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International Journal of NanoScience and Nanotechnology

ISSN 0974– 3081 Volume 1, Number 1-2 (2009), pp.81-90

© International Research Publications House

http://www.irphouse.com/sci/ijnn.htm

Visible Emission from ZnO Nanorods Synthesized by

a Simple Wet Chemical Method

P. K. Samanta*, S. K. Patra, A. Ghosh and P. Roy Chaudhuri

Department of Physics& Meteorology, Indian Institute of Technology, Kharagpur

*Corresponding author E-mail: [email protected]

Abstract

In this article, we report our investigation results of a strong visible emission

from ZnO nanorods fabricated by us using a template-free aqueous solution

based simple chemical route. Through repeated fabrication and

characterization studies, the emission around 421 nm (violet) from the

prepared nanorods is established. Attributed to the recombination of electron

at Zn interstitial and a hole in the valance band, this emission is accompanying

those caused by few weaker defect states’ emissions owing to several oxygenvacancies. This result has been further confirmed from the Raman shift

measurements. UV-visible spectroscopy was also carried out for further

studies of the optical properties of the nanorods. Our investigation, focused

around this visible emission, therefore, makes useful contribution to the ZnO

nanostructure studies, and can be used to explore potentials applications in

luminescence, lasing, nano-photonic and optoelectronic devices.

Key Words: ZnO nanorods; visible emission; photoluminescence; wet

chemical method

IntroductionZnO based nanostructure research has drawn considerable attention in the last few

years as a multi-functional material due to its versatile properties like near UV [1] and

visible (green [2], blue [3] and violet [4]) emission, optical transparency [5], electrical

conductivity [6], piezoelectricity [7] and many others promising applications in

electroacoustic transducers, gas sensors, transparent conducting coating materials,

photovoltaic devices and optical solar cells [8]. With its wide band gap of 3.37 eV at

room temperature and a large exiton bonding energy of 60 meV, it is being

extensively investigated due to its short-wavelength light emitting, and room

temperature ultraviolet lasing [9]. In this front, various ZnO nanostructures have been

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82 P. K. Samanta, S. K. Patra, A. Ghosh and P. Roy Chaudhuri

fabricated and their photoluminescence were studied by many researchers. The

emission peak in the UV range that originates due to the band edge transition or the

exciton transition is the characteristic emission for zinc oxide. But researchers had

also shown the visible emissions from nanostructured ZnO that mainly originate from

different defect states (several oxygen vacancies, Zn interstitials etc.) of ZnO.

However, these emissions from ZnO depend highly on the shape and size of the

nanostructures which indeed owe much to the technique in general, and the process

variables, in particular, of fabrication. Low dimensional nanostructures are now being

extensively explored for applications in advanced devices. Various methods such as

chemical vapour deposition [10], laser ablation [11], vacuum arc deposition [12],

sputtering [13], and hydrothermal process [14, 15] have already appeared in the

literature to have been implemented for synthesizing ZnO nanostructures. But mostly

these fabrication techniques appear to be involved process with many complex steps,require sophisticated equipments and rigorous experimental conditions. Therefore, it

is important to develop a simple method to synthesize ZnO nanorods in laboratory

environment with a view to characterizing such structures for a wide range of

applications through repeated fabrication and modification. The wet chemical method

provides a better route to fabricate multi-dimensional nanostructures in a large scale.

It is an inexpensive technique which does not involve complicated processing or huge

infrastructures/sophisticated equipments as needed in physical or chemical vapour

deposition, pulsed laser deposition, molecular beam epitaxy. Furthermore, the

fabrication based on this table-top experimental setup provides repeated preparation

that enables one to varying the process conditions for an investigational study and

repeatability in the yield. In our attempt, we have succeeded in realizing ZnOnanorods grown at room temperature that show strong emission in the visible

window. In the following sections, we outline our experimental approach towards

growing of a few targeted nanostructures followed by some typical characterization

results of our experiments. We also discuss the possible growth mechanism of these

ZnO-structures in the relevance of the chemical route of fabrication.

ExperimentalPreparation of nanorods

As an initial attempt, we started with the well-known basic fabrication technique as

reported by Wu et al [16] and followed quantitatively almost the similar type ofrecipe. But the difference is that we have done our fabrication process at room

temperature. So maintenance of a constant higher temperature is not required which

makes the process much simpler. All the reagents we used were of analytical grade

(MERCK) that did not necessitate any more purification. Under constant stirring of

NaOH solution (1M) at room temperature, zinc nitrate solution of 0.5M was added

dropwise for 15 min. Stirring was continued for 3 hours till a white precipitate

deposited at the bottom of the flask. The precipitate was then filtered and washed 2-3

times with distilled water. Then the powdered sample was dried at 600C in a furnace

for further characterizations. With this basic recipe, we then varied the key process

parameters to provide different fabrication conditions in order to grow different

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Visible Emission from ZnO Nanorods Synthesized 83

structural yields. The resulting nano-powders were characterized repeatedly for

studying their optical luminescence properties.

