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Chemical Physics Letters 451 (2008) 70–74

Forster resonance energy transfer from poly(9-vinyl carbazole) tosilicon nanoparticles in their composite films

Nan Liu, Hong-Zheng Chen *, Fei Chen, Mang Wang *

Department of Polymer Science and Engineering, State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China

Key Laboratory of Macromolecule Synthesis and Functionalization (Zhejiang University), Ministry of Education, Hangzhou 310027, PR China

Received 3 September 2007; in final form 20 November 2007

Abstract

A novel organic–inorganic hybrid composite was prepared from poly(9-vinyl carbazole) (PVK) and silicon nanoparticles (NPs). Theoptical properties of the composites were characterized by photoluminescence (PL), photoluminescence excitation (PLE), and PL decays.The results indicated that an efficient Forster resonance energy transfer (FRET) took place from PVK (donor) to silicon NPs (acceptor),which was necessary for the optimization of polymer/inorganic NPs hybrid light-emitting diodes.� 2007 Elsevier B.V. All rights reserved.

1. Introduction

Silicon is the basic material of modern electronic indus-try, but the low luminescence efficiency due to its indirectband gap excludes its further application in optoelectronicintegration, which is important for future information tech-nology. Great interest in silicon nanoparticles (NPs) hasbeen aroused because of the visible luminescence observedfrom them as a result of the quantum confinement effect[1]. To date, silicon NPs are believed to have potential appli-cations in light-emitting diodes [2,3], bioprobes [4], solarcells [5] and so on. Many preparation methods for siliconNPs have been introduced, such as silicon wafer anodiza-tion [1], laser ablation [6], and nonthermal plasma synthesis[7]. Recently, the solution-phase synthesis of silicon NPswas reported by Kauzlarich and co-workers [8–11]. Com-pared with others, this method can provide a better meansto control surface passivation/termination of the NPs whichis crucial for the application of silicon NPs.

0009-2614/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2007.11.057

* Corresponding authors. Address: Department of Polymer Science andEngineering, State Key Lab of Silicon Materials, Zhejiang University,Hangzhou 310027, PR China.

E-mail addresses: hzchen@zju.edu.cn (H.-Z. Chen), mwang@zju.edu.cn (M. Wang).

On the other hand, polymer light-emitting diodes(PLEDs) have attracted a broad range of attention due totheir high potential application in large area displays [12–14]. However, a significant stumbling block to the rapidcommercialization of PLEDs is their high susceptibility tooxidation under ambient conditions which leads to theshort lifetime of devices [15,16]. Contrariously, inorganicNPs have excellent ambient stability [17]. So, combiningthe characteristics of inorganic NPs and the advantages ofpolymer (including spin coatability and use on large andflexible substrates) might produce PLEDs with designeddevice performance. Some pioneer works on polymer/inor-ganic NPs hybrid LEDs have been reported [18–20], wherepolymer was employed as a carrier transporting layer andinorganic NPs (such as Au and CdSe) acted as luminescencecenters. Efficient Forster resonance energy transfer (FRET)from polymer (donor) to NPs (acceptor) is required for theoptimization of such kind of devices.

In this Letter, silicon NPs were synthesized by using asolution-phase method. Poly(9-vinyl carbazole) (PVK), ahole transport polymer, was employed to composite withsilicon NPs, and the PVK/silicon NPs composites with dif-ferent silicon NPs content were fabricated by solution-blending method. The energy transfer from PVK to siliconNPs was demonstrated in this work.

50nm

(1)

20 30 40 50 60 70 80 90

88.0676.3869.18

56.20In

tens

ity

(a.u

.)

Two-theta (degree)

28.48

47.38

(2)

Fig. 1. (1) TEM image and (2) XRD pattern of synthesized siliconnanoparticles.

300 400 500 600 700 800

(d)

(c)(2)

Abs

(a.

u.)

Wavelength/nm

(b)

(a)

PL

intensity (a.u.)P

L intensity (a.u.)

Abs

(a.

u.)

