Untraditional approach to complex hierarchical periodic arrays with trinary stepwise architectures of micro-, submicro-, and nanosized structures based on binary colloidal crystals and their fine structure enhanced properties.

Abstract

A unique approach for fabricating complex hierarchical periodic arrays with trinary stepwise architectures of micro- and submicro- as well as nanosized structures by combining a novel double-layered binary colloidal crystal with pulsed laser deposition techniques is developed. The present strategy is universal and nanostructures with different materials can be easily prepared in the complex hierarchical periodic arrays. This approach offers the advantage of low costs compared to conventional lithographic techniques. These as-prepared unique structures cannot be directly fabricated by conventional lithography. These special hierarchically structured arrays demonstrate fine structure-enhanced performances, including superhydrophilicity without UV irradiation and surface enhanced Raman scattering (SERS), which is highly valuable for designing micro/nanodevices, such as biosensors or microfluidic devices.

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October 29, 2011
C2011 American Chemical Society
Untraditional Approach to Complex
Hierarchical Periodic Arrays with
TrinaryStepwiseArchitecturesofMicro-,
Submicro-, and Nanosized Structures
Based on Binary Colloidal Crystals and
TheirFineStructureEnhancedProperties
Yue Li,†,‡,*Naoto Koshizaki,‡,*Hongqiang Wang,‡and Yoshiki Shimizu‡
†Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences
(CAS), Hefei 230031, Anhui, China, and‡Nanosystem Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
H
structures and nanostructures and there-
fore have many applications in optoelectro-
nic devices, microfluidic devices, biomedical
science, field emission, etc.1?5If these hier-
archical structures can be arranged into
ordered arrays, this will further improve ap-
plications in micro/nano-devices because of
their more stable and uniform properties on
the array surfaces.3?5Generally, hierarchical
structures could be synthesized by replica-
tion induced by an electric field, chemical
vapor deposition, or electron irradiation.6?10
Unfortunately, it is difficult to cause them to
form ordered arrays by self-assembly due to
limitations of the geometric configuration
once the hierarchical structures have been
prepared. Conventional approaches to hier-
archical micro/nanostructured arrays are
generallybasedonlithographicaltechniques.
Microsize-structured arrays are first fabri-
cated by lithography, including photolitho-
graphy, X-ray lithography, e-beam litho-
graphy, etc.11?14and then nanosized
structures are created based on the micro-
sized ones.15Thus, hierarchical micro/na-
nostructured arrays can be achieved.
However, an approach based on tradi-
tional lithography can only be afforded
by a few laboratories due to the low
sample throughput and high cost, hinder-
ing further applications of the special
structured arrays. More importantly, one
disadvantages.
ierarchical
possess unique properties and offer
the advantages of both microsized
micro/nanostructures
hardly devises and manufactures more
complex hierarchical arrays by such an
approach. Therefore, an unconventional
approach to create micro- or nanosized
arrays has attracted much attention, and
it is urgent to develop a new technique
for fabricating complex hierarchically struc-
tured arrays to overcome the above
*Address correspondence to
yueli@issp.ac.cn,
koshizaki.naoto@aist.go.jp.
Received for review March 2, 2011
and accepted October 28, 2011.
Published online
10.1021/nn203239n
ABSTRACT
A unique approach for fabricating complex hierarchical periodic arrays with trinary stepwise
architectures of micro- and submicro- as well as nanosized structures by combining a novel
double-layered binary colloidal crystal with pulsed laser deposition techniques is developed.
The present strategy is universal and nanostructures with different materials can be easily
preparedinthecomplexhierarchicalperiodicarrays.Thisapproachofferstheadvantageoflow
costs compared to conventional lithographic techniques. These as-prepared unique
structures cannot be directly fabricated by conventional lithography. These special hierarchi-
cally structured arrays demonstrate fine structure-enhanced performances, including super-
hydrophilicity without UV irradiation and surface enhanced Raman scattering (SERS), which is
highly valuable for designing micro/nanodevices, such as biosensors or microfluidic devices.
