AERTAS 8 D0L10.1002adma.20070153 Transparent,Low-Electric-Resistance Nanocomposites of Self-Assembled Block Copolymers and SWNTs** By Jinwoo Sung,Pil Sung Jo,Hyein Shin,June Huh,Byung Gil Min,Dong Ha Kim,and Cheolmin Park The recent demand for trar ran ent film because of the emerging industry of new-generation displays an industrial point of.spin coating is corporated in fabricating SWNT-based transparent substrates mpac In spite the cont prev substrate has been currently prepared by processes without ow spin-coating of an SWNT suspension with a polymeric ilm a relatively high process temperature.fabrication cos bility.For successful spin-coating.one first epares a go and so oAn ultrathin networkedm of SWNTs with its SWNT suspension stabilized with polymeric dispersants whose an red se the film is pared in ambient conditions small-moleculc surfactants in neral results in serious substrate has successfully fabricated a highly conductive thin compositeilm of SWNTs and polymers sufficiently con ductive for a certain device application.For high conductivity the entire film.However Engineering cntrat on of SWNTs.whose length is typically (o Prof.Huh ence and Engineering als and s Engineering a few works have addressed the electricalp s and Department of Chemistry micrometer ig that are even no longer ransparent i -Gu,Seoul 120-750 (Korea) reported,mainly because of the intrinsic charge localization RD:KR ing from insulating polymer dispersant 1 Tpolymer nanoce very low e ctric film,prepared by spin ess D dby the Se Adv Mater..2008.9999,16 2008 WILEY-VCH Verlag GmbH Co.KGaA,Weinheim iterScience
DOI: 10.1002/adma.200701535 Transparent, Low-Electric-Resistance Nanocomposites of Self-Assembled Block Copolymers and SWNTs** By Jinwoo Sung, Pil Sung Jo, Hyein Shin, June Huh, Byung Gil Min, Dong Ha Kim, and Cheolmin Park* The recent demand for transparent electrodes is tremendous because of the emerging industry of new-generation displays and solar energy generators such as organic light-emitting diodes (OLEDs), transistors, and solar cells.[1] Furthermore, either folding or rolling capability of the electrodes, resulting from their flexible characteristics, broadens their usage for a variety of highly compact and wearable devices.[2] Although thin laminated indium tin oxide (ITO) film on a polymer substrate has been currently prepared by processes without low vacuum, such as a continuous sputtering process, there are still several disadvantages such as possible delamination of ITO film, a relatively high process temperature, fabrication costs, and so on.[3] An ultrathin networked film of SWNTs with its superior conductivity and flexibility originating from the nature of SWNTs themselves has been considered as an alternative because the film is prepared in ambient conditions based on a solution process. Either direct spraying of nanotubes or transfer of a filtered nanotube network onto a plastic substrate has successfully fabricated a highly conductive transparent film utilized for, amongst others, OLEDs and organic transistors.[4–8] From an industrial point of view, however, spin-coating is one of the most desirable methods for large-area uniform film formation and thus provides a great benefit when it is incorporated in fabricating SWNT-based transparent substrates. In spite of the controlled flocculation method previously reported, which contains a spin-coating step of an SWNT suspension stabilized by small surfactant molecules,[9] the spin-coating of an SWNT suspension with a polymeric dispersant would provide much better uniformity and flexibility. For successful spin-coating, one first prepares a good SWNT suspension stabilized with polymeric dispersants whose chain length is sufficiently long, ensuring homogeneous film formation during the process. In contrast, a suspension with small-molecule surfactants in general results in serious reaggregation of the nanotubes with sporadic distribution. Another factor that should be considered is how to make a thin composite film of SWNTs and polymers sufficiently conductive for a certain device application. For high conductivity, the concentration of SWNTs in the film should be