NANO LETTERS
Photoconductivity of Packed Homotype Bundles Formed by Aligned Single-Walled Carbon Nanotubes
2008 Vol. 8, No. 3 968-971
Antonio Serra,† Daniela Manno,† Emanuela Filippo,† Antonio Tepore,† Maria Letizia Terranova,*,‡ Silvia Orlanducci,‡ and Marco Rossi§ Dipartimento di Scienza dei Materiali, UniVersità del Salento, Via per Monteroni, 73100 Lecce, Italy, Dipartimento di Scienze e Tecnologie Chimiche and MINAS, UniVersità di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italy, and Dipartimento di Energetica and CNIS, UniVersità di Roma “La Sapienza”, Via A. Scarpa 16, 00161 Roma, Italy Received November 22, 2007; Revised Manuscript Received January 16, 2008
ABSTRACT Photoconductivity properties of aggregated single-walled carbon nanotubes have been studied by performing measurements on macroscopic ribbons, obtained by the aggregation of a large number of SWCNT bundles. Structural analysis performed by electron diffraction revealed that the nanotubes forming each bundle have the same chirality. The experimental results, regarding the region 1.2–3.6 eV and the pressure range 103-10-3 mbar, suggest that the photoexcitation of nanotubes, packed in bundles and organized in ribbons, generates electron–hole pairs within a band structure and that bond excitons are formed by Coulomb interactions between spatially confined charge carriers.
The photoconductivity properties of single-walled carbon nanotubes (SWCNT) are a subject lying at the forefront of research in condensed matter physics. SWCNTs represent nearly ideal one-dimensional (1D) structures with outstanding, but not yet fully understood, electronic transport properties.1 The relatively few experimental and theoretical data on photoconductivity reported in the literature regard mostly isolated nanotubes;2,3 the state density of nanotubes close packed in a bundle and how the bundle’s aggregation influences the electronic properties of a bulk nanotube material4 are still open questions. Fundamental problems related to electron transport in systems with nanotube components could be solved through a deep investigation of the electronic properties of nanotubes organized at the micro- and mesoscopic scale. However, experimental studies of how the aggregation modifies the density of states of nanotubes with respect to that of an isolated nanotube and, as a consequence, influences the effective opto-electronic properties, require either selected placement of CNT arrays or CNT assembling in well-defined morphologies. The difficulty to achieve such controlled systems is probably the reason for which the basic studies of photoconductivity in * Corresponding author. E-mail:
[email protected]. † Dipartimento di Scienza dei Materiali, Università del Salento. ‡ Dipartimento di Scienze e Tecnologie Chimiche and MINAS, Università di Roma “Tor Vergata”. § Dipartimento di Energetica and CNIS, Università di Roma “La Sapienza”. 10.1021/nl073052w CCC: $40.75 Published on Web 02/12/2008
2008 American Chemical Society
nanotube ensembles are rather rare5 despite the intense experimental and theoretical research regarding the electronoptical properties of carbon nanotubes. In this context, we investigated the photoconductivity of aggregated homotype bundles produced using single-walled carbon nanotubes (SWCNTs) grown by hot-filament chemical vapor deposition, following a procedure similar to the one already reported.6 This approach that leads to the formation of macrosized ribbons is based on a recondensation of SWCNT oxidized by KMnO4/H2SO4 and dispersed in aqueous solutions. Ribbons with lengths of some millimeters and transversal size of tens of micrometers were obtained by an 18 h lasting reaction at RT. The ribbons have been submitted to treatments using 1 M HNO3 solutions in order to eliminate the graphitic layers present on their external surface. The morphology of the ribbons has been studied by using a Jeol 4210 scanning tunnelling microscope (STM) and a Jeol 2010 high resolution transmission electron microscope (HRTEM). STM analysis reveals that ribbons consist of well-aligned close-packed bundles (Figure 1a). The image was collected in the constant current mode in air, at room temperature, using mechanically sharpened Pt-Au wires as tips. HRTEM observations were performed at 160 kV on a number of isolated bundles obtained by mechanical stressing of the ribbons. A HRTEM image of some typical isolated bundles is reported in Figure 1b. In all the HRTEM images,
Figure 1. Morphologies of the investigated bundles: (a) STM images evidencing the inner texture of the ribbon formed by aggregated bundles. (b) HRTEM of two isolated bundles showing that each bundle is formed by identical SWCNT
Such diffraction features are exactly the ones expected7 from the theoretical simulation of a transmission electron diffraction pattern generated by a SWCNT in a plane perpendicular to the electron beam. On the base of theoretical predictions, the spots along the equatorial line should be characterized by a faint intensity. The strong intensity of our spots can be ascribed to the superposition of the contributions from identical well-aligned SWCNTs, belonging to the same bundle, probed by the incident electron beam. Because the distribution of the layer lines depends on the chiral angle of the SWCNTs and the modulation of the intensity along the equatorial line depends on tube diameter, from the analysis of the electron diffraction pattern, it is possible to determine both chiral angle and diameter of carbon nanotubes forming the bundle. The distances d1 and d2 on Figure 2a are related to the chiral angle R8 by
(
R ) atan
Figure 2. Electron diffraction pattern performed onto an isolated bundle (a), experimental equatorial line profile (solid line) and a simulation (dotted line) for F ) (1.10 ( 0.02) nm (b).
