Photoconductivity from Carbon Nanotube Transistors Activated by

J. Phys. Chem. C 2008, 112, 18201–18206

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Photoconductivity from Carbon Nanotube Transistors Activated by Photosensitive Polymers Yumeng Shi,† Xiaochen Dong,† Hosea Tantang,† Cheng-Hui Weng,† Fuming Chen,† Chunwei Lee,† Keke Zhang,† Yuan Chen,‡ Junling Wang,† and Lain-Jong Li*,† School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore, 639798, and School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, Singapore, 637459 ReceiVed: June 16, 2008; ReVised Manuscript ReceiVed: September 12, 2008

We perform electrostatic force microscopic measurements to reveal the mechanisms of the photoresponse from polymer-coated carbon nanotube transistors, where the effective gating due to the trapping of photogenerated electrons at SiO2 dielectric surfaces is found dominant. The distant photoresponse and the unique spatial-dependent photoconductivity for polymer-coated SWNT-network transistors are explored for the first time. The photoconductivity depends strongly on the polarity of the applied voltage across the contact pads, which suggests that a secondary effect (electrical field dependent exciton dissociation) needs to be included in addition to the photoinduced electrostatic gating. These spatial photoresponses are generally observed for various substrates and polymers. These results suggest a new strategy for achieving remote light detection, position sensors, or antenna devices. Introduction Single-walled carbon nanotubes are promising electronic materials for variety of applications such as field-effect transistors (FETs),1,2 memory devices,3,4 and chemical/biological sensors.5-8 The photoconductivity of SWNTs has been attributed to the direct excitation of SWNTs.9-15 Recently, the unique photoinduced electron transfer from the encapsulated fullerenes to SWNTs has been observed.16 Resistor type of SWNT networks are also proposed as photodetecters.17 In parallel, the optoelectronic switching behaviors due to the interaction between photosensitive polymers and SWNT transistors have been proposed to serve as memory devices.18,19 It has been suggested that the photogenerated holes in polymers were directly transferred to nanotubes, resulting in a current increase.18 An alternative mechanism “photoinduced electrostatic gating” has also been proposed,19 stating that the photocurrent was due to the trapping of photogenerated electrons at SiO2 dielectric surfaces, which then effectively gated the transistor. We performed electrostatic force microscopy (EFM) measurements to reveal the dominating mechanism. In addition, the distant photoresponses and the unique “spatial-dependent photoconductivity” for polymer-coated SWNT-network transistors are explored. The spatial-dependent photoconductivity depends strongly on the relative position of the light spot to the sourcedrain electric field. We suggest that the overall photoresponse is dominated by photoinduced electrostatic gating.19 Nevertheless, a secondary effect needs to be included to better explain the observation. It has been suggested that excitons are able to dissociate at the polymer-SWNT interface.20 We proposed that the dissociation is also affected by the voltage applied on the SWNTs, which then determines the injection efficiency of holes to the SWNTs and subsequently changes the photoconductivity. * Corresponding author. E-mail: [email protected]. † School of Materials Science and Engineering. ‡ School of Chemical and Biomedical Engineering.

Figure 1. Typical transfer curves for the bare SNFET used in this study, where no specific procedure is applied to remove the metallic tubes. The on-off ratio is around 100.

Experimental Section SWNT networks were synthesized by the CVD process21 using cationized Ferritin as catalysts. SWNT-network field-effect transistors (SNFETs) were fabricated in a top contact device geometry, where a highly p-doped silicon wafer with an 80 nm thick SiO2 layer was used as a back gate and 30 nm of Ta electrodes separated by 100 µm were patterned on top of it using standard lithography techniques. The SNFETs were then protected by photoresists, and the undesired SWNT networks outside the devices were removed by oxygen plasma. The typical transfer curves for this type of transistors are shown in Figure 1, where no specific procedure to selectively remove metallic tubes is needed in this study. Poly[(9,9-dioctyl-fuorenyl-2,7diyl)-co-(bithiophene)] (F8T2) was purchased from American Dye Source and used without further purification. All electrical measurements were carried out in ambient using a Kiethley semiconductor parameter analyzer model 4200-SCS in a lighttight enclosure. A double monochromator (Horiba JY Gemin180), attached to a broadband light source (450 W short arc Xe lamp or 250 W Tungsten lamp), was used to select the desired wavelength of light for the experiments. For the experiments requiring more intense light, the band-pass filters

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Figure 2. (a) Drain current (Id) response of the bare SNFET to the illumination cycle of 450 nm of light at gate voltage (Vg) ) 20 V and -20 V, respectively. (b) Percentage increase in Id upon the illumination of various wavelengths.

