Research Article
www.acsami.org
Hybrid Top-Down/Bottom-Up Strategy Using Superwettability for
the Fabrication of Patterned Colloidal Assembly
Yuezhong Wang,†,‡ Cong Wei,§ Hailin Cong,*,† Qiang Yang,§ Yuchen Wu,§ Bin Su,§ Yongsheng Zhao,§
Jingxia Wang,*,‡ and Lei Jiang‡
†
Laboratory for New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, Qingdao
266071, China
‡
The Laboratory of Bio-Inspired Smart Interface Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Science,
Beijing 100190, China
§
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Key Laboratory of Organic Solids, Key
Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S Supporting Information
*
ABSTRACT: Superwettability of substrates has had a profound influence on
the production of novel and advanced colloidal assemblies in recent decades
owing to its effect on the spreading area, evaporation rate, and the resultant
assembly structure. In this paper, we investigated in detail the influence of the
superwettability of a transfer/template substrate on the colloidal assembly from
a hybrid top-down/bottom-up strategy. By taking advantage of a superhydrophilic flat transfer substrate and a superhydrophobic groove-structured
silicon template, the patterned colloidal microsphere assembly was formed
including linear and mesh-, cyclic-, and multistopband assembly arrays of
microspheres, and the optic-waveguide of a circular colloidal structure was
demonstrated. We believed this liquid top-down/bottom-up strategy would
open an efficient avenue for assembling/integrating microspheres building
blocks into device applications in a low-cost manner.
KEYWORDS: colloidal assembly, pattern colloidal, superwettability, top-down/bottom-up, optic waveguide
1. INTRODUCTION
Superwettability1−6 has produced many important applications
in antiwetting materials,7 anti-icing, oil−water separation,8
chemical reactions,9,10 directional water collection,11 lowenergy-consuming frosting prevention,12 subcooled water
nonstickiness,13 condensation heat and mass transfer enhancement,14−20 and pattern crystals.21−48 Particularly, the superwettability of a substrate has had a profound influence on the
production of novel and advanced colloidal assembly in recent
decades22−48 owing to its effect on the spreading area,
evaporation rate, and the resultant assembly structure.
Typically, superhydrophilic substrate25−30 (with water contact
angle (CA) approaching 0°) benefits the fully wetting/
spreading of colloidal suspension, which promotes the
formation of continuous and high-quality colloidal assembly.
In contrast, a low-adhesive superhydrophobic substrate (with
water CA > 150°) has been known as a specific substrate to
fabricate the colloidal assembly with unique properties.31−33
For example, its low-adhesive property results in a continuously
receded three phase contact line (TCL) during the solvent
evaporation procedure, which induces the formation of
spherical colloidal assembly32,33 or crack-free colloidal crystals
with narrow stopband. 31 It is worth noting that the
combination of a hydrophilic/hydrophobic substrate produces
© 2016 American Chemical Society
an effective approach for the creation of a patterned colloidal
microsphere assembly,40−42 wherein the hydrophilic part is
beneficial for the latex assembly owing to the pinning
TCL,34−37 while the hydrophobic part prevents the assembly
owing to its receding TCL.38−40 Recently, some novel
approaches are developed for the patterned colloidal microsphere assembly based on the modulation of substrate
wettability.44−49 For example, Song et al. inkjet-printed a series
of colloidal patterns by smartly designing the pattern from the
hydrophobic region surrounded by hydrophilic point.44,45 Su et
al.46−49 developed a novel assembly approach for the pattern
with precise orientation and position based on the elaborate
design of wetted and dewetted regions by sandwiching the latex
suspension in between the template and transfer substrate. In
this case, the assembly pattern can be directly transferred to the
desired substrate; it is convenient for further device application
of the as-prepared pattern array. In contrast, for the traditional
template assembly method50−54 the latex particles can be easily
and precisely assembled on the template in large scale and
suitable time, but the necessary pattern transfer or template
Received: December 7, 2015
Accepted: January 29, 2016
Published: January 29, 2016
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DOI: 10.1021/acsami.5b11945
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Research Article
ACS Applied Materials & Interfaces
Scheme 1. Schematic Illustration for the General Strategy to Align Poly(St-MMA-AA) Latex in One Direction Based on a
Sandwich Assemblya
a
(a, b) Colloid latex was carefully dropped onto the superhydrophobic groove-structured template and covered by a flat superhydrophilic transfer
substrate, yielding a fixed-gap sandwich assembly system. The assembly system was put at constant template/humidity for 12 h. (c) Schematic of the
contact position for the colloid latex between the superhydrophobic template and superhydrophilic transfer substrate; the inset is the shape of the
water droplet on the transfer substrate and the template, respectively. (d) With the evaporation of solvent, the groove wall array served as wetting
defects to control the rupture of colloid latex, yielding a micrometer-scaled liquid film between the substrate and the top surface of the groove wall.
