Abstract Nanosphere lithography (NSL) is a successful technique for fabricating highlyordered arrays of ZnO nanowires typically on sapphire and GaN substrates. In this work, we investigate the use of thin ZnO films deposited on Si by pulsed laser deposition (PLD) as the substrate. This has a number of advantages over the alternatives above, including cost and potential scalability of production and it removes any issue of inadvertent n-type doping of nanowires by diffusion from the substrate. We demonstrate ordered arrays of ZnO nanowires, on ZnO-coated substrates by PLD, using a conventional NSL technique with gold as the catalyst. The nanowires were produced by Vapor Phase Transport (VPT) growth in a tube furnace system and grew only on the areas pre-patterned by Au. We have also investigated the growth of ZnO nanowires using ZnO catalyst points deposited by PLD through an NSL mask on a bare silicon substrate

E Mccarthy

E Mcglynn

J.-P Mosnier

Ncpst

S Garry

CiteSeerX

 
 - 2 - 
Control of ZnO nanowire arrays by Nanosphere Lithography (NSL) on laser-
produced ZnO substrates. 
S. Garry, E. McCarthy, J.-P. Mosnier, E. McGlynn*.  
School of Physical Sciences and National Centre for Plasma Science and Technology 
(NCPST),  
Dublin City University,  
Glasnevin, Dublin 9, Ireland. 
Abstract 
Nanosphere lithography (NSL) is a successful technique for fabricating highly-
ordered arrays of ZnO nanowires typically on sapphire and GaN substrates. In this 
work, we investigate the use of thin ZnO films deposited on Si by pulsed laser 
deposition (PLD) as the substrate. This has a number of advantages over the 
alternatives above, including cost and potential scalability of production and it 
removes any issue of inadvertent n-type doping of nanowires by diffusion from the 
substrate. We demonstrate ordered arrays of ZnO nanowires, on ZnO-coated 
substrates by PLD, using a conventional NSL technique with gold as the catalyst. The 
nanowires were produced by Vapor Phase Transport (VPT) growth in a tube furnace 
system and grew only on the areas pre-patterned by Au. We have also investigated the 
growth of ZnO nanowires using ZnO catalyst points deposited by PLD through an 
NSL mask on a bare silicon substrate. 
 
*Author to whom correspondence should be addressed: enda.mcglynn@dcu.ie. 
 
 
 
 
 - 3 - 
1. Introduction  
 
There is currently a great deal of interest in 1D nanoscale materials with the 
aim of developing nanodevices with improved functionality. One such potential 
application is in the development of new field emission (FE) sources for use as 
miniature X-ray sources or flat panel displays [1,2]. Zinc oxide (ZnO) is a promising 
candidate for such devices which has applications in various technologies [3] and in 
particular is well suited for FE applications. However, strong control of the 
morphology of nanowire arrays produced for this purpose is required. Array density 
has been shown to have a large impact on the efficiency and uniformity of FE devices 
[4]. This control over array morphology can be achieved using nanosphere 
lithography (NSL) and there are many examples in the literature of well ordered, 
periodical arrays of ZnO nanowires grown via the use of NSL [5-9] to create an initial 
ordered template of metal islands on a substrate. NSL relies on producing a 
lithographic mask comprised of a close packed monolayer of polystyrene beads which 
can be produced with differing aperture sizes and separation. It is often preferable to 
other patterning techniques such as e-beam lithography and mask lithography [9] due 
to its low cost and the ability to easily pattern large areas. This technique is most 
commonly used in conjunction with vapor phase transport growth for ZnO nanowires 
where, under suitable conditions, the nanowire growth occurs at the metal catalyst 
point only. NSL has been used to create well ordered arrays of ZnO nanowires on 
substrates such as sapphire [5-7] and GaN thin films [8], however to our knowledge 
this technique has not been applied to growth on ZnO substrates, ZnO buffer layers or 
even directly on Si substrates. The development of ordered arrays on these types of 
substrate would have a number of advantages such as removing the possibility of 
 
