Controlled Decoration of Single-Walled Carbon Nanotubes with Pd

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Controlled Decoration of Single-Walled Carbon Nanotubes with Pd Nanocubes Aaron D. Franklin,†,‡ Joshua T. Smith,†,‡ Timothy Sands,†,‡,§ Timothy S. Fisher,†,| Kyoung-Shin Choi,⊥ and David B. Janes*,†,‡ Birck Nanotechnology Center, School of Electrical and Computer Engineering, School of Materials Engineering, School of Mechanical Engineering, and Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907 ReceiVed: June 7, 2007; In Final Form: July 18, 2007

Although there have been many reports of nanoparticle-decorated single-walled carbon nanotubes (SWCNTs), the morphology of the resulting nanoparticles has lacked consistency and control. The present work demonstrates a process for decorating SWCNTs with Pd nanoparticles that have a tendency toward a selective and distinct cubic shape. SWCNTs were synthesized from an embedded catalyst in a porous anodic alumina (PAA) template. A single galvanostatic electrodeposition created Pd nanowires that contacted the SWCNTs within the pores and Pd nanoparticles that decorated the SWCNTs above the surface of the PAA. A distinct change in potential was observed as nanoparticles nucleated on the SWCNTs. The effects of current density and deposition time on the morphology of the nanoparticles were studied. Optimal deposition parameters yielded Pd nanocubes with smooth and flat facets. The electrochemical response and resulting nanocubic deposits provide insights into the difference in electrochemistry between metallic and semiconducting SWCNTs that are consistent with a disparity in the electron-transfer kinetics. Obtaining Pd nanoparticles of consistent shape that are electrically addressed by SWCNTs provides an improved structure for a variety of nanoparticle applications.

Introduction Metal nanoparticles have useful characteristics for applications in hydrogen detection and storage,1 chemical sensing,2 biological sensing,3 catalysis,4 and electronics.5 As with most nanostructures, the properties of nanoparticles are tuned by their size, shape, and crystal structure.5,6 Additionally, many applications of nanoparticles require them to be electrically addressedsa task not easily achieved with nanoparticles that are randomly dispersed during synthesis. Although a wide variety of techniques have been demonstrated for producing nanoparticles, from solution-phase chemical processes to templated physical vapor deposition, very few of these processes allow for controlling the size, distribution, and crystal structure of the nanoparticles while also providing a means for electrically contacting them for applications. The report from Quinn et al.7 concerning the electrodeposition of metal nanoparticles on single-walled carbon nanotubes (SWCNTs) initiated new research into this unique decorating technique. By electrochemically forming nanoparticles on the sidewalls of SWCNTs, an inherent electrical contact is created. In this way, the SWCNTs serve as both a template and a contact for the nanoparticles. Since this development, many reports1a,2,3,8 have considered the mechanisms of nanoparticle formations on SWCNTs. Apparently, SWCNTs achieve enhanced nanometerscale mass transport and high current densities in electrochemical experiments because of their high surface-to-volume ratio.8b,9 * Corresponding author. E-mail: [email protected]. † Birck Nanotechnology Center. ‡ School of Electrical and Computer Engineering. § School of Materials Engineering. | School of Mechanical Engineering. ⊥ Department of Chemistry.

Small changes in the diameter of SWCNTs can cause large changes in their electronic density of states (DOS). Regardless of the growth process, a mixture of metallic SWCNTs (mSWCNTs) and semiconducting SWCNTs (s-SWCNTs) will result. Such changes in the DOS have been theoretically shown to result in varied electron-transfer kinetics from tube to tube and are expected to alter the electrochemistry at one tube versus another.9 However, observations have differed regarding the distinction between metallic and semiconducting SWCNTs in electrochemical experiments. Whereas one report noted that at low overpotential, electrodeposition occurred selectively on certain (possibly metallic) SWCNTs,8a another observed little or no difference in the voltammetric responses of metallic and semiconducting SWCNTs.8b Recently, Heller et al.9 suggested that the electrochemical double-layer capacitance is so much larger than the quantum capacitance of an SWCNT that an applied electrochemical potential will be almost entirely used to tune the Fermi energy of the SWCNT as opposed to being dropped over the double layer. In such a scenario, electrodeposition should occur on all available s-SWCNTs provided that the overpotential is large enough to shift the Fermi energy to align with available reactant states. To date, reports involving the electrochemical decoration of SWCNTs have generally shown nanoparticles of random and uncontrolled geometries. Even in recent work in which nearly uniform nanoparticle size was achieved,8a an apparent lack of consistency in morphology from particle to particle exists. One possible factor in the variation of morphology is that reported experiments have involved SWCNTs supported on, and likely bound to, silicon dioxide substrates by their confinement beneath contact pads and/or van der Waals forces. With every atom of an SWCNT exposed at its surface, the supporting substrate can