KEYWORDS: hierarchical array.trinary.binary colloidal crystals.enhanced
properties
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It has been proven that the monolayer colloidal
crystal template technique is an efficient approach to
micro- or nanostructured patterns including particle
arrays, pore arrays, and ring arrays, etc.16?23If one
choosesacolloidalmonolayerwithmicrosizedspheres
and then deposits nanomaterials (i.e., nanoparticles,
nanotubes, and nanorods), the desired hierarchical mi-
cro/nanostructured arrays will be easily obtained.24?27
However, more complicated hierarchically structured
arrays are required in some cases for special applica-
tions in biotechnology or other fields. It is crucial to
design and create complex hierarchically structured
arrays featuring low cost and facile manipulation.
On the basis of the colloidal monolayer templates,
althoughvariousperiodicpatternshavebeendevelo-
ped, it is still difficult to devise more complex
hierarchical structured arrays due to the simple, gen-
eral hcp alignment of their colloidal monolayer. With
development of the colloidal assembling technique,
more complex colloidal crystals, such as binary col-
loidalcrystals,canbefabricatedbyspincoating,28dip
coating,29?32horizontal deposition,33?36and coself-
assembly at the air?water interface,37,38where
small colloidal spheres were filled in the interstices
of colloidal crystals according to certain rules. The
success of binary colloidal crystals might bring a
greater chance to achieve more complicated hier-
archical structured arrays in combination with other
techniques.
We present a novel approach to prepare complex
hierarchical periodic arrays with microsize, submicro-
size,andnanosize trinary stepwisestructures based on
a double layered binary colloidal crystal with different
sphere sizes in each layer. Such a hierarchical binary
colloidal crystal was obtained by float-transferring the
second-layered colloidal monolayer with submicro-
sizedspheresontothefirstlayeredcolloidalmonolayer
with microsized spheres that was prefixed on the
substrate. Such binary colloidal crystals possess the
advantagesoforiginalhcpalignmentsforbothbottom
and top layers of colloidal crystals, which is quite
different from previous reports.28?38Finally, the na-
nostructured materials were created on the hierarch-
ical colloidal crystals by pulsed laser deposition (PLD).
Herein, CuO deposition was used as an example to
demonstrate the fabricating process of such complex
structured arrays. We successfully obtained complex
hierarchical periodic arrays with trinary stepwise archi-
tecturesofmicro-,submicro-,andnanosizedstructures
by this strategy, and such complex hierarchical peri-
odic arrays demonstrated fine structure-enhanced
performance, that is, superhydrophilicity without UV
irradiation. Generally, superhydrophilicity could be
created by increasing the surface roughness of materi-
als with high free energy according to the Wenzel
model.39The superhydrophilicity demonstrated by
such complex hierarchical periodic arrays should be
attributed to their rough enough surfaces and the PLD
process. Besides superhydrophilicity, such a complex
hierarchical array after being coated with a thin gold
layer also exhibited an excellent surface-enhanced
Raman scattering (SERS) effect for detecting organic
molecules when it was used as an active substrate of
SERS. Additionally, the feature of hierarchical ordered
arrays with trinary stepwise architectures was almost
kept after the removal of the binary colloidal crystal
by the annealing process, and microsized and sub-
microsized holes were templated by the double-
layered binary colloidal crystals. More importantly, it
is almost impossible to create such complexhierarch-
ical arrays by traditional lithographical techniques.
Such specially structured arrays have highly valuable
applications in optical, biotechnical, separation
science, and microfluidic devices as well as bionic
devise.
In this approach, a colloidal monolayer composed of
largespheres(microsizedPSspheres:2or5μm)wasfirst
fabricated on a cleaned Si orglass substrate byinterface
self-assemblyasGiersigM.etal.described,40?42andthis
monolayer was fixed onto the substrate by heating it
at 120 ?C for 3 min. The fixed colloidal monolayer on
the substrate was treated by ozone cleaner in order to
make its surface hydrophilic to guarantee the subse-
quent transferring of another colloidal monolayer. An-
othercolloidal monolayerwithsmallspheres(submicro-
sized PS spheres: 350 or 200 nm) prepared on another
substrate was slowly dipped into water. It was then
gradually peeled off from the substrate and floated on
the water surface. A binary colloidal crystal of double
layers with micro-/submicrosized structures was thus
Scheme 1. Schematic illustration of the fabrication process
for a complex hierarchical periodic array with trinary step-
wise architectures.
