A facile layer-by-layer assembly method for the fabrication of fluorescent polymer/quantum dot nanocomposite thin films

Article abstract of DOI:10.1039/c4ra04779f

We demonstrated an efficient and robust approach for the preparation of polymer/quantum dot (QD) nanocomposite thin films in nonpolar media. By employing photopolymerization based on the thiol-ene reaction, thiol-containing polymer with affinity groups for CdSe@CdS QDs was produced for the buildup of the polymer/CdSe@CdS QD nanocomposite thin films utilizing layer-by-layer assembly. The polymer/QD nanocomposite films were characterized by SEM, TEM, fluorescence spectrometry, thermogravimetric analysis, confocal laser scanning microscopy and time-resolved fluorescence spectrometry. The resulting free-standing polymer/QD nanocomposite films exhibit high fluorescent intensity, moderate transparency, good photostability and thermal stability. We further applied photolithographic patterning in conjunction with an LBL assembly method to fabricate a polymer/CdSe@CdS QD nanocomposite pattern on a flexible substrate. This versatile and facile method provides prospects for the design of novel photonic devices based on the polymer/QD nanocomposite thin films. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04779f

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PAPER
A facile layer-by-layer assembly method for the
fabrication of fluorescent polymer/quantum dot
nanocomposite thin films†
Cite this: RSC Adv., 2014, 4, 33206
a
a
ac
a
Feng Jin, Mei-Ling Zheng, Mei-Lin Zhang, Zhen-Sheng Zhao
ab
and Xuan-Ming Duan*
We demonstrated an efficient and robust approach for the preparation of polymer/quantum dot (QD)
nanocomposite thin films in nonpolar media. By employing photopolymerization based on the thiol-ene
reaction, thiol-containing polymer with affinity groups for CdSe@CdS QDs was produced for the buildup
of the polymer/CdSe@CdS QD nanocomposite thin films utilizing layer-by-layer assembly. The polymer/
QD nanocomposite films were characterized by SEM, TEM, fluorescence spectrometry,
thermogravimetric analysis, confocal laser scanning microscopy and time-resolved fluorescence
spectrometry. The resulting free-standing polymer/QD nanocomposite films exhibit high fluorescent
intensity, moderate transparency, good photostability and thermal stability. We further applied
photolithographic patterning in conjunction with an LBL assembly method to fabricate a polymer/
CdSe@CdS QD nanocomposite pattern on a flexible substrate. This versatile and facile method provides
prospects for the design of novel photonic devices based on the polymer/QD nanocomposite thin films.
Received 21st May 2014
Accepted 23rd July 2014
DOI: 10.1039/c4ra04779f
www.rsc.org/advances
3,4
5,6
cells,
biological sensors,
and so forth. The common
1
. Introduction
methods for producing polymer/QD nanocomposite thin lms
7,8
Polymer/quantum dot (QD) nanocomposite thin lms are of include layer-by-layer (LBL) assembly,
great importance for their applications in photonic and opto- deposition, and self-assembly of block copolymer, in which
electronic devices, such as light-emitting diodes, photovoltaic the QDs were incorporated into polymer matrix. Among these
approaches, LBL assembly is the most versatile and robust
Langmuir–Sch ¨a fer
9
10
1,2
protocol to create the nanocomposite thin lms with controlled
Laboratory of Organic NanoPhotonics and Key Laboratory of Functional Crystals and morphology, thickness, functionality, and unique properties.
11
a
Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Numerous polymer/QDs nanocomposite thin lms have been
Sciences, No. 29, Zhongguancun East Road, Beijing 100190, P. R. China. E-mail:
fabricated utilizing the LBL assembly approach based on the
xmduan@mail.ipc.ac.cn; Fax: +86-10-82543597; Tel: +86-10-82543596
complementary interactions, including electrostatic attrac-
b
Chongqing Institutes of Green and Intelligent Technology, Chinese Academy of
12
13
14
tion, hydrogen-bonding, and covalent bonds.