larger than the percolation threshold, provided that SWNTs are homogeneously dispersed over the entire film. However, the threshold concentration of SWNTs, whose length is typically 1 mm, is roughly 1–5 wt%, which becomes even larger when the SWNTs are chopped, while homogenous dispersion of concentrated SWNT films usually necessitates laborious chemical/ mechanical processes, for example, mechanical mixing or covalent/noncovalent functionalization of the SWNTs’ surface. In spite of many studies dealing with successful dispersion of SWNTs using various macromolecular stabilizing agents such as homo and block copolymers, DNAs, and proteins,[10–16] only a few works have addressed the electrical properties of bulk or micrometer thick composites that are even no longer transparent in visible light.[17–22] Transparent SWNT/polymer nanocomposites with low electric resistance have been rarely reported, mainly because of the intrinsic charge localization arising from insulating polymer dispersant. Here we present a new practical and robust method for fabricating a transparent SWNT/polymer nanocomposite with very low electric resistance. The conductance of the thin composite film, prepared by spin-casting a well-dispersed SWNT solution with a self-assembled poly(styrene-block- 4vinylpyridine) (PS-b-P4VP) copolymer as a dispersant, was significantly enhanced by the selective incorporation of COMMUNICATION [*] Prof. C. Park, J. Sung, P. S. Jo, H. Shin Department of Materials Science and Engineering Yonsei University Seoul (Korea) E-mail: cmpark@yonsei.ac.kr Prof. J. Huh Department of Materials Science and Engineering Seoul National University Seoul (Korea) Prof. B. G. Min School of Advanced Materials and System Engineering Kumho Institute of Technology Kumi 730-701 (Korea) Prof. D. H. Kim Division of Nano Sciences and Department of Chemistry Ewha Womans University 11-1 Daehyun-Dong, Seodaemun-Gu, Seoul 120-750 (Korea) [**] This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD; KRF-2005- 042-D00110), Ministry of Commerce, Industry and Energy(MOCEI) through the project of NGNT (No. 10024135-2005-11) and a grant (f0004091) from the Information Display R&D Center, one of the 21st Century Frontier R&D Programs, and the 0.1 Terabit Non-volatile Memory Development and Seoul Research and Business Development Program (10701 and 10816). This work was supported by the Second Stage of Brain Korea 21 Project in 2006 and by the Seoul Science Fellowship. Supporting Information is available online from Wiley InterScience or from the authors. Adv. Mater. 2008, 9999, 1–6 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim 1
X思 HAuCl-3H2O doping agent into P4VP domains without An OTI nel and tra ratio of approximately10 VE ous distribution of the nanotubes without any thic 00 nm nanotube bundles typically observed in a sample with no block tached cop c mic es with P4V 6 provide sufficient space to overcome van der Waals attraction giving rise to the good dispersion o of the metal salt in P4VP domains did not deteriorate the stabilit shown Figure I the electron beam in TEM,is provided to tal 400nm nergy f suc characterized by TEM in bright-feld mode where the Au articles educed selectively in the core reg ns of the micelle with a diameter of approximately 5nm.A HRTEM image in copolyme of the ing-mode atomic force micr in height contrast (Fig 1e).consists of SWNTs evenly distributed with the block throot-mean-square roughness of which We examined the transmittance of the composite film 400nm prepared on glas was first nared as a function of the ure 1.Bright- s of SWNT SWNTs in the thin composites (Fig.2a).A typical UV- om of a af间 th is 0.07 for ood dispersion and the maximum concentratio on of the bl renare a www.advmat.de 2008 WILEY-VCH Verlag GmbH Co.KGaAWeinheim Adv Mater 2008,9999.1-6
