Photon-Drag Effect in Single-Walled Carbon Nanotube Films

Nov 23, 2011 - Institute of Applied Mechanics of Ural Branch of Russian Academy of Sciences, ul. T. Baramzinoy 34, 426067, Izhevsk, Russia. ‡...
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Photon-Drag Effect in Single-Walled Carbon Nanotube Films Gennady M. Mikheev,*,† Albert G. Nasibulin,*,‡ Ruslan G. Zonov,† Antti Kaskela,‡ and Esko I. Kauppinen‡ †

Institute of Applied Mechanics of Ural Branch of Russian Academy of Sciences, ul. T. Baramzinoy 34, 426067, Izhevsk, Russia NanoMaterials Group, Department of Applied Physics and Center for New Materials, Aalto University, P.O. Box 15100, 00076, Espoo, Finland

‡

ABSTRACT: We observed an interaction of single-walled carbon nanotube films with obliquely incident nanosecond laser radiation in visible and infrared regions generating unipolar voltage pulses replicating the shape of the laser pulses. The photoelectric signal significantly depends on the laser polarization and has maximum value at the laser beam incidence angle of ±65° and at the film thickness of 350 nm. The results are explained in the framework of the photon-drag effect.

KEYWORDS: Carbon nanotube film, laser, polarization, photon-drag effect, photogalvanic effect

P

radiation. Obviously, the surface photogalvanic effects must disappear in semitransparent films having a thickness of several tens of nanometers due to the equivalence of the two surfaces of the film in terms of scattering of the photoexcited electrons. Therefore, studies on the interaction of laser radiation with thin semitransparent single-walled carbon nanotubes (SWCNTs) films14,15 are important to observe pure photon-drag effect, that is, clearly separating it from the photogalvanic effect. Moreover, the high photon−electron coupling should lead to successful observation of the photon-drag effect in SWCNTs. SWCNT films are considered to be promising material for photonics and optoelectronics.15−21 The SWCNT films have demonstrated their numerous applications in light and heat detection22−25 as laser absorbers over a wide wavelength region (see, for example, refs 26−29). Furthermore, optically transparent conductive and flexible SWCNT films are suitable for various applications in widely used electrical components such as touch sensors and thin displays.20 In this paper, we report for the first time the photon-drag effect in SWCNT films. This phenomenon opens new SWCNT film applications in photonics and optoelectronics. It can be utilized for nonbolometric detection of the intensity of the laser irradiation. The SWCNT films can be used to create various types of competitive high-speed photodetectors, laser polarization analyzers, and optoelectronic angular position encoders. The possibility to create terahertz electrical signals using picoand femtosecond lasers based on the proposed approach is

hotoelectric current produced due to the momentum transfer from the incident photons to the charge carriers during interband or intraband energy transitions is known as a photon-drag effect. By a classical language it can be interpreted as the dynamic ac Hall effect.1 The photon-drag effect was first experimentally observed by Danishevskii et al.2 and Gibson et al.3 during carbon dioxide laser excitation of holes and electrons in bulk germanium crystals. This effect caused by different optical excitation mechanisms (interband transitions, direct and indirect transitions by free carrier absorption, by impurity ionization, etc.) has been widely investigated by other researchers in various materials (e.g., Ge, Si, and GaP).4−7 From practical point of view, investigations of the photondrag effect in semimetals8 and metals9,10 are the most important. In those materials, this effect is observed as the surface currents that uniquely depend on the incident angle and the polarization of the exciting radiation. However, the drag effect in highly absorbing materials with the surfaces without mirrorlike reflection of the photoexcited electrons is always accompanied with the surface photogalvanic effect with similar angular and polarization dependences.9 The surface photogalvanic effect appears due to the anisotropy of the quasimomentum distribution of photoexcited electrons during their transition from the valence to the conduction band and their diffuse scattering on the sample surface.11 For this reason, an interpretation of the experimental results12 describing the dependence of the photoelectric signal on the laser incidence angle in strongly absorbing carbon nanotube yarns with a diameter of about 10 μm is not unambiguous. Indeed, according to ref 13 the photon-drag and photogalvanic effects in few micrometer thick porous resistive nanographite films occur simultaneously in a wide range of wavelengths of the incident © 2011 American Chemical Society

Received: August 29, 2011 Revised: November 8, 2011 Published: November 23, 2011 77