Such a liquid bridge provides a gradually reducing confined space for latex aggregation. (e) Close-packed, linear assembly arrays of colloids can be
formed upon the substrate. (f) After removal of the groove-structured template by physical peeling, a precisely positioned colloidal assembly pattern
can be generated.
methacrylate (MMA,10.00 mM), acrylic acid (AA,13.89 mM), and
styrene (St, 182.60 mM), were added into a four-necked flask.
Subsequently, the above mixture was stirred at 70 °C for 10 h after
charging the initiator of ammonium persulfate. The as-prepared latex
particles can be used directly without purification. The fluorescent
latex was obtained on the basis of a similar procedure except with
charging a fluorescent molecule such as coumarin 6 (about 1 wt % of
St) into the monomer system.
Surface Modification for Groove-Template and Glass
Substrate. The hydrophobic treatment groove-template or substrate
was generated by silanizing the template/substrates with heptadecafluorodecyltrimethoxysilane (FAS) in a decompression environment at
room temperature for 0.5 h and then heated at 80 °C for 0, 1, 2, and 3
h, respectively, yielding different hydrophobic surfaces. The superhydrophobic templates were obtained by combining the low-surfaceenergy treatment with surface roughness structure after being FAS
modified for longer than 3 h. Superhydrophilic glass and silicon wafer
substrate were obtained by oxygen plasma treatment. The relevant
operating parameters are as follows: feed gas is oxygen; gas flow is 40
SCCM; backing vacuum degree (working pressure) is 40 Pa; discharge
power (working power) is 200 W; working time is 10 min. The water
CA of the treated substrate is ca. 0°, and that of the clean silicon wafer
is 33.5° ± 4.2°.
Generation of Colloidal Pattern from Hybrid Top-Down/
Bottom-Up Approach. A FAS modified groove-structured silicon
substrate with groove width of 5 μm, gap of 5 μm, and height of 20 μm
was held horizontally. Then poly(St-MMA-AA) latex with the colloidal
suspension’s weight concentration of 8.67% (see Figures S7−S8 for
the influence of the colloidal concentration on assembly structure) was
carefully dropped onto the template and covered by a flat substrate,
yielding a sandwich assembly. With the assembly system kept at 20°
for 12 h, long poly(St-MMA-AA) microsphere linear assemblies were
achieved. In this case, controlling the gap between the chosen
substrate and the groove-structured template at ca. 20 μm led to
continuous liquid stripes. The assembly process for other pattern
colloidal was similar except for the change in the kind of templates.
The two or multistopband assembly array was obtained by repeating
the above-mentioned assembly process except with use of the asprepared the assembly array as the transfer substrate.
Characterization. The morphologies of groove-structured silicon
substrates and aligned pattern colloidal assemblies were investigated by
scanning electron microscopy (SEM, JEOL, JSM-7500F, Japan) at an
removal is required for further device applications. Accordingly,
the newly developed sandwich assembly approach46−49
provides a simple and direct assembly approach for the pattern
array on the desired substrate, completely avoiding the
additional substrate transfer process. In this paper, we
demonstrated a full investigation of the influence of the
superwettability of the template/transfer substrate on the
patterned colloidal microsphere assembly through extending
the above-mentioned sandwich method. By taking advantage of
a superhydrophilic flat transfer substrate and a superhydrophobic groove-structured silicon template, the wellordered pattern colloidal was formed, including linear and
mesh-, cyclic-, and multistopband assembly arrays of microspheres. The optic-waveguide behavior of circular colloidal was
investigated. We believed this liquid top-down/bottom-up
strategy would open an efficient avenue for assembling/
integrating microspheres building blocks into device applications in a low-cost manner.
