A novel approach to grow ZnO nanowires and nanoholes by combined colloidal lithography and MOCVD deposition

Article abstract of DOI:10.1039/b816251d

A hybrid approach of colloidal lithography and metalorganic chemical vapour deposition (MOCVD) has been used to fabricate ZnO nanowire bundles and nanoholes by using a silver metalorganic precursor as the growth catalyst. The Royal Society of Chemistry.

Full text of DOI:10.1039/b816251d

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A novel approach to grow ZnO nanowires and nanoholes by combined
colloidal lithography and MOCVD depositionw
Maria Elena Fragala,*a Cristina Satrianob and Graziella Malandrinoa
Received (in Cambridge, UK) 16th September 2008, Accepted 21st November 2008
First published as an Advance Article on the web 24th December 2008
DOI: 10.1039/b816251d
A hybrid approach of colloidal lithography and metalorganic
chemical vapour deposition (MOCVD) has been used to fabri-
cate ZnO nanowire bundles and nanoholes by using a silver
metalorganic precursor as the growth catalyst.
simply obtained by combined use of colloidal lithography.
A silver film is deposited on two-dimensional arrays of colloi-
dal polystyrene nanospheres self-assembled by dewetting.15
The Ag(hfa)tetraglyme precursor has been dissolved in
EtOH–H2O 1 : 1 solution to obtain 0.1 M and 0.05 M
concentrations, respectively.
The synthesis and size control of multidimensional composite
materials is receiving growing attention due to potential
applications in catalysis and gas sensing.1 Metallic nanoring
and nanohole arrays are particularly attractive for their
unique optical properties, persistent currents, and multiple
magnetic states.2–4 ZnO nanostructures, especially nanowires
and nanorods, offer remarkable physical and chemical proper-
ties, with potential applications as nanoscale electronic, photo-
nic, field emission, sensing, and energy conversion devices.5,6
The common vapour transport deposition methods usually
grow ZnO nanowires on silicon, sapphire, or nitride substrates
coated with Au catalytic nanoparticles.7 Nanorods placement
can be predefined via location of metal catalyst islands or
particles.8 Au is the most used catalyst for ZnO nanowires
growth, since the use of Ag as an alternative metal has the
drawback of temperature limitation (up to 500 1C) related to
fast Ag oxidation which results in low-quality nanowires.9
Here we present an original method to grow ZnO nanowires
by using a new Ag catalyst obtained from thermal reduction of
silver(I) hexafluoroacetylacetonate tetraglyme [Ag(hfa)tetra-
glyme] metalorganic complex.10,11 This precursor has been
already successfully applied to deposit silver films, silver
nanoparticles embedded in polymeric and silica thin films or
silver nanorods through a single-step route involving its
thermal reduction to metallic silver.12–14 The main advantage
of this approach is the employment of liquid precursor solu-
tions that can be easily deposited by spin coating on any type
of substrate as films. Moreover, no Ag2O formation is detected
after the ZnO deposition by MOCVD performed at reduced
pressure in an Ar–O2 atmosphere and temperatures up to
700 1C. The fabrication of a nanostructured ZnO layer is
Two types of substrates have been prepared by spin coating
of an Ag precursor solution, respectively on (1) quartz sub-
strates (homogeneous Ag film, h-Ag) and (2) PS monolayers
transferred on quartz substrates (nanostructured Ag film,
ns-Ag). The preparation of a PS monolayer by dewetting
driven self-assembly is described elsewhere.15 After spin
coating, the samples have been thermally treated for 30 min
in a MOCVD hot wall tubular reactor, under an Ar flow
(100 sccm, P = 1 Torr) at T = 150 1C. XRD analysis
performed after thermal treatment (not shown here) confirms
the presence of metallic silver (111 preferentially oriented) on
both substrates. AFM analysis reveals that Ag precursor
solution fully incorporates the PS monolayer array with the
formation of very tiny nanoneedle grains, while the layer
deposited on quartz exhibits a regular distribution of large
grains (height = 60 Æ 40 nm, diameter = 94 Æ 27 nm)
(see Fig. S1 ESIw). Fig. 1 shows the AFM images of ns-Ag
films obtained after the PS colloidal mask removal, for the two
concentrations of investigated precursor solution. The ns-Ag
layer deposited from 0.1 M solution exhibits a dual grain size
distribution, which consists of circular patterns of large grains
(height = 3.8 Æ 1 nm, diameter = 114 Æ 10 nm) and tiny
nanoneedles (height = 1.4 Æ 0.3 nm, diameter = 12.5 Æ
1.3 nm) in the centre, corresponding to the PS sphere masked
area. On the other hand, the ns-Ag layer deposited from
a Department of Chemical Sciences of Catania University and INSTM
UdR Catania, Viale Andrea Doria 6, 95125 Catania, Italy.
E-mail: me.fragala@unict.it; E-mail: gmalandrino@dipchi.unict.it;
Fax: +39 095 580138; Tel: +39 095 7385055
b Laboratory for Molecular Surfaces and Nanotechnology
(LAMSUN) at Department of Chemical Sciences of Catania
University and CSGI, Viale Andrea Doria 6, 95125 Catania, Italy.
E-mail: csatriano@unict.it; Fax: +39 095 580138;
Tel: +39 095 7385136
w Electronic supplementary information (ESI) available: (1) Lengthy
experimental details; (2) AFM images of dense PS monolayer array,
Ag precursor-coated PS monolayer, Ag precursor-coated quartz;
(3) EDX spectra of ZnO nanowires grown on different Ag films;
(4) XRD patterns of ZnO nanowires grown on different Ag films. See
