Annealing effects on the optical and morphological properties of ZnO nanorods on AZO substrate by using aqueous solution method at low temperature
Nanoscale Research Letters
Annealing effects on the optical and morphological properties of ZnO nanorods on AZO substrate by using aqueous solution method at low temperature
Da-Ren Hang 0 1
Sk Emdadul Islam 1
Krishna Hari Sharma 1
Shiao-Wei Kuo 0 1
Cheng-Zu Zhang 1
Jun-Jie Wang 1
0 Center for Nanoscience and Nanotechnology, National Sun Yat-sen University , 70 Lienhai Rd., Kaohsiung 804 , Taiwan
1 Department of Materials and Optoelectronic Science, National Sun Yat-sen University , 70 Lienhai Rd., Kaohsiung 804 , Taiwan
Vertically aligned ZnO nanorods (NRs) on aluminum-doped zinc oxide (AZO) substrates were fabricated by a single-step aqueous solution method at low temperature. In order to optimize optical quality, the effects of annealing on optical and structural properties were investigated by scanning electron microscopy, X-ray diffraction, photoluminescence (PL), and Raman spectroscopy. We found that the annealing temperature strongly affects both the near-band-edge (NBE) and visible (defect-related) emissions. The best characteristics have been obtained by employing annealing at 400°C in air for 2 h, bringing about a sharp and intense NBE emission. The defect-related recombinations were also suppressed effectively. However, the enhancement decreases with higher annealing temperature and prolonged annealing. PL study indicates that the NBE emission is dominated by radiative recombination associated with hydrogen donors. Thus, the enhancement of NBE is due to the activation of radiative recombinations associated with hydrogen donors. On the other hand, the reduction of visible emission is mainly attributed to the annihilation of OH groups. Our results provide insight to comprehend annealing effects and an effective way to improve optical properties of low-temperature-grown ZnO NRs for future facile device applications.
Zinc oxide; Photoluminescence; Raman; Annealing
ZnO is a promising II-VI compound semiconductor
because of its excellent catalytic, optoelectronic, and
piezoelectric properties. It has been demonstrated to have
diverse applications in electronic, optoelectronic, and
electrochemical devices, such as ultraviolet (UV) lasers,
light-emitting diodes, high-performance nanosensors, and
solar cells [
]. In addition to the low cost, ease of
availability, and chemical stability, the wide direct bandgap
of 3.37 eV and large excitonic binding energy (60 meV at
300 K) make ZnO a highly competitive material to GaN. It
was also reported that textured ZnO films may have higher
quantum efficiency than GaN films . Nowadays, ZnO
thin films and nanostructures can be synthesized by using
various deposition techniques, such as molecular beam
epitaxy (MBE) [
], pulsed laser deposition (PLD) [
metal-organic chemical vapor deposition (MOCVD) [
chemical vapor deposition (CVD) [
], and aqueous
solution deposition [
High-temperature techniques such as CVD and thermal
evaporation have been mainly employed to grow aligned
ZnO nanostructures, for example, nanorods (NRs). These
processes have disadvantages of high energy consumption
and requirement of expensive infrastructure. Here, we
adopt an inexpensive and simple method to prepare
uniformly distributed and well-aligned vertical ZnO NRs,
whereas no catalyst or seeding step is required to initiate
controlled growth. This approach is based on a one-step
electrochemical processing of reliably nontoxic and
abundant materials in aqueous solution at low temperature
(≤80°C). Moreover, it allows for large-scale processing at
low cost and facile integration for complex devices. The
substrate of our choice is aluminum-doped zinc oxide
(AZO). Meanwhile, transparent and conductive AZO
substrate is an alternative to the ITO glass. AZO has
better lattice matching than ZnO, so the development of
ZnO/AZO devices is now a hot pursuit.
It is known that due to oxygen vacancies (VO), inherent
n-type ZnO is formed and its carrier concentration
depends on post-growth annealing treatment. The intrinsic
defects in ZnO are always associated with various
deposition processes. The understanding of defect properties is
very useful to improve the quality of ZnO. Generally,
post-deposition annealing treatment is a convenient and
appropriate way to modify intrinsic defects and improve
the crystallinity of ZnO. Proper annealing is an effective
way to obtain high-quality ZnO material. There are many
reports on the thermal treatment of ZnO for different
annealing conditions such as annealing temperatures and
gas environments to improve the optical properties of
ZnO. In this paper, ZnO NRs were synthesized by an
aqueous solution deposition method and effects of
postgrowth annealing were studied. The structural,
morphological, and optical characteristics have been studied after
annealing processes. Mechanisms that are responsible for
the annealing effects are investigated.
