polymer composite infrared sensors

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available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

Advanced carbon nanotube/polymer composite infrared sensors Basudev Pradhana, Ryan R. Kohlmeyera, Kristina Setyowatia, Heather A. Owenb, Jian Chena,* a

Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, 3210 N. Cramer St., Milwaukee, WI 53211, USA Department of Biological Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA

b

A R T I C L E I N F O

A B S T R A C T

Article history:

We demonstrate that both single-walled carbon nanotube (SWCNT) types and nanotube-

Received 21 October 2008

matrix polymer-nanotube (CNT–P–CNT) junctions have profound impact on electro-optical

Accepted 22 February 2009

properties of SWCNT/polymer composites. Composite IR sensors based on CoMoCAT-pro-

Available online 3 March 2009

duced SWCNTs (SWCNTsCoMoCAT) significantly outperform those based on HiPco-produced SWCNTs (SWCNTsHiPco). Higher semiconducting nanotube concentration in a SWCNT material is critical to enhance the photo effect of IR light on SWCNT/polymer nanocomposites, whereas CNT–P–CNT junctions play a dominant role in the thermal effect of IR light on supported SWCNT/polymer composite films.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Optoelectronic materials that are responsive at wavelengths in the near-infrared (NIR) region (e.g., 800–2000 nm) are highly desirable for various demanding applications such as telecommunication, thermal imaging, remote sensing, thermal photovoltaics, and solar cells [1]. Significant advances have been made recently in developing organic photoconductive materials that offer ease of materials processing and integration, low cost, physical flexibility, and large device areas; however, these organic optoelectronic materials have yet to demonstrate sensitivity beyond 850 nm [1–4]. Single-walled carbon nanotubes (SWCNTs) have strong absorptions in the NIR region owing to the first optical transition (S11) of semiconducting nanotubes [5–7]. The inverse diameter dependence of S11 optical transition energy enables the wavelength-tuning of SWCNT’s NIR absorptions, which, coupled with the rapid progress in nanotube separation [8], will ultimately allow the development of SWCNT IR sensors tailored to specific regions of the NIR spectrum.

There are a number of reports on the IR photoresponse in the electrical conductivity of individual SWCNTs [9–12], SWCNT films [13–15], and multi-walled carbon nanotube films [16]. Levitsky and coworkers demonstrated that the arc-produced SWCNT (SWCNTarc) film is capable of generating a very weak photocurrent upon continuous-wave IR illumination (12 mW/mm2) in the air at room temperature, and the current increase upon IR illumination is only about 0.2% [13]. In addition, they observed a dark current drift due to oxygen adsorption and a relatively slow rise/decay (4–5 s relaxation time) of the photocurrent in response to the on/off illumination. Itkis and coworkers reported that the IR photoresponse in the electrical conductivity of a SWCNTarc film is dramatically enhanced when the nanotube film is suspended in vacuum at low temperature [15]. For example, at 50 K, the SWCNTarc film suspended in vacuum shows a resistance drop of 0.7% under an extremely low incident power of 0.12 lW of IR radiation. Matsuoka and coworkers investigated the temperature dependence of photoconductivity in SWCNTarc films, and they found that photoconductive response increases by approximately 60-fold with a temperature decrease from

* Corresponding author: Fax: +1 414 229 5530. E-mail address: [email protected] (J. Chen). 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.02.021

