Stepwise Self-Assembly of P3HT/CdSe Hybrid Nanowires with Enhanced Photoconductivity

Communication

Stepwise Self-Assembly of P3HT/CdSe HybridNanowires with Enhanced Photoconductivity

Jingjing Xu, Jianchen Hu, Xinfeng Liu, Xiaohui Qiu, Zhixiang Wei*

A facile approach to prepare poly(3-hexylthiophene) (P3HT)/cadmium selenide quantum dot(CdSe QD) hybrid coaxial nanowires by a stepwise self-assembly process is reported. P3HTnanowires of �20nm diameter are first prepared by self-assembly in a poor solvent such ascyclohexanone, and then as-prepared CdSe QDsare deposited compactly onto the P3HT nanowiresby non-covalent interactions between P3HT andCdSe. When illuminated with white light, thehybrid nanowires show enhanced photoconduc-tivity comparedwith the pristine P3HT nanowiresand the blended nanocomposites.

Introduction

Heterojunctions of electron donor and acceptor materials

play an important role in optoelectronic devices, such as

light-emitting diodes,[1] photodetectors,[2–4] and solar

cells.[5–7] Hybrid heterojunctions composed of a p-type

conjugated polymer and n-type inorganic crystals have

attracted much interest recently,[1,4,8–11] and they combine

the merits of inorganic nanocrystals and conjugated

polymers. Inorganic crystals have a high charge mobility

and band-gap tunable by altering their radius, while

conjugated polymers possess the properties of ready

processability, flexibility, and versatility at low cost. Using

hybrid heterojunctions of poly(3-hexylthiophene) (P3HT)

and cadmium selenide (CdSe) nanorods, Alivisatos et al.[8]

reported a hybrid solar cell device with a power conversion

J. Xu, J. Hu, X. Liu, X. Qiu, Z. WeiNational Center for Nanoscience and Technology, China, Beijing100190, P. R. ChinaE-mail: [email protected]. Xu, X. LiuGraduate School of Chinese Academy of Sciences, Beijing, 100039,P. R. China

Macromol. Rapid Commun. 2009, 30, 1419–1423

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

efficiency as high as 1.7%. On the other hand, Stupp et al.[4]

prepared nanometer-scale lamellar conjugated molecule/

ZnO hybrid materials by synergistic assembly, which

showed a stable photoconductive performance.

One-dimensional nanostructures of heterojunctions,

which possess merits such as an extremely efficient

donor/acceptor interface, phase separation at the nan-

ometer scale, and one-dimensional anisotropy that allows

the photochemical generation of spatially separated charge

carriers and the supply of a quick route for carrier

transportation to the electrode, are expected to have

improved efficiency in energy converters[6] or an enhanced

photoconductive response in photodectors.[12,13] Many

efforts have been applied to obtain one-dimensional radial

coaxial nanostructures of p-type and n-type layers, such as

controlled molecular self-assembly,[12] chemical vapor

deposition approaches,[6] template methods,[11] and elec-

trostatic interaction absorptions.[14] For instance, the

controlled self-assembly of a trinitrofluorenone (accep-

tor)-appended gemini-shaped amphiphilic hexabenzocor-

onene (donor) gave rise to coaxial nanotubes that show a

quick photoconductive response with a large on/off ratio

greater than 104.[12] One the other hand, p-type/intrinsic/

n-type silicon nanowires have been proved to form an

DOI: 10.1002/marc.200900132 1419

J. Xu, J. Hu, X. Liu, X. Qiu, Z. Wei

Figure 1. Schematic of the stepwise self-assembly of P3HT/CdSehybrid coaxial nanowires. P3HT is self-assembled into nanowiresin the first step (i), and CdSe QDs are self-assembled onto P3HTnanowires in a second step (ii).

1420

efficient nanometer-scale solar cell, which could be used as

a nanoelectronic power source.[6] However, although

hybrid materials of conjugated polymers and inorganic

nanocrystals have been proved as an efficient active layer

for solar cells, there is no report on the preparation and

property investigations of their radial coaxial nanostruc-

tures.

