Photocurrent Response from Vertically Aligned Single-Walled

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Photocurrent Response from Vertically Aligned Single-Walled Carbon Nanotube Arrays Mark A. Bissett and Joseph G. Shapter* School of Chemistry, Physics and Earth Sciences, Flinders UniVersity, Sturt Road, Bedford Park, Adelaide SA 5001, Australia ReceiVed: October 29, 2009; ReVised Manuscript ReceiVed: March 2, 2010

The unique electronic properties of single-walled carbon nanotubes (SWNTs) can be used to generate a current response from visible light. Vertically aligned single-walled carbon nanotube arrays were created on an optically transparent electrode (fluorine-doped tin oxide coated glass, FTO) by a self-assembly process using the hydrophobicity of the nanotube side walls and chemical treatment of both the nanotubes and the FTO substrate. This leads to arrays of SWNTs chemically attached to the substrate that, when exposed to visible light, exhibited a prompt current response (4.7 µA/cm2, e200 ms) and a voltage of ∼40 mV. This photoresponse behavior was investigated by modifying the attachment conditions and also the SWNT treatment procedures. The nanotube arrays were found to have a tunable current and voltage response and serve as a possible scaffold for further functionalization. Introduction Carbon nanotubes are a unique material. As well as having high tensile strength and rigidity, the high degree of sp2 hybridization leads to a complex electronic behavior.1 They have been the focus of much research into solar energy generation. Such applications have, so far, be geared toward the improvement of existing dyesensitized solar cells, including improving the conductivity of counter electrodes,2-6 improving interconnectivity,7-9 replacing TiO2 as the working electrode scaffold,10-18 and, to a lesser degree, as the photoactive element themselves.19-25 The photoresponse of carbon nanotubes was first published in 1999,26 and further photocurrent generation was investigated over the next decade. The unique density of states that occurs in carbon nanotubes allows for electron-hole generation when exposed to light. Single-walled carbon nanotubes (SWNTs) can exist as direct semiconductors, depending on several factors, such as chirality and tube diameter. A tube diameter of ∼1 nm will have a band gap of ∼1 eV, easily within the energy produced by solar irradiation. The difficulty arises from capturing and utilizing the electron-hole pairs created by irradiation. One method that has been investigated is the use of an electrochemical cell. A layer of carbon nanotubes acts as the working electrode while an electrolyte solution containing an iodide/triiodide redox couple acts as an electron shuttle to a platinized counter electrode.21,24 Existing work has included the photoresponse of individual nanotubes23 or physisorbed nanotube films.19,24,27 This approach, however, could be improved by the chemical attachment and use of a well-ordered array to prevent electron-hole recombination and increase the photocurrent produced. There have been several previous studies into the alignment and chemical attachment of SWNTs to surfaces and utilizing their conductivity to act as electrodes.15,28-31 These previous studies have shown that it is possible to create vertically aligned single-walled carbon nanotube arrays on silicon and utilize the nanotubes as conductive wires, allowing conduction * To whom correspondence should be addressed. E-mail: Joe.Shapter@ Flinders.edu.au. Phone: +61 8 8201 2005. Fax: +61 8 8201 2905.