room temperature to 10 K [14]. How to significantly increase the IR photoresponse of SWCNT films at room temperature in the air remains to be a great challenge. We have recently discovered that the IR photoresponse in the electrical conductivity of SWCNTs is dramatically enhanced by embedding SWCNTs in an insulating polymer matrix such as polycarbonate (PC) in the air at room temperature [17]. In contrast to the gradual photoresponse and weak conductivity change (1.10%) observed in a HiPco-produced SWCNT (SWCNTHiPco) film in the air at room temperature, the 5 wt% SWCNTHiPco/PC nanocomposite under the same IR illumination (power intensity: 7 mW/mm2) shows a sharp photoresponse and strong conductivity change (4.26%), which is twenty-one times of that (0.2%) observed in the pure SWCNTarc film in the air at room temperature (12 mW/mm2) [17]. The dark current drift owing to oxygen adsorption is also minimized by embedding nanotubes in the polymer matrix. There are numerous commercial SWCNT materials available, which are manufactured and purified via various processes. Monte Carlo simulations indicate that the tunneling resistance of nanotube-matrix polymer-nanotube (CNT–P– CNT) junctions plays a dominant role in the electrical conductivity of carbon nanotube (CNT) composites [18]. Despite tremendous research interest and effort in the field of CNT/ polymer composites [19,20], there is little experimental information in the literature about the influence of nanotube types and CNT–P–CNT junctions on electrical and electro-optical properties of CNT/polymer composites [21,22]. In this article we report that both SWCNT types and CNT–P– CNT junctions have profound impact on electro-optical properties of SWCNT/polymer composites. Composite IR sensors based on CoMoCAT-produced SWCNTs (SWCNTsCoMoCAT) significantly outperform those based on SWCNTsHiPco. The 5 wt% SWCNTCoMoCAT/PC nanocomposite demonstrates a very strong conductivity change of 23.45% upon the IR illumination (7 mW/mm2) in the air at room temperature, which is 5.5 times of that (4.26%) observed in the 5 wt% SWCNTHiPco/PC composite film (7 mW/mm2) [17], nearly 27 times of that (0.88%) observed in the pure SWCNTCoMoCAT film (7 mW/mm2), 21 times of that (1.10%) observed in the pure SWCNTHiPco film (7 mW/ mm2) [17], and 117 times of that (0.2%) observed in the pure SWCNTarc film in the air at room temperature (12 mW/mm2) [13].

2.

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Experimental section

Purified SWCNTsCoMoCAT (Lot#: UW1-A001) and SWCNTsHiPco (Lot#: P0279) were purchased from SouthWest NanoTechnologies Inc. and Carbon Nanotechnologies Inc., respectively, and were solubilized in chloroform with standard PPE along with vigorous shaking and/or short bath sonication [23–25]. The resulting SWCNT solution was then mixed with a PC solution in chloroform to produce a homogeneous SWCNT/PC composite solution, which was cast on a glass dish and dried very slowly to give a free-standing film after peeling from the substrate. SWCNT/PC composites with various SWCNT types and loadings were prepared according to the above procedure [17,21,24,27]. The pure SWCNT film was obtained from the PTFE membrane filter after filtration of a suspension of pris-

tine SWCNTs in chloroform. The mass ratio of PPE:SWCNT was kept at 0.4. The SWCNT loading values quoted in this communication are based on purified SWCNTs only, and exclude the PPE. The typical film thickness was in the range of 25–60 lm. The UV–Vis–NIR absorption spectra were recorded with a Cary 5000 UV–Vis–NIR spectrophotometer. SEM was performed using a Hitachi S-4800 field emission scanning electron microscope (accelerating voltage: 15 kV). No sample coating was used in the SEM experiment in order to avoid possible artifacts induced by the metal coating. The schematic IR sensor device structure is shown in Fig. 1a. Electrical contacts to the sample film were made with

IR light

(a)

(b)

(c) Absorbance (a.u)

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1.2

1.0 M11

PPE

0.8 S22

S11

0.6

0.4 300

600

900

1200

1500

1800

Wavelength (nm) Fig. 1 – (a) Schematic experimental setup. (b) SEM image of 5 wt% SWCNTCoMoCAT/PC composite film (scale bar: 1 lm). (c) UV–Vis–NIR spectrum of 5 wt% SWCNTCoMoCAT/PC composite thin film. M11, S11, and S22 represent optical transitions in metallic and semiconducting SWCNTs, respectively.

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silver paste. The sample film was kept on a ceramic plate with proper thermal contact and was annealed at 80 C for 24 h in order to reach the equilibrium state [17,21]. The two-point probe electrical conductivity measurement was performed using a Keithley 2400 source meter and Keithley 6485 picoammeter (for low current) instrument through the computer controlled LabVIEW program. The typical dc bias voltage of 20 mV–1 V was applied across the SWCNT composite film and the output current was monitored. The IR light source for SWCNTCoMoCAT/PC nanocomposites was a xenon light bulb (XNiteFlashSTG, LDP LLC) with a long-pass NIR filter (X-Nite850 filter; 850 nm cutoff at 50%; 950 nm passband >99%). The incident power of the IR radiation was measured using a Newport power meter model 1918-C with an IR detector (918D-IR-OD3). To collect the temperature dependent conductivity data, a nanocomposite film was heated to the desired temperature by digital hotplate and waited for 10 min to reach its equilibrium state before each temperature dependence conductivity measurement. The film temperature was measured by using a digital thermometer with a type-K thermocouple probe.

