Chemical Physics Letters 380 (2003) 201–205

www.elsevier.com/locate/cplett

Optical limiting performance of two solublemulti-walled carbon nanotubes

Chao Li, Chunling Liu *, Fushan Li, Qihuang Gong

Department of Physics and State Key Laboratory for Mesoscopic Physics, Peking University, Beijing 100871, China

Received 27 June 2003; in final form 19 August 2003

Published online: 26 September 2003

Abstract

The optical limiting properties of two polymer-bound multi-walled carbon nanotubes (MNWTs), poly(N-vinylc-

arbazole)-MWNTs (PVK-MWNTs) and polybutadiene-MWNTs (PB-MWNTs), were investigated using 532 nm

nanosecond laser pulses. Both of them, especially PVK-MWNTs, showed much better optical limiting performance

than that of C60 in toluene solution. A nonlinear absorption mechanism was proposed and explanation was given on the

interesting difference of the optical limiting properties of these two soluble MWNTs.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

The search for new compounds for passive op-

tical limiters to protect sensors and eyes from in-

tense light pulses has attracted great attention

[1,2]. Among widely investigated materials for

optical limiting, some carboneous materials haveespecially been shown to be good candidates.

Fullerenes and their derivatives performed excel-

lent optical limiting properties for their larger

excited-state absorption [3,4]. Carbon black sus-

pensions undergo dramatic changes in transmit-

tance because of nonlinear scattering [5–7]. Since

they were discovered by Iijma in 1991 [8], carbon

nanotubes have been the focus of extensive studies

* Corresponding author. Fax: +86-10-62756567.

E-mail address: clliu@pku.edu.cn (C. Liu).

0009-2614/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.cplett.2003.08.078

for their unique physical properties and various

potential applications [9]. They have also been

considered as excellent broadband optical limiting

materials [10–13]. However, it should be noted

that many experiments were made on carbon na-

notubes suspended in liquids due to their poor

solubility in most solvents, and such suspensionswere unstable at high concentrations and high la-

ser intensity. Hence it is highly desirable to obtain

stable carbon nanotubes solutions for detailed

study on their optical limiting properties. More

recently, there have been a few reports on the

optical limiting properties about soluble carbon

nanotubes and the poor optical limiting properties

have been reported [14–16].In this Letter two polymer-bound multi-walled

carbon nanotubes (MWNTs) – poly(N-vinylcar-

bazole)-MWNTs (PVK-MWNTs) and polybuta-

diene-MWNTs (PB-MWNTs) were synthesized

ed.

202 C. Li et al. / Chemical Physics Letters 380 (2003) 201–205

and dissolved in tetrahydrofuran (THF) and o-dichlorobenzene (ODCB) respectively for optical

limiting behavior measurement. Compared with

that of C60 in toluene solution at same conditions,

the results show that both these two soluble

MWNTs possess better optical limiting propertiesthan that of their well-studied cousin, C60. A

nonlinear absorption mechanism was proposed for

the perfect fitting of the optical limiting results

using a phenomenological model. To our interest,

PVK-MWNTs shows better optical limiting re-

sponse than that of PB-MWNTs. The corre-

sponding limiting threshold of PVK-MWNTs is

about 1.7 times lower than that of PB-MWNTs.The superiority of PVK-MWNTs over PB-

MWNTs for optical limiting was attributed to the

stronger electron-donating competence of PVK

than PB.

Fig. 1. Schematic diagram of molecular structure of PVK-

MWNTs and PB-MWNTs.

Fig. 2. Linear absorption spectra of PVK-MWNTs in THF

(––––––), PB-MWNTs in ODCB (� � � � � �), MWNTs in ODCB

(- - - - - -) and C60 in toluene (�–�–�–�).

2. Experiments

PB-MWNTs was synthesized by the following

route: dissolving 400 mg of cis-1,4-polybutadiene

and 50 mg MWNTs in 50 ml of dry, degassed

cyclohexane under an N2 atmosphere at room

temperature. Then, 0.7 ml BuLi solution in hex-

anes (1.6 M) was added with stirring and to the

stirred solution tetramethylethylene-diamine

(TMEDA) was subsequently injected at a 1:1molar ratio with respect to BuLi (TMEDA was

used to enhance the efficiency of the metallation of

diene polymers). The reaction mixture was further

stirred at room temperature for 72 h. The reaction

was quenched by one drop of methanol. After

removing the residual insoluble materials through

centrifugation, the solution was then evaporated

to remove cyclohexane. The resulting solid wasdissolved in ODCB and purified by repeated pre-

cipitation from ODCB solution into hexane. The

resulting precipitation was then dissolved in

ODCB and filtered through a 0.45 lm polytetra-

fluoroethylene membrane, a homogeneous black

solution was obtained. PVK-MWNTs was also

synthesized by the similar grafting reaction of

PVK with MWNTs. More details of the prepara-tion and full characterizations will be reported

elsewhere [17].