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obtained by picking up the colloidal monolayer with
small spheres from the water surface using the colloidal
monolayer with large spheres prefixed to the substrate.
Finally, nanostructures were created on the colloidal
doublelayersbypulsedlaserdeposition(PLD).Complex
hierarchical ordered arrays with trinary stepwise struc-
tures of micro-, submicro- and nanosize were obtained,
as schematically illustrated in Scheme 1.
RESULTS AND DISCUSSION
Morphology and Structure. Figure 1 presents SEM
images of typical hierarchically structured arrays with
trinary stepwise architectures using CuO as a target in
the PLD process. One can see that these special
structured arrays possess micro-, submicro-, and nano-
sized structures. A microsized structure with a size of
2 μm exhibited a hexagonal close-packed (hcp) arrange-
ment, as seen in Figure 1A. Each microsized unit was
composed of hcp-aligned submicrosized structures
with a size of 350 nm, and each submicrosized unit
consistedofnanostructureswithsharptips(Figure1B).
Fromthecross-sectionalFE-SEMimage,themicrosized
structure unit is reflected from the microsized PS
colloidal sphere, and the submicrosized structure unit
is reflected from the submicrosized PS sphere on the
microsized sphere. The nanostructures on the submi-
crosized PS spheres were formed by the PLD process.
We can also observe that the spheres at the bottom of
thecolloidallayerhadacontactareawiththesubstrate
or with each other (Figure 1C), caused by heating
deformationwhenthecolloidalmonolayerwasheated
slightly above the PS glass transition temperature
(Tg).43?45The contact between sphere and substrate
or between two neighboring spheres guaranteed that
thecolloidalmonolayerwaswellfixedonthesubstrate
and it could not be peeled off from the substrate
during the subsequent transfer of a second colloidal
monolayer in water. The microsized colloidal sphere
layeratthebottomwasfabricatedbyaself-assembling
approach at the interface of air and water, which
producedthetypicalhcparrangement,finallyresulting
in an hcp structure in the hierarchical structures. The
second submicrosized sphere colloidal monolayer was
obtained on a substrate using the same approach
and then transferred onto a microsized colloidal layer
supported on the substrate, while maintaining its
integrity and hence also the hcp alignment. Nano-
structures with columnar structures were formed by
PLDonthetopofsubmicrosizedspheresinthesecond
layer of the final hierarchical arrays. The column-
like nanostructures deposited on the second colloidal
monolayerwerecausedbyboththeshadoweffectand
multidirectional deposition at the high pressure of the
background gas in the PLD process.46
The X-ray photoelectron spectroscopy (XPS) survey
spectrum indicates that the materials deposited by
PLD are composed of Cu and O from the CuO target
(SupportingInformation,FigureS1).TheXRDspectrum
(Supporting Information, Figure S2) further indicates
that the materials are crystalline CuO with a much
higher relative intensity of the (002) peak compared
with the standard data for bulk CuO (JCPDS 801268),
suggesting that the deposited CuO nanostructures are
well aligned and have a preferential orientation along
the c-axis, attributable to a template-induced fabrica-
tioneffect.Asimilarorientationhasalsobeenobserved
in a lithography-induced growth process.47?49
A typical TEM image of a complex, hierarchical
trinary structured unit is presented in Figure 2. It can
clearly be seen that the structure is composed of
microsized and submicrosized spheres and nanostruc-
ture on a submicrosized sphere surface, which agrees
Figure 1. FE-SEM images of CuO periodic arrays with tern-
ary stepwise hierarchical structures of micro-, submicro-,
and nanosize: (A, B) top view image; (B) magnified image of
one unit from image A; (C) cross-section image.
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with the SEM sectional image of the sample. The
separated nanostructures with their supporting small
PS spheres display radially aligned nanorod structures
with sharp tips that grow almost vertically on the small
PS sphere surface (inset on the left top). The selected
area electron diffraction (SAED, inset on the right top)
pattern demonstrates that the nanorods deposited on
the small PS spheres by PLD are polycrystalline CuO,
coinciding well with the XRD result.