ZnO NRs were deposited on AZO substrates by an
aqueous solution method. Zinc nitrate hexahydrate (Alfa Aesar,
Ward Hill, MA, USA) was used as the zinc source. Ethanol
amine (Merck, Whitehouse Station, NJ, USA) and
hexamethylenetetramine (HMTA) were used as the stabilizer
and base, respectively. Firstly, the AZO substrates were
cleaned through sonication in a mixture of acetone and
isopropyl alcohol (1:1), followed by cleaning with deionized
water and drying in N2 atmosphere before use. The zinc
precursor solution was prepared by dissolving equimolar
zinc nitrate hexahydrate, HMTA, and ethanol amine in
deionized water. The above solution was stirred by using a
magnetic stirrer at 60°C for 10 min. NRs were grown by
dipping the as-cleaned substrate horizontally into the
prepared solution and were covered with a lid for 30 min at
80°C on a regular laboratory hot plate. The prepared NRs
were annealed in air for 2 h by using a
microprocessorcontrolled furnace for different annealing temperatures
ranging from 200°C to 600°C. X-ray diffraction (XRD) was
conducted to examine the structure and orientation of
ZnO. The surface morphology of the prepared NRs was
investigated by scanning electron microscopy (SEM) (JEOL
6380, JEOL Ltd., Akishima-shi, Japan). For the
photoluminescence (PL) investigations, the samples were excited
by a chopped He-Cd laser beam working at 325 nm. The
PL signal was dispersed by a Jobin Yvon Triax 550
monochromator (Jobin Yvon Inc., Edison, NJ, USA) equipped
with a 2,400 rules/mm grating. A Hamamatsu R928
photomultiplier tube (Hamamatsu Photonics K.K., Iwata, Japan)
equipped with a lock-in amplifier was used to record the
optical intensity of the selected emission. A closed-cycle
optical cryostat was used for low temperatures down
to 10 K. Room-temperature (RT) Raman scattering
measurements were performed in a backscattering
configuration on a micro-Raman setup equipped with a
Jobin Yvon iHR320 spectrometer and a multi-channel
TE-cooled (−70°C) CCD detector.
Results and discussion
Evaluation of the as-grown sample
Figure 1a shows the tilt-view SEM image of the
asgrown ZnO NRs on an AZO substrate. A high density of
ZnO NRs grew vertically on the substrate. The diameter
of the nanorods is about 200 nm. The crystallinity of the
grown ZnO NRs was investigated by using XRD. As
shown in Figure 1b, the XRD pattern of θ-2θ scan of the
as-grown ZnO NRs shows only the ZnO (002) peak
(black solid curve), indicating that the c-plane of ZnO is
oriented parallel to the basal plane of the AZO substrate.
It indicates that individual ZnO NRs, crystallized along
Figure 1 SEM image and XRD patterns of the ZnO NRs on AZO.
(a) SEM image of the as-grown ZnO NRs on AZO. (b) XRD patterns
of the as-grown and annealed ZnO NRs on AZO.
the c-axis direction of ZnO, were all vertically aligned on
the AZO substrate.
The black solid curve in Figure 2 shows the full RT PL
spectrum of the as-prepared ZnO NRs. The PL spectrum
comprises one UV emission band with a peak at 3.28 eV,
which is attributed to the near-band-edge (NBE) emission.
In addition, there is a broad visible emission band with
comparable intensity, centered at approximately 2.15 eV,
which can be ascribed to the defect emission (DE) [
It is frequently observed in ZnO prepared by aqueous
solution and is often considered to be caused by atomic
defects, such as oxygen vacancies in ZnO. It means
that a lot of photo-generated carriers in ZnO NRs do
not recombine close to the band edge, resulting in a
poor NBE efficiency.
Effects of annealing on structural and optical properties
In order to improve the optical properties, we performed
post-deposition thermal treatment in air at different
temperatures for 2 h. Figure 1b displays XRD patterns of
the annealed ZnO NRs. The intensities of the (002)
diffractions are relatively higher for the samples annealed
at various temperatures, suggesting the improvement of
the crystalline quality. Then, the effects of thermal
annealing on the PL properties of ZnO NRs were investigated.
