Optofluidic Assembly of Colloidal Photonic Crystals with Controlled Sizes, Shapes, and Structures

Article (PDF Available)inAdvanced Materials 20(9):1649 - 1655 · May 2008with204 Reads
DOI: 10.1002/adma.200703022
Abstract
The fabrication of various photonic structures with negligible cracking through evaporation-free colloidal self-assembly using silica particles dispersed in ethoxylated trimethylolpropane triacrylate (ETPTA) resin was investigated. The spherical photonic crystal balls, all of the same size, was prepared by using simple and high-throughput microfluidic devices. The emulsion drops were elongated initially but relaxed to a spherical shape if the particle concentration was not too high to immobilize the interface. The microfluidic device produced highly monodisperse emulsion drops, of which the size was proportional to the diameter of the inner capillary. It was observed that the proposed fabrication scheme was effective for creating non-close-packed colloidal crystals with well-controlled shapes and lattice constants.

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DOI: 10.1002/adma.200703022
Optofluidic Assembly of Colloidal Photonic Crystals with
Controlled Sizes, Shapes, and Structures**
By Shin-Hyun Kim, Seog-Jin Jeon, Gi-Ra Yi, Chul-Joon Heo, Jae Hoon Choi, and
Seung-Man Yang*
The spontaneous crystallization of colloidal particles
provides a simple and cheap alternative to nanolithography
for creating three-dimensional periodic structures. Colloidal
crystals with long-range order diffract light and display
photonic bandgaps, which is of practical significance for
photonic and phononic crystal devices,
[1–4]
biological and
chemical sensors,
[5]
and tunable lasers.
[6]
Also, the phenom-
enon of colloidal crystallization has been studied as a model
system for understanding the phase behavior of matter, with
the colloidal particles considered as artificial atoms and
molecules.
[7]
A major obstacle to the practical application of
colloidal crystals is a lack of simple and reliable methods for
consolidating the colloidal particles into crystals with well-
controlled shapes, sizes, and structures over fast time scales
without disturbing the fragile crystal structure formed in a
suspension state. Previous research on the generation of
colloidal crystals has predominantly focused on two
approaches: (i) flow- or evaporation-induced packing of
colloidal particles;
[8–11]
and (ii) the use of electrostatics to
induce non-close packing, followed by immobilization.
[12,13]
However, these methods involve slow and complicated
processes or produce intrinsic defects. Although Kanai et al.
reported recently that the air-pulse-driven crystallization can
produce single crystals of charged colloids in large area within
short times,
[14]
in situ solidification of crystalline colloidal
arrays is still challenging. Here, we report a novel and simple
method for creating various structural motifs of colloidal
crystals by photoinduced consolidation over fast time scales.
Our strategy, in conjunction with a high-throughput optofluidic
technique, allows unprecedented control over the three-
dimensional organization of the colloidal particles, as well as
the combination of different materials over multiple length
scales. Specifically, we demonstrate shaping colloidal crystals
into various usable solid objects such as photonic (Janus) balls,
cylinders, and films in a controlled manner, thus expanding the
potential for specific applications such as mobile information
displays, e-papers,
[15]
photonic crystal sensors,
[16]
and light-
emission modulators.
[17]
Conventional evaporation-induced colloidal crystallization
uses emulsion or aerosol drops,
[18–22]
cylindrical capil-
laries
[23,24]
and patterned microchannels
[25,26]
as templates to
mold the crystals into desired shapes. However, evaporation-
induced crystallization requires impractically long times for
complete consolidation and inevitably yields crystals with
severe cracks. In the present work, by using silica particles
dispersed in ethoxylated trimethylolpropane triacrylate
(ETPTA) resin, we successfully fabricated various photonic
structures with negligible cracking via evaporation-free
colloidal self-assembly. The success of this system stems from
the highly viscous nature of the ETPTA resin, which causes the
crystalline structure formed in suspension to be more robust to
disturbances from external flows or Brownian diffusion. The
ETPTA resin is photopolymerizable under UV exposure,
enabling the capture of the colloidal crystal structures with no
disturbance within a time period of 1 s. More importantly, the
silica particles spontaneously organize into an ordered phase at
low concentrations on account of a strong repulsive potential
relative to a diminishing van der Waals attraction, because the
ETPTA resin is highly polar and has a refractive index
(n
m
¼ 1.4689) that matches that of silica (n
p
¼ 1.45).