((10 nm) were used to select the desired band of light from the short arc lamp. Electrostatic force microscopy has been used to study electrical properties on a nanometer scale.22-24 We perform an EFM study using the phase measurement mode. It is a dualpass technique. Two scans are conducted in tapping mode, where the tip is mechanically driven around its resonance frequency. During the first scan, topography information is acquired. The tip is then lifted, and the line scan is repeated at a constant distance from the surface based on the recorded profile. During the second scan (interleave scan), a DC voltage is applied to the tip. The long-range electrostatic force between the tip and the sample surface alters the tip resonance frequency, inducing a change in both the phase and amplitude signals. Attractive and repulsive forces will give rise to an opposite phase shift. Recording the phase shift reveals information about charge/ potential distribution on the sample surface. An Asylum Research MFP-3D system with Olympus (OMCL-AC240TM) Pt-coated cantilevers is used for the experiments. The tip curvature radius is ∼15 nm, with a quality factor 109, spring constant 2 N/m, resonance frequency ∼70 kHz, and cantilever length 240 µm. Results and Discussion Recently, the fluorene-based polymers have been reported to interact strongly with carbon nanotubes,25,26 and therefore we choose one of the relatively stable fluorine polymers, poly[(9,9dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2), for the optoelectronic study in ambient. Before examining the photoconductivity for the F8T2-coated SWNT-network field-effect

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Figure 3. (a) Photoconductivity response (increase in Id) versus varied wavelengths of illumination from 400 to 580 nm for an F8T2-coated SNFET (solid square). The solid curve shows the absorption spectrum for an F8T2 polymer thin film. The results demonstrate that the photocurrent of the F8T2-SNFET results from the absorption of the coated F8T2 polymer. (b) Typical Id response for an F8T2-SNFET to the illumination cycle of the 450 nm wavelength of light (Vg ) 20 V).

transistors (SNFETs) on SiO2/Si substrates, first we test the photoresponses from bare SNFET devices, where the device structure is illustrated in the inset of Figure 2a. The drain current (Id) in Figure 2a shows a steep increase upon the exposure of 450 nm of light at Vg ) -20 V, whereas no detectable Id change is observed at Vg ) 20 V. Figure 1b demonstrates the percentage increase in Id upon the illumination of various wavelengths (spot size: 75 µm; at similar power densities, ∼6 × 10-3 W/cm2). The Id increase has no obvious wavelength dependence from 450 to 900 nm and steeply decreases when the excitation wavelength is beyond 900 nm. The observed cutoff wavelength, ∼1100 nm, is consistent with the absorption edge (band-toband transition) for Si,27 directly proving that the photoconductivity for bare SNFETs is dominated by the photovoltage which electrically gates the transistor,28,29 where the inset in Figure 2b schematically illustrates the photogenerated electrons trapped at the SiO2/Si interface, resulting in the photovoltage. It is noted that this photoresponse becomes negligible at Vg ) 20 V, suggesting that the photovoltage is not formed at positive Vg due to the unsuitable Si band bending, and therefore the subsequent experiments are performed at Vg ) 20 V to avoid the photovoltage effect from the Si substrate. The observation also indicates that the direct excitation of SWNTs does not significantly contribute to the photoresponse in our experiments. Second, the photoconductivity of a pure F8T2 polymer is nondetectable, which is due to the fact that exciton diffusion lengths in various conjugated polymers are in a very short range of 5-14 nm30 and most of the excitons are recombined without generating photoconductivity. When a 60 nm layer of F8T2 is

Photoconductivity from Carbon Nanotube Transistors

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Figure 4. EFM phase images upon light switching for (a) F8T2-coated SiO2/Si substrates and (b) an F8T2-SNFET, with a tip bias at -3 and 3V, and AFM topography images for (a) and (b) are shown in (c) and (d), respectively.