Characterizations of the nanorods

The X-ray diffraction data were collected on a RIGAKU diffractometer using Cu K α-

radiation over an angular range 200< 2θ<600. The morphology of the sample was

observed in field emission scanning electron microscopy (FESEM) using ZEISS

scanning electron microscope. Transmission Electron microscopy was carried out in a

JEOL JEM-2100F microscope with the accelerating voltage of 200 kV. For TEM

study a very small amount of the powder sample was first dispersed in acetone by

ultra-sonication. A drop of that solution was taken on a carbon coated grid for TEM

imaging. The room temperature PL spectrum was recorded in PERKIN ELMER LS-

55 with a Xenon lamp with the excitations of 275 nm, 300 nm and 325nm. ReinshawRaman System: RM-1000B (coupled with LEICA microscope DMLM) was deployed

to carry out the Raman shift measurement of the sample. As the excitation source, a

20mW Argon ion laser was used which operated at 514nm wavelength using an edge

filter of 200/cm as cut-off. UV-visible absorption data of the sample was collected in

Perkin-Elmer Lambda-45 spectrophotometer in the wavelength range 380-800 nm to

study further the optical absorption.

Results and DiscussionsX-ray diffraction

A typical XRD pattern of the prepared nanorods (powders) is shown in figure 1. The pattern is indexed with hexagonal unit cell structure (JCPDS card no.36-1451). We

also compare the observed relative peak intensities to that of their standard values (see

table-1).There is a small difference in the relative peak intensities of the (100) to

(002) as observed in our case imply that the ZnO nanorods fabricated by different

methods exhibit different preferred orientations. Further more no impurities were

found in the XRD pattern. Also the diffraction peaks are intensive and very sharp.

Thus high purity hexagonal ZnO nanocrystals could be obtained by this synthesis

process.

Field emission scanning electron microscopy

Figure 2 shows the morphology of the as prepared ZnO nanorods. It reveals the moststriking feature of the as obtained product is ZnO nanorods. The powder contains ZnO

nanorods of diameter ~30-50 nm and length ~ (100-150) nm. We have taken the

FESEM images from different region of the distributed sample and it was observed

that the rods are randomly distributed in the powder sample and gathered together.

Growth mechanism

The growth mechanism has been proposed by several researchers according to the

condition of the experiments [17, 18]. In the context of our synthesis, we depict the

mechanism of ZnO nanorod formation as follows: When zinc nitrate solution is added

with the NaOH solution under constant stirring it produces Zn(OH)2 colloidal

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particles. During the hydrothermal decomposition, a part of this colloidal Zn(OH)2

dissolves into Zn2+ and OH- and it continues further to form Zn(OH)42-. When their

concentration reaches to the degree of supersaturation, ZnO nuclei form. The basic

reactions are:

3 2 2 2 3 2( ) .6 2 ( ) 2 6 Zn NO H O NaOH Zn OH NaNO H O+ = + +

2 2

2 2 2 4( ) 2 2 2 ( ) 2 Zn OH H O Zn OH H O Zn OH H + − − +

+ = + + = + 2

4 2( ) 2 Zn OH ZnO H O OH − −

= + +

Transmission electron microscopy study

Further studies on the structure of the ZnO nanorods were done using transmission

electron microscopy. Figure 3(A-C) shows the TEM image of the fabricated

nanorods. The length of the nanorods varies from 100-200 nm and their diameters lies between 20-40 nm. The corresponding selected area electron diffraction (SAED)

pattern is also shown in figure 3D, which reveals the single crystalline nature of thenanorods.

Raman shift measurement

Nanocrystals or quantum dots, produced by chemical methods, normally have moredefects than corresponding bulk crystals. Raman spectra of such nanocrystalline

semiconductors are red shifted and broadened due to the relaxation of the selection

rule for the q-vector (conservation of crystal momentum) due to small crystallite sizeof the nanocrystals. The phonon uncertainty roughly goes as Δq ~ 1/d , d being the

diameter of the nanocrystals [19]. The phonon scattering will not be limited to thecenter of the Brillouin zone and phonon dispersion near the center of the Brillouin

zone should be taken into consideration .Thus, symmetry forbidden modes will also be observed along with the shifting and broadening of the first-order optical phonon

scattering modes. ZnO has hexagonal wurtzite structure which falls in the space group

