Copyright © 2009 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol.9, 1–4, 2009
1
1
1
1
3
3
4
1 ∗
2
2
4
1 School of Advanced Materials and System Engineering, Kumoh National Institute of Technology,
Gumi, Gyeongbuk 730-701, Korea
Department of Advanced Materials Engineering and SAINT, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Korea
3 School of Advanced Materials Engineering and Research Center for Advanced Materials Development,
Chonbuk National University, Chonju 561-756, Korea
R&D Affairs Department, Gumi Electronics and Information Technology Research Institute, Gyeongbuk 730-853, Korea
ZnO nanorods and thin films were synthesized on Si(100) substrates at a low temperature by controlling the ZnO seed formation in an aqueous solution process. Vertically well-aligned ZnO nanorods with a single-crystalline hexagonal structure and c -axis growth orientation were obtained despite having a different crystal structure than a large lattice mismatch with the Si(100) substrate.
In addition, the authors suggest that the lateral growth of ZnO nanorods causes them to merge together into ZnO thin films during growth into the aqueous solution. It is also suggested that the significant blueshift of the main emission band in the photoluminescence spectrum obtained from the nanorod sample at 10 K can be mainly attributed to Burstein-Moss effects in the ZnO nanorods.
ZnO, Nanorod, Thin Film, Aqueous Solution Process, Si Substrate,
Photoluminescence, Blueshift.
ZnO is a wide band gap (3.37 eV) semiconductor with a large exciton binding energy of 60 meV that exhibits ultraviolet (UV) light emission, chemical and biological sensitivity, piezoelectricity, and transparent conductivity.
1–4
Recently, one-dimensional (1D) ZnO nanostructures such as nanowires, 5 and nanotubes 7 nanorods, 6 have been extensively investigated due to their great potential for use in fundamental research on the novel physical and chemical properties of nano-structured materials as well as for their application in optoelectronic nano-scale devices.
In order to fabricate ZnO 1D nanostructures and thin films, various techniques such as metal organic chemical vapor deposition, 6 the vapor–liquid–solid epitaxial pulsed laser deposition, 8 mechanism, 1 and electrodeposition 10 spray pyrolysis, 9 have been used.However, these methods require extreme reaction conditions such as high temperature and pressure control, which are impractical for the large-scale production of ZnO 1D nanostructures and thin films.Thus, the fabrication of ZnO thin films and well-aligned 1D ZnO nanostructure arrays
∗ Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2009, Vol. 9, No. xx over a large substrate area or a flexible substrate has been very challenging.Recently, the growth of ZnO nanorods and thin films in aqueous solution at a low temperature was reported.
11 12 Aqueous solution processes have shown potential for applications in transparent thin-film transistors and solar cells due to their low growth temperatures (below 100 C).
4 13 However, the device applications might be reinforced if the growth mode of ZnO can be controlled to a high degree of precision either nanostructures or thin films as a function of growth condition.The authors report the low-temperature growth of ZnO nanorods and thin films on Si(100) substrates by controlling the ZnO seed formation in a simple aqueous solution route.
First, nanocrystal ZnO seed particles for the synthesis of
ZnO nanorods and thin films were prepared in a solution of
0.01 M zinc acetate [Zn(C
2
H
3
O
2 2
] dissolved in de-ionized
(DI) water.The reaction temperature and reaction time were about 80 C, 15 min.The
p -type Si(100) substrates were cleaned ultrasonically in acetone, methanol, and DI water for 5 min.Then, the seed solution was coated onto p -type Si(100) substrates by a spin coater at 1000 rpm for
1533-4880/2009/9/001/004 doi:10.1166/jnn.2009.1762
1

Low-Temperature Solution-Based Growth of ZnO Nanorods and Thin Films on Si Substrates
80 s.Subsequently, the seed-coated substrates were preheated at 100 C for 30 min in an air ambient to remove the residual solvent.The procedures from spin coating to preheating were repeated four and eight times for the growth of ZnO nanorods and thin films, respectively.Then, only eight-layer seed films were thermally annealed in a
(10%) tube furnace at 500 C for 30 min in an Ar and O
2 mixed ambient.