Figure 5. (a) Light-induced phase shift for a F8T2-SNFET in EFM measurements (Vtip ) 3 V), where we indicate that labels “a” and “b” represent the traces with and without SWNT underneath for the EFM image in the inset. (b) Schematic illustration for the distribution of photoexcited charges in the F8T2 layers and also the dissociated charges around SWNTs.

coated on an SNFET (F8T2-SNFET), the transfer characteristics and on-off ratio of the SNFET exhibit no obvious change in dark ambient conditions (data not shown). However, the photoconductivity becomes detectable if the illuminating light is absorbed by F8T2. Figure 3a demonstrates that the optical absorption feature of an F8T2 film is well correlated to the Id increase of the F8T2-SNFET for at least seven wavelengths, suggesting that the photocurrent mainly results from the absorption of the F8T2 polymer. Figure 3b shows the typical Id response for an F8T2-SNFET to the 450 nm illumination cycle (75 µm spot size illuminating at the center of the channel; F8T2 has a strong absorption at 450 nm). Borghetti et al. have proposed that the photogenerated electrons are trapped at the

Figure 6. (a) Low-magnification SEM and (b) AFM images for the bare SNFET.

polymer-SiO2 interface and therefore induce a pronounced electrostatic gating effect resulting in the Id increase.19 We study the effect of illumination on charge distribution of the F8T2-dielectric interface using EFM.31-35 In EFM, the tip is driven at its resonant frequency during the interleave scan. The force gradient sensed by the tip changes the effective spring constant of the cantilever, modulating its resonant frequency. The shift in the phase lag between the drive frequency and the cantilever oscillation is measured when a DC voltage (+3 V or -3 V) is applied between the tip and sample. Therefore, EFM can map out the phase shift of the cantilever and link it to the electrical force gradient sensed by the tip.34 It is noted that the definition of phase value in the system is different from that typically reported in the literature.35 When a cantilever is driven at its resonance frequency, there is a π/2 phase shift between the cantilever vibration and the driving force. This π/2 shift is conventionally assigned as 0, but not in the MFP-3D system. Therefore, a positive phase shift in our EFM images indicates

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Figure 7. (a) Schematic illustration for the spatial arrangement of the SWNT networks, pads, and the definition for the spatial angle of illumination. (b) The effect of distance between the illumination spot and channel center on Id increase along the R direction. (c) The dependence of Id increase on the orientation (angle) of incident light. (d) The scheme showing the exciton dissociation and redistribution at SWNT and SiO2 interface.

Figure 8. Typical method of extracting the Id increase from Id vs time measurement.

the existence of an attractive force between the tip and the sample surface. Figure 4a demonstrates that the phase lag for F8T2-coated SiO2/Si substrates is positively shifted (more attractive force between the tip and polymer surface) when the light is turned on in the case of Vtip ) -3 V. By contrast, the phase lag is negatively shifted (more repulsive force) when Vtip is applied with 3 V. The result suggests that the surface of the F8T2 polymers is positively charged after light exposure. Figure 4b shows the result of parallel EFM measurements for an F8T2SNFET, where we can clearly identify the SWNTs under the F8T2 polymers. For both Vtip ) -3 and 3 V, the shift direction of the phase lag at either the blanket polymer area or the location with SWNTs underneath is consistent with the observation in Figure 4a, indicating that the phase shift is dominantly from the photon-induced change of the polymer F8T2. It is explained

by the fact that the photogenerated excitons in F8T2 are dissociated into electrons and holes, where some of the electrons are then trapped at the defect sites at the polymer-dielectric interface.36 The positive charges are therefore located closer to the F8T2 surface and sensed by EFM tips. Our EFM results provide direct evidence for Borghetti’s proposal that the trapping of electrons at the polymer-SiO2 interface in the direct vicinity of the SWNT governs the Id increase in SNFET. Figures 4c and 4d demonstrate the AFM topography of the samples studied for Figures 4a and 4b, respectively, where we conclude that the observations from EFM are valid, not due to the possible false signals from the topography. Figure 5a compares the typical light-induced phase shift for an F8T2-SNFET in EFM measurement (Vtip ) 3 V), where we indicate that labels “a” and “b” represent the traces with and without SWNT underneath for the EFM image in the inset. The change in phase shift along the trace with SWNT underneath is larger than the trace with the polymer only, showing that light exposure induces more charge separations at the polymerSWNT interfaces. It is noted that the reference EFM results for bare SWNTs (without the F8T2 coating) on SiO2/Si substrates do not show any phase change upon light exposure. It is suggested that the excitons dissociate at the polymer-SWNT interface,20 where the holes are preferentially injected into the SWNT due to the relative band energy alignment between F8T2 and SWNTs.29 The electrons stays with F8T2 are then trapped by the SiO2 surface as discussed previously. Thus, the EFM tip is able to effectively sense more positive charge around SWNTs upon light exposure. Figure 5b schematically illustrates the distribution of photoexcited charges in the F8T2 layers and also the dissociated charges around SWNTs.

