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Generic Approach to Modulate Conductivity and Coat Discontinuous Gate Dielectrics of Carbon Nanotubes Lei Fu, Xianglong Li, Yunqi Liu,* Zhimin Liu, Lingchao Cao, Dacheng Wei, Yu Wang, Gui Yu, Wenping Hu, and Buxing Han Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: February 9, 2007; In Final Form: March 30, 2007
Supercritical fluids in the presence of metal nitrate are used for the modulating conductivity of carbon nanotubes (CNTs). At the same time, CNTs are coated with discontinuous nanometer thin metal oxide dielectrics. To validate the effect of modulation, we have made multiwalled CNTs (self-coating with gate dielectrics) transistors using electron beam lithography deposition technology and studied their electronic properties. In particular, the method appears promising to improve the performance of existing field-effect transistors consisting of single-walled CNT networks, which display poor switching ratios in normal cases due to the coexistence of metallic and semiconducting tubes. Moreover, the mechanism of the multifarious morphology of a CNT/ metal oxide has been discussed.
Introduction Carbon nanotubes (CNTs) are promising candidates for the next generation of nanometer-scale electronic devices due to their unique electronic properties.1-3 For many electronic applications, it is extremely important to have nanotube ensembles that exhibit uniform electrical properties. While metallic nanotubes are desirable for nanoscale electrical interconnections, the fabrication of transistor devices requires exclusively semiconducting nanotubes. The key problem for electronic device development is how to control or tailor the electronic properties of a given nanotube. At present, few of the synthesis methods allow for a reliable control over the electrical properties of the produced nanotubes.4 Avouris et al. reported that structural deformations of multiwalled CNTs (MWCNTs) changed their electronic properties and could lead to a significant gate effect.2 Dai et al. used atomic force microscope (AFM) tips to manipulate single-walled CNTs (SWCNTs).5 The results show that mechanical deformation of SWCNTs may lead to a metal-to-semiconductor transition (MST) in their electronic properties. This experiment suggests a means for SWCNT-based electronic devices: a metallic SWCNT can act as a good conducting lead, and when mechanical deformation leads to semiconducting areas along the tube, a nanometer-scale electronic device is produced. Because the efficiency of mechanical deformations using AFM is very low, this method cannot be carried out in large-scale MSTs. Thus, understanding MST in CNTs is a necessary step in realizing nanotube-based electronic devices. It is by now well-established that the functionalizing and physisorption of CNTs greatly affects their electronic properties. For example, the conductivity of SWCNTs exposed to NO2, NH3, or O2 changes in a reliable, reproducible manner, which opens up the possibility of using nanotubes as chemical sensors or detectors.6,7 Hence developing controlled methods of functionalizing the nanotubes is crucial to tuning the electrical * To whom correspondence should be addressed. E-mail: liuyq@ mail.iccas.ac.cn.
properties of CNTs. Much progress has been made toward the goal of making nanometer-scale electronic devices out of carbon nanotubes, and the chemical alteration of the nanotubes is an important component of this work.8-12 Nanometer-scale p-n13 and p-n-p14 junctions have been made from individual semiconducting SWCNTs. The p-type sections of the nanotube are made by modification with poly(methyl methacrylate) (PMMA), whereas the n-type sections are made by modification with K atoms. Upon switching off the semiconducting tubes under appropriate gate control, the metallic tubes are exclusively provided with a high density of covalently coupled phenyl radicals reductively generated from aromatic diazonium salts.15 As a consequence, the modified metallic tubes become insulating, and the resulting ensemble shows purely semiconducting behavior. However, in all of these cases no extensive alteration of the nanotubes could be achieved. Among various surface functionalization techniques, oxidation is probably the most widely studied. Early treatment techniques have involved gas-phase oxidation in air and oxidative plasmas,16 but these techniques have led to an overoxidation of CNTs, often removing or severely damaging the CNTs. A low yield of oxidized CNTs subsequently results. Liquid-phase oxidation involving acidic etching with nitric acid and/or sulfuric acid was generally considered to be the dominant method.8,17 Acidic reflux processes need a prolonged oxidization period, often as long as 3 days for