Fabrication of artificial opals by electric-field-assisted vertical deposition.

Abstract

We present a new technique for large-scale fabrication of colloidal crystals with controllable quality and thickness. The method is based on vertical deposition in the presence of a DC electric field normal to the conducting substrate. The crystal structure and quality are quantitatively characterized by microradian X-ray diffraction, scanning electron microscopy, and optical reflectometry. Attraction between the charged colloidal spheres and the substrate promotes growth of thicker crystalline films, while the best-quality crystals are formed in the presence of repulsion. Highly ordered thick crystalline layers with a small amount of stacking faults and a low mosaic spread can be obtained by optimizing the growth conditions.

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DOI: 10.1021/la902793bLangmuir 2010, 26(4), 2346–2351Published on Web 10/02/2009
pubs.acs.org/Langmuir
©2009 American Chemical Society
FabricationofArtificialOpalsbyElectric-Field-AssistedVerticalDeposition
Kirill S. Napolskii,*,†Nina A. Sapoletova,†Dmitriy F. Gorozhankin,†Andrey A. Eliseev,†
Dmitry Yu. Chernyshov,‡Dmytro V. Byelov,§Natalia A. Grigoryeva,)
Wim G. Bouwman,^Kristina O. Kvashnina,#Alexey V. Lukashin,†Anatoly A. Snigirev,#
Alexandra V. Vassilieva,3Sergey V. Grigoriev,3and Andrei V. Petukhov§
Alexander A. Mistonov,)
†Department of Materials Science, Moscow State University, Leninskie Hills, 119991 Moscow, Russia,
‡Swiss-NorwegianBeamlinesattheEuropeanSynchrotronRadiationFacility,BP220,F-38043GrenobleCedex,
France,§Van’t Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science,
University of Utrecht, Padualaan 8, 3584CH Utrecht, The Netherlands,
State University, 198504 St. Petersburg, Russia,^Faculty of Applied Sciences, Delft University of Technology,
Mekelweg 15, 2629 JB Delft, The Netherlands,#European Synchrotron Radiation Facility, BP220, F-38043
Grenoble Cedex, France, and3Petersburg Nuclear Physics Institute, Gatchina, 188300 St. Petersburg, Russia
)
Department ofPhysics, St. Petersburg
Received July 29, 2009. Revised Manuscript Received September 20, 2009
We present a new technique for large-scale fabrication of colloidal crystals with controllable quality and thickness.
ThemethodisbasedonverticaldepositioninthepresenceofaDCelectricfieldnormaltotheconductingsubstrate.The
crystal structure and quality are quantitatively characterized by microradian X-ray diffraction, scanning electron
microscopy, and optical reflectometry. Attraction between the charged colloidal spheres and the substrate promotes
growthofthickercrystallinefilms,whilethebest-qualitycrystalsareformedinthepresenceofrepulsion.Highlyordered
thickcrystallinelayerswithasmallamountofstackingfaultsandalowmosaicspreadcanbeobtainedbyoptimizingthe
growth conditions.
Introduction
Photonic crystals are materials with a modulated refractive
indexonthescaleoftheorderoflightwavelengthsinthevisibleor
near-infraredregionofthespectrum.Thesematerialsattractgreat
attention due to optical properties that allow one to control and
manipulate the flow of light.1,2Despite the significant progress
achieved in understanding the optical phenomena in photonic
crystals, large-scale fabrication of high-quality photonic crystals
remains a challenge.
The use of colloidal crystals for the production of 3D
photonic crystals is a fast and cheap pathway that gives
large-scale periodic structures. Colloidal self-assembly, how-
ever, often yields a high density of defects, which can sig-
nificantlyaffecttheopticalpropertiesbybreakingtheaverage
symmetry of the crystal and creating defect states.2-4The
quality of colloidal crystals leaves much to be desired in
comparison with structures obtained by lithography5,6or
holography7,8techniques.
Nowadays,variousmethodstofabricatecolloidalcrystalshave
been developed such as sedimentation,9,10spin coating,11colloi-
dal epitaxy,12,13electrophoresis,14-16and convective assembly,
also known as controlled drying or vertical deposition.17-21All
these techniques yield large arrays of colloidal particles, but
cracks, stacking faults, dislocations, and point defects are still
abundant. Moreover, reproducibility of the results is often a
problem.
