Texture and light-induced anisotropic terahertz properties of free-standing single-walled carbon nanotube films with random networks

Materials Chemistry and Physics 162 (2015) 743e747

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Texture and light-induced anisotropic terahertz properties of freestanding single-walled carbon nanotube films with random networks Xinlong Xu*, Zehan Yao, Yanping Jin State Key Lab Incubation Base of Photoelectric Technology and Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon-Technology, School of Physics, Northwest University, Xi'an 710069, China

h i g h l i g h t s
Texture formation was found in single-walled carbon nanotube random networks.
Modulation of anisotropy in the networks can be done by mechanical stretching.
The anisotropy can also be enhanced by light illuminating.
THz spectroscopy offers a way to estimate orientation order of the networks.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 January 2015 Received in revised form 23 June 2015 Accepted 29 June 2015 Available online 7 July 2015

We present an anisotropic property investigation of the free-standing single-walled carbon nanotube (SWCNT) random networks by terahertz time-domain spectroscopy (THz-TDS). Strongly anisotropic transmission in the spectral region 0.3e2.5 THz is observed. The experimental observation implies that our pristine SWCNT networks show preferential order in micrometer scale, which are attributed to the formation of the texture pattern. Modulation of this exceptional anisotropy by mechanical stretching and light illuminating indicates that the anisotropic response can be enhanced. The light-induced anisotropic property shows a hyperbolic tangent dependence with the pump power. The results suggest the potential applications and manipulations of SWCNT networks for THz electro-optical and polarized devices. © 2015 Elsevier B.V. All rights reserved.

Keywords: Nanostructures Thin films Infrared spectroscopy Optical properties

Single-walled carbon nanotubes (SWCNTs) as intrinsically anisotropic materials have stimulated much attention due to their unique physical properties and their potential applications in electro-optical devices [1]. Spectroscopic techniques employed microwave, far-infrared [2,3], terahertz [4e6], optical absorption/ reflection spectroscopy [7e10] and Raman spectroscopy [11,12] have confirmed that the remarkable polarization dependence of the absorption, transmission, and resonant scattering in the inplane and vertically aligned SWCNT films, or the selectively isolated SWCNTs. These reported results have indicated that the polarized absorption or scattering signal exhibit a maximum when the light is polarized along the nanotube axis, while it is strongly suppressed as the polarized orientation is rotated from parallel to perpendicular. Theoretical studies [11,13] have also corroborated that the anisotropic properties come from the antenna effect due to

* Corresponding author. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.matchemphys.2015.06.050 0254-0584/© 2015 Elsevier B.V. All rights reserved.

the one-dimensional characteristic of SWCNT. This means one isolated SWCNT can act as a dipolar antenna polarized along the tube axis and more than one nanotube in aligned films or fibers can behave as a complex multipolar antenna [11]. On the other hand, the electrical transport [14], thermal transport [15], shear stress [16] and charged induced distortion [17] in aligned SWCNT films or in suspensions also show the anisotropic response. With regard to the disordered SWCNT films or networks, few experiments show the anisotropy is relatively high. According to the optical theory [18] with the effective medium assumption [19], a mixture of randomly oriented compositions with SWCNT becomes evidently averaged in all orientations, reducing the anisotropic information that might be obtained by macroscopic probes when the wavelength l and the unit dimension d satisfy the restriction of d≪l. In this paper, orientation-dependent measurement by THz time-domain spectroscopy (THz-TDS) reveals that our pristine disordered SWCNT networks grown by chemical vapor deposition technique can show high anisotropy ascribed to the formation of

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texture pattern. When the preferential texture axis and the polarization of THz pulse wave are parallel, the transmission is low. While in the perpendicular orientation, the transmission is high. This exceptional anisotropic response can be enhanced artificially by mechanical stretching and 800 nm light illuminating. The lightinduced anisotropic property shows a hyperbolic tangent dependence with the pump power. These indicate that THz-TDS provides a convenient and sensitive measurement for the evaluation of the order degree of SWCNT networks. Furthermore, a sound understanding of the anisotropic properties of the SWCNT networks in THz region is essential for applications of SWCNT networks as polarization-sensitive and electro-optical devices. In the past decade, many efforts have been contributed to synthesize large-sized SWCNT materials for filter and sensor membranes, electromagnetic shields, field emission displays and so on by easy methods [20]. Our large-sized samples were synthesized by the method of optimum floating chemical vapor deposition technique as described in reference [21]. The area of the pristine largescale SWCNT non-woven material can reach several tens of square centimeters. These large pieces of SWCNT networks can be handed and manipulated easily. Simple purification including oxidation in air and treatment with concentrated HCl without destroying the networks’ compact structure was performed to eliminate impurities in the original SWCNT non-woven material. Scanning electron microscopy (SEM. S-5200) image of the SWCNT networks is shown in Fig. 1 with the scale bar of 300 nm (a), of 500 nm (b), and of 1 mm (c). It is evident that the SWCNT networks are highly entangled with each other, resulting in a highly disordered freestanding films. The diameter of the carbon nanotubes in the bundles is about 1e2 nm and the diameter of the assembled bundles is about 30 nm. Our home built THz-TDS system is similar to the experimental setup in references [22,23]. Ti-sapphire femtosecond laser (Spectra-Physics Laser Inc.) possesses the physical parameters as follows: a central wavelength of 800 nm, a repetition rate of 82 MHz, a pulse width of 100 fs, and an average power of 0.70 W. The laser pulses are divided into three beams by beam splitters, one beam for generating THz pulse wave using the transient optical