[27]
Indeed,
as shown in Figure S1 of the Supporting Information, silica
particles dispersed in ETPTA resin diffracted light and
displayed iridescent colors at volume fractions as low as
10%. The reflection wavelength l depends on the particle
radius a and volume fraction f, and can be estimated by
Bragg’s law.
For practical applications, we prepared spherical photonic
crystal balls, all of the same size, by using simple and
high-throughput microfluidic devices. Previously, similar
schemes have been used to create monodisperse double
emulsions
[28]
and photolithographically featured 2D particles
of various shapes.
[29]
As shown in the schematic optofluidic
system in Figure 1a, we used co-current flows in a system of two
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[*] Prof. S.-M. Yang, S.-H Kim, S.-J. Jeon, C.-J. Heo, J. H. Choi
National Creative Research Initiative Center for Integrated Opto-
fluidic Systems
Korea Advanced Institute of Science and Technology
Daejeon, 305-701 (Korea)
E-mail: smyang@kaist.ac.kr
Prof. S.-M. Yang, S.-H Kim, S.-J. Jeon, C.-J Heo, J. H. Choi
Department of Chemical and Biomolecular Engineering
Korea Advanced Institute of Science and Technology
Daejeon, 305-701 (Korea)
Dr. G.-R. Yi
Nano Bio System Research Team
Korea Basic Science Institute
Seoul, 136-713 (Korea)
[**] This work was supported by a grant from the Creative Research
Initiative Program of the Ministry of Science & Technology for
‘‘Complementary Hybridization of Optical and Fluidic Devices for
Integrated Optofluidic Systems.’’ The authors also appreciate
partial support from the Brain Korea 21 Program. Supporting
Information is available online from Wiley InterScience or from
the authors.
Adv. Mater. 2008, 20, 1649–1655 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1649
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coaxial microcapillaries, in which the silica-in-ETPTA suspen-
sion is fed through the inside microcapillary and the aqueous
phase is flowed through the annulus region between the inner
and outer capillaries. At the tip of the inner capillary, the flow
of aqueous phase sweeps to break the stream of the silica
suspension into oil-in-water emulsion drops. During shear-
induced emulsification, the emulsion drops were elongated
initially but relaxed to a spherical shape if the particle
concentration was not too high to immobilize the interface.
[30]
Specifically, when f is less than 0.33, the relaxation time scale
to a spherical shape is less than 1 s (see the Supporting
Information, S2). To see the internal arrangement of the
colloidal particles within spherical emulsion droplets of
different sizes, we used silica particles of diameter 1 mm and
observed the cross sections of emulsion drops using optical
microscopy. The results, displayed in Figure 1b for three drop
sizes (d
d
¼ 15.9, 24.5, and 53.7 mm), show that the 1 mm silica
particles formed layered structures of concentric spherical
shells. When the suspension is broken into spherical droplets,
the particles begin to form concentric spherical shells of
hexagonal arrays from the outermost layer, thereby reducing
the repulsive energy. However, the core of the structure is less
ordered because of the high curvature in this region, which
cannot accommodate the hexagonal arrangement.
Figure 1. a) Schematic illustration for the preparation of photonic balls of equal size. b) Cross-sectional images of three colloidal photonic balls with
different diameters of 15.9, 24.5, and 53.7 mm. Each ball is composed of 1 mm silica particles embedded in an ETPTA matrix at f ¼ 0.33. c) Optical
microscopy images of monodisperse silica-in-ETPTA droplets. d) Counter-clockwise from top-right, optical microscope images of blue, green, and red
photonic balls composed of 145 nm silica particles at f ¼ 0.33, 152 nm silica particles at f ¼ 0.25, and 190 nm silica particles at f ¼ 0.25 and corresponding
reflectance spectra. e) scanning electron microscopy image of green photonic balls composed of 165 nm silica particles embedded in ETPTA matrix at
f ¼ 0.25. Scale bars: b) 10 mm, c–e) 100 mm.