Inthecaseofverticaldeposition,anumberofcompetingforces
areinvolvedinthecolloidself-assembly.Capillaryforcesarevery
strong but act only on the particles at the liquid-air interface. In
the liquid bulk, it is the combination of the convection, sedimen-
tation, diffusion, and interparticle forces that determine the
particle trajectories. Recently, the relation between the growth
dynamics and the structural quality of colloidal crystal films has
beenestablishedusing optical reflectometry.22,23Photonic crystal
*Corresponding author. E-mail: napolsky@inorg.chem.msu.ru.
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Napolskii et al.Article
engineering can use a number of parameters, which can influence
theforcesresponsiblefortheformationofacrystal.Forexample,
by adjusting the density mismatch between the particles and the
solvent,onecanmodifythestrengthoftheeffectivegravityforce.
Variation of the growth temperature can slightly affect the
diffusion and can have a strong effect on the evaporation rate
and, therefore, on the convection force. Changing the particle
chargecaninfluencetheinterparticleforces.Still,onemayneedto
haveadditionalmeanstovarytheforcebalanceand,therefore,to
fine tune the crystal structure and quality.
The electric field could be an excellent tool to control the
crystallization process. Itwas shown that the electric field can act
as the main driving force, which moves colloidal particles to the
substrate and induces the crystallization process.14,15The electric
field can also play an auxiliary role by, for example, activating
shear deformations and therefore improving the crystal structure
during the formation of artificial opals by the inclined deposition
method.24The electric field was successfully utilized for prepara-
tion of colloidal crystals from monodisperse14and even binary
colloidal particles.25It was also used to induce dipole-dipole
interparticle interactions, which can improve the crystal quality
and even change the crystal structure.26
Here we present results on the application of the DC electric
field normal to the substrates during vertical deposition of
colloidal crystals. To the best of our knowledge, for such a
geometry, the influence of the electric field on the structure of
colloidal crystals has not been studied before. Moreover, inves-
tigations of the formation of colloidal crystals in an electric field
so far have been performed for rather thin structures.14,24The
stacking order is mostly addressed using electron15or optical27
microscopy, which are local and can be applied to very limited
sample volumes. Here we show that a detailed quantitative
characterization of structural order as a function of the electric
field can be performed by combining scanning electron micro-
scopy,highresolutionmicroradian X-ray diffraction, and optical
reflectometry. The single crystal X-ray diffraction on colloidal
crystals was first reported in ref 28. Since then, this method has
been successfully used for the characterization of the structural
order of artificial opals.29-31Despite the negative charge of the
particles, the crystals are found to form on both positively and
negativelychargedelectrodes. It isfoundthatelectrostaticattrac-
tion between the particles and the substrate increases the crystal
thickness while repulsion between them promotes higher crystal
quality.Opticalreflectometryalsoisusedtoillustratetheeffectof
the crystal quality on its optical properties.
Experimental Section
Materials and Substrates. Styrene (C6H5CHdCH2, 99.5%)
and potassium persulfate (K2S2O8, 99.99%) were obtained from
Sigma-Aldrich.Purewater(>18MΩ3cm)wasuseddirectlyfrom
a Milli-Q water system. Glass slides covered with an indium tin
oxide (ITO) layer with a surface resistivity of 15-25 Ω/sq were
purchased from 3M. The slides were cut into 4 cm?1.5 cm pieces
and used as substrates for deposition of artificial opals.
Sample Preparation. The polystyrene (PS) microspheres
were synthesized by emulsifier-free emulsion polymerization of
styrene using potassium persulfate as an initiator.32Prior to the
polymerization, styrene was purified from a stabilizer (4-tert-
butylcatechol) by vacuum distillation. The mixture with molar
ratio1C8H8/0.003K2S2O8/58H2Owasvigorouslystirredfor24h
at 343 K.
Colloidal crystals made of polystyrene microspheres were
grown by the vertical deposition technique in the presence of an
externalelectricfield.DepositionofnegativelychargedPSspheres
onto ITO glass vertically aligned electrodes was performed in a
cylindrical glass cell (5 cm diameter) using a Solartron 1287
potentiostat. The distance between the electrodes was 3 cm. The
electrodes were fixed precisely parallel to each other. Before the
deposition, ITO glass substrates were carefully washed under
sonicationin pure grade ethanol and water. The concentration of
PS particles in the colloidal solution was ∼0.2 vol %, and the pH
of the mixture was 4. Deposition of PS spheres has been carried
out at constant voltage U ranging from 0.1 to 3 V for 24 h. The
temperature of the film growth was 60 ( 3 ?C.