rectification effect in GaAs (110), the second beam for probing THz pulse wave by the linear electro-optical effect in ZnTe (110), and the third beam for illuminating samples to demonstrate the light influence. Using polarization detection and changing the motorized delay-line between the generating and probing beams, we can obtain the whole electric component of the THz pulse wave. All the optical elements from generation to detection of THz pulse wave should be encapsulated in a vacuum chamber to eliminate the influence of the water vapor in the air [24]. The free-standing samples are mounted into a metallic sample holder with a central hole of 1 mm diameter. The metallic sample holder can also guarantee the same diffraction condition at the cases with the sample and without the sample. The sample holder is then placed between two parabolic mirrors and can be rotated along the normal direction of the samples. All our experiments are done at the normal incidence. To demonstrate the anisotropic response of the samples, the samples are rotated along the THz wave vector k axis, while we monitor the peak transmission of the THz pulse wave. Fig. 2(a) presents an angular dependent transmission of the peak of THz pulse wave. This response should come from the projection of the electric field E onto the preferred texture axis of the SWCNT networks. It is noted that the SWCNT networks observed in SEM (Fig. 1(a)e(c)) is in a random orientation on the scale of several hundred nanometer. But to our surprise, the sample shows exceptionally anisotropic response in Fig. 2(a) and the ratio of T⊥ =Tk is as high as 2.5. This value is higher than the results obtained by Jeon et al. [4,25], who aligned SWCNT on a bar coater with a simple mechanical squeezing. According to Ajiki and Ando's theoretical assumption [26,27], the absorption ratio of light polarized parallel versus perpendicular to the tube axis is up to about 20 times for a single carbon nanotube and our result could be reasonable. The difference with Jeon's results would come from two aspects. One is the influence of the substrate (our sample is freestanding), and the other comes from the different sample synthesis and preparation. We also tested our other random networks by controlling carefully the preparation condition and we got the similar results. As the network pattern would show the preferential texture axis, all the individual SWCNT can project onto the

Fig. 1. SEM image of random SWCNT networks with the scale bar of 300 nm (a), scale bar of 500 nm (b), and scale bar of 1 mm (c). (d) Fourier transformation of SEM image of (c) to the moment space, which suggests the texture generation.

X. Xu et al. / Materials Chemistry and Physics 162 (2015) 743e747

(a) A

Normal Left 15o

(b) 120

90

Right 15o

60 30

150

180

B

0 360

330

210 240

C

270

300

Fig. 2. (a) Angular dependence of the transmission of the peak of THz pulse wave through the SWCNT networks. A, B, C are the position, at which we have recorded the whole THz waves as shown in Fig. 3(b). (b) Polar diagram of the angular dependence of the transmission of the peak of THz pulse wave through the SWCNT networks, when the incident wave is at normal, left-rotated 15 and right-rotated 15 to the sample plane.