As Figure 2 shows, we found that there is pronounced
influence on the NBE emission after annealing. The inset
of Figure 2 shows the intensity ratio of NBE to visible DE
against annealing temperature. It can be found that the
ratio is 1, 4.38, 6, and 1.45 for the as-grown and 200°C-,
400°C-, and 600°C-annealed samples, respectively. We get
the strongest NBE from the sample annealed at 400°C.
However, the NBE decreases again with the high
temperature treatment at 600°C. Moreover, it was noted
that the annealing time of 2 h is the optimized duration.
Figure 3 presents the RT PL spectra of the samples
annealed at 400°C in different duration time ranging from
60 to 150 min. It shows that the intensity of NBE is
enhanced with increasing annealing time until 120 min.
However, it is weakened with prolonged annealing time.
To understand the influence of annealing on the
morphological properties, we performed SEM
measurements. Figure 4 shows the SEM images of the as-grown
ZnO NRs and annealed ZnO NRs at various annealing
temperatures. It can be observed that the hexagonal
crystallite appears in all the samples, having average diameter
ranging from 200 to 300 nm. The whole surface looks
smooth and uniform in the nonannealed and annealed
(at 200°C and 400°C) samples. However, at 600°C
annealing temperature, surface smoothness and uniformity
reduce dramatically and some void space is presented on
the surface. It suggests that with increasing annealing
temperatures, small crystallites start to coalesce together to
form larger crystallites [
]. It may be attributed to the
annealing-induced coalescence of small grains by grain
boundary diffusion [
]. SEM images also indicate that
hexagonal crystal phase is less distinct for the as-grown
sample. Distinct hexagonal phase appears gradually with
increasing annealing temperatures. Furthermore, it
appears that the sample with 400°C annealing is best with
respect to the homogeneous crystallinity. Together with
previous PL results, it implies that 400°C is the optimum
annealing temperature to get high-quality ZnO NRs.
Low-temperature PL characterization
To reveal more optical properties, the 10 K PL spectra
for the as-grown and 400°C-annealed samples are shown
in Figure 5. It is clear that the NBE for the annealed
sample is stronger and sharper than that for the
asgrown sample. The full width at half maximum is 47
and 23.5 meV for the as-grown and 400°C-annealed
samples, respectively. It gives a quantitative measure of
the improved optical quality of our annealed sample.
Moreover, we find that there are 3-meV redshifts of NBE
emission after annealing at 400°C, as shown by the
vertical dashed lines in Figure 5. In order to understand the
nature of the enhanced NBE emission, we check the
temperature-dependent activated behavior in the inset of
where I(T) is the integrated PL intensity at T (K), I0 is a
scaling factor, and P is a process rate parameter [
It provides important information to the origin of carrier
recombination in various semiconductors [
dashed line in the inset of Figure 5 is the least square fit
of data with Equation 1. The fitted value of P and Ea is
5.2 and 19.5 meV, respectively. We obtain a high thermal
activation energy. It is in close agreement with the
activation energy of the hydrogen donor in ZnO deduced by
PL (21 to 25 meV) [
]. Therefore, it is indicative that
excitonic recombination at the hydrogen donor (HO)
dominates the NBE after annealing treatment. Next, the
temperature dependence of the emission peak E(T) was
studied, as shown in Figure 6. It is known that the
bandgap energy of ZnO decreases with increasing temperature.
The change of bandgap energy with temperature is
described by Varshni's empirical equation. Assuming that
the peak positions of the NBE vary with the temperature
as the energy bandgap, the dependence of E(T) on
temperature can be fitted with the following expression:
EðT Þ ¼ Eð0Þ− β þ T ;
where E(0) is the transition energy at zero temperature
and α and β are fitting parameters referred to as Varshni's
coefficients . Both α and β are material-dependent.
The β value is expected to be correlated with the Debye
temperature, but a range of values were reported for
]. The fitting results, which are denoted
by the solid line in Figure 6, yield E(0) = 3.364 eV, α =
5.5 × 10−4 eV/K, and β = 250 K.
Origin of enhanced optical properties
Based on the results above, we discuss annealing effects
on the emission properties of our low-temperature-grown
ZnO NRs. The enhancement of NBE by annealing might
be explained by two previously proposed mechanisms.
