A hybrid technique for the selective growth of ZnO nanorod arrays on wanted areas of thin cover glass substrates was developed without the use of seed layer of ZnO. This method utilizes electron-beam lithography for pattern transfer on seedless substrate, followed by solution method for the bottom-up growth of ZnO nanorod arrays on the patterned substrates. The arrays of highly crystalline ZnO nanorods having diameter of 60 ± 10 nm and length of 750 ± 50 nm were selectively grown on different shape patterns and exhibited a remarkable uniformity in terms of diameter, length, and density. The room temperature cathodluminescence measurements showed a strong ultraviolet emission at 381 nm and broad visible emission at 585–610 nm were observed in the spectrum

A Dorfman

C Yan

DF Liu

GJ Strom

Jeong Hyun Kim

JH Fan

Jin Hwan Kim

K Govender

L Nie

LE Greene

MH Huang

NK Reddy

P Ravirajan

Q Ahsanulhaq

Q Li

Q. Ahsanulhaq

V Srikant

Y Masuda

Y Tak

Y. B. Hahn

Z Wang

ZL Wang

English

PubMed

Crossref

Seedless Pattern Growth of Quasi-Aligned ZnO Nanorod Arrays on Cover Glass Substrates in Solution

Abstract A hybrid technique for the selective growth of ZnO nanorod arrays on wanted areas of thin cover glass substrates was developed without the use of seed layer of ZnO. This method utilizes electron-beam lithography for pattern transfer on seedless substrate, followed by solution method for the bottom-up growth of ZnO nanorod arrays on the patterned substrates. The arrays of highly crystalline ZnO nanorods having diameter of 60 &#177; 10 nm and length of 750 &#177; 50 nm were selectively grown on different shape patterns and exhibited a remarkable uniformity in terms of diameter, length, and density. The room temperature cathodluminescence measurements showed a strong ultraviolet emission at 381 nm and broad visible emission at 585&#8211;610 nm were observed in the spectrum.</p

