Journal of the Korean Physical Society, Vol. 68, No. 2, January 2016, pp. 210∼214
Polarization-selective Alignment of a Carbon Nanotube Film by Using Femtosecond Laser Ablation S. B. Choi Department of Physics, Incheon National University, Incheon 22012, Korea
C. C. Byeon School of Mechanical Engineering, Kyungpook National University, Daegu 41566, Korea
D. J. Park∗ Department of Physics, Hallym University, Chuncheon 24252, Korea
M. S. Jeong† Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea (Received 26 November 2015, in final form 9 December 2016) We report on a considerable alignment of single-walled carbon nanotubes (SWCNTs) in randomly oriented bundles, by using a simple drop-and-dry method and irradiation with high-intensity femtosecond laser pulses. A remarkable third-harmonic generation was observed after irradiation with the laser pulses, whereas a narrow-band white-light continuum was generated in the as-prepared films. This observation, combined with scanning electron microscopy images, confirmed the high degree of alignment of the SWCNTs. In contrast to the pulsed irradiation of carbon soot, the powerdependent laser irradiation of a highly-purified SWCNT film show polarization-dependent ablation of individual nanotubes caused by polarization-dependent absorption. Raman spectroscopy results confirmed the presence of fractured nanotubes caused by the ablation processes. Polarizationresolved absorption spectroscopy results revealed that the aligned SWCNT film had potential usage in optical polarizers. PACS numbers: 42.25.Ja, 42.62.Eh Keywords: Polarization, Carbon nanotubes, Laser ablation DOI: 10.3938/jkps.68.210
I. INTRODUCTION
applications in optical polarizers operating in the visible frequency region, the average spacing between neighboring CNTs should be kept within the wavelength of the incident light, i.e., a few hundred nanometers. Various methods have been introduced to achieve wellaligned CNTs, such as using polymer-CNT composites [7], chemical and mechanical self-assembly processes, or applying mechanical forces on vertically-aligned CNT arrays [8]. Even though those methods demonstrate excellent alignment, they have limitations in real applications. For example, the methods using polymer-CNT composites deteriorate the electrical and the optical properties to a certain extent, and the methods using verticallyaligned CNT arrays are limited in the freedom of choosing the average spacing between individual CNTs so that the design of an optical polarizer becomes unsatisfactory. In this report, we propose a simple drop-and-dry method to fabricate well-aligned single-walled CNT films from randomly-oriented CNT bundles. The polarization-
The one-dimensional geometrical shape, together with the excellent electrical property of high conductivity, is the basic nature of carbon nanotubes (CNTs) [1]. These properties are attractive for the design of nanoscale electrical devices [2] using a single CNT as a functional material for switching [3,4] and for charge-transfer layers in solar cells [5]. Additionally, recent interests have arisen regarding the application of CNTs to optical polarizers that may operate in a broad spectral range, especially in the THz frequency region [6]. However, difficulties in realizing well-aligned nanotubes on a nanometer scale prevent such practical applications. Hence, the development of methods for the ordering of individual CNTs along the desired direction is essential. Moreover, for ∗ E-mail: † E-mail:
[email protected] [email protected]
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Fig. 1. (Color online) Experimental set-up (a) Experimental setup for laser ablation of a random CNT-network film. Incident laser radiation comes from a femtosecond regenerative amplifier with a 1.27 mm central wavelength and a 1-kHz repetition rate. A part of the laser pulse is diverted to an optical power meter to monitor the incident intensity. A translation stage moving the sample slowly in the lateral direction is used to avoid any thermal damage to the sample. (b−c) Schematic presentation of the sample preparation. (b) Singlewalled CNT films are dispersed by using ultrasonic agitation in a deionized water and IPA solution. (c) This solution is dropped onto a sapphire plate and dried to form the CNT network film.
dependent absorption of CNTs enabled their selective ablation by irradiation with strong, ultrafast laser pulses emitted from an optical parametric amplifier. Scanning electron microscopy (SEM) images and polarizationdependent absorption spectroscopy confirm the considerable alignment of individual CNTs.