Traditional LBL assembly is carried out in polar solvent,
Sciences, No. 266 Fangzheng Ave, Shuitu Technology Development Zone, Beibei
District, Chongqing 400714, P. R. China
c
University of Chinese Academy of Sciences, No. 29, Zhongguancun East Road, Beijing mainly in water and/or alcohol, in which cationic and anionic
15
1
00190, P. R. China
layers are alternatively adsorbed on the substrate. Aqueous-
based LBL approach is not suitable for the buildup of thin
†
Electronic supplementary information (ESI) available: Experimental procedures
for the preparation of stock solutions. Synthesis route of trimethylolpropane

lms containing uncharged polymer and hydrophobic QDs,
1
tris(3-mercaptopropionate)
(Scheme
S1),
H
NMR
spectrum
(Fig. S1); high
resolution TEM image of CdSe@CdS QDs (Fig. S2); EDS of the PMMA with and
of
which are usually synthesized in nonpolar solvents using
organic capping ligands. As a result, LBL assembly of polymer/
3
trimethylolpropane tris(3-mercaptopropionate) (in CDCl )
without thiol group aer QDs solution immersion (Fig. S3), molecular QD thin lms oen involves the phase transfer of QDs from
structures of the components of the photopolymerizable resin (Fig. S4); FTIR
spectra of PMMA-SH, OA-CdSe@CdS QDs, and PMMA-SH/CdSe@CdS lms with
different bilayer number (Fig. S5); photograph of free-standing
PMMA-SH/CdSe@CdS QDs nanocomposite lm under ambient light and UV
light (Fig. S6); TEM image of the PMMA-SH/CdSe@CdS QDs nanocomposite
16
nonpolar media to aqueous solution through ligand exchange.
This may lead to some drawbacks: (i) the photoluminescence
PL) of the resultant nanocomposite lms would be diminished
due to the poor surface passivation during the phase transfer
(
17–19

lm (Fig. S7); PL spectra of the (PMMA-SH/CdSe@CdS)
7
lm before and aer and/or self-assembly.
(ii) Chemical degradation will occur
CW laser irradiation (Fig. S8); photographic images of the patterned
PMMA-SH/CdSe@CdS nanocomposite microdots on different substrates, e.g.
glass, ITO, Si and plastics (Fig. S9). Biexponential tting results of the CdSe@
CdS QDs toluene solution and PMMA-SH/CdSe@CdS QDs nanocomposite lm,
respectively (Table S1). See DOI: 10.1039/c4ra04779f
for QDs irradiated under the humidity environment due to the
residual moister during LBL assembly.
Hence, LBL assembly of polymer/QD thin lms is preferable
to be carried out in the nonpolar solvents to preserve the
20
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original optical properties of the QDs. Lee and coauthors have 2.2. Synthesis of CdSe@CdS QDs
demonstrated the LBL growth of polymer/CdSe@ZnS QD
CdSe@CdS QDs were synthesized according to the previously
nanocomposite multilayers in organic media through LBL
24–26
reported procedure with minor modication.
First, CdSe
In a typical synthesis,
(1 mmol), cadmium myristate (1 mmol, ESI†), and ODE (63
21,22
assembly adopting a two-step grating approach.
Alterna-
25,26
QDs were synthesized and puried.
SeO
tively, polymer/QD thin lms could also be fabricated through
direct affinity interaction between QDs and polymers contain-
2
mL) were added and stirred in a 150 mL three-neck round ask,
23
ing terminal anchoring moieties in nonaqueous solution.
ꢀ
and the reaction was performed at 240 C in the air atmosphere.
However, growth of highly uorescent polymer/QDs thin lms
with hydrophobic properties in nonpolar media has not yet
been reported based on the one-step LBL assembly employing
the QDs and functionalized polymers with affinity groups.
In this study, we proposed a robust and versatile approach
for the growth of polymer/QDs nanocomposite thin lms via
LBL assembly in nonpolar media. The LBL assembly was carried
out utilizing the affinity interaction between the CdSe@CdS
QDs and photopolymerized PMMA containing thiol groups
introduced by thiol-ene reaction. Free-standing PMMA-SH/
CdSe@CdS QDs nanocomposite thin lms with bright PL
were successfully fabricated without the necessity of ligand
exchange and the substrate's surface functionalization before
the deposition of the rst layer. The nanocomposite lms
exhibit highly luminescent property, good photostability and
thermal stability, and moderate transparence. In addition,
microstructured patterns of PMMA-SH/CdSe@CdS QDs nano-
composite on exible substrate were fabricated employing
photolithography processing in conjugated with an LBL
assembly approach. The present work provides the potential
prospects for the development of novel photonic and opto-
electronic devices such as exible light-emitting diodes.
ꢀ
Three minutes aer the temperature reached to 240 C, 1 mL of
OA was added dropwise into the reaction solution to further
stabilize the CdSe QDs. The reaction was stopped by cooling
down the ask to room temperature. CdSe QDs were puried by
adding excess amount of acetone and centrifugation. The
resultant CdSe QDs were dispersed in hexane for the further
measurement and preparation of CdSe@CdS core–shell QDs.
Second, CdSe@CdS QDs were synthesized using one-pot injec-
24
tion approach. In a typical synthesis, 2 mL of hexane solution
of CdSe QDs (35 mM), 8 mL of trioctylamine, and 600 mL of
ꢀ
cadmium oleate (0.5 M, ESI†) in OA were degassed at 70 C
ꢀ
during 1 h. The solution was then heated to 260 C under
nitrogen atmosphere, and a mixture of 3.4 mL of TOA, 600 mL of
TOPS (0.5 M, ESI†) in TOP, and 1 mL of OA was injected in 4 h.