3.

SWCNTCoMoCAT/PC composite film shows characteristic absorptions corresponding to the first (800–1400 nm) and second (500–800 nm) optical transitions of semiconducting SWCNTsCoMoCAT (Fig. 1c) [22]. The relatively weak absorptions corresponding to optical transitions of metallic SWCNTsCoMoCAT overlap with the broad and strong p–p* transition band (505 nm) of the planarized PPE [25]. The photoexcitation of semiconducting SWCNTs with IR light leads to generation of excitons instead of free carriers [5–7,9,10]. The IR light have two main effects on the conductivity of SWCNTs: (1) Photo effect: excitons can be dissociated into free electrons and holes thermally or by a large electric field [5,14,15], which increases the conductivity of SWCNTs; (2) Thermal effect: excitons decay into heat and the strong warming of SWCNT reduces its resistance [15]. The overall conductivity change (IRtotal) upon IR illumination is therefore the sum of the conductivity change due to the photo effect (IRphoto) and the conductivity change owing to the thermal effect (IRthermal): IRtotal = IRphoto + IRthermal. The determination of IRphoto and IRthermal contributions is based on our previous method.17 The IRtotal, IRphoto, and IRthermal data are included in Table 1. In contrast to the weak photoresponse (0.88% change in conductivity) of the pure SWCNTCoMoCAT film upon the on/ off IR illumination (power intensity: 7 mW/mm2; on/off time period: 30 s; Table 1), which is characterized by the gradual rise and decay of the conductivity (Fig. 2a), the 5 wt% SWCNTCoMoCAT/PC nanocomposite demonstrates a very strong photoresponse (23.45% change in conductivity) upon the same IR illumination (Table 1), which is characterized by the sharp rise and decay of the conductivity (Fig. 2b). This finding further supports that the IR photoresponse in the electrical conductivity of SWCNTs is dramatically enhanced by embedding SWCNTs in an insulating polymer matrix such as PC in the air at room temperature [17]. Most interestingly, the 5 wt% SWCNTCoMoCAT/PC composite film drastically outperforms the 5 wt% SWCNTHiPco/PC nanocomposite in IRtotal (23.45% vs 4.26%), IRphoto (14.95% vs 3.53%), and IRthermal (8.50% vs 0.73%), respectively (Table 1 and Fig. 2c). The dependence of the photocurrent on the incident IR light intensity demonstrates a linear relationship as expected (Fig. 2d). The 5 wt% SWCNTCoMoCAT/PC composite film shows a detectable

Results and discussion

We developed a versatile, non-damaging chemistry platform that enables us to engineer specific CNT surface properties while preserving CNT’s intrinsic properties [17,21,23–30]. We discovered that rigid, conjugated macromolecules, poly(pphenylene ethynylene)s (PPEs), can be used to noncovalently functionalize and solubilize CNTs [23,25,28,29], and disperse CNTs uniformly in various polymer matrices [17,21,24, 26,27,30]. The resulting multifunctional CNT/polymer composites not only show excellent electrical and mechanical properties [21,24,26,27], but also are promising candidates for uncooled IR sensors and remotely controlled actuators [17,30]. SWCNTCoMoCAT/PC composite films were prepared according to a literature procedure (see Section 2) [17]. The fabrication of IR sensor devices (Fig. 1a) is described in the Experimental section. Scanning Electron Microscopy (SEM) shows the uniform dispersion of PPE-SWCNTsCoMoCAT in the PC matrix (Fig. 1b). The UV–Vis–NIR spectrum of the 5 wt%

Table 1 – The IR photoresponse in the electrical conductivity of SWCNT/PC nanocomposite films in the air at room temperature. Sample No.

p (wt%)a

SWCNT type

rdark (S/cm)