2. EXPERIMENTAL SECTION
Groove-Structured Silicon Templates. The silicon wafers with
template (N doped, ⟨100⟩ oriented, 400 μm thick, 10 cm diameter)
were structured by direct laser writing apparatus (DWL200,
Heidelberg Instruments Mikrotechnik, Germany) that transferred
the computer-predefined design onto the photoresist-coated (Shipley
Microposit S1800 series) wafer with about 1 μm precision. After
irradiation and development, the wafers were etched using deep
reactive ion etching (DRIE, Alcatel 601E) with fluorine-based reagents
for different times (10 s to 6 min) depending on the desired height of
the structures. Groove-structured silicon substrates with groove width
of 5 μm, gap of 5 μm, and height of 20 μm were fabricated. After resist
stripping (Shipley Microposit Remover 1165), the substrates were
cleaned with ethanol and acetone prior to use.
Preparation of Polystyrene-Methyl Methacrylate-Arylic Acid
(Poly(St-MMA-AA)) Microspheres. The monodispersed latex
particles of poly(St-MMA-AA) were prepared by modified emulsion
polymerization based on our previous method.59 First, aqueous
solution of sodium dodecyl benzenesulfonate (0−0.072 mM) and
ammonium bicarbonate (6.30 mM), and monomer mixture of methyl
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Figure 1. Optical microscopy images and SEM images (inserted) of pattern latex assembly from the hybrid top-down/bottom-up approach: (a) blue,
(b) green, (c) red linear colloidal, (d) mesh colloidal, (e, f) cyclic colloidal. As-prepared latex assembly shows homogeneous structure color from the
well-ordered latex arrangement, that can be clearly confirmed from the inserted SEM images. The inserted image of the right-top is the optical
microscopic images, and the inserted images of the left-bottom are the SEM images of the corresponding samples.
Figure 2. SEM images of the pattern latex assembly from (a−d) groove-structured template with water CA of (a) 147.7° ± 4.5°, (b) 103.5° ± 5.6°,
(c) 72.7° ± 3.8°, (d) 4.2° ± 2.5° combined with the superhydrophilic transfer substrate. The inserted left-bottom images in parts a−d are the FFT
transfer of the corresponding SEM image of as-prepared samples. (e−g) SEM images of the latex assembly on the glass substrate with water CA of
(e, f) 33.4° ± 2.4°, (g) 105.6° ± 4.2° in combination with the superhydrophobic template. (h, i) SEM images of the latex particles leaving the
superhydrophobic template in combination with the hydrophobic transfer substrate. The photograph of the inserted right-top image is the shape of
the water droplet on the as-used groove template (a−d), and on the transfer substrate (e, g). The superhydrophobic template combined with
superhydrophilic transfer substrate is the optimal choice for the well-ordered latex assembly and perfect transfer.
average CA was obtained by measuring more than five different
positions of the sample.
Calculation of the Ordering of the Latex Assembly. The
quantitative analysis of the ordering of latex assembly is according to
the literature method.55 The detailed calculation process includes
transferring the SEM micrograph into a binary image, and locating the
centroid of each particle via the MATLAB image processing toolbox.
The curves of g(r) were calculated between r = 0.05R0 and r = 30R0
using shell thicknesses of 0.016R0, where R0 is the particle radius, r is
the distance from the origin of the radial distribution function, and
g(r) is the calculated distance. Each SEM image was measured against
accelerating voltage of 5.0 kV. Bright-field optical images and
fluorescence microscopy images were taken by system microscopy
(Olympus BX51, Japan), by exciting the samples with a mercury lamp.