COMMUNICATION HAuCl4 3H2O doping agent into P4VP domains without harming the film transparency. Control of the amount of the doping agent allows us to tune the conductance of the thin transparent films ranging from 0.01 to approximately order of 1 S cm1 . An OTFT with a pentacene channel and transparent source/drain SWNT/PS-b-P4VP composite films exhibited a good mobility of approximately 0.05 cm2V1 s 1 with an on/off ratio of approximately 105 . The origin of the good stability of SWNTs with PS-b-P4VP in solution was elucidated with bright-field transmission electron microscopy (TEM), as shown in Figure 1a. As we reported previously,[13] a TEM sample, prepared by depositing a droplet of a diluted SWNT solution onto a Cu grid, exhibits a very homogeneous distribution of the nanotubes without any thick nanotube bundles typically observed in a sample with no block copolymer dispersant. Block copolymer micelles with P4VP core and PS corona attached on the surface of the nanotubes provide sufficient space to overcome van der Waals attraction between nanotubes, giving rise to the good dispersion of SWNTs. The inset of Figure 1a depicts a schematic of block copolymer micelles separating SWTNs. The properties of a SWNT solution well-dispersed with the block copolymer have been examined in our previous work.[13] The selective doping of the HAuCl4 3H2O metal salt in P4VP domains did not deteriorate the stability of the SWNT solution, as shown in the inset of Figure 1c. Individual metal salt molecules are coordinated and stabilized by lone pairs of electrons in a 4VP unit unless sufficient energy, for example, the electron beam in TEM, is provided to overcome the activation photon energy for metal reduction. The successful doping in the P4VP core of block copolymer micelles was thus characterized by TEM in bright-field mode where the Au nanoparticles were visualized after reduction by strong electron beam of 50 kV.[23] Figure 1b clearly shows Au nanoparticles reduced selectively in the core regions of the micelles with a diameter of approximately 5 nm. A HRTEM image in the inset of Figure 1b displays that many Au nanoparticles are formed in a micelle core adhering to the surface of nanotubes. Spin-casting of a solution results in a homogeneous thin SWNT/block copolymer film without re-aggregates of the nanotubes over a large area. The surface of the film, visualized by tapping-mode atomic force microscopy in height contrast (Fig. 1c), consists of SWNTs evenly distributed with the block copolymer micelles, the root-mean-square roughness of which is approximately 5 nm. We examined the transmittance of the composite films prepared on glass substrates having various SWNT and metal salts content. The background-subtracted transmission at 550 nm was first compared as a function of the amount of SWNTs in the thin composites (Fig. 2a). A typical UV-vis spectrum of a composite film is shown in the inset of Figure 2a. In our system, the maximum ratio of SWNTs to block copolymer is 0.07 for good dispersion and the maximum concentration of the block copolymer in toluene is approximately 1 wt% above which SWNT solution with the block copolymer becomes a viscous gel, making it hard to prepare a Figure 1. Bright-field TEM images of SWNTs dispersed and stabilized by a) PS-b-P4VP copolymer, and b) PS-b-P4VP with the maximum doping of HAuCl4 3H2O with respect to P4VP domains. The insets of (a) and (b) show a schematic illustration of SWNT stabilization by PS-b-P4VP and a high-resolution (HR)TEM image of a micelle adhering to the surface of nanotubes with the reduced Au nanoparticles, respectively. c) Tappingmode atomic force microscopy image in height contrast, displaying the surface of an SWNT/PS-b-P4VP composite film spin-cast onto a glass substrate. The inset of (c) shows a homogeneous SWNT suspension stabilized with the block copolymer prior to spin-coating. 2 www.advmat.de 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim Adv. Mater. 2008, 9999, 1–6
X思 10 The sheet resistan c ofthrWT-PVp 1 compensates the contact resistance between a compositeilm curve 20 PS-b-P4VP thin film and decreases slowly to 1 with 3wt%of SWNIs,which indicates the formation of conti SWNTs(wt.%to PS-P4VP) calculated threshold volume fraction of SWNTs is approxi % mately 0 by xtrapolating ntal data.co drops again slowly decreaseswth tely7×10n 40 YONSEI UK is still very high for application as a low electric resistance film. 20 a) 10' 020406080 10 HAuCl3H2O(wt.%to PS-P4VP) 10" gth of 550 nm SWNT/ SWNTs/PS-b-P4VF f (a)an 10 SWNTs(wt.%PS-bP4VP) thin film by spin-coating.As shown in Figure 2a.transmittance of a composite with with the of SWNT 0 (85%)as shown in Figure 2h The reduction of the metal salt 10 20 and maximum doping shown or the inset of Figure2p.In particular.the composite film io of 10 mm bending radius typically used in other works. Adv Mater..2008.909g.1-6 2008 WILEY-VCH Verlag GmbH Co.KGaA.Weinhein www.advmat.de 3