Aer the reaction, CdSe@CdS QDs were puried by ethanol
precipitation followed by centrifugation. The resultant
CdSe@CdS QDs were dispersed in toluene for the further
measurement and LBL assembly.
2.3. Synthesis of trimethylolpropane tris(3-
mercaptopropionate)
Trimethylolpropane tris(3-mercaptopropionate) (denoted as
trithiol) was synthesized according to the reported procedure
27
(
(
Scheme S1†). In brief, 1,1,1-tris(hydroxymethyl) propane
6.715 g, 0.05 mol), toluene (40 mL), and H SO (0.401 g) were
2
. Materials and methods
2
4
added and mixed in a three-neck ask equipped with reux
condensing tube, constant pressure drop funnel, and Dean–
Stark trap. 3-mercapto-propanoic acid (17.504 g, 0.15 mol) was
dropped slowly into the reaction system through the constant
pressure drop funnel. The reaction was carried out by heating
the ask to 130 C. Four hours later, the reaction was termi-
nated by cooling down to room temperature. Then, the reaction
solution was washed with deionized water to neutrality. The
2.1. Materials
1
9
-Octadecene (ODE, tech. 90%), cadmium oxide (CdO,
9.998%), polymethylmethacrylate (PMMA) and 3-mercapto-
propanoic acid (MPA, 97%) were purchased from Alfa Aesar.
Rhodamine 6G (R6G) was purchased from Sigma. Tri-
octylphospine (TOP, tech. 90%) was purchased from Fluka.
ꢀ
Deuteriochloroform (CDCl
3
) was purchased from J&K Chemical
0
Ltd. Benzil and 2-benyl-2-(dimethylamino)-4 -morpholinobu-
tyrophenone (BDMBP) were purchased from Sigma-Aldrich.
Pemaerythritol-triacrylate (PE-3A) was purchased from Kyoei-
sha Chemical Co. Ltd., Japan. Trioctylamine (TOA, 95%) was
purchased from Shanghai Indole Biological Technology Co.,
4
organic phase was separated, and dried over anhydrous MgSO .
The resultant product was obtained by the removal of toluene
using the rotary evaporator. Viscous transparent liquid was
collected and kept in the moister-proof cabinet for the further
measurement and preparation of the photopolymerizable resin.
1
Ltd. Selenium dioxide (SeO , AR) was purchased from Zhengz-
hou Kefeng Chemical reagent Co., Ltd. Anhydrous methanol
2
Yield ¼ 92%. H NMR (400 MHz, CDCl , d ppm): 0.9 (m, 3H,
3
3 2 2
CH ), 1.45 (m, 2H, CH ), 1.65 (m, 3H, SH), 2.65 (S, 6H, –CH –
(
CH
hydroxide (NaOH, AR), acetone (CH
CH , AR), myristic acid (MA, AR), anhydrous magnesium
sulfate (MgSO , AR), oleic acid (OA, 90%), sulphur (S, AR),
hexane (CH (CH CH , AR), sulfuric acid (H SO
, 98%), methyl Photopolymerizable resin containing thiol groups was prepared
3
OH, AR), anhydrous ethanol (CH
3
CH
2
OH), sodium
, AR), toluene
2 2
CO–), 2.8 (S, 6H, –CH –S–), 4.1 (S, 6H, –CH –O–) (Fig. S1†).
3
COCH
3
(C
6
H
5
3
2.4. Preparation of photopolymerizable resins
4
3
2
)
4
3
2
4
28
methacrylate (MMA, AR), chloroform (CHCl , AR) and 1,1,1- according to the previously reported procedure. In brief,
3
tris(hydroxymethyl) propane (TMP, AR) were purchased from methyl methacrylate (MMA), trithiol, PE-3A, benzil, and BDMBP
Sinopharm Chemical Reagent Beijing Co., Ltd and used without were mixed overnight in the dark room to make the photo-
further purication.
polymerizable resin. For comparison, photopolymerizable resin
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1
Table 1 Components of the photopolymerizable resins with and
without trimethylolpropane tris(3-mercaptopropionate) (trithiol)
Bruker, Cu Ka radiation, l ¼ 1.5406 A). H NMR spectra were
˚
recorded on an Avance II-400 spectrometer (Bruker) using
CDCl as a solvent and all shis are referred to tetramethylsi-
3
MMA
Trithiol
PE-3A
Benzil
BDMBP
lane (TMS). Vibrational absorbance spectra were characterized
using FTIR spectroscopy (Excalibur 3100, Varian). Thermogra-
vimetric analysis (TGA) was measured by using a SDT Q600
Resin 1 (wt %)
Resin 2 (wt %)
64.7
28.4
0
31.4
33.3
38.3
1.0
1.1
1.0
0.8
ꢀ
ꢁ1
system (TA instrument) with a heating rate of 10 C min
under nitrogen atmosphere. Time-resolved photoluminescence
without thiol groups was also prepared under the same exper- decays were recorded using single-photon count uorescence
imental condition. The components of the photopolymerizable spectrometer (F900, Edinburgh Instrument Co.). The specimens
resins are shown in Table 1.