the bundles appear to be constituted by parallel and aligned SWCNTs. Moreover, the set of fringes corresponding to a bundle has the same spacing, within the limits of the HRTEM resolution. This suggests that the bundle is made by the aggregation of identical tubes. In the case of bundles reported in Figure 1b, it is possible to estimated a diameter F ) 1.05 ( 0.05 nm. Such a result is quite surprising, considering that production methods generally give a mixture of different SWCNT sorts.1 However, this significant finding may be rationalized by considering that, due to differences in their chemical reactivity, tubes with different chirality3 are being reacted progressively during the redox titration. Therefore, at each stage of chemical process, a fraction of functionalized nanotubes, with the same band structure and same dielectric constant, is ready to be assembled in homotype bundles by self-governed processes. To confirm that the bundles are formed by nanotubes of the same type, a series of isolated bundles have been investigated using electron diffraction under selected area conditions. The electron diffraction patterns obtained from isolated bundles show two peculiar features that can be observed in Figure 2. The first one is the presence of intense spots regularly positioned along a line perpendicular to the bundle axis and crossing the 000 central spot, the so-called equatorial line. The other notable feature is the distribution of nonequatorial spots in the so-called layer lines, elongated in the direction normal to the tube axis and localized on two concentric circles (related to the graphite reflections) having their centers on the 000 spot. Nano Lett., Vol. 8, No. 3, 2008
1 2d2 - d1 √3 d1
)
(1)
These relationships are not affected by the tilting angle of the tube. The distances d1 and d2 can be measured from the digitalized patterns, with errors estimated to be within 1%. Concerning the electron diffraction in Figure 2a, by using eq 1, we estimated for the chiral angle of the SWCNTs forming the bundle a value of 26.0 ( 0.3°. Considering possible chiral vectors,4 the best match is with the SWCNT type (9,7), which has a diameter and chiral angle of 1.088 nm and 25.872°, respectively. The closest alternative would be the SWCNT type (10,8), having diameter of 1.223 nm and chiral angle of 26.329°, but the discrepancy with measured values is well beyond the experimental error. The SWCNT diameter can be alternatively evaluated by a study of the intensity profile along the equatorial line using the kinematical theory of diffraction.9 Because the distances (in real space) associated with low momentum transfer10 (k < 25 nm-1) are larger than the C-C distance, we can use the continuum theory in order to analyze the equatorial lines. In this model, the individual tubes are viewed as a continuous of carbon atoms on a cylinder surface.11 Within this approximation, the scattering form factor of a single tube of radius F is given by A(k) ) f(k)FJ0(kF), where J0 is the Bessel function of order 0 and f(k) is the carbon atomic scattering factor. For our bundle, the intensity along this line is modulated by the form factor of nanotubes and the structure factor of the bundle packing. The scattering form factor of a set of N identical and closely aligned SWCNTs become: AN(k) ) Nf(k)FJ0(kF). In Figure 2b, we have plotted the experimental equatorial line profile (solid line) and, for comparison, the line obtained from the simulation (dotted line) for F ) (1.10 ( 0.02) nm. Similar results have been achieved by investigating other bundles obtained by disentangling the same ribbon. The results achieved with the two methods are consistent, confirming that the various bundles are formed substantially by identical SWCNTs. The ED pattern features let us to exclude the concomitant presence in the same bundle of nanotubes with different chiralities. Of course, depending on the chirality of the nanotubes constituting the bundles, 969
Figure 3. Ratio between current measured under illumination and in dark conditions (Ilight/Idark) vs photon energy, in the range 1.2–3.6 eV. Measurements were performed at a constant temperature of 250 K and at a pressure of 10-3 mbar. The inset shows the ribbon mounted on the top of a multifinger device with interdigitated Au electrodes on Al2O3 substrates. The electrodes thickness was 90 nm, and the distance between the opposite electrodes was about 200 µm.