surface oxidation. Unfortunately, even with a prolonged treatment, refluxed CNTs failed to result in a uniform mild surface oxidation of the CNTs.18 Since CNTs have a hydrophobic surface, they tend to form aggregates in polar solvents. During the acidic reflux process, some CNTs inside these aggregates may not be attacked by the oxidative agents but remain unmodified. To modulate conductivity of CNTs, the entire surfaces of CNTs must be mildly oxidized. Herein, an effort to prepare uniform mild surface oxidation of the CNTs, supercritical carbon dioxide and metal oxide were successfully employed for oxidation treatment of multiwalled CNTs. It was found that treated CNTs resulted in being coated discontinuously with nanometer oxide.19 Despite this observa-
10.1021/jp0711186 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/11/2007
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tion, the effects of this processing on the surface oxidation of CNTs were not characterized. We report here a detailed study of the supercritical treatment with emphasis on its effects on surface structure. Particular attention is paid to delineating the effect of various metal oxides. The supercritical CO2 based method is found to be an easy and rapid method, avoiding the use of strong acids. Moreover, we present a novel method to fabricate carbon nanotube field-effect transistors (CNTFETs). This task is performed by nitrate decomposing reactive NxO that is able to oxidize the nanotube sidewall, resulting in an increase in resistance. On this basis, this reaction at the surface can reduce the carrier density at zero gate voltage, yielding a desirable semiconducting CNT channel between source and drain electrodes. During the modification, the CNTs are discontinuously coated with a nanometer thin alumina gate dielectric shell. It is a more low-cost, simple, and efficient method for oxide gate dielectric deposition than conventional atomic layer deposition (ALD) and molecular beam epitaxy (MBE) grown dielectric films.20,21 The major advantage of the supercritical-solution approach is the alteration of CNTs and the fabrication of gate dielectric shell simultaneously. Experimental Section MWCNTs synthesized by chemical vapor deposition (SWCNTs synthesized by arc discharge) were first suspended in an ethanol solution of aluminum nitrate nonahydrate in a high-pressure vessel. Then CO2 was charged into the vessel to a desired pressure at the temperature of 35 °C, resulting in a mixed supercritical fluid containing ethanol and CO2 for dissolving aluminum nitrate nonahydrate. Heating the high-pressure vessel at 80 °C for about 6 h, the surfaces of CNTs were oxidized and CNTs coated discontinuously with alumina were obtained. The thickness of alumina sheaths was controllable by changing the concentration of the aluminum ion. Subsequently, field-effect transistors were prepared using individual coated nanotubes. Overall, nanotube-based devices were prepared on a silicon substrate with a 300-nm-thick thermally grown SiO2 layer. The process was as follows: (1) Two Pt crosses were predeposited by the focused ion beam (FIB) method for use as the markers. (2) Poly(methyl methacrylate) (PMMA) was coated on top of this substrate. (3) Coated CNTs in acetone solution were dispersed by ultrasonic agitation and were dropped on SiO2/Si substrates with Pt cross markers. The density of the solution was adjusted to yield approximately one CNT in an area of ∼10 × 10 µm2. (4) After drying, the location and the appearance of the discontinuously coated nanotubes were identified using a Raith 150 Electron Beam Lithography System (Raith Company). (4) Once a right one was found, two electrodes were patterned by electron-beam exposure on alumina coating (the gate electrodes) and the other two were patterned on bare nanotubes (the source and drain electrodes). (5) After the PMMA was removed by the lift-off process, electrodes of Ti (20 nm)/Au (80 nm) were formed on a CNT by evaporation of metals. All the samples were prepared with exactly the same procedure and doses. The electrical properties were measured using probe station (Micromanipulator Company) and semiconductor parameter analyzer (Keithley 4200) at room temperature in air. Results and Discussion The supercritical fluid assisted process resulted in a discontinuously coated product in very high yield. Figure 1a reveals that the product consists of a large quantity of discontinuously
Figure 1. (a) SEM image of the discontinuously coated product recorded at 15 kV without surface coating. Several uncoated spaces of the MWCNTs are indicated by arrows. (b) TEM image of alumina coating on MWCNTs. The inset shows a 3D schematic diagram of alumina-coated MWCNTs. (c) Enlarged images of the tube wall.
