synthesis of cdse–poly(n-vinylcarbazole) nanocomposite by atom transfer radical polymerization for...

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Synthesis of CdSe–Poly(N-vinylcarbazole)Nanocomposite by Atom Transfer RadicalPolymerization for Potential OptoelectronicApplications

Tzong-Liu Wang,* Chien-Hsin Yang, Yeong-Tarng Shieh, An-Chi Yeh

A hybrid inorganic–polymer nanocomposite using CdSe nanocrystals with high electronmobility has been successfully synthesized by atom transfer radical polymerization (ATRP).First the hydroxyl-coated CdSe nanoparticles (i.e., CdSe–OH) were prepared via a wet chemicalroute. A polymerization initiator was then prepared for ATRP of N-vinylcarbazole. FT-IR,1H NMR, and XRD analyses confirmed the successful synthesis of CdSe–poly(N-vinylcarbazole)(PVK) nanohybrid. UV–Vis spectra and photolu-minescence data revealed that grafting of PVKonto the surface of CdSe nanocrystals wouldreduce the band gap of PVK and cause the redshift of emission peak. TEM and SEM micrographsexhibited CdSe nanoparticles that were well-coated with PVK polymer.

Introduction

Semiconductor materials can be tailored to prepare

quantum dots with special optoelectronic properties. In

the extensive research of colloidal II-VI semiconductor

nanoparticles, many works on the synthesis of CdSe

nanocrystals have been reported. However, several

reported studies used high toxicity, high reactivity

dimethyl cadmium as the precursor, and hazardous,

expensive trioctylphosphine (TOP) or TOP oxide (TOPO) as

the coordinating solvent. Therefore, it remains a challenge

to produce CdSe nanocrystals via an economical and simple

method.

T.-L. Wang, C.-H. Yang, Y.-T. ShiehDepartment of Chemical and Materials Engineering, NationalUniversity of Kaohsiung, Kaohsiung 811, Taiwan, Republic of ChinaFax: (þ886) 7 5919277; E-mail: [email protected]. YehDepartment of Chemical and Materials Engineering, Cheng ShiuUniversity, Kaohsiung County 833, Taiwan, Republic of China

Macromol. Rapid Commun. 2009, 30, 1679–1683

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Recently, inorganic-polymer nanocomposites have

attracted much attention due to their unique size-

dependent magnetic, optical, and material properties.[1–4]

However, the exploitation of these properties requires a

homogeneous dispersion of the inorganic particles in the

polymer matrix. The use of atom transfer radical poly-

merization (ATRP) would conduct a controlled/‘‘living’’

radical polymerization from the surface of an inorganic

nanoparticle macroinitiator, yielding nanoparticles with

an inorganic core and an outer layer of covalently attached,

well-defined polymer chains.

In the past decade, many research groups have reported

on the preparation and applications of CdSe–polymer[5–11]

nanocomposites that were usually prepared by mixing of

polymer with inorganic nanoclusters. Based on the poly(N-

vinylcarbazole) (PVK) matrix, in a previous study of CdS–

PVK nanocomposite, Wang et al. presented the new

nanocomposite, prepared by a chemically hybridized

approach, which has a significant photoconductivity

enhancement as compared to pure PVK and CdS/PVK

nanoblends.[12] Using a similar approach to prepare the

DOI: 10.1002/marc.200900349 1679

T.-L. Wang, C.-H. Yang, Y.-T. Shieh, A.-C. Yeh

1680

CdSe–PVK nanocomposite, Park et al. have shown that

the composite has a significantly enhanced photorefractive

gain value compared with that of a CdSe/PVK nanoblend

system.[11] Nonetheless, there are still relatively few

experimental studies reported to date performed on

the CdSe–polymer nanohybrids based on a PVK

matrix, not to mention the synthesis of nanohybrids by

using ATRP.

Herein, we have employed a cheaper and greener non-

TOP-based approach for the synthesis of in situ thiol-capped

CdSe nanocrystals through a wet chemical route[13,14] in

aqueous solution, and then ATRP of N-vinylcarbazole was

carried out on the functionalized CdSe nanocrystal surfaces

to obtain the CdSe–PVK nanocomposite. In this work, a PVK

matrix was investigated due to its hole-transporting

feature and the photoactivity of the carbazole sidegroup.

The basic structural characterizations and optical proper-

ties of thiol-capped CdSe nanoparticles and CdSe–PVK

nanocomposite are presented, while a more detailed study

in applications such as light-emitting and photovoltaic

devices is still in progress.

Experimental Part

2-Mercaptoethanol (ME), cadmium chloride, selenium, sodium

sulfite, sodium hydroxide, triethylamine (TEA), 4-(dimethylamino)

pyridine (DMAP), and N-vinylcarbazole were used as received.