were excited at 488 nm with a pulsed laser (150 ps pulse dura-
tion), and the recorded emission wavelength was 626 nm. The
photoluminescence decays exhibited a multiexponential decay
and were tted with a biexponential function, and we reported
the resulting average lifetime savg (ESI†). Steady state and time
resolved uorescence spectrometer (FLSP920, Edinburgh
Instruments) equipped with integrated sphere was used to
determine the absolute photoluminescence quantum yield
2.5. Preparation of PMMA-SH/CdSe@CdS QDs
nanocomposite lms
30
The preparation of PMMA-SH/CdSe@CdS QDs nanocomposite
thin lms was shown in Scheme 1. First, transparent PMMA

lm containing thiol groups (PMMA-SH) was produced by
photopolymerization of the thiol-containing resin based on the
(PLQY) of CdSe@CdS QDs toluene solution and PMMA-SH/
29
thiol-ene reaction.
The spin-coating of the photo-
CdSe@CdS QDs nanocomposite lm, respectively. For the
PLQY measurement, the excitation wavelength was xed at 550
nm, and the optical density of the CdSe@CdS QDs toluene
solution was tuned to 0.1 at the excitation wavelength.
A confocal laser scanning microscope (A1R MP, Nikon) was
used to investigate the photostability of the PMMA-SH/CdSe–
CdS QDs nanocomposite lm, and the morphologic and uo-
rescent performance of the nanocomposite patterns. For the
photostability measurement, the samples were excited on the
confocal laser scanning microscope A1R MP by using
continuous-wave (CW) 561 nm diode-pumped solid-state laser
polymerizable resin was carried out on the KW-4 spin-coater
over 4000 rpm for 60 s. The photopolymerizable resin was
polymerized by exposing to the UV light (320 W high-pressure
Hg Arc lamp) for 4 min. Second, the PMMA-SH lm was
immersed in the toluene solution of CdSe@CdS QDs (QDs
ꢁ
1
concentration: 0.55 g L ) for the adsorption of the QDs on the
surface of the PMMA-SH lm. The substrate was then removed
from the QDs solution, rinsed with toluene and dried. Third,
PMMA-SH/CdSe@CdS QDs nanocomposite multilayers were
obtained by repeating the rst and second step for several
cycles. The resultant lm is denoted as “(PMMA-SH/
(Coherent Sapphire). The laser beam was focused using an
n
CdSe@CdS) ”, which means the lm is fabricated from PMMA-
SH and CdSe–CdS QDs with a bilayer (assembly cycle) number
of n.
objective with a numerical aperture of 0.5 (20ꢂ, NA ¼ 0.5,
Nikon). The PL was collected with PMT (32 channels) through
the same objective. The PL spectra were taken at 1s intervals
during the laser exposure. The excitation laser power was
recorded with Newport optical power meter (Model 1916 R),
2.6. Characterization
Transmission electron microscopy (TEM) and high-resolution equipped with the detector (Model 818-SL). All of the
transmission electron microscopy (HR-TEM) images were measurements were carried out at room temperature.
collected using a JEM 2100F (JEOL) instrument working at
2
00 kV. Scanning electron microscopy (SEM) images and
3
. Results and discussion
energy-dispersive spectroscopy (EDS) were obtained with a
FE-SEM equipped with an EDS (Hitachi S-4800). The absorption
3.1. Synthesis and characterization of CdSe@CdS QDs
and PL spectra were collected by using Shimadzu UV-2550 Zinc blende (ZB) CdSe@CdS QDs were synthesized using the
spectrometer and Hitachi F-4500 uorescence spectrometer, seed-growth method. First, nearly monodispersed ZB CdSe QDs
respectively. X-ray diffraction (XRD) measurements were per- about 3.4 nm were acquired, as shown in Fig. 1a. The ZB
formed on a D8 focus Powder X-ray Diffractometer (XRD, structure of the CdSe QDs was recognized by the HR-TEM image
(inset of Fig. 1a). The lattice spacing was 0.35 nm, which was
consistent with the value of ZB CdSe (111) (International center
for diffraction data, no. 19-0191).