Sample Characterization. Scanning electron microscopy
(SEM) pictures were recorded on a Supra 50 VP instrument
(LEO).SamplesforSEMwerecoveredbyathinconductivelayer
of carbon using a Scancoat sputterer (Edwards). A Lambda 950
spectrophotometer (Perkin-Elmer) is used to record optical re-
flection spectra at incident angle of 8? with respect to the normal
to the sample. The size of the light spot was 4?4 mm2.
X-raystudieswereperformedattheDutch-Belgianbeamline
BM-26 DUBBLE of the European synchrotron radiation
facility (ESRF) in Grenoble (France) using a microradian
X-ray diffraction setup similar to the one described in ref 33.
In brief, to achieve the maximum transverse coherence length
of the beam, any focusing of the beam before the experimental
hutch was avoided. Instead, the X-ray beam was focused by a
set of compound refractive lenses (CRLs)34at the phosphor
screen of the CCD (charge-coupled device) X-ray detector
(Photonic Science VHR, 4008 ? 2671 pixels of 9 ? 9 μm2)
located at a distance of 8 m from the lenses. The samples
were placed just after the CRLs. This scheme, which differs
from an ordinary small-angle X-ray scattering setup by
the novel approach to beam focusing, allows one to achieve
angularresolution(full-widthathalf-maximum)oftheorderof
5 μrad, corresponding to 3.3 ? 10-4nm-1in the reciprocal
space.A12keVX-raybeam(wavelengthλ=0.1nm,band-pass
Δλ/λ=2?10-4, size 0.5?0.5 mm2at the sample position) was
used. The colloidal films were first mounted perpendicular to
the X-ray beam. Samples were then rotated around the vertical
axis within the range -75? e ω e 75?, and the diffraction
patternswererecordedateachdegreeofrotation.Thecollected
images represent sections of reciprocal space projected on the
flat area detector. This allowed for the mapping of most of
the three-dimensional reciprocal lattice of the crystal. It is
worth noting that the curvature of Ewald’s sphere in small-
angle experiments could be neglected due to the gigantic
difference between the structure period and the X-ray wave-
length. The background subtraction and reconstruction of
reciprocal space from the scattering data were performed by
a Mathcad code developed in-house.35
(24) Schope, H. J. J. Phys.: Condens. Matter 2003, 15, L533–L540.
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2004, 92, 058301.
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Grigoriev, S. V. JETP Lett. 2009, 90, 297–303.

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Results and Discussion
The representative scanning electron micrographs of as-grown
colloidal crystals of polystyrene spheres obtained on a cathode
and on an anode are shown in Figure 1. Panels A and B
correspond to the top surface and cross section of the crystal
depositedatU=1.5Vonacathode,respectively,andCandDat3
V on an anode. On the cathode, one can see that the particles are
neatlyarrangedintoperiodicarrays.Still,somedefectsarevisible
such as a few vacancies and lines (Figure 1A), where the spheres
have a squarelike arrangement. The apparent line defects are
typical for crystals grown by convective assembly4,36and are
openings of double stacking faults at the angle of 70.5? with the
substrate.36
Figure1Cillustratesthatanodepolarizationleadstoformation
of highly defective colloidal crystal structure. In this case, the
electrostatic attraction between negatively charged particles and
the positive electrode is presumably too strong, so that the
polystyrene spheres have little chance to rearrange their position
to find a better place in the crystal. Only top layers of the formed
films are ordered due, probably, to screening of Coulomb inter-
action by the bottom layers (Figure 1D). Sample cross sections
seeninSEMrevealthatthethicknessofcolloidalfilmsformedon
the cathode (-) is smaller than the corresponding value for
crystals on the anode (þ). The dependence of film’s thickness
on applied voltage is shown in Figure 2.
Toquantifytheeffectoftheelectricfieldonthestructuralorder
of colloidal films, we complemented the SEM observations with
microradian X-ray diffraction accompanied by 3D mapping of
the reciprocal space.35Figure 3 shows typical examples of micro-
radian diffraction patterns measured for different orientations of
colloidal crystals, corresponding to lowest index zones of fcc
structure,thatis,(111)atω=0?,(101) atω=-35.3?,and(010) at
ω=54.7? (ω is the rotation angle around the vertical axis, which
alignswiththe[101]axisofthecrystal).Inthediffractionpatterns,
onecanclearlyidentifyalargenumberofBraggreflections,which
can be assigned to the reciprocal lattice of the ideal fcc crystal
structure with the cubic cell size of a0=750 nm. Corresponding
indexes are shown in Figure 3.