into the surface. This “plugging in” to the surface by covalent attachment was shown to increase the conduction into the surface.29-31 We report herein the photocurrent response from these vertically aligned SWNT arrays on optically transparent conductive electrodes (fluorine-doped tin oxide glass, FTO) and investigate the relationship between the electronic nature of these nanotubes and the formation of these arrays. Figure 1 shows schematics of the surface preparation and cell design. Materials Carbon nanotube arrays were prepared by modifying an existing technique.31 It is outlined here in brief. Purified low functionality single-walled carbon nanotubes (P2-SWCNT, Carbon Solutions, Inc.) were purchased and sonicated at 0 °C in a 3:1 (v/v) mixture of H2SO4/HNO3 for 2, 4, and 8 h, as specified. This acid treatment does several things; it shortens the nanotubes, dissolves catalyst particles, removes amorphous carbon, and introduces carboxylic acid groups to defect sites along the tube walls and ends. The resulting functionalized nanotube solution was then filtered through a 0.4 µm membrane (Isopore HTTP, Millipore) and dispersed in dimethyl sulfoxide (DMSO) at 0.2 mg/mL. Dicyclohexyl carbodiimide (DCC) was added at 1:1 (w/w) of cut nanotubes, and the solution was stored under nitrogen in a glovebox. Fluorine-doped tin oxide glass was purchased (TCO22-15, Solaronix SA, 15 Ω/square) and hydroxylated by, first, treating with H2O2 and NH4OH and then with H2O2 and HCl. The hydroxylated FTO glass substrates were then submerged in the SWNT solution and stored at 80 °C for 2, 4, 6, 18, and 24 h, as specified. The aligned nanotube arrays were rinsed with acetone and dried with nitrogen before being used. Electrochemical solar cells were constructed by using the SWNT-modified FTO glass as the working electrode, whereas counter electrodes were produced by taking FTO glass with fill holes already in place and sputtering a 10 nm platinum coating to act as a catalyst. Gaskets were made from 60 µm thick Surlyn (SX1170-60, Solaronix SA), and this was sandwiched between the counter and working electrodes and heated to 100 °C in an oven for 10 min. The cell was then filled with a solution of 0.8

10.1021/jp1003193  2010 American Chemical Society Published on Web 03/18/2010

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Figure 1. Schematics for CNT attachment and cell design.

M 1-methyl-3-propylimidazolium iodide (Sigma-Aldrich), 0.1 M iodine (Sigma-Aldrich), and 0.3 M benzimidazole (SigmaAldrich) in 3-methoxypropylamine (Sigma-Aldrich) to make an iodide/triiodide (I-/I3-) redox couple. The fill hole was then sealed with a glass microscope coverslip. I-V measurements were taken using a Keithley 2400 SourceMeasure unit interfaced with Labview based software written in-house. Measurements were taken under illumination from an optic fiber light source (Dolan-Jenner Fiber-Lite 190-1) at ∼35 mW cm-2, measured with a light meter (Newport Power Meter, model 1815-C). It was noticed that cells improved in photocurrent when aged for 24 h, after which the response was stable. As such, results are presented as freshly prepared cells and those that have been aged by storing in the dark for 24 h. Confocal Raman measurements were taken on an alpha300R microscopy/spectroscopy setup. (Witec, Germany) A 40× aperture was used with a 532 nm laser (2.33 eV), with a maximum power of 60 mW. Spectral maps were constructed from 50 × 50 scans with an integration time of 0.5 s per scan. Results and Discussion To first ascertain if nanotubes had been attached to the FTO glass and to gather information on the electronic state of the aligned nanotube bundles, confocal Raman images and spectra were taken and are shown in Figure 2. From the spectra taken, it is clear that the nanotubes have successfully been attached to the FTO glass as nanotube specific peaks, such as the RBM (148 cm-1) and G band (1570 cm-1), can be seen.32 We also see that, due to the shape and position of the G+ and G- peaks, the nanotubes present are predominately semiconducting. The FTO glass spectrum has only a single peak at ∼1030 cm-1 and a region of increased intensity between 250 and 750 cm-1. These features are seen in the FTO-CNT sample as well but with lower intensity due to the nanotubes blocking some of the signal. This is seen in the images in Figure 2, where, on the left, high CNT intensity is represented as white areas and these same areas appear as black when we plot the glass peak intensity. Following this, complete cells were produced to test the photoresponse of the arrays. Figure 3 shows the photocurrent produced when the light source was switched on and off. The data points shown are separated by 200 ms. Clearly, the array produced a noticeable increase in current, approximately 4.5 µA/cm2, when exposed to light. Additionally, essentially no current (0.01 µA/cm2) is observed when blank FTO is irradiated with light. Of note is that the response time to both the on and the off cycles was e200 ms, indicating that it is not a gradual

