Fabrication, Characterization, and Optoelectronic Properties of

The introduction of terpyridine groups on the surface of MWCNTs provides a .... Photo of acetonitrile solutions of pristine MWCNTs (left) and tpy-MWCN...
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Fabrication, Characterization, and Optoelectronic Properties of Layer-by-Layer Films Based on Terpyridine-Modified MWCNTs and Ruthenium(III) Ions Yuexiu Pan,† Bin Tong,† Jianbing Shi,*,† Wei Zhao,† Jinbo Shen,† Junge Zhi,‡ and Yuping Dong*,† College of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China, and College of Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ReceiVed: October 15, 2009; ReVised Manuscript ReceiVed: March 20, 2010

A novel full-conjugated 4-(2,2′:6′,2′′-terpyrid-4′-yl) benzenediazonium tetrafluoroborate (diazo-tpy) was synthesized and used for surface modification of materials, such as quartz wafers, ITO glass, silicon, and multiwalled carbon nanotubes (MWCNTs). Under UV irradiation, the diazonium group of diazo-tpy is decomposed and the residual terpyridine group is covalently anchored to the surface of substrates. The obtained tpy-modified MWCNTs (tpy-MWCNTs) have good solubility in common organic solvents. TGA and HRTEM analyses confirmed that terpyridine groups have been symmetrically grafted on MWCNTs. The thickness of the tpy-modified monolayer is about 2.3 nm, which is approximately 2 times the axial length of the 4-(2,2′: 6′,2′′-terpyrid-4′-yl)phenyl group. The introduction of terpyridine groups on the surface of MWCNTs provides a coordination site to complex with metal ions. Multilayer films were fabricated from tpy-MWCNTs and ruthenium ions [Ru(III)] via the layer-by-layer self-assembled (LBL SA) technique on the tpy-modified quartz wafer, ITO glass, or silicon. The UV-vis results indicate that (1) Ru(III)-tpy-MWCNT SA multilayer films are successfully formed based on the coordination interaction between ruthenium ions and terpyridine groups, and (2) a progressive assembly occurred regularly with almost an equal amount of deposition in each cycle. The SEM image showed a highly covered Ru(III)-tpy-MWCNT film on the substrate. Moreover, the optoelectronic conversion was also studied by assembling Ru(III)-tpy-MWCNT multilayer films on ITO substrates. Under illumination, the LBL SA films on ITO showed an effective photoinduced charge transfer because of their conjugated structure and the ITO current density changed with the number of bilayers. As the number of bilayer increases, the photocurrent intensity increases and reaches its maximum (∼65 nA/cm2) at six bilayers. These results allow us to design novel materials for applications in optoelectronic devices by using LBL SA techniques. 1. Introduction Since being discovered by Iijima in 1991,1 carbon nanotubes (CNTs) have attracted considerable interest due to their unique high mechanical rigidity, low density, and special electrochemical, magnetic, optical, and thermodynamic properties. Recently, they are considered as the most promising materials for applications in many fields, such as catalyst carriers,2 nanosensors,3 nanoelectrodes,4 quantum wire,5 molecular switches,6 photoelectrochemical devices,7 nanotransistors,8 and nonlinear optical material.9 A wide variety of methodologies were employed to prepare the CNT-incorporated films. Among them, the layer-by-layer self-assembled (LBL SA) technique is an interesting approach that offers several advantages, for example, (i) generation of the most thermodynamically stable structure, (ii) possibility of self-repair, and (iii) formation of a precise arrangement of molecules into the desired structure.10,11 It is thus possible to easily control the organization of the molecules within the film, which allows building complex molecular architectures.12 Such structures can be used to develop interesting applications for * To whom correspondence should be addressed. Fax: +86-10-68948982. E-mail: [email protected] (J.B.S.), [email protected] (Y.P.D.). † College of Materials Science and Engineering. ‡ College of Science.

