Carbon Nanotubes as Injection Electrodes for Organic Thin Film

NANO LETTERS

Carbon Nanotubes as Injection Electrodes for Organic Thin Film Transistors

2009 Vol. 9, No. 4 1457-1461

C. M. Aguirre,† C. Ternon,†,‡ M. Paillet,§ P. Desjardins,† and R. Martel*,§ Regroupement Que´be´cois sur les Mate´riaux de Pointe (RQMP) and De´partement de Ge´nie Physique, E´cole Polytechnique de Montre´al, Montre´al, Quebec H3C 3A7, Canada, Laboratoire des Technologies de la Microe´lectronique (LTM), Grenoble, France, and De´partement de Chimie, UniVersite´ de Montre´al, Montre´al, Quebec H3T 1J4, Canada Received November 3, 2008; Revised Manuscript Received January 15, 2009

ABSTRACT We have investigated the charge injection efficiency of carbon nanotube electrodes for organic semiconducting layers and compared their performance to that of traditional noble metal electrodes. Our results reveal that charge injection from a single carbon nanotube electrode is more than an order of magnitude more efficient than charge injection from metal electrodes. Moreover, organic thin film transistors that use arrays of carbon nanotube electrodes display considerable effective mobilities (0.14 cm2/(V·s)) and nearly ideal linear output characteristics. These results indicate that carbon nanotubes should be considered a viable alternative to metal electrodes for next-generation organic fieldeffect transistors.

Achieving efficient carrier injection into organic semiconducting layers has proven to be a significant challenge to improving the performance of organic thin-film transistors (OTFTs).1 Indeed, inefficient carrier injection leads to parasitic contact resistances that compare in magnitude to the channel resistance of these devices.2,3 Barriers to charge injection are caused by dipoles located at the interface between the electrode and the organic semiconductor.4,5 As a number of theoretical and experimental studies have highlighted, the electrode work function,5-7 doping levels,8 and interfacial traps9 are key factors in determining the height of the barriers at the electrode-organic semiconductor interface. It has also been suggested that the morphology of the interface played a primary role with respect to the injection efficiency of the contact.10-12 Strategies for reducing parasitic contact resistances have thus far almost exclusively centered on lowering the height of dipole barriers by modifying the energetics at the surface of noble metal electrodes.13-16 The search for alternative electrode materials is motivated by the limited success these strategies have had in eliminating contact resistances. Carbon nanotubes have been suggested as a potential electrode material for organic semiconductors.17-19 Owing

to their high work function (∼5.0 eV),20,21 carbon nanotubes are well suited as hole injection electrodes (anodes) for OTFTs. Moreover, it is possible to adjust the carbon nanotube work function through charge transfer doping in order to align it to the conduction band of a given organic semiconductor.22 Transparent and conducting carbon nanotube sheets have already demonstrated their effectiveness as anodes in both organic photovoltaic devices23-25 and organic lightemitting diodes.26-28 It has also been shown that an individual carbon nanotube could be used to inject charge into either nanoscale semiconducting islands18,19 or individual conducting molecules.17,29,30 Both the chemical compatibility of carbon nanotubes and the favorable one-dimensional (1D) electrostatics have been suggested to account for the performance of individual carbon nanotube electrodes. Despite these encouraging results, only a few studies have explored the suitability of carbon nanotubes as electrodes for OTFTs having technologically relevant dimensions.31-33 A systematic evaluation of the injection efficiency of carbon nanotube electrodes must be carried out if they are to be considered a viable alternative to the traditional metal contacts for both nanoscale and macroscale organic fieldeffect transistors.

* Corresponding author, [email protected]. † Regroupement Que´be´cois sur les Mate´riaux de Pointe (RQMP) and De´partement de Ge´nie Physique, E´cole Polytechnique de Montre´al. ‡ Laboratoire des Technologies de la Microe´lectronique (LTM). § De´partement de Chimie, Universite´ de Montre´al.