2-Bromoisobutyryl bromide (BrIB), 1-bromoethyl benzene (BrEB),

and tetrahydrofuran (THF) were distilled before use. CuCl and 2,20-

bipyridyl (bpy) were purified before use.

Preparation of CdSe Colloidal

Initiator (CdSe–BrIB)

Scheme 1. Synthetic route for the preparation of the CdSe–PVK nanohybrid.

A Na2SeSO3 precursor was firstly

synthesized according to the solution

growth approach reported by Ma

et al.[15] ME was then used as the

organic ligand to prepare the hydroxyl-

coated CdSe nanocrystals (CdSe–OH).

Next, a solution of CdSe–OH, DMAP,

and TEA in THF was added with excess

BrIB drop by drop in an iced bath and

flushed with N2. The reaction was

carried out at room temperature for

one day. The obtained particles were

purified by repeated centrifugation/

resuspension to remove the unreacted

BrIB and other organics. This initiator

was designated as CdSe-BrIB. The reac-

tion is shown in Scheme 1.

FT-IR (KBr, cm�1): 2 983 and 2 940

(nC�H, �CH3, and �CH2�), 1 740 (nC¼O),

1 040 (nC�O C�O�C). 1H NMR (d, ppm,

CDCl3): 1.33–1.38 (t, 6H, �C(CH3)2�Br),

3.06–3.14 (m, 4H, �S�(CH2)2�O�).



Atomic Transfer Radical Polymerization of

N-Vinylcarbazole from BrIB Functionalized CdSeColloids

Suitable amounts of N-vinylcarbazole, CuCl, and bpy were

introduced into a glass flask, which was then capped by a three-

way stopcock and purged with nitrogen by repeated vacuum/

nitrogen cycles. The CdSe colloidal initiator was then added under

nitrogen with a syringe. The reaction mixture was then heated at

90 8C for 4 h. The solution was filtered through a column with

neutral alumina to remove the catalyst. The CdSe–polymer hybrid

was recovered by precipitation in methanol, followed by drying at

70 8C for 24 h under high vacuum. The nanocomposite was

designated as CdSe–PVK. For comparison of physical and optical

properties, neat PVK was also synthesized by ATRP using BrEB as

the initiator.

FT-IR (KBr, cm�1): 3 048 (nC�H carbazyl ring), 1 594, 1 484, and

1 453 (nC¼C carbazyl ring), 1 371 and 1 327 (nC�N). 1H NMR (d, ppm,

CDCl3): 1.09 (d, 2H, �N�CH�CH2�), 3.46 (s, 1H, �N�CH�CH2�),

7.18–8.08 (m, 8H, carbazyl ring).

Characterization

IR spectra of samples were obtained using a PerkinElmer spectrum

GX FT-IR spectrometer. 1H NMR spectra were recorded using a

Varian UNITY INOVA-500 FT-NMR spectrometer. Mn, Mw, and

polydispersity index (PDI, Mn=Mn) were determined by gel

permeation chromatography (GPC) using Young Lin Acme 9000

liquid chromatograph. The CdSe cores of the hybrid materials were

etched with aqueous HCl solution to afford the neat polymers for

GPC analysis. Wide-angle X-ray diffractograms (WAXDs) were

obtained on a Bruker D8 ADVANCE diffractometer, using CuKa

radiation at 2u¼28 �608 with a step size of 0.058 and a scanning

speed of 48min�1. UV–Vis spectroscopic analysis was conducted on

a PerkinElmer Lambda 35 UV–Vis spectrophotometer. Room

temperature photoluminescence (PL) spectrum was recorded on

DOI: 10.1002/marc.200900349

Synthesis of CdSe–Poly(N-vinylcarbazole) Nanocomposite by . . .

605040302010

PVK

CdSe-PVK

CdSe-OH

Rela

tive In

ten

sit

y

2θ (degree)

Figure 1. XRD patterns of CdSe–OH, PVK, and CdSe–PVK nano-composites.

a Hitachi F-7000 fluorescence spectrophotometer. Transmission

electron microscopy (TEM) images were taken with a JEOL JEM-

1230 TEM. The distributions of Cd and Se atoms in the hybrid

material were obtained from scanning electron microscopy (SEM)

EDS mapping (JEOL 5610, Japan).

Results and Discussion

Synthesis and Structural Characterization

The synthetic route for the preparation of CdSe–PVK

nanocomposite was summarized in Scheme 1. At first,

CdSe–OH was prepared via wet chemical route. Next, a

thiol-group containing ME was used as the organic ligand

and hydroxyl-coated CdSe nanocrystals (CdSe–OH) were

prepared in the presence of ME by the in situ reaction of

cadmium and selenide ions. The hydroxyl-group end-

capped ligands were then attached with halide groups by

the reaction of BrIB with the hydroxyl groups on the CdSe

surface to form the surface-modified nanocrystal initiator

CdSe-BrIB. The controlled/‘‘living’’ characteristics of ATRP of

N-vinylcarbazole were characterized with GPC and shown

in Table 1.