electronics, photonics, optoelectronic energy transfer devices,13-15 and two-dimensional conjugated polymers.16 The very low interfacial adhesion and strong π-π interaction, however, lead to poor dispersion and high self-aggregation of carbon nanotubes in the host matrices.17 For extending the applicable range of CNTs, CNT functionalization has thus been widely developed, including noncovalent18,19 or covalent methods.20-26 Covalent reactions on CNTs can be classified as surface-carboxylation,20 nucleophilic substitution after fluoridation,21 carbine,22 radical addition,23 electrophilic addition,24 electrochemical redox,25 electrochemical and thermochemical reduction of aryl diazonium compounds,26 etc. Among them, diazonium coupling to CNTs has become the most popular chemical functionalization route of CNTs because diazonium compounds show an intrinsically high reactivity associated with good shelf stability. Moreover, their binding to CNTs provides a stable, covalent anchorage. Multifarious (mono-, bis-, or ter-)2,2′:6′,2′′-terpyridine,27 due to its unique electrical,28 optical,29 magnetic,30 and catalytical31 properties, as well as the strong coordination ability with transition metals, is attractive as a building block for the construction of supramolecular self-assembly functional materials32 and surface modification of functional materials, such as Au,33 Si,34 Pt,35 TiO2,36 fullerenes,37 etc. However, only a few studies have been reported concerning complexation and

10.1021/jp909904t  2010 American Chemical Society Published on Web 04/02/2010

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SCHEME 1: Schematic Illustration of Anchoring tpy to MWCNTs by Covalent Bonds under UV Irradiation

coordination chemistry of terpyridyl-containing compounds with CNTs. In these studies, most of the terpyridine/metal ion complexes were covalently attached onto the surface of the carboxylated single-walled carbon nanotubes (SWCNTs) based on esterification.38 A major drawback of the above approach is that the charge transport properties are strongly blocked because of nonconjugated linkers. Recently, electrochemical functionalization of SWCNTs via the diazonium salt of the terpyridine ruthenium complex was first described by Bidan and coworkers.39 Their experimental results indicated that the diazonium-containing compounds are stable and have been electrografted on CNTs using a bucky paper network. To the best of our knowledge, there is no published paper about the preparation of full-conjugated multilayer films from CNTs covalently modified by terpyridine or its derivants up to now. In our previous papers,40 full-conjugated 4-(2-(4-pyridinyl)ethynyl)benzenic or 4-(2,2′:6′,2′′-terpyrid-4′-yl) benzenediazonium tetrafluoroborate was used to modify the surface of substrates through the photochemical decomposition of the diazonium group under UV irradiation. The resultant pyridinecontaining monolayer film anchored on substrates is very stable toward acid, base, strong electrolytic solution, and organic solvents. Furthermore, the LBL SA films from bis(4,4′-bipyridine) (phthalocyaninato) ruthenium (RuPc(bipy)2, BPR)-triruthenium dodecacarbonyl (Ru3(CO)12, TRDC)40a or 4′,4′′′-(1,4phenyl)-bis(2,2′:6′,2′′-terpyridine) (bitpy)-different transitionmetal ions (Pt4+, Ru3+, Rh3+, Pd2+)40b were successfully fabricated on the modified substrates. To verify the formation of these films based on the coordination interaction between bitpy and metal ions, we attempted to use bitpy itself to fabricate the regular multilayer films via π-π interactions but ended up in disappointment.40b Herein, we reported on the synthesis of terpyridine-modified multiwalled carbon nanotubes (tpyMWCNTs) (Scheme 1) and the multilayer films fabricated from tpy-MWCNTs and ruthenium ions using the LBL SA technique. A new conjugated 4-(2,2′:6′,2′′-terpyrid-4′-yl) benzenediazonium tetrafluoroborate (diazo-tpy) was synthesized and employed as