After the seed-layer formation, the samples were immersed in the same main growth solution.ZnO
nanorods and thin films were formed by the continuous supply of zinc ions and hydroxyl radicals into the aqueous solution consisting of 0.025 M zinc nitrate hexahydrate [Zn(NO
[C
6
H
12
N
4
3 2
· 6H
2
O], 0.025 M hexamethylenetetramine
] (HMT), and DI water.The main growth of
ZnO nanorods and thin films was carried out at 90 C for four hours.Finally, the grown ZnO nanorod and thin film samples were dried at 100 C for several hours in an air ambient.The grown ZnO nanorods and thin films were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements in order to evaluate their morphological, structural, and optical properties.
Jin et al.
Figure 1 shows the FE-SEM images for the surface morphologies of ZnO nanorods and thin films.A high density of ZnO nanorods grew vertically on the Si substrates, as shown in Figure 1(a).The diameters of the nanorods ranged from 80 to 250 nm and the length was approximately 1.2
m.Achieving large-scale and low-temperature growth of vertically well-aligned ZnO nanorods on Si(100) substrates is one of the most important issues in ZnO-based nanodevice fabrication because
Si as a substrate material for ZnO nanostructure synthesis has many advantages such as low cost, high electrical conductivity, and well-developed Si-based semiconductor process technologies.
Figure 1(b) illustrates the surface morphology of ZnO thin films with large and densely packed grains.Though
they share the same main growth conditions, the formation of the thin film rather than nanorods occurs because the
ZnO seeds have a higher annealing temperature and larger seed density in the thin film.This result is consistent with previous work 14 suggesting that a high annealing temperature enhances the interaction among the seed particles and causes the formation of bigger ZnO seeds.This fact indicates that the size of the seed particles is one of the key factors that influence the formation of ZnO thin films with a large grain size.In addition, although the preferred c -axis vertical growth is energetically prevailing in ZnO, lateral growth of ZnO also occurs simultaneously during the main growth.The lateral growth of ZnO nanorods causes them to merge together into a ZnO thin film.
2
Fig. 1.
FE-SEM images of (a) ZnO nanorods and (b) ZnO thin films on
Si(100) substrates at low temperature of 90 C via the aqueous solution method.
Figure 2 shows the XRD patterns of the ZnO nanorods and the thin film grown on Si(100) substrates.As shown in Figure 2(a), the XRD –2 scan of the ZnO nanorods on the Si(100) substrate shows only the ZnO(002) and
(004) peaks, indicating that the ZnO(001) planes are oriented parallel to the basal plane of the substrate.This
result indicates that individual ZnO nanorods were grown along the preferred c -axis [0001] direction of hexagonal ZnO, which corresponds well to the FE-SEM image shown in Figure 1(a).However, an additional (100) peak is observed in the ZnO thin film, although the intensities of the ZnO(002) and (004) peaks are much stronger compared to the ZnO nanorods, indicating the polycrystalline nature of the ZnO thin film grown on Si via the aqueous solution method.
To investigate the optical properties of the ZnO nanorod and thin film samples presented in Figure 1, PL measurements were performed using the 325 nm line of a
He-Cd laser as an excitation source.PL spectra measured at room temperature (RT) are shown in Figure 3.The
spectra obtained from both samples are composed of an emission band at approximately 3.26 eV (thin film) and
3.32 eV (nanorods) with a strong deep-level emission band around 2.2 eV. The first emission band coincides with the well-known excitonic emission band of ZnO, which can be attributed to the recombination of free excitons.
15 16
J. Nanosci. Nanotechnol. 9, 1–4 , 2009

Jin et al.
Low-Temperature Solution-Based Growth of ZnO Nanorods and Thin Films on Si Substrates
300 K
ZnO thin film
3.26 eV
ZnO nanorods
(x 4)
3.32 eV
Fig. 2.
XRD –2 scan results of (a) ZnO nanorods and (b) ZnO thin film grown on Si(100) substrates.
The broad deep-level emission band around 2.2 eV, which is known to be related to the presence of the slightly ionized oxygen vacancies or other point defects, is relatively strong.