Ahsanulhaq Q

Kim Jin Hwan

Kim Jeong Hyun

Hahn YB

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NANO PERSPECTIVES
Seedless Pattern Growth of Quasi-Aligned ZnO Nanorod Arrays
on Cover Glass Substrates in Solution
Q. Ahsanulhaq • Jin Hwan Kim • Jeong Hyun Kim •
Y. B. Hahn
Received: 28 August 2009 / Accepted: 1 December 2009 / Published online: 27 December 2009
 The Author(s) 2009. This article is published with open access at Springerlink.com
Abstract A hybrid technique for the selective growth of
ZnO nanorod arrays on wanted areas of thin cover glass
substrates was developed without the use of seed layer of
ZnO. This method utilizes electron-beam lithography for
pattern transfer on seedless substrate, followed by solution
method for the bottom-up growth of ZnO nanorod arrays on
the patterned substrates. The arrays of highly crystalline
ZnO nanorods having diameter of 60 ± 10 nm and length
of 750 ± 50 nm were selectively grown on different shape
patterns and exhibited a remarkable uniformity in terms
of diameter, length, and density. The room temperature
cathodluminescence measurements showed a strong ultra-
violet emission at 381 nm and broad visible emission at
585–610 nm were observed in the spectrum.
Keywords ZnO nanorod arrays 
Seedless pattern growth  Hybrid method
Introduction
One-dimensional (1D) ZnO nanostructure arrays aligned on
substrates are highly desirable for promising device applica-
tions. For example, it has been demonstrated that ZnO nano-
rod/nanowire arrays on proper substrates can emit ultraviolet
laser at room temperature [1], enhance biofluorescence
detection [2], act as piezoelectric nanogenerators [3], and can
be used to construct nanowire dye-sensitized solar cells [4].
Moreover, ZnO nanorod arrays (NRAs) are expected to find
potential applications as nano-optoelectronic devices [5–11].
For the growth of aligned and quasi-aligned ZnO NRAs on
substrates, various low temperature solution-phase approa-
ches have recently been developed because of their good
potential for scale-up production and commercial feasibility.
A selective area growth of well-aligned ZnO nanostructures
has also been achieved using various techniques like template-
directed approach [12], nanosphere lithography [13], poly-
styrene microsphere-based self-assembly monolayers [14],
silane-based self-assembled monolayers [15], and conven-
tional photolithography [16]. However, these patterning
methods require expensive mask, complex multi-step pro-
cesses, and metal catalysts in some cases, and deposition of
seed layer on substrates. In general, it is known that the seed
layer of ZnO is required for the growth of ZnO nanorods
without the use of catalyst.
The ability to control the nanorod orientation and
position during the growth is critically important, because
it allows the growth and assembly of nanorods to be
combined into one step, facilitates the subsequent fabri-
cation processes such as the formation of electrical contacts
to nanorods and opens up the design space for novel
devices. Achieving spatially, specific, selective, highly
ordered, and uniform growth of ZnO NRAs on wanted
areas of substrates via a one-step approach by solution
method without using seed layer remains a prominent
challenge, although impressive progresses have been made
using seeded substrates [17, 18].
In this paper, we report the growth of quasi-aligned ZnO
NRAs on a thin cover glass substrate in solution without
the use of any seed layer. The position and location of
Q. Ahsanulhaq  J. H. Kim  J. H. Kim  Y. B. Hahn (&)
School of Semiconductor and Chemical Engineering,
Department of BIN Technology, and Nanomaterials Processing
Research Centre, Chonbuk National University, Jeonju
561-756, South Korea
e-mail: ybhahn@chonbuk.ac.kr
Q. Ahsanulhaq
Faculty of Engineering, Toyama University, 3190 Gofuku,
Toyama 930-8555, Japan
123
Nanoscale Res Lett (2010) 5:669–674
DOI 10.1007/s11671-009-9504-6
NRAs was controlled by electron-beam lithography patterns.
The ZnO NRAs were grown on the patterned cover glass
substrate in solution at 90 C. We chose cover glass substrate
to grow NRAs because it possesses many attractive proper-
ties including biocompatibility, lightweight, and transpar-
ency. Particularly, the ZnO NRAs on cover glass open a wide
variety of application; examples are enhancement of fluo-
rescence detection, single native DNA molecule detection
and can act as novel carriers for mammalian cell transfection
[2, 19].
Experimental
The nanofabrication process used in this work was accom-
plished in four steps as shown in Fig. 1: (a) thin layer of resist
polymer spun over cover glass substrate; (b) electron-beam
writing; (c) growth of ZnO nanorods arrays in solution; and
(d) lift-off resist material and residue from the substrate.
Polymethylmethacrylate (PMMA) was used as electron-