The single composite microring was locally excited by a focused 400
nm pulse laser beam (200 fs, 1000 Hz). The oxygen plasma instrument
(DT-03) is purchased from Suzhou OPS oxygen plasma Technology
Co., Ltd. Static CAs were measured on a DataPhysics (Germany)
OCA20 contact-angle system at ambient temperature. The adhesion
force is characterized by the adhesion force tester DCTA21 machine,
which was made in Germany by Dataphysics Instruments Gmbh. The
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Figure 3. (a−e) Curves of g(r) calculated for pattern latex assembly (a) with perfect array and samples fabricated from groove template with
wettability of (b) 147.7° ± 4.5°, (c) 103.5° ± 5.6°, (d) 72.7° ± 3.8°, and (e) 4.2° ± 2.5°, combined with the superhydrophilic transfer substrate,
respectively. Inserted images from parts a−e are the corresponding Fourier transforms (FTs) that transfer in a 2D way for latex assembly from the
corresponding template. (f−i) Single-sided power spectra FT of g(r) − 1 compared to that of the corresponding perfectly ordered arrays for the
samples obtained from the template with wettability of (f) 147.7° ± 4.5°, (g) 103.5° ± 5.6°, (h) 72.7° ± 3.8°, and (i) 4.2° ± 2.5° in combination
with the superhydrophilic transfer substrate. The power spectra were scaled to have identical maximal at f/f 0 = 1. Inserted images in parts f−i are the
corresponding SEM images of the latex assembly.
a perfect array with comparable period, image resolution, and number
of particles.
microstructures aligned in one direction at precise positions
(Figure 2a). Top width (3.9 ± 0.5 μm) of the colloidal
assembly line is more narrow than that of the bottom one (6.6
± 0.8 μm) (Figure 2a), which is affected by the gap between
the template and the substrate as shown in Figure S6. The
poly(St-MMA-AA) microspheres exhibited a hexagonal closepacked structure on the top of the colloidal assembly line,
showing iridescent structure color. Importantly, when further
replacing the template with well-designed mesh and cyclic
patterns, the large-scale colloidal assembly with the correspondingly pattern can be fabricated in Figure 1d−f. An as-prepared
patterned colloidal microsphere assembly shows iridescent
structure color owing to the well-ordered latex assembly in the
inserted image. Accordingly, this hybrid top-down/bottom-up
approach provides a high throughput and precise strategy to
produce a guided microsphere pattern assembly. In this way, we
can precisely control the shape, location, and area of the
colloidal assembly in one step, omitting the process of
removing the template and transferring to the substrate.
The wettability of the groove-shaped template produces an
important impact on the resultant latex assembly in Figure 2a−
d. Evidently, a distinct pattern morphology and latex assembly
were observed when changing the wettability of the template.
As shown in Figure 2a, a perfect and smooth linear colloidal
structure with well-ordered latex arrangement was obtained on
the superhydrophobic template. In contrast, an increased defect
formed on the linear colloid in Figure 2b−d when the sample
was assembled using the less hydrophobic template. The
increased hydrophilicity of the template induced an obvious
growth of the defect area when the CA of the template changed
from 147.7° ± 4.5°, to 103.5° ± 5.6°, to 72.7° ± 3.8°, and to
4.2° ± 2.5° as shown in Figure 2a−d, respectively. Concretely,
only a somewhat disordered assembly of microparticles can be
observed in Figure 2b (103.5° ± 5.6°) in a comparison to that
in Figure 2a (147.7° ± 4.5°). The sample from template with
■
RESULTS AND DISCUSSION
Scheme 1 showed the fabrication process of a linear colloidal
assembly from the hybrid top-down/bottom-up approach. First,
the milky poly(St-MMA-AA) latex (5 μL, the colloidal
suspension’s weight concentration is about 8.67%; Figure S7
presents the influence of colloidal concentration on assembly
structure) was sandwiched into a superhydrophobic groovestructured template and a superhydrophilic flat transfer
substrate (glass or silicon wafer, oxygen plasma processing 10
min, working power is 200 W). Copper wires with diameter
20−100 μm were employed to control the gaps between the
substrate and groove-structured template. With the assembly
system kept at constant humidity/temperature for 12 h, the
linear assemblies with precise position can be generated. In this
process, the key step is to control the size of the gaps between
the transfer substrate and the groove-structured template. Only
an appropriate gap is favorable for producing a confined effect
of the latex film between the substrate and template.
Specifically, the large gap will weaken the rupture behavior of
the liquid film, resulting in a continuous film rather than a
patterned colloidal microsphere assembly on the substrate.
Furthermore, the superhydrophobic template is almost
completely clean after being transferred, and it can be reused
immediately as shown in Figure S3.
Figure 1 demonstrated the optical microscopy images and
SEM images of an as-prepared patterned colloidal microsphere
assembly from the hybrid top-down/bottom-up approach.