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The microfluidic device produced highly monodisperse
emulsion drops, of which the size is proportional to the
diameter of the inner capillary.
[31]
The optical microscopy
image in Figure 1c and the still shots of microfluidic
emulsification in the Supporting Information (Fig. S3) show
that highly monodisperse silica-in-ETPTA drops of spherical
shape are created, with a narrow size distribution. In fact, the
coefficient of variation of the drop sizes remains less than 1.5%.
Downstream of the tip of the inner channel, the generated
droplets were solidified by photopolymerization for 0.5 s under
a UV lamp mounted on an inverted optical microscope,
without coalescence as the droplets flowed through the
channel. The emulsion drops remained spherical during
UV-induced solidification. The consolidated photonic balls
of silica-in-ETPTA colloidal crystals reflected light at a
frequency that depended on the particle size and concentra-
tion. Indeed, the blue, green, and red photonic balls in Figure
1d were composed of 145 nm silica particles at f ¼ 0.33, 152 nm
silica particles at f ¼ 0.25, and 190 nm silica particles at
f ¼ 0.25, respectively. The corresponding reflectance spectra,
shown in Figure 1d, are well-matched to Bragg’s law. Also, a
scanning electron microscopy (SEM) image of a photonic ball
(Fig. 1e) shows a smooth spherical surface with hexagonally
packed silica particles in the ETPTA matrix. These results
clearly indicate that the surface of the spherical suprastructure
is the (111) plane of the face-centered cubic (fcc) lattice, and
that the reflected colors correspond to the L gaps of the
photonic balls.
According to Bragg’s law, the photonic band gap is
modulated by the particle concentration as well as the particle
size. For example, when we used 165 nm silica particles, the
photonic balls were blue at a particle concentration of f ¼ 0.33,
green at f ¼ 0.25, and red at f ¼ 0.17. Figure 2a shows the
reflectance spectra and the corresponding optical microscope
images of these blue, green, and red photonic balls. Also shown
in Figure 2b are optical images of aqueous dispersions of
photonic balls reflecting blue, green, and red light from normal
incident white light. Comparison of the spectra reveals that
lowering the volume fraction increases the lattice constant and
causes a red-shift in the reflection spectra. The changes in
lattice constants for three different particle concentrations can
be seen from the particle packing structures on surface of the
photonic balls in the SEM images in Figure 2c.
Although the small refractive-index mismatch between the
silica particles and ETPTA is useful for the self-assembly of
silica particles at low concentrations, the attenuation length of
Bragg diffraction of silica-in-ETPTA suprastructures is
relatively large (ca. 100 mm). Therefore, the silica particles
must be selectively removed to increase the refractive index
mismatch, thereby reducing the attenuation length. In the
present study, we used hydrofluoric acid (HF) solution as a
selective etchant that removes the silica particles, leaving
behind spherical air cavities in the ETPTA matrix. Indeed,
SEM imaging of a fractured porous photonic ball (Fig. 2d)
revealed a layered arrangement of air spheres. The reflectance
spectra and optical microscopy images of these porous balls are
shown in Figure 2e. After removal of the silica spheres, the
reflectance peak is blue-shifted and the band gap becomes
much broader owing to the increased refractive index
mismatch.
The viscous nature of the ETPTA resin is of practical
significance for fabricating other colloidal suprastructures,
such as photonic Janus balls, cylinders, and patterned films. To
fabricate photonic Janus balls, we modified the microfluidic
device used above. As shown schematically in Figure 3a, the
modified device consists of a pair of microcapillaries through
which two different colloidal suspensions are forced to flow. At
the exit tips, the aqueous fluid flow in the outer region breaks
the two suspension streams into a pair of drops which
immediately coalesce to form a larger drop. A train of larger
emulsion drops is then solidified by UV exposure to form Janus
balls. In creating Janus balls, it is essential that the two different
colloidal particles in each large drop are not mixed together
either by flow or by diffusion. In our system, the ratio of
ETPTA suspension to aqueous phase viscosities is as large as
O(10
2
) and the aqueous outer flow, which is strong enough to
break up the suspension streams at the tips, cannot induce
appreciable flow inside the ETPTA droplets. Trivially weak
internal flow is evident from the particle arrangement in the
photonic balls of Figure 1b, which do not display any trace of
the twin recirculatory flows that would otherwise be formed
inside the drops. Moreover, the diffusion coefficient D
0
of silica
particles, which is given by the Stokes–Einstein law in dilute
suspensions, D
0
¼ kT/6pha, is very small and O(10
14
m
2
s
1
)
for 200 nm silica particles dispersed in ETPTA at 27 8C. In
concentrated suspensions, the self-diffusion coefficient D is
much smaller than the Stokes–Einstein limit due to hydro-
dynamic and thermodynamic interactions.
