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High-Performance Field-Effect Transistors Based on Langmuir-Blodgett Films of Cyclo[8]pyrrole Hai Xu, Gui Yu, Wei Xu,* Yu Xu, Guanglei Cui, Deqing Zhang, Yunqi Liu,* and Daoben Zhu* Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received January 14, 2005. In Final Form: March 31, 2005 We demonstrate the field-effect transistors (FETs) made of cyclo[8]pyrrole thin films prepared by the Langmuir-Blodgett (LB) method. The cyclo[8]pyrrole molecule possesses a 30-π-electron system and narrower highest-occupied molecular orbital-lowest-unoccupied molecular orbital energy gap (0.63 eV), forms a stable, reproducible monolayer at the air-water interface, and transfers onto a substrate with a nearly unity transfer ratio and face-to-face configuration due to its strong π-π interaction. The LB films are uniform characterized by atomic force microscopy and in ordered form confirmed by X-ray diffraction. The FET exhibited high performances with one of the highest hole mobilities (0.68 cm2 V-1 s-1) for thin-film transistors and a high on/off ratio, implying a promising material in the FET family.
Introduction In the past decade, there are growing interests in the field of organic field-effect transistors (OFETs). As a key component of plastic circuits, OFETs are requested for the fabrication of identification tags, smart cards, and display drivers.1 To date, the most promising molecular materials that can be used for such applications are pentacene2 and thiophene oligomers.3 Searching for new organic semiconductor as the active layer for OFETs is a current research focus in the field of OFETs to meet with the requirement of high performance and easy processibility.4 Porphyrin is an important active component involved in natural and biological systems and studied for centuries. A typical porphyrin molecule has an 18-π-electron system and coplanar disk-like structure; these features seem to be favorable for the formation of a conducting channel through close π-π stacking. FET activities of porphyrins have been reported recently with mobilities of 0.012 cm2 V-1 s-15 and ∼1.3-2.2 × 10-4 cm2 V-1 s-1,6 respectively. (1) (a) Bao, Z. N. Adv. Mater. 2000, 12, 227. (b) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99. (c) Katz, H. E.; Bao, Z. N.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359. (d) Katz, H. E. J. Mater. Chem. 1997, 7, 369. (2) (a) Garnier, F. Chem. Phys. 1998, 227, 253. (b) Horowitz, G.; Hajilaoui, R.; Bouchriha, R. B.; Hajlaoui, M. Adv. Mater. 1998, 10, 923. (c) Nelson, S. F.; Lin, Y. Y.; Gundlach, D. J.; Jackson, T. N. Appl. Phys. Lett. 1998, 72, 1854. (d) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.; Wudl, R.; Bao, Z. N.; Siegrist, T.; Kloc, C.; Chen, C. H. Adv. Mater. 2003, 15, 1090. (3) (a) Akimichi, H.; Waragai, K.; Hotta, S.; Sakaki, H. Appl. Phys. Lett. 1991, 58, 1500. (b) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716. (c) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69, 4108. (d) Facchetti, A.; Nushrush, M.; Katz, H. E.; Marks, T. Adv. Mater. 2003, 15, 33. (4) See, for example: (a) Mas-Torrent, M.; Durkut, M.; Hadley, P.; Ribas, X.; Rovira, C. J. Am. Chem. Soc. 2004, 126 (4), 984. (b) Yasuda, T.; Fujita, K.; Tsutsui, T.; Geng, Y.; Culligan, S. W.; Chen, S. H. Chem. Mater. 2005, 17 (2), 264. (c) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.; Tierney, S.; McCulloch, I. J. Am. Chem. Soc. 2005, 127, 1078. (d) Wu, Y.; Li, Y.; Gardner, S.; Ong, B. S. J. Am. Chem. Soc. 2005, 127, 614. (5) Checcoli, P.; Conte, G.; Salvatori, S.; Paolesse, R.; Bolognesi, A.; Berliocchi, M.; Brunetti, F.; D’Amico, A.; Di Carlo, A.; Lugl, P. Synth. Met. 2003, 138, 261. (6) Noh, Y. Y.; Kim, J. J.; Yoshida, Y.; Yase, K. Adv. Mater. 2003, 15, 699.