DOI: 10.1039/b816251d
Fig. 1 AFM images and section analysis of Ag nanostructures from
0.1 M (upper panels) and 0.05 M (lower panels) precursor solutions.
(1 Â 1) mm2 scan area.
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0.05 M solution presents a ring pattern with a wider Ag
grain size distribution (height = 15 Æ 3 nm, diameter =
22 Æ 10 nm).
substrates as well as the relative intensity of substrate and
overlayer related peaks observed in the EDX spectra, indicate
that the detection of Ag on ZnO films deposited on h-Ag
substrates cannot be related to a film thickness lower than that
of the film deposited on ns-Ag.
ZnO has been deposited from a Zn(tta)2tmeda metalorganic
precursor16,17 using standard deposition conditions (70 min at
600 1C under Ar–O2 gas flows). Fig. 2 shows high yield
nanowires formation, whose length is 410 mm, obtained
for ZnO films grown both on ns-Ag and h-Ag substrates
(0.1 M Ag precursor concentration). On ns-Ag (Fig. 2a)
nanowire diameters range from 100 to 200 nm and bundle
formation is observed. The insert shows a magnified bundle
area with a circular pattern having almost 300 nm diameter
(comparable with the spheres’ dimensions). These features are
likely related to the local enhancement of a silver catalytic
effect, in turn due to a high concentration of silver catalyst in
the large grains formed around the circular sphere area
(see Fig. 1). EDX analysis (see Fig. S2a ESIw) confirms the
presence of ZnO, while no Ag peaks are detected. This finding
can be explained by the simultaneous occurrence of poly-
styrene calcination and ZnO growth during the deposition
process, which helps the formation of locally depleted silver
areas. On the other hand, the ZnO layer deposited on h-Ag
(Fig. 2b) exhibits thinner wires (B20 nm) homogeneously
distributed over the whole surface area. A small Ag peak is
detected by EDX analysis (see Fig. S2b ESIw) in addition to
Zn and O related signals. It is to stress that the comparable
deposition conditions used for ZnO growth on the two
XRD analysis (Fig. 3) confirms the polycrystalline nature of
the ZnO nanowires and the diffraction peaks are consistent
with values indexed for the wurtzite in the standard card
(JCPDS 36-1451). The Ag peaks 111 and 200 are visible in
the spectrum of ZnO grown on spin coated film (h-Ag) and
confirm that silver is not oxidised during ZnO deposition at
T = 600 1C (using O2 flows). The ZnO peak intensity ratios
slightly change for the two samples: in particular in the case of
ZnO nanowires grown on h-Ag, a small increase of the
002 reflection is observed with respect to the one on ns-Ag
(the latter well matching the bulk ZnO spectrum). This
observation points to a modest preferential growth orientation
of thinner nanowires along the 002 direction. Further studies
focused on growth mechanisms of ZnO nanowires as a func-
tion of different Ag catalyst morphology (in terms of height
and particle distribution rather than particle dimensions) are
in progress.
It must be noted that the patterning shown in Fig. 1 is
covered by the massive ZnO nanowire growth induced by the
enhanced catalytic effect of ns-Ag. Milder deposition condi-
tions need to be defined to observe the nano-patterning effect.
Preliminary results obtained by using shorter time (30 min)
and lower temperature (500 1C) deposition conditions are
shown in Fig. 4. An ordered nanoring ‘‘Cheerios-like’’ array
is visible, confirming the dual role of silver as catalyst and
patterning agent.
Semi-quantitative analysis of EDX data (Fig. 5) indicates a
reduced ZnO deposition yield with respect to the previously
discussed samples (while no Ag peak is present).
The XRD analysis (see Fig. S3 ESIw) confirms formation of
the ZnO crystalline phases showing a predominant 002
orientation, typical of ZnO nanostructured layers, while the
002 preferential orientation is lost and the 100 and 101 peaks
become predominant when long nanowires are formed
(Fig. 3).
We have shown an original method to grow highly crystal-
line ZnO nanostructures (nanowires and nanoholes) via
MOCVD combined with colloidal lithography methods.
Fig. 2 SEM images of ZnO films grown on ns-Ag (a) and h-Ag
(b) substrates.
Fig. 3 XRD patterns of ZnO nanowires grown on h-Ag and ns-Ag.
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The combined use of closely packed PS self-assembled
colloids and Ag(I) metalorganic precursor offers the advantage
of growing ‘‘Ag free’’ ZnO nanowire bundles (at higher
temperature and time), characterised by very large surface
areas, or ‘‘Cheerios-like’’ nanohole arrays (at lower tempera-
tures and times), which replicate the template of the PS
monolayer assembly.
This approach leads to formation of porous materials
having a wide variety of applications in bioengineering,
catalysis, environmental engineering, and sensor systems
because of their high surface-to-volume ratio.
The authors acknowledge the CNR-ISTM and INSTM
within the PROMO ‘‘Nanostrutture organiche, organometalliche
polimeriche ed ibride: ingegnerizzazione supramolecolare
delle proprieta’ fotoniche e dispositivistica innovative per
optoelettronica’’ project and MIUR (Ministero dell’Istruzione,
dell’Universita’ e della Ricerca) that support this work.
Fig. 4 SEM image of a ZnO nanoholes array on ns-Ag obtained
Notes and references
using low temperature and short time deposition conditions.
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