X-ray diffractogram (XRD) analysis for the hydroxyl-

coated CdSe nanoparticles is shown in Figure 1. As expected,

the XRD peaks of the CdSe nanocrystals are considerably

broadened compared to those of the bulk CdSe due to the

finite size of these nanoparticles. As seen in this figure, three

peaks at 2u¼ 25.48, 42.68, and 49.88 corresponding to

the (111); (220); and (311) plane reflections of cubic CdSe

are observed (JCPDS no. 19-191). This result indicates that

the as-prepared CdSe particles by wet chemical route have

the typical zinc blende structure of CdSe, whereas it has

been indicated that CdSe nanocrystals prepared by using

hot TOPO[16] or microwave approaches[17] acquire the

hexagonal structure. The difference in the width of

diffraction peaks can be attributed to the difference in

the particle sizes. To analyze the apparent particle sizes, the

Table 1. GPC results and physical properties of CdSe nanocrystalsand CdSe–PVK nanohybrids.

Specimens Mn Mw (Mw=Mn) Particle

size

Eg

g �mol�1 g �mol�1 PDI nm eVa)

dXRD ¼ 3.3

CdSe–OH N/A N/A N/A dUV ¼ 3.4 2.86

dTEM ¼ 6.2

PVK 1 450 1 520 1.05 N/A 3.50

CdSe–PVK 1 290 1 450 1.12 dTEM ¼ 650 3.45

a)Data obtained from the Tauc relation.



Scherrer equation was used:[18]

Dhkl ¼Kl

bhkl cos u(1)

where Dhkl is the mean crystallite size along the [hkl]

direction, K is the shape factor (the Scherrer constant, a

value of 0.9 is used in this case), l is the wavelength

(0.154 nm), bhkl is the line breadth (usually taken as the

fwhm of a (hkl) diffraction, fwhm is the value of the full

width at the half-maximum), and u is the half-scattering

angle. The average particle size, estimated from the (111);

(220); and (311) reflections of the XRD pattern with Scherrer

formula, was ca. 3.3 nm for CdSe–OH nanoparticles.

The XRD patterns for PVK and the CdSe–PVK nanohybrid

are also illustrated in Figure 1. For PVK, the pattern shows

broad peaks around 2u¼ 13.38 and a shoulder at 2u¼ 20.58,suggesting the sample is typical of semicrystalline/

amorphous material. On the other hand, the pattern of

CdSe–PVK exhibits detectable peak characteristics arising

from (111); (220); and (311) plane reflections of CdSe

nanocrystals. The X-ray patterns further confirmed the

successful synthesis of CdSe–PVK nanocomposite.

Optical Properties

The UV–Vis absorption spectrum of the as-prepared CdSe

nanocrystals dispersed in THF is presented in Figure 2(b).

The optical absorption threshold at 442 nm corresponds to

the band gap (Eg) of the CdSe nanocrystals. According to

Brus’s model,[19] the excited energy of nanocrystals is in

reverse of their particle size. If the excited energy of

nanocrystal is higher, the maximum absorption peak of its

UV–Vis spectrum will be blue-shifted. To obtain a more

accurate optical band gap of the CdSe nanocrystals, the

www.mrc-journal.de 1681

T.-L. Wang, C.-H. Yang, Y.-T. Shieh, A.-C. Yeh

4.03.53.02.52.0

0.0

0.1

0.2

0.3

(αh

ν)2

hν (eV)

CdSe-OH

800700600500400300

Inte

nsit

y (

a.u

.)

Wavelength (nm)

CdSe-OH

CdSe-PVK

PVK

550500450400350

Inte

nsit

y (

a.u

.)

Wavelength (nm)

CdSe-OH

CdSe-PVK

PVK

a)

c)

b)

Figure 2. a) Plot of (ahn)2 versus hn for the CdSe–OH nanocrystals.b) UV–Vis absorption spectra, and c) PL spectra of CdSe–OH, PVK,and CdSe–PVK nanocomposites. All measurements were taken onthin films except CdSe–OH (in THF).

1682

fundamental equation ahn¼B(hn� Eopt)n developed in the

Tauc relation[20] was used, where a is the absorption

coefficient, hn is the energy of absorbed light, n is equal

to 1/2 for the direct allowed transition and B is the

proportionality constant. The optical band gap calculated



by this equation is 2.86 eV, as shown in Figure 2(a) and listed

in Table 1. Based on this band gap, the estimated size of CdSe

nanocrystals was approximately 3.4 nm by the effective

mass approximation (EMA) model proposed by Brus.[19]

Next, as evident from Figure 2(b), the absorption

spectrum of PVK film shows a doublet positioned at about

344 and 331 nm which is due to the p–p� transition of

carbazole groups that are pendent to the PVK backbone. The

CdSe–PVK absorption is also characterized by the presence

of the doublet at about 345 and 331 nm, ascribed to the PVK

energy levels and a visible shoulder, ranging from 360 to

600 nm, due to the CdSe absorption. Hence, our successful

synthesis of CdSe–PVK nanocomposite is further con-

firmed. In addition, due to the effect of low band gap CdSe

nanocrystals, it was expected that the band gap of CdSe–

PVK composites would be lower than the neat polymer PVK.