COMMUNICATION thin film by spin-coating. As shown in Figure 2a, transmittance of a composite film remains approximately 95% of the transmittance observed for the film without SWNT and did not significantly change with the concentration of SWNTs (90%). Also, the selective doping of HAuCl4 3H2O marginally reduced the transparency of the film when the maximum amount of the metal salt was loaded in the P4VP block (85%), as shown in Figure 2b. The reduction of the metal salt into Au nanoparticles by either reducing agents or heat above 250 8C renders the film a little reddish owing to the characteristic color of Au nanoparticles (see Supporting Information).[24] The photograph of a transparent composite film with SWNTs (7 wt%) and maximum doping agent is shown on both glass and poly(ethylene terephtalate) (PET) substrates in the inset of Figure 2b. In particular, the composite film prepared on a 180 mm thick PET substrate was hardly delaminated from the substrate even after bending more than 1000 times, as tested with a lab-made apparatus under the condition of 10 mm bending radius typically used in other works.[25,26] The sheet resistance of the transparent SWNT/PS-b-P4VP composite, determined by 4-point probe measurement which compensates the contact resistance between a composite film and a measuring metallic needle probe, is in general very consistent with that by current-bias curves from 2 gold contacts, implying negligible measurement contact resistance.[27] The sheet resistance larger than approximately 1014 V/& is maintained up to 1 wt% of SWNT loading in PS-b-P4VP, apparently similar to that of pure insulating PS-b-P4VP thin film and decreases slowly to 1010 V/& with 3 wt% of SWNTs, which indicates the formation of continuously networked SWNT pathways between two measuring electrodes, as shown in Figure 3a (solid squares). The calculated threshold volume fraction of SWNTs is approximately 0.01 by extrapolating our experimental data, consistent with the values reported by others.[28] The resistance rapidly drops to 106 V/& with 5 wt% and again slowly decreases with further increase of SWNTs, as shown in Figure 3a. The minimum resistance we obtained in our transparent composite film without the metal salt is approximately 7 105 V/&, which is still very high for application as a low electric resistance film. Figure 2. Variation of transmittance at the wavelength of 550 nm SWNT/ PS-b-P4VP composite films spin cast on glass substrates a) as a function of SWNTs in the composite films and b) as a function of HAuCl4 3H2O with respect to P4VP in the composite with 7 wt% SWNTs. The insets of (a) and (b) show a typical UV-vis spectrum of a composite film and a photograph of a composite with 7 wt% SWNTs and the maximum doping of the metal salt on a glass substrate (left) and a poly(ethylene terephtalate) (PET) one (right). Figure 3. Variation of sheet resistance of SWNT/PS-b-P4VP composite films spin cast on glass substrates a) as a function of SWNTs in the composite films (squares) and b) as a function of HAuCl4 3H2O with respect to P4VP in the composite with 7 wt% SWNTs. The circles in (a) correspond to the sheet resistance of the composites when the maximum amount of HAuCl4 3H2O is doped into each composite. The mean value from 3 different samples was used for each point. Adv. Mater. 2008, 9999, 1–6 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim www.advmat.de 3