Figure 1. Chemical structure of cyclo[8]pyrrole molecule.
To increase the mobility of the porphyrin-based FET, extension of the π system should be a reasonable strategy, because the highest-occupied molecular orbital (HOMO) energy level in the porphyrin will be significantly increased with an extended π system, which facilitates the formation of radical cations (holes) at the active layer. An extended porphyrin analogue will also increase the intermolecular overlap of π-π orbitals in the solid states and lead to high mobility. Very recently, an extended porphyrin-like molecule, cyclo[8]pyrrole (see Figure 1), possessing a 30-πelectron system was synthesized by J. L. Sessler and coworkers using a creative one-pot strategy from bipyrrolic fragments.7 Different from those large expanded porphyrins (those containing eight or more pyrrole rings) that have ever been obtained, which often adopt twisted conformations,8 this molecule shows a nearly coplanar conformation. These two characteristics, highly extended π systems and coplanar conformation, made this molecule fulfill the requirement of the above purpose. In this work, FETs using Langmuir-Blodgett (LB) films of cyclo[8]pyrrole as an active layer have been demonstrated. The preliminary results revealed that this molecule could act as a high-performance semiconductor for OFETs. Results and Discussion According to the crystal structure reported by Sessler,7 cyclo[8]pyrrole possesses a nearly flat coplanar macrocyclic (7) Seidel, D.; Lynch, V.; Sessler, J. L. Angew. Chem., Int. Ed. 2002, 41, 1422. (8) See, for example: (a) Sessler, J. L.; Weghorn, S. J.; Lynch, V.; Johnson, M. R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1509. (b) Setsune, J. I.; Katakami, Y.; Iizuna, N. J. Am. Chem. Soc. 1999, 121, 8957. (c) Shin, J. Y.; Furuta, H.; Yoza, K.; Igarashi, S.; Osuka, A. J. Am. Chem. Soc. 2001, 123, 7190.
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Figure 2. Surface pressure-area isotherm of cyclo[8]pyrrole at room temperature.
system with a sulfate anion centrally located in the cavity. With a hydrophilic center and a hydrophobic periphery, cyclo[8]pyrrole seems to be an amphiphilic molecule. Although it is different from the typical amphiphilic molecules, it can form a stable, reproducible Langmuir film at the air-water interface. The surface pressurearea isotherms of cyclo[8]pyrrole are shown in Figure 2. The smooth inclining part corresponding to formation of the solid monolayer and the relatively high surface pressure of the collapse point of the monolayer indicate the good film-forming behavior of the cyclopyrrole compounds. The extrapolating area from the linear part of the isotherm was 120 Å2 molecule-1. The crystal structure analysis revealed that cyclo[8]pyrrole can be regarded as
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a square disk with a side length of 17 Å and a thickness of 4 Å. So if the cyclo[8]pyrrole molecules are densely stacked in a face-to-face orientation and standing perpendicularly on the water surface, the average area per molecule would be about 68 Å2. And if the molecules were laid down flat on the water surface, the average area per molecule would be about 290 Å2. Since the limiting area per molecule from the isotherm is between the above two calculated values, it is reasonable to consider that cyclo[8]pyrrole molecule is in a tilted arrangement on the water surface, and the tilt angle is calculated to be about 65°. The LB film of cyclo[8]pyrrole can transfer onto different substrate with the vertical deposition. At a constant pressure of 15.5 mN m-1, the film transfer ratio was about ∼1.0-0.9 and was nearly unity up to more than five layers. The deposition was monitored by optical absorption. Figure 3a shows the UV-vis spectra of the films deposited on the quartz with different number of layers. These films display a Soret-type absorbance at 432 nm which is consistent with the absorption spectra of cyclo[8]pyrrole observed in solution. The linear relation between the absorbance of the film at 432 nm and the number of layers clearly demonstrates the reproducibility of the transfer process and indicates the films were built up uniformly into multilayer structure. The morphology of the LB films was investigated by atomic force microscopy (AFM). A sample of 1-layer cyclo[8]pyrrole LB film deposited directly on mica was observed in a tapping mode. The large square image (2.0 µm × 2.0 µm, Figure 4a) shows that the LB film deposited from the water surface has a uniform surface morphology and is
Figure 3. (a) UV-vis absorption spectra of 1-, 3-, and 5-layer cyclo[8]pyrrole LB films. (b) The UV-vis absorbance of cyclo[8]pyrrole LB films as a function of the number of layers deposited, monitored at 432 nm.