a covalent anchor layer on the surface of substrates. Further, we obtained full-conjugated LBL SA multilayer films based on the coordination interaction between tpy-MWCNTs and RuCl3 on the designated substrates (Scheme 2). UV-vis absorption spectroscopy was used to investigate the presence and stability of the monolayer on the MWCNTs as well as to monitor the growth of the tpy-MWCNT/Ru(III) SA films. The formation and morphology of tpy-MWCNTs are characterized by TGA and TEM. The electrochemical and photoelectrochemical properties of the coordinated SA films fabricated from tpy-MWCNTs and ruthenium ions were simultaneously studied to develop viable energy transferring materials through molecular engineering. This work offers a basic method for developing CNT/ conjugated spacer/metal hybrid film. 2. Results and Discussion 2.1. Modification of MWCNTs by tpy Groups. When the substrates or MWCNTs were immersed in the diazo-tpy acetonitrile solution, the diazo-tpy moieties are attached to the surface of substrates or MWCNTs through electrostatic interaction (Scheme 1). Under UV irradiation, the light-sensitive diazonium group of the diazo-tpy is decomposed and the residual 4′-(2,2′:6′,2′′-terpyridinyl)phenyl groups are covalently connected to the substitute surface. Such functionalization has made the resultant MWCNTs very stable toward electrolyte aqueous solutions or strong polar organic solvents.41 The change in the solubility of the hybrid in organic solvents can reflect the functionalization efficiency.42 Figure 1 shows the photo of pristine MWCNTs and tpy-MWCNTs with the same concentration in acetonitrile. Both solutions were ultrasonicated for 30 min and then placed subsequently in the quiescent condition. The pristine MWCNTs are quickly deposited, whereas tpyMWCNTs can be dispersed well in acetonitrile for a long time. It indicates that modification of MWCNTs with tpy groups can efficiently improve their solubility in organic solvents due to the hydrophobicity of the 4′-(2,2′:6′,2′′-terpyridinyl)phenyl moiety.

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SCHEME 2: Schematic Representation of the Formation of LBL SA Films Fabricated from Transition-Metal Ions and tpy-MWCNTs on the diazo-tpy-Modified Functional Surface

2.2. Characterization of tpy-MWCNTs. The tpy-MWCNTs were characterized by UV-vis absorption spectra, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). Figure 2 shows the UV-vis spectra of pristine MWCNTs and tpy-MWCNTs in acetonitrile solutions (50 mg/ L), which were treated by ultrasonication for 2 h and placed overnight before measurement. The spectrum of pristine MWCNTs has two characteristic absorption bands at 232 and 258 nm assigned to the π-π* transition of the hexagonal carbon. Introducing terpyridine groups onto pristine MWCNTs has extended the conjugation, and the maximum absorption of tpyMWCNTs is located at 262 nm. The peak intensity at 232 nm is also higher than that of pristine MWCNTs. This further confirms the success of the preparation of tpy-MWCNTs. High-resolution transmission electron microscopy (HRTEM) was used to further characterize the tpy-functionalized MWCNTs.

Figure 3 shows the HRTEM micrographs of pristine MWCNTs and tpy-MWCNTs. Compared with pristine MWCNTs (Figure 3a), an ultrathin and flat film was formed on the sidewall of MWCNTs in macroscopical aspect (Figure 3b). The thickness of the film is about 2.3 nm (Figure 3c), which is about 2-fold the axial length of the tpy molecule. Thus, we conclude that the tpy-containing film is probably composed of tpy dimers, as shown in Scheme 1. This phenomenon had been reported in earlier published articles when diazonium salts were used to modify surfaces of functional substrates, such as carbon, glass, metal, etc.43 The attachment of aryl groups to the substrate surfaces can be assigned to the reaction of an aryl radical formed by a one-electron reduction of the diazonium salt. That is, the multilayer formation has been assigned to the attack of an aryl radical on the grafted layer along an SH homolytic substitution reaction.44 The structure of the layer should look somewhat like a substituted polyphenylene. However, the monolayer is only

Figure 1. Photo of acetonitrile solutions of pristine MWCNTs (left) and tpy-MWCNTs (right).

Figure 2. UV-vis absorption spectra of pristine MWCNTs and tpyMWCNTs dispersed in CH3CN.

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Figure 4. TGA thermograms of pristine MWCNTs, diazo-tpy, and tpy-MWCNTs.