ADVANCED MATERIALS The sheet resistance of the composite films was also 00 PAVP blocks (circles in Fig).The doping of the sat was no 80 SWNTs/PS-b-P4VF ving I wt of har deed pld fo channels between electrodes were formed as observ ved in 0.5001000150020002500 Wavelength(nm) ron of more than orders of magnitude with the doping of HAuCl-3H2O in P4VP domains. Th loading of 3Hwas the most effective for reducing the resistance among and shifts toward lower energy with the diameter of the various metal sals that could be selectively incorporated The distribution of the diameter of our SWNTs raction of metalsat wth P4VP significant depres sion of the characteristic peaks,as shown in ure of de decrease sup to roximately4×1n/ doping c f nanotubes facilitat ed by P4VP core 6x100y th made after slow de ease at the intermediate regime, displayed in Figure 3b.Our method d allows us to tune the sheet carriers in the nanotubes,apparently leading to significan spa NT/b copolym es.Th nples as we observed in nanocomposite film was also confirmed by the field-effec a composite film ctively The main reason for the higher resistance of ou cithe ks reported previously is tha sed,which ting Information)The increased metallic channels nan the charge transfer doping in the 1n30 anic parent SWNT/PS-b-P4VP composite film with the er to gain o controlling the resistan e ar ith ho pectroscopy.The characteristic absorbance was compared in the lift-off of a patterned photoresist on which transparent WO SWN nple without the salt clearly sl d h respectively The device exhihits peaks in the range between 1200 to 1500nm that correspond to havior at operating voltages as shown in Figure5a. The s calc on both charalities and diameters of nanotubes.A typical Ist the saturated regime.using the following equation.Ips electro Hipty of state L)Cu(Va-Vi) and L are the width and approximately capacitance of the gate insulator.the ehannel mobility and www.advmat.de 2008 WILEY-VCH Verlag GmbH Co.KGaA.Weinhein Adv.Mater.2008,9999.1-6
COMMUNICATION The sheet resistance of the composite films was also examined as a function of the amount of SWNTs with the maximum incorporation of HAuCl4 3H2O with respect to P4VP blocks (circles in Fig. 3a). The doping of the salt was not effective for electric resistance for a composite having 1 wt% SWNTs, resulting in almost no resistance change. The addition of the metal salt, however, gave rise to a significant resistance drop of more than 3 orders of magnitude in a composite with 3 wt% SWNTs, which implies that the doping indeed played a role for reducing the resistance once continuous SWNT channels between electrodes were formed as observed in Figure 3a (solid squares). All composite films containing SWNTs higher than 3 wt% exhibited a significant resistance drop of more than 2 orders of magnitude with the maximum doping of HAuCl4 3H2O in P4VP domains. The lowest electric resistance we obtained was approximately 6000 V/& with a transmittance of 85%. It should be noted that HAuCl4 3H2O was the most effective for reducing the resistance among the various metal salts that could be selectively incorporated into P4VP microdomains (Supporting Information). We also investigated in details the effect of the doping fraction of metal salt with respect to P4VP blocks on electric resistance in a 7 wt% SWNTs composite film. The sheet resistance of approximately 7 105 V/& without the doping agent suddenly decreases up to approximately 4 104 V/& with 20% doping of the metal salt in the block copolymer and finally becomes about 6 103 V/& when a full 100% doping is made after slow decrease at the intermediate regime, as displayed in Figure 3b. Our method allows us to tune the sheet resistance of a transparent SWNT/block copolymer composite film from insulating to conductive over the very broad range of more than 10 orders of magnitude. The highest conductance of a composite film obtained is approximately 1 S cm1 with the resistance and film thickness of 6 103 V/& and 700 nm, respectively. The main reason for the higher resistance of our CNT composites, even after metal salt treatment, than that of either ITO or SWNT networks reported previously is that nanotubes are still embedded in insulating polymer matrix and thus make it difficult for them to form continuous pathways by direct contact between the nanotubes. Nevertheless it is worthwhile to note that the resistance of 6103 V/& is sufficiently low for the electrodes based on SWNTs in organic transistor as will be demonstrated