response but a very quick and well-defined increase. Also, once exposed to light, the current appears to be constant. To verify the trend in current with time, light-soaking experiments were conducted on the array to monitor the affect of constant cycling. These results show that, over an extended period, there is a consistent current output by the cell, with no noticeable drop in current. By modifying the attachment time of the functionalized nanotubes to the FTO glass, we can modify the amount of nanotube present in the array, which should have a correlation with the photocurrent produced as the number of photoactive elements changes. Figure 4 shows the trend in peak photocurrent with nanotube attachment time for both freshly prepared cells and 24 h aged cells. Of note is the marked increase of both current and voltage produced from the aged cells. This has been attributed to the slow diffusion of the electrolyte into the carbon nanotube bundles, effectively increasing the active surface area or electrical contact. Clearly, there is an increase in photocurrent produced as attachment time increases from 2 to 8 h, and then it plateaus after 18 h. This is expected as the surface coverage is approaching saturation. A similar trend in nanotube coverage has also been seen in AFM images of the surfaces after varying attachment times.33 The peak current seen from the 18 h cell is ∼4.7 µA/cm2. Figure 4 also shows the short-circuit voltage produced by the cells; interestingly, there is a gradual decrease with attachment time. This may indicate that, as the number of nanotube bundles attached to the surface increases, the shunt resistance decreases, leading to a drop in voltage. This affect is overcome in TiO2 cells by the application of an insulating blocking layer; this procedure would be significantly more difficult for the nanotube arrays as the attachment of any insulating molecules may, in fact, have a diminishing effect on the nanotube conductivity and alter its band structure.34 The effect of the low shunt resistance is also noticeable in the fill factor of the I-V curves taken for each of the different cells. Figure 5 shows the I-V curves for the varying attachment times of aged CNT arrays. The high degree of linearity is attributed to the low shunt resistance of the nanotube array. The fill factor is on average 30%, considerably lower than the 70% achieved in most TiO2-based cells. It is not well-understood how the electron-hole generation of large bundles of tubes each with slightly differing density of states will affect recombination within the bundles themselves. Interestingly, the current direction is opposite to that of typical dye-sensitized cells. The direction of electron flow depends on the arrangement of the energy levels of the cell. Using the iodide electrolyte and titania gives a certain

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Figure 2. Confocal Raman images showing nanotube coverage (A, B). Image A is a plot of the intensity of the G band present in the spectra shown in (C) (1570 cm-1). Image B is the plot of the glass specific peak (1030 cm-1). The sample was prepared using nanotubes cut for 8 h and 24 h of nanotube attachment time.

Figure 3. On-off light response from the FTO-CNT cell and blank cell containing no CNTs. The arrows indicate the light on and off cycles (CNT attachment time was 24 h, and cutting time was 8 h; points are separated by 200 ms) (A). Light-soaking response of the same cell left under illumination for over 200 min (B).

“downhill” flow of electrons in a certain direction. For our system, that flow is in the opposite direction because the CNTs have very different energy levels from that of the titania. By modifying the attachment time, we are able to modify the photocurrent by increasing the number of photoactive elements on the surface; however, it may also be possible to further tune the output of the cells by adjusting the mixed acid cutting time. The reaction with the mixed acid produces defect sites along the tubes’ length, and this disrupts the electronic structure of the tubes by either altering their band gap or

converting metallic tubes to semiconducting.1 This modification of the band structure should also be present as changes in both the Raman spectra and the photoresponse of the array. In a typical Raman SWNT spectrum, there are several peaks that can tell us very specific information. The radial breathing mode (RBM) is present only in single-walled carbon nanotubes and not graphitic carbon, such as HOPG, and relates to the diameter of the tube.32 The “D” peak or disorder peak can tell us information about defects along the tube walls, which, in turn, tells us about the band gap present. As defects along the side

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Figure 4. Peak current versus CNT attachment time (A). Peak voltage versus CNT attachment time (B).