 - 4 - 
unwanted doping from the substrate (e.g., Al from sapphire substrates) and also 
allowing reductions in cost due to the use of a range of cheaper substrate materials 
and enable potential scalability to large area substrates [10]. While ordered growth on 
ZnO substrates is desirable, it presents problems which are not addressed in the 
literature since these same problems are not encountered for growth on substrates 
such as sapphire and GaN. This issue is that ZnO naturally presents a highly 
favourable surface for ZnO nucleation, causing growth to occur on all areas of the 
substrate making selective growth difficult or impossible. To overcome this issue, 
very close control over the Zn vapor pressure in the VPT growth is required.  
In this paper we present the results of studies attempting to grow well ordered 
arrays of vertically aligned ZnO nanowires by VPT on ZnO buffer layers deposited by 
PLD onto Si(100) substrates. Ordered arrays were achieved by patterning the ZnO 
substrate with Au islands using NSL. These arrays cover large area substrates (of 
dimensions > 1 cm x 1 cm) and are uniform in terms of nanowire height and density 
of growth across these areas. The problem of growth over the entire substrate has 
been overcome by careful placement of the sample in the VPT tube furnace relative to 
the source materials. We also investigate the possibility of growth of patterned arrays 
of nanowires without the use of Au catalyst points onto Si substrates. This is a very 
desirable goal as it entirely removes the possibility of unwanted doping/contamination 
from the substrate or from metallic catalyst points.  
 
2. Experimental details 
 
Si (100) substrates were cleaned by ultrasonication in acetone and isopropanol for 15 
minutes each and then a ZnO buffer layer was deposited by PLD. Due to the 
 
 - 5 - 
deposition of a ZnO buffer layer, the native oxide layer is not expected to impact 
result and no attempt was made to remove the native oxide prior to growth. The 
thickness of the ZnO buffer layer was maintained between 150 nm and 250 nm. PLD 
was carried out with a frequency quadrupled Nd:YAG laser with a wavelength of 266 
nm focussed on to a sintered compressed ZnO target. The fluence was kept at ~3 
J/cm
2
. The base pressure of the system was ~5 x 10
-12
 Pa (5 x 10
-8
 mbar) and 
depositions were carried out at 10 Pa (0.1 mbar) with a 30 sccm flow of oxygen. The 
samples were heated to 900
0
C before deposition to remove any impurities and 
deposition was carried out at 300
0
C. The temperature was then ramped up to 700
0
C to 
anneal the deposited layer and slowly cooled to room temperature.  
These substrates were then coated with a close packed monolayer of 
polystyrene (PS) nanospheres. The spheres used (Thermo Scientific 5100A) were 1 
m in diameter. The nanosphere solution was mixed with ethanol in a 1:1 ratio and 
carefully applied to the surface of water in a Petri dish. Once in contact with the water 
surface the spheres are seen to disperse across the surface before organising into areas 
of close packed monolayer. The ZnO substrate is then immersed in the water and 
slowly withdrawn through an area where the spheres have packed together. The 
quality of this layer can be easily verified by visual inspection as these layers display 
very clear opalescence under white light illumination. The coated substrates are then 
allowed to dry in air before a layer of Au (of nominal thickness 2.5 nm) is deposited 
over the nanosphere layer using a bell jar thermal evaporator. To control the size of 
the deposited Au islands the substrates were held at angles from 10 to 20 degrees 
relative to the Au source. This allows a rough degree of control over the size of the 
islands. The nanospheres are finally removed by ultrasonic bath in toluene, leaving 
only an ordered array of Au islands across the substrate. 
 