According to the optical band gaps calculated by Tauc

relation and shown in Table 1, it seemed that our Eg data

were reasonable.

Figure 2(c) shows the characteristic emission for CdSe

nanocrystals present at about 535 nm. For the neat PVK,

there is a strong luminescence at 364 nm attributed to the

excimer emission of PVK. On the other hand, the spectrum

of CdSe–PVK seems to be the sum of the emission spectra of

the constituent parts of the composite film, that is, the

emission peak of PVK and CdSe. However, the intensity of

the luminescence is reduced, compared to that of PVK. It is

worth noting that there is a significant quenching of the PL,

implying that the occurrence of interfacial electron

transfer, leading to the formation of separated electron–

hole pairs that subsequently recombine non-radiatively.

Nonetheless, we note that the quenching of this lumines-

cence is not complete. This may be due to the insufficient

contact area in the interface of PVK and CdSe nanocrystals.

In addition, there is a red shift for the emission spectrum as

compared to that of neat PVK. The shift of emission peak is

due to the decrease of HOMO–LUMO band gap for the CdSe–

PVK nanohybrid, as mentioned above.

TEM Analysis and SEM Mapping

Figure 3(a) shows a typical TEM overview image of CdSe

nanoparticles. The average size of the CdSe nanoparticles is

ca. 6.2 nm, based on estimates from the TEM image. The

average size estimated from TEM micrograph is generally

larger than that obtained from the XRD pattern or UV–Vis

spectrum, a trend already observed for thiol-stabilized CdSe

clusters with comparable sizes.[21] The difference between

the result of XRD or UV–Vis with TEM may be due to the

aggregation of smaller particles of the CdSe nanocrystals.

The data calculated from the XRD or UV result reflected the

size of a ‘‘single’’ crystal, while the TEM micrograph showed

the aggregates of the particles, which were formed because

of the high surface energy of the nanometer-sized crystals.

DOI: 10.1002/marc.200900349

Synthesis of CdSe–Poly(N-vinylcarbazole) Nanocomposite by . . .

Figure 3. TEM images of a) CdSe–OH and b) CdSe–PVK. SEMmapping micrographs of c) Cd and d) Se in the CdSe–PVKnanohybrid.

Figure 3(b) shows the micrograph of the CdSe–PVK

composite film, in which particles with an average size of

650 nm are observed. This image displays CdSe particles

which were well coated with PVK. Compared to the particle

size of CdSe, it seems that the inorganic cores are composed

of the aggregates of CdSe nanoparticles. The grain sizes

estimated by TEM are also listed in Table 1. The distribu-

tions of Cd and Se in the CdSe–PVK hybrid were also

observed by a SEM mapping technique. In the two mapping

photographs [Figure 3(c) and (d)], the white dots represent

Cd and Se atoms, respectively. Both images demonstrate

that CdSe particles are uniformly dispersed in the PVK

polymer matrix.

Conclusion

In this article, the first synthesis of CdSe–PVK nanocompo-

site by grafting PVK onto CdSe nanocrystals through ATRP

technique has been achieved. Since the reaction was carried

out at moderate temperature, the as-prepared CdSe

particles have the typical zinc blende structure of CdSe.

UV–Vis absorption spectra indicated that the band gap of

PVK was reduced by grafting PVK onto the surface of CdSe



nanocrystals. PL measurements revealed that there was a

significant quenching of the PL for the CdSe–polymer

nanocomposite grafted with a hole-transporting polymer

PVK. TEM and SEM micrographs displayed the CdSe–PVK

hybrid particles which were well separated and coated with

PVK polymer. Combining the above results, it is suggested

that optimum physical and optical properties could be

tuned by adjusting the kind and chain length of the tethered

polymer grafting onto the CdSe quantum dots for possible

applications such as optoelectronic appliances including

light-emitting and photovoltaic devices.

Acknowledgements: We gratefully acknowledge the support ofthe National Science Council of Republic of China (Grant no. NSC97-2221-E-390-005-MY2).

Received: May 19, 2009; Published online: August 13, 2009;DOI: 10.1002/marc.200900349

Keywords: atom transfer radical polymerization; CdSe; nano-composites; nanocrystals; poly(N-vinylcarbazole)

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