Field Emission Properties of Hybrid Carbon Nanotube−ZnO

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17702

J. Phys. Chem. C 2008, 112, 17702–17708

Field Emission Properties of Hybrid Carbon Nanotube-ZnO Nanoparticles Yong Min Ho, Wei Tao Zheng,* Ying Ai Li, Jian Wei Liu, and Jun Lei Qi Department of Materials Science, State Key Laboratory of Superhard Materials, and Key Laboratory of Automobile Materials of MOE, Jilin UniVersity, Changchun, 130012, People’s Republic of China ReceiVed: May 23, 2008; ReVised Manuscript ReceiVed: September 18, 2008

ZnO nanoparticles are uniformly coated on the walls of carbon nanotubes (CNTs) via a straightforward process, and the particle size and interparticle distance can be controlled by coating time. The appropriate amount of coated nanoparticles effectively reduces the formation of various structural defects induced by oxygen or hydrogen atoms on the walls of CNTs, which can be evaluated through a decrease in the intensity ratio of disorder graphitic band (D peak) over graphitic C-C stretching band (G peak) in the Raman spectrum. An overincrease in coating time simultaneously causes an increase in interstitial zinc and oxygen vacancies in ZnO. The high local electric field around the ZnO particle on the walls of CNTs can increase the tunneling probability at CNTs-ZnO heterojunction, significantly enhancing the field emission property for CNTs. 1. Introduction Carbon nanotubes (CNTs) have been known to be one of the most promising candidates for field emission devices due to their small curvature radius and high thermal stability and conductivity.1,2 In addition, ZnO, a semiconductor with a wide band gap (3.37 eV) and large exciton binding energy (60 meV), has more chemical stability in harsh environments and exhibits good field emission properties comparable to those of CNTs.3,4 Recently, in order to improve optical and field emission properties of CNTs or ZnO, several groups have investigated the coating processes of ZnO nanoparticles and the growth of ZnO nanowires on CNTs using various methods,5-10 such as chemical surface modification, chemical vapor deposition, and vapor-phase transport. In these coating processes, the average size of ZnO particles and interparticle distance can be controlled, resulting in a change of surface electronic properties of CNTs-ZnO. In general, for good field emission (FE) properties of hybrid CNTs-ZnO nanoparticles, it is necessary to employ the CNTs with minor surface defects as the composites because these defects cause a decrease in conductivity owing to the damage of graphite lattice. In a previous work, we have realized a direct coating of ZnO nanoparticles on CNTs via a straightforward process by plasma-enhanced chemical vapor deposition (PECVD) using hydrogen, and showed that this method had a great effect on preventing the formation of defects on the CNTs walls in synthesis process.11 Since the FE properties in CNTs-ZnO materials are significantly influenced by the geometric factors of the structure, it is important to pay attention to the variation in geometrical shapes of CNTs surface in the coating process. Moreover, until now, many efforts have focused on the enhanced FE properties of CNTs-ZnO materials with increasing coated ZnO nanoparticles, but there have been no reports on the effect of the variation in the geometrical factors for ZnO particles on the FE properties for CNTs. Herein, we report the effect of the variation in the size and interparticle distance for ZnO nanoparticles coated on the walls of CNTs, brought about by controlling coating time, on the FE properties for CNTs. * To whom correspondence should be addressed. Tel/fax: +86431 5168246. E-mail: [email protected].

Figure 1. Schematic drawing of RF-PECVD apparatus.

2. Experimental Section Figure 1 shows the schematic drawing of the radio frequency (RF)-PECVD apparatus used in our experiments (RF power 13.56 MHz). A MgO-supported Ni catalyst was used for CNTs synthesis. A 1 g portion of Mg(NO3)2 · 6H2O and 1 g of Ni(NO3)2 · 6H2O were dissolved together in 50 mL of ethanol, and the catalyst suspension was dropped onto Si substrates. High-purity ZnO powders (0.02 g, particle size 50-100 nm, purity 99.999%) were put in a flat-lying cylindrical ceramic tube with a diameter of 6 mm. Two ends of the ceramic tube were covered with flexible steel plates by thin and short aluminum lines. The aluminum lines looped through small holes on the tops of the ceramic tube and steel plate will be loosed at the melting point of aluminum (660 °C). The Si substrate was placed downstream site in the vapor flow direction at a distance of 5 mm from the ceramic tube. The PECVD reactor chamber was evacuated to a pressure of 20 Pa, and the chamber pressure was maintained at 200 Pa with a H2 gas flow of 20 sccm. After Si substrate was heated to 800 °C for 40 min, methane with a gas flow of 80 sccm was introduced into the chamber, and a plasma power of 200 W was utilized to grow CNTs. When CNTs were grown for 6-7 min, the aluminum wires were melted because of the heating from Si substrate at high temperature. Since the