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Figure 2. C 1s X-ray photoelectron spectroscopy spectra of (a) pristine MWCNTs, (b) Al2O3-coated MWCNTs, (c) Eu2O3-coated MWCNTs, and (d) Co3O4-coated MWCNTs.
coated MWCNTs. Several uncoated spaces of the MWCNTs are indicated by arrows, and the surface is smooth. The in situ energy-dispersive X-ray spectrum (EDX) shows only the elements of Al and O (as well as C, which presumably arises from the carbon nanotube), indicating the formation of alumina. Figure 1b shows a transmission electron microscopy (TEM) image of typical coated MWCNTs found in the composite. Notably, some of the tube bodies were discontinuously sheathed with coating structures. The inset of Figure 1b is the schematic diagram of the alumina discontinuous coating over the outer surfaces of the nanotubes. The thick-thin and thin-thin white lines respectively mark the cylindrical outer layer and the tube walls of MWCNT in the magnified TEM image (Figure 1c). The thickness of the coating layer and the outer diameters of the MWCNTs determined from this magnified TEM image are 7 and 35 nm, respectively. These TEM images reveal that alumina material coats the MWCNTs conformally; the surface of CNTs is relatively smooth and an ordered graphite layer structure. The morphology and tubular structure of the functionalized MWCNTs were observed in scanning electron microscopy (SEM) and TEM, suggesting that the functionalization did not deteriorate the structural integrity of MWCNTs. The wide-survey X-ray photoelectron spectroscopy (XPS) spectrum of coated MWCNTs shows the predominant presence of carbon (36 atom %), oxygen (45 atom %), aluminum (18 atom %), and nitrogen (1 atom %). Compared with the uncoated MWCNTs (composition 91% C, 7% O, 2% N), the atomic composition reveals a prominent increase of oxygen, attributed to the oxygen atoms in the coating and air absorbed on the surfaces of MWCNTs. The trace content of nitrogen indicates the coating is alumina. By using a simple low-temperature supercritical fluid assisted method, we have successfully synthesized several kinds of CNT-metal oxide heterostructures introducing appropriate precursors (metal nitrate) as coating sheaths. For example, MWCNTs were coated discontinuously with nanometer alumina gate dielectric shell,19 beaded with cobalt oxide nanoparticles,22 or coated uniformly with multiwalled europium oxide nanotubes.23 Figure 2 shows the C 1s core level photoemission spectra of MWCNT starting material (spectrum a), MWCNTs coated with Al2O3 (spectrum b), MWCNTs beaded with Eu2O3 (spectrum c), and MWCNTs coated with Co3O4 (spectrum d). The untreated CNT shows a dominant peak structure for the C 1s core level at a binding energy (BE) of 284.6 eV, which corresponds to the bare, untreated CNT surface.24 The peak has a slightly asymmetric line shape with a high binding energy tail. The peaks at about 289.6 and 286.1 eV originate from carbon atoms bound to
Fu et al. oxygen atoms, such as C-O and -COO groups, from the mildly oxidized MWCNT.25 In the process of modification in supercritical fluid, metal nitrate precursors decomposed to form metal oxide at an appropriate temperature, releasing reactive NxO. In this oxidizing ambience, the nanotube sidewall can be oxidized in the cases of decompositon of aluminum(III) nitrate nonahydrate and europium(III) nitrate hexahydrate. Cobalt(II) nitrate hexahydrate decomposed to form Co3O4, which indicated that cobalt(II) was oxidized prior to the nanotube. This is why there are oxidized MWCNT peaks in spectra b and c, while there is no clear XPS peak due to surface oxygen groups observed in spectrum d (Figure 3). Figure 4a shows the morphologies of the coated nanotubes before patterning leads. The locations of leads are marked with dotted line frames. An example of the resulting connections is shown in Figure 4b. It is clear that the source and drain electrodes were deposited at bare nanotubes in regions 1 and 4, the two gate electrodes were deposited at coated nanotubes in regions 2 and 3. Figure 5 shows a schematic of the FET device. In our electrical transport studies on MWCNTs, we have measured many individual tubes (in excess of 20) and find two types of behavior at room temperature. The metallic variety of tubes reported previously has linear current-voltage curves26 and shows no dependence on the gate voltage (Vg). The difference from our previous experiment19 is that the content of metallic variety is much smaller, approximately 15%. Here we present measurements on the second type of sample. Figure 6a shows the typical drain current (Isd) versus gate voltage (Vg) characteristics for a coated nanotube transistor when top-gated through an Al2O3 dielectric layer from electrode 2. The subthreshold swing, defined as S ) ∂Vg/∂(log Isd), is a key parameter to transistor miniaturization. Small S is desired for low threshold voltage and low power operation of FETs scaled