One is the elimination of unwanted functional groups
acting as nonradiative centers on the surface of ZnO, and
the other is the improvement of the crystal quality
resulting from removal of intrinsic defects [
]. But the
anomalous behavior after 400°C treatment, which shows reduced
NBE, cannot be explained by the mechanisms above. In
our aqueous solution growth of ZnO, generation of
interstitial H is highly possible. Hydrogen defects can be
introduced as a result of incomplete dehydration during the
formation of ZnO [
]. Most of these initial hydrogen
states, interstitial H or H complex (hydroxyl group and
complex with other defects), are not active donors. During
annealing at 400°C, intrinsic defects are passivated and
initially trapped hydrogen is released. The interstitial H
has high mobility and can move around the lattice. Since
there are oxygen vacancies in the ZnO NRs, the interstitial
hydrogen can be trapped inside oxygen vacancies, forming
HO. Substitutional hydrogen at the oxygen site, from
many experimental and theoretical investigations, is
believed to be an important shallow donor in ZnO [
The existence of HO exactly accounts for the observed
activation energy in our PL study [
]. The decrease of NBE
intensity under prolonged annealing or annealing higher
than 400°C can then be attributed to the dissociation of
hydrogen donors. Therefore, we attributed our NBE
enhancement to the hydrogen donor formation activated by
thermal annealing. The inset of Figure 3 shows illustrative
diagrams for recombination processes that have taken
place in (1) NBE in the as-grown sample and (2) NBE in
the annealed sample. From the schematic diagrams, we
can understand the redshift of NBE after annealing
treatment, which is due to the annealing-induced activation of
The visible emission drops exactly in the opposite way
as NBE enhances with annealing temperature, that is, it
decreases with increasing annealing temperature up to
400°C and then it again increases at 600°C. Figure 3 also
shows that visible emission decreases with increasing
annealing time as well, but it is noted that after 120 min,
further reduction of visible emission is insignificant. The
chemical origin of the DE is intriguing. Our result shows
that the broad DE is already reduced even with a
lowtemperature annealing at 200°C. It is indicative of the
presence of OH groups whose desorption temperature is
at approximately 150°C [
]. The assignment of this
visible emission to the presence of hydroxyl groups is in
agreement with a previous report on visible luminescence
in ZnO nanocrystals [
]. Moreover, a previous study on
the O-H local vibrational modes in ZnO also confirmed
that such a low-temperature annealing results in the
removal of OH groups [
]. Therefore, we attribute the
corresponding reduction of visible (defect-related)
emission to the annihilation of OH groups.
Finally, we carried out RT Raman scattering
measurements to understand the light-scattering properties.
Raman spectra for the nonannealed and 400°C-annealed
samples are shown in the inset of Figure 6. Two dominant
peaks have been clearly resolved. One peak is observed
around 446 cm−1, which is attributed to the nonpolar
optical phonon E2 (high) mode of wurtzite ZnO [
the other peak at 588 cm−1, known as E1 (LO), which is
ascribed to the defect formation of oxygen vacancies
]. No other mode related to defect-induced local
vibration mode is observed. The weak intensity of E1 (LO)
suggests that there are relatively low oxygen vacancies.
Since both spectra contain E1 (LO) contribution, the low
oxygen vacancies should be responsible for the residue
visible emission after annealing treatment.
In conclusion, we present the investigation of annealing
effect on the optical and structural properties of
vertically aligned ZnO NRs on AZO substrates by a
singlestep aqueous solution method at low temperature. The
annealing temperature strongly affects both the NBE
and visible emissions. We found the optimum annealing
temperature to be 400°C. It yields a sharp and intense
NBE emission and effectively suppressed visible
emission. The enhancement of NBE is due to the activation
of radiative recombinations associated with hydrogen
donors while the reduction of DE is ascribed to the
annihilation of OH groups. These results are useful to understand
and optimize ZnO NRs grown in a low-temperature
solution. Our approach has the advantages of low cost, fine
quality, and straightforward one-step synthesis, which is
promising to realize facile and controlled ZnO-based
The authors declare that they have no competing interests.
DRH directed the project and finalized this manuscript. SEI carried out the
sample preparation and XRD, SEM, PL, and Raman measurements. KHS and
JJW helped perform the PL measurements. SWK provided equipment
support in the synthesis work. CZZ helped prepare the samples. All the
authors read and agreed with the final version of the paper.
This work was supported by the Ministry of Science and Technology of the
Republic of China under Grant No. MOST 103-2112-M-110-001.