C46v with two formula units per primitive cell and the Raman- active modes predicted

by the group theory for wurtzite ZnO are A1 + 2E2 +E1 . The polar modes A1 and E1

can split into transverse optical (TO) and longitudinal optical (LO) modes. Thenonpolar E2 mode is composed of two modes with a low and high frequency. Figure 4

shows the Raman spectra of the ZnO nanorods. Three prominent vibration peaks areobserved at 334, 441 and 583 cm-1. The peak at 583 cm-1, positioned between A1

(LO) and E1 (LO) optical phonon mode can be attributed to the oxygen deficiency. Itis in good agreement with the theoretical calculations of Fonoberov and Balandin

[20]. The peak at 441cm-1 corresponds to the nonpolar E2 optical phonon mode. The

334 cm-1

peak is attributed to the second order Raman process and is assigned to the2E2 mode. The strong intensity of this peak reveals that the ZnO nanorods grown at

room temperature have several oxygen deficiencies. No TO phonons are observed inthe nonresonant Raman spectrum of the grown ZnO nanorods.

Photoluminescence Spectroscopy

Room temperature photoluminescence (PL) was recorded with the excitation

wavelengths of 275 nm, 300 nm and 325nm respectively. In figure 5 the strong

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emission peak around 421nm can be assigned to the recombination of an electron atzinc interstitial and a hole in the valance band. Some other peaks are also observed at

406nm (3.05eV), 456nm (2.72eV), 485nm (2.59eV), and 527nm (2.35eV) which are

attributed to different defect state emissions. Vanheusden et al [2] had reported thatthe visible luminescence of ZnO mainly originates from different defect states such as

oxygen vacancies and Zn interstitials. Oxygen, in general, exhibit three types ofcharge states of oxygen vacancies such as Vo0, Vo+, and Vo2+. The oxygen vacancies

are located below the bottom of the conduction band (CB) in the sequence of Vo0,

Vo+, and Vo2+, from top to bottom. The peak around 527 nm can be related to singly

ionised oxygen vacancy. The green emission is the results of the recombination of a photogenerated hole with a singly ionised charged state of the specific defect. Shallow

acceptor levels are created at 0.3eV and 0.4eV above the top of the valance band (VB)

due to zinc vacancy (VZn) and oxygen interstitial (Oi) respectively. Again, Zincinterstitial (Zni) produces a shallow donor level at 0.5ev below the bottom of CB [21,

22, 23]. The positions of different defect levels are schematically shown in figure 6.From figure 6, the emission at 421nm can be assigned to the recombination of an

electron at Zni and a hole in the VB. We also observed that the PL intensity decreaseswith the increase of the excitation wavelength. The peak positions were almost same

but a very small blue-shift was observed which may be resulting due to the

polydispersity in the shape of the tips of the nanorods.

UV-visible spectroscopy

UV-visible spectroscopy was carried out to study further the optical property of the

nanorods. The room temperature UV-absorption spectra of the ZnO nanorodsdispersed in tetrahydrofuran is shown in figure 7. It shows a prominent exciton band

at 382 nm corresponds to the ZnO nanostructures. This absorption peak is red shiftedas compared to the bulk exciton absorption of ZnO (373 nm) [22] which is due to the

size effect of the nanostructures. This absorption in the visible range of wavelengthimplies that there exist more defect energy levels in the synthesized ZnO

nanostructures that are due to the specific experimental synthesis conditions.

Table 1: Comparison of the X-ray diffraction peak intensities of the standard JCPDSdata and the observed data.

XRD peak(hkl)

2θ (degree)from JCPDS

2θ (degree)observed

Intensityfrom JCPDS

Intensityobserved

(100) 31.770 31.811 57 61.95

(002) 34.422 34.495 44 68.84

(101) 36.253 36.307 100 100

(102) 47.539 47.501 23 30.63

(110) 56.603 56.565 32 47.67

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20 30 40 50 60

I n t e n s i t y

( a . u . )

( 1 1 0 )

( 1 0 2 )

( 1 0 1 )

( 0 0 2 )

( 1 0 0 )

2 (degree)

Figure 1: X-ray diffraction of the ZnO nanorods prepared at room temperature.

Figure 2: Field emission scanning electron microscopy image of the ZnO nanorods.

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Figure 3: (a)Transmission microscope image of the grown nanorods. (b) The SAED

pattern of the nanorods.

300 400 500 600 700

I n t e n s i t y ( a . u . )

583

441

334

Raman Shift (cm-1)

Figure 4: Raman scattering spectrum of the ZnO nanorods grown at room

temperature.