prefectional axis or onto the perpendicular axis. When the preferential texture axis and the polarization of THz pulse wave are parallel, the transmission is low at the position C in Fig. 2(a). While in the perpendicular orientation, the transmission is high at the position A in Fig. 2(a). This would induce the angular anisotropy for the terahertz transmission as shown in Fig. 2. Fig. 2(b) shows the polar diagram of the angular dependent anisotropy of another sample with the incident angle normal, leftrotated 15 and right-rotated 15 to the sample plane. They exhibit the same anisotropic response. A reasonable explain to this phenomena is that although the sample shows disordered in a small scale, in a large scale such as in several hundred mm scale (in our experiment, the wavelength l1THz z300mm), the statistical distribution will emerge and the sample will show a preferentially macroscopical orientation to some degree. This means the SWCNT random networks are the topological disordered in small scale but show preferentially statistic order in large scale. This is similar to the texture formation in liquid crystalline [28] as well as in polycrystal materials [29], which are important in many fields of material science. Preferred orientation or texture forms usually occurs in material growth or deformation, which can also modified by phase transformations or recrystallization [28]. Texture characteristic in our SWCNT networks could come from the influence of the sample preparation such as mechanical influence or gas flowing in the sample synthesis. As the texture characterization are usually done with x-ray diffraction in the moment space in polycrystalline materials, we transform SEM image (Fig. 1(a)) with a Fourier transformation to a moment space, which demonstrates a dispersive bright spot in the center (Fig. 1(b)), suggesting the texture generation. Similar to analysis with the absorption in in-plane aligned SWCNT in polymer [30], the experimental data in Fig. 2(a) can be expressed as:

Zp=2 TðqÞ ¼ a

745

nanotube. f ð4Þ is a Gaussian angular distribution function. b is a constant to describe the background transmission. The adjustable fitting parameters a, b and the width of the angle-distribution function can be used to reproduce the experimental result well. Our fitted full width at half maximum of the obtained angle distribution is about 82 . This means that the statistical distribution shows the preferentially orientation to a certain degree. The transmission of the peak of THz pulse wave only reflects the average transmission properties of the sample. To view the frequency-dependent transmission, total input and output waves should be recorded. A typical input THz pulse wave (Fig. 3(a)) and the output THz pulse waves passing through the position A, B, C (Fig. 3(b)) are recorded in sequence. The marked A in Fig. 2(a) and Fig. 3(b) means the preferential orientation axis is perpendicular to the polarization of the input THz pulse wave. While the labelled C presents the parallel orientation between the preferred axis and the polarization of the input wave. B is the case when the included angle between the preferred orientation axis and the polarized wave is 45 . It is interesting to see that the output waves change in shape and amplitude compared with the input wave and this change comes from the absorption and dispersion variation of the sample. On the other hand, the output waves also change with the angle between the preferential orientation axis and the polarization of the input wave, which is consistent with the results in Fig. 2(a). Fig. 3(c) corresponds to the raw optical transmission spectroscopy T∞ expðndsÞ, for THz pulse waves polarized with the perpendicular, 45 tilt and parallel along the preferred axis in the spectral region 0.3 THz ~ 2.5 THz. Here, n is the density of nanotubes, d is the thickness of the samples and s is the effective cross-section (absorption plus scattering). Similar to in-plane aligned SWCNT films, the electron can move easily along the preferred orientation axis and the transmission is low when the polarization of THz pulse wave is parallel to the preferential orientation. While the polarization of the wave and the preferential orientation are perpendicular, the electrons can not move smoothly and the transmission is high. To enhance the orientation order of the SWCNT materials may be the key point for the application of SWCNT materials. Some methods such as mechanical shear [31], uniaxial pressure [32], anisotropic flow [33], magnetically alignment [34], laser ablation [35] etc have been put forward. Mechanical stretching may be a simple method to get the partially aligned samples. Fig. 4 shows the sensitive anisotropic response when the SWCNT networks are stretched artificially. After stretching, the anisotropic response is enhanced efficiently (up to ~150% peak-to-peak). This suggests

8 (a)

0.3 Peak of THz wave

A B C

(c)

0

0.2

-8 2

A B C

(b)

0

0.1

-2 f ð4Þcos2 ðq  4Þd4 þ b;

(1)

1

2

3

0.8

1.6

2.4

0.0

p=2

Where q is the angle between the preferred orientation axis and the polarization of the incident THz wave and 4 is the orientation distribution angle between the preferred axis and the axis of every

Fig. 3. (a) Input THz pulse wave. The arrow on the wave indicates the peak of THz pulse wave. (b) Output THz pulse waves passing through the sample with the orientation same as A, B, C denoted in Fig. 2(b). (c) The transmission spectra of the output waves at A, B, C.