down to small size. Subthreshold swings for all of our FETs are reproducibly measured in the S ) 200-520 mV per decade range for various bias voltages of Vsd ) -0.5 to -1.5 V. In the measurement of Isd while changing Vg at a fixed Vsd of -2.0 V, Isd changes from 10-6 to 10-5 A as Vg changes from 2.0 V to -1.5 V. Importantly, the tunneling leak current through a ∼7nm-thick Al2O3 dielectric layer is negligible (Figure 6b). At a voltage between the source-drain voltage and gate voltage of 1.0 V the gate leakage current is about 10-11 A. The drain current exceeds the gate current by a factor of 104-105, confirming the excellent gate insulation provided by the alumina dielectric. Figure 6c shows typical current versus source-drain bias voltage curves (Isd-Vsd) obtained from a typical device made this way under various gate voltages Vg in steps of 0.12 V for the Al2O3/MWCNT transistors at room temperature. The Isd decrease with increasing Vg, indicating a p-type FET device, is consistent with previous observations.6,26 Although the interest in supercritical fluids (SCFs), in particular, supercritical carbon dioxide (scCO2), has received much attention within academic and chemical industrial laboratories as a green alternative to conventional organic solvent, the application of CO2 is not comprehensive in the nanomaterial field. The nontoxicity, low cost, abundance, ease of recycling, and ability to make new materials under mild conditions27 are some of the key attributes of this environmentally benign solvent.28-31 A fluid is supercritical when its temperature and pressure are higher than their critical-point values. At these conditions, the fluid exists as a single phase having some of the advantageous properties of both a liquid and a gas: it has sufficient density to give appreciable dissolving power, but the diffusivity of solutes in SCFs is higher than in liquids, and the
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Figure 3. Schematic illustration for the reaction at the surface of CNTs involving (a) aluminum nitrate, (b) europium nitrate, and (c) cobalt nitrate.
Figure 5. Bird’s-eye view of schematic device structure of CNTFETs with alumina coated as the gate dielectrics. The nominal thickness of alumina used for our FETs is 7 nm. The thickness of thermally grown SiO2 on the Si substrate is 300 nm.
Figure 4. SEM image of an example of MWCNT patterning with leads. (a) Morphologies of the coated nanotubes before patterning leads; the locations of patterning leads are marked with a white dotted line frame. (b) Resulting connections.
viscosity of SCFs is lower, facilitating mass transport. The low viscosities of SCFs and high diffusivities of solutes in SCFs combined with very high buoyant forces (which cause significant density gradients across the interface) may result in superior mass transfer characteristics compared with conventional solvents. Thus the reagents (metal salts) can penetrate through the aggregates. In addition, the low viscosity and rapid convection of SCFs increase the possibility of individually dispersed CNTs
in solution. MWCNTs are dispersed individually in supercritical CO2 (Figure 1). It has been reported that MWCNTs can be thinned and opened in carbon dioxide.32 In the experiment, the more reactive outer carbon layers reacted with the mild oxidizing agent carbon dioxide and were subsequently stripped off. Chang and Ling reported that supercritical water was used for opening and thinning of MWCNTs.33 Those experimental conditions are quite intense, and the pristine structure of CNTs has been damaged. Utilization of the distinctive properties of the supercritical carbon oxide in nanometer-scale surface material science could provide a new way to control properties of the surfaces and the materials. The oxidation of CNTs in scCO2 solvents offers great advantages, which include rapid solvent separation, accelerated reaction rates (due to high diffusivities), and the possibility of depositing particles in situ in materials, thereby taking advantage of the unique properties of the SCF phase.34,35 It is known that the nucleation of oxide crystals is usually too fast in aqueous solution because the surface hydroxyls can lead to the aggregation of metal oxide nanoparticles,22,36 while nanocrystals aggregating in the nonaqueous solution are kinetically slower than in aqueous solution due to fewer surface hydroxyls. In this work, the formation of the resultant structure of the Al2O3-coated MWCNTs may result from the special properties of supercritical solution, such as low viscosity, high diffusivity, and near-zero surface tension. Under the experimental conditions, CO2 and
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Figure 6. FET characteristics of CNTFETs at room temperature. (a) Isd-Vg characteristics for Vsd ) -0.5 to -2.0 V. (b) Leakage current at a gate voltage of 1.0 V. (c) Isd-Vsd characteristics at various Vg’s applied to the gate.