Figure 4. 2.0 µm × 2.0 µm (a) and 400 nm × 400 nm (b) AFM images of 1-layer cyclo[8]pyrrole film transferred onto the mica surface.
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Figure 6. Schematic view of an OFET based on the LB films of cyclo[8]pyrrole.
Figure 5. X-ray diffraction pattern of the cyclo[8]pyrrole LB films.
very flat. From the enlarged image (Figure 4b), large grains with a size of about 20-40 nm and a height lower than 1.1 nm can be noticed. These figures show that the cyclo[8]pyrrole molecules are homogeneously arranged in the LB film. The structures of the LB films were further investigated by X-ray diffraction studies on multilayer samples deposited on a glass slip. The X-ray diffraction pattern of a 30-layer sample is shown in Figure 5. An intense Bragg diffraction peak at 2θ ) 1.58 was observed, indicating the existence of an ordered layer structure (001) parallel to the substrates. This peak maximum yields a Bragg spacing of 56.0 Å. As the films were transferred on to the substrate with a Y-type vertical deposition method, the multiplelayer film obtained should have a bilayer structure. If cyclo[8]pyrrole molecules have a fully face-to-face conformation in the film and the center of the molecules are located at a plane parallel to water surface, this bilayer should have a thickness (40.8 Å) about twice the side length of cyclo[8]pyrrole molecule (17 Å) and a van der Waals radii (3.4 Å). So, the Bragg spacing is larger than what was predicted for such an ordered bilayer structure. This indicates that the centers of cyclo[8]pyrrole molecules are not located at a plane parallel to water surface; the molecules just partially overlapped with each other. Studies of the electrochemical behaviors of the cyclo[8]pyrrole are essential for the understanding of the redox properties of the molecule which is considered as an indicator of the HOMO and the lowest-unoccupied molecular orbital (LUMO). As previously reported,9 cyclo[8]pyrrole displayed four reversible one-electron oxidation waves and one two-electron reduction wave under cyclic voltammogram (CV) investigation. In this work, the electrochemical behaviors of cyclo[8]pyrrole were reinvestigated. For comparison, 5,10,15,20-tetra-phenyl porphyrin (H2-TPP), 2,3,7,8,12,13,17,18-octaethyl-porphyrin (H2-OEP), and a well-known electron donor bisethylenedithiatetrafulvalene (BEDT-TTF) were also characterized by CV under the same conditions. The redox potentials of these compounds were summarized in Table 1. Similar to
Figure 7. (a) Drain current (IDS) as a function of gate voltage (VG) for a FET made of 30-layer cyclo[8]pyrrole LB films. (b) IDS and IDS1/2 vs VG plots at VD ) -12 V for the same device.
what reported by Sessler, five redox couples of cyclo[8]pyrrole can be observed. By comparison with the first oxidation potential of H2-TPP, H2-OEP, and BEDT-TTF, cyclo[8]pyrrole displays a better electron-donating ability than H2-TPP, H2-OEP, and even the most widely studied electron donor BEDT-TTF. The energy difference between the first reduction and the first oxidation is related to the band energy gap of HOMO and LOMO orbitals. The electrochemical results indicate that, compared to H2TPP (2.00 eV) and H2-OEP (2.27 eV), cyclo[8]pyrrole possesses a much narrower HOMO-LUMO gap (0.63 eV). The LB films were subsequently made into FET devices to test their charge-carrier mobilities. The cross-sectional view of the transistor device structure is shown in Figure 6, and its fabrication was described previously.10 Figure 7 shows typical I-V curves acquired from devices operating in the accumulation mode and cyclo[8]pyrrole acts as
Table 1. Half-Wave Potentials (V vs SCE) of Cyclo[8]pyrrole, H2-TPP, H2-OEP, and BEDT-TTFa E01/2 (V) Compound H2-TPP H2-OEP BEDT-TTF cyclo(8)pyrrole a
(4)
1.51
ER1/2 (V)
HOMO-LUMO gap (eV)
(3)
(2)
(1)
(1)
(2)
gap (eV)
0.95 0.81 0.49 0.33
-1.05 -1.46
-1.47 -1.86
2.00 2.27
1.25
1.28 1.30 0.84 0.72
-0.30
10-4 M in CH2Cl2 containing Bu4NPF6 (10-1 M) as supporting electrolyte; scan rate 50 mV s-1.