Figure 5. I-V curves of aged CNT arrays for differing attachment times of 2, 6, and 24 h, as indicated.

walls disrupt the electronic state of the tube, it increases the band gap, making it semiconducting with differing band-gap energies.35 As mentioned previously, the treatment of the raw nanotubes with mixed acid introduces these defects and this can be tracked with Raman spectra. By taking the ratio of intensity of the D band to the G+ band, we can verify if the level of disorder or number of defects is increasing with increasing cutting time. We have to use a ratio and not the absolute value to compensate for slightly differing laser energies that will affect peak intensities. We can also take the ratio of the G- to the G+ peak. Normally, the presence of both peaks alone tells us the sample consists of SWNTs and not multiwalled tubes, but we can also look at the line shape and intensity to get an idea about the percentage of semiconducting tubes in the sample. Extended cutting times should lead to an increased number of semiconducting tubes and an increasing band gap. Figure 6 shows the graph of the D/G+ ratio as well as the G-/ G+ ratio; the D/G+ is seen to be increasing with cutting time up until 4 h and then plateaus out to 8 h, indicating that the number of defect sites is increasing and then remaining constant. This plateau effect seen after 4 h of cutting time is thought to be due to the consumption amorphous carbon and catalyst particles early on, leading to a greater change in disorder, which then becomes a more gradual change after 4 h of treatment time. The G-/G+ ratio is seen to be decreasing to 4 h and then plateauing also. This indicates that the percentage of semiconducting tubes is increasing to a point and then remaining constant. Confocal Raman images were also obtained from a 5 × 5 µm area for CNTs with differing cutting times attached to the FTO glass. These scans were made up of 50 × 50 individual spectra with an integration time of 0.5 s at identical laser power. A histogram was then generated by taking the intensity of the

Figure 6. Graph of D and G+ band ratio and G- and G+ ratio versus cutting time. Each point represents the average of several individual spectra taken across a wide area on the sample.

Figure 7. Histogram of G-band intensity for differing cutting times (2, 4, and 8 h, as indicated) of CNTs attached to the FTO glass for 24 h and identical Raman laser energy.

G band from 1485 to 1685 cm-1 (x axis, counts) and the number of pixels with that intensity (y axis, pixels). The graph in Figure 7 shows the histogram for the differing cutting times. The 2 h cutting time has the lowest intensity with its center at 2500 counts, and then the 4 h cutting time is centered at 13 000 and the 8 h cutting time at 24 000. This increase in total intensity with cutting time is an indication of the amount of CNTs attached to the surface, as an average over the 5 µm square. The width of distribution also tells us about the bundle size present on the surface as wider bundles will lead to Raman intensity over more pixels. The 2 h sample has a much narrower distribution, indicating the presence of thinner bundles. All the nanotube suspensions used to prepare the samples have the same

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Figure 8. Graph of current versus CNT cutting time (A) and voltage versus CNT cutting time (B).

TABLE 1: Integrated Area under Histogram Peaks from Figure 7 and Current from Figure 8A cutting time (h)

max current (µA/cm2)

integrated peak area (arb units/1E5)