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shows the scanning electron microscope image of a typical SWCNT film.

another very attractive way for the future development of the optoelectronics. SWCNTs were synthesized by an aerosol (floating catalyst) method based on thermal decomposition of ferrocene vapor in a carbon monoxide (CO) atmosphere.30 The synthesis was carried in a scaled-up version of the reactor with a tube of 150 mm in diameter and 1.5 m in length at a total CO flow rate of 4 L/min (with 1% volumetric content of CO2) and at a temperature of 880 °C.31 Ferrocene evaporated at room temperature and thermally decomposed in a high temperature gradient resulting in the supersaturated conditions, formation of nanoparticles, and subsequent carbon monoxide decomposition on the surface of catalyst particles leading to the SWCNT growth. The synthesized SWCNTs were collected downstream of the reactor by passing the aerosol flow through nitrocellulose membrane filters to form SWCNT films, which were transferred from the low adhesive force filter to a polyethylene terephthalate (PET) substrate by a simple room temperature press transfer process.14 The SWCNT collection time was varied from 3 to 30 min, which corresponded to the SWCNT film thickness change from 25 to 625 nm and film transmittance from 94 to 22%. Absorbance spectra measured from the as-deposited SWCNT films at different thicknesses is presented in Figure 1a.

Figure 2. Schematic presentation of the experimental setup: 1, laser; 2, quarter-wave plate; 3, polarizer; 4, Galileo’s telescope; 5, beam splitter; 6, SWCNT film; 7, insulating substrate; 8, digital oscilloscope; 9, photodetector. A and B are electrodes, α is the angle of the laser beam incidence, Φ is the angle between the electric field vector, E, and the plane of incidence, σ, determined by the wave vector, k, and the normal vector, n; ξ is an axis on plane σ (ξ⊥ k).

To study the photoelectrical properties of the SWCNT films we utilized YAG:Nd3+-laser32 with passive Q-switching, which generates pulses of 20 ns in a single-frequency regime and shorter pulses with 1 ns duration resulting in the multimode partial-locking. To examine the amplitude and polarization characteristics of the photoelectric response single-mode lasing was averaged over 30 flashes. To study the time-dependent photoelectrical pulse shapes we used both generation regimes. The experiments were conducted at the wavelengths of the first λ1 = 1064 nm and second λ2 = 532 nm laser harmonics. Linearly polarized light from laser (1) after passing through quarter-wave plate (2) was directed to polarizer (3) as shown in Figure 2. The polarizer was utilized to change the angle, Φ, between the electric field vector, E, and the plane of incidence, σ, determined by the wave vector, k, and the normal vector, n, relative to the surface of SWCNT film (6). Galileo’s telescope (4) with a 2.5× magnification was used to reduce the radiation power density by increasing the diameter of the beam. To measure the energy of the laser pulses we used calibrated photodetector (9) together with beam splitter (5). The electric unipolar voltage pulses (electromotive force, emf), arising owing to the interaction of the laser pulses with the SWCNT film, were measured by digital oscilloscope (8) (Tektronix TDS7704B) with bandwidth of 7 GHz with the help of two parallel electrodes (A) and (B) pressed to the film surface. The electrodes were protected from the light by a nontransparent dielectric coating. The distance between the electrodes (2x0) was kept 12 mm. Depending on the SWCNT film thickness, the resistance of a 6 mm wide (2y0) film between the electrodes was ranged from 180 to 3380 Ω. These film dimensions were found to be the most optimum in terms of maximum measured signal, as will be shown below. The SWCNT film with electrodes was located on the insulating PET substrate (7), locked in a special alignment

Figure 1. (a) Optical absorbance spectra of as-deposited SWCNT films used for the photon-drag investigations. Van Hove transitions S11 and S22 of semiconducting SWCNTs and M11 of metallic SWCNTs are clearly visible, indicating the high quality of synthesized SWCNTs. (b) Experimental dependence of thickness−absorbance for SWCNT films.