In addition to the assigned Bragg peaks, the diffraction
patterns also show features which cannot be assigned to the fcc
structure (marked by arrows in Figure 3A). They can be truly
Bragg’s reflexes (e.g., corresponding to an hcp structure coexist-
ingwithfcc),orcanbesectionsofdiffuseobjects inthereciprocal
space. The latter can be related to the finite film thickness and/or
to the presence of stacking faults along Æ111æ cubic directions.37
Complete information on this type of disorder could be inferred
from the distribution of diffracted intensity in the three-dimen-
sional (3D) reciprocal space. The latter is reconstructed from the
collecteddiffractiondata.35Anexampleofa3Dmapinreciprocal
Figure 1. SEM imagesofcolloidal crystalspreparedbyverticaldepositionmethodinthe presenceofanexternalelectricfield perpendicular
tothesubstrates.PanelsAandBcorrespondtothetopsurfaceandcrosssectionofthecrystaldepositedatU=1.5Vonacathode,respectively,
and C and D at U=3 V on an anode. The distance between electrodes is 3 cm.
Figure 2. Dependence of the thickness of a colloidal film on
applied voltage U (according to SEM data). Negative values of
U correspond to a cathode polarization, and positive to an anode
polarization. Error bars indicate a standard deviation from the
mean value. The dashed line is the linear fit to the experimental
data.
(36) Hilhorst, J.; Abramova, V. V.; Sinitskii, A.; Sapoletova, N. A.; Napolskii,
K.S.;Eliseev,A.A.;Byelov,D.V.;Grigoryeva,N.A.;Vasilieva,A.V.;Bouwman,
W. G.; Kvashnina, K.; Snigirev, A.; Grigoriev, S. V.; Petukhov, A. V. Langmuir
2009, 25, 10408–10412.
(37) Loose, W.; Ackerson, B. J. J. Chem. Phys. 1994, 101, 7211–7220.

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Napolskii et al.Article
space is shown in Figure 4A for the colloidal crystal formed on a
cathodeatU=1.5V.Oneclearlyseesthepresenceoftheextended
rods of diffuse scattering and of localized reflections with well-
defined round shape. This reciprocal lattice is typical for a close-
packed structure with stacking faults.37-40
Weanalyzethedistributionofthediffractedintensityalongthe
Bragg rod with help of Wilson’s theory,37,38which assumes a
crystal consisting of a random sequence of close-packed layers.
Themainparameterofthistheory,R,istheprobabilityoffinding
aclose-packedlayerinthefccenvironment.ThevalueofRcanbe
evaluated from the distribution of scattered intensity along a
Bragg rod.
Figure 4B shows the normalized intensity profiles along a
Braggrodextractedfrom3Dreconstruction forcolloidal crystals
grown at U=1.5 V on the cathode and the anode. These profiles
are compared to the predictions of Wilson’s theory37with
different values of the probability R of finding an fcc sequence
of stacked layers. The calculated intensity profiles I(l) ? S(l) F(l),
where S(l) is a structure factor and F(l) is the form factor of 5%
polydisperse uniform spheres, are shown by lines in Figure 4B.
One can see that while for the crystal grown on the cathode the
stacking is mostly fcc, a nearly random hexagonal close packed
(rhcp) structure is found onthe anode (R ≈ 0.6). The dependence
of the parameter R on the applied potential is summarized in
Figure 5A. Since stacking disorder deteriorates the optical prop-
erties of photonic crystals,3,4application of cathode polarization
will allow improvement of the crystal quality.