II. MATERIALS AND METHODS The pristine CNT samples used in this experiment were synthesized by using arc discharge (Hanwha Chemical) and the thick CNT film were prepared by the simple drop-and-dry method. Single-walled CNTs were dispersed via ultrasonic agitation for 1 h in deionized water and isopropyl alcohol (IPA). This as-prepared solution was then dropped onto a sapphire substrate and dried to remove water and IPA (drop-and-dry method). By repeating this process several times, a sufficiently thick CNT bundle was prepared (see Fig. 1(b) and (c)). This sample was irradiated with ultrafast laser pulses having an ∼80-fs pulse duration and a 1.27 μm central wavelength emitted from an optically parametric amplifier driven by a Ti:Sapphire regenerative amplifier with a 1kHz repetition rate. During irradiation, the laser pulses were focused lightly to have a diameter of 30 μm by
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Fig. 2. (a) SEM image of the as-prepared CNT film. (b) Luminescence spectrum of the CNT film irradiated with highintensity femtosecond laser pulses. A clear signature of whitelight continuum is observed. (c) SEM image of the aligned CNT film after polarized femtosecond laser pulse irradiation. The inset shows a high-resolution transmission electron micrograph revealing nanometer-sized particles attached to an individual nanotube. (d) Third-harmonic spectrum after the CNT alignment is finished.
using a conventional lens with a focal length of 10 cm and the sample was slowly translated to avoid complete burnout. We assume that the spatial shape of the beam at the sample position should have a uniform Gaussian distribution. The intensity of the irradiating laser pulses was monitored by an optical power meter. To monitor the optical response of the CNT bundle during irradiation, we analyzed the light scattered from the sample was analyzed by using a spectrometer equipped with a CCD camera.
III. RESULTS AND DISCUSSION Figure 2(a) shows a SEM image of the prepared thick pristine CNT film without a substrate. Due to the low purity of the sample, individual nanotubes are not observable in this image [9]. During laser irradiation having a polarization along the horizontal direction in the image (see white arrow in figure) with a power of 50 mW (50 μJ per pulse), corresponding to a fluence per pulse of 6 J/cm2 , radiation from the sample was observed by using a spectrometer, and the recorded optical spectra are shown in Figs. 2(b) and (d). Remarkably, the spectra undergo considerable changes with increasing irradiation time. For a short irradiation time, a white-light continuum having a center at 600 nm with an ∼200 nm width is generated, together with a distinct peak at ∼420 nm which is related to third-harmonic generation (Fig. 2(b)). Here, the white-light continuum is assumed to be attributed to a laser-induced incandescence associated with
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invasive burnout of individual CNTs [10], and the third harmonic generation may originate from the highly nonlinear optical properties of individual CNTs [11]. No second-harmonic generation was observed because the intrinsic nonlinear properties of the CNT do not contain a meaningfully large second-order tensor component, as reported previously [11,12]. On the contrary, as the irradiation time is increasesd, the broad spectral distribution associated with the white-light continuum is completely removed, and only the sharp peak associated with third-harmonic generation remains (Fig. 2(d)). This observation implies that the strong laser pulses introduce ablation of nanotubes, which yields a white-light continuum associated with plasma radiation [13]. Moreover, the fact that the white-light continuum disappears after a certain irradiation time indicates that the nanotubes that absorb the laser radiation are completely ablated from that moment onwards. To confirm this assumption, a SEM image was taken after the white-light continuum had disappeared, as depicted in Fig. 2(c). Clear lines along the up-down direction are observed, which denote that only those nanotubes that are aligned along the left-right direction of the image absorbed the pulsed laser radiation and were finally ablated. Also, the period of the line pattern was roughly measured as ∼600 nm, which is half of the incident laser wavelength. Such period is suspected to be attributed to the formation of a standing wave at the CNT surface, which resembles the results reported in refs. [14] and [15]. The high-resolution transmission electron microscopy image depicted in the inset of Fig. 2(c) shows a few nanometersized particles attached to a CNT. These particles are thought to be either carbon particles generated during the ablation process or amorphous carbon synthesized along with the CNTs during the synthesis process. Motivated by this result, we prepared a high-quality CNT film on a sapphire substrate with better dispersion and higher purity [9] by using the spraying method. In this method, high-purity CNTs are dispersed in IPA and exposed to ultrasonic agitation for ∼30 min. This solution is then sprayed onto the sapphire substrate by using a spray gun. As evidenced by the SEM image depicted in Fig. 3(a), the film prepared by this method shows high quality without any signature of carbon impurities. To elucidate the ablation mechanism, we investigated the film after irradiation with laser pulses under the same condition, as those applied when obtaining Fig. 2, except that the pulse energy was lowered to 2 J/cm2 by adjusting the neutral density filter in experimental setup. As shown in the SEM image of this film depicted in Fig. 3(b), those CNTs that are aligned along the direction of the incident polarization are broken into pieces due to the larger absorption whereas other CNTs that aligned perpendicular to the laser polarization are not damaged. Another interesting point is that no distinct feature of periodic patterns is observed in this irradiation intensity, which means that the absorption of light in an individual nanotube may possess
Fig. 3. (a) SEM image of the as-prepared CNT film produced by using the spraying method. (b) SEM image of CNT film after laser pulse irradiation at a relatively low energy. (c) SEM image after laser pulse irradiation at a higher intensity. (d) SEM image of carbon soot after laser pulse irradiation.