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Properties of Hybrid CNTs-ZnO Nanoparticles

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Figure 2. SEM images for CNTs coated ZnO at 800 °C for (a) 3 min, (b) 6 min, and (c) 10 min.

Figure 3. TEM images for CNTs coated with ZnO at 800 °C for (a) 3 min, (b) 6 min, (c) 10 min, and (d) 20 min, and HRTEM images for CNTs coated with ZnO for (e) 10 min and (f) 20 min.

thickness of the aluminum wire was much thinner in the contacting part of ceramic tube and steal plate, the wire of this contacting part broke earlier. At this moment, the steel plates were easily deviated from the tube ends and fell down to the bottom because the down-side length (37 mm) of the ceramic tube was shorter than the up-side (40 mm). Since the coating time was short, the other part of the aluminum wire still remained in a partially melted state on the top of the ceramic tube. The plasma power was off after the two steel plates were found to deviate from the tube ends through the window of the chamber. For about 3-6 min, the growing CNTs were simultaneously coated by a flow of ZnO vapor particles sublimated in the ceramic tube. Then the substrate was annealed at 400 °C for 60 min without any gas flow and cooled down to room temperature. Scanning electron microscope (SEM) (JEOL JSM6700F), transmission microscope (TEM) (Hitachi H-8100 at 175kV), high resolution electron microscope (HRTEM) (JEOL TEM-2010 at 300kV), and Jobin-Yvon LabRam Infinity Raman spectroscopy with a 514.5 nm Ar+ laser excitation were used to study the morphology and structure of hybrid CNTs-ZnO material. The photoluminescence (PL) of hybrid CNTs-ZnO was investigated using a He-Cd laser (325 nm) as the excitation source. Field-emission measurements were carried out using a diode configuration, a cathode (sample), and a parallel anode

plate in a vacuum chamber maintained at a pressure of 2 × 10-7 Pa with an oil-free turbo-molecular pump. 3. Results and Discussion Figure 2a-c shows the SEM images for CNTs coated with ZnO at 800 °C for 3, 6, and 10 min, respectively, in which the diameter and length of CNTs is 10-30 nm and about 4 µm. We can see that the walls of the CNTs are more densely covered with ZnO nanoparticles upon increasing the coating time. Parts a, b, c, and d of Figure 3 display the TEM images for CNTs coated with ZnO for 3, 6, 10, and 20 min, and parts e and f of Figure 3 show the HRTEM images for CNTs coated with ZnO for 10 and 20 min, respectively. A number of bead-shaped ZnO nanoparticles with a small radius of 1-12 nm are attached to the CNTs walls. TEM and HRTEM images show that the obtained CNTs have a multiple-walled structure (MWCNTs) and ZnO nanoparticles coated on the CNT have approximately a spherical shape. The lattice spacing between adjacent walls of the CNT has been observed to be 0.34 nm, which is good agreement with the layer spacing of graphite. The values of the interplanar spacing for ZnO nanoparticle in parts e and f of Figure 3 have been measured to be 0.25 and 0.28 nm, respectively, which agree well with the interplanar distances of the (002) and (100) plane for ZnO with a hexagonal structure.

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Figure 5. Raman spectra for CNTs coated with ZnO for 3, 6, and 20 min. Inset: Raman spectra in the range of 300-700 cm-1 for CNTs coated with ZnO for 20 min.

Figure 4. (a) Size distribution for ZnO particles in CNTs coated with ZnO for 3 and 6 min, respectively, and (b) average interparticle distance (curve A) and average particle radius (curve B) as a function of coating time. The solid line is just to guided to the eyes.