1. Alivov YI , Özgür Ü , Dogan S , Johnstone D , Avrutin V , Onojima N , Liu C , Xie J , Fan Q , Morkoç H : Photoresponse of n-ZnO/p-SiC heterojunction diodes grown by plasma-assisted molecular-beam epitaxy . Appl Phys Lett 2005 , 86 : 241108 .
2. Zhu H , Shan CX , Yao B , Li BH , Zhang JY , Zhao DX , Shen DZ , Fan XW : High spectrum selectivity ultraviolet photodetector fabricated from an n-ZnO/p-GaN heterojunction . J Phys Chem C 2008 , 112 : 20546 .
3. Hsueh HT , Chang SJ , Weng WY , Hsu CL , Hsueh TJ , Hung FY , Wu SL , Dai BT : Fabrication and characterization of coaxial p-copper oxide/n-ZnO nanowire photodiodes . IEEE Trans Nanotechnol 2012 , 11 : 127 .
4. Huang H , Fang G , Mo X , Yuan L , Zhou H , Wang M , Xiao H , Zhao X : Zero-biased near-ultraviolet and visible photodetector based on ZnO nanorods/n-Si heterojunction . Appl Phys Lett 2009 , 94 : 063512 .
5. Chu S , Lim JH , Mandalapu LJ , Yang Z , Li JL : Sb-doped p-ZnO/Ga-doped n-ZnO homojunction ultraviolet light emitting diodes . Appl Phys Lett 2008 , 92 : 152103 .
6. Wu J-K , Chen W-J , Chang Y-H , Chen Y-F , Hang D-R , Liang C-T, Lu J-Y : Fabrication and photo-response of ZnO nanowiress/CuO coaxial heterojunction . Nanoscale Res Lett 2013 , 8 : 387 .
7. Yu P , Tang ZK , Wong GK , Segawa Y , Kawasaki M : Stimulated emission at room temperature from ZnO quantum dot films . In ICPS'96. 23th International Conference on the Physics of Semiconductors: 21-26 July 1996 ; Berlin. Edited by Scheffler M , Zimmermann R . Singapore: World Scientific; 1996 : 1453 .
8. Wang H-C , Liao C-H, Chueh Y-L , Lai C-C , Chen L-H, Tsiang RC-C: Synthesis and characterization of ZnO/ZnMgO multiple quantum wells by molecular beam epitaxy . Opt Mater Express 2013 , 3 : 237 .
9. Gluba MA , Nickel NH , Hinrichs K , Rappich J : Improved passivation of the ZnO/Si interface by pulsed laser deposition . J Appl Phys 2013 , 113 : 043502 .
10. Hauschild R , Lange H , Priller H , Klingshirn C , Kling R , Wang A , Fan HJ , Zacharias M , Kalt H : Stimulated emission from ZnO nanorods . Phy Status Solidi B 2006 , 243 : 853 .
11. Chou MM-C, Hang D-R , Chen C , Wang SC , Lee CY : Nonpolar a-plane ZnO growth and nucleation mechanism on (1 0 0) (La, Sr)(Al, Ta)O3 substrate . Mater Chem Phys 2011 , 125 : 791 .
12. Chou MM-C, Hang D-R , Chen C , Liao YH : Epitaxial growth of nonpolar m-plane ZnO (10-10) on large-size LiGaO2 (100) substrates . Thin Solid Films 2010 , 519 : 3627 .
13. Chen C , Lan YT , Chou MM -C, Hang D-R , Yan T , Feng H , Lee CY , Chang SY , Li CA : Growth and characterization of vertically aligned nonpolar [110̅0] orientation ZnO nanostructures on (100) γ-LiAlO2 substrate . Cryst Growth Des 2012 , 12 : 6208 .
14. Ting C-C , Li C-H, Kuo C-Y, Hsu C-C, Wang H-C, Yang M-H : Compact and vertically-aligned ZnO nanorod thin films by the low-temperature solution method . Thin Solid Films 2010 , 518 : 4156 .
15. Chen W-J , Wu J-K , Lin J-C , Lo S-T, Lin H-D , Hang D-R , Shih MF , Liang C-T , Chang YH : Room-temperature violet luminescence and ultraviolet photodetection of Sb-doped ZnO/Al-doped ZnO homojunction array . Nanoscale Res Lett 2013 , 8 : 313 .