Clearly, linear colloidal assemblies with structure colors of red,
green, and blue were shown in Figure 1a−c, respectively. The
line length approaches 0.5 cm, and the adjacent spacing
between two lines is ca. 4.0 μm in Figure 1a−c, wherein,
poly(St−MMA−AA) microspheres were assembled into linear
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Figure 4. (a−c) Optic microscopy images (in top-view) and (d−f) the corresponding schematic illustration for the in situ observation of the
formation process for linear colloidal assembly (in cross-section view). (a, d) The initial state of the poly(St-MMA-AA) latex in the sandwich
assembly system. (b, e) Assembled process, with the latex film retraction line indicated by red arrows. (c, f) Complete microspheres assembly.
CA leads to little particles being transferred in Figure 2g,
suggesting that the latex particles prefer to keep in the groove
template in Figure 2h,i for the transfer substrate with higher
CA. Interestingly, when using a superhydrophobic template,
most of the microspheres stay on the top of the groovestructured template rather than infiltrate into the inside of the
groove template as shown in Figure 2h,i, which also implies that
the superhydrophobic groove-structured template prevents the
latex from infiltration into the interior of the template.
Accordingly, a combined use of superhydrophobic template
and the superhydrophilic transfer substrate is the most optimal
choice for the perfect latex assembly though a hybrid topdown/bottom-up approach.
To understand the influence of the substrate wettability on
the latex assembly, we conducted a quantitative analysis in
Figure 3 for the ordering of the linear colloidal arrays obtained
from a different wettability template (Figure 2a−d). The
ordering of the latex assembly can be evaluated by calculating
the linear colloidal pair correlation function, g(r), given in eq 1
according to the literature method55
CA of 72° (Figure 2c) showed a more peeling and disordered
structure, which indicates a degraded latex ordering in the
samples when improving the hydrophilicity of the template.
Worse, a large part of blank region can be observed for the
sample from the hydrophilic template (with water CA of 4.2° ±
2.5°) in Figure 2d. This implies that most of the latex particles
cannot be effectively transferred to the substrate, or plenty of
latex particles will be infiltrated/remained in the groovestructure of the template as shown in Figure S4d. Accordingly,
the successful fabrication of the line structure in Figure 2a can
be evaluated as a well-ordered latex assembly, and there is
complete transfer of the structure from template to the
substrate. These can be attributed to the synergistic effect of
superhydrophobic template and superhydrophilic transfer
substrate. In this case, the low adhesion force (Figure S2 for
the adhesion force of the template) of the superhydrophobic
template can prevent latex from infiltration or adhesion into the
groove template (in Scheme 1c),31 which is favorable not only
for the easy peeling procedure of the template from the
substrate but also its recycle use, contributing to the complete
transfer and smooth assembly surface. In contrast, increasing
the hydrophilicity of the template promotes the spreading and
infiltrating of the latex into the groove structure of template,
resulting in more particles in contact with the template as
shown in Figure S4. This corresponds to the increased defect
area that resulted from increased hydrophilicity of the template
in Figure 2d. Otherwise, the combination of superhydrophobic
groove template and superhydrophilic transfer substrate
provides an effective platform for a well-ordered latex assembly
owing to its slow evaporation time and additional assembly
force from the sliding TCL during the solvent evaporation
process.31
On the other hand, the wettability of the transfer substrate
affects the resultant latex assembly as well. Figure 2e,g
presented SEM images of the as-prepared samples from
transfer substrate with different wettability in combination
with the superhydrophobic template. Transfer substrate with
lower water CA is favorable to an increased transfer of the latex
particles onto the substrate in Figure 2e, while increased water
g (r ) =
1 dn(r , r + dr )
⟨ρ⟩ da(r , r + dr )
(1)
Here, a is the shell area, and n(r, r + dr) is the number of
particles that lies within the shell considered. As shown in
Figure 3a, g(r) shows a series of broad peaks that coincides with
those calculated for a perfect array. The peak intensities of g(r)
decrease under the different samples, indicating that the 2D
ordering decreases. The linear colloidal arrays obtained by the
superhydrophobic groove-structured template exhibit significant correlation in Figure 3b beyond the tenth normalized
distance r/2R0 in comparison to the perfect array in Figure 3a,
indicating its perfectly well-ordered structure, while samples
from the other cases show little corresponding to the perfect
array. Furthermore, a quantitative measure of ordering can be
evaluated by the value (k/k0) of the full width at half-maximum
(fwhm) based on a literature method23,24 to quantitatively
investigate the wettability influence on latex assembly. k/k0 is
the first peak in the Fourier transform of the function g(r − 1)
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Figure 5. Schematic illustration for the preparation of (a, b) pattern colloidal with double bandgap, (c) heterogeneous colloidal arrays, and (g) dot−
line composite structure. Optical photograph (d) and SEM images (e, f) of the heterogeneous colloidal arrays. (h−j) SEM images of the
heterogeneous dot−line colloidal arrays with different magnification.