Figure 3e also shows that, in fact, two independent ZnO particles are connected with each other on the surface of CNTs, implying the conversion process from ZnO particle to the film layer as the coating time increases, and Figure 3d,f shows the perfect formation of the ZnO coating layer on the walls of the CNTs. Figure 4a shows the size distribution for ZnO particles, determined from TEM images for CNTs coated with ZnO for 3 and 6 min. As the coating time is 3 min, most of ZnO particles have size of 1-4 nm, and the width of distribution curve is narrow. At the beginning stage of coating, many vapor-phase ZnO particles sublimated in the ceramic tube are instantly covered on the walls of CNTs, which causes the homogeneous attachment of particles to the walls. At this moment, the diameters of coated particles will be mostly equal to those of species in the vapor phase transported by the gas flow, resulting in a narrow width in the distribution curve. As the coating time increases, the species in the vapor phase will attach not only on the walls of CNTs, but also to the previously coated ZnO particles. This will cause a decrease in interparticle distance and a wide distribution in particle sizes. Determined from SEM and TEM images, the average interparticle distance (curve A) and the average particle radius (curve B) with coating time are shown in Figure 4b. In section I, the average interparticle distance is larger than the average particle radius, while section II shows the opposite tendency. Especially, since in section III the walls of CNTs have completely been coated by ZnO nanoparticles, it is reasonable

for the particle radius to be expressed as the thickness of the coating layer. In Figure 5, Raman spectra for CNTs coated with ZnO for 3, 6, and 20 min show a G band at about 1593 cm-1 corresponding to sp2-hybridized carbon and a D band at about 1348 cm-1 originating from disordered carbon. The origin of this disorder band in CNTs can be primarily due to defects in the tube wall, such as bending in the nanotube, the finite size of crystalline domains, sp3-hybridized bonds, and functional groups created by oxidation.12,13 Therefore, the intensity ratio of D over G peak (ID/IG) band can be used to evaluate the degree of disorder in the walls of CNTs.14-17 The intensity ratios of D over G band (ID/IG) are 1.12, 0.93, and 0.85, respectively, showing a substantial decrease in the degree of disorder in the walls of CNTs as coating time increases. This can be interpreted as a decrease in defects due to oxidation and formation of C-H bonds on the walls of CNTs annealed without hydrogen gas flow.11 Among the nanoparticles coated on the walls of CNTs by the vapor phase transport process, Zn atoms or ZnO nanoparticles (i.e., nanoparticles with more oxygen vacancies) partially reduced by discharge hydrogen gas can exist. Generally, purified CNTs also have a considerable number of the oxygen atoms adsorbed on the surface of CNTs, forming C-O, CdO, and O-CdO bonds,18 and Zn atoms have a coordinative affinity to the oxygen of these oxygen-containing bonds.19 Hence when the amount of coated nanoparticles is not enough, Zn or ZnO particles with oxygen vacancies can be selectively anchored to the oxygen atom of the oxygen-containing groups on the surface of CNTs because of the interaction between electropositive Zn and electronegative oxygen atom. As the substrate is annealed at 400 °C without hydrogen gas flow, the residual oxygen atoms in the chamber can oxidize these nanoparticles and the surface of CNTs. After coating, the nanoparticles adsorbed on CNTs will cause an increase in the number of oxygen atoms binding to the coated nanoparticles in the annealing process, and the oxidation of CNT will be attenuated significantly. On the other hand, the additional deformation of C-H bonds due to the hydrogen atoms existing in the chamber at an annealing temperature of 400 °C causes the partial conversion from sp2to sp3-hybridized carbon on the walls of CNTs during the annealing process and gives another reason why the ratio of ID/IG in the Raman spectra increases with decreasing coating time. Here, the difference between the ratio of ID/IG for CNTs coated with ZnO for 6 min and that coated for 20 min is much smaller than that for the samples coated with ZnO for 3 and 6

Properties of Hybrid CNTs-ZnO Nanoparticles

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Figure 6. Photoluminescence (PL) spectra for CNTs coated ZnO for 3, 6, 10, and 20 min, respectively.