Herein, we report a facile approach to prepare CdSe

quantum dot (QD)-decorated P3HT nanowires by a two step

self-assembly process (Figure 1). P3HT nanowires of

�20 nm in diameter were first prepared by self-assembly

in a poor solvent such as cyclohexanone, and then as-

prepared CdSe QDs were compactly deposited onto the

P3HT nanowires by non-covalent interactions between

P3HT and CdSe. As is commonly known, one-dimensional

P3HT nanostructures can act as efficient ‘conduits’ for hole

carrier transport[15] and improve the field-effect mobi-

lity.[16,17] On the other hand, the CdSe QDs densely

deposited on the P3HT nanowires not only can supply a

large donor/acceptor interface, but can also form a one-

dimensional distribution along the P3HT nanowires, and as

such can act as efficient ‘conduits’ for electron carrier

transport. Thus, a radial coaxial nanowire was formed for

facile hole and electron transfer. Furthermore, with the

20 nm diameter of P3HT (on the order of the exciton

diffusion length in P3HT, i.e., 10 nm[18]), excitons generated

in P3HT by illumination diffused easily to the donor/

acceptor interface. Therefore, P3HT/CdSe coaxial nano-

wires, with a high electron donor/acceptor interfacial area

and a bicontinuous and nanoscopic separated phase, are

promising building blocks for nanometer-scale photode-

tectors and solar cells. Because of its simple and easy

handling procedure, the stepwise self-assembly approach

could possibly be developed into a versatile strategy for the

preparation of one-dimensional hybrid nanostructures

based on conjugated polymers and inorganic crystals.

Experimental Part

Materials

Regioregular P3HT (Mn � 64 000, regioregularity greater than

98.5%), cadmium oxide (CdO) (99.99%), selenium (99.5%, 100 mesh),

trioctylphosphine oxide (TOPO, 99%), tributylphosphine (TBP, 97%),



octadec-1-ene (ODE), oleic acid (OA, 90%), stearic acid (98.5%), and

octadecylamine (ODA, 97%) were purchased from Aldrich. Cyclo-

hexanone (99.8%) was purchased from Acros. All organic solvents

were purchased from Beijing Chemicals Company. Cyclohexanone

was distilled under reduced pressure and other chemicals were

used directly without any further purification.

Preparation of P3HT Nanowires

P3HT nanowires were prepared by self-assembly in its poor solvent

cyclohexanone as previously reported.[19] P3HT (2 mg) was

dissolved in 5 mL of cyclohexanone by heating to 150 8C under

agitation. During the dissolution process, the solution was

protected from light and ambient air to avoid P3HT from the

chemical oxidation and/or photooxidation. After turning a limpid

orange in color, the solution was slowly cooled to room

temperature at a controlled rate, generally 20 8C �h�1. The final

P3HT nanowire solution was obtained as an opaque violet colored

solution.

Preparation of CdSe Nanocrystals

CdSe nanocrystals were prepared according to published proce-

dures.[20] For a typical reaction, a mixture of 0.2 mmol of CdO,

0.8 mmol of stearic acid, and 2 g of ODE in a 25 mL three-necked

flask were heated to about 200 8C to obtain a colorless clear solution.

After this solution was cooled to room temperature, ODA (1.5 g) and

0.5 g of TOPO were added into the flask. Under argon flow, this

system was reheated to 280 8C. At this temperature, a selenium

solution made by dissolving 2 mmol of Se in 0.472 g of TBP and

further diluting with 1.37 g of ODE was quickly injected. The

growth temperature was then reduced to 250 8C and reacted for

10 min. The reaction mixture was allowed to cool to room

temperature and an extraction procedure was used to purify the

nanocrystals from side products and unreacted precursors. The

nanocrystals remained in the hexane/ODE layer, and the unreacted

precursors and excess amines were extracted into the methanol

layer. To remove the TOPO, the purified CdSe nanocrystals were

precipitated into acetone and redispersed by hexane more than

three times, and finally they were redispersed in cyclohexanone for

further use.

Preparation of P3HT/CdSe Coaxial Nanowires

A stock solution of CdSe QDs in cyclohexanone was mixed with the

P3HT nanowire solution in cyclohexanone to ensure 0.1 mg �mL�1

of P3HT. In order to make the P3HT combine adequately with the

QDs, the mixture was left at 40 8C with stirring for more than two

days. P3HT/CdSe coaxial nanowires were obtained in the mixture.