As was shown in our previous paper,14 absorbance is a linear function of the SWCNT film thickness. The thickness of the SWCNT film, h, was estimated on the basis of the calibration curve obtained with the help of transmission electron microscope (Figure 1b)15 according to the experimental dependence of h (nm) = 417 × Absorbance (at 550 nm). An inset in Figure 2 78

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device. The electrodes were oriented either perpendicular (transverse arrangement) or parallel (longitudinal arrangement) to the plane of incidence, σ. The alignment device allowed us to vary the angle of laser beam incidence, α, to the film from −90° to +90°. The shape of the laser pulses were measured with the help a high-speed photodetector (Thorlabs SIR5-FC). Our preliminary experiments showed that under oblique incident laser beam unipolar voltage pulses with nanosecond duration were detected between electrodes (A) and (B). The dc recorded electrical pulses can be characterized by the rise, τrise , dc and fall, τfall , times (respectively, defined at 0.1 and 0.9 of the signal maximum), time duration at half of the signal maximum, dc , and peak voltage U, which can have either positive or τhw negative values. It was found that the temporal shapes of the pulsed voltage signals appeared between the electrodes have practically the same shape as the incident laser pulses. This can be confirmed by the measurements of the electric pulses obtained with the oscilloscope from the SWCNT film (traces 1 and 3) and using the high-speed photodetector (traces 2 and 4) and, when the SWCNT film is excited with the laser pulses in a single-frequency regime (Figure 3a) and in a partial modelocking regime (Figure 3b).

Figure 4. The experimental dependences of photoelectrical conversion efficiency ηx in the transverse arrangement on the angle of the laser beam incidence at Φ = 0 (p-polarized excitation) obtained at wavelengths of: 532 nm (curve 1) and 1064 nm (curve 2). Inset shows the voltage signal dependence on the laser power obtained in the transverse arrangement at α = 55°.

densities below the threshold power density, Sbd, needed to destroy the SWCNT films by excessive heating and subsequent oxidation. Typical Sbd value for the SWCNT films was found to be around 0.7 MW/cm2, which is lower than that for thick nanographite films of Sbd = 18 MW/cm2. It is worth noting that for a fixed distance between the electrode and the film width, the signal U is independent of the laser power density, and therefore of the laser diameter, which is in an agreement with our previous experimental results obtained for nanographite films.33 The linear dependence of the recorded signals, Ux and Uy, on laser pulse power in the transverse and longitudinal arrangements allowed us to examine the dependences of the photoelectric conversion efficiencies, ηx = Ux/P and ηy = Uy/P, on the film thickness, h, and the angle of incidence, α, at a fixed angle Φ, as well as on the angle of polarization, Φ, at fixed angle α. The experimental dependences of ηx(α) at Φ = 0 (ppolarized radiation) obtained at the wavelengths of 532 and 1064 nm are shown in Figure 4. To measure the signal at large incidence angles, α, the experiments were carried out with the film size of 2x0 = 27 mm and 2y0 = 6 mm. As can be seen, the photoelectric conversion dependences can be approximated by an odd function of ηx(α) with zero values at normal and grazing beam incidence relative to the SWCNT film surface. In general, the dependences of ηx(α, Φ = 0) obtained at both wavelengths have the same behavior. At both wavelengths, the function |ηx(α, Φ = 0)| has extrema at αextr = ±65° and both experimental curves can be approximately described by linear functions in the range of angles of −45 < α < 45°. It is worth noting that the modulus of αextr is 15° larger than that of the angle obtained for multiwalled carbon nanotube yarns.12 The obtained experimental data can be approximated by an odd function of Fλ = sin 2α/(nλ cos α+1)2, where nλ is a coefficient slightly dependent on the wavelength (nλ1 =1.43, nλ2 = 1.54). The modulus of the conversion efficiency at λ2 is higher than that at λ1. It was additionally found that the polarization angle, Φ, has a very small effect on the position of αextr. At the same

Figure 3. Comparison of the photoelectric pulses measured by oscilloscope from the SWCNT film (traces 1 and 3) and by the highspeed photodetector (traces 2 and 4), when the laser was operated (a) in a single-frequency regime and (b) in a partial mode-locking regime. dc = Trace 1 is characterized by the following parameters: τrise dc dc 14.1 ns, τfall = 36 ns, τhw = 20.5 ns. These are very close to those dc = 13.7 of trace 2 obtained by the high-speed photodetector: τrise dc dc = 41.3 ns, τhw = 20.9 ns. Shorter duration laser pulses ns, τfall corresponding to the partial mode-locking regime recorded with the photodetector and from the SWCNT film are practically identical within 1 ns (see Figure 3b, traces 3 and 4). Thus, the temporal shape of the photoelectric pulses appeared in SWCNT films duplicates the shape of the incident laser pulses in the nanosecond range. No signal was detected at normal incidence of the laser beam (α = 0). For the positive and negative angles of the incidence (the reference for the positive direction of the angle, α, is shown by an arrow in Figure 2) signal has positive and negative polarity, respectively. It was found that the voltage signal, U, changes linearly with increasing the laser pulse power, P, p p , where ε and τhw are the energy and determined as P = ε/τhw duration of laser pulses. The experimental dependence of Ux(P) obtained in the transverse arrangement at α = 55° is shown in inset of Figure 4. It behaves linearly as Ux = ηxP, where ηx = 30 mV/MW. This relationship is valid for the incident power