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A

1.5

Original Streched

1.0 0.5 0.0 -0.5

C

-1.0 0

60

120

180

240

300

360

Fig. 4. Angular dependence of the transmission of the peak of THz pulse wave through original (square-dotted line) and stretched (circle-dotted line) SWCNT networks. The amplitude is normalized to show clearly.

more SWCNT aligned along the uniform orientation. Mechanical stretching can reform the network pattern and put more SWCNTs along the preferential texture axis. As the transmission of THz wave is proportional to the T∞ expðndsÞ as described above, with the increasing of the density of nanotubes n along the preferential axis by mechanical stretching, this results in the enhanced angular anisotropy for the terahertz transmission as shown in Fig. 4. The transmission is low at the position C in Fig. 4 as the polarization of THz pulse wave are parallel to the preferential texture axis. While in the perpendicular orientation, the transmission is high at the position A in Fig. 4. Due to the inhomogeneous deformation, the anisotropic response curve after pulling (circle-dotted line in Fig. 4) does not show the symmetry with the angle and this does not obey the Gaussian angular distribution function as described in Eq. (1). Even though, THz-TDS can help us to quickly estimate the degree of the alignment by stretching. In order to further understand the interaction mechanism between the light field and the SWCNT networks, we illuminated the SWCNT networks to bring the photoexcited extra electrons or holes. The pump power is changed from 0 w to 0.28 w, as is shown in Fig. 5(a) and this power level will not bring the selective ablation to our samples [35]. The beam diameter of the pump light is about 1 mm. There are approximately two kinds of optical processes in the SWCNT networks: promotion of electrons from one orbit to another in the SWCNTs or motion of free carriers from one place to another. The extra carriers injected into an SWCNT will bring the

1.2

(a) A

0.8

(b)

0 0.05 0.10 0.21 0.28

0.4 0.0 -0.4 -0.8

C 0

120

240

360

Fig. 5. (a) Enhancing the modulation of the anisotropic responses by lightilluminating. The power is changed in sequence 0, 0.05, 0.10, 0.21, 0.28 W. (b) Peakto-peak modulation amplitude versus pump power. The solid line is a fitting result using Eq. (2).

adjusting of carbonecarbon bonds length, anisotropic deformations, and also providing the basis for electromechanical actuators as Gartsein et al. discussed [17,36]. Fig. 5(a) shows that this extra injection enhances the anisotropic response of the SWCNT networks. The inherent anisotropy of the SWCNTs can be preferentially photoexcited as the polarization of the pump beam is along the preferential texture axis [37]. The resulting carrier population predominates on the preferential texture axis and thus will enhance the anisotropy. This demonstrates the decreasing of the transmission at the position C of Fig. 5(a) and increasing the tranmission at the position A in Fig. 5(a). On one hand, the adjustment of the carbonecarbon interaction by excitation would influence the distribution of the SWCNT in the networks. On the other hand, photoexcitation would introduce extra carrier number n. The carrier may not move equivalently along all directions. Along the preferred axis, it is easy, whereas it is hard when it moves perpendicular to the preferred axis. This in turn will also bring the enhancement of the anisotropic response. The increasing n will be reflected in the transmission T∞ expðndsÞ. Define the peak-topeak modulation M as the difference of the positive and negative extreme of the angular dependency curve (Fig. 5(a)). Combined with Eq. (1), we can deduce that the peak-to-peak modulation amplitude versus pump power can be written as:

M ¼ B þ C tanhð  ndsÞ:

(2)

Where tanh is a hyperbolic tangent function. B and C are the constants. n is the density of nanotubes, which is increasing linearly with the pump power. d is the thickness of the samples and s is the effective cross-section. This formula is similar to the peak-to-peak modulation with the magnetic field [20]. Fig. 5(b) gives the power dependence of the peak-to-peak modulation and the solid line is the fitting result by Eq. (2). As the pump power is increased from zero, a significant enhanced modulation of the peak-to-peak signal develops. This characteristic can be applied to make controllable Feussner-type polarizers in THz region. Feussner-type polarizers in effective polarization [38] usually use organic films. However, inorganic material such as anisotropic carbon nanotube films [9,30] may be robust to meet different environment. Recently, the anisotropic dynamic dielectric response of aligned SWCNT has been observed with optical-pump THz-probe spectroscopy [39] and an ultrafast THz polarization modulator has been demonstrated [37]. Kono et al. also has developed a broadband THz polarizers with highly aligned SWCNT [40]. All these research suggest that the potential applications and manipulations of SWCNT for THz electro-optical and polarized devices both in a static method and in a dynamic method. In conclusion, anisotropic characterizations of the random and free-standing SWCNT networks are measured using THz-TDS. We have observed obviously structural anisotropy, which could come from the formation of texture pattern due to the preferentially statistical distribution. The anisotropic response can be enhanced by mechanical stretching and light illuminating. Specially, these observations have great potential applications in THz region. For example, the anisotropic response of SWCNT networks in THz region can lead to the Feussner-type polarized optical devices for THz community. SWCNT networks can also be used as attenuation plates for strong THz radiation source. Moreover, THz-TDS offers a simple, effective, and rapid way to estimate orientation order of the random SWCNT networks. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 11374240), Key Laboratory Science Research Plan

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