ethanol formed a mixed supercritical medium for dissolving aluminum nitrate and for its decomposition. Despite their liquidlike solvation power, SCFs have poor solvent shielding. This serves to promote self-assembly of solvated molecules onto favorable substrates. The positive metal ions can be first randomly adsorbed onto the surface of MWCNTs due to electrostatic attraction.37 However, they may be preferably absorbed on some sites due to imperfections of the CNT surface. The absorbed substances act as preferential nucleation sites, and larger crystals are formed. The aluminum nitrate decomposed and the outer layers of MWCNTs oxidized in situ at a temperature of 80 °C. Similar to the previous experiment,22 we carried out a control experiment to validate the pivotal effect of a supercritical medium: MWCNTs were soaked in nitrateethanol solution under the same conditions but without supercritical CO2. The oxide tended to homogeneous nucleation and
Figure 7. (a) TEM image of alumina coating on SWCNT bundles. Isd-Vsd characteristics at various Vg’s applied to the gate of (b) pristine SWCNT networks and (c) modified SWCNT networks at room temperature.
could not form the coated MWCNTs without a supercritical medium. Therefore, the introduction of a supercritical medium is a key factor in forming the coated MWCNTs. The morphology of a CNT/metal oxide depends on the interfacial energy between the metal oxide and CNTs during the growth of the metal oxide on the CNT surface. Based on above experiments, the supercritical medium can reduce the surface energy of metal oxide, γoxide. Most oxides tend to homogeneous nucleation without a supercritical medium. If γoxide is larger than the interfacial energy between the metal oxide and CNTs, γCNT-oxide, as is the case for CNT/Y2O3, metal oxide
Discontinuous Gate Dielectrics of CNTs nanoparticles will not grow on CNTs (figures not provided here). However, when γoxide is much smaller than γCNT-oxide, such as for CNT/Eu2O3, metal oxide atoms tend to be located on the CNT surfaces, causing them to be coated with nominally complete metal oxide layers.23 When γoxide is smaller than γCNT-oxide, such as for CNT/Al2O3, metal oxide atoms tend to homogeneous nucleation on the CNT surfaces, causing them to be coated with discontinuous metal oxide layers.23 When γoxide is similar to γCNT-oxide, as is the case for CNT/Co3O4, metal oxide atoms tend to be located on both CNTs and the metal oxide. In this case, the metal oxide particles take on a spherical shape and are threaded by CNTs, resulting in the pearl-necklacestructured CNT/Co3O4.22 Previous electrical conductivity measurements of individual CNTs showed that defective CNTs indeed possess higher resistivities than do graphite CNTs,38,39 which is due to an increase in electron scattering by the defects. Theoretical studies by Rochefort et al. proposed that the introduction of defects (e.g., C-O) into CNT lattices would reduce CNT conduction by 30-50%,40 a result that is consistent with experiment.38 Avouris et al. reported that structural deformations of MWCNTs changed their electronic properties and could lead to a significant gate effect. Since the intershell interaction in MWCNTs is weak because electron hopping across 3.4-Å spaces from the inner layers to the outer layers is difficult, it is reasonable to assume that transport is confined to the outermost shell of the nanotubes. In our work, the outermost shell of MWCNTs was mildly oxidized as confirmed by XPS, similar to the oxidation in supercritical water.33 Moreover, the thin top-gate oxide of several nanometers also contributed to the improved electrical properties in our devices. As a result, the coated MWCNTs exhibit distinct p-type characteristics. On the other hand, it is worth mentioning that, in a previous experiment, we used FIB systems to deposit metal electrodes for field-effect transistors.19 Although we found no evidence for significant ion damage or penetration, quite a number of MWCNTs presented metal characteristics after modification. In fact, the injection of metal ions caused severe impact to the surfaces of CNTs because the structure of the outer layer was brittle. The device fabrication process made us underestimate the ability of our method to modulate conductivity. The penetration depth of electron beam lithography is quite short, so electron beam lithography is a better choice to use to fabricate the device, and then to evaluate the degree of modulating conductivity. We have also performed the analogous modification with SWCNTs (Figure 7a). SWCNTs like to form bundles because the cohesive energy between the SWCNTs, due to van der Waals interactions, is significantly greater than the solvation energies. The surfaces of SWCNTs bundles do not look smooth, indicating that they are coated by discontinuous nanometer thin metal oxide. As is well-known, SWCNTs exist in two types: metallic and semiconducting. If there are metallic SWCNTs in the bundle, the performance of the device will severely deteriorate. The SWCNT networks exhibited a small dependence on gate voltage (Figure 7b), resulting in pathways with a metallic character.41 One of the great challenges is to separate metallic from semiconducting tubes in substantial quantities. We have made a FET using SWCNT networks and found good electronic properties (Figure 7c). With this process we receive about a 103 reduction in off current, and achieve effectively a 100-fold increase of the on/off current ratio. Our method provides a possibility to steer clear of separation and still receive good device performance.