min, in spite of long coating time interval, which implies that the additional defect formation on the walls of CNTs during the annealing process can be probably ignored in the samples coated for more than 6 min. Additionally, we also find some Raman peaks from ZnO nanoparticles in CNTs coated with ZnO for 20 min, wherein the relatively strong peaks at 437 and 570 cm-1 correspond to E2 and E1(LO) modes, respectively, and the peaks at 391, 464, and 585 cm-1 are attributed to A1(TO), E2, and E1(LO) modes,20,21 respectively. The peak at 520 cm-1 may be from the silicon substrate. The appearance of E1(LO) modes associated with the local vibration of impurities suggests that an increase in coating time can cause the formation of defects in ZnO, which will be further discussed in the following by performing PL measurements. Figure 6 shows the room temperature PL spectra for CNTs coated with ZnO for different coating times, in which the ultraviolet (UV) emission peak at about 380 nm corresponds to the near band-edge emission of ZnO crystal.22 The UV emission peak shows a small blue shift with increasing coating time, which can be related to an increase in thickness of ZnO coating layer on the walls of CNTs.23 In addition to the UV emission, we also find a blue emission peak at about 440 nm, originating from the transition between the photoexcited holes and interstitial zinc, and two weak peaks at about 473 and 495 nm, attributed to the singly ionized oxygen vacancies.24,25 From the PL spectra in Figure 6, we believe that the ZnO nanoparticles deposited for 3 and 6 min have a high crystalline quality, since no interstitial zinc and ionized oxygen vacancies are detected. This can be attributed to the small sizes of the coated ZnO particles in these samples and the pure ZnO raw powders in the ceramic tube. Since the oxidation rate due to the absorption of oxygen atoms is very rapid as the depth of the ZnO layer is thin,26 active oxygen atoms at the annealing temperature of 400 °C can be rapidly absorbed on the coated ZnO nanoparticles before coming into contact with carbon atoms of CNTs, which causes a decrease in the number of interstitial zinc atoms and oxygen vacancies, i.e., a perfect oxidization of the particles. For a similar reason, an increase in visible emission intensity with increasing coating time can be ascribed to the thick ZnO layer and the confined amount of residual oxygen atoms in the chamber. The FE experiments of the samples were carried out in a vacuum chamber with a pressure of 2 × 10-7 Pa. Indium tin oxide (ITO) coated glass was used as an anode, and the silicon substrate with CNTs-ZnO nanostructure acting as cathode was

Figure 7. (a) Field emission current density as a function of electric field for (A) pure CNTs and CNTs coated with ZnO for (B) 3 min, (C) 6 min, (D) 10 min, and (E) 20 min, respectively. (b) The magnified curve of part (a) at the low-field region. (c) Fowler-Nordheim (F-N) plots corresponding to A-D, respectively.

mounted on a stainless-steel plate. During our experiments the anode-cathode distance was kept constant by two Teflon spacers with a thickness of 300 µm and the measured emission area was 10 × 10 mm2. Figure 7a displays the current density as a function of electric field strength for (A) pure CNTs and CNTs coated with ZnO for (B) 3 min, (C) 6 min, (D) 10 min, and (E) 20 min, respectively. Here, the results for the samples