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400 450 500 550

50

100

150

200

250

300

Wavelength(nm)

527

485

421

----- 275 nm

___ 300 nm

-ooo- 325 nm

I n t e n s i t y ( a . u . )

Figure 5: Room temperature photoluminescence spectrum of the ZnO nanorods.

Figure 6: A schematic illustration of different defect levels of ZnO [22].

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400 450 500

2.0

2.2

2.4

2.6

2.8

3.0

381

A b s o r b a n c e ( a . u . )

Wavelength (nm)

Figure 7: UV-visible absorption spectroscopy of the nanorods.

ConclusionsWe report here the implementation of chemical reaction based nanostructure

fabrication method in the laboratory atmosphere with extensive studies to achieverepeatability and structure optimization. This approach offered producing a largequantity of ZnO nanorods at relatively high purity and with very low cost. We discuss

here in this occasion a possible formation mechanism of these structures. An

important observation is that, ZnO nanorods have a very strong photoluminescence(PL) band at visible range accompanied by few weaker defect states emissions whichagrees well the measurements from Raman spectroscopy. Further studies on the

optical properties of the nanorods agree well with the standard reported data. Thus our

experimental nanostructure studies would help exploring more potential applicationsin luminescent, nano-photonic and optoelectronic devices.

AcknowledgementsThe authors sincerely acknowledge IIT Kharagpur for providing research facilities.Two of the authors, P. K. Samanta, S. K. Patra sincerely acknowledge University

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Grant Commission (UGC), Govt. of India and Council of scientific & IndustrialResearch (CSIR) respectively for the financial support to carryout this work.

References

[1] Eva M. Wong and Peter C. Searson, Appl. Phys. Lett. 74(20) (1999)2939.[2] K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt,

Appl. Phys. Lett. 68(3)(1996) 403.[3] H.Q.Wang, G Z Wang, L C Jia, C J Tang and G H Li, J. Phys. D: Appl. Phys.

40 (2007) 6549.[4] Q.P Wang, D.H.Zhang, Z.Y.Xue, X.T.Hao, Applied Surface Science

201(2002) 123.[5] Ma Jina,, Ji Fenga, Zhang De-hengb, Ma Hong-leia, Li Shu-ying, Thin Solid

Films. 357 (1999) 98.

[6] Jason B. Baxter and Charles A. Schmuttenmaer, J. Phys. Chem. B., 110(2006),25229.

[7] Xiang Yang Kong and Zhong Lin Wang, Nano Letters. 3(2003 1625.[8] Yasuhide Nakamura MATERIALS, NNIN REU, Research Accomplishments.

(2006)74.[9] Michael H. Huang et al Science. 292 (2001) 1897.

[10] Jih-Jen Wu and Sai-Chang Liu, Adv. Mater. 14(2002)215.

[11] A. Fouchet et al, J. Appl. Phys. 96 (2004) 3228.

[12] Hirofumi Takikawa at al, Thin Solid Films. 74-80 (2000) 377.[13] Q. P. Wang, D. H. Zhang, Z. Y. Xue and X. T. Hao, Applied Surface Science.

201(2002)123.

[14] Hanmei Hu et al, Materials Chemistry and Physics 106 (2007) 58.[15] Ali Eftekhari, Foroogh Molaei, Hamed Arami, Materials Science and

Engineering A. 437 (2006) 446.[16] C. Wu et al, Materials Letters 60 (2006) 1828.

[17] C. X. Xu, A. Wei, X. W. Sun and Z. L. Dong, J. Phys. D: Appl. Phys.

39(2006)1690.[18] J. Zhang, L. Sun, J. Yin, H. Su, C.Liao and C. Yan, Chem. Mater. 14(2002)

4172.

[19] K. A. Alim, V. A. Fonoberov, M. Shamsa and A. A. Baladinl, J. Appl. Phys.97(2005)124313.

[20] V. A. Fonoberov and A. A. Balandin, Physical Review B. 70(2004)233205.

[21] P. S. Xu, Y. M. Sun, C. S. Shi, F. Q. Xu, H. B. Pan, Nucl. Instrum. Methods

Phys. Res., Sect. B. 199(2003) 286.[22] S. B. Zhang, S. H. Wei, A. Zunger, Phys. Rev. B. 63(2001) 075 205.

[23] Y. Chen, D. M. Bagnall, Z. Zhu, T. Sekiuchi, K.-T. Park, K. Hiraga, T. Yao,S. Koyama, M. Y. Shen, T. Goto, J. Cryst. Growth. 181(1997) 165.

[24] M. Haase, H. Weller and A. Henglein, J. Phys. Chem.92 (1988) 482.

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