Figure 3. TEM images of pristine MWCNTs (a) and tpy-MWCNTs (b, c).

composed of tpy dimers in this paper because the chemical activity of pyridine and its substituent is much lower than that of benzene. TGA provides a quantitative estimation of the amount of aryl diazonium functionalized on MWCNTs. As shown in Figure 4, diazo-tpy has two decomposition stages. The one at about 110 °C is attributed to the decomposition of the diazonium group, while the second step at 240 °C is due to the degradation

of the tpy group. Although pristine MWCNTs are thermally stable under a nitrogen atmosphere up to 500 °C, tpy-modified MWCNTs start to decompose at about 190 °C. Clearly, this is attributed to the loss of the organic moiety. The weight loss at 500 °C is 18%, which also reflects the amount of the organic portion grafted on the MWCNTs.45 The residual mass is attributed to MWCNTs and can be used to determine the mass equivalent of carbon present. Thus, the functional group coverage (number of functional groups per gram of MWCNTs) based on TGA analysis is about 1.278 mmol/g. 2.3. Fabrication of SA Multilayer Films Based on tpyMWCNTs and Transition-Metal Ions. For the fabrication of SA multilayer films from tpy-MWCNTs and RuCl3, the surface of substrates should first be modified by diazo-tpy. The introduction of the terminal terpyridine group on the substrates provides a binding site that can form a coordination bond with ruthenium through consecutive reactions.40 Consequently, the multilayer films from diffluent tpy-MWCNTs and transitionmetal ions were fabricated on the tpy-modified substrates with a tpy monolayer through coordination bonds via the LBL SA method. Fabrication of SA multilayer films was carried out at room temperature. UV-vis spectroscopy was used to monitor the LBL multilayer fabrication process. Figure 5a displays the absorption spectra of the tpy-MWCNT-Ru(III) SA film from 1 to 10 bilayers on a tpy-modified quartz substrate. The absorption band at 283 nm originates from the π-π* transitions of tpyMWCNTs, while the weak peak at around 344 nm corresponds to the Ru(III) ion-to-ligand transition.46 The emergence of the absorption peak at 583 nm proves the existence of Ru(III) complexed with terpyridine groups.32a All these results confirm the successful fabrication of self-assembled films from tpyMWCNTs and Ru(III). With increasing of the bilayer number, the amount of tpy-MWCNTs deposited increases and the quartz slide is gradually darkened due to strong absorption of CNTs in the visible region.47 Figure 5b shows that the absorbance at 283, 344, and 583 nm increases linearly with the number of bilayers, suggesting that the same amount of tpy-MWCNTs and Ru(III) is deposited at each deposition cycle and the fabrication of multilayers is a stepwise and regular process. The morphology of the multilayer films was investigated by SEM. As shown in Figure 6, the magnified SEM image shows a high coverage of tpy-MWCNTs bound to the tpy-modified silicon substrate via the coordinate SA method. Meanwhile, the small-scaled image shows that all the nanotubes are isolated well, although some small bundles are observed. The architec-

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Figure 7. Cyclic voltammogram curves of three-bilayer Ru(III)/tpyMWCNT films built on the diazo-tpy-modified ITO surface with a scan rate of 100 mV/s.

Figure 5. (a) UV-vis absorption spectra of tpy-MWCNT/RuCl3 LBL SA multilayer films on tpy-quartz substrates. (b) Absorption intensity at 283 (black), 344 (red), and 583 nm (blue) versus the number of bilayers.

Figure 6. SEM image (scale bar, 5 µm) of five-bilayer Ru(III)/tpyMWCNT LBL SA films built on the tpy-Si surface and its magnified picture (scale bar, 500 nm).

tures are highly robust and able to withstand rinsing, as well as many organic solvents, and even long time sonication. Besides, the deposition can be controlled quantitatively by controlling the number of SA bilayers. 2.4. Optoelectrical Properties of the tpy-MWCNT/RuCl3 LBL SA Film. An independent quantitative analysis of the typMWCNT-Ru(III) SA films deposited on tpy-modified ITO glass slides was carried out. Figure 7 shows the cyclic voltammograms (CV) of the tpy-modified ITO and three-bilayer film fabricated on tpy-modified ITO immersed into 0.1 mol/L [NBu4+][ClO4-] acetonitrile solution. The scan rate is 100 mV/s over a voltage range of -1.5 to 1.5 V. On comparison with tpy-modified ITO, the two redox couples (-0.566 V (A1)/ -0.752 V (B1)), 0.394 V (A2)/-0.362 V (B2)) and one irreversible oxidation peak (0.745 V (A3)) of the three-bilayer film were detected. The first redox couple (A1/B1) is assigned