ratio of current/ peak area

2 4 8

2.99 ( 0.022 3.03 ( 0.025 5.04 ( 0.11

2.23 ( 0.11 8.73 ( 0.44 8.91 ( 0.45

1.34 0.347 0.566

amount of carbon content. In the case of the 2 h cutting, the nanotubes are longer, meaning there are fewer to bind to the surface or subsequently add to adsorbed tubes to make wider bundles. Thus, even after 24 h of attachment time, narrow bundles are observed. The distribution then widens for the next two samples, indicating an increasing bundle size distribution and surface coverage. For longer cutting times, the shorter nanotubes mean more species are available to attach to the surface or increase the bundle size, leading to more material on the surface in larger bundles. Differing cutting times and the resultant differing band-gap energies present in the tubes would be expected to give differing photoresponse properties. Cells were constructed out of arrays containing SWNTs of differing cutting times, but identical attachment times of 24 h, and the photoresponse versus cutting time is shown in Figure 8. Uncut tubes are seen to have the lowest produced current. This is expected not only because of the more metallic nature of the pristine tubes, which will decrease the electron-hole generation, but also because the low functionality will mean a minimum attachment to the FTO surface, leading to much fewer photoactive elements on the surface. Thus, it is hard to discern the contribution of changing the electronic state of the tube because the increased functionality will increase the nanotube density on the surface and lead to an increased photoresponse. Unlike the attachment time graph (Figure 4) that displayed a gradual increase and then leveled out, the current as a function of cutting time appears to increase greatly to 3 µA/cm2 at the 2 h cutting time and then stays at that level before rising to 5 µA/cm2 at the 8 h cutting time. Part of this increased photocurrent will be caused by the increased density of SWNTs on the surface, as seen in the Raman data (Figure 7), but also, the increasing semiconducting nature of the tubes should increase the chance of charge separation and an increase in photocurrent. If we integrate the area under the peak in the histogram (Figure 7), we can get an idea about the total amount of nanotubes on the surface. These areas are shown in Table 1. The ratio of current per integrated peak area, which is proportional to the amount of nanotubes on the surface, is

highest for the 2 h cutting time, followed by the 8 h, and then, last, the 4 h cutting time. This is because, at the shorter cutting time, the CNTs have enough semiconducting nature to be photoresponsive but are also spread out thinly enough to minimize recombination effects and not lower the shunt resistance. The 8 h sample has a greatly increased percentage of semiconducting tubes, and this gives the increased current; but the detrimental recombination effects mean that, even with 4 times the amount of CNT on the surface, the current is only 1.7 times higher. Thus, the 2 h cutting time is the most efficient converter of light to current for this given system. Although the cells described produce relatively low currents and voltages (4.7 µA/cm2 and 40 mV), one must take into account the low amount of material present on the working electrode. Using the average fill factor of 30% and the power of the incident light, the efficiencies of the cells are calculated to be 4.7 × 10-5%. Traditional TiO2 cells use a layer thickness of 10-15 µm36 and produce a 10.6% efficiency.37 The nanotube thickness here is 0.5-1 µm, so a low efficiency is not surprising. However, this work does demonstrate that aligned SWNT arrays are a viable method for producing a quick and steady photocurrent response. Additionally, further modification of the nanotubes with light-capturing species and other species to reduce electron recombination will increase the efficiency dramatically. Conclusion The results presented show that nanotubes can indeed be attached to an optically transparent working electrode. These arrays are capable of very quickly producing a current, when exposed to visible light, with response times of e200 ms, indicating that the electron-hole pair generation and separation occurs rapidly in the nanotubes. The current produced, ∼4.7 µA/cm2, was seen to be stable over extended periods with little or no degradation. The aging affect seen in each of the constructed cells is attributed to the diffusion of the electrolyte solution into the attached bundles of nanotubes on the surface, increasing the contact with the working electrode. For an attachment time of 24 h, the most efficient electrode for converting light to current on a per nanotube basis is seen for a nanotube cutting time of 2 h. The trend of increasing photocurrent with attachment time matches the results seen in the AFM of nanotube coverage using a similar attachment technique, indicating a direct correlation between current and number of nanotubes on the surface. Variation of nanotube cutting time allows modification of both nanotube functionality