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time, the photoelectric conversion dependences at other polarization angles ηy(α, Φ ≠ 0, 90°) showed very similar results. The dependence of the photoelectric conversion ηx on the film thickness at α = 55°, Φ = 0 for wavelength λ2 = 532 nm (curve 1) and λ1 = 1064 nm (curve 2) is shown in Figure 5.

Figure 5. Thickness dependences of the photoelectrical conversion efficiency at wavelength of 532 nm (curve 1), at wavelength 1064 nm (curve 2), and sheet resistance of the SWCNT films (curve 3).

The photoelectric conversion nonmonotonically depends on the film thickness and has a maximum value at around 350 nm. It should be also noted that the thickness increase is accompanied with a monotonic decrease in the resistance, RAB, between electrodes (A) and (B) as shown in Figure 5 (curve 3). Experiments carried out in the transverse arrangement at α = 55° at λ2 = 532 nm with a laser beam diameter of about 4 mm showed that the detected signal depends on the width of the film, 2x0, and the interelectrode distance, 2y0 (Figure 6a,b). The initial increase in film area between the electrodes was accompanied with the increase in the signal, which could be explained by the increase of the irradiated area of the film. After the signals reach their maximum values at some optimum 2x0 and 2y0, the photoelectric signals drop with the width and interelectrode distance increase, which will be discussed later. The measurements carried out at λ2 = 532 nm without the telescope (with a beam diameter of 1.6 mm) and the sizes of the film of 2x0 = 6 mm and 2y0 = 12 mm showed that during the film scanning with a laser beam the signal practically did not changed within the region of interaction of the laser beam with the film surface. Figure 7 shows the results of these studies in the transverse arrangement during the scanning along the axes of x′ and y, where x′ is parallel to the axis ξ (Figure 2). Curve 1 (Figure 8) shows the polarization angle dependence of the conversion factor ηx,λ1 (Φ, α = 55°), obtained in the transverse arrangement, at the wavelength of λ1 = 1064 nm, which can be approximated by an even function

ηx , λ = η0x , λ (c λ1 + cos 2Φ) 1 1

Figure 6. Dependences of the photoelectric signal, Ux, (a) on the interelectrode distance, 2x0, at 2y0 = 6 mm and (b) on the width of the film, 2y0, at 2x0 = 12 mm. The dependences were obtained at the laser beam incidence angle of α = 55° at the wavelength of 532 nm and with a beam diameter of 4 mm.

Figure 7. Dependences of the photoelectric signal, Ux, obtained during the laser beam scanning in the plane perpendicular to the incident laser beam (α = 55°). The measurements were carried out with a beam diameter of 1.6 mm (a) along the axis y and (b) along the axis x′. Upper inset shows the experimental design. Lower insert shows the view of the film in the direction of the laser beam.

(1)

0 where ηx,λ = 9 mV/MW and cλ1 = 2.1. Whereas, the 1 experimental dependence of the conversion coefficient of ηy,λ1 (Φ, α = 55°), obtained in a longitudinal arrangement, can be fitted by a curve described by an odd function (curve 2 in Figure 8)

ηy, λ = η0y, λ sin 2Φ 1

1

(2)

0 ηy,λ 1

where = 8.9 mV/MW. Similar results were obtained at the wavelength of λ2 = 532 nm for ηx,λ2 (Φ, α = 55°) and 80