17706 J. Phys. Chem. C, Vol. 112, No. 45, 2008 coated with ZnO for 3 and 6 min reflect the Fowler-Nordheim (FE) property in section I, and the samples coated with ZnO for 10 and 20 min exhibit the properties in sections II and III, respectively. The values of turn-on field defined as the electric field required for extracting a current density of 1 µA/cm2 show an apparent decrease with increasing coating time and are much lower than that (2.1 V/µm) for pure CNTs, as shown in Figure 7b. This demonstrates that the FE property for the obtained hybrid CNTs-ZnO nanoparticles has been improved significantly compared to pure CNTs. It is worth noting that, although the turn-on field (0.3 V/µm) of the sample coated with ZnO for 10 min is lower than that (0.6 V/µm) coated for 6 min, its value (6.0 V/µm) of the threshold field required to produce a current density of 1 mA/cm2 is higher than that (4.9 V/µm) for 6 min. Especially, the current density for the sample coated with ZnO for 20 min increases proportionally to the applied field as the applied field is over 0.3 V/µm. This implies that the FE property for the samples with different ZnO coating times in every section has a different emission mechanism. As shown in Figure 7c, the Fowler-Nordheim (F-N) plots for pure CNTs and CNTs coated with ZnO for 3 and 6 min fit well to the linear relationship given by ln(J/E2) ) ln(Aβ2/φ) - Bφ3/2/βE, which indicates that the measured currents are mostly due to vacuum tunneling from CNTs or CNTs-ZnO, where A ) 1.54 × 10-6 A eV V-2, B ) 6.83 × 103 eV-3/2 V µm-1, β is the field enhancement factor, and φ is the work function of emitters. Also, the F-N plot of CNTs coated with ZnO for 10 min obeys the F-N theory in low-field region but shows a significant saturation tendency in the high-field region. From the slopes of F-N plots, the field enhancement factor for CNTs (φCNT ) 5.0 eV) is calculated to be 8.5 × 103, whereas those for CNTs coated with ZnO (φZnO ) 5.3 eV) for 3, 6, and 10 min are estimated to be about 2.7 × 104, 4.3 × 104, and 3.0 × 104, respectively, which are much higher than that for CNTs. In general, it is known that a significant increase in field enhancement factor β is strongly attributed to the geometrical parameters of field emitter, particularly to its small curvature radius and high density on the substrate.27 In our case, the bead-shaped ZnO nanoparticles in the hybrid CNTs-ZnO can act as additional emission sites, because their small sizes and spherical shapes leave many small and sharp tips on CNTs, leading to an enhanced local field at the tip region. From the TEM images and the results in Figure 4b, the number of ZnO nanoparticles per unit area on the walls of CNTs for the samples coated with ZnO for 3, 6, and 10 min can be estimated to be about 2.5 × 1011, 2.8 × 1011, and 3.7 × 1011 cm-2, respectively. Therefore, it can be expected that these well-dispersed ZnO nanoparticles with a small size and spherical shape also act as independent emitters on the walls of CNTs, in addition to tips of CNTs, and hence significantly enhance the field emission of CNTs. However, even though the sample coated with ZnO for 10 min have a greater particle density on the walls of CNTs, its field enhancement factor is smaller than that for the sample coated with ZnO for 6 min. This implies that the efficient FE property in hybrid CNTs-ZnO nanoparticles cannot be obtained by only increasing the number of ZnO particles on the walls of CNTs. In order to reveal the mechanism of the enhanced field emission property, we use an equipotential model of the electrostatic field on the wall of CNT coated with ZnO nanoparticles, as shown in Figure 8a, where r is the average radius of coated ZnO particle (corresponding to section I and II), d is the average thickness of the ZnO layer coated on the wall of the CNT (corresponding to section III), and r is generally smaller than d. CNTs-ZnO is placed to be parallel to the anode,

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Figure 8. (a) Equipotential model of electrostatic field for CNT coated with ZnO nanoparticles, and (b) a schematic band diagram of field emission for CNTs-ZnO nanoparticles.

because the CNTs have mostly bent shape due to the weight of ZnO particles after coating, as shown in Figure 2a-c. Since the local field strength predominates, the emission current is stronger at the top of coated ZnO particle than on the wall of CNTs, the electrons can emit from the top of the ZnO particle to the vacuum, and the emission current will increase with an increase in the number of ZnO particles on the wall of the CNTs. However, the too short interparticle distance with increasing coating time, which can correspond to a flat ZnO coating layer on the wall of CNT, will lead to an opposite effect: an apparent decrease in the local field strength owing to an electrostatic screening effect, resulting in a damage of FE property. The variation of FE in sections II and III can be more concretely analyzed via a band diagram of the emission mechanism for CNTs-ZnO nanoparticles in Figure 8b. Since the band gap of CNTs is a few hundred millielectronvolts at room temperature, which is much narrower than that of ZnO,28 the CNTs-ZnO junction can be considered to be the metal-semiconductor junction. The electron affinity of ZnO is about 4.3 eV,29 and its work function is reported to be nearly independent of the geometrical size of ZnO.30 Under the external field condition, the electrons are injected from the Fermi level of a CNT into the conduction band of ZnO through tunneling because of the Schottky barrier established at the CNT-ZnO junction. These electrons are transported to the ZnO surface and are emitted to the vacuum through subsequent tunneling. Here, the size of the coated ZnO particle and the local field strength at the top of the particle are the main factors in improving the tunneling effect at the CNT-ZnO junction. The electrons can more effectively tunnel at the boundary of the particle with smaller size, and especially, the higher local field near the sharp emitter on