0.63
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Figure 8. Leakage current-gate voltage characteristic (drain and source electrodes are connected together and the voltage is applied to the gate terminal).
Figure 9. Drain current-drain voltage characteristic (gate electrode floating) showing ohmic behavior of the contacts.
a typical p-type semiconductor. The drain-source currents of negative sign scale up with negative gate voltage. All curves show a linear and saturation region. The leakage current was measured by connecting together drain and source electrodes and applying to them 0 to -15 V against the grounded gate electrode. Figure 8 shows the drainsource current-gate voltage characteristic. The leakage current is at the 10-2 nA order of magnitude, which is smaller than 1% of the current in the saturation region (Figure 7a, at the µA order of magnitude for the saturation current). The electrode contact behavior was checked by measuring I-V curve between drain and source electrodes with the gate electrode floating. Figure 9 shows the drain current-source voltage characteristic. A near-linear line through V ) 0 V was observed, indicating that the contacts are ohmic. Therefore, the field mobility can be calculated using the current in saturation region. At the saturated region (VD > VG), IDS can be described using eq 111
IDS )
WCi µ(VG - V0)2 2L
(1)
where µ is the field-effect mobility, W is the channel width, L is the channel length, and Ci is the capacitance per unit (9) T. Ko¨hler, D.; Seidel, V.; Lynch, F. O.; Arp, Z.; Ou, K. M.; Kadish, J. L.; Sessler, J. Am. Chem. Soc. 2003, 125, 6872. (10) (a) Xiao, K.; Liu, Y. Q.; Huang, X. B.; Xu, Y.; Yu, G.; Zhu, D. B. J. Phys. Chem. B 2003, 107, 9226. (b) Xiao, K.; Liu, Y. Q.; Yu, G.; Zhu, D. B. Appl. Phys. A 2003, 77, 367. (11) Sze, S. M. Physics of Semiconductor Devices; John Wiley & Sons: New York, 1981.
area of the insulating layer (SiO2, 400 nm; Ci, 10 nF cm-2). A plot of IDS1/2 vs VG (Figure 6b) can be used to obtain V0, the extrapolation to the VG axis. The field-effect mobility can then be calculated from eq 1. The field-effect mobility measured in the saturation regime was about 0.68 cm2 V-1 s-1 with a on-off ratio above 8 × 104 for a device based on a 30-layer LB film of cyclo[8]pyrrole. The mobilities determined for 4 devices fabricated at the same conditions ranged from 0.09 to 0.68 cm2 V-1 s-1. Up to date, this maximum mobility (0.68 cm2 V-1 s-1) achieved is one of the best results that can be obtained from an organic thin film FETs. However, it is worth noting that such a mobility might be effected by oxygen doping since the cyclo[8]pyrrole molecule has a very low HOMO-LUMO gap (0.63 eV) and our measurements were carried out in air. Usually, solution methods (casting and spin coating) and vacuum evaporation method are used for the fabrication of organic thin-film transistors. For cyclo[8]pyrrole, it is unable to prepare its thin film via thermal evaporation method because it will decompose during the heat up process. And no field-effect activity could be obtained from the thin film prepared by spin coating and casting method. This is due to the poor film formation abilities of cyclo[8]pyrrole using such solution methods. LB techniques is known as a promising and versatile method for the preparation of ordered thin films with well-defined architecture and has been widely used for the construction of molecular devices. Recently this technique was also applied for the preparation of organic thin film transistors.10a,12 We found that cyclo[8]pyrrole can form a stable monolayer at the water-air interface, although it is not a typical amphiphilic molecule. The most important aspect is that these thin films spread on the water surface can be