J. Phys. Chem. C, Vol. 111, No. 22, 2007 8103 Conclusion Mild surface oxidation is a feasible way for modification of the conductivity of CNTs for device applications. A facile approach to the purpose is presented using supercritical carbon dioxide to promote oxide etching and increase the population density of surface oxide groups. The relatively low temperature and short time for CNT oxidation/functionalization is desirable for minimizing structural damage to the CNTs that could lead to changes in their properties. The results we found here are in sharp contrast to those of noncovalent functionalization, where slight modification of band structures of SWCNTs was found. The modification of electronic band structures of SWCNTs by covalent functionalization thus provides a clear pathway for controlling the electronic properties of CNTs, for band structure engineering, for electronic applications, and for chemical sensor applications. This process can be used to reduce the carrier density of CNTs, resulting in high-performance FETs. In particular, the method appears promising to improve the performance of existing FETs consisting of SWCNT networks, which display poor switching ratios due to the coexistence of metallic and semiconducting tubes. Acknowledgment. This work was supported by the Major State Basic Research Development Program, the National Natural Science Foundation of China (20573115, 90206049, 20472089, 20421101, 50673093, 60671047), and the Chinese Academy of Sciences. References and Notes (1) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49. (2) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Appl. Phys. Lett. 1998, 73, 2447. (3) Javey, A.; Qi, P. F.; Wang, Q.; Dai, H. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 13408. (4) Wang, Y. H.; Kim, M. J.; Shan, H. W.; Kittrell, C.; Fan, H.; Ericson, L. M.; Hwang, W. F.; Arepalli, S.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2005, 5, 997. (5) Tombler, T. W.; Zhou, C. W.; Alexseyev, L.; Kong, J.; Dai, H. J.; Lei, L.; Jayanthi, C. S.; Tang, M. J.; Wu, S. Y. Nature 2000, 405, 769. (6) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622. (7) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (8) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853. (9) Sun, Y. P.; Fu, K. F.; Lin, Y.; Huang, W. J. Acc. Chem. Res. 2002, 35, 1096. (10) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105. (11) Banerjee, S.; Kahn, M. G. C.; Wong, S. S. Chem.sEur. J. 2003, 9, 1899. (12) Banerjee, S.; Hemraj-Benny, T.; Wong, S. S. AdV. Mater. 2005, 17, 17. (13) Zhou, C. W.; Kong, J.; Yenilmez, E.; Dai, H. J. Science 2000, 290, 1552. (14) Kong, J.; Cao, J.; Dai, H. J.; Anderson, E. Appl. Phys. Lett. 2002, 80, 73. (15) Balasubramanian, K.; Sordan, R.; Burghard, M.; Kern, K. Nano Lett. 2004, 4, 827. (16) Ebbesen, T. W.; Ajayan, P. M.; Hiura, H.; Tanigaki, K. Nature 1994, 367, 519. (17) Lordi, V.; Yao, N.; Wei, J. Chem. Mater. 2001, 13, 733. (18) Xing, Y. C. J. Phys. Chem. B 2004, 108, 19255. (19) Fu, L.; Liu, Y. Q.; Liu, Z. M.; Han, B. X.; Cao, L. C.; Wei, D. C.; Yu, G.; Zhu, D. B. AdV. Mater. 2006, 18, 181. (20) Ye, P. D.; Wilk, G. D.; Yang, B.; Kwo, J.; Chu, S. N. G.; Nakahara, S.; Gossmann, H. J. L.; Mannaerts, J. P.; Hong, M.; Ng, K. K.; Bude, J. Appl. Phys. Lett. 2003, 83, 180. (21) Hong, M.; Kwo, J.; Kortan, A. R.; Mannaerts, J. P.; Sergent, A. M. Science 1999, 283, 1897. (22) Fu, L.; Liu, Z. M.; Liu, Y. Q.; Han, B. X.; Hu, P. G.; Cao, L. C.; Zhu, D. B. AdV. Mater. 2005, 17, 217. (23) Fu, L.; Liu, Z. M.; Liu, Y. Q.; Han, B. X.; Wang, J. Q.; Hu, P. G.; Cao, L. C.; Zhu, D. B. AdV. Mater. 2004, 16, 350.
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