Second, CdSe@CdS QDs were synthesized employing the 3.4
nm CdSe QDs as seeds. As shown in Fig. 1b, cubic and tetra-
hydral CdSe@CdS QDs were obtained, which were the typical
morphology of ZB crystal structure. The deposition of CdS shell
on the CdSe seeds was veried by the HR-TEM image (inset of
Fig. 1b). The lattice spacing was 0.29 nm, which was consistent
with the ZB CdS (200) (International center for diffraction data,
no. 41-1049). Growth of the CdS shell on the CdSe seeds resulted
Scheme 1 Schematic diagram showing the fabrication of PMMA-SH/
CdSe@CdS QDs nanocomposite thin films adopting LBL assembly.
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in the toluene solution of CdSe@CdS QDs for 30 min. An
obvious PL band centered at the wavelength of 626 nm was
observed for PMMA-SH lm aer dipping, as shown in Fig. 2a.
The PL band was consistent with that of the toluene solution of
CdSe@CdS QDs, indicating that CdSe@CdS QDs were
successfully adsorbed on the surface of the PMMA-SH lm. In
contrast, we did not observe any noticeable PL emission for the
PMMA lm without thiol groups under the identical condition.
The bright red colour of the polymer lm with thiol groups
under UV light also veried that CdSe@CdS QDs were anchored
on the surface of the PMMA-SH lm (Fig. 2b). The absorption of
the CdSe@CdS QDs on the surface of PMMA-SH lm was
further conrmed by the EDS spectra, in which the emergence
of the Cd and Se atoms corroborated the introduction of the
CdSe@CdS QDs on the PMMA-SH lm (Fig. S3†). Therefore,
thiol group is critical for the adsorbing of the CdSe@CdS QDs,
owing to the strong affinity interaction between Cd atoms and
Fig. 1 TEM images of (a) CdSe and (b) CdSe@CdS QDs. The insets are
the corresponding high-resolution TEM images of the CdSe and
CdSe@CdS QDs, respectively. Scale bar in the inset is 5 nm. (c) XRD
patterns of the CdSe and CdSe@CdS QDs, respectively. (d) Absorption
(
dashed line) and photoluminescence (solid line) spectra of the CdSe
and CdSe@CdS QDs, respectively.
in the increase of size from 3.4 nm to 7.3 nm. Owing to the
monodispersed property, the CdSe@CdS QDs could be assem-
bled into a two-dimensional ordered structure when dropped
on the carbon-supported copper grid (Fig. 1b).
The powder XRD patterns of CdSe and CdSe@CdS QDs are
plotted in Fig. 1c. CdSe QDs show sharp reection peaks at 2q ¼
ꢀ ꢀ ꢀ
5.3 , 42.2 , and 49.7 , which are consistent with the (111),
2
(220), and (311) planes of the bulk ZB CdSe (JCPDS 19-0191).
Aer the growth of CdS shell, the diffraction features appearing
ꢀ
ꢀ
ꢀ
at 2q ¼ 26.5 , 44 , and 52.1 correspond to the (111), (220), and
311) planes of the bulk ZB CdS (JCPDS 10-0454). The growth of
(
the CdS shell induced the narrowing of the diffraction patterns
owing to the smaller lattice constant compared to that of the ZB
31
CdSe core, which is consistent with the reported result.
The absorption and PL spectra of the CdSe and CdSe@CdS
QDs are shown in Fig. 1d. The absorption and PL peaks of the
CdSe QDs located at 565 and 578 nm, respectively. The growth
of CdS shell led to a red-shi of the rst excitonic transition and
PL peaks to 615 and 626 nm, respectively, due to the delocal-
ization of the electron wave function into CdS shell, which
32
agreed well with the previously reported result. The growth of
CdS shell also induced some broadening of the PL spectrum, in
which the full width at half-maximum increased from 25 to 32
nm. The CdSe@CdS QDs with good surface passivation and
bright uorescence provide prospective applications in con-
structing functional organic–inorganic nanocomposite thin

lms.
3.2. LBL assembly of the PMMA-SH/CdSe@CdS QDs
nanocomposite thin lms
Fig. 2 (a) PL spectra of the PMMA and PMMA-SH films immersed into
the toluene solution of CdSe@CdS QDs. Photographs of the PMMA
and PMMA-SH/CdSe@CdS QDs films under (b) 365 nm UV light and (c)
To demonstrate the effect of thiol groups on the adsorption of
the CdSe@CdS QDs, we characterized the PL spectra of the
PMMA lms with and without thiol groups aer dipping them ambient light, respectively.
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33,34
37
thiol groups.
FTIR characterization further suggested that in the LBL assembly of QDs in previous report. Furthermore,
the LBL assembly could be ascribed to the in-site ligand SEM images of the PMMA-SH lms immersed in CdSe@CdS
exchange between oleic acid and thiol moieties of polymer QDs toluene solution for different time were presented in Fig. 4.