(1)

Fig. 2. (1) UV–vis absorption (a) and PL (b) spectra of silicon NPs films,the excitation wavelength is 415 nm; (2) UV–vis absorption (c) and PL (d)spectra of PVK films, the excitation wavelength is 350 nm.

N. Liu et al. / Chemical Physics Letters 451 (2008) 70–74 71

2. Experimental

SiCl4 (99.998%) and poly(9-vinyl carbazole) (PVK)( �Mn ¼ 90000) were purchased from Aldrich. Sodium,naphthalene, chloroform, hexane, 1,2-dimethoxyethane(glyme) and 1-octanol were purchased from SinopharmChemical Reagent Co. Ltd. The latter two chemicals weredried by distillation over sodium before the reaction.

Silicon NPs were synthesized via the reduction of silicontetrachloride with sodium naphthalide according to a pub-lished method [9] and all reactions were carried out in aSchlenk line. 0.18 g sodium and 0.74 g naphthalene werestirred in 20 ml glyme for 24 h to produce sodium naphtha-lide, which was added rapidly via a syringe into 0.26 g SiCl4in 60 ml glyme. The reaction lasted for 40 min before1.25 ml 1-octanol was added. Then the reaction mixturewas stirred for 12 h, giving an orange solution with whiteprecipitates. The solvent and the unreacted naphthalenewere removed under reduced pressure in a water bath.The resulted orange solid was then washed three times withchloroform or hexane, giving an orange solid of siliconnanoparticles.

A certain amount of PVK and silicon NPs were dis-solved in chloroform together to give a mixed PVK/siliconNPs solution. The concentration of PVK was kept constantand that of the silicon NPs varied with 0%, 0.2%, 0.4%, and0.8% silicon mass loading in PVK/silicon NPs composites,respectively. The mixed PVK/silicon NPs solutions werespin-coated onto glass substrates and dried at 110�C in avacuum oven for 3 h, giving the PVK/silicon NPs compos-ite films in identical thickness.

Transmission electron microscopy (TEM) image ofsilicon NPs was observed on a JEM 200CX TEM. X-raydiffraction (XRD) patterns were obtained on PigakuD/Max-38. The photoluminescence (PL) and photolumi-nescence excitation (PLE) spectra were taken on a HIT-ACHI 4500 fluorescence spectrophotometer. UV–visabsorption was recorded on a Cary Bio100 spectrophoto-meter. PL decays were measured on a Laserstrobe fluores-cence lifetime spectrometer.

3. Results and discussion

IR spectrum of the final product indicated the presenceof octanoxide groups on the surface of the silicon nanopar-ticles. The bands corresponding to alkyl C–H stretching at�3000 cm�1 and Si–OR stretching at �1080 cm�1 wereobserved. The silicon NPs were also soluble in chloroformand hexane, making wet-processing possible.

TEM image of the synthesized silicon NPs is shown inFig. 1(1). The size is in the range of 8–25 nm. As seen fromits XRD pattern shown in Fig. 1(2), the peaks are fully con-sistent with the crystalline silicon [11].

The optical properties of octanol capped silicon NPsfilms were investigated by UV–vis and PL spectra andthe results are shown in Fig. 2(1). Absorption peaks at�290 and �390 nm are observed for silicon NPs. An

absorption onset at �750 nm (1.65 eV) is shown in theUV–vis absorption spectrum, which is blue-shifted fromthe band gap of bulk silicon (�1.2 eV). The PL peak of sil-icon NPs films can also be observed at �510 nm with a415 nm excitation.