15 16 The observation of a strong and broad, deep-level emission band from ZnO nanostructures or thin films grown via an aqueous solution at low temperature is in accordance with previous studies.
11 12 16 Interestingly, the free exciton emission band around 3.32 eV from the
ZnO nanorod sample at RT, which is at a slightly higher energy compared to that of ZnO bulks, thin films, or nanostructures may be attributed to Burstein-Moss (BM) effects or low-dimensional quantum confinement effects.
17–19
To examine the origin of the UV emission, PL measurements were carried out at a low temperature.In the
PL spectrum from the ZnO thin film, a main peak labeled
“D o X” at 3.350 eV is attributed to the neutral donor-bound exciton emission.For the ZnO nanorods, the relatively broader D o X emission band at 3.371 eV was observed with a tail up to about 3.55 eV located at the higher energy with respect to D o X emission of ZnO thin films where the D is generally located from 3.355 to 3.366 eV.
20 o X
There are two possible explanations for the blueshift: One is lowdimensional quantum confinement effects from the ZnO nanorods, and the other is band gap widening due to BM effects, which could occur due to the unintentional doping effect of impurities into nanorod source materials and a
J. Nanosci. Nanotechnol. 9, 1–4, 2009
2.2
2.4
2.6
2.8
3.0
Photon energy (eV)
3.2
3.4
Fig. 3.
RT PL spectra obtained from ZnO nanorod and thin film samples.
large number of point defects, which are naturally formed during growth at low temperature (below 100 C).
The quantum confinement effect is not likely to be responsible for the blueshift of the emission band here since the dimensions of the nanorods, as shown in
Figure 4(a), are much larger than the excitonic ZnO Bohr radius of 2.34 nm.
21 In addition, the emission band shift toward a lower-energy side with temperature increase is clearly observed in the temperature-dependent PL measurements (not shown), which is very similar to the optical
(a)
ZnO thin film
(b)
ZnO nanorods
D o
X
(3.350 eV)
DºX
(3.371 eV)
10 K
10 K
2.8
3.0
3.2
Photon energy (eV)
3.4
3.6
Fig. 4.
PL spectra, measured at 10 K, from ZnO thin film (a) and nanorod (b) samples.
3

Low-Temperature Solution-Based Growth of ZnO Nanorods and Thin Films on Si Substrates behavior of ZnO thin films.It was found that the band gap shrinkage due to thermal energy conforms to the Varshni equation 22 in both samples, indicating that the blueshift is less related to the quantum confinement effect.Interestingly, the band gap shrinkage of the nanorod sample with increasing temperature is more dramatic than that of the thin film sample.This result suggests that the nanorod sample has a much higher carrier concentration than the thin film sample.
Alternatively, the higher energy shift of the PL spectrum obtained from the nanorod sample can be explained by the BM shift.The Fermi Level in the conduction band of the degenerate semiconductor leads to the widening of the energy band following by 19
E =
8 h 2 m ∗
3 n 2 / 3
The electron effective mass in ZnO is expressed as m ∗ and n is the carrier concentration.The main peak position of the D o X emission from the thin film sample is 3.350 eV at 10 K; whereas, the main peak position from the nanorod sample is 3.371 eV. In order to obtain this energy blueshift of 21 meV by the BM shift, the carrier concentration estimated from the above equation should be approximately
1 1 × 10 19 cm − 3 , which could be a possible carrier concentration.Considering the clear redshift of the emission band with increasing temperature and estimated carrier concentration, we suggest that the blueshift of the PL spectrum from the nanorods is mainly due to the BM effect in the
ZnO nanorods.
Low-temperature growth of ZnO nanorods and thin films on Si(100) substrates by controlling the ZnO seed particle size in a simple aqueous solution route was accomplished.
It was found that vertically well-aligned ZnO nanorods on
Si(100) have a single-crystalline hexagonal structure and the preferred c -axis growth orientation in XRD measurements.In addition, the authors suggest that the formation of ZnO thin films via this aqueous solution route can be mainly attributed to the lateral growth of ZnO nanorods.
The higher energy shift of the PL spectrum obtained from the nanorod sample can be explained by the BM effect in the ZnO nanorods.