(
Fig. S5†), which is similar to the previously reported results for For the immersion time of 2 min, only a spot of surface of the
35,36
metal (or metal oxide) nanoparticles LBL assembly.
Mean- PMMA-SH lm was covered by the CdSe@CdS QDs. Neverthe-
while, the polymer lm maintained transparency aer the less, majority of the surface of the polymer lm was covered by
adsorption of the CdSe@CdS QDs as shown in Fig. 2c, indi- the QDs for the immersion time of 20 min, and 30 min,
cating the well dispersion of CdSe@CdS QDs on PMMA-SH lm. respectively. Consequently, the immersion time was optimized
In sequence, the inuence of immersion time on the to be 30 min in order to obtain PMMA-SH/CdSe@CdS QDs
adsorption of the CdSe@CdS QDs was studied to optimize the nanocomposite lm with high PL intensity in appropriately
assembly condition. Fig. 3a presents the PL spectra of the short time.
PMMA-SH lms dipped in the toluene solution of CdSe@CdS
QDs for different time. The PL emission intensity of the polymer CdSe@CdS QDs on assembly cycles (n) was monitored by the
lms increased monotonically with the prolonged immersion UV-vis absorption and PL spectra measured aer each deposi-
Furthermore, the dependence of the assembly of the

time. Accordingly, the emission colour changed from weak to tion cycle. As shown in Fig. 5a, the absorbance at 615 nm (the
strong with the ascending immersion time, as shown in the rst excitonic transition of the CdSe@CdS QDs) increased
inset of Fig. 3a. Fig. 3b shows that the PL intensity rstly monotonically with the increase of n. The linear relationship
increases with the immersion time, and then gradually between this absorption peak and n (inset of Fig. 5a) suggests
approaches saturation. The phenomenon is probably ascribed that the growth process is well controlled, leading to stepwise,
21
to the Langmuir absorption isotherm, which has been observed uniform, and layer to layer deposition. This can be further
veried by the monotonic increase of the PL intensity at 626 nm
(Fig. 5b). The inset of Fig. 5b shows the photograph of (PMMA-
SH/CdSe@CdS) under 365 nm excitation, demonstrating a
7
bright red emission from the nanocomposite thin lm. Mean-
while, a blue-shi of the PL peaks is observed during the
preparation of the polymer/CdSe@CdS QDs multilayers. The
phenomenon is attributed to the oxidation of the CdSe@CdS
QDs upon the UV irradiation during the polymerization of the
photopolymerizable resin, which is consistent with the similar
38,39
QDs/polymer nanocomposites.
It's worth noting that free-
standing polymer/QDs nanocomposite lm can be obtained
by stripping the multilayer lm from the glass substrate
(Fig. S6†). As a result, mechanically stable polymer/CdSe@CdS
QDs nanocomposite lms with different PL intensities and
thickness can be easily assembled by optimizing the deposition
cycles.
Fig. 3 (a) PL spectra and photograph of PMMA-SH/CdSe@CdS QDs Fig. 4 SEM images of the PMMA-SH film immersed in CdSe@CdS QDs
with the increasing of the immersion time from 2 min to 60 min. (b) solution for (a) 0 min, (b) 2 min, (c) 20 min, and (d) 30 min, respectively.
The plot of the fluorescence intensity at 626 nm versus time.
The scale bar is 150 nm.
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7
toluene solution and the (PMMA-SH/CdSe@CdS) lm, respec-
tively. Fig. 6 plots the PL decay curves of the CdSe@CdS QDs
toluene solution and (PMMA-SH/CdSe@CdS)7 lm, respec-
tively. Biexponential tting results are shown in Table S1.† The
average PL lifetime of the CdSe@CdS QDs in diluted toluene
solution is determined to be 25.67 ns, which is similar to the
43,44
value in previous study.
attributed to the thin CdS shell.
The relative short PL lifetime is
45,46
Aer assembly into the polymer matrix, the PL lifetime of the
CdSe@CdS QDs is reduced to 16.93 ns, which is shorter than
that of the pristine CdSe@CdS QDs. The declined radiative
lifetime was attributed to the interaction of the polymer with
the CdSe@CdS QDs surface. In CdSe@CdS QDs, the small
conduction band offset allows for partial delocalization of the
47
electron wave function from core into the shell. Photo-
generated holes will transfer quickly from the QDs to thiol
groups in the polymer matrix. Charge separation at the interface
between the CdSe@CdS QDs and thiol groups resulted in
shorter radiative lifetime, which is consistent with the previous
41
study. The accelerated radiative decay agrees well with the
reduced PLQY aer LBL assembly, both arising from the carrier
transfer from the QDs to the thiol groups in the polymer matrix.