The molecular structure of PVK-MWNTs and

PB-MWNTs are shown in Fig. 1. Their linear ab-

sorption spectra in the 200–1100 nm wavelength

region were recorded on an Agilent 8453 UV–vis-

ible spectrophotometer at room temperature. Fig. 2

shows the linear absorption spectra of PVK-MWNTs in THF solution, PB-MWNTs in ODCB

solution, MWNTs in ODCB suspension and C60 in

toluene solution, respectively. In the visible and

near-infrared region, the spectra of PVK-MWNTs,

PB-MWNTs and MWNTs are very similar and

there is not any absorption peak, which are differ-

ent from that of C60. However, compared with that

of MWNTs suspension, there are characteristicabsorption peaks for PVK-MWNTs and PB-

MWNTs in the ultraviolet region, which can be

Fig. 4. Comparison of optical limiting properties of PVK-

MWNTs in THF ðjÞ, PB-MWNTs in ODCB ðOÞ and C60 in

toluene ð�Þ with the same linear transmittance of 70% at

532 nm.

C. Li et al. / Chemical Physics Letters 380 (2003) 201–205 203

attributed to the polymers PVK and PB linked

covalently with MWNTs.

PVK-MWNTs in THF and PB-MWNTs in

ODCB solutions were contained in a 1-mm thick

glass cell for optical limiting measurements. The

concentrations of all the samples were adjusted sothat their linear transmittances are 70% at 532 nm.

For comparison, C60 in toluene solution with the

linear transmittance of 70% was also measured at

the same conditions. The optical limiting mea-

surements were performed with 10ns linearly

polarized laser pulses generated from a frequency-

doubled Quantel Q-switched Nd:YAG laser at 532

nm. The experimental setup is shown in Fig. 3. Thespatial profiles of the pulses were of nearly

Gaussian form after passing through a spatial fil-

ter. The laser beams were divided by a beam

splitter into two parts. The reflected part was

employed as the reference representing the incident

light energy and the transmitted beam was focused

by a 12 cm focal length lens L1. Both the incident

and transmitted pulse energies were measured si-multaneously by two energy detectors (Rjp-735

and Rjp-734, Laser Precision Corporation, USA).

The sample was placed at a place (before the focus)

where the spot diameter of the pulses was about

246 lm, which was measured by a razor method.

L2, a lens with large aperture was put before the

detector DB (Rjp-734) and the solid angle for the

collection of the transmitted light is about 120�. Inaddition, compared with the light-spot dimension,

the much larger acceptance area of the energy

probes ensured the collection of all the energy of

the pulse and thus the contribution of nonlinear

refraction and scattering was excluded. To avoid

the influence of thermal effect and get the major

Fig. 3. Experimental setup used for optical limiting measure

optical limiting mechanism, a single-pulse mea-

surement was used with pulse repetition every

thirty seconds so that each pulse of light was cer-

tain to encounter fresh molecules in the sample.

3. Results and discussion

The observed variations of output fluence with

input fluence for these samples are shown in Fig. 4.

It is evident that the optical limiting effects of

PVK-MWNTs and PB-MWNTs are stronger than

that of C60 at the same linear transmittance of

70%. Especially, PVK-MWNTs plays the bestperformance. Furthermore, the optical limiting

properties can be quantitatively compared by the

ments: (A) aperture; (L) lens; (S) sample; (D) detector.

Fig. 5. Fluence-dependent transmittance of PVK-MWNTs in

THF ðjÞ, PB-MWNTs in ODCB ðOÞ and C60 in toluene ð�Þ.

204 C. Li et al. / Chemical Physics Letters 380 (2003) 201–205

limiting threshold defined as the input fluence at

which the transmittance falls to 50% of the linear

transmittance. As shown in Fig. 5, the limitingthresholds of PVK-MWNTs, PB-MWNTs and C60

are 0.24, 0.40 and 0.70 J/cm2, respectively. The

limiting threshold of C60 is comparable to our

previous work with the collimated beam geometry

[18]. The limiting thresholds of PVK-MWNTs and

PB-MWNTs are about 2.9 and 1.8 times lower

than that of C60. This concludes that PVK-

MWNTs and PB-MWNTs have much better op-tical limiting response than that of C60, and also

are the best optical limiting materials among the

reported carbon nanotubes [10–16].