Photocurrent Response from SWCT Arrays and degree of semiconducting nature. This is seen in both the Raman data and the photoresponse of the cells. The increasing nanotube coverage and bundle size show an increase in photocurrent, while showing a decrease in voltage. References and Notes (1) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (2) Ramasamy, E.; Lee, W. J.; Lee, D. Y.; Song, J. S. Nanocarbon counterelectrode for dye sensitized solar cells. Appl. Phys. Lett. 2007, 90, 173103. (3) Hino, T.; Ogawa, Y.; Kuramoto, N. Dye-sensitized solar cell with single-walled carbon nanotube thin film prepared by an electrolytic micelle disruption method as the counterelectrode. Fullerenes, Nanotubes, Carbon Nanostruct. 2006, 14, 607–619. (4) Koo, B.-K.; Lee, D.-Y.; Kim, H.-J.; Lee, W.-J.; Song, J.-S.; Kim, H.-J. Seasoning effect of dye-sensitized solar cells with different counter electrodes. J. Electroceram. 2006, 17, 79–82. (5) Wang, Q.; Moser, J. E.; Gratzel, M. Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells. J. Phys. Chem. B 2005, 109, 14945–14953. (6) Hoshikawa, T.; Yamada, M.; Kikuchi, R.; Eguchi, K. Impedance analysis for dye-sensitized solar cells with a three-electrode system. J. Electroanal. Chem. 2005, 577, 339–348. (7) Jung, K. H.; Hong, J. S.; Vittal, R.; Kim, K. J. Enhanced photocurrent of dye-sensitized solar cells by modification of TiO2 with carbon nanotubes. Chem. Lett. 2002, 31, 864–865. (8) Jung, K. H.; Jang, S. R.; Vittal, R.; Kim, V. D.; Kim, K. J. Photocurrent improvement by incorporation of single-wall carbon nanotubes in TiO2 film of dye-sensitized solar cells. Bull. Korean Chem. Soc. 2003, 24, 1501–1504. (9) Jang, S. R.; Vittal, R.; Kim, K. J. Incorporation of functionalized single-wall carbon nanotubes in dye-sensitized TiO2 solar cells. Langmuir 2004, 20, 9807–9810. (10) Lee, W.; Lee, J.; Min, S. K.; Park, T.; Yi, W.; Han, S.-H. Effect of single-walled carbon nanotube in PbS/TiO2 quantum dots-sensitized solar cells. Mater. Sci. Eng., B 2009, 156, 48–51. (11) Landi, B. J.; Evans, C. M.; Worman, J. J.; Castro, S. L.; Bailey, S. G.; Raffaelle, R. P. Noncovalent attachment of CdSe quantum dots to single wall carbon nanotubes. Mater. Lett. 2006, 60, 3502–3506. (12) Landi, B. J.; Castro, S. L.; Ruf, H. J.; Evans, C. M.; Bailey, S. G.; Raffaelle, R. P. CdSe quantum dot-single wall carbon nanotube complexes for polymeric solar cells. Sol. Energy Mater. Sol. Cells 2005, 87, 733–746. (13) Lee, T. Y.; Yoo, J.-B. Adsorption characteristics of Ru(II) dye on carbon nanotubes for organic solar cell. Diamond Relat. Mater. 2005, 14, 1888–1890. (14) Kymakis, E.; Amaratunga, G. A. J. Photovoltaic cells based on dye-sensitisation of single-wall carbon nanotubes in a polymer matrix. Sol. Energy Mater. Sol. Cells 2003, 80, 465–472. (15) Yu, J.; Mathew, S.; Flavel, B. S.; Johnston, M. R.; Shapter, J. G. Ruthenium porphyrin functionalized single-walled carbon nanotube arrays; a step toward light harvesting antenna and multibit information storage. J. Am. Chem. Soc. 2008, 130, 8788. (16) Ren, D. M.; Guo, Z.; Du, F.; Zheng, J. Y.; Chen, Y. S. Nanohybrid material of SWNTs covalently functionalized with porphyrin for light harvesting antenna: synthesis and photophysical properties. J. Nanosci. Nanotechnol. 2007, 7, 1539–1545. (17) Sgobba, V.; Guldi, D. M. Carbon nanotubes-electronic/electrochemical properties and application for nanoelectronics and photonics. Chem. Soc. ReV. 2009, 38, 165–184.