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domains with a large temperature gradient (up to 108 K/cm) on the sample surfaces.36 Indeed, for oblique incidence of the beam (e.g., at α = 55°), coefficients of the light absorption for polarization angles of Φ = 45 and 135° are the same, but for Φ = 0 and Φ = 90° they are different. This can be verified by the analysis of the light reflection from the isotropic absorbing media surface.37 Therefore, if we observed an photothermoelectric effect in the SWCNT films, the signal level at Φ = 45 and 135° should be the same, but remarkably different at Φ = 0 and 90°. However, according to Figure 8 (curve 2), the signal level for the polarization angles of 45 and 135° is different by both value and sign. The signal for the polarization angles of 0 and 90° disappears completely, despite the fact that the light absorption by the film takes place in both cases. Thus, the observed phenomenon cannot be explained by the photothermoelectric effect, because it does not depend on the polarization of the incident irradiation and cannot be described by alternating in sign sinusoidal function. Similarly, the dependences of the conversion factor, ηx, on the angle α, having a form of an odd function (Figure 4, curves 1 and 2), also cannot be explained within the framework of photothermoelectric effects as well as photovoltaic Dember effect. During our measurements of the angular and polarization dependences the electrodes were protected from the light. It was found that if the dielectric protective coating were irradiated by the laser, no photoelectrical signal was observed. Therefore, the photovoltaic effects, which might appear due to the irradiation of the CNT films near the electrodes due to charge separation (Schottky barrier),38 should not be discussed here. The photoelectric conversion dependences similar to (5) and (6) were obtained in our previous papers,13,39 where we investigated the photoelectric laser pulse rectification caused by the surface photogalvanic effect11 and the photon-drag effect.9 As discussed earlier, the first effect appears due to the anisotropy of the quasimomentum distribution of photoexcited electrons and their diffuse scattering on the sample surface. This effect can be observed on any strongly absorbing surfaces with the sample thickness much larger than the excitation skin depth. In our case, we have dealt with thin and semitransparent SWCNT films. Consequently, in terms of the interaction of the light emission and scattering of photoexcited electrons, both surfaces of the film are almost equivalent, which eliminates the existence of the surface photogalvanic effect. According to ref 9, in the case of the photon-drag effect, the densities of surface current along the directions of the incidence light plane, jx, (transverse arrangement) and in the plane perpendicular to the incidence light plane, jy, (longitudinal arrangement), respectively, depend on the polarization angle as jx ∝ (b + cos 2Φ) and jy ∝ sin 2Φ, where b is a constant. These expressions coincide with eqs 5 and 6, describing the polarization effects in our experiments. The experimental dependence of ηx(α), shown in Figure 4, is also in a good agreement with the dependence of the surface current jx on α, obtained for a thin bismuth film also explained in the framework of the photon-drag effect.8 Since increasing the thickness of the SWCNT films leads to the increase in the light absorption,14 under oblique excitation incidence this should be accompanied with the increase in the number of electrons gaining momentum from the incident photons. This should result in the light-induced emf signal increase, which was experimentally observed at the initial stage of curves 1 and 2 in Figure 5. However, the thickness increase is

Figure 8. The polarization angle dependence of the conversion efficiency obtained in the transverse (curve 1) and longitudinal (curve 2) arrangements at the wavelength of λ = l064 nm.

ηy,λ2 (Φ, α = 55°), for the transverse and longitudinal arrangements

ηx , λ = η0x , λ (c λ 2 + cos 2Φ) 2 2

(3)

ηy , λ = η0y , λ sin 2Φ 2 2

(4)

0 0 where ηx,λ = 17.3 mV/MW, cλ2 = 1.5 and ηy,λ = 9.3 mV/MW. 2 2 Comparison of the experimental results presented above, obtained at different wavelengths, shows that the experimental conversion efficiencies, ηx,λ2 and ηy,λ2, at 532 nm are greater than the corresponding efficiencies, ηx,λ1, ηy,λ2, obtained at 1064 nm in the transverse and longitudinal arrangements. This slight increase in the efficiency of the photoelectric conversion with the decrease in the light wavelength 1064 to 532 nm can be explained by the increase in the absorbance of the film (Figure 1a). Generalizing the above-mentioned results one can write the equation for the photoelectric conversions in transverse and longitudinal arrangements, ηx,λ and ηy,λ, depending on the wavelength

ηx , λ (α , Φ) = η0x , λ Fx , λ(α , Φ)(cλ + cos 2Φ)

(5)