[30]
Therefore, the
diffusive displacement length O(Dt)
1/2
is much smaller than the
particle radius a in time interval t (approx. less than 1 min)
between the formation of a drop and the UV-induced
solidification. Under these circumstances, the colloidal parti-
cles that originated from the two different suspension streams
form structures within their own hemispherical domains of a
moving ETPTA drop in the microfluidic channel without
experiencing diffusive or flow-induced disturbances. To
demonstrate this, we produced photonic Janus balls using
silica-in-ETPTA suspensions of two different particle sizes,
145 nm and 190 nm. In this experiment, to ensure stable drop
generation at the tips, the viscosities of the two silica
suspensions were matched by keeping an identical volume
fraction of f ¼ 0.20 in both silica suspensions. Because the
colloidal particles of different sizes were not mixed in the
emulsion drop, the resulting photonic ball after UV-induced
solidification displayed different reflection colors from differ-
ent sides. Figure 3b and c shows optical microscopy images of
the created Janus balls at different magnifications. Indeed, the
two faces of the photonic Janus balls exhibit different reflection
colors. In addition, the photonic Janus balls have clear
boundaries around their equators (see inset of Fig. 3c),
providing further evidence of negligible flow-induced and
diffusive mixing effects within the balls. We also made porous
Adv. Mater. 2008, 20, 1649–1655 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 1651
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photonic Janus balls by removing the silica spheres; optical
microscopy images of the resulting structures are shown in
Figure 3d and e. As expected, the reflected colors from both
sides were blue-shifted by the removal of the silica spheres.
Using the same approach, we created photonic crystals with
various shapes by inducing capillary-force-driven flow through
microchannels with different geometries. We prepared, for
example, crack-free composite photonic crystal films, line
patterns, and striped cylinders by inserting the suspension via
capillary forces into microcapillary molds. Because of the
high viscosity of the suspension, the infiltrating capillary flow
will not disturb the ordered state of colloidal silica particles
formed in the ETPTA resin (see Supporting Information Fig.
S4 for a detailed explanation). The reflection spectra of
colloidal thin films in Figure 4a show that the enhanced index
mismatch led to substantial increases in the intensity and full
width at half maximum (FWHM) as well as a blue-shift of the
reflection peak after removal of the silica particles. Cross-
sectional SEM images of the silica-in-ETPTA and porous
ETPTA structures are shown in Figure 4b and c, respectively.
Figure 2. a) Normalized reflectance spectra and optical microscopy images of colloidal photonic balls composed of 165 nm silica particles embedded in
ETPTA at f ¼ 0.33, f ¼ 0.25, and f ¼ 0.17. b) Optical images of the aqueous dispersions of three different photonic balls showing their respective reflection
colors for normal incident light. c) SEM images showing the surface arrangements of silica particles in the photonic balls at three different volume fractions.
d) SEM image of a fractured porous photonic ball templated from 165 nm silica particles at f ¼ 0.25. e) Normalized reflectance spectra and optical
microscopy images of photonic balls for f ¼ 0.25 before and after 165 nm silica particles are removed from the ETPTA matrix. Scale bars: c) 1 mm, d) 2 mm.
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Also, Figure 4d and e show the reflection color change of the
line patterned photonic crystals by the removal of the silica
particles. These images show that, indeed, the photonic films
and line patterns have no cracks. In addition, large-area SEM
images of photonic films and optical and SEM images of
free-standing striped photonic cylinders are available in the
Supporting Information Figure S4.