The monotonic improvement of the crystal quality can also be
derived from the width of the diffraction spots. The results
obtained from the patterns measured at ω=0? are summarized
in Figure 5B,C. The full-width at half-maximum (fwhm) of the
diffraction maxima in the azimuthal (δj) and radial (δq) direc-
tions characterizes the mosaicity of the colloidal films and the
average crystallite size (Λ), respectively. One can see that the
mosaicity δj of the colloidal crystals decreases from 12? to 5?
whentheapplied voltage changes fromþ1.5 V to -1.5V. Higher
valuesofappliedpotentialleadtosignificantdisorientationofthe
domains in colloidal films grown at both anode and cathode
polarizations. We believe that the given behavior is connected
with water electrolysis accompanied by gas evolution on a
substrate’s surface at high values of U. The apparent peak width
in the radial direction δqappis determined from the fit by a
Lorenzian. The intrinsic peak width δqintrwas subsequently
estimated assuming the following relation:
ðδqappÞ2¼ Δ2þðδqintrÞ2
ð1Þ
where Δ is the instrument resolution Δ=(3.9 ( 0.2)?10-4nm-1
(fwhmoftheprofileofthedirectbeam).OnecanseeinFigure5C
that the application of a negative potential as far as U g -1.5 V
leadstoasignificantincreaseoftheaveragesizeofcrystallitesΛ=
2πB/δqintr, where B is a factor of the order 1. Then gas evolution
process at |U| g 2 V leads to rapid decrease of domain’s size,
which manifest itself in increase of radial width of diffraction
spots.
Thus, the increase of a cathode polarization leads to the
improvementofcrystalquality,butatthesametimethethickness
ofafilmdecreases(seeFigure2).Inordertopreventgasevolution
Figure 3. MicroradianX-raydiffraction patternsmeasuredwiththeX-raybeamorthogonal(A)tothesubstrate(ω=0?) andafterasample
rotation aroundthe verticalaxisbyω=-35.3? (B)and ω=54.7? (C).These patternscorrespondtotheindexzones(111), (101),and(010)of
thefccstructure.ThecolloidalcrystalisobtainedonthecathodeatU=1.5V.Peaks,whichcannotbeassignedtothefcclattice,aremarkedby
arrows in panel A.
Figure 4. (A)3DreconstructionofthereciprocalspaceforthecolloidalcrystalobtainedonthecathodeatU=1.5V(A).Thereflectionsonly
within q < 0.03 nm-1are shown. The hexagonal basis is depicted by the vectors b1, b2, and b3. The inset illustrates a view on the reciprocal
space along b3. (B) Normalized intensity variation along rods for two samples prepared on electrodes with opposite polarization at 1.5 V.
Intensity profiles calculated within Wilson’s theory for R equal to 0.85 and 0.60 are shown by solid and dashed lines, respectively.
(38) Wilson, A. J. C. Proc. R. Soc. London, Ser. A 1941, 180, 277–285.
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H. N. Phys. Rev. Lett. 2003, 90, 028304.

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Article Napolskii et al.
on the electrodes, which is responsible for structural defects, the
highvaluesofappliedpotentialsshouldbeavoided.Theoptimum
value of an applied voltage U for the conditions we have used
(given concentration of the suspension, pH, charge of colloidal
particles, temperature, distance between electrodes, etc.) is
around -1.5 V. These conditions allow growing crystals that
are 20 layers thick and have dominant fcc stacking (R ≈ 0.85).
The crystal quality of the best sample obtained in the present
work(seeforexamplecolloidalfilmgrownoncathodeat1.5V)is
comparable with the quality of artificial opals grown on glass at
optimal conditions;36as for example, the mosaicity is 8.4? for a
crystal grown on glass under optimal conditions, while the same
parameter for the best sample obtained by the suggested method
on ITO is 5?. The comparison for δqintr/q220values leads to the
same conclusion: δqintr/q220equal to 0.068 and 0.025 for films
grown on glass and ITO, respectively.
The quality of the obtained colloidal crystals reflects on their
optical properties. Figure 6 shows reflectance spectra for the
colloidalcrystalsobtainedatU=1Vwithoppositepolarizations.
The high reflectance baseline over 1300 nm is caused by high
reflectivityoflightfromITOinthisspectralregion.Thereflection
spectra exhibit a main peak at 1230 nm with two additional low
intensity peaks at 680 and 620 nm. These optical stop bands
originate to the Bragg reflection caused by the (111), (220), and
(222)planesofthefccstructure,respectively.Forthemostintense
reflectance, the reflectivity peak amplitude exceeds 50% and falls
down to 35% for crystals obtained on the negative and the
positive electrode, respectively. Moreover, in the case of the
colloidal crystal obtained on the cathode, Fabry-Perot oscilla-
tions are observed in the low-energy region, corresponding to a
constant thickness of the sample over the beam area (4?4 mm2).