In this work we explore the performance of carbon nanotubes as electrodes for injecting holes into high mobility organic semiconducting layers. We begin by evaluating the injection efficiency of an individual carbon nanotube elec-

10.1021/nl8033152 CCC: $40.75 Published on Web 03/16/2009

 2009 American Chemical Society

Figure 1. (a) Schematic diagram (top) of an individual pentacene island contacted by two metallic carbon nanotubes that act as source and drain electrodes. The ∼50 nm gap is created by the electrical breakdown of a double-walled metallic carbon nanotube. A composite SEM/AFM image of a nanoscale pentacene transistor (bottom). (b) Schematic of a carbon nanotube array electrode in the configuration used to fabricate the pentacene OTFT devices (top). SEM image of an array-contacted OTFT device showing the carbon nanotubes sticking out from the titanium electrodes (bottom).

trode into a nanoscale pentacene island. We show that currents injected using a carbon nanotube are more than an order of magnitude greater than currents injected using traditional metal electrodes. In order to evaluate the potential of carbon nanotubes in OTFTs having larger dimensions, we contacted devices with arrays of carbon nanotubes attached to Ti electrodes. These devices exhibited not only better injection performance but overall effective mobility values that were greater than OTFTs contacted using the traditional Au contacts. Using a simple, solution-based method, we fabricated nanoscale field-effect transistors from a nanoscale pentacene island placed between the ends of two metallic carbon nanotubes. Our devices have similar geometries to those studied by Qi et al.18 A schematic illustration of the back-gated pentacene nanotransistor configuration used in this study is shown in Figure 1a, along with a composite of the scanning electrode microscopy (SEM) and the atomic force microscopy (AFM) images of one such device (see Supporting Information for image processing details). The fabrication protocol for these devises is as follows: first, double-walled nanotubes were dispersed in a dichloroethane solvent and spin-coated onto a degenerately doped n+ silicon wafer covered by a 20 or 100 nm thick thermal oxide layer. Electrical contacts were patterned by optical lithography, followed by e-beam evaporation of 0.5 nm Ti and 20 nm Pd layers. The devices were annealed in vacuum (1 × 10-6 Torr) at 550 °C for 1 h in order to decrease the metal-carbon nanotube contact resistances.34 The electrical characteristics of the connected carbon nanotubes were then probed in order to identify devices exhibiting purely metallic behavior (i.e., no conductance dependence of the gate voltage). Finally, gaps were opened at the center the metallic carbon nanotubes by electrical breakdown.35 The resulting gap length was measured to be in the range of ∼20-200 nm. Each carbon nanotube end will serve as the source and drain electrodes of the nanoscale transistor. 1458

Pentacene islands were deposited using a soluble pentacene precursor route. The pentacene adduct 13,6-N-sulfinylacetamidopentacene was purchased from Sigma-Aldrich and used without further purification. Spin-coated films of the precursor undergo a solid-phase conversion to pentacene when heated at 170-200 °C. These pentacene films have been shown to exhibit field-effect mobilities (∼0.8 cm2/(V·s)) that are comparable to vacuum-sublimed pentacene films (∼1 cm2/(V·s)).36 For this study, adduct films, 10 nm in thickness, were spin coated from a 2 mg/mL chloroform solution onto the wafer containing the carbon nanotube electrodes (after breakdown). During their conversion into pentacene, these molecules assemble into pentacene islands ∼100 nm in width, which preferentially nucleate at carbon nanotube locations (see Supporting Information). As a result, islands were found to grow preferentially in the gaps between the carbon nanotube ends, which enabled the fabrication of pentacene nanotransistors with high yield. We also fabricated OTFTs contacted with arrays of carbon nanotube electrodes. Figure 1b shows a schematic diagram of the OTFT devices together with an SEM image of the region in the proximity of the carbon nanotube array electrode. Because of the different shape and orientation of pentacene grains deposited on carbon nanotubes,37 the array of nanotubes, although covered with a 50 nm pentacene layer, can be clearly resolved. (See Supporting Information for additional images.) A subtractive technique was used for the fabrication of these “hairy electrodes”. First, a dense network (∼10 nanotubes/µm2) of single-walled carbon nanotubes was transferred onto the surface of a degenerately doped n+ silicon wafer covered by a 100 nm oxide layer by using a vacuum filtration method.38 Titanium contacts 30 nm in thickness were made using conventional optical lithography and lift-off techniques. These electrode patterns define OTFTs having channels 200 µm wide and 20 µm in length. The carbon nanotubes that are not directly attached to the metal electrodes were removed by subjecting the structures to a sonication treatment, while immersed in a n-methylpyrrolidone-based resin stripper (AZ300T Clariant). The electrode patterns were annealed in vacuum (1 × 10-6 Torr) at 550 °C for 1 h prior to the deposition of pentacene thin films. Pentacene was purchased from Sigma-Aldrich and used without further purification. Pentacene layers, 50 nm in thickness, were deposited onto the electrodes by vacuum sublimation at a rate of 0.2 nm/s. In order to benchmark our nanotube electrodes, standard OTFT devices having both Au and Ti 30 nm thick electrodes (without carbon nanotube arrays) were fabricated and characterized simultaneously. A composite SEM/AFM from a typical pentacene nanotransistor connected by carbon nanotube source and drain electrodes separated by a ∼40 nm gap is displayed in Figure 2a, while the transfer and output characteristics of this device are presented in Figure 2b. The on-off ratio (Ion/Ioff) for this particular pentacene nanotransistor is 102 and the On-state current is 2 nA at a source-drain voltage of Vds ) 8 V. All 11 similar devices fabricated and characterized for this study exhibited similar p-type transport characteristics with Onstate currents in the 1-10 nA range (Vds ) 8 V). These large Nano Lett., Vol. 9, No. 4, 2009