In the presented strategy, complex trinary stepwise
architectures with micro-/submicro- /nanostructures
can be tuned by changing the periodicities of the
bottom colloidal monolayer and the second colloidal
crystal, as well as by changing the deposition condi-
tions during the PLD process (e.g., the background gas
pressure and deposition time). For instance, by simply
changing the PS sphere size for the second-layer
colloidal crystal from 350 to 200 nm without any other
alteration of the fabrication process, a similar complex
hierarchically structured array with the same micro-
sized and nanosized structures but different submicro-
sizedstructureswasobtained,asdepictedinFigure3A.
If we changed the periodicity of the bottom colloidal
monolayer from 2 to 5 μm, we could also achieve a
similar hierarchical array with the submicrosized and
nanosized structures unchanged (Figure 3B). If the
periodicities of both the bottom and second colloidal
layers were changed, a similar complex hierarchically
structured array can also be created (Figure 3C). The
nanostructurescouldbeusuallytunedbychangingthe
background gas pressure and deposition time.
Additionally, we found that morphologies of such
hierarchical structured arrays with trinary stepwise
architectures with micro-/submicro-/nanostructures
could be almost inherited from the former ones after
being annealed at high temperature in ambient air;
even the binary colloidal crystal was removed due to
pyrolysis of PS in the annealing process. Herein we
present in Figure 4 the CuO hierarchical trinary struc-
tured arrays based on the binary colloidal crystal of
2 μm/350 nm after annealing. The microsized units in
the hierarchical array still kept the hcp alignment, and
the submicrosized units on the spherical surface of the
microsizedunitalsokeptthehcppattern.However,the
nanostructures were a little bit different from those
beforeannealing.Afterbeingannealed,thenanostruc-
tures changed from radially aligned nanorods with
sharp tips to spherical nanoprotuberances on the
submicrosized unit surface (insets in Figure 4A), which
originate from the atomic diffusion of the nanomater-
ials with sharp tips in the annealing process. Addition-
ally,itwasdiscoveredthatmicrosizedholesandsubmicro-
sized ones were formed under hierarchical trinary
Figure 2. TEM image of a unit in an as-prepared trinary
stepwisestructuredarray.Insetatthelefttopisasubmicro-
sphere with nanostructures peeled off from the trinary
structure unit. Inset on the right top is the selected area
electron diffraction (SAED) pattern of the nanostructures
deposited by PLD.
Figure 3. Hierarchical periodic structured arrays based on
doublelayeredbinarycolloidalcrystalswithvariablemicro-
sized and submicrosized PS sphere. Microsize/submicro-
size: (A) 2 μm/200 nm; (B) 5 μm/350 nm; (C) 5 μm/200 nm.
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stepwise arrays (Figure 4B), which templated from
binary colloidal crystals in the annealing process. The
crosssectionimagedemonstratedthatCuOwallswere
also created on the microsized PS sphere surface in
PLD (Figure 4C), which supported the whole hierarch-
ical structured arrays on the substrate after removal of
the PS templates by pyrolysis of PS. The hierarchical
arrayfilmadheredtightlytothesubstrateafteranneal-
ing and could not be detached from the supporting
substrate even when it was ultrasonicated in water for
20min.Thehierarchicalperiodicarraywithhierarchical
holes with microsized and submicrosized structures
has important applications in separation science.
Theproposedstrategyisuniversalforcreating such
special hierarchical structured arrays by changing the
target to various materials in the PLD process. Besides
CuO, other materials such as ZnO, Fe2O3, TiO2, NiO,
WO3, SnO2, and C with similar hierarchical micro-/
submicro-/nanostructured arrays can be fabricated.
Figure 5 presents a polycrystalline-structured ZnO
(Figure 5A,B) and Fe2O3(Figure 5C,D) array with trinary
hierarchical architectures obtained by PLD on a binary
colloidal crystal using ZnO and Fe2O3 as targets
(oxygen pressure, 6.7 Pa; deposition time, 0.5 h for
ZnO and 1 h for Fe2O3). Figure 5 panels E and F depict
amorphous TiO2
micro-/submicro-/nanostructured
arrays by the same approach, using TiO2as a target
at 6.7 Pa oxygen pressure for 40 min deposition time.