The thickness of the colloidal film (h) can be calculated from the
position of the Fabry-Perot oscillations as
h ¼
λi
2neffcos θ 1-
λi
λiþ1
??
ð2Þ
where λiand λiþ1are positions of neighboring reflection maxima
(λi< λiþ1), θ is the incident angle of light with respect to the
normal to the sample, and neffis the effective (average) refractive
index of the polystyrene/air medium. neffis defined as
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
wherefPS≈0.74isthevolumefractionoccupiedbypolystyrenein
a closed packed structure; nPS≈ 1.57 and nair≈ 1 are refractive
indexes of polystyrene and air in the near IR region, respec-
tively.41Thus, for colloidal closed packed crystals made of
polystyrene microspheres, neffis equal to 1.44.
Accordingtoeq2, theobservedpeakpositionsatλ1=1335nm
and λ2=1415 nm correspond to h=8.3 μm for the colloidal film
obtained on the cathode at U=1 V. In assumption of an average
sphere diameter of 530 nm (according to the SEM data), the
crystal consists of 19 layers. The calculated value is in good
agreement with SEM data (see Figure 2).
Finally, it should be noted that the tendencies found for
colloidal crystals on ITO substrates have also clearly been seen
forotherkindsofconductingsubstrates,forexample,glass,mica,
and silicon single crystals covered by a thin layer of gold.
neff ¼
nPS2fPSþ nair2ð1- fPSÞ
q
ð3Þ
Conclusions
Ourobservationsrevealtheeffectofanexternalelectricfieldon
the structure of colloidal crystals made by the vertical deposition
Figure 5. U dependence of the main structural parameters of
artificial opals according to microradian X-ray diffraction data:
stacking probability R (A), mosaicity of the structure (B), and
radialwidthof (220)reflexes(C).NegativevaluesofU correspond
to a cathode polarization, and positive to an anode polarization.
Figure 6. Optical reflection spectra for artificial opals fabricated
on ITO. The spectra were measured at incident angle of 8? with
respect to the normal to the sample. A beam with a size of 4 ?
4 mm2was utilized. The samples were obtained at U = 1 V.
(41) Kasarova,S.N.;Sultanova,N.G.;Ivanov,C.D.;Nikolo,I.D.Opt.Mater.
2007, 29, 1481–1490.

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DOI: 10.1021/la902793b
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Langmuir 2010, 26(4), 2346–2351
Napolskii et al. Article
method. Although we do not directly compare the quality of our
crystals to those grown on dielectric substrates, our results
unambiguously show that application of a negative potential
can significantly improve the quality of the synthetic opals. The
suggested syntheticapproachisa highlyreproduciblewayfor the
formation of large-scale high-quality dry colloidal crystals with
controllable thickness on conducting substrates. This kind of
sample is attractive as a template material for preparation of
inverse photonic crystals by the electrochemical approach.42,43
The applicability of the suggested method for preparation of
colloidal crystals on dielectric substrates should be clarified in
future experiments.
Characterizationofthestructureofartificialopalsbenefitsalot
when standard techniques, such as SEM, are complemented by
microradian X-ray diffraction. Short acquisition times, modern
area detectors, and progress in computing techniques make 3D
reconstructions of the reciprocal space routinely available. This
method provides extremely valuable information on a real struc-
ture of mesoscopic materials, which cannot be easily obtained by
other analytical approaches.
Acknowledgment. This work is partially supported by the
Russian Federal Agency of Science and Innovations (Grant
Nos. 02.740.11.0135 and 02.513.12.3001) and Russian Foun-
dation for Basic Research. We thank the personnel of the
DUBBLE beamline and, in particular, Dirk Detollenaere for
their excellent support, and Swiss-Norwegian Beamlines for
allocation of part of the beam time in the frame of the SNBL-
DUBBLE agreement. The authors are grateful to G. A.
Tsirlina (Moscow State University) for numerous fruitful
discussions. The “Nederlandse organisatie voor Wetenschap-
pelijkOnderzoek(NWO)”isthankedforgrantingusthebeam
time.
(42) Braun, P. V.; Wiltzius, P. Adv. Mater. 2001, 13, 482–485.
(43) Meng, X. D.; Al-Salman, R.; Zhao, J. P.; Borissenko, N.; Li, Y.; Endres, F.
Angew. Chem., Int. Ed. 2009, 48, 2703–2707.