later. In order to gain insight into controlling the resistance via the metal salt, we performed absorbance measurements by NIR spectroscopy. The characteristic absorbance was compared in two SWNT/PS-b-P4VP composite samples containing 7 wt% SWNTs with and without the metal salt, as shown in Figure 4. The sample without the salt clearly shows several characteristic peaks in the range between 1200 to 1500 nm that correspond to the optical transitions between the pair of van Hove singularities of semiconducting SWNTs in the DOS depending on both charalities and diameters of nanotubes. A typical 1st gap between singularities of electronic density of state of semiconducting SWNTs fabricated by HiPCo process is approximately 0.8 eV corresponding to 1500 nm in wavelength and shifts toward lower energy with the diameter of the nanotube.[29] The distribution of the diameter of our SWNTs results in the occurrence of the multiple distinct absorbance peaks. The doping of SWNTs by the metal salt gave rise to the significant depression of the characteristic peaks, as shown in Figure 4 (dotted line), which is the signature of depletion of electrons from van Hove singularites of valence band by charge transfer doping.[30] The effective contact of HAuCl4 3H2O on the surface of nanotubes facilitated by P4VP cores of the micelles adhering to the nanotubes generates the chargetransfer doping and thus enhances the number of mobile hole carriers in the nanotubes, apparently leading to significant decrease of sheet resistance of the samples as we observed in Figure 3. The effect of the metal salt on the resistance of a nanocomposite film was also confirmed by the field-effect electric transport of the composite film in a transistor with the nanocomposite active layer. The p-type drain-source current modulation with hysteresis typically found in semiconducting SWNT films[31] was diminished as the content of the metal salt in the composite film increased, which implies that the composite film became more and more metallic with the metal salt. (Supporting Information) The increased metallic channels arising from the effective charge transfer doping in the composite enhanced its conductance. A transparent SWNT/PS-b-P4VP composite film with the sheet resistance of approximately 6 103 V/& was utilized for both source and drain electrodes in an OTFT with pentacene channel layer. A transistor with bottom gate was fabricated by the lift-off of a patterned photoresist on which transparent source and drain composite film was spin cast. The output and transfer characteristics of the OTFT are shown in Figure 5a and b, respectively. The device exhibits good linear/saturation behavior at operating voltages as shown in Figure 5a. The field-effect mobility is calculated from the slope of a plot of the square root of the drain current (IDS) versus gate voltage (VG) in the saturated regime, using the following equation, IDS ¼ (W/2L)Cim(VG Vth) 2 , where W and L are the width and length of channel and Ci, m, and Vth correspond to the capacitance of the gate insulator, the channel mobility and Figure 4. NIR spectra of SWNT/PS-b-P4VP composite films containing 7 wt% SWNTs with (dotted line) and without (solid line) the maximum loading of HAuCl4 3H2O metal salt. 4 www.advmat.de 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim Adv. Mater. 2008, 9999, 1–6
X a 5.0x10 40x10 mold where hexagonal posts of 20m are arrayed into 6mm 30x10 82.0x10 mold/the compositefilm with conformal contact at 175C 1.0x10 90 are well defined with sharp edges as shown in the inset of 4030 20 10 Figure 5c Vos(V) 1E- 0.00 In particula the tive doping o adherine on the surface of nanotubes enabled us to lower the 1E. 0.00 .00 A HAuCh doped SWNT/PS--P4VP somposite V-(V with approximately transmittance and 6000 resis Experimental (PS--PAVP A 50 um Figure 5.a)Output char P co mperature A the y to the P4V olutio aindd ive doping of the et electrodes fabricated at the same batch,which indicates that iPS-b-P4VP/S with re the contact tto P4VE previously reported ing a solu Furthermore.our transparent electrode,capable of being sobthc molding and thus micropatterned arrays of the composite Adv Mater..2008.909g.1-6 2008 WILEY-VCH Verlag GmbH Co.KGaA.Weinhein www.advmat.de 5