more importance in ablation than the variation on the intensity distribution due to the interference in the lowintensity region. This confirms that the polarizationdependent absorption is responsible for the high degree of alignment of the individual CNTs inside of the film [16]. Additionally, for laser powers less than 2 J/cm2 , such alignment was not observed, which denotes that the threshold for polarization-selective alignment was 2 J/cm2 . At a larger pulse energy of 6.2 J/cm2 , however, even those CNTs aligned perpendicular to the incident polarization also ablated as shown in Fig. 3(c). This indicates a damage threshold, where complete destruction of CNT rather than polarization selective alignment, is ∼6 J/cm2 . In addition to those results on the CNT networks, such alignment is not manifested in carbon soot, as revealed by the SEM image shown in Fig. 3(d), because polarization-dependent absorption does no occur. To investigate the change in the chemical and the structural properties of the remaining CNTs during laser irradiation, we performed confocal micro Raman scattering measurements for pristine and ablated CNT films and the results are presented in Fig. 4. In the case of laser irradiation at a low intensity of 25 mW (Fig. 4(a)), the D band evolved, originating from fragments of ablated CNTs. However, the G and the G bands show no significant changes, which denotes that the aligned CNTs are only slightly damaged by laser irradiation. At a higher laser intensity of 55 mW (Fig. 4(b)), however, the G band decreases distinctly, which indicates that the remaining aligned CNTs starts to be seriously damaged at this intensity level, which agrees well with the previous SEM observation depicted in Fig. 3(d). To test the suitability of these aligned CNT arrays
Polarization-selective Alignment of a Carbon Nanotube Film · · · – S. B. Choi et al.
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their successful application in broadband, achromatic polarizers, which can be used in the optical frequency region. The wavelength-integrated absorption as a function of the incident polarization angle is also depicted in Fig. 5(b). A good cosine-like variation in the absorbance is observed, which confirms the polarization-sensitive absorption of our film. The rather small visibility value may originate from the reduced absorption contrast between different polarizations in wavelength regions beyond that displayed in Fig. 5(a). We believe that such unidirectionally-aligned CNT films can be used as optical polarizers in the visible range even though the visibility needs to be further improved. Fig. 4. (Color online) (a) Raman spectra of the asprepared CNT film (red) and of the film after irradiation with a 25-mW pulsed laser (blue). The D band is pronounced due to broken individual CNTs, but the G and the G bands show no significant difference after irradiation. (b) Raman spectra of the as-prepared CNT film (red) and of the film after irradiation with a 55-mW pulsed laser (blue). Unlike case (a), the G band shows a significant decrease, which indicates that the aligned CNTs suffer structural damage due to the high laser intensity.
Fig. 5. (Color online) (a) Absorption spectrum of a pure CNT film (black curve), together with the polarizationresolved absorption spectra of an aligned CNT film for hpolarization (red curve) and v-polarization (green curve). (b) Total absorption of aligned CNTs as a function of incident polarization.
for optical polarizers, we performed polarization-angleresolved absorption spectroscopy. As shown in Fig. 5(a), pure CNTs show some characteristic peaks corresponding to different chiralities of the individual CNTs in the film, along with a gradual overall increase in the absorption with increasing photon energy (black curve). For the aligned CNT film, remarkably, the absorbance is considerably different for different incident polarizations. Important is the observation that the absorbance is almost two times higher for h-polarization (parallel to the CNT alignment) than for v-polarization (perpendicular to the CNT alignment) throughout almost the entire investigated spectral region. Such a highly anisotropic optical property strongly suggests that the polarizationselective absorption of aligned CNT films may lead to
IV. CONCLUSION In conclusion, we have demonstrated the successful alignment of a randomly-oriented CNT-network film by using simple irradiation with high-intensity femtosecond laser pulses. Polarization-sensitive absorption of individual CNTs and the corresponding polarization-dependent ablation is the origin of such an alignment. Raman spectroscopy confirmed the structural invariance of the aligned CNTs at an appropriately low irradiation intensity. Angle-resolved absorption spectroscopy showed a considerable difference in absorption for different incident polarizations, leading to the possibility of using our film in achromatic, broadband optical polarizers. We believe those films with aligned CNT networks may be used in high-quality electronic and electro-optic devices and may be applied in the development of various novel optical devices.
ACKNOWLEDGMENTS This work was supported by an Incheon National University Research Grant in 2014.
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