beam resist material and spin-coated over the substrate. After
writing, the patterned substrates were developed in a solution
of methyl isobutyl ketone and isopropyl alcohol (volume
ratio of 1:3) for 30 s. For the growth of ZnO nanostructures
on the patterned substrates, 10 mM aqueous solution of zinc
nitrate hexahydrate (Zn(NO3)26H2O, Sigma–Aldrich) and
water-soluble hexamethylenetetramine (C6H12N4, Sigma–
Aldrich) were used as reagents. In a typical reaction, the pH
of solution was kept at 6 * 7. The pre-patterned cover glass
substrates were immersed in the solution, and the tempera-
ture of flask was kept constant at 90 C for 6 h. After the
successful growth of nanorods, the electron-beam resist
material was lifted off from the substrate using organic sol-
vents. The surface morphologies of as-grown ZnO nanorod
arrays were examined by field emission scanning electron
microscopy (FESEM), (FESEM, Hitachi, Japan). An X-ray
diffractometer (XRD) (Rigaku III/A, Japan) was used to
analyze crystal phase and crystallinity of ZnO NRAs. X-ray
wavelength used in the analysis was 0.154 nm of Cu Ka. The
room temperature cathodluminescence (CL) spectra and
images were taken using CL system equipped on a high
resolution FESEM (FESEM, Hitachi, Japan).
Results and Discussion
Figure 2 shows the FESEM images of ZnO NRAs, selec-
tively grown on cover glass substrates. The interdistance
Fig. 1 Schematic illustration of
selective growth process of ZnO
nanorod arrays: a thin layer of
resist polymer (PMMA) spun
over cover glass substrate;
b pattern transfer by electron-
beam lithography; c growth of
aligned ZnO nanorod arrays in
solution, and d lift-off resist
material and residue from the
substrate
670 Nanoscale Res Lett (2010) 5:669–674
123
between two strips is kept 3 lm, and each strip has length
of 20 lm (a). The magnified image of (a) clearly identifies
high degree site-selective growth of NRAs (b). The gap
between the two strips and length of patterns was same;
however, the patterned site further reduced to 1.5 lm and
shown in (c). As can be seen in the magnified FESEM
images of ZnO NRAs (b and d), the patterned sites are fully
occupied with ZnO nanorods. The ability to scale up the
pattern to a larger size has also been examined. Figure 2e
shows the low-magnification FESEM image of large size
square-shape pattern (100 9 100 lm), where each white
spot represents the patterned ZnO NRAs. The inset image
of Fig. 2e shows ZnO nanorods on a thin cover glass
substrate. The magnified image (f) represents that each
nanorod exhibits hexagonal nanotip. The inset in (f) rep-
resents a magnified square pattern of 4 lm 9 4 lm. It can
be seen that high-density NRAs were grown only on both
microstrip and microsquare patterns, while no NRAs were
grown in the interstrip and intersquare areas. Each nanorod
has a diameter of 60 – 10 nm and length of 550 – 50 nm,
respectively. More interestingly, these quasi-aligned ZnO
NRAs on different shape patterns exhibit remarkable uni-
formity in terms of diameter, length, and density since they
are grown under same experimental conditions.
Figure 3a shows the XRD pattern of the ZnO NRAs
selectively grown on pre-patterned cover glass substrates.
A sharp, strong, and dominant peak of ZnO (0002) at
34.24 observed, indicating that the synthesized nanorod
arrays are single crystalline in nature and grown along the
c-axis direction. Moreover, the strong intensity of (0002)
reflection with narrow width also exhibits that the ZnO
NRAs are well oriented along the normal direction of the
substrate surface, which is almost consistent with the FE-
SEM observations. TEM image of a ZnO nanorod is pre-
sented in Fig. 3b, and SAED pattern of ZnO nanorod is
represented as Fig. 3c. These results confirmed that the
ZnO NRAs were single crystal in nature.
ZnO nanorods were formed by the hydrolysis of zinc
nitrate hexahydrate in water in the presence of hexameth-
ylenetetramine (HMTA). HMTA is a nonionic cyclic ter-
tiary amine that can act as a Lewis base with metal ions and
a bidentate ligand capable of bridging two zinc(II) ions in
Fig. 2 FESEM images of ZnO
nanorod arrays: a low- and
b high-magnification images of
ZnO NRAs grown on 2-lm strip
patterns; c low- and d high-
magnification images of ZnO
NRAs grown on 500-nm strip
patterns; e low- and f high-
magnification images of ZnO
NRAs grown on square patterns
Nanoscale Res Lett (2010) 5:669–674 671
123
solution [20]. HMTA can also hydrolyze to generate
formaldehyde and ammonia under the known reaction
conditions [21]. It is considered that HMTA slowly
decomposes and afford steady supply of ammonia, which
can form ammonium hydroxide and complex with zinc (II)
to form Zn(NH3)4
2? [22, 23]. Zinc (II) is thought to exist
mainly as Zn(OH)4
2 and Zn(NH3)4
2? under the given pH and
temperature. Consequently, the formation of ZnO occurs