UV–vis absorption and PL spectra of a PVK film arepresented in Fig. 2(2). The absorption edge of PVK is

72 N. Liu et al. / Chemical Physics Letters 451 (2008) 70–74

observed at �370 nm, which is consistent with Ref. [21].The excellent overlap between the PL of PVK and theabsorption of silicon NPs can also be observed, which isthe fundament of FRET from PVK to silicon NPs. Accord-ing to FRET mechanism, the energy transfer rate is decidedby the following formula [22]:

kET ¼1

sD

R0

R

� �6

ð1Þ

where kET is the energy transfer rate, sD is the PL lifetimeof the donor in absence of the acceptor, R is the separationdistance between the donor and the acceptor and R0 is For-ster critical distance corresponding to a rate of FRETequivalent to the rate of radiative decay which is given asfollows:

R60 ¼

9000ðln 10Þk2/d

128p6N AVn4

ZfDðmÞeAðmÞ

m4dm ð2Þ

where n is the film refractive index (n � 1.683) [23], N indi-cates Avogadro number, Ud is the donor emission quantumefficiency which is assumed as 0.50 [24], k2 is an orientationfactor which for random relative orientation of the donorand acceptor dipoles is 2/3, m is the wave number andR fDðmÞeAðmÞ

m4 dm represents the overlap integral of the normal-

400 450 500 550 600 650

(a)(b)(c)

(c)(b)(a)

PL

inte

nsit

y (a

.u.)

Wavelength/nm

(1)

450 500 550 600

(3)

(e)

(b)

Wavelength/nm

PL

inte

nsit

y (a

.u.)

Fig. 3. PL spectra of PVK/silicon NPs composite films with (a) 0.2%, (b) 0.4%350 nm for them. (d), (e) and (f) indicate the PL spectra of the composite fiexcitation, respectively.

ized donor emission spectrum (fDðmÞ) and the acceptorabsorption extinction coefficient spectrum (eAðmÞ). Thus aForster critical distance of about 51 A was calculated fromEq. (2).

Fig. 3 shows PL spectra of the composite films with 350and 415 nm excitations. It can be observed from Fig. 3(1)that the PL of PVK is quenched gradually with increasingthe content of silicon NPs in the composite. On the otherhand, 415 nm is obviously at the red side of the absorptionedge of PVK, so only the silicon NPs in the composite filmsare excited in Fig. 3(2)–(4). They reveal the enhancement ofthe PL intensity of silicon NPs in the composite films whenboth PVK and the silicon NPs are excited.

Fig. 4 presents PLE spectra of a pure PVK film, siliconNPs, and PVK/silicon NPs composite films with varioussilicon NPs mass loading. Comparison of Fig. 4(1) withFig. 4(2) reveals that the PL of silicon NPs in the compositefilms originates from photoexcitations in both PVK and sil-icon NPs. Considering what is shown in Fig. 3, it is sug-gested that PVK should transfer the energy it absorbs tosilicon NPs.

Fig. 5 shows time-resolved PL decays of a pure PVKfilm, silicon NPs, and PVK/silicon NPs composite filmswith various silicon NPs mass loading. The fit results are

450 500 550 600 650

(a)

Wavelength/nm

PL

inte

nsit

y (a

.u.)

(d)

(2)

450 500 550 600 650

(4)

(c)

(f)

Wavelength/nm

PL

inte

nsit

y (a

.u.)

, and (c) 0.8% silicon NPs mass loading, and the excitation wavelength islms with 0.2%, 0.4%, and 0.8% silicon NPs mass loading with a 415 nm

300 350 400 450 500

(2) (e)(d)(c)

PL

E in

tens

ity

(a.u

)

Wavelength/nm

(b)

PL

E in

tens

ity

(a.u

.)

(a) (1)

Fig. 4. (1) PLE spectra of (a) a PVK film and (b) silicon NPs. Theemission wavelength is 420 nm and 510 nm, respectively. (2) PLE spectraof PVK/silicon NPs composite films with (c) 0.2%, (d) 0.4%, and (e) 0.8%silicon NPs mass loading. For all of them, the emission wavelength is510 nm.

403020100

PL

inte

nsit

y (a

.u.)

Time/ns

403020100Time/ns

(2)

PL

inte

nsit

y (a

.u.)