array of regularly microspheres linear assemblies (Figure 4e,f).
It should be pointed out that these used templates can be
recycled by a simple organic solvents washing process to get rid
of deposited microspheres.
The developed method is versatile for creating the
heterogeneous film for linear colloidal assembly. Figure 5
shows the schematic of pattern colloidal with double bandgap
as shown in Figure S8 using a two-step procedure. In this case,
the as-prepared linear colloid was used as the transfer substrate,
which sandwiched the milky poly(St-MMA-AA) latex in a
direction orthogonal to the initial stamp orientation in
combination with the groove-template. After solvent evaporation, heterogeneous linear colloidal structures were obtained,
which showed two different uniform colors due to light
diffraction. These iridescent colors mainly depended on the two
kinds of size of the microspheres and the refractive indexes,
suggesting the well-ordered latex assembly. The magnified SEM
images of a crossover of two lines of crystal films (Figure 5e,f)
display the heterogeneous structures of colloidal assembly; one
line is made of 223 nm microspheres, and the other is made of
264 nm microspheres (the colloidal suspension’s weight
concentration is about 8.34%, see Figure S8). Some microsphere residue appeared between the two lines of crystal film
possibly because the first step colloidal lines can also be pinned
and dominated the rupture of the liquid films. The gap between
the groove tops and the linear colloidal substrate cannot be
uniform during the second printing process due to the
thickness of the primary patterned crystal. From Figure 5f,
we could observe that no microspheres (264 nm) appeared on
the primary linear patterned crystal film made of microspheres
(223 nm) after the second assembly process, which suggests
that the interaction between the two microspheres is smaller
than the liquid film shrink traction. Some interesting
phenomena can be observed from Figure 5g−j. Dot−line
composite structures can be obtained by reducing the poly(StMMA-AA) latex concentration (the colloidal suspension’s
weight concentration is about 4.17%). The dots were
assembled on the top of the line rather than between the
neighboring lines, because the latex prefers to assembly at the
lower gap region.
of the sample and that of a perfectly ordered array, respectively.
Figure 3f−i demonstrated k/k0 values calculated for samples
prepared from a template with different wettability. The values
of the sample from a template with water CA of 147.7° ± 4.5°
(Figure 2a) and 103.5° ± 5.6° (Figure 2b) are 1.17 and 1.24,
respectively (Figure 3f,g), which indicates that the poly(StMMA-AA) arrays from these substrate are highly ordered.57−59
In comparison, the k/k0 values, calculated for samples prepared
from template with water CA of 72.7° ± 3.8° (Figure 2c) and
4.2° ± 2.5° (Figure 2d), curves are 2.06 and 2.94, respectively
(Figure 3h,i), implying that the latex assemblies are highly
disordered.56−59 The decrease of k/k0 of latex assembly from
the template substrate with higher water CA confirms that the
combination of superhydrophobic template with superhydrophilic substrate is favorable for efficient latex assembly and
transfer.