 - 6 - 
VPT growth was carried out on the Au patterned substrates using the method 
described in [11] at a temperature at 925
0
C for one hour and allowed to cool to room 
temperature over a period of several hours. 0.06g of  ZnO and 0.06g graphite powder 
was mixed thoroughly using a pestle and mortar and loaded into an alumina boat with 
the substrate loaded facing down approximately 1 cm away from the source powders 
in the direction of the Ar flow (i.e. downstream).  
For the growths carried out without the use of Au catalysts, cleaned Si (100) 
substrates were coated with a layer of close packed nanospheres as described above 
and then coated with a layer of ZnO by PLD. For these substrates, the deposition was 
carried out at room temperature as the polystyrene beads cannot withstand high 
temperatures. The spheres were removed by ultrasonic bath in toluene and VPT 
nanowire growth was carried out under the same conditions as above, but with the 
substrate loaded directly over the source powders as under these conditions growth on 
non-patterned bare Si/SiO2 regions is not expected as Si/SiO2 does not present an 
attractive surface for the nucleation of ZnO. 
Characterisation was carried out using scanning electron microscopy (Karl-
Zeiss EVO series). 
 
3. Results and Discussion 
 
3.1 Growth of ordered nanowire arrays on continuous PLD ZnO buffer layers using 
NSL patterned Au catalysts. 
 
Au was deposited through a NSL mask, leaving islands of Au patterned regions of 3 
mm diameter which were then grown upon. Strong growth was seen on the samples 
 
 - 7 - 
once removed from the VPT furnace. Invariably there were three distinct regions after 
growth. The first region, that closest to the source powders, is white/grey in colour 
and is dominated by very strong nanowire growth, however this growth is unaligned 
and does not show any patterning. It is thought this is due to a high Zn vapour 
pressure in this region. 
The second region is that where ZnO deposits solely on areas catalyzed with 
Au. Areas not patterned with Au are unchanged post-growth. Where catalyzed, the 
substrate appeared matt-white, an indicator of nanowire growth. The substrate also 
retained the opalescent qualities of the NSL mask used to pattern it. This is a strong 
indicator of spatially ordered growth, as deduced from comparison of SEM data with 
visual inspection. It is in this region where ordered, aligned nanowire growth occurs. 
The final region is the furthest from the source powders. Some ZnO nucleation 
occurs here but does not develop into nanowires. Visually its appearance is 
unchanged. It is thought that this is due to low Zn vapour pressure, as it has been 
depleted through deposition on other regions of the substrate. 
 
Growths were carried out using arrays of Au catalyst points deposited at 
various angles with respect to the Au source as described in [12]. This produced 
samples with Au islands of varying sizes depending on both the angle of deposition 
and the orientation of nanosphere domains on each sample. It was observed that on 
samples where Au was deposited perpendicular to the source that multiple nanowires 
would grow from each point in the catalyst pattern, in fact the pattern itself is 
obscured except for regions with quite short nanowires. Depositing Au at higher 
angles reduced multiwire nucleations but only to a certain degree. Au deposited at an 
angle of ~10º would result in nanowire arrays that would also have multiple 
 
 - 8 - 
nucleations from each point, however there were fewer nanowires growing from each 
point and the hexagonal pattern was still apparent. There are also regions where a 
single nanowire would grow from a single dot. Increasing the deposition angle to ~16º 
or higher further reduced the occurrence of multiple nucleation from each point, 
however it introduced another issue impacting the quality of the arrays grown. In this 
case areas of the sample were observed with no nanowire growth at all. It is suspected 
that this is due to the random orientation of nanosphere domains across the sample, 
causing some of the nucleation points to be too small to catalyze growth or possibly 
not be present at all.  
 Growths were also carried out with varying thicknesses of Au. Two nominal 
thicknesses were used, 2.5 nm and 5 nm. The growths using the thicker, 5 nm, coating 
were seen to be less spatially ordered than those coated with 2.5 nm thickness of Au. 
We believe that this is due to the thicker Au layers beading up at high temperature and 
moving slightly across the surface of the substrate, causing growth of multiple rods 
from each point and some growth from areas not initially patterned by Au. 
The best results were achieved using 2.5 nm thick Au catalyst pattern 
deposited ~10º from normal as demonstrated in Fig 1 and Fig 2. These figures show 
areas of highly patterned nanowires in plan view with tilted views inset. It is clear 
from these images that there is multiple wire growth from many catalyst points and 
that there is no growth where the substrate is not catalysed with Au. Fig. 1 also 
includes XRD data showing -2 and rocking curve of ZnO (0002) peak. In plan view 
the hexagonal pattern is clear and the majority of nanowires appear as points due to 
good alignment. There are also a number of defects in the pattern visible, including 
nanowires that are not perfectly aligned.  
 