Properties of Hybrid CNTs-ZnO Nanoparticles the flat surface can lower the Schottky barrier height at the CNT-ZnO interface, increasing the tunneling probability. In addition, since the electric field inside the coated ZnO layer is reduced by its dielectric constant, ε, which is lower than in a vacuum, the tunneling effect at the CNT-ZnO heterojunction can be reduced as the distance from the CNT, i.e., the thickness of the ZnO layer, increases. On the other hand, the FE property in sections II and III can be influenced by the formation of the defects such as interstitial zinc and oxygen vacancies in ZnO. Generally, the interstitial zinc and oxygen vacancies can act as shallow donors in ZnO, and the donor energy level near below the conduction band bottom is very shallow (0.03-0.40 eV).24,31,32 These native defects can provide electrons, causing an increase in the electron concentration in ZnO. This leads to a lifting of the Fermi level to the conduction band, and it is possible for more electrons to emit from the ZnO-vacuum interface under high electric field, in addition to the electrons injected from CNTs into the conduction band of ZnO. However, from the fact that the linear relationship of J-E curve occurs at the current density almost equal to the current limiting density Jlim ) enµnE/ε (e, electronic charge; n, electron concentration, µn, electron mobility),33 we believe that a great increase in the electron concentration due to the defects can result in a linear dependence of J-E, as shown in sections II and III. By the way, since these defects totally behave as the obstacles for electron moving in ZnO structure, the overincrease in interstitial zinc and oxygen vacancies with the increase of coating time causes the conspicuous decrease in the conductivity of CNTs-ZnO. Eventually, the FE property in section III is worse than in section I or II, although these defects can provide more emission electrons. Additionally, when CNTs were completely coated with a thin layer of ZnO, not only CNTs but the ZnO layer can offer an independent canal for electron transfer from the silicon substrate. Nevertheless, the FE property in section III is conspicuously worsened, which can be also concerned with the average diameter of CNTs-ZnO. In general, the emission current density is inversely proportional to the square of the emitter radius, and the relatively large diameter of ZnO nanorod or nanowire is considered to be a limitation to further improve the FE property of ZnO nanostructures.3,34 In section I, the ZnO particles on CNTs act as the additional emitters, not effecting the average diameter of CNTs-ZnO particles. But, the average diameter of the CNTs-ZnO layer in section III is about twice that in section I, and the surface morphology of the flat ZnO layer on CNTs causes a reduce in the additional emitting ability due to the decrease in local field strength on the CNTs walls, resulting in the poor FE property in this section. These results and analyses show that an appropriate control of coating time causes a significant change in the geometrical morphology of ZnO nanostructure on the CNTs walls, playing an important role in the FE property of CNTs-ZnO hybrid material. In this work, the most efficient FE property is obtained at the boundary region between sections I and II, corresponding to a coating time of about 6-10 min. 4. Conclusions The hybrid CNTs-ZnO noparticles can be synthesized via a straightforward method at the final growth process of CNTs. The PL spectra for the hybrid CNTs-ZnO reveal that the complete UV emission appears for the samples coated with ZnO for 3 and 6 min, and the structural defects such as