Dr/rdark (%) IRtotal

A B C D E F G a b c d

4 5 1 3 5 100 100

CoMoCAT CoMoCAT HiPco HiPco HiPco CoMoCAT HiPco

4

8.09 · 10 2.96 · 103 5.10 · 102 7.00 · 101 2.48 80 150

15.85 23.45 2.56 3.47 4.26 0.88 1.10

b

IRphoto 6.01 14.95 1.40 2.61 3.53 0.50 0.06

p (wt%) represents the SWCNT mass fraction. IRtotal represents the overall conductivity change upon IR illumination (power intensity: 7 mW/mm2). IRphoto represents the conductivity change due to the photo effect of the IR illumination. IRthermal represents the conductivity change due to the thermal effect of the IR illumination.

c

T1 (K) IRthermald 9.84 8.50 1.16 0.86 0.73 0.38 1.04

733.2 634.5 205 137.6 122 N/A N/A

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(a)

81.8

(b)

Pure SWCNTCoMoCAT film

σ x 10 (S/cm)

off

-3

σ (S/cm)

81.6 81.4 81.2 81.0 80.8 80.6

on 50

100

150

200

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3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0

250

5 wt% SWCNTCoMoCAT/PC off

on 50

0

100 150 200 250 300

Time (s) 5 wt% SWCNT

HiPco

σ/σdark

1.25 on

1.20 1.15 1.10 1.05

off

1.00 0

50

(d)

2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -1 0

100

150

200

Data Linear fit

-3

5 wt% SWCNTCoMoCAT

σ x 10 (S/cm)

1.30

Photocurrent (μA)

(c)

Time (s)

2.983 2.982

5 wt% SWCNTCoMoCAT/PC 2

23.4 μW/mm

2.981 2.980 2.979 2.978 30

1

2

3

4

5

60

90 120 150

Time (s)

6

7

2

8

Light intensity (mW/mm )

Time (s)

Fig. 2 – (a) Conductivity response of pure SWCNTCoMoCAT film to the on/off IR illumination (power intensity: 7 mW/mm2). (b) Conductivity response of 5 wt% SWCNTCoMoCAT/PC nanocomposite film to the on/off IR illumination (power intensity: 7 mW/ mm2). (c) Relative conductivity (r/rdark) response of 5 wt% SWCNTCoMoCAT/PC and 5 wt% SWCNTHiPco/PC nanocomposite films to the on/off IR illumination (power intensity: 7 mW/mm2). (d) Dependence of photocurrent of 5 wt% SWCNTCoMoCAT/PC nanocomposite on the IR light intensity. (Inset) Conductivity response of 5 wt% SWCNTCoMoCAT/PC nanocomposite film to the on/off IR illumination at 23.4 lW/mm2 light intensity.

contain more semiconducting nanotubes than SWCNTsHiPco. (4) The featureless absorption background in a UV–Vis–NIR spectrum of SWCNTs comes from the p-plasmon of metallic SWCNTs and carbon impurities [33]. The intensity of this background in the SWCNTHiPco solution is several times higher than that in the SWCNTCoMoCAT solution at the same concen-

2.0

Absorbance (a.u)

IR photoresponse at a light intensity as low as 23.4 lW/mm2 (Fig. 2d), which is 30 times lower than the lowest light intensity (700 lW/mm2) required by the 5 wt% SWCNTHiPco/PC nanocomposite [17]. The remarkable difference in IR photoresponse between the SWCNTCoMoCAT/PC nanocomposite and SWCNTHiPco/PC nanocomposite does not originate from the difference in IR absorptions between SWCNTsCoMoCAT and SWCNTsHiPco. In fact, the PPE-SWCNTCoMoCAT solution (0.02 mg/ml in CHCl3) shows a much weaker absorption than the PPE-SWCNTHiPco solution at the same concentration in the NIR region (Fig. 3), which explains the lower IRthermal (0.38%) in the pure SWCNTCoMoCAT film as compared with that (1.04%) in the pure SWCNTHiPco film (Table 1). We believe that the difference in semiconducting nanotube concentrations between SWCNTsCoMoCAT and SWCNTsHiPco is primarily responsible for the substantial difference in IRphoto (14.95% vs 3.53%) between the 5 wt% SWCNTCoMoCAT/PC composite film and the 5 wt% SWCNTHiPco/PC nanocomposite. This assessment is based on following considerations: (1) Our previous doping experiments demonstrate that semiconducting SWCNTs are critical to the IR photoresponse of CNT/ polymer nanocomposites [17]. (2) Jorio and coworkers found that SWCNTsCoMoCAT contain 91% semiconducting nanotubes whereas SWCNTsHiPco contain 80% semiconducting nanotubes [22,31,32]. (3) IRphoto (0.50%) in the pure SWCNTCoMoCAT film is much higher than that (0.06%) in the pure SWCNTHiPco film (Table 1), which indicates SWCNTsCoMoCAT