Acknowledgments: This work was supported by the
Korea Research Foundation Grant funded by the Korean
Government (MOEHRD, Basic Research Promotion Fund)
(KRF-2007-313-D00475).
Jin et al.
1.
M.H.Huang, S.Mao, H.Feick, H.Yan, Y.Wu, H.Kind, E.Weber,
R.Russo, and P.Yang, Science 292, 1897 (2001) .
2.
J.Goldberger, D.J.Sirbuly, M.Law, and P.Yang, J. Phys. Chem. B
109, 9 (2005) .
3.
Z.L.Wang and J.Song, Science 312, 242 (2006) .
4.
Y.Y.Noh, X.Cheng, H.Sirringhaus, J.I.Sohn, M.E.Welland, and
D.J.Kang, Appl. Phys. Lett.
91, 043109 (2007) .
5.
Z.L.Wang, Mater. Today 7, 26 (2004) .
6.
W.I.Park, G.C.Yi, M.Kim, and S.J.Pennycook, Adv. Mater.
14,
1841 (2002) .
7.
P.X.Gao, C.S.Lao, Y.Ding, and Z.L.Wang, Adv. Funct. Mater.
16, 53 (2006) .
8.
H.J.Son, K.A.Jeon, C.E.Kim, J.H.Kim, K.H.Yoo, and S.Y.
Lee, Appl. Surf. Sci.
253, 7848 (2007) .
9.
M.G.Ambia, M.N.Islam, and M.O.Hakim, J. Mater. Sci.
29,
6575 (1994) .
10.
N.Saito, H.Haneda, T.Sekiguchi, N.Ohashi, I.Sakaguchi, and
K.Koumoto, Adv. Mater.
14, 418 (2002) .
11.
J.H.Kim, E.M.Kim, D.Andeen, D.Thomson, S.P.DenBaars, and F.F.Lange, Adv. Funct. Mater.
17, 463 (2007) .
12.
L.Vayssieres, K.Keis, S.Lindquist, and A.Hagfelt, J. Phys. Chem.
B 105, 3350 (2001) .
13.
M.Law, L.E.Greene, J.C.Johnson, R.Saykally, and P.Yang, Nat.
Mater.
4, 255 (2005) .
14.
J.S.Huang and C.F.Lin, J. Appl. Phys.
103, 014304 (2008) .
15.
S.W.Kim, Sz.Fujita, M.S.Yi, and D.H.Yoon, Appl. Phys. Lett.
88, 253114 (2006) .
16.
L.E.Greene, M.Law, J.Goldberger, F.Kim, J.C.Johnson,
Y.Zhang, R.J.Saykally, and P.Yang, Angew. Chem., Int. Ed.
42,
3031 (2003) .
17.
E.Burstein, Phys. Rev.
93, 632 (1954) .
18.
S.W.Kim, Sz.Fujita, and Sg.Fujita, Appl. Phys. Lett.
81, 5036
(2002) .
19.
S.W.Kim, M.Ueda, M.Funato, Sg.Fujita, and Sz.Fujita, J. Appl.
Phys.
97, 104316 (2005) .
20.
A.Teke, U.Ozgur, S.Dogan, X.Gu, H.Morkoc, B.Nemeth,
J.Nause, and H.O.Everitt, Phys. Rev. B 70, 195207 (2004) .
21.
Y.Gu, I.L.Kuskovsky, M.Yin, S.O’Brien, and G.F.Neumark,
Appl. Phys. Lett.
85, 3833 (2004) .
22.
E.Y.P.Varshni, Physica 34, 149 (1967) .
23.
N.K.Park, Y.J.Lee, J.Y.Jung, W.G.Lee, Y.J.Bae, S.H.
Yoon, G.B.Han, S.O.Ryu, and T.J.Lee, J. Nanosci. Nanotechnol.
8, 4653 (2008) .
24.
F.K.Shan, G.X.Liu, W.J.Lee, K.R.Bae, B.C.Shin, and H.S.
Kim, J. Nanosci. Nanotechnol.
8, 5203 (2008) .
Received: 4 October 2008.Accepted: 9 February 2009.
4 J. Nanosci. Nanotechnol. 9, 1–4 , 2009