Photoconductivity from Carbon Nanotube Transistors One striking feature observed for the polymer-coated SNFET is that the photoconductivity shows strong dependence on the spatial location of the incident light spot. We performed the location-selective illumination using 450 nm of wavelength light as the excitation source (focused by an objective down to 25 µm diameter). The low-magnification SEM for the device edge and AFM images for the channel area of the bare SNFET are shown in Figure 6. Figure 7a schematically illustrates the spatial arrangement of the SWNT networks, pads, and the definition for the spatial angle of illumination. Interestingly, the photoresponse can be detected when the illumination spot is several millimeters away from the device channel. Figure 7b shows that the effect of distance between the illumination spot and channel center on Id increases along the R direction, where we illustrate in Figure 8 the method of extracting the photocurrent from Id vs time measurement. The device can remotely sense the light at least up to 350 µm distance (light power density ∼0.24-0.30 W/cm2). The curve in Figure 7b is well fitted with a simple exponential decay function. There is no abrupt change in curve shape when the light spot is moving across the border of the underneath SWNT networks, indicating that the distant photoresponse is not directly related to the excitons generated in the F8T2-SWNT network composites distant from the channel center. Instead, it may be suggested from the long-range traveling of light from the illumination site to the devices. Poly(fluorene)s are known as good waveguide materials (refractive index (RI) is 1.75 at 466 nm)37 on SiO2 (RI ) 1.41-1.45). Therefore, the light is likely to travel within F8T2 via waveguide mode. The observed exponential decay of photocurrent with distance, regardless of the presence of underneath SWNT networks, seems to corroborate the expected exponential decay of light intensity due to light absorption in polymers. Figure 7c shows the dependence of Id increase on the orientation (angle) of incident light. When the light spot is 350 µm away from the channel center, the photoconductivity varies with the illumination angle (defined in Figure 7a; the power density is around 0.20 W/cm2). The Id increase at 0° illumination is much larger than that at 180°, suggesting that the pad polarity (determined by the applied bias) plays a role. Note that the SNFET is electrically symmetric, and the observation (preference of negative bias) is also valid if the source-drain bias is reversely switched. In general, a higher Id increase is observed for F8T2-SNFET when the illumination is close to the pad with a relatively negative bias (0 V at Source). As discussed previously, the overall photoresponse is suggested dominated by photoinduced electrostatic gating. However, this effect is not able to explain the spatial-dependent photoresponses, and therefore a secondary effect needs to be included. The experimental results suggest that the electric field is another dominant factor for the spatial-dependent photoresponse. It has been reported that photogenerated excitons in polymers are able to dissociate at the polymer-SWNT heterojunctions.20 We believe that the dissociation of excitons at the polymer-SWNT interface could be affected by the voltage applied on the SWNTs. It is likely that the exciton dissociation is favorable at the location where SWNT is applied with more negative voltage due to the fact that the negative voltage at SWNTs should attract positive charge carriers. Therefore, the injection of hole into SWNT is preferred from F8T2 polymers. The injected holes result in the increase in Id because the SWNT transistors are known to be p-type semiconductors in ambient.1 Thus, the position of illumination spot relative to the source and drain pads determines the response of photoconductivity. Figure 7d schematically illustrates that more excitons are