1.5

PPE-SWCNT HiPco

1.0

0.5

300

PPE-SWCNTCoMoCAT 600

900

1200

1500

Wavelength (nm) Fig. 3 – UV–Vis–NIR spectra of PPE-SWCNTCoMoCAT solution in chloroform (0.02 mg/ml) and PPE-SWCNTHiPco solution in chloroform (0.02 mg/ml). * indicates the artifact caused by chloroform.

4 7 ( 2 0 0 9 ) 1 6 8 6 –1 6 9 2

rdc / exp½T1=ðT þ T0 Þ

ð1Þ

where T1 represents the energy required for an electron to cross the insulator gap between nanotubes and T0 is the temperature above which the thermal activated conduction over the barrier begins to occur. Our previous analysis of temperature dependent conductivity data reveals that T0 is very small (<5 K) for uniform SWCNT/composites [21].

(a)

Side view

Top view

CNT

Polymer matrix

CNT 0.5

(b)

A, T1 =733.2 K B, T1 =634.5 K

0.4

C, T1 =205 K

295K

)

D, T1 =137.6 K

0.3

E, T 1 =122 K

T

tration (Fig. 3). Since various experimental evidences show that purified SWCNTsHiPco contain less carbon impurities than purified SWCNTsCoMoCAT [22,29], the much higher p-plasmon in the UV–Vis–NIR spectrum of SWCNTsHiPco suggests SWCNTsHiPco contain more metallic nanotubes than SWCNTsCoMoCAT. (5) We found that replacing 20% of SWCNTsCoMoCAT with SWCNTsHiPco in a 5 wt% SWCNT/PC nanocomposite (4 wt% SWCNTCoMoCAT and 1 wt% SWCNTHiPco) leads to a 5-fold drop in IR photoresponse (4.86% change in conductivity) as compared with the 5 wt% SWCNTCoMoCAT/PC nanocomposite (23.45% change in conductivity). The IR photoresponse (4.86%) of the (4 wt% SWCNTCoMoCAT + 1 wt% SWCNTHiPco)/PC nanocomposite is also much lower than that (15.85%) of the 4 wt% SWCNTCoMoCAT/PC nanocomposite (Table 1). This experiment supports that SWCNTsHiPco contain more metallic nanotubes than SWCNTsCoMoCAT and metallic nanotubes can significantly diminish the IR photoresponse of SWCNT/polymer nanocomposites. Monte Carlo simulations indicate that the tunneling resistance of CNT–P–CNT junctions plays a dominant role in the electrical conductivity of CNT composites [18]. Our previous study suggests that the electrically and thermally insulating polymer matrix may have a significant impact on the IR photoresponse mechanism of SWCNT/polymer nanocomposites [17]. The effect of CNT–P–CNT junctions on electro-optical properties of CNT/polymer composites, however, remains essentially unknown, which warrants further investigation. Direct imaging of CNT–P–CNT junctions has proven to be extremely challenging due to following reasons: (1) Li and coworkers’ simulations suggest that the maximum tunneling distance (i.e., the thickness of matrix polymer between two nanotubes, Fig. 4a) is 1.8 nm in CNT composites [18]. Based on Li et al.’s work and electrical conductivities of SWCNT/PC composites (Table 1), the tunneling distances in our SWCNT/ PC composites are estimated to be in the range of 0.5– 0.8 nm, which are beyond typical high-resolution SEM’s resolution (1 nm). (2) The diameters of the SWCNTs of CNT/polymer composites shown in SEM are significantly inflated because the SEM-image contrast stems from local potential differences between the conductive nanotube and the insulating polymer matrix [34,35]. (3) The extremely low contrast between SWCNTs (single layer of carbon) and the polymer matrix makes the high-resolution Transmission Electron Microscopy (TEM) imaging of SWCNTs embedded in the polymer matrix extremely difficult. (4) In order to get the high quality TEM images, the sample thickness has to be 50 nm or less. Microtoming of CNT/polymer composite samples has proven to align CNTs in the polymer matrix, therefore changing polymer gaps between neighboring CNTs [36]. The conductivity of CNT/polymer composites can be described by a thermal fluctuation-induced tunneling model [37,38] eveloped by Sheng and coworkers as follows:

ln( σ /σ

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3.0

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3.2

3.3

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3.5

-1

1000/T (K )

(c)

0.12

Data

0.10

ln(σ T(IR)/σ 295K(dark))

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Linear fit

0.08 0.06 0.04 0.02 0.00 -0.02 0

150

300

450

600

750

T1 (K) Fig. 4 – (a) Schematic representation of top view and side view of a CNT–P–CNT junction with a tunneling distance t. (b) Temperature dependences of ln(rT/r295 K) for five CNT/ polymer composites (A–E) listed in Table 1. (c) Plot of ln(rT(IR)/ r295 K(dark)) as a function of T1 for five nanocomposites (A–E) listed in Table 1.

We found that if T P 295 K, the small T0 can be ignored for the simplicity and Eq. (1) can be rewritten as follows [21]: lnðrT =r295K Þ ¼ T1 ð0:00339  1=TÞ

ð2Þ

The analysis of temperature dependent conductivity data (e.g. from room temperature to 343 K) allows us to extract the value of T1, which provides crucial information about CNT–P–CNT junctions.

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Fig. 4b demonstrates that the temperature dependent conductivity data of five CNT/polymer composites (A–E) listed in Table 1 are in good agreement with the prediction of Eq. (2). Solid lines are theoretical fittings to Eq. (2) with values of T1 given in Fig. 4b. Careful examination of Table 1 suggests there exists a strong correlation between IRthermal and T1, that is, IRthermal increases with an increasing T1. To better understand the effect of T1 on IRthermal, Eq. (2) can be expressed as: lnðrTðIRÞ =r295KðdarkÞ Þ ¼ T1 ð0:00339  1=TIR Þ

[5]

[6]

ð3Þ

where r295 K(dark) and rT(IR) represent the electrical conductivity of a CNT/polymer composite before and after the IR illumination, respectively, and TIR is the temperature of a CNT/ polymer composite upon IR illumination. Since the nanocomposite film was in good contact with a thick ceramic plate, which acted as a heat sink, we found that the TIR of nanocomposites (A–E) is in a very narrow temperature range of 306– 308 K upon 30 s of IR illumination. Therefore TIR in Eq. (3) can be considered approximately a constant in our experiment. As a result, Eq. (3) predicts an approximate linear relationship between ln(rT(IR)/r295 K(dark)) and T1, which has been confirmed by our experimental data (Fig. 4c). Therefore CNT–P–CNT junctions play a dominant role in the thermal effect of IR light on supported CNT/polymer composite films.

4.

[4]

Conclusions

In conclusion, we demonstrate that both SWCNT types and CNT–P–CNT junctions have profound impact on electro-optical properties of SWCNT/polymer composites. Composite IR sensors based on SWCNTsCoMoCAT significantly outperform those based on SWCNTsHiPco. Higher semiconducting nanotube concentration in a SWCNT material is critical to enhance the photo effect of IR light on CNT/polymer nanocomposites, whereas CNT–P–CNT junctions play a dominant role in the thermal effect of IR light on supported CNT/polymer composite films.

[7]

[8] [9]

[10]

[11]

[12] [13]

[14]

[15]

[16]

[17]

Acknowledgements [18]

J.C. thanks the financial support from the National Science Foundation (DMI-06200338), UWM start-up fund, and UWM Research Growth Initiative award. H.A.O. thanks the financial support from the National Science Foundation (CHE-0723002) for the acquisition of a field emission scanning electron microscope (Hitachi S-4800).

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