some have metallic behavior and some other have semiconducting behavior. The photoconductivity of homotype SWCNT bundles assembled in macroscopic ribbons, as described in the following, has to be related to the presence of bundles made by semiconducting SWCNT. For the photocurrent investigation, a segment of the same ribbon was mounted on the top of a multifinger device with interdigitated Au electrodes fabricated on Al2O3 substrates using conventional lithography, gold evaporation, and liftoff processes. The electrodes thickness was 90 nm, and the distance between the opposite electrodes was about 200 µm. A Keithley 6517A electrometer was employed as V source and current meter. The on–off photoconductivity was investigated by using a tungsten-halogen lamp with a long pass NIR filter (850 nm) as light source, equipped with a series of neutral filters to reduce the light power. Photoconductivity measurements were performed at a constant temperature of 250 K, using a liquid nitrogen cryostat BIORAD. The photoresponse as a function of photon energy was detected with a Jobin-Yvon H10 single grating monochromator. The trend of photocurrent vs power was linear in the integrated power range 0.5–30.0 mW/mm2. Figure 3 shows the photoconductivity behavior of a ribbon of SWCNT bundles under a light illumination of 30 mW/ mm2 at a pressure of 10-3 mbar. The figure reports the ratio between current measured under illumination and in dark conditions (Ilight/Idark) in the spectral range 1.2–3.6 eV. The inset shows the ribbon, with a size of about 50 µm × 1 mm, mounted on the multifinger device described above. In the photoconductivity excitation spectra, three clear peaks are observed at 1.45, 1.95, and 2.95 eV, respectively. These values of energy are very close to the second and third interband gaps of semiconducting SWCNTs with diameters ranging between 0.75 and 1.1 nm.12 The on–off light excitation response is shown in Figure 4. After the excitation, a prompt generation of reproducible 970
Figure 4. Dynamic on–off photocurrent response under an irradiance power of 30 mW/mm2 with a frequency of 25 mHz and a duty cycle of 50%. The response and recovery times obtained from the reported fit are about 0.8 s.
and stable photocurrent is observed. The features of the photocurrent response can be interpreted in the frame of a simple kinetic model, taking into account processes of photocarrier generation and relaxation. To analyze the data using a dynamic model, one can start from the continuity equation for minority carriers13 d∆n d∆n d2∆n ∆n ) -µE + Dn 2 +G dt dx τ dx
(2)
where ∆n is the nonequilibrium minority carrier concentration, Dn the electrons diffusion coefficient, τ the lifetime of electrons, G the generation rate of nonequilibrium charge carriers, µ the mobility, and E the electrical field. By assuming the carriers uniformly distributed in the carbon nanotubes and neglecting the electric field, the following equation can be derived ∆n d∆n )+ ηI dt τ
(3)
In eq 3, the term G has been substituted with ηI, η being the quantum efficiency and I the light intensity. Under steadystate conditions, d∆n/dt ) 0, so that ∆n ) τηI. Assuming the concentration of nonequilibrium carriers proportional to the photocurrent, the steady-state photocurrent would be a linear function of the light power intensity. In nonsteady state, the solution of eq 3 in the presence of light gives a photocurrent increase
( -tτ )]
[
∆n ) τηI 1 - exp
(4)
and in the absence of light a photocurrent drop
[ ( -tτ )]
∆n ) τηI exp
(5)
Fitting the experimental data with eqs 4 and 5, one obtains an electron lifetime of the order of 0.8 s, which is shorter than other data reported in literature.14 While in previous studies15 the electron lifetime τ was attributed to photodesorption of gas molecules from the nanotubes, here, according to Lu et al.,16 present results strongly suggest an annihilation of charge carrier pairs at the metal–carbon nanotube contacts. Nano Lett., Vol. 8, No. 3, 2008
Figure 5. Photo and dark-current vs pressure from assembled SWCNTs at RT.