 - 9 - 
The patterned arrays of nanowires however do not extend over the full 
substrate. The regions of successful growth are confined to regions that are 
approximately 4 or 5 mm wide and are found approximately 5 mm away from the 
source powders. 
 
3.2 Growth of nanowire arrays on NSL patterned ZnO buffer layers deposited by PLD 
with no Au catalyst.  
 
The samples were examined by SEM both before and after VPT growth. Once 
the nanosphere layer was removed from the sample it had the same appearance as a 
bare Si substrate. However when examined by SEM a ZnO honeycomb pattern was 
noted across the sample. Unlike Au deposition through a nanosphere mask, which 
results in discrete islands across the sample, depositing ZnO through this mask by 
PLD resulted in a honeycomb pattern. After VPT growth the sample had taken on a 
matt white appearance. SEM studies showed that randomly oriented ZnO nanowire 
growth occurred all over the sample and not solely at NSL patterned regions. Low 
temperature growth is needed as to not destroy the PS nanospheres, however it results 
in poor texture of ZnO seeds [13], which we expect caused unaligned growth. In areas 
near the edge of the substrate where nanowire growth did not occur it was clear that 
the ZnO honeycomb pattern formed a ZnO film when subjected to VPT growth. This 
film growth completely obscured the NSL pattern making patterned ordered arrays 
impossible to grow with this method. 
 
 
 
 
 
 - 10 - 
4. Conclusions 
 
In summary, we have presented a procedure for producing well ordered arrays of 
vertically aligned ZnO nanowires on an NSL patterned Au catalyzed ZnO buffer layer 
deposited by PLD on Si. Patterned arrays of ZnO nanowires have been obtained but 
much more control over parameters such as the size of the Au catalyst points and Zn 
vapor pressure inside the VPT furnace are required to enable reproducibility and 
scalability. The second route investigated involved depositing NSL patterned ZnO 
seeds onto Si directly by PLD. This method was unsuccessful because (a) the low 
deposition temperatures required to deposit through the PS nanosphere mask mean 
that the deposited ZnO seeds were unaligned, leading to randomly oriented VPT 
growth, and also (b) because ZnO film growth from the NSL honeycomb patterned 
ZnO initial deposit leads to complete substrate coverage and subsequent unpatterned 
nanowire growth over the entire substrate.  
Further work towards the production of large area patterned arrays of 
nanowires will include replacing PS with silica as the nanosphere material, allowing 
high temperature PLD depositions though an NSL mask, allowing for the deposition 
of ordered arrays of high quality textured/aligned ZnO seeds to catalyze subsequent 
VPT growth. 
 
Ackowledgements 
The authors gratefully acknowledge funding from SFI under the RFP programme 
(Grant title: Novel Strategies to enhance the Field Emission Properties of Zinc Oxide 
Nanowires). 
 
 
 - 11 - 
References 
 
[1] G.Z Yue, Q. Qiu, Bo Gao, Y. Cheng, J. Zhnag, H. Shimoda, J.P. Lu, O. Zhou, 
Appl. Phys. Lett. 81 (2002) 355. 
 
[2] J.D. Carey, Philos. Trans. R. Soc. London A 361 (2003), 2891. 
 
[3] Özgür Ü, Alivov Ya I, Liu C, Teke A, Reshchikov M A, Dogan S, Avrutin V, Cho 
S-J, Morkoç H, J. Appl. Phys. 98 (2005) 041301 
 
[4] Rajendra Kumar R.T, McGlynn E, McLoughlin C, Chakrabarti S, Smith R.C, 
Carey J.D, Mosnier J.P. and Henry M.O, Nanotechnology 18 (2007) 215704 
 
[5] X. Zhang, D. Liu, L. Zhang, W. Li, M. Gao, W. Ma, Y. Ren, Q. Zeng, Z. Niu, W. 
Zhou, S. Xie, J. Mater. Chem., 19 (2009) 962-969. 
 