In conclusion, we have demonstrated that colloidal
suprastructures with various shapes can be fabricated by using
silica particles dispersed in a photopolymerizable ETPTA
phase of high viscosity and strong polarity. We found that, by
adjusting the particle size and concentration, the proposed
fabrication scheme was quite effective for creating non-
close-packed colloidal crystals with well-controlled shapes and
lattice constants. The strong polarity of the ETPTA matrix
induces long-range repulsions and enables the silica particles to
self-assemble into periodic non-close-packed structures dis-
persed in the matrix. These self-organized structures can then
be rapidly solidified by UV exposure to produce colloidal
crystals without cracks. Moreover, colloidal crystals with
different lattice constants can be incorporated into a single
suprastructure (e.g., Janus balls) in which each crystal phase is
Figure 3. a) Schematic illustration of microfluidic fabrication of photonic Janus balls. b,c) Photonic Janus balls reflecting red and green colors in both sides at
different magnifications before the silica particles are removed. Red and green colors are reflected from 190 nm and 145 nm silica particles at f ¼ 0.20,
respectively. d,e) PhotonicJanus balls at different magnifications after the silica particlesare removed from the ETPTAmatrix. Scalebars: b,d)1 mm, c,e) 100mm.
Adv. Mater. 2008, 20, 1649–1655 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 1653
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kept undisturbed. In addition, the high chemical resistivity and
physical rigidity of crosslinked ETPTA open a wide range of
applications such as color pigments in reflection mode displays
and e-paper, and emission modulators of fluorescing dyes or
quantum dots.
Experimental
Preparation of Silica Particles Dispersed in ETPTA: Monodisperse
silica particles of various sizes were synthesized through sol–gel
chemistry by the Sto
¨
ber–Fink–Bohn method and seeded growth. The
prepared particles were washed with ethanol and mixed with
ethoxylated trimethylolpropane triacrylate (ETPTA; SR454, Sarto-
mer) containing 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP;
Darocur1173, Ciba Chemical) as a photoinitiator at various volume
fractions. Ethanol was selectively evaporated from the mixture at 70 8C
in a convection oven for 1 day. After complete evaporation, the silica
suspensions showed iridescent colors.
Emulsification and Photopolymerization: To fabricate polydisperse
photonic balls, 200 mL of ETPTA suspension was injected into 5 mL of
water with 1 wt% block copolymer surfactant (Pluronic F108,
ethyleneoxide propyleneoxide tri-block copolymer; BASF). The
mixture was shaken in a vortex mixer, and ETPTA droplets were
generated in the aqueous medium. The block copolymer surfactant
adsorbed on the water-ETPTA interface stabilized the emulsions.
Subsequent UV exposure under a mercury arc lamp for 10 s solidified
the emulsion droplets into rigid photonic balls. To produce mono-
disperse photonic balls, we used microcapillary devices that were
prepared by pulling glass capillary tubes and assembling them into a
fluidic chip. Typically the inner and outer capillaries were 100 and
400 mm in diameter. ETPTA and aqueous phases were pumped into the
inner and outer capillaries at volumetric flow rates of about 5 mLmin
1
and 500 mLmin
1
, respectively. Emulsion droplets were exposed to a
UV light source mounted on an inverted optical microscope (Nikon,
TE2000-U). The synthesized photonic balls were rinsed with deionized
water several times to remove residual surfactant. To make porous
photonic balls, silica particles embedded in the ETPTA matrix were
etched out by treatment with 5% HF solution (50%, Sigma–Aldrich)
for 12 h. Emulsion droplets and colloidal crystal structures were
observed using optical microscopy (Nikon, TE2000-U and L150) with a
high-speed video camera (Redlake, Motionscope M1), and by scanning
electron microscopy (Philips, XL30) after Au coating.
Infiltration and Photopolymerization: To fabricate colloidal crystal
films, we used slide glass and cover glass with 50 mm thick polyimide
tape (Kapton) as a spacer. To fabricate patterned photonic crystal
arrays, we used a poly(dimetyl siloxane) PDMS mold with patterned
microchannels prepared by a soft-lithography technique. In both cases,
capillarity-induced infiltration of the silica-in-ETPTA suspension took
ca. 1 h. In addition, we used glass capillary tubes of 1.1 mm inner
diameter (NO.2502, Chase Scientific Glass) to make cylindrical
colloidal crystals. Capillary forces pulled the silica-in-ETPTA suspen-
sion at a rate of about 1 cm min
1
along the capillary tubes. To avoid
cracking during photopolymerization, UV irradiation for 1 s was
repeated 5 times with a 2 s time interval between exposures.