ηy , λ (α , Φ) = η0y , λ Fy , λ(α , Φ)sin 2Φ

(6)

where Fx,λ(α,Φ) = sin 2α/(nx,λ cos α+1) f x(Φ) and f y,λ(α,Φ) = sin 2α/(ny,λ cos α+1)2f y(Φ) are odd functions vanishing at α = 0, ±90° and depending notably on the excitation wavelength; f x(Φ) and f y(Φ) are weakly depending on the polarization 0 0 angle functions; ηx,λ , cλ, and ηy,λ are coefficients, which depend on the excitation wavelength. The experimental dependences, expressed by (5) and (6), cannot be explained by any known photothermoelectric effects observed in the films of CNTs.22,34 They can be understood neither within the framework of anisotropic Seebeck effect, observed, for example, during the laser irradiation of metallic nanometer thick films,35 nor due to the Benedicks effect, which is observed in experiments with powerful lasers capable to create the 2

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also accompanied by a decrease in the resistance (RAB) between electrodes (A) and (B) connected parallel to the oscilloscope with the load resistance of Rload = 50 Ω. As a result, at relatively large thicknesses the film resistance, RAB, becomes comparable with the load resistance, Rload, which leads to the reduction of the amplitude of the detected signal. In other words, due to the reduction in the surface resistance electrical power of the lightinduced emf is scattered on the film surface. Similarly, in accordance with our model,40 which we proposed to explain the size factor in nanographite films, the photoelectric signal decrease in the CNT films after its maximum (Figure 6) can be explained by an increase in the leakage (parasitic) current through the closed contours on the surface of the film. The future possible applications of the SWCNT films in photonics and optoelectronics should discussed on the basis of the following experimental results. Figure 3 demonstrates the ability to utilize the SWCNT films as a high-speed photodetector for recording nanosecond light pulses. Nevertheless, we believe that our films will also work in a terahertz range applying pico- and femtosecond lasers. An inset in Figure 4 shows the voltage signal dependence on the laser power, which can be utilized for nonbolometric detection of the intensity of the laser irradiation. Although this study shows the possibility to create SWCNT sensors at two wavelengths (at 532 and 1064 nm), such kind of detector is believed to operate over a wide spectral range including the ultraviolet and middle infrared regions. It is expected that due to higher absorbance, the photoelectric conversion in the UV range is higher. The odd function signal dependence on the incidence angle with a monotonically growing segment in the range of angles from −65° to +65° (Figure 4) affords ground for the development of the laser angle sensors based on SWCNT films. Last but not least, due to high sensitivity of the photoelectric conversion on the polarization angle (Figure 8, curve 2), the SWCNT films can be utilized to build a polarization analyzer of the laser radiation. Comparing the SWCNT films with other carbon nanomaterials (yarns and nanographite) for various types of the photoelectric devices, the extreme adaptability and technical characteristics of the films significantly exceed the relevant parameters for the other materials. This is determined by easiness of the SWCNT film fabrication on practically any substrates (and even in their free-standing state) and by physicochemical properties of this material. We also believe that further studies of the laser interaction with SWCNT films may lead to the discovery of new phenomena and other useful film properties. Thus, in this study for the first time we have experimentally shown the generation of high-speed unipolar voltage (emf) in the films of SWCNTs under oblique laser excitation with nanosecond pulse duration. The temporal shape of the electric pulses coincides with the shape of the incident laser pulses. The amplitude of the pulses increases linearly with the laser power and depends on the incidence angle in accordance with the odd function law. The dependences of the surface current on the angle of polarization in the light incidence plane and the plane perpendicular to the light incident plane have the form of even and odd number functions, respectively. The photoelectric response in the SWCNT films was explained by the excitation of the surface electric current due to the photon-drag effect.

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AUTHOR INFORMATION

Corresponding Author *(G.M.) E-mail: [email protected]. Tel: +7 3412 218955. Fax: +7 3412 507959. (A.N.) E-mail: [email protected]. Tel: +358 50 33 975 38. Fax: +358 9 2451 3517.

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ACKNOWLEDGMENTS The authors gratefully thank Mr. V. M. Styapshin for his help during the experiments. This work was partially supported by RFBR (Project No.10-02-96017-r_Ural_a), Finnish Academy of Science through a number of projects and by a MIDE program of Aalto University.

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REFERENCES

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dx.doi.org/10.1021/nl203003p | Nano Lett. 2012, 12, 77−83