The morphology was elucidated by transmission electron

microscopy (TEM, Tecnai G220 S-TWIN TEM operating at 200 kV).

UV-Vis absorption spectra were measured with a PE Lambda 650/

850/950 UV-Vis spectrophotometer. The charge transfer property

between the P3HT and CdSe was investigated by photolumines-

cence quenching in the composite (LS-45/55 Fluorescence Spectro-

meter, excitation at 441 nm). The photoresponse character and I–V

DOI: 10.1002/marc.200900132

Stepwise Self-Assembly of P3HT/CdSe Hybrid Nanowires . . .

Figure 2. TEM images of a) P3HT nanowires, b) CdSeQDs, and c) P3HT/CdSe hybrid coaxial nanowires. d) EDX of P3HT/CdSe hybrid nanowires,in which S, Cd, and Se elements are clearly identified. e) Phase separation of P3HT and CdSe after blending for a short time. f) Coaxialnanostructures of P3HT and CdSe by a two-step evaporation.

characteristics were recorded by a computer-controlled Keithley

4200 source meter in the dark or in white light (mercury-vapor

lamp, 100 W).

Results and Discussion

P3HT nanowires (Figure 2a) and monodispersed CdSe QDs

(Figure 2b) were all prepared according to published

procedures,[19,20] which are summarized briefly in the

experimental section. The sulfur atom of P3HT can combine

with the CdSe QDs by a coordination interaction through

replacing part of the ligands of the CdSe QDs. Therefore, it

was expected that CdSe could be uniformly deposited on the

P3HT nanowires (Figure 1). CdSe QDs dissolved in

cyclohexanone were mixed with a P3HT nanowire disper-

sion, and the mixture was left for more than two days under

stirring at 40 8C. In order to ensure the P3HT combined with

CdSe by replacing ligands, a long stirring time and high

temperature are necessary. It was found that the P3HT and

CdSe could not be combined efficiently if the stirring time

was shorter than two days or if at room temperature.

Figure 2c shows that the CdSe QDs were deposited

uniformly and compactly onto the P3HT nanowires to

form one-dimensional coaxial nanowires, and their com-

ponents were proven by energy diffraction X-ray (EDX)

analysis (Figure 2d).



To confirm that the CdSe deposited onto P3HT was

attached by a non-covalent interaction between P3HT and

CdSe, and not by capillary phenomena or interfacial effects,

P3HT-CdSe composites were also prepared by simply

blending for short time and by a two step evaporation. A

macroscopic phase separation of P3HT (grey region) and

CdSe (black region) could be seen in the blended composite

(see Figure 2e), which confirmed that no interaction existed

in simple blended composites for a short time. By a two-step

evaporation, P3HT nanowires were first deposited onto a Cu

grid and the solvent entirely evaporated to dryness, and

then CdSe QDs were subsequently deposited. Coaxial

nanostructures were also formed as a result of interfacial

effects (Figure 2f). CdSe QDs distributed uniformly at both

sides of the P3HT nanowires, and the contrast difference

between the P3HT and CdSe made the nanowires appear as

nanotubes.

In order to confirm the interaction between P3HT and

CdSe in the coaxial nanowires, UV-vis spectra (the mass

ratio of P3HT/CdSe is 1/4) were characterized (Figure 3a). In

comparison, the pristine P3HT was also treated with

stirring for two days at 40 8C. The short-time blended

composite was obtained by simply mixing treated pristine

P3HT and CdSe. In the UV-vis spectra (Figure 3a), the

maximum absorption wavelength (lmax) in the range of

440–650 nm corresponds to the p–p� transition of the

conjugated P3HT structure.[21,22] Compared with pristine

P3HT nanowires, lmax of the coaxial nanowires was red-

www.mrc-journal.de 1421

J. Xu, J. Hu, X. Liu, X. Qiu, Z. Wei

Figure 3. a) UV-Vis and b) photoluminescence spectra of pristine P3HT, P3HT/CdSe hybridcoaxial nanowires, and the blended composite (excitation at 441 nm).