Optical limiting in carbon nanotube suspen-

sions has been well studied. They have been illus-

trated to be good optical limiting materials based

on the same nonlinear scattering mechanism with

that of carbon black suspension [19–22]. As faras scattering is concerned, Vivien et al. [20–22]

identified two mechanisms using pump-probe ex-

periments: solvent microbubble growth and subli-

mation of carbon nanotubes. However, the optical

limiting mechanism of soluble carbon nanotubes

has been proposed to be a nonlinear absorption

mechanism [14–16]. For example, Sun et al. [15]

synthesized aminopolymer modified solubilizedcarbon nanotubes and found their optical limiting

performance was weaker than that of C60 solution

for 532-nm nanosecond pulsed laser irradiation.

They considered that the observed optical limiting

of the solubilized carbon nanotubes is mainly re-

sulted from the nonlinear absorption of carbon

nanotubes but the polymer. Similarly, in our ex-

periment, both PVK and PB have no meaningful

optical limiting responses. So the observed opticallimiting effect can be attributed to the nonlinear

absorption of carbon nanotubes.

A phenomenological model [23] has been de-

veloped to describe the optical limiting perfor-

mance based on nonlinear absorption of C60 by

Golovlev et al. The absorption cross-section r is a

function of the laser fluence / and can be repre-

sented in the form of a series expansion in powersof /

rð/Þ ¼ r0 þ l1/þ l2/2 þ � � � ð1Þ

Taking the first two terms of the expansion and

substituting the rð/Þ expression into the Beer�s lawand integrating, the expression of output laser

fluence can be easily found:

/out ¼ T0/inp=½1þ ð1� T0Þ/inp=/nln�: ð2Þ

The corresponding transmittance is obtained asfollows:

T ¼ T0=½1þ ð1� T0Þ/inp=/nln�; ð3Þ

where T0 ¼ expð�r0N0LÞ is the linear transmit-

tance, /inp is the incident laser fluence, /out is the

transmitted laser fluence and /nln ¼ r0=l1 is the

parameter characterizing the nonlinear absorption

of the material. Obviously, in this model the op-

tical limiting performance of a sample can conve-

niently be assessed by only the value of /nln. Thesmaller magnitude of /nln denotes the better opti-

cal limiting performance of the sample.

The experimental results were fitted with Eq. (2)

for Fig. 4 and Eq. (3) for Fig. 5, as shown with

solid curves. It is obtained that the values of /nln of

PVK-MWNTs, PB-MWNTs and C60 are 85, 147

and 237 mJ/cm2. This result is coincident with the

conclusion drawn from the comparison of thelimiting thresholds. The excellent fittings of PVK-

MWNTs and PB-MWNTs indicate that the

observed optical limiting performance is mainly

resulted from the nonlinear absorption. It is well

known that carbon nanotubes are a p-conjugatedsystem. The bounded polymer with strong elec-

C. Li et al. / Chemical Physics Letters 380 (2003) 201–205 205

tron-donating or electron-accepting abilities can

increase the intramolecular electron transfer, and

thus can heighten the delocalization of the p-con-jugated system and probably improve the nonlin-

earities of carbon nanotubes. Furthermore, charge

transfer nature between the bounded polymer andcarbon nanotubes has been demonstrated by ESR

measurement [17]. So the superiority of PVK-

MWNTs over PB-MWNTs for optical limiting can

be attributed to the stronger electron-donating

ability of PVK than PB.

4. Summary

In conclusion, we investigated the optical lim-

iting performances of two soluble MWNTs –

PVK-MWNTs and PB-MWNTs at 532 nm with

10ns laser pulses and found that both of them had

better optical limiting effects than that of C60 un-

der identical conditions. A nonlinear absorption

mechanism based on a phenomenological model isrevealed and the superiority of PVK-MWNTs

over PB-MWNTs for optical limiting was attrib-

uted to the stronger electron-donating ability of

PVK than PB.

Acknowledgements

The carbon nanotube samples were provided

by Dr. Zhi-Xin Guo of Institute of Chemistry,

Chinese Academy of Sciences. The work was

supported by the National Natural Science

Foundation of China (Grant Nos. 10204003

10104003, 90206003 and 90101027), National Key

Basic Research Special Foundation of China un-

der Grant No. TG1999075207.

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