Additionally, we clearly observed that fine nanostruc-
tures of ZnO, Fe2O3, and TiO2were slightly different,
which could be mainly resulted from their various
chemical andphysical properties, such as crystal facets
of the interface with different energies.
Fine Structures Enhanced Properties. Superhydrophilicity
without UV Irradiation. These special hierarchical micro-/
submicro-/nanostructuredarraysexhibitcertainstructure-
enhanced properties due to their unique structures. For
example, CuO hierarchically structured arrays with trinary
architectureswithoutannealingexhibitedsuperhydrophi-
licity without UV irradiation and the water contact angle
(CA) was5.2?,asshowninFigure6a.(Superhydrophilicity
is usually defined as occurring when the water CA is
less than 10? on the material surface and it is generally
achieved by UV irradiation on semiconductor films.50,51)
This superhydrophilicity was mainly induced by its
special structures. Such superhydrophilicity is highly
useful for devising the microfluidic devices and self-
cleaning surfaces. A CuO nanoparticle film was also
created by PLD ona flat substratewithoutusingany PS
sphere colloidal monolayer, under the same experi-
mental conditions as for the hierarchically structured
array with trinary architectures. The water CA of the
nanoparticle film was 22.9? (Figure 6b). Hierarchically
structured arrays with binary hierarchical structures
were obtained by combining a colloidal monolayer
with the PLD technique. The water CAs for binary
hierarchically structured arrays based on a colloidal
monolayer with PS sphere sizes of 350 nm and 2 μm
were 19.2? and 15.3?, as seen in Supporting Informa-
tion,Figure S3. Generally, theWenzel mode isusedto
explain the wettability for a rough surface, where a
water drop has complete wetting on it.39
cos θr ¼ r cos θ
(1)
Here,risthesurfaceroughness,whichistheratioofthe
total surface area to the projected area on the horizon-
talplane,andθrandθaretheCAsofaparticlefilmanda
Figure 4. CuO hierarchical structured array with trinary
stepwise architectures after annealing at 600 ?C for
3 h in ambient air. Microsized sphere/submicrosized
sphere: (A) 2 μm/350 nm, low magnification FE-SEM
image; left and right inset in panel A shows FE-SEM
images with high magnification before and after an-
nealing, respectively. Scale bars in inset: 500 nm;
(B) damaged sample scraped by tweezers; (C) cross-
section image.
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smooth native film. This equation indicates that the
wettability will be enhanced by increasing the sur-
face roughness. Compared with the CuO film on a
flat substrate produced by PLD without using any
colloidal monolayers, the binary hierarchical micro-/
nanostructured or submicro-/nanostructured array
increased the roughness of the CuO surfaces, there-
by enhancing the wettability while decreasing the
water CA. However, the roughness is not enough to
induce the surface wettability to superhydrophili-
city. The special hierarchical micro-/submicro-/na-
nostructured arrays based on the binary colloidal
monolayer greatly increase the surface roughness,
leading to superhydrophilicity according to eq 1.
Therefore, such superhydrophilicity without UV irra-
diation originated from fine structure enhanced
properties and special process of PLD.
Active SERS Substrate. Such special hierarchical mi-
cro-/submicro-/nanostructured arrays with 10 nm Au
coating exhibited excellent SERS effects using the R6G
as a probe molecule (Curve e in Figure 7). For compar-
ison, SERS performances of different substrates with
the same Au coatings including smooth silicon wafer,
CuO film by PLD without using colloidal monolayer
(sample in Supporting Information, Figure S3A), hier-
archical submicro-/nanostructured arrays produced
by PLD using a colloidal monolayer with 350 nm PS
spheres (sample in Supporting Information, Figure
S3B) as well as micro-/nanostructured arrays produced
by PLD using a colloidal monolayer composed of 2 μm
PS spheres (sample in Figure S3C), were also investi-
gated, as shown in Figure 7. Smooth Au coating on Si
substrateonlyproducedveryweakSERSsignal(curvea
in Figure 7). The CuO film by PLD without using the
Figure5. FE-SEMimagesofcomplexhierarchicalmicro-/submicro-/nanostructuredarraysofZnO,Fe2O3,andTiO2fabricated
by the proposed strategy. (A, B) ZnO; (C, D) Fe2O3; (E, F) TiO2. Scale bars are 1 μm.