To explore the growing details of the aligned linear latex
assembly, Figure 4a−c presented an in situ assembly procedure
for linear assembly monitored by video recording through an
optical microscope in a top-view way. Figure 4d−f shows the
detailed assembly process in a cross-view. Owing to a suitable
gap that presents between the groove tops and the glass plate, a
poly(St-MMA-AA) latex film (Figure 4a,d) can form. Following
the evaporation of water at the edges of this sandwich system,
the gas−solid−liquid TCL of poly(St-MMA-AA) latex underwent unidirectional shrinkage in Figure 4b,e. Since the ordered
groove tops pinned and dominated the rupture of the liquid
films, an array of ca. 5 μm width parallel to liquid bridges was
formed (Figure 4c,f). In contrast to cylindrical liquid bridges
suspended in air,34−55,59 three-dimensional (3D) liquid bridges
with a meniscus in both the horizontal and vertical directions
are formed in between groove tops and substrate owing to the
superhydrophobic nature of not only the top substrate but also
the neighboring groove-wall. In this case, poly(St-MMA-AA)
microspheres are restricted followed by the shrinkage of the 3D
liquid bridges, yielding trapezoid-shaped linear assemblies in
Figure 4f with a gradually reducing number of microspheres
from bottom to top as shown in Figures S5 and S6. In brief, the
ordered groove-walls dominated in guiding the rupture of the
latex and formed an array of parallel liquid bridges, yielding an
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Figure 6. Microring colloidal and its waveguide property. (a) Fluorescence optic microscopy images and (b) fluorescent spectrum of colloidal
microrings. (c) Schematic illustration of optical waveguiding along microring-pattern after irradiation by laser. A laser beam was focused on the
microring to investigate the optical propagation. (d) Photoluminescence (PL) images of microring-pattern under a focused 400 nm laser beam F
excitation. The excitation positions are marked with the red circles.
substrate, high-quality latex assembly can be achieved, and
the basic optic waveguide test was carried out for the circular
assembly. We anticipated that this fabrication approach will
become a useful platform for achieving various functional
nanophotonic devices.The as-fabricated latex assembly can also
act as prototype models in theory simulation fields for optical
materials.
Waveguide behavior is a typical property for the determination of optic manipulation and the transport.48,60,61 To
demonstrate the waveguide property of the patterned colloidal
microsphere assembly, we fabricated the circular colloidal by
using the microring template and the P(St-MMA-AA) latex
containing fluorescent coumarin 6 according to Scheme 1.
Figure 6 shows the microscopic optic images of the as-prepared
microring assembly. The as-prepared microring has the interior
and exterior diameters of 20 and 26 μm, respectively, with the
ring width of 3 μm, respectively. The microrings showed
homogeneous bright green color in Figure 6a owing to the
fluorescent signal. The assembled latex particles are wellordered in the circular as inserted in Figure 6a, indicating the
good optic manipulation behavior. The waveguiding behavior
of these microrings was investigated using far-field microscopy
and spectroscopy,60,61 as shown in Figure 6c. When focused
with a continuous wave laser onto one position of the
microring (Figure 6c), the generated photoluminescence was
strongly guided by the regularly arranged structure, causing the
light to propagate through the colloidal microring wall. This
optic waveguide behavior can be fully confirmed in Figure 6d. A
shining blue signal is observed around the whole microring
after a laser beam focused on one point of the microring. In the
meantime, an obvious fluorescent spectrum can be captured
from the microring as shown in Figure 6b.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.5b11945.
Morphological observations and wetting behaviors of
FAS modified groove-wall-structured template (Figure
S1), adhesion force and the inserted water CA for
different groove-structured template (Figure S2), more
details of assembled template and substrate (Figures S3−
S4), lateral and top view of SEM images of more detailed
assembled linear colloidal structure (Figures S5−S6),
SEM images of linear and heterogeneous colloidal arrays
with different morphology (Figures S7−S8), and SEM
images of as-prepared colloidal crystal microrings (Figure
S9) (PDF)
■
4. CONCLUSION
In summary, we have reported a simple fabrication approach
toward patterned colloidal structures including linear-, mesh-,
circle-, and multistopband pattern colloidal structures from the
hybrid top-down/bottom-up approach, where the effective
control of the template/transfer substrate wettability produces a
decisive role on the final latex assembly. By combining the
superhydrophobic template and superhydrophilic transfer
AUTHOR INFORMATION
Corresponding Authors
*E-mail: Jingxiawang@mail.ipc.ac.cn.
*E-mail: hailincong@163.com.
Author Contributions
Y.W. and C.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acsami.5b11945
ACS Appl. Mater. Interfaces 2016, 8, 4985−4993
Research Article
ACS Applied Materials & Interfaces
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ACKNOWLEDGMENTS
The authors acknowledge the financial support by the National
Nature Sciences Foundation (Grants 51373183, 21421061,
91127029, and 21074139) and 973 program (no.
2013CB933000) and Key Research Program of the Chinese
Academy of Sciences.
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