16. Alvi NUH , Hussain S , Jensen J , Nur O , Willander M : Influence of helium-ion bombardment on the optical properties of ZnO nanorods/p-GaN light-emitting diodes . Nanoscale Res Lett 2011 , 6 : 628 .
17. Özgür Ü , Alivov YI , Liu C , Teke A , Reshchikov MA , Doğan S , Avrutin V , Cho SJ , Morkoç H : A comprehensive review of ZnO materials and devices . J Appl Phys 2005 , 98 : 041301 .
18. Sengupta J , Sahoo RK , Mukherjee CD : Effect of annealing on the structural, topographical and optical properties of sol-gel derived ZnO and AZO thin films . Mater Lett 2012 , 83 : 84 .
19. Pankove JI : Optical Properties in Semiconductors. New York: Dover; 1971 .
20. Hang D-R , Islam SE , Sharma KH , Chen C , Liang C-T , Chou MM -C: Optical characteristics of nonpolar a-plane ZnO thin film on (010) LiGaO2 substrate . Semicond Sci Tech 2014 , 29 : 085004 .
21. Hang D-R , Sharma KH , Islam SE , Chen C , Chou MM -C: Resonant Raman scattering and photoluminescent properties of nonpolar a-plane ZnO thin film on LiGaO2 substrate . Appl Phys Express 2014 , 7 : 041101 .
22. Hang D-R , Chou MM-C , Chang L , Lin JL , Heuken M : Optical characteristics of m-plane InGaN/GaN multiple quantum well grown on LiAlO2 (100) by MOVPE . J Cryst Growth 2009 , 311 : 2919 .
23. Huang XH , Tay CB , Zhan ZY , Zhang C , Zheng LX , Venkatesan T , Chua SJ : Universal photoluminescence evolution of solution-grown ZnO nanorods with annealing: important role of hydrogen donor . Cryst Eng Comm 2011 , 13 : 7032 .
24. Dev A , Richters JP , Sartor J , Kalt H , Gutowski J , Voss T : Enhancement of the near-band-edge photoluminescence of ZnO nanowires: important role of hydrogen incorporation versus plasmon resonances . Appl Phys Lett 2011 , 98 : 131111 .
25. Lee SH , Lee JS , Ko WB , Sohn JI , Cha SN , Kim JM , Park YJ , Hong JP : Photoluminescence analysis of energy level on Li-doped ZnO nanowires grown by a hydrothermal method . Appl Phys Express 2012 , 5 : 095002 .
26. Yang LL , Zhao QX , Willander M , Yang JH , Ivanov I : Annealing effects on optical properties of low temperature grown ZnO nanorod arrays . J Appl Phys 2009 , 105 : 053503 .
27. Brauer G , Anwand W , Grambole D , Skorupa W , Hou Y , Andreev A , Teichert C , Tam KH , Djurisic AB : Non-destructive characterization of vertical ZnO nanowire arrays by slow positron implantation spectroscopy, atomic force microscopy, and nuclear reaction analysis . Nanotechnology 2007 , 18 : 195301 .
28. Du MH , Biswas K : Anionic and hidden hydrogen in ZnO . Phys Rev Lett 2011 , 106 : 115502 .
29. Xie R , Sekiguchi T , Ishigaki T , Ohashi N , Li D , Yang D , Liu B , Bando Y : Enhancement and patterning of ultraviolet emission in ZnO with an electron beam . Appl Phys Lett 2006 , 88 : 134103 .
30. Norberg NS , Gamelin DR : Influence of surface modification on the luminescence of colloidal ZnO nanocrystals . J Phys Chem B 2005 , 109 : 20810 .
31. Shi GA , Stavola M , Pearton SJ , Thieme M , Lavrov EV , Weber J : Hydrogen local modes and shallow donors in ZnO . Phys Rev B 2005 , 72 : 195211 .
32. Exarhos GJ , Sharma SK : Influence of processing variables on the structure and properties of ZnO films . Thin Solid Films 1995 , 270 : 27 .
33. Xing YJ , Xi ZH , Xue ZQ , Zhang XD , Song JH , Wang RM , Xu J , Song Y , Zhang SL , Yu DP : Optical properties of the ZnO nanotubes synthesized via vapor phase growth . Appl Phys Lett 2003 , 83 : 1689 .
34. Rajalakshmi M , Arora AK , Bendre BS , Mahamuni S : Optical phonon confinement in zinc oxide nanoparticles . J Appl Phys 2000 , 87 : 2445 .