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colloidal monolayer shows a stronger SERS property
due to its relatively rough surface (curve b in Figure 7).
The hierarchical micro-/nano- or submicro-/nanostruc-
tured array by PLD using colloidal monolayers with
differentPSspheresizes(PSspheresize:2μm,350nm)
as templates show much stronger signals owing to
their rougher surfaces with two-scaled structures
(curve c and d in Figure 7). Interestingly, the complex
trinary micro-/submicro-/nanostructured array by PLD
usingbinary colloidal crystal demonstrate dramatically
increasing SERS property compared with the above
four cases.
Generally, SERS performance is determined by mat-
ter and morphology of substrates.52?54Noble metals
(i.e., Au) are excellent candidates for SERS matter with
rough surfaces. The sharp protrusion and gaps in
nanoscale in a rough metallic matter surface will pro-
duce much larger electromagnetic enhancement at
these sites, and therefore the intensity of the Raman
spectraofthemoleculeabsorbedonmetalsurfacescan
be dramatically enhanced. So the rough structure units
are usually called active “hot” spots, which could be
mainly attributed to SERS properties.55,56The as-pre-
paredcomplexhierarchicalarrayswithtrinarystepwise
architectures
structures possess many active “hot” spots for SERS-
detecting organic molecules after coating the Au thin
layer, which mainly contributes to their excellent SERS
performances. From the CuO film directly by PLD, to
hierarchical micro-/nano- or submicro-/nanostructured
structuredarraysby PLDbased oncolloidal monolayers,
andfinallytocomplextrinarymicro-/submicro-/nanos-
tructured arrays by PLD using binary colloidal crystals,
the roughness of their surfaces increases, which leads
to increasing active “hot” spots by SERS effect, finally
resulting in the dramatical increase of the SERS signal.
Besides increasing surface roughness, the feature of
array periodicity also contributes to the excellent
SERS effect to some extent. The periodic structures
may produce the redistribution of the photon density
ofstates,whichleadstoanincreaseofdensityofoptical
modes and thus the enhancement of the Raman
scattering of the detected organic molecules. This fact
has been proven by experiments and theoretical
calculations.57?59
ofmicro-/submicro-/and nanosized
CONCLUSION
In summary, a unique approach to fabricating com-
plex hierarchical periodic arrays with trinary architec-
tures of micro- and submicro- as well as nanosized
structures, developed by combining a double layered
binarycolloidalcrystalwithPLDtechniquesbeforeand
after annealing is presented. The strategy is universal
and nanostructures with different materials could be
easily prepared in the complex hierarchical periodic
arrays. Compared to conventional lithographic techni-
ques, this approach offers the advantage of low cost.
Importantly,theseuniquestructurescannotbefabricated
byconventionallithography.Thesespecialhierarchically
Figure 6. (a) Structure-enhanced wettability of hierarchical
trinary stepwise structured arrays fabricated by using mi-
crosizespheresof2μmandsubmicrosizespheresof350nm
by PLD without annealing, showing a water CA of 5.2?.
(b)WaterCAonathinfilmonthesiliconsubstrateobtained
directlybyPLDwithoutusingcolloidaltemplates:waterCA,
22.9?.
Figure 7. SERS spectra of R6G on different substrates after
being coated with 10 nm gold layer: (a) smooth silicon
wafer. (b) CuO film by PLD without using colloidal mono-
layer; (c) hierarchical micro-/nanostructured arrays pro-
duced by PLD using a colloidal monolayer with PS sphere
sizes of 2 μm; (d) hierarchical submicro-/nanostructured
arrays produced by PLD using colloidal monolayer with PS
sphere sizes of 350 nm; (e) complex hierarchical trinary
stepwise structured arrays with micro-/submicro-/nano-
sized units.