(1)

Fig. 5. (1) PL decays monitoring the PL peak of PVK in a pure PVK (j)and the PVK/silicon NPs composite films with 0.2% (s), 0.4% (N), and0.8% (O) silicon NPs mass loading. (2) PL decays monitoring the PL peakof silicon NPs in the pure silicon NPs (j) and PVK/silicon NPs compositefilms with 0.2% (s), 0.4% (N), and 0.8% (O) silicon NPs mass loading. Thesolid lines in (1) and (2) are the best fit curves which are obtainedaccording to [25,26].

N. Liu et al. / Chemical Physics Letters 451 (2008) 70–74 73

given in Table 1. It is found that the PL lifetime of siliconNPs is prolonged from 1.11 ns to 1.44, 1.65, and 1.91 nswith increasing the content of silicon NPs from 0% to0.2%, 0.4% and 0.8% in the composite films, while that ofPVK is decreased from 7.24 ns to 5.24 ns, 5.90 ns, and4.19 ns, correspondingly. This is what expected in the pres-ence of FRET [25,26].

Forster critical distance can also be obtained from Fig. 5assuming the transfer rate is the same for all the PVK mol-ecules [25]:

nD;cðtÞ ¼ nD;pðtÞ exp �cptsD

� �1=2" #

ð3Þ

where. c ¼ c 43pR3

0

� �nD,c(t) and nD,p(t) are the best fit decay

curves of the donor in the composite films and pure films.sD is the PL lifetime of the donor. C is the concentration ofthe acceptor. The decay kinetics yields R0 = 47 A which isclose to the one (51 A) calculated by Eq. (2), thus strength-ening the deduction that FRET takes place from PVK tosilicon NPs in their composite films. The energy transferefficiency can be given by [24]:

ET ¼ 1� sDA

sD

ð4Þ

Table 1The PL lifetime values fitted from Fig. 5 and the values of ET, R, and kET

Silicon NPs mass loading sPVK sSi

(%) (ns)a (ns)b

0 7.24 s1 = 0.86, s2 = 2.30.2 5.94 s1 = 1.01, s2 = 2.70.4 5.20 s1 = 1.17, s2 = 2.90.8 4.19 s1 = 1.23, s2 = 3.16

a sPVK indicates the PL lifetime of PVK which is obtained from Fig. 5(1).b sSi indicates the PL lifetime of silicon NPs which is obtained from Fig. 5(2c savg is the average PL lifetime which is calculated from

Paisi and a is the

where ET is the energy transfer efficiency. sDA is the PL life-time of the donor in presence of the acceptor. The values of

ET R kET

(A) (107 s�1)

6, savg = 1.11c

6, savg = 1.44 0.18 61 3.028, savg = 1.65 0.28 55 5.37, savg = 1.91 0.42 50 11.10

).preexponential factor.

74 N. Liu et al. / Chemical Physics Letters 451 (2008) 70–74

ET are shown in Table 1 as 0.18, 0.28, and 0.42 for the com-posites with 0.2%, 0.4% and 0.8% silicon NPs mass load-ing, respectively. ET can also be given by [24]:

ET ¼R0

R

� �6

R0

R

� �6 þ 1ð5Þ

R can be calculated from (5) as 61, 55, and 50 A for thecomposites with 0.2%, 0.4% and 0.8% silicon NPs massloading, respectively (shown in Table 1). Thus, kET canbe given from (1) using the values of R obtained as 3.02,5.37 and 11.10 � 107 s�1.

4. Conclusions

PVK/silicon NPs composite films could be obtained viawet-processing method from PVK and silicon NPs. Forsterresonance energy transfer (FRET) in the composite filmswas studied by combining UV–vis, PL, PLE, and PLdecays, which revealed that FRET took place from PVK(donor) to silicon NPs (acceptor). The results indicate thatthe prepared composites have potential application in poly-mer/inorganic NPs hybrid light-emitting diodes.

Acknowledgements

We are grateful for the financial support of the NationalNatural Science Foundation of China (Grant Nos.50433020, 50673083 and 50520150165) and of the develop-ing program of Changjiang Scholar and Innovation Teamfrom Ministry of Education of China (Grant No.IRT0651). The authors also would like to thank the Fundfor the Major State Basic Research Development Program(2007CB613400).

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