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Lee et al.
posite films with tailored electrical, mechanical, or optical
properties. An important advantage of this method is that it
enables the preparation of films with controlled thickness,
composition, and functionality through complementary interac-
tions (i.e., electrostatic,4,12 hydrogen-bonding,5,13-15 or covalent
interaction16). However, a disadvantage of the conventional LbL
method based on dip5-13 or spin-coating17-19 is in terms of
efficiency, especially for thicker films as layer deposition
requires adsorption of the desired materials via a self- or forced
diffusion process followed by rinsing of weakly adsorbed
materials. For example, for the fabrication of functional mul-
tilayer films with micron-scale thickness, these films should have
hundreds of layers (i.e., hundreds of deposition cycles) because
each deposition process adds up to only a few nanometers thick
layer after rinsing. Furthermore, the conventional LbL approach
cannot be applied for the buildup of multilayers containing
nonpolar or uncharged polymers and nanoparticles. In addition,
it is not possible to easily incorporate layers of differing
chemistry of hydrophilic/hydrophobic balance in the traditional
LbL process.20 Resultantly, this assembly has much difficulty
in the preparation of various functional free-standing multilayers
with micron-scale thickness and hydrophobic properties due to
the disadvantages as mentioned above.
provide the basis for the preparation of many potential applica-
tions such as highly flexible optical and electronic devices.
Experimental Section
Materials. Photocross-linkable polymers, PS-N3 (Mn ) 28.0
kg/mol) and PS-N3-SH (Mn ) 6.5 kg/mol), were synthesized via
reversible addition fragmentation transfer (RAFT) polymerization.21
For PS-N3, styrene (5.0 g, 0.048 mol), 4-vinylbenzyl chloride (0.5
g, 3.33 mmol), 2,2′-azobis(2-methylpropionitrile) (AIBN) (2 mg,
0.014 mmol), and RAFT agent (27 mg, 0.09 mmol) were mixed
and degassed. The reaction was carried out at 70 °C for 48 h. The
reaction mixture was then precipitated into methanol, resulting in
the random copolymer as a pink powder. To avoid the coupling
during the azidation, the dithioester end group was removed by
the reaction with AIBN under nitrogen (80 °C for 12 h). The
solution was precipitated into methanol, obtaining the white powder.
The change in color suggested that the dithioester group was
removed. Then, the polymer was stirred with 3 equivalents of
sodium azide in dimethylformamide at the ambient condition for
12 h. The solution was filtered and precipitated into methanol to
give a final product, PS-N3, as a white powder. From size exclusion
chromatography (SEC), the Mn and PDI were 28.0 kg/mol and 1.1,
respectively. The composition of the azide group in PS-N3 was
found to be 0.10 from proton NMR. The thiol-terminated random
copolymer, PS-N3-SH, was synthesized by the similar procedure
as PS-N3. The only difference is the reaction with AIBN was
replaced by the reaction with hexylamine to convert the dithioester
group to the thiol group. The detailed procedure regarding this
reaction is described in ref 1. The Mn and PDI were 6.5 kg/mol
and 1.1, respectively. The composition of the azide group in
PS-N3-SH was 0.10.
Oleic acid-stabilized CdSe@ZnS with green and red emissive
colors was synthesized as previously reported.22 It was reported
that high quality QDs can be easily synthesized using oleic acid
stabilizers.23 For blue emissive QDs, 38.5 mg of CdO, 700 mg of
zinc acetate, 17.6 mL of oleic acid, and 15 mL of 1-octadecene
were put into a 250 mL round flask. The mixture was heated to
150 °C with N2 gas blowing and further heated to 300 °C to form
a clear solution of Cd(OA)2 and Zn(OA)2. At this temperature, 31
mg of Se powder and 128.2 mg of S powder both dissolved in 2
mL of trioctylphosphine were quickly injected into the reaction
flask. After the injection, the temperature of the reaction flask was
set to 300 °C for promoting the growth of QDs, and it was then
cooled to room temperature to stop the growth. QDs were purified
by adding 20 mL of chloroform and an excess amount of acetone
(3 times). After this purification, PS-N3-SH of 2 wt % was added
to a QD solution of 15 mL for the stabilizer exchange from oleic
acid to N3-PS-SH and then was heated at 40 °C for 2 h.
Water-soluble Au or Pt nanoparticles (AuNP or PtNP) were
synthesized as reported in our previous papers.8,24 Briefly, 250 mL
of 1.79 mM HAuCl4 or H2Cl6Pt·6H2O was maintained at room
temperature with vigorous stirring. Rapid addition of 20 mL of 68
mM sodium citrate to the vortex of the solution and successive
addition (1 mL) of 70 mM NaBH4 resulted in a color change from
dark yellow to dark brown. The diameters of synthesized AuNP and
PtNP were about 8 and 6 nm, respectively, as confirmed by TEM
images. These nanoparticles were dispersed in aqueous solution at
pH 5. The AuNP or PtNP solution of about 20 mL was mixed with
PS-N3-SH toluene solution of about 7 mL for phase transfer of
AuNP or PtNP from water to toluene phase. In this case, metal
nanoparticles dispersed in toluene were highly concentrated.
Herein, we introduce a novel strategy for the preparation of
free-standing nanocomposite multilayers with various length
scales, functionalities, and adjustable internal structures. The
strategy is based on the use of hydrophobic polymers containing
UV cross-linkable units and/or high affinity groups with the
polymers and/or polymer-coated nanoparticles being consecu-
tively deposited by spin-coating. The photocross-linking of each
layer allows various thicknesses to be achieved without any
rinsing or intermediate purification steps. When optically active
nanoparticles such as QDs are incorporated into the photocross-
linkable polymer layers, the photoluminescent durability is
significantly enhanced while also allowing facile color tuning.
Remarkably, we show that the internal structure of multilayer
films can be controlled via segregation between polymer-coated
nanoparticles and polymers during the spin-coating step.
Furthermore, it is demonstrated that this novel strategy can be
extended to the functional free-standing multilayers through use
of sacrificial ionic substrates (NaCl) and adjustment of internal
structures or photopatterning. We highlight that our photocross-
linking LbL assembly is a simpler and more versatile process
for the fabrication of functional nanocomposite multilayers
including free-standing films than any other method introduced
to date. Therefore, we strongly believe that our approach can
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