COMMUNICATION threshold voltage, respectively. The field-effect mobility is 0.05 cm2V1 s 1 and the on/off ratio is approximately 105 with the subthreshold swing of 2.97 mV decade1 . The performance of the device with the SWNT/PS-b-P4VP composite electrodes is very comparable with one based on Au source and drain electrodes fabricated at the same batch, which indicates that the contact between the composite and pentacene becomes sufficiently stable, consistent with the CNT based transistors previously reported.[7] Furthermore, our transparent electrode, capable of being spin cast from a solution, is easily combined with various soft lithographic techniques such as microimprinting and capillary molding and thus micropatterned arrays of the composite electrode film can be fabricated. For example, a patterned SWNT/PS-b-P4VP composite film as shown in Figure 5c is obtained by microimprinting lithography. We applied a PDMS mold where hexagonal posts of 20mm are arrayed into 6mm symmetry to confirm the capability of the pattern formation with the composite film. The pressure of 200 kPa on the PDMS mold/the composite film with conformal contact at 175 8C successfully produced a micropattern of the composite as shown in Figure 5c. The pressed hexagonal regions of the film are well defined with sharp edges as shown in the inset of Figure 5c. In summary, we developed a method based on spin coating process for fabricating a transparent SWNT/block copolymer composite film with the sheet resistance tunable from the order of 1014 to 103 V/&. In particular the selective doping of HAuCl4 3H2O metal salt into polar P4VP microdomains adhering on the surface of nanotubes enabled us to lower the resistance more than two orders of magnitude mainly due to the increased number of mobile charge carriers in the nanotubes facilitated by the effective charge transfer doping. A HAuCl4 3H2O doped SWNT/PS-b-P4VP composite film capable for micropatterning over large area was successfully utilized for source and drain electrodes in a pentacene OTFT with approximately 85% transmittance and 6000 V/& resistance, leading to the mobility and the on-off ratio of approximately 0.05 cm2V1 s 1 and 105 , respectively. Experimental Nanocomposite Preparation: Purified high pressure carbon monoxide (HiPCO) SWNTs exhibiting 7 wt% residue at 800 8C in air produced at Rice University were used as received [32]. The samples consist of SWNT ropes. A poly(styrene-block-4vinyl pyridine) (PS-b-P4VP) copolymer was purchased from Polymer Source Inc. Doval, Canada with 47,600 g mol1 of PS block and 20,900 g mol1 of P4VP one, respectively. Polydispersity Indexes (PDI) of PS-b-P4VP is 1.14. The PS-b-P4VP was first dissolved in toluene, a solvent in which PS block is well solvated and P4VP core, PS corona micelles are readily formed. The block copolymer solution appears bluish by the formation of the block copolymer micelles. Solutions of SWNTs in toluene with various concentrations were prepared from 0.01 to 0.07 wt% and briefly sonicated for 10 min with a power of 80W, a frequency of 50/60 Hz. The 1 wt% block copolymer solution prepared previously was added to the SWNTs solution. The mixture was finally sonicated for 2 hours at room temperature. All solutions show good stability of SWNTs without significant precipitation more than 1 month. To control the conductance of the polymer dispersant, we coordinated a metal salt, HAuCl4 3H2O selectively to the P4VP block. We first dissolved HAuCl4 3H2O, purchased from Aldrich, Korea, in a PS-b-P4VP solution, which resulted in selective doping of the metal salt into P4VP domains. The solutions with doping agent were also mixed with the SWNT suspension prepared (solvent: toluene). The amount of the metal salt was varied from 20% up to 100% with respect to P4VP block. A thin PS-b-P4VP/SWNTs composite film was prepared by spin coating a solution with 1000 rpm. Characterization: Transmittance of the composite films was measured by UV-vis spectrometer (JASCO V-530) and that at the wavelength of 550 nm was chosen for plots as a function of either nanotubes or metal salts. The sheet resistance of the films was obtained by both two and four point probe measurements. For the two point measurement, Au electrodes of approximately 100 nm thick were Figure 5. a) Output characteristics of a pentacene OTFT as function of the gate voltage in an organic transistor having transparent SWNT/PS-b-P4VP composite source and drain electrodes. b) Transfer characteristics of the OTFT. c) A micropatterned SWNT/PS-b-P4VP composite film prepared by microimprinting lithography using an elastomeric PDMS mold at 175 8C. Adv. Mater. 