1422

shifted from �450 nm to �470 nm. A coordination inter-

action occurs between CdSe and P3HT by ligand replace-

ment in the coaxial nanowires, which results in the red-

shift by a partial electron transfer from P3HT to CdSe. There

is no obvious shift of lmax for the blended composite, which

indicates that no obvious electron transfer occurs in the

blended composite. The results also indicate that a long

stirring time of more than two days and an enhanced

temperature (40 8C) are necessary for the formation of

coaxial nanowires.

As is well known, the incorporation of inorganic crystals

into P3HT benefits exciton dissociation and charge genera-

Figure 4. a) Schematic of the experimental setup for the measurement of photocon-ductivity. b) I–V characteristics of hybrid nanowires to on/off of white light illumination.c) Current–time (I–t) characteristics of hybrid nanowires to on/off white light illumina-tion at 5 V of bias voltage. d) I–V characteristics of the blended composite and pristineP3HT nanowires to on/off white light illumination. The mass ratio of P3HT/CdSe is 1/4 inall samples.



tion, and subsequently induces photo-

luminescence quenching, which can

be characterized by a fluorescence

spectrometer. In order to compare

the efficiency of exciton dissociation

and charge generation, the photolu-

minescence spectra of pristine

P3HT, the coaxial nanowires, and the

blended composite were measured

(Figure 3b). Photoluminescence

quenching of P3HT was observed for

both the coaxial nanowires and

the blended composite, but the

quenching is more obvious for the

coaxial nanowires than for the blended composite.

Compared with pristine P3HT, the photoluminescence

efficiency is reduced by 50% for the coaxial nanowires,

while it is only 20% for the blended composite. Therefore,

there is a more efficient photo-induced exciton dissociation

and charge generation in the coaxial nanowires, caused by

the one-dimensional CdSe QDs being densely deposited on

the P3HT nanowires with a high electron donor/acceptor

interfacial area, and a bicontinuous and nanoscopic

separated phase.

As previously mentioned, compared with the blended

composite, the one-dimensional coaxial nanowires not

only can favor the exciton dissociation

and charge generation, but can also be

efficient ‘conduits’ for both hole and

electron carrier transport and improve

field-effect mobility. The coaxial nano-

wires should be more photoconductive

than the blended composite and P3HT

nanowires. Consequently, we measured

the photoresponse of the composites on

white light illumination in ambient

atmosphere at room temperature

(Figure 3).

The configuration for the photocon-

ductive measurements of the nanowire

film is shown in Figure 4a. Films of the

coaxial nanowires, the blended compo-

site, and P3HT nanowires were prepared

by drop casting on interdigitated gold

electrodes on an oxidized silicon sub-

strate, followed by annealing for 10 min

at 150 8C under vacuum. The current of

the coaxial nanowires increased pro-

nouncedly under white light excitation

over a bias voltage range of �5 to 5 V

(Figure 4b), which indicates a photo-

induced electron transfer from the

P3HT to the CdSe QDs. The current of

the coaxial nanowires increased from

DOI: 10.1002/marc.200900132

Stepwise Self-Assembly of P3HT/CdSe Hybrid Nanowires . . .

4.5 to 10mA under white light excitation at 5 V bias, which

suggests a two-fold on/off current ratio. From the energy

level diagram for CdSe nanocrystals and P3HT, CdSe is

electron-accepting and P3HT is hole-accepting.[8] The rise

and decay of the current were reproducibly observed (on/

off time period: 120 s), which indicates the relative stability

of the photo-responsive properties. The gradual rise and

decay of the current may originate from the charging/

discharging of the organic/inorganic interface. On the other

hand, the current of the blended composite or P3HT

nanowires increased only slightly under white light

excitation, which indicates that significantly fewer charge

carriers are induced by white light because of an

unoptimized interface structure. Therefore, the photocon-

ductive property of the coaxial nanowires is much better

than that of the blended composite and P3HT, which further

proves that the one-dimensional structure of the coaxial

nanowires contributed to the charge transfer significantly.

Conclusion

In conclusion, we have successfully exploited a facile

approach for the preparation of one-dimensional coaxial

nanowires of inorganic crystals/conjugated polymer. CdSe

QDs were densely and uniformly deposited onto P3HT

nanowires by a weak interaction between the P3HT and

CdSe QDs. The photoluminescence quenching and photo-

conductive characterization showed that the coaxial

nanostructure was beneficial to the charge carrier separa-

tion and transportation.