The photostability of the PMMA-SH/CdSe@CdS QDs nano-
composite lm is a crucial issue for the practical applications.
We further investigated the photostability of the PMMA-SH/
CdSe@CdS QDs nanocomposite lm under laser excitation.
Fig. 7 shows the time-resolved PL intensities of the (PMMA-SH/
CdSe@CdS) lm excited with a CW laser irradiation at 561 nm
7
under ambient condition. The power intensity of the laser
PMMA/SH-CdSe@CdS QDs)
n
measured with increasing the bilayer irradiation was 0.508 kW cm (ESI†). For comparison, time-
Fig. 5 (a) UV-vis absorption and (b) photoluminescence spectra of
(
ꢁ2
number from 1 to 7. Dashed lines are the UV-vis and photo-
luminescence spectra of the PMMA-SH film without CdSe@CdS QDs
deposition for comparison. The inset in panel (a) shows the linear
increase of absorbance measured at 615 nm wavelength as a function
of the deposition cycles (n). The inset in panel (b) shows the photo-
7
graph of (PMMA/SH-CdSe@CdS QDs) under the 365 nm UV light
irradiation.
3.3. Photophysical properties of the PMMA-SH/CdSe@CdS
QDs nanocomposite lm
We investigated the photophysical properties of the PMMA-SH/
CdSe@CdS QDs nanocomposite lm, including the PLQY,
radiative dynamics and photostability. Self-assembly of the
CdSe@CdS QDs on the PMMA-SH lm resulted in the decrease
of the absolute PL QY of the CdSe@CdS QDs from 45.8% to
16.9%. The decline of the PL QY of the CdSe@CdS QDs was
mainly attributed to the quenching effect of thiol groups
through QDs surface trap states, which suppressed the radiative
40,41
recombination of the photogenerated exciton.
The PL
quenching result is consistent with the reported phenomenon
that the PL intensity of thiol capped CdSe@CdS QDs decreased
42
compared with the amine capped CdSe@CdS QDs.
In order to gain further insight into the effects of the thiol
groups on the exciton recombination of the CdSe@CdS QDs, we
studied radiative dynamics of the PMMA-SH/CdSe@CdS QDs
nanocomposite lm. Time-resolved decay curves were
measured to determine the PL lifetimes of the CdSe@CdS QDs
Fig. 6 PL decays of the CdSe@CdS QDs (black) and PMMA-SH/
CdSe@CdS QDs nanocomposite film (red), respectively.
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Fig.
CdSe@CdS QDs nanocomposite film (red) and R6G doped PMMA film
black), respectively.
7 PL intensity versus laser irradiation time for PMMA-SH/
(
resolved PL intensity of R6G doped PMMA (R6G/PMMA ¼ 0.5
wt%, ESI†) was also measured under the same condition. All the
PL intensities are normalized with their initial values at t ¼ 0. As
shown in Fig. 7, the PL intensity of the (PMMA-SH/CdSe@CdS)
7

lm shows a monotonic decrease under the CW laser exposure
for 2 h. The PL intensity declined to 92% of the initial value at
the beginning 5 min, and then kept stable during the remained
evolution time. For R6G doped PMMA lm, the PL intensity
decreased dramatically to only 20% of the initial value aer 2 h
laser exposure. The PL intensity decrease of the composite lm
was attributed to the photobleaching and/or photoinduced
Fig. 8 (a) Transmittance spectra of the PMMA-SH film and (PMMA-
SH/CdSe@CdS)
7
nanocomposite film, respectively. Inset is the
nanocomposite film. (b)

uorescence dark of the CdSe@CdS QDs under laser exposure, photograph of the (PMMA-SH/CdSe@CdS)
7
48,49
TGA curves of the PMMA-SH film and PMMA-SH/CdSe@CdS nano-
composite film, respectively.
which is consistent with the previously reported results.
The
proportion of the PL much smaller than that of the R6G doped
PMMA lm, which suggests that the PMMA-SH/CdSe@CdS QDs
nanocomposite lm possesses much better photostability than
the commonly used organic dye- doped polymer lm. Although
slight photobleaching occurred for PMMA-SH/CdSe@CdS QDs
nanocomposite lm under laser irradiation, the PL spectra of
the composite lm almost kept constant aer laser exposure
decreased transmission between 450 and 650 nm due to the
absorption of the CdSe@CdS QDs. The red arrow in Fig. 8a
shows the dip caused by the absorption of the CdSe@CdS QDs,
which agrees well with the absorption spectrum of the toluene
solution of CdSe@CdS QDs (Fig. 1d). The inset of Fig. 8a shows
(Fig. S8†). The photostability of the PMMA-SH/CdSe@CdS QDs
the photograph of the (PMMA-SH/CdSe@CdS)
7
nanocomposite
nanocomposite lm can benet numerous applications from
solid-state lasers to light emitting diodes (LEDs).