2008, 9999, 1–6 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim www.advmat.de 5
X迅 MMSm..A ogen curve o ined hy Asilent E5270 T n.T. Gaur.S.Jcon.M.LUsrey.M.S.Strano.J.A organic tr 12R10172 E.Nativ-Roth.O.Regev.R.Yerushalmi-Rozer nol Rapid Commu 141 .SungM.Park.U-H Choi.J.Huh.B. B.G.Min.C.H.Ahn on [15]M R.S.Me ean.S.R. The ele 【IgG.RDie N.G A.B.D nd2000 the OTFT R.H.Baughman,R. 03.2 in of a sw 4VP med wit油 F.Du.E F PDMS 184 by st the PDM .A.R.Meharabi,M.V.Bannon,J.L Bahr,Adv.Maer Published online: 阅8 moLe.20N44.2513 2 M.J. LS.MB时 Mo e.M.S Ma.R.H.Hause.RB.Weisman.R.ESmalley.Sclence 20022 anel Displays.Wiley.Hoboken.NI a.M.K.O .J.A.E.M n.F.A 60.156 [3I]H.E.Una i.A.Kanwal.A.D.Pasquier.M.Chhowalla ou.K. 24 www.advmat.de 2008 WILEY-VCH Verlag GmbH Co.KGaA.Weinhein Ad Mater0.1-6
COMMUNICATION deposited onto a composite film spin cast on a glass substrate using thermal evaporation. The sheet resistance was calculated from the low bias slope of I–V curve obtained by Agilent E5270. The nanostructure of SWNTs with PS-b-P4VP was characterized by TEM, Hitachi H-600 and HRTEM, Jeol 2100 operated at 50 and 200 kV in bright field, respectively, and Atomic Force Microscope (AFM) (Nanoscope IVa Digital Instruments) in tapping mode. The samples for TEM were prepared by depositing droplets of suspension onto carbon-coated TEM grids, allowing the grids to dry without further staining. The electronic band characteristics of the composite films were observed by Near IR spectroscope (Model: Cary 5000) in the range of the wavelength from 400 to 2500 nm. The samples for the spectroscopy were prepared by drop casting of suspensions on quartz substrate. Organic Transistor Fabrication: An organic transistor with pentacene channel and SWNT/PS-b-P4VP composite source and drain electrodes was fabricated on a degenerately doped Si substrate with a 200 nm thermally grown oxide that can be used as a bottom gate. The source and drain of the composite films were prepared by spin coating and subsequent lift-off of a photo-resist film developed by conventional photolithography. Approximately 50-nm pentacene active layer was evaporated on the defined source and drain composite film using a metal shadow mask under pressure of 106 mTorr and a rate of 0.1–0.2 A˚ per second. The channel length (L) and width (W) were 100 and 2000mm, respectively. The electrical characteristics of the OTFT were measured in air at room temperature using Agilent E5270. Micro-Imprinting: Micro-imprinting of a SWNT/PS-b-P4VP composite film was performed with a house-made microimprinting apparatus. [33] The elastomeric PDMS mold was fabricated by curing a PDMS precursor (Sylgard 184, Dow Corning Corp) on a prepatterned silicon master. The prepatterned photoresist master was prepared by standard photolithography, and the surface was fluorinated before casting the PDMS precursor on the master. After the PDMS precursor was cured at 40 8C for 12 h using vacuum oven, the mold was separated from the master. The ranges of temperature and pressure applied were 167 to 190 8C and 200 to 800 kPa, respectively. The samples were held at constant pressure and temperature for certain period of time and slowly cooled down to room temperature. The imprinting of the thin composite film with a PDMS mold was performed for 10 min at 175 8C with 200 kPa. Received: June 27, 2007 Revised: September 20, 2007 Published online: [1] R. G. Gordon, MRS Bull. 2000, 25, 52. [2] G. P. Crawford, Flexible Flat Panel Displays, Wiley, Hoboken, NJ 2005. [3] Y. Leterrier, L. Medico, F. Demarco, J.-A. E. Manson, U. 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