Acknowledgements: The authors gratefully acknowledge theNational Natural Science Foundation of China (Grants20604008), National Basic Research Program of China(2006cb932100, 2009CB930400) and Chinese Academy of Sciencefor financial support.

Received: February 24, 2009; Revised: April 19, 2009; Accepted:April 20, 2009; Published online: May 26, 2009; DOI: 10.1002/marc.200900132



Keywords: coaxial nanowires; hybrids; luminescence; photocon-ductivity; poly(3-hexylthiophene); quantum dots; self-assembly

[1] N. Tessler, V. Medvedev, M. Kazes, S. Kan, U. Banin, Science2002, 295, 1506.

[2] G. Konstantatos, I. Howard, A. Fischer, S. Hoogland, J. Clifford,E. Klem, L. Levina, E. H. Sargent, Nature 2006, 442, 180.

[3] J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, C. M. Lieber, Science2001, 293, 1455.

[4] M. Sofos, J. Goldberger, D. A. Stone, J. E. Allen, Q. Ma, D. J.Herman, W. Tsai, L. J. Lauhon, S. I. Stupp, Nature Mater. 2009,8, 68.

[5] H. J. Snaith, G. L. Whiting, B. Sun, N. C. Greenham, W. T. S.Huck, R. H. Friend, Nano Lett. 2005, 5, 1653.

[6] B. Tian, X. Zheng, T. J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang,C. M. Lieber, Nature 2007, 449, 885.

[7] K. S. Leschkies, R. Divakar, J. Basu, E. Enache-Pommer, J. E.Boercker, C. B. Carter, U. R. Kortshagen, D. J. Norris, E. S. Aydil,Nano Lett. 2007, 7, 1793.

[8] W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 2002, 295,2425.

[9] K. R. Choudhury, Y. Sahoo, T. Y. Ohulchanskyy, P. N. Prasad,Appl. Phys. Lett. 2005, 87, 073110.

[10] P. Wang, A. Abrusci, H. M. P. Wong, M. Svensson, M. R.Andersson, N. C. Greenham, Nano Lett. 2006, 6, 1789.

[11] Y. Xi, J. Zhou, H. Guo, C. Cai, Z. Lin, Chem. Phys. Lett. 2005, 412,60.

[12] Y. Yamamoto, T. Fukushima, Y. Suna, N. Ishii, A. Saeki, S. Seki,S. Tagawa, M. Taniguchi, T. Kawai, T. Aida, Science 2006, 314,1761.

[13] C. Yang, C. J. Barrelet, F. Capasso, C. M. Lieber, Nano Lett. 2006,6, 2929.

[14] D. M. Guldi, G. M. A. Rahman, V. Sgobba, N. A. Kotov,D. Bonifazi, M. Prato, J. Am. Chem. Soc. 2006, 128, 2315.

[15] M. Surin, Ph. Leclere, R. Lazzaroni, J. D. Yuen, G. Wang,D. Moses, A. J. Heeger, S. Cho, K. Lee, J. Appl. Phys. 2006,100, 033712.

[16] D. H. Kim, Y. Jang, Y. D. Park, K. Cho, J. Phys. Chem. B 2006, 110,15763.

[17] J. A. Merlo, C. D. Frisbie, J. Phys. Chem. B 2004, 108, 19169.[18] B. R. Saunders, M. L. Turner, Adv. Colloid Interface Sci. 2008,

138, 1.[19] S. Berson, R. D. Bettignies, S. Bailly, S. Guillerez, Adv. Funct.

Mater. 2007, 17, 1377.[20] J. J. Li, A. Wang, W. Guo, J. C. Keay, T. D. Mishima, M. B.

Johnson, X. Peng, J. Am. Chem. Soc. 2003, 125, 12567.[21] J. Boucle, S. Chyla, M. S. P. Shaffer, J. R. Durrant, D. D. C.

Bradley, J. Nelson, Adv. Funct. Mater. 2008, 18, 622.[22] Y. Chang, W. Su, L. Wang, Macro. Rapid Commun. 2008, 29,

1303.

www.mrc-journal.de 1423

Documents

Stepwise Self-Assembly of P3HT/CdSe Hybrid Nanowires with Enhanced Photoconductivity