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6783 (18) Kongkanand, A.; Martinez Dominguez, R.; Kamat, P. V. Single wall carbon nanotube scaffolds for photoelectrochemical solar cells. Capture and transport of photogenerated electrons. Nano Lett. 2007, 7, 676–680. (19) Li, Z.; Kunets, V. P.; Saini, V.; Xu, Y.; Dervishi, E.; Salamo, G. J.; Biris, A. R.; Biris, A. S. Light-harvesting using high density p-type single wall carbon nanotube/n-type silicon heterojunctions. ACS Nano 2009, 3, 1407–1414. (20) Liu, G.; Liu, Z.; Zhao, Y.; Zheng, K.; Huang, H.; Ma, W.; Gu, C.; Sun, L.; Xie, S. Large photocurrent generated by a camera flash in singlewalled carbon nanotubes. J. Phys. D: Appl. Phys. 2007, 40, 6898–6901. (21) Kamat, P. V. Harvesting photons with carbon nanotubes. Nano Today 2006, 1, 20–27. (22) Castrucci, P.; Tombolini, F.; Scarselli, M.; Speiser, E.; Gobbo, S. D.; Richter, W.; Crescenzi, M. D.; Diociaiuti, M.; Gatto, E.; Venanzi, M. Large photocurrent generation in multiwall carbon nanotubes. Appl. Phys. Lett. 2006, 89, 253107. (23) Lee, J. U. Photovoltaic effect in ideal carbon nanotube diodes. Appl. Phys. Lett. 2005, 87, 073101. (24) Barazzouk, S.; Hotchandani, S.; Vinodgopal, K.; Kamat, P. V. Single-wall carbon nanotube films for photocurrent generation. A prompt response to visible-light irradiation. J. Phys. Chem. B 2004, 108, 17015– 17018. (25) Mele, E. J.; Kra´l, P.; Toma´nek, D. Coherent control of photocurrents in graphene and carbon nanotubes. Phys. ReV. B 2000, 61, 7669. (26) Zhang, Y.; Iijima, S. Elastic response of carbon nanotube bundles to visible light. Phys. ReV. Lett. 1999, 82, 3472. (27) Shi, Y.; Fu, D.; Marsh, D. H.; Rance, G. A.; Khlobystov, A. N.; Li, L.-J. Photoresponse in self-assembled films of carbon nanotubes. J. Phys. Chem. C 2008, 112, 13004–13009. (28) Yu, J.; Flavel, B. S.; Shapter, J. G. Optical and electrochemical properties of single-walled carbon nanotube arrays attached to silicon(100) surfaces. Fullerenes, Nanotubes, Carbon Nanostruct. 2008, 16, 18–29. (29) Yu, J. X.; Shapter, J. G.; Quinton, J. S.; Johnston, M. R.; Beattie, D. A. Direct attachment of well-aligned single-walled carbon nanotube architectures to silicon (100) surfaces: a simple approach for device assembly. Phys. Chem. Chem. Phys. 2007, 9, 510–520. (30) Yu, J.; Shapter, J. G.; Johnston, M. R.; Quinton, J. S.; Gooding, J. J. Electron-transfer characteristics of ferrocene attached to single-walled carbon nanotubes (SWCNT) arrays directly anchored to silicon(1 0 0). Electrochim. Acta 2007, 52, 6206–6211. (31) Yu, J. X.; Losic, D.; Marshall, M.; Bocking, T.; Gooding, J. J.; Shapter, J. G. Preparation and characterisation of an aligned carbon nanotube array on the silicon (100) surface. Soft Matter 2006, 2, 1081–1088. (32) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Raman spectroscopy of carbon nanotubes. Phys. Rep. 2005, 409, 47–99. (33) Flavel, B. S.; Yu, J.; Shapter, J. G.; Quinton, J. S. Patterned attachment of carbon nanotubes to silane modified silicon. Carbon 2007, 45, 2551–2558. (34) Filho, A. G. S.; Jorio, A.; Samsonidze, G. G.; Dresselhaus, G.; Saito, R.; Dresselhaus, M. S. Raman spectroscopy for probing chemically/ physically induced phenomena in carbon nanotubes. Nanotechnology 2003, 14, 1130–1139. (35) Jorio, A.; Pimenta, M. A.; Filho, A. G. S.; Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Characterizing carbon nanotube samples with resonance Raman scattering. New J. Phys. 2003, 5, 139. (36) Oregan, B.; Gratzel, M. A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737–740. (37) Gratzel, M. Conversion of sunlight to electric power by nanocrystalline dye-sensitized solar cells. J. Photochem. Photobiol., A 2004, 164, 3–14.

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