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structured arrays demonstrate structure-enhanced
performances, including superhydrophilicity without
UV irradiation and excellent SERS for detecting or-
ganic molecules. Additionally, such hierarchical trin-
ary stepwise structured arrays with hole-architec-
tures could be obtained after removal of the PS
binary colloidal template by use of the annealing
process, which has important applications in separa-
tion science. Overall, this strategy is highly valuable
for designing micro-/nanodevices, such as biotech-
nological or microfluidic devices, based on these
special structured arrays.
MATERIALS AND METHODS
A2.5wt%colloidalsuspensionwithmicrosize(2μm,5μm)or
submicrosize (350 nm, 200 nm) monodispersed PS spheres was
purchased from the Alfa Aesar Co. First, monolayer colloidal
crystals with microsized PS spheres were fabricated on cleaned
Si substrates by self-assembly at the interface between air and
water and subsequent formation on the cleaned Si substrate
after complete evaporation of the water, as previously
described.40?42The microsized PS sphere colloidal monolayer
washeatedat120?C(slightlyhigherthanthePSglasstransition
Tg=100?C)for3mininordertotightlyfixittothesubstrateand
then was treated by ozone cleaning for 45 min to make a good
hydrophilic surface. A submicrosized PS sphere monolayer was
fabricated using the same approach without heating that
gradually floated to the water surface. It was then picked up
by the microsized PS sphere monolayer fixed on the substrate,
and thus a double-layered binary colloidal crystal was formed
with the bottom of the microsize PS sphere monolayer and the
top of the submicrosize sphere layer on the substrate.
The double-layered binary colloidal crystal, on its supporting
substrate, was placed in a chamber for PLD in an off-axis
configuration. A laser beam with a wavelength of 355 nm from
a Q-switched Nd:YAG laser (Continuum, Precision 8000) oper-
atedat10Hzwith100mJpulse?1andapulsewidthof7ns,was
applied and focused on the target surface with a diameter of
2mm.Thetargetcouldbechangedfordifferentmaterials(CuO,
Fe2O3,ZnO,orTiO2).Thesubstrateandtargetwererotatedat40
and30rpm.ThePLDwascarriedoutfordifferenttimesatabase
pressure of 2.66 ? 10?4Pa and abackgroundO2pressure of 6.7
Pa for CuO, or at other suitable pressures depending on the
deposited materials.
The water CA was measured with a VCA Optima XE from AST
Products, Inc.
Previous to the investigation of the SERS spectra, all the
sampleswerecoatedwith10nmofagoldlayerontheirsurfaces
bysputtering.Thesampleswithgoldcoatingweredippedinthe
10?6M rhodamine 6G (R6G) aqueous solution for 10 min, and
thenrinsedwithpurewateranddriedbynitrogenflow.TheSERS
properties were examined by a confocal microprobe Raman
system (Renishaw inVia Raman Microscope). The excitation
wavelength was 532 nm. The data integration time was 5 s,
and five different positions were measured at each sample.
Themorphologiesoftheas-preparedsampleswereobserved
usinganFE-SEM(HitachiS-4800)andaTEM(JEOLJEM-2000FX).
The composition and chemical state of the samples were
examined using X-ray photoelectron spectroscopy (XPS, PHI
5600ci).ThewaterCAwasmeasuredwithaVCAOptimaXEfrom
AST Products, Inc.
Acknowledgment. We are grateful to J. Wang, G. Duan
from the Institute of Solid State Physics, CAS for experimental
assistance in SERS measurements and for helpful discussions.
This work was partially supported by the National Natural
Science Foundation of China (Grant No. 10974203), and Anhui
ProvincialNatural ScienceFunds forDistinguishedYoungScho-
lar (Grant No. 1108085J20).
Supporting Information Available: XPS spectrum (Figure S1)
and XRD pattern (Figure S2) of the sample after PLD of CuO and
schematic illustration of water CAs on different surfaces (Figure
S3). This material is available free of charge via the Internet at
http://pubs.acs.org.
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