[6] W. Wang, C.J. Summers, Z.L. Wang, Nano Lett., 4(3) (2004) 423-426. 
 
[7] D.F. Liu, Y.J. Xiang, X.C. Wu, Z.X. Zhang, L.F. Liu, L. Song, X. W. Zhao, S. D. 
Luo, W.J. Ma, J. Shen, W.Y. Zhou, G. Wang, C.Y. Wang, S.S. Xie, Nano Lett., 6(10) 
(2006) 2375-2378. 
 
[8] H.J. Fan, W. Lee, R. Scholz, A. Dadgar, A. Krost, K. Nielsch, M. Zacharias, 
Nanotechnology 16 (2005) 913-917.  
 
[9] J. Rybczynski, D. Banerjee, A. Kosiorek, M. Giersig,
 
Z. F. Ren Nano Lett., 4(10) 
(2004) 2037-2040.  
 
[10] J.-R. Duclère, B. Doggett, M. O. Henry, E. McGlynn, R. T. Rajendra Kumar, J.-P. 
Mosnier, A. Perrin, and M. Guilloux-Viry, J. Appl. Phys., 101 (2007) 013509.  
 
[11] M. Biswas, E. McGlynn, M. O. Henry, M. McCann, A. Rafferty, J. Appl. Phys. 
105, 094306 (2009) 
 
[12] C.L. Haynes, A.D. McFarland, M.T. Smith, J.C. Hulteen, R.P. Van Duyne, J. 
Phys. Chem. B 106 (2002) 1898-1902. 
 
[13]  B.L Zhu, X.H. Sun, X.Z. Zhao, F.H. Su, G.H. Li, X.G. Wu, J. Wu, R. Wu, J. Liu. 
Vacuum 82 (2008) 495-500. 
 
 
 
 
 
 
 
 
 
 
 - 12 - 
List of Figure Captions 
 
Fig 1: ZnO nanowire array on continuous PLD ZnO buffer layer using NSL patterned 
Au catalysts (plan view), insets show 45
0
 tilted view (upper left, scale bar indicates 2 
m) and XRD data (lower right, showing both -2 and rocking curve of ZnO (0002) 
peak) illustrating ordering and alignment of nanowire growth. 
 
Fig 2: ZnO nanowire array on continuous PLD ZnO buffer layer using NSL patterned 
Au catalysts (plan view), insets show 45
0
 tilted views at different magnifications 
(scale bar in inset at upper left is 3 m and at lower right is 2 m) illustrating ordered 
growth. 
 
Fig 3: ZnO film covering substrate after VPT growth on a NSL patterned ZnO buffer 
layer in area with no observed VPT nanowire growth. Inset shows the honeycomb 
ZnO patterned buffer layer prior to growth. 
 
 - 13 - 
10 20 30 40 50 60 70 80 90
In
te
n
s
it
y
 (
A
.U
.)
2 theta (degrees)
15 16 17 18 19 20
10000
15000
20000
25000
omega (Degrees)
In
te
n
s
it
y
 (
A
.U
.)
In
te
n
s
it
y
 (
A
.U
.)
 
Figure 1. 
 
 - 14 - 
 
 
Figure 2. 
 
 
 
 
 
 
 
 - 15 - 
 
Figure 3 
 
 


Control of ZnO nanowire arrays by nanosphere lithography (NSL) on laser-produced ZnO substrates

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=A91188B80D86B7730B34F99CB907A6E0?doi=10.1.1.1041.4520&rep=rep1&type=pdf

Control of ZnO nanowire arrays by nanosphere lithography (NSL) on laser-produced ZnO substrates

Abstract

Similar works

Full text

Available Versions

CiteSeerX