transferred onto substrate for device fabrication with vertical deposition, and the good electrical properties of these films have not been destroyed during the transfer process. Analysis of the pressure-area isotherms and the X-ray diffraction data of the transfer multilayer films revealed that these LB films have an ordered bilayer structure, in each layer cyclo[8]pyrrole molecules stacked in a face-to-face conformation (tilting angle 65°) with the π orbital partially overlapped. This partially overlapped π system should contribute to the flow of electric current. Obviously, cyclo[8]pyrrole is a high-performance semiconductor for organic thin-film transistors. Its special fully conjugated 30-πelectron system provides good electron donating ability and favors intermolecular π-π overlapping. In summary, we fabricated and characterized the LB films of cyclo[8]pyrrole. The cyclo[8]pyrrole molecules were found to be stacked face-to-face with a tilting configuration on the substrate surface. The OFET based on cyclo[8]pyrrole LB films were successfully prepared. A typical p-channel FET activity was observed. The hole mobility in the unoptimized device was 0.68 cm2 V-1 s-1. Experimental Section The chemical structure of cyclo[8]pyrrole molecule used in our experiments is shown in Figure 1. It was synthesized following the procedures described previously.7 1HNMR spectrum were recorded with Bruker ARX400 spectrometers. High-resolution mass spectrum was determined with Bruker ApEX II. 1H NMR (400 MHz, CDCl3): -0.64 (s, 8H, NH), 2.06 (t, JH,H ) 7.4 Hz, 24H, CH2CH3), 3.81 (s, 24H, CH3), 4.19 (br, 16H, CH2CH3). HR-MS: m/z 952.5398; calcd for C56H72N8O4S1: 952.5397. Cyclic voltammetric measurements were conducted on an EG&G 328 System. Before measurements, the solution was deoxygenated by argon bubbling for about 15 min (10-4 M in CH2Cl2, Bu4NPF6 as supporting electrolyte, Pt electrode as (12) Xu, G. F.; Bao, Z. N.; Groves, J. T. Langmuir 2000, 16, 1834.
Cyclo[8]pyrrole, A Promising Candidate for OFET working electrode, scan rate 50 mV s-1, reported voltage vs a standard calomel electrode). Surface pressure-area isotherm measurements and deposition experiments were performed on a fully automatic KSV-5000 instrument (Finland). Cyclo[8]pyrrole dissolved in CHCl3 (1 mg mL-1) was spread onto pure water. At a constant pressure of 15.5 mN m-1, the floating layers on the subphase were transferred to SiO2/Si substrates by the vertical dipping method at a speed of 1 mm/min. Millipore-Q water (18 MΩ cm) was used in all of the cases. The UV-vis spectra of the LB films deposited on the quartz were recorded on a Shimadazu UV-3100 spectrometer using blank quartz as a reference. The topography and morphology of cyclo[8]pyrrole LB films deposited on mica were studied in air using a Digital Instruments nanoprobe atomic force microscope in tapping mode with a 10-µm scanner. The LB films deposited on hydrophobic glass slides were used for X-ray diffraction
Langmuir, Vol. 21, No. 12, 2005 5395 measurements, which were recorded on a Rigaku (D/max-RB) instrument. The schematic structure of the FET is shown in Figure 6.The n-doped Si substrate functions as the gate, and an oxide layer of 400 nm is the gate dielectric having a capacitance per unit area of 10 nF cm-2. The drain and source electrodes were vacuum deposited using a shadow mask. These electrode have widths W ) 34 mm and channel length L ) 500 µm. The electric characteristics of these devices were measured at atmosphere. The current-voltage characteristics were obtained with a HewlettPackard 4140B parameter analyzer at room temperature.
Acknowledgment. This work was financially supported by the Chinese Academy of Sciences, NSFC, and the State Basic Research Development Program. LA050106D