to the reversible one-electron oxidation-reduction of ruthenium(II)/ruthenium(III), and the second (A2/B2) one originates from the tpy group. The voltammogram also shows an irreversible oxidation peak A3 corresponding to the terpyridine ligandcentered oxidation.32f The current intensity is high and almost equals that of 0.11 mA hexagonal terpyridine-ruthenium macrocyclic complex solution in CH3CN,32c thanks to MWCNT moieties in this full-conjugated SA film, which act as a transmission bridge, though there are not any typical redox peaks themselves. To understand the structure and optoelectronic property relationship, an “on” and “off” photoswitchable photocurrent experiment was conducted to explore its photochemical characterization. The ITO coated with multilayer films was utilized as working electrodes. A three-electrode method was used to study the photoelectric conversion property of tpy-MWCNTsRu(III) SA films coated on ITO (0.2 cm2) under the illumination with a 500 W xenon-mercury lamp. The relationship between the photocurrent value and the irradiated time for three-bilayer tpy-MWCNTs-Ru(III) films is shown in Figure 8a. Upon irradiation, photocurrent is generated, which indicates that the tpy-MWCNT-Ru(III) film is an excellent photosensitizer in the visible region. At the initial stage, the photocurrent generated in the first cycle is higher than that in the second when illuminated, but it becomes relatively stable after the third irradiation cycle. It should be noted that the photocurrent production is a net effect of initial charge transfer to MWCNTs, followed by its transport to the electrode and regeneration of the tpy-MWCNT-Ru(III) moieties by the redox couple.48 The photogenerated excited-state moieties can likely react with some yet unidentified substances, which then leads to the degradation of the photoactive layer, and thus, the photocurrent decays slightly. It should also be mentioned that photocurrent decays have been observed previously in Ru(bpy)32+-based systems.49 Figure 8b shows the relationship between the photocurrent value and the number of bilayers. The photoelectric current increases with the number of bilayers from zero to six. Further increments of the bilayer have, however, lowered the photocurrent. The maximum photocurrent achievable by the film is 65 nA/cm2. Clearly, the film thickness strongly affects the overall energy conversion efficiency. Thinner films adsorb fewer functional molecules and hence generate lower photocurrent. In contrast, when the film is relatively thicker, the cell resistance will become larger, which is unfavorable for electron transfer.40,41b-d,50

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Figure 8. (a) The “on” and “off” photoswitchable photocurrent generation responses of three-bilayer SA films of Ru(III)/tpy-MWCNTs built on the ITO subtrate. (b) Average current intensity (ACI) as a function of the number of bilayers.

3. Conclusion A novel full-conjugated 4-(2,2′:6′,2′′-terpyrid-4′-yl) benzenediazonium tetrafluoroborate (diazo-tpy) was synthesized and used for surface modification of substrates, such as MWCNTs, quartz wafers, ITO glass, and silicon. The diazonium group of diazo-tpy is decomposed in the presence of UV light, resulting in covalent attachment of the residual terpyridine group on the substrates. The solubility of the MWCNTs modified by tpy groups is enhanced in common organic solvents. Analysis by TGA and HRTEM demonstrates the success of functionalization of the MWCNTs. The introduction of terminal terpyridine groups on the MWCNTs provides a binding site for ruthenium ions, generating multilayer films of tpy-MWCNTs and ruthenium ions on substrates with a tpy monolayer through coordination bonds using the LBL SA method. Furthermore, the multilayer films based on tpy-MWCNTs and Ru(III) ions were fabricated on tpy-modified substrates. The UV-vis results indicate that (1) LBL tpy-MWCNT-Ru(III) SA multilayer films fabricated from ruthenium ions and terpyridine groups anchored on the MWCNTs were successfully formed and (2) a progressive assembly process was carried out regularly with almost an equal amount of deposition in each cycle. The TEM image shows a high coverage of tpy-MWCNTs SCHEME 3: Synthetic Route for diazo-tpy