lm under ambient light. The clear lm with orange color
veries the transparency of the nanocomposite lm containing
CdSe@CdS QDs.
The thermal stability of the PMMA-SH/CdSe@CdS QDs
nanocomposite has been investigated by the TGA analysis. As
shown in Fig. 8b, the decomposition temperature of the PMMA-
3
.4. Transparent and thermal properties of the PMMA-SH/
CdSe@CdS QDs nanocomposite lm
We further investigated the transparent and thermal properties
of the PMMA-SH/CdSe@CdS QDs nanocomposite lm. The
optical transmittance spectra of the PMMA-SH and PMMA-SH/
CdSe@CdS nanocomposite lms were plotted in Fig. 8a. The
PMMA-SH lm without CdSe@CdS QDs has a transmission of
ꢀ
SH/CdSe@CdS QDs lm is 330 C (here the decomposition
temperature is dened as the temperature of 10 wt% weight loss
ꢀ
ꢁ1
under nitrogen at a heating rate of 10 C min ). In addition,
TGA curves also indicate that about 2.04 wt% of CdSe@CdS QDs
is incorporated in the PMMA-SH/CdSe@CdS QDs nano-
composite lm through the LBL assembly. The thermal stability
of PMMA-SH/CdSe@CdS QDs is better than that of the bare
95% across the range of 450–800 nm, with a signicant trans-
mittance drop between 300–450 nm due to the absorption of the
residual photo initiator and photosensitizer. For the (PMMA-
SH/CdSe@CdS)
ꢀ
PMMA-SH lm aer 340 C. The improvement of the thermal
7
lm, the optical transmittance spectrum
stability of the PMMA-SH/CdSe@CdS QDs nanocomposite
shows a similar transmission of 95% between 650–800 nm, and
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coating. Second, the resin lm is photocross-linked by UV
irradiation through a shadow mask. Third, the patterned
PMMA-SH microstructures are immersed in the toluene solu-
tion of CdSe@CdS QDs for 30 min. Finally, the uorescent
pattern is obtained aer the removal of the unadsorbed
CdSe@CdS QDs. As shown in Fig. 9b, highly uorescent dots (9
ꢂ 9) are patterned on the exible plastic lm. Optical and

uorescent microscopic images show that the size of the
patterned dot is about 400 mm with bright uorescence. The
pink color is attributed to the combination of the red color of
the CdSe@CdS QDs and the blue colour of the plastic lm.
Similarly, the uorescent structures could be also patterned on
other substrates, such as glass, ITO, and silicon (Fig. S9†). These
patterned structures open up the prospect in the design and
construction of advanced optical devices such as exible display
panel and LEDs.
4. Conclusions
We have demonstrated a facile and efficient approach for the
fabrication of polymer/QDs nanocomposite thin lms adopting
LBL assembly. The key feature is the use of the thiol-ene reac-
tion to incorporate thiol groups in polymer network, which will
adsorb the CdSe@CdS QDs through the thiol-cadmium affinity
via in situ ligand exchange process. Highly uorescent nano-
composite thin lms are obtained with good photostability,
thermal stability, and optical transparency. Furthermore,
patterned uorescent structures can be constructed based on
the combination of photolithographic process and LBL
assembly of CdSe@CdS QDs. Therefore, this work solves the
problem of the self-assembly of hydrophobic QDs in nonpolar
media and provides a robust strategy for fabricating functional
polymer/QDs nanocomposite lms for the purpose of prom-
ising applications in optical and optoelectronic devices.
Fig. 9 (a) Schematic for the preparation of the PMMA-SH/CdSe@CdS
QDs nanocomposite pattern on the flexible substrate. (b) Photo-
graphic image of the patterned PMMA-SH/CdSe@CdS QDs nano-
composite microdots on the flexible substrate. (c) Optical and (d)
fluorescent microscopic images of single PMMA-SH/CdSe@CdS QDs
nanocomposite microdot. The size of the patterned dot is about 400
mm.
Acknowledgements
The authors are grateful to the National Natural Science Foun-
dation of China (Grant no. 51003113, 61205194, 61275171 and
5
0973126), the National Basic Research Program of China
(2010CB934103), and CAS-JSPS Joint Research Project
GJHZ1411) for nancial support.
could be assigned to the interaction between the CdSe@CdS
50
QDs and PMMA-SH polymer chains. The further crosslinking
in the PMMA-SH/CdSe@CdS QDs nanocomposite hindered the
movement of the PMMA-SH chains and prevented its thermal
decomposition. The superior thermal stability of the PMMA/
QDs nanocomposite is of great importance for applications in
device manufacturing.
(
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