in the samples which is in a good agreement with the Raman results as

discussed before.

0 300 600 900

Raman shift (cm-1)

(e)

(d)

(c)

(b)

(a)

Advanced Materials Research Vol. 832 751

Fig. 3, The room temperature PL spectra of ZnO nanorod annealed at 400 ᵒC

Conclusion

ZnO nanorods with hexagonal structure were successfully synthesized by mist-atomiser and

solution-immersion technique. Raman measurement confirms that the samples produced have good

crystallinity with hexagonal wurtzite phase. The intense and broad photoluminescence is because of

the presence of structural defects or high oxygen deficiency in the ZnO nanorod. The result

obtained from PL measurements is in a good agreement to Raman results.

Acknowledgement

The author would like to acknowledge Universiti Teknologi MARA (UiTM) for the funding

through the project 600-RMI/FRGS 5/3 (18/2012).

References

[1] Lori E. Greene, B.D.Y., Matt Law, David Zitoun, and Peidong Yang, Solution-Grown Zinc

Oxide Nanowires. Inorg. Chem. 45 (2006) 7535-7543.

[2] Chung-Yuan Kung, S.-L.Y., Hone-Zern Chen, Ming-Cheng Kao, Lance Horng, Yu-Tai Shih,

Chen-Cheng Lin, Teng-Tsai Lin and Chung-Jen Ou, Influence of Y-doped induced defects on

the optical and magnetic properties of ZnO nanorod arrays prepared by low-temperature

hydrothermal process. Nanoscale Research Letters. 7 (2012) 372.

[3] H.J. Pandya, S.C., A.L. Vyas, Integration of ZnO nanostructures with MEMS for ethanol

sensor. Sensors and Actuators B. 161 (2012) 923– 928.

[4] L. Motevalizadeh, Z.H., M. Ebrahimizadeh Abrishami, A facile template-free hydrothermal

synthesis and microstrain measurement of ZnO nanorods. (2012) 1-28.

[5] Chien-Yie Tsay, K.-S.F., Chien-Ming Lei, Synthesis and characterization of sol–gel derived

gallium-doped zinc oxide thin films. Journal of Alloys and Compounds. 512 (2012) 216– 222.

[6] N. D. Md Sin, M.H.M., M. Rusop, Z. Zulkifli, Electrically Conductive Zinc Oxide (ZnO)

Nanostructures Prepared By Sol-gel Spin-coating. ESciNano 2010, (2010).

[7] A. Barnabé, M.L., L. Presmanes, J.M. Soon, Ph. Tailhades, C. Dumas, J. Grisolia, A.

Arbouet, V. Paillard, G. BenAssayag, M.A.F. van den Boogaart, V. Savud, J. Brugger, P.

Normand, Structured ZnO-based contacts deposited by non-reactive rf magnetron sputtering

on ultra-thin SiO2/Si through a stencil mask. Thin Solid Films. 518 (2012) 1044-1047.

100 200 300 400 500 600 700 800 900 1000

Wavelength (nm)

Bandgap emission

Defect emission

752 Nanoscience, Nanotechnology and Nanoengineering

[8] M. Stoehr, S.J., T. M. Kyaw, and J. G. Wen, Doping effect on the optical properties of ZnO

nanostructures. phys. stat. sol. 4 (2007) 1432– 1437.

[9] Z. Khusaimi, S.A., H. A. Rafaie, M. H. Mamat, N. Abdullah, S. Abdullah, and M. Rusop,

Photoluminescence and Structural Properties of Gold-Assisted Zinc Oxide Nanorods.

Malaysian Journal of Science (Special Edition). 28 (2009) 197-201.

[10] Kim, P.R.a.J., Facile and fast synthesis of flower-like ZnO nanostructures. Materials Letters,

93 (2013) 52-55.

[11] N. Kamal Singha, S.S., Shyama Ratha, S. Annapoornia, Optical and room temperature

sensing properties of highly oxygen deficient flower-like ZnO nanostructures. Applied

Surface Science. 257 (2010) 1544–1549.

[12] Qiu YF, Y.S., ZnO nanotetrapods: Controlled vapour-phase synthesis and application for

humidity sensing. Adv Funct Mater. 17 (2007) 1345-1352.

[13] B. Mari, F.J.M., M. Mollar, J. Cembrero, R. Gomez, Photoluminescence of thermal-annealed

nanocolumnar ZnO thin films grown by electrodeposition. Appl. Surf. Sci. 252 (2006) 2826–

2831.

Advanced Materials Research Vol. 832 753

Nanoscience, Nanotechnology and Nanoengineering 10.4028/www.scientific.net/AMR.832 Low Temperature Growth and Optical Properties of Zinc Oxide Nanostructure Prepared by Two-Step

Solution-Based Deposition Method 10.4028/www.scientific.net/AMR.832.749

DOI References

[3] H.J. Pandya, S.C., A.L. Vyas, Integration of ZnO nanostructures with MEMS for ethanol sensor. Sensors

and Actuators B. 161 (2012) 923- 928.

http://dx.doi.org/10.1016/j.snb.2011.11.063