To clarify this point, we measured the light-induced photocurrent of the same samples varying the pressure in the range 103-10-3 mbar. Under light conditions, the amplitude of the photocurrent exhibited a remarkable increase during the pumping, as shown in Figure 5, revealing that at 10-3 mbar, the photocurrent intensity is about six times higher than that at a pressure of 103 mbar. This behavior demonstrates that the mechanism of photocurrent emission is not due to the molecular desorption from nanotubes. Conversely, these results revealed that the presence of gaseous species inhibit the photocurrent generation. This effect can be rationalized by taking into account that, at atmospheric pressure, a relative large number of gas molecules are adsorbed onto nanotube surfaces. Under these conditions, they may act as recombination centers or carrier traps to facilitate electron–hole pair recombination15 before they are separated by the metal–carbon nanotube contacts. Furthermore, photocurrent can be limited by the heating of carbon nanotubes upon light absorption.16 As reported in ref 15, the carbon nanotubes are capable to absorb significant amounts of photon energy; as a consequence, under light conditions, the local temperature of nanotubes could increase. This temperature rise, generated by multiphonon processes, involves the nonilluminative recombination of photopumped electron–hole pairs.1 Under higher temperature, more photogenerated electrons become “hot” electrons with higher kinetic energy, which means that they have a greater probability of crossing the metal–carbon nanotube contacts and can produce a higher photocurrent.17 In this context, we are led to consider the possibility that the detected photocurrent was produced by charge separation of photogenerated electron–hole pairs at the metal-nanotube contacts but was limited by the processes of sample heating by light absorption. In air and at higher pressures, the process of sample heating by light absorption is not as effective as under
Nano Lett., Vol. 8, No. 3, 2008
vacuum conditions and it would result in a lowering of the photocurrent in comparison with vacuum. In our experiments, under an irradiance power of 30 mW/mm2, we have monitored after 1 h an increase in the temperature of about 0.5 K. Such a low increase of temperature makes it impossible to entirely justify the remarkable increase of the measured photocurrent in vacuum conditions on the basis of “hot” electrons. In other words, the contribute to the photocurrent given by the “hot” electrons, able to cross the metal–carbon nanotube contacts, is not the prevalent one. Conversely, our experimental data can be rationalized on the base of the hypothesis that photoexcitation of homotype SWCNTs bundles, having a semiconducting behavior and regularly organized in ribbons, is capable to generate electron–hole pairs within band structure. Coulomb interaction between spatially confined charge carriers results in the formation of bond excitons, which can relax via intraband transitions to the lower levels of the SWCNT band structure to produce a successive sub-band gap excitons or unbounded electron–hole pairs. Acknowledgment. The research work has been partially supported by the EU Commission in VI FP, contract no. 011935 EUROFEL-DS1. A. R. De Bartolomeo is gratefully acknowledged for her valuable technical assistance in measurements. References (1) Dresselhaus, M. S.; Dresselhaus, G. and Avouris, Ph. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Springer: Berlin, 2001. (2) Freitag, M.; Martin, Y.; Misewich, J. A.; Martel, R.; Avouris, Ph. Nano Lett. 2003, 3, 1067. (3) Qiu, X.; Freitag, M.; Perebeinos, V; Avouris, Ph. Nano Lett. 2005, 5, 749. (4) Okada, S.; Oshiyama, A.; and Saito, S. Phys. ReV. B 2000, 62, 7634. (5) Bunning, J. C.; Donovan, K. J.; Scott, K.; Somerton, M. Phys. ReV. B 2005, 71, 085412. (6) Terranova, M. L.; Orlanducci, S.; Fazi, E.; Sessa, V.; Piccirillo, S.; Rossi, M.; Manno, D.; Serra, A. Chem. Phys. Lett. 2003, 381, 86. (7) Qin, L.-C. Rep. Prog. Phys. 2006, 69, 2761. (8) Gao, M.; Zuo, J. M.; Twesten, R. D.; Petrov, I.; Nagahara, L. A.; Zhang, R. Appl. Phys. Lett. 2003, 82, 2703. (9) Colomer, J. F.; Henrard, L.; Van Tendeloo, G.; Lucas, A.; Lambin, P. J. Mater. Chem. 2004, 14, 603. (10) Flahaut, E.; Peigney, A.; Laurent, Ch.; Rousset, A. J. Mater. Chem. 2000, 10, 249. (11) Henrard, L.; Loiseau, A.; Journet, C.; Bernier, P. Eur. Phys. J. B 2000, 13, 661. (12) Venema, L. C.; Janssen, J. W.; Buitelaar, M. R.; Wildoer, J. W. G.; Lemay, S. G.; Kouwenhoven, L. P.; Dekke, C. Phys. ReV. B 2000, 62, 5238. (13) Manno, D.; Di Giulio, M.; Serra, A.; Siciliano, T.; Micocci, G. J. Phys. D: Appl. Phys. 2002, 35, 228. (14) Levitsky, I. A; Euler, W. B. Appl. Phys. Lett. 2003, 83, 1857. (15) Robert, J. C.; Nathan, R. F.; Jing, K.; Jien, C.; Thomas, W. T.; Yuegang, Z.; Hongjie, D. Appl. Phys. Lett. 2001, 79, 2258. (16) Lu, S.; Panchapakesan, B. Nanotechnology 2006, 17, 1843. (17) Muller, R. S.; Kamins, T. I.; and Chan, M. DeVice Electronics for Integrated Circuits; Wiley: New York , 2002.
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