4. Experimental Section 4.1. Materials. All reagents and chemicals were used as received unless otherwise noted. MWCNTs (purity >95%, diameter ) 10-20 nm) were obtained from Tsinghua University. Ruthenium(III) chloride anhydrous (purity g99.6%) was purchased from Urchem. Nitrosonium tetrafluoroborate (NOBF4, purity g98%) was from Alfa Aesar. 4-(2,2′:6′,2′′-Terpyrid-4′yl) benzenediazonium tetrafluoroborate (diazo-tpy) was prepared according to the previous literature51 with some amelioration. Silicon, ITO, and quartz wafers were used as substrates. 4.2. Instrumentation. UV-vis spectra were obtained by a Shimadzu 1901 UV-vis spectrophotometer. SEM images were collected by an S4800 scanning electronic microscope (Hitachi Corporation, Japan). TGA data were collected by a Q50 thermogravimetic analysis instrument (TA, U.S.A.). TEM images were obtained from a TECNAI F30 field emission transmission electron microscope. Cyclic voltammetry and photoelectric transformation property measurements were performed on an IM6e Zahner elektrik electrochemical instrument (Germany) using a CHF-XM-500W short arc xenon lamp as the light source. The working electrode was Ru(III)/tpyMWCNT films that were built on the tpy-ITO surface with different numbers of layers. The counter electrode was a Pt sheet, and Ag/AgCl was used as the reference electrode. Bu4NClO4 (0.1 M) spectroscopically pure CH3CN solution was used as the supporting electrolyte. 4.3. Synthesis of 4-(2,2′:6′,2′′-Terpyrid-4′-yl) Benzenediazonium Tetrafluoroborate (diazo-tpy). The synthesis route is shown in Scheme 3. 4-[4-(2,2′:6′,2′′-Terpyridinyl)]aniline (aminotpy) (0.324 g, 1 mmol) was dissolved in 10 mL of sulfolane and 3 mL of acetonitrile mixture and then dropped in NOBF4 solution (prepared by dissolving 0.128 g of powder in 10 mL of acetonitrile and 3 mL of sulfolane mixture under -30 °C). After stirring for about 40 min under -40 to -30 °C and under light protection, the mixture was poured into 500 mL of ethyl ether. The precipitation was filtrated and washed with ethyl ether three times. Orange-red 4-(2,2′:6′,2′′-terpyridinyl) benzenediazonium tetrafluoroborate powder was obtained after air-drying without further purification (yield 85%). 1H NMR (400 MHz, CD3CN,) δ (ppm): 8.68 (d, 2H), 8.64-8.58 (m, 4H), 8.0-7.96 (t, 2H), 7.74-7.70 (d, 2H), 7.42-7.48 (t, 2H), 6.60-6.64 (d, 2H); IR (KBr): V (cm-1): 3390, 2266. 4.4. Pretreatment of Si, ITO, and Quartz Wafers. Quartz slides, ITO, and silicon wafers were used as substrates to fabricate self-assembled films, and the films were characterized by UV-vis spectra, cyclic voltammetry, and SEM, respectively.