J. Phys. Chem. C, Vol. 112, No. 46, 2008 18205 dissociated at the polymer-SWNT interface close to the electrode applied with a relatively negative voltage. Conclusions In summary, our EFM results suggest that upon light illumination the excitons dissociate at the polymer-SWNT interface, where the holes are preferentially injected in to the SWNT due to the relative band energy alignment between the polymer and SWNTs, and electrons are trapped at the polymer-SiO2 interface in the direct vicinity of the SWNT which governs the Id increase in SNFET. In other words, our results are in line with the recent argument that the photocurrent is due to the trapping of photogenerated electrons at SiO2 dielectric surfaces rather than the direct transfer of the photogenerated carrier into SWNTs. Also, remarkable locationdependent photoresponses in photosensitive polymer-SNFET have been observed. These spatial photoresponses are general phenomena, which are also observed for network resistors on other substrates and for other polymers. The design of metal patterns is crucial for enhancing the spatial photoresponses. These results suggest a new strategy for achieving remote light detection, position sensors, or antenna devices. Acknowledgment. We acknowledge with thanks the support from MINDEF and Nanyang Technological University. L.J.L. thanks Prof. J.A. Rogers (UIUC) for providing some of the nanotube transistor devices. Y. Shi and X. Dong contributed equally to this work. References and Notes (1) Tan, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49–52. (2) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, Ph. Appl. Phys. Lett. 1998, 73, 2447–2449. (3) Fuhrer, M. S.; Kim, B. M.; Durkop, T.; Brintlinger, T. Nano Lett. 2002, 2, 755–759. (4) Radosavljevic, M.; Freitag, M.; Thadani, K. V.; Johnson, A. T. Nano Lett. 2002, 2, 761–764. (5) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622–625. (6) Star, A.; Gabriel, J. C. P.; Bradley, K.; Gruner, G. Nano Lett. 2003, 3, 459–463. (7) Snow, E.; Perkins, F.; Houser, E.; Badescu, S.; Reinecke, T. Science 2005, 307, 1942–1945. (8) Gui, E. L.; Li, L. J.; Zhang, K.; Xu, Y.; Dong, X.; Ho, X.; Lee, P. S.; Kasim, J.; Shen, Z. X.; Rogers, J. A.; Mhaisalkar, S. J. Am. Chem. Soc. 2007, 129, 14428–14432. (9) Freitag, M.; Martin, Y.; Misewich, J. A.; Martel, R.; Avouris, Ph. Nano Lett. 2003, 3, 1067–1071. (10) Balasubramanian, K.; Fan, Y.; Burghard, M.; Kern, K.; Friedrich, M.; Wannek, U.; Mews, A. Appl. Phys. Lett. 2004, 84, 2400–2402. (11) Ohno, Y.; Kishimoto, S.; Mizutani, T.; Okazaki, T.; Shinohara, H. Appl. Phys. Lett. 2004, 84, 1368–1370. (12) Itkis, M. E.; Borondics, F.; Yu, A.; Haddon, R. C. Science 2006, 312, 413–416. (13) Wei, J.; Sun, J. L.; Zhu, J. L.; Wang, K.; Wang, Z.; Luo, J.; Wu, D.; Cao, A. Small 2006, 8-9, 988–993. (14) Lien, D. H.; Hsu, W. K.; Zan, H. W.; Tai, N. H.; Tsai, C. H. AdV. Mater. 2006, 18, 98–103. (15) Ma, Y. Z.; Valkunas, L.; Bachilo, S. M.; Fleming, G. R. J. Phys. Chem. B 2005, 109, 15671–15674. (16) Li, Y. F.; Kaneko, T.; Hatakeyama, R. Appl. Phys. Lett. 2008, 92, 183115/1–183115/3. (17) Shi, Y.; Fu, D. L.; Marsh, D. H.; Rance, G. A.; Khlobystov, A. N.; Li, L. J J. Phys. Chem. C. 2008,112,13004–13009. (18) Star, A.; Lu, Y.; Bradley, K.; Gruner, G. Nano Lett. 2004, 4, 1587– 1591. (19) Borghetti, J.; Derycke, V.; Lenfant, S.; Chenevier, P.; Filoramo, A.; Goffman, M.; Vuillaume, D.; Bourgoin, J. P. AdV. Mater. 2006, 18, 2535–2540. (20) Yang, C.; Wohlgenannt, M.; Vardeny, Z. V. Physica B 2003, 338, 366–369. (21) Hur, S. H.; Kocabas, C.; Gaur, A.; Shim, M.; Park, O. O.; Rogers, J. A J. Appl. Phys. 2005, 98, 114302/1–114302/6.

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