Figure 2. (a) Pentacene nanotransistor having a channel length of 40 nm and width of 2.5 nm (nanotube diameter). (b) Transfer and output (inset) characteristics of the corresponding device. The transfer characteristic is shown using Vds ) 5 V.

current densities are remarkable if we consider that the width of the channel is within the range of the carbon nanotube diameter, i.e., only 2-4 nm. We also note that the superlinear increase in the drain-source current seen in Figure 2 was observed for all devices. This behavior is characteristic of short channel effects that result from inefficient gate coupling in devices having thick gate dielectrics. That is, when the length of the channel (e.g., 40 nm) is similar to the thickness of the gate dielectric (e.g., 20 nm), the field at the channel is poorly controlled by the gate electrode, thus leading to the superlinear behavior.39 Scaling the gate dielectric down to the 1 nm range should drastically improve the linearity of the output characteristics. The importance of the contact resistances of our devices could then be estimated. Indeed, amorphous carbon produced during electrical breakdown of the nanotubes could hinder the direct contact between semiconductor island and the carbon nanotube electrode. Interestingly, although the length of the gaps (effective channel length) varied considerably from device to device (from 20 to 200 nm), no correlation was found between this length and the magnitude of either the On-state current or the transfer characteristics of the devices. While Ion/Ioff values were typically of 102 with a subthreshold slope S ∼ 1.3 V/dec, we noted a considerable device-to-device variability Nano Lett., Vol. 9, No. 4, 2009

(Ion/Ioff from 10 to 104), which reflects the inherent differences in electrostatics that are inevitable in devices having such small dimensions. For instance, small differences in the diameter of the carbon nanotubes will lead to substantially different channel fields at the vicinity of the injection area. It is interesting to compare the behavior of our devices to that of Au-contacted pentacene nanoscale transistors such as those reported by Kagan et al.40 In their study, pentacene field effect transistors were fabricated on a 2 nm thick silicon dioxide layer having channel widths W ) 250 nm and lengths down to 20 nm. The soluble precursor route that enabled the fabrication of the nanoscale pentacene transistors described in the present study was also used in Kagan’s work in order to place pentacene islands between lithographically defined Au electrodes. The Au-contacted nanoscale pentacene transistors described by Kagan et al. exhibited ratios Ion/Ioff ∼ 1 × 102 and On state currents of 2 nA at Vd ) -4 V with a subthreshold slope of S ∼ 1 V/dec Although these characteristics are similar to those measured for the nanotube-pentacene transistors, the width of the channel for Au-contacted devices is approximately 2 orders of magnitude larger. Thus, as a first approximation, by normalizing for the channel width we can estimate that the On-state currents of nanotube-contacted pentacene nanotransistors as being roughly 100× greater than that obtained for Au-contacted devices of similar widths. It is important to note, however, that this estimate is an upper bound and does not take into account the difference in fringing fields and in electrode contact areas of these devices. Nevertheless, this value alone clearly indicates that better injection performances are achieved with carbon nanotube contacts. The behavior observed in devices asymmetrically contacted by carbon nanotube and Pd electrodes provides further evidence to the considerably better injection efficiency of carbon nanotube electrodes. Devices contacted by metallic carbon nanotubes at both ends (Figure 3a) display very similar output characteristics irrespective to which end was used as the source electrode. In fact, the On-state current of devices contacted in this manner differed, on average, by a factor of only 2 when injecting charge from either carbon nanotube end. This behavior is very different from that presented in Figure 3b where On-state currents are 21 times smaller when injecting holes from the Pd contact compared to when currents are injected from the carbon nanotube contact. Thus, although previous studies have shown Pd to be among the best metals for injecting holes into pentacene layers10,41 and despite the much greater contact area between the Pd electrode and the pentacene island, the use of carbon nanotube leads to much greater current densities. The improvement over the injection efficiency that is observed in nanoscale pentacene transistors can be used to improve the performance of pentacene OTFTs having commercially viable dimensions. Instead of a single carbon nanotube contacting a single nanoscale pentacene island, arrays are attached to Ti electrodes in order to fabricate OTFTs having channels 200 µm in width and 20 µm in length such as those shown in Figure 1b. The output characteristics in the low bias regime typical of carbon 1459