by dehydration of these reaction intermediates [24]. The
comprehensive chemical reaction represented below.
C6H12N4 þ 6H2O $ 6CH2O þ 4NH3 ð1Þ
C6H12N4 þ Zn2þ ! ½ZnðC6H12N4Þ2þ ð2Þ
NH3 þ H2O ! NHþ4 þ OH ð3Þ
Zn2þ þ 4NH3 ! Zn(NH3Þ2þ4 ð4Þ
Zn2þ þ 4OH ! Zn(OH)24 ð5Þ
Zn(NH3Þ2þ4 þ 2OH ! ZnO þ 4NH3 þ H2O ð6Þ
Zn(OH)24 ! ZnO þ H2O þ 2OH ð7Þ
½Zn(C6H12N4Þ2þ þ 2OH ! ZnO þ H2O þ C6H12N4
ð8Þ
We assume that the reaction pH and temperature
facilitates the positively charged [Zn(C6H12N4)]
2? and
Zn(NH3)4
2? intermediates to migrate on patterned spots on
the cover glass substrate. This causes elevated concen-
tration of ZnO precursors to become confined to a small
area on patterned sites of the cover glass substrate. In this
way, enhanced nucleation can be obtained on the seedless
patterned cover glass substrates. The orientation of crystal
is highly uniform with in each patterned site, implying that
atomic level interfacial structure may also control the
nucleating plane of crystals. This method demonstrates that
it is possible to obtain highly specific growth of crystals in
a constrained crystallographic direction by combining
patterning process with bottom-up solution approach. The
patterned spot helps the nucleated ZnO to grow in quasi-
aligned direction on the electron-beam exposed sites.
Controlling the size of microstrip and microsquare-shape
patterns simultaneously controls the density and quasi-
alignment of NRAs. Thus, by adjusting the concentration
of zinc nitrate, temperature, reaction time, and substrate
patterning we could control the most important charac-
teristics of anisotropic crystallization process: that is,
location, density, and anisotropic crystallographic orien-
tation. In this way, we can grow nanorod arrays on
patterned substrates with different desirable configurations.
The microscopic spectral-emission characteristic of ZnO
NRAs was directly imaged by highly spectrally and spa-
tially resolved scanning CL microscopy. Since the CL
microscopy has the advantage of a much higher lateral and
depth resolution when compared to photoluminescence, it
is a unique tool for the investigation of nanometer-scaled
structures. Figure 4c shows room temperature CL spectrum
of periodically grown ZnO NRAs on cover glass substrates.
A strong ultraviolet (UV) emission at 381 nm and a broad
visible emission at 585–610 nm were observed in the
spectrum. The UV emission is the band-edge emission
resulting from the recombination of excitonic centers [25].
The visible luminescence band appears broad, composed of
yellow-orange and red part of the spectrum and is known to
be attributed to native defects, oxygen interstitial defects
and/or nonradiative recombination centers. Figure 4b and c
show the low-magnification FESEM image of ZnO NRAs
grown on square pattern, showing clearly well-defined
squares of ZnO NRAs, and the corresponding monochro-
matic CL images taken from the samples, respectively. The
CL images were taken under the same microscope and
monochromator conditions. Bright areas in Fig. 4c
revealed the luminescence sites, which coincide with the
location and position of patterned NRAs.
Summary and Conclusion
In summary, the ZnO nanorod arrays were successfully
grown on patterned glass substrates without the use of seed
Fig. 3 a XRD pattern of ZnO nanorod arrays grown on glass substrate;
b TEM image of single ZnO nanorod and c its corresponding SAED
pattern
672 Nanoscale Res Lett (2010) 5:669–674
123
layer of ZnO by solution method at 90 C. Electron-beam
lithography process controlled the alignment, position, and
location of NRAs. The arrays of highly crystalline ZnO
nanorods having diameter of 60 – 10 nm and length of
750 – 50 nm were selectively grown on glass substrates,
which have different shapes of patterns and exhibited a
remarkable uniformity in terms of diameter, length, and
density. XRD spectrum showed a strong and dominant
peak of ZnO (0002), grown along c-axis direction. The
room temperature CL measurements showed a strong
ultraviolet (UV) emission at 381 nm and a broad visible
emission at 585–610 nm. The developed hybrid method is
a simple, environmentally friendly system for the direct
production, deposition, and patterning of nanostructures on
a seedless surface. This seedless procedure could replace
the multitude of competing complex processes presently in
use. Moreover, this approach is generic and could easily be
modified for any desired metal oxide with very few
limitations.
Acknowledgment This work was supported by the Korean gov-
ernment (MEST) through the Pioneer Research Center for Artificial
Mechanosensory System (grant number 2009-0082837).
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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Seedless Pattern Growth of Quasi-Aligned ZnO Nanorod Arrays on Cover Glass Substrates in Solution

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