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Before the experiments, these substrates were treated according to reported methods,52,53 which creates negatively charged surfaces. 4.5. Modification of MWCNTs by diazo-tpy. A 10 mg portion of MWCNTs was dissolved in 20 mL of acetonitrile and ultrasonicated for 30 min. Afterward, the suspension was added into 30 mL of diazo-tpy acetonitrile solution (0.2 mg/ mL). After stirring for 48 h at 0-10 °C under exclusion of light, the mixture was irradiated under 400 W ultraviolet light for 30 min under stirring. After filtration, the black power was washed with acetonitrile, chloroform, aqueous sodium hydroxide solution, deionized water, and chloroform sequentially. After centrifugation and drying in vacuo, tpy-modified MWCNTs (tpy-MWCNTs) were obtained. 4.6. Modification of Si, ITO, and Quartz Wafers by diazotpy. Si, ITO, and quartz wafers were immersed in diazo-tpy acetonitrile solution (0.2 mg/mL) for about 48 h. After exposure to 400 W ultraviolet light for 5 min, the substrates were purified using procedures similar to those of tpy-MWCNTs and gave tpy-Si, tpy-ITO, tpy-quartz with naked terpyridine groups on their surfaces. 4.7. Fabrication of Full-Conjugated Coordinate LBL SA Films. Tpy-MWCNTs (5 mg) were dissolved in 50 mL of acetonitrile to give 0.1 mg/mL tpy-MWCNT solution. Ruthenium(III) chloride was dissolved in ethanol to obtain 1 mmol/L solution. Multilayer films were assembled following the literature procedure.40,41a,b Tpy-Si, tpy-ITO, and tpy-quartz wafers were first immersed in RuCl3 ethanol solution for 10 min, rinsed with ethanol for 3 min, and dried by a hot flow of air. Afterward, they were immersed in tpy-MWCNT acetonitrile solution for another 10 min, followed by washing with acetonitrile for 3 min and drying in air. In each cycle, a bilayer tpy-MWCNT film was deposited on both surfaces of tpy-quartz but a monolayer on tpy-Si and tpy-ITO surfaces. Acknowledgment. The authors are grateful to the National Natural Scientific Foundation of China (Grant Nos. 50573008 and 20634020), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant Nos. 20050007018 and 20091101110031), and the Basic Research Foundation of Beijing Institute of Technology for financial support of this work. References and Notes (1) Iijima, S. Nature 1991, 354, 56–58. (2) (a) Georgakilas, V.; Gournis, D.; Tzitzios, V.; Pasquato, L.; Guldi, D. M.; Prato, M. J. Mater. Chem. 2007, 17, 2679–2694. (b) Wu, B. H.; Hu, D.; Kuang, Y. J.; Liu, B.; Zhang, X. H.; Chen, J. H. Angew. Chem., Int. Ed. 2009, 48, 4751–4754. (c) Shao, Y. Y.; Liu, J.; Wang, Y.; Lin, Y. H. J. Mater. Chem. 2009, 19, 46–59. (3) (a) Wang, J.; Musameh, M.; Lin, Y. H. J. Am. Chem. Soc. 2003, 125, 2408–2409. (b) Lynam, C.; Gilmartin, N.; Minett, A. I.; O’Kennedy, R.; Wallace, G. Carbon 2009, 47, 2337–2343. (4) (a) Heller, I.; Kong, J.; Williams, K. A.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2006, 128, 7353–7359. (b) Heller, I.; Mannik, J.; Lemay, S. G.; Dekker, C. Nano Lett. 2009, 9, 377–382. (5) Nojeh, A.; Lakatos, G. W.; Peng, S.; Cho, K.; Pease, R. F. W. Nano Lett. 2003, 3, 1187–1190. (6) (a) Yang, X. J.; Guillorn, M. A.; Austin, D.; Melechko, A. V.; Cui, H. T.; Meyer, H. M.; Merkulov, V. I.; Caughman, J. B. O.; Lowndes, D. H.; Simpson, M. L. Nano Lett. 2003, 3, 1751–1755. (b) Zhao, Y. L.; Stoddart, J. F. Acc. Chem. Res. 2009, 42, 1161–1171. (c) Martins, T. B.; Fazzio, A.; da Silva, A. J. R. Phys. ReV. B 2009, 79, 115413. (d) Liu, Y.; Wang, Y. X.; Jin, J. Y.; Wang, H.; Yang, R. H.; Tan, W. H. Chem. Commun. 2009, 665–667. (7) (a) Martin, R. B.; Qu, L. W.; Lin, Y.; Harruff, B. A.; Bunker, C. E.; Gord, J. R.; Allard, L. F.; Sun, Y. P. J. Phys. Chem. B 2004, 108, 11447– 11453. (b) Lin, S. J.; Keskar, G.; Wu, Y. N.; Wang, X.; Mount, A. S.; Klaine, S. J.; Moore, J. M.; Rao, A. M.; Ke, P. C. Appl. Phys. Lett. 2006, 89, 143118.

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