J. Phys. Chem. C, Vol. 112, No. 45, 2008 17707 interstitial zinc and oxygen vacancies significantly increase with prolonging coating time. The well-dispersed ZnO nanoparticles with a small size and spherical shape can act as independent emitters on the walls of CNTs, in addition to the tips of CNTs, which significantly enhance the FE property of hybrid CNTs-ZnO nanoparticles. However, a further increase in the number of ZnO particles causes a decrease in local field strength near the particle and an increase in the structural defects, which leads to a decrease in tunneling probability at the CNTs-ZnO junction and an increase in the resistance for carrier-moving in ZnO structure. This results in conspicuously worsening the FE property. Therefore, we can conclude that the FE property for hybrid CNTs-ZnO nanoparticles significantly depends on both the ZnO particles on the walls of CNTs and the local electric field near the particles, and it is possible that the most efficient FE property can be obtained by optimizing ZnO coating time. Acknowledgment. The authors would like to thank the support from National Natural Science Foundation of China (Grant No. 50525204 and 50832001), the National Key Basic Research and Development Program (Grant No. 2004CB619301), and Project 985sAutomotive Engineering of Jilin University. References and Notes (1) Xu, C. X.; Sun, X. W. Appl. Phys. Lett. 2003, 83, 3806. (2) Kim, K. K.; Lee, S. H.; Yi, W. K.; Kim, J. M.; Choi, J. W.; Park, Y. S.; Jin, J. I. AdV. Mater. 2003, 15, 1618. (3) Zhu, Y. W.; Zhang, H. Z.; Sun, X. C.; Feng, S. Q.; Xu, J.; Zhao, Q.; Xiang, B.; Wang, R. M.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 144. (4) Li, S. Z.; Gan, C. L.; Cai, H.; Yuan, C. L.; Guo, J.; Lee, P. S.; Ma, J. Appl. Phys. Lett. 2007, 90, 263106. (5) Olek, M.; Bu¨sgen, T.; Hilgendorff, M.; Giersig, M. J. Phys. Chem. B 2006, 110, 12901. (6) Kim, H. S.; Sigmund, W. Appl. Phys. Lett. 2002, 81, 2085. (7) Green, J. M.; Dong, L. F.; Gutu, T.; Jiao, J.; Conley, J. F.; Ono, J. Y. J. Appl. Phys. 2006, 99, 094308. (8) Yu, K.; Zhang, Y. S.; Xu, F.; Li, Q.; Zhu, Z. Q.; Wan, Q. Appl. Phys. Lett. 2006, 88, 153123. (9) Yan, X. B.; Tay, B. K.; Miele, P. Carbon 2008, 46, 753. (10) Pak, C. J.; Choi, D. K.; Yoo, J. K.; Yi, G. C.; Lee, C. J. Appl. Phys. Lett. 2007, 90, 083107. (11) Ho, Y. M.; Liu, J. W.; Qi, J. L.; Zheng, W. T. J. Phys. D: Appl. Phys. 2008, 41, 065308. (12) Li, W. Z.; Zhang, H.; Wang, C. Y.; Zhang, Y.; Xu, L. W.; Zhu, K.; Xie, S. S. Appl. Phys. Lett. 1997, 70, 2684. (13) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853. (14) Kim, N. S.; Lee, Y. T.; Park, J. H.; Han, J. B.; Choi, Y. S.; Choi, S. Y.; Choo, J. B.; Lee, G. H. J. Phys. Chem. B 2003, 107, 9249. (15) Arcos, T. D. L.; Garnier, M. G.; Oelhafen, P.; Mathys, D.; Seo, J. W.; Domingo, C.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. Carbon 2004, 42, 187. (16) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761. (17) Ritter, U.; Scharff, P.; Siegmund, C.; Dmytrenko, O. P.; Kulish, N. P.; Prylutskyy, Y. I.; Belyi, N. M.; Gubanov, V. A.; Komarova, L. I.; Lizunova, S. V.; Poroshin, V. G.; Shlapatskaya, V. V.; Bernas, H. Carbon 2006, 44, 2694. (18) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. J. Phys. Chem. B 1999, 103, 8116. (19) Ravindran, S.; Andavan, G. T. S.; Ozkan, C. Nanotechnology 2006, 17, 723. (20) Rajalakshmi, M.; Arora, A. K.; Bendre, B. S.; Mahamuni, S. J. Appl. Phys. 2000, 87, 2445. (21) Bae, S. Y.; Seo, H. W.; Choi, H. C.; Park, J. H.; Park, J. C. J. Phys. Chem. B 2004, 108, 12318. (22) Huang, M. H.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. AdV. Mater. 2001, 13, 113. (23) Wang, X. D.; Ding, Y.; Summers, C. J.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 8773. (24) Lin, B. X.; Fu, Z. X.; Jia, Y. B. Appl. Phys. Lett. 2001, 79, 943. (25) Yang, C. L.; Wang, J. N.; Ge, W. K.; Guo, L.; Yang, S. H.; Shen, D. Z. J. Appl. Phys. 2001, 90, 4489.

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