Received: December 5, 2007
Revised: February 14, 2008
Published online: April 15, 2008
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    • "Increasing humidity involves a reduction of the evaporation rate and an increase of the internal drop pressure that may lead to swelling and/or disintegration of particles. Regarding the correlation between microparticle quality and colloidal particle concentration in suspension, two concentration values responsible for radical changes in particle morphology should be distinguished: 16 vol%., the percolation limit below which void particles may frequently occur [26] ; 50 vol%, the concentration that initiates a liquid–solid phase transition in the colloidal system [15]. In the optimal concentration range, 16–33 vol%., the quality of microparticles is independent of concentration (but may depend on other parameters, for instance, the evaporation rate). "
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    Full-text · Article · Nov 2015 · Journal of Nanoparticle Research
    • "To progress in our understanding of the stability behavior of Pickering emulsions, it is therefore important to prepare droplets of both controlled size and controlled surface coverage using well defined NPs as stabilizers. The past decade has seen the rise of microfluidic tools, see for a non-exhaustive review [6], that allow, among other, a good control of emulsions, i.e. monodisperse droplets [7] and surface coverage by particles [8] [9]. In this article we have set-up a microfluidic system allowing such a coupled control. "
    [Show abstract] [Hide abstract] ABSTRACT: We study the stability of a model Pickering emulsion system. A special counter-flow microfluidics set-up was used to prepare monodisperse Pickering emulsions, with oil droplets in water. The wettability of the monodisperse silica nanoparticles (NPs) could be tuned by surface grafting and the surface coverage of the droplets was controlled using the microfluidics setup. A surface coverage as low as 23$\%$ is enough to stabilize the emulsions and we evidence a new regime of Pickering emulsion stability where the surface coverage of emulsion droplets of constant size increases in time, in coexistence with a large amount of dispersed phase. Our results demonstrate that the previously observed limited coalescence regime where surface coverage tends to control the average size of the final droplets must be put in a broader perspective.
    Full-text · Article · Apr 2015 · Journal of Nanoparticle Research
    • "Colloidal photonic crystals (CPCs) with unique light manipulation and inhibition capability have been extensively investigated in areas ranging from chemical sensing, light-emitting devices (LED) to biological labels (Ge and Yin 2011; Wang et al. 2011; Kim et al. 2008; Dinsmore et al. 2002; Pallavidino et al. 2006; Razo et al. 2008). Recently, the ability to finely bind CPCs with various semiconductor nanocrystals (NCs) is of special interest, because not only a full photonic bandgap with high refractive index contrast is expected but also great fluorescence enhancement of the incorporated NCs is achieved through the resonance with CPCs (Xu et al. 2008; Kubo et al. 2002; Hong et al. 2010). "
    [Show abstract] [Hide abstract] ABSTRACT: Versatile hydrogel-based nanocrystal (NC) microreactors were designed in this work for the construction of uniform fluorescence colloidal photonic crystal (CPC) supraballs. The hydrogel-based microspheres with sizes ranging from 150 to 300 nm were prepared by seeded copolymerization of acrylic acid and 2-hydroxyethyl methacrylate with micrometer-sized PS seed particles. As an independent NC microreactor, the as-synthesized hydrogel microsphere can effectively capture the guest cadmium ions due to the abundant carboxyl groups inside. Followed by the introduction of chalcogenides, in situ generation of higher-uptake NCs with sizes less than 5 nm was finally realized. Additionally, with the aid of the microfluidic device, the as-obtained NC–latex hybrids can be further self-assembled to bi-functional CPC supraballs bearing brilliant structural colors and uniform fluorescence. This research offers an alternative way to finely bind CPCs with NCs, which will facilitate progress in fields of self-assembled functional colloids and photonic materials.
    Full-text · Article · Nov 2014
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