with either Au or Ti electrodes displayed the nonlinear behavior that is indicative of the presence of large contact barriers at the metal-organic semiconductor interface. Most importantly, we see that the lowering of injection barriers leads to an improvement of the effective mobility of the devices. For the devices displayed in Figure 3, the effective linear mobility of nanotube array-, Au-, and Ti-contacted OTFTs were measured to be 0.14, 0.09, and 0.001 cm2/(V·s), respectively. (See Supporting Information for further details.) We note that mobilities ∼0.1 cm2/(V·s) are within the best for pentacene OTFTs fabricated from commercial, vacuumsublimed pentacene.42 The low mobility and nonlinear output characteristics of Ti-contacted OTFTs are not surprising given that the presence of an oxide at this interface is expected to lead to important tunnel barriers. Similarly to Pd, Au is among the best contact metals for injecting holes into pentacene layers;10,41 however, unmodified gold bottom contacts are known to exhibit nonlinear output characteristics and large contact resistances as those measured for the device displayed in Figure 4.15,16

Figure 3. Output characteristics of pentacene nanotransistors: (a) carbon nanotube electrodes at both ends (continuous red curves) or (b) asymmetric carbon nanotube-Pd electrodes. The continuous red curve corresponds to injection by the nanotube electrode, while the continuous blue curve is for injection from the Pd electrode.

Figure 4. Low bias output characteristics of OTFTs having either carbon nanotube array electrodes attached to Ti contacts (red curve with squares), Au electrodes (blue curve with circles), or Ti electrodes (green curve with triangles).

nanotube array contacted devices are displayed (red squares) in Figure 4. Also shown are the characteristics of equivalent OTFTs contacted by either Au (blue circles) or Ti (green triangles) electrodes. As evidenced from these curves, only the carbon nanotube array contacted-OTFT displayed ideal linear current output characteristics. The OTFTs contacted 1460

The origin of the improved performance of carbon nanotube electrodes remains unclear. It is possible that intrinsic injection barriers at the pentacene-nanotube interface are lower than those for metals. However, given the scale of the improvement, we believe that the contact geometry most likely induces electrostatic effects that shift the molecular energy levels near this interface (e.g., bandbending-like shifts) resulting in an effective dipole barrier reduction. We propose that the enhancement of the applied electric field at the tip of carbon nanotubes is at the origin of the dipole barrier reduction. Indeed, it is well-known that the form factor of carbon nanotubes results in the enhancement of the applied external field at the nanotube apex. This, in turn, narrows the effective barrier for an electron to escape the carbon nanotube through Fowler-Nordheim tunneling. This effect is exploited in carbon nanotube vacuum fieldeffect emitters.43,44 The same effect also appears to facilitate the tunneling of charges across the carbon nanotube-organic semiconductor interface. We have demonstrated that carbon nanotubes are indeed efficient electrodes in both nanoscale and thin-film pentacene transistors. Injection from carbon nanotube electrodes led to devices displaying better performances than devices contacted using traditional noble metal electrodes. Individual nanotube-pentacene nanotransistors displayed output currents that are 2 orders of magnitude greater than that for similar Au contacted devices, whereas OTFTs contacted using carbon nanotube arrays exhibited barrier-free-like injection and overall better effective mobilities. The deviceto-device variability in nanoscale pentacene transistors suggests that the 1D geometry of carbon nanotubes is central to the observed improvement in injection efficiency. Namely, the electrostatic field at the nanotube-organic junctions and, consequently, the narrowing of injection barriers result in better tunneling injection even without any surface treatment. We expect that further improvements in device performance will be achieved by optimizing the density of the carbon nanotube arrays in order to minimize electrode static screenNano Lett., Vol. 9, No. 4, 2009

ing by neighboring nanotubes,45 by shifting the work function of carbon nanotube electrodes by charge transfer doping, and by exploiting covalent chemistry at carbon nanotube surfaces. Acknowledgment. The authors thank Professor Ricardo Izquierdo for valuable discussions, technical assistance, and access to the organic thin-film deposition equipment. We are grateful to Dr. Emmanuel Flahaut and Dr. Benoit Simard for the generous donation of the double-walled and singlewalled carbon nanotubes, respectively. This project is supported by the Canada Research Chair (CRC), NSERC Discovery, and FQRNT E´quipe programs. Part of this work was carried in the GCM Central Facilities, which are supported by the NSERC MRS and RQMP programs. Supporting Information Available: Image processing details, nucleation of pentacene crystals on carbon nanotubes, fabrication protocol for carbon nanotube array electrodes, high magnification image of carbon nanotube array electrodes, and output characteristics for pentacene OTFTs. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Shen, Y.; Hosseini, A. R.; Wong, M. H.; Malliaras, G. G. ChemPhysChem 2004, 5, 16–25. (2) Hamadani, B. H.; Natelson, D. J. Appl. Phys. 2004, 95, 1227–1232. (3) Pesavento, P. V.; Puntambekar, K. P.; Frisbie, C. D.; McKeen, J. C.; Ruden, P. P. J. Appl. Phys. 2006, 99, 094504–10. (4) Hill, I. G.; Rajagopal, A.; Kahn, A.; Hu, Y. Appl. Phys. Lett. 1998, 73, 662–664. (5) Ishii, H.; Seki, K. IEEE Trans. Electron DeVices 1997, 44, 1295– 1301. (6) Kahn, A.; Koch, N.; Gao, W. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2529–2548. (7) Rajagopal, A.; Wu, C. I.; Kahn, A. J. Appl. Phys. 1998, 83, 2649– 2655. (8) Kahn, A.; Zhao, W.; Gao, W.; Vazquez, H.; Flores, F. Chem. Phys. 2006, 325, 129–137. (9) Vazquez, H.; Flores, F.; Oszwaldowski, R.; Ortega, J.; Perez, R.; Kahn, A. Appl. Surf. Sci. 2004, 234, 107–112. (10) Bock, C.; Pham, D. V.; Kunze, U.; Ka¨fer, D.; Witte, G.; Wo¨ll, C. Physica E 2008, 40, 2107–2109. (11) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A. Chem. Mater. 1996, 8, 2542–2544. (12) Veinot, J. G. C.; Marks, T. J. Acc. Chem. Res. 2005, 38, 632–643. (13) Marmont, P.; Battaglini, N.; Lang, P.; Horowitz, G.; Hwang, J.; Kahn, A.; Amato, C.; Calas, P. Org. Electron. 2008, 9, 419–424. (14) Vanoni, C.; Tsujino, S.; Jung, T. A. Appl. Phys. Lett. 2007, 90, 193119– 3. (15) Hamadani, B. H.; Corley, D. A.; Ciszek, J. W.; Tour, J. M.; Natelson, D. Nano Lett. 2006, 6, 1303–1306. (16) Gundlach, D. J.; Li Li, J.; Jackson, T. N. IEEE Electron DeVice Lett. 2001, 22, 571–573. (17) Guo, X.; Small, J. P.; Klare, J. E.; Wang, Y.; Purewal, M. S.; Tam, I. W.; Hong, B. H.; Caldwell, R.; Huang, L.; O’Brien, S.; Yan, J.;

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