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Controlled Decoration of SWCNTs with Pd Nanocubes play a critical role in the mechanisms of electrochemical formation of nanoparticles. Recently, we developed a structure that allows for Pd to be electrodeposited on SWCNTs that are tethered at one end by nanowire contacts in a porous anodic alumina (PAA) template.10 In that approach the porous alumina surface provides a unique support for the SWCNTs compared to the typical planar insulating surfaces. SWCNTs are synthesized from a catalyst embedded in the PAA.11 Subsequent pulsed galvanostatic electrodeposition of Pd forms nanowires that nucleate from the pore bottoms and eventually contact the SWCNTs at their innerpore nucleation sites, thereby adding them to the working electrode. The porous alumina surface provides a unique exposure of the SWCNTs to the electrolyte, causing Pd deposits to form annularly with a tendency toward cubic shapes. In the present work, we report a detailed study of the formation of Pd nanocubes on SWCNTs and present deposition conditions that yield well-controlled Pd nanocubic formations. After electrodeposition at high current densities, the resulting Pd nanocubes exhibited rough and inconsistent surfaces, but at sufficiently lowered current density, Pd nanocubes with smooth and flat facets were obtained. Variation of current density and deposition time provides data that advance the understanding of the mechanisms controlling nanocube formation. Furthermore, the electrochemical response and resulting nanocube deposits provide insights into the differences in electrochemistry between metallic and semiconducting SWCNTs. Obtaining shape-selective Pd deposits on SWCNTs provides a structure that can be tuned for specific applications in electronic, magnetic, or optical devices and enables further research into highly functionalized catalysts and face-selective reactions.4,12 Because the nanoparticle shape dictates the arrangement of surface atoms, the ability to form specific shapes, such as cubes, creates the possibility of enhancing the sensing ability and catalytic properties of Pd. In contrast, nanoparticles of less-controlled or uncontrolled shapes, such as rough spheres, have unpredictable arrangements of surface atoms. Experimental Section PAA templates were fabricated as outlined in our previous work with minor variations.10,11 A metal film stack of Ti/Al/ Fe/Al/Fe/Al (100 nm/150 nm/1 nm/2 nm/1 nm/350 nm) was electron-beam-evaporated at a base pressure of 5.0 × 10-7 Torr. Each sample was anodized at 40 V in 0.3 M oxalic acid maintained at 5 °C, yielding pores with an average diameter of 20 nm. In one step, SWCNTs were synthesized from the embedded Fe catalyst (exposed on the inner sidewalls of the vertical PAA pores), and the alumina barrier was penetrated to partially expose the Ti at each pore bottom13 using hydrogensupported microwave plasma chemical vapor deposition (MPCVD). Figure 1a,b schematically illustrates this process. Using a BAS Epsilon Electrochemical System, Pd was galvanostatically electrodeposited on the PAA/SWCNT templates. A three-electrode setup was used with a Pt gauze auxiliary electrode, Ag/AgCl reference electrode, and the sample (initially the underlying Ti at the pore bottoms) as the working electrode. Pulses of 0.5 s were applied between the working and auxiliary electrodes, while the potential was monitored between the working and reference electrodes. Applied current density and total deposition time were varied systematically. The deposition bath was 2 mM PdCl2 in 0.1 M HCl. A schematic of a sample following an ideal Pd electrodeposition is given in Figure 1c. Field-emission scanning electron microscope (FESEM) images were obtained using a Hitachi S-4800 instrument to characterize the size, shape, and density of the Pd nanocubes.

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Figure 1. Tilted cross-sectional schematic of the process showing (a) an evaporated metal film stack on a Si substrate, (b) the structure following anodization and alumina barrier penetration/SWCNT synthesis by microwave plasma CVD, and (c) the result of electrodepositing Pd to form nanowires in the pores and nanocubes on the surface portion of the SWCNTs.

Results and Discussion In our previous report involving the electrodeposition of Pd on the PAA/SWCNT structure, we noted that Pd nanoparticles form on the exposed portions of the SWCNTs once the growth front of the Pd nanowires reaches the base nucleation site of the SWCNTs on the pore walls.10 We also reported that these Pd deposits are often geometrically cubic, with rough and inconsistent surfaces. One task in this study was to optimize the deposition current density to obtain Pd nanocubes with smooth surfaces on the SWCNTs. Accurate determination of current density requires a consistent electrode area from sample to sample. In the present case, a working electrode area of approximately 0.16 cm2 was defined by applying an acidresistant coating to the PAA surface. The macroscopic definition of the area created some variation from sample to sample, but was consistent within a reasonable margin. All electrodepositions were performed by applying 0.5-s pulses of constant current between the auxiliary and working electrodes while monitoring the potential between the reference and working electrodes. Figure 2a shows the working electrode potential transient during select 0.5-s pulses at 2.0 mA/cm2 from a single electrodeposition. The first pulse begins with a sharp decrease in the potential related to the establishment of a sufficient electrochemical double layer at the Ti surface and

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Figure 2. (a) Working electrode potential as a function of deposition time for various pulses at 2.0 mA/cm2 showing the steady-state potential. (b) Plot of the steady-state potential from each pulse as a function of deposition time for three different depositions at 2.0 mA/cm2. Region I corresponds to the nucleation of Pd on Ti and Pd nanowire growth, region II involves the gradual charging of the SWCNTs’ capacitances as they are contacted by nanowires, and region III relates to the deposition of Pd to both the nanowires and nanocubes.

Figure 3. (a-c) FESEM images of Pd nanocubes on SWCNTs from samples deposited at 2.0 mA/cm2, demonstrating the increase in nanocube size with increasing deposition time. (d-f) Cross-sectional FESEM images showing increasing Pd nanowire length with increasing deposition time. The dotted line indicates the approximate location of the Fe catalyst layer from which the SWCNTs grow. Electrodeposition data from these samples are plotted in Figure 2b.

initial Pd nucleation, which requires a more negative potential because of the presence of native TiOx. Note that, for cathodic electrodeposition, a more positive potential corresponds to a lower deposition rate. For a constant current deposition, the potential required to maintain the current will change over time as a result of changes in contact barriers, double-layer effects, and effective electrode area. In this case, a more positive potential corresponds to a situation in which maintaining the current is easier. As expected, each pulse beyond the first begins at a relatively high potential and then proceeds to a Cottrelian decay, indicating a linearly diffusive deposition. A rest time of approximately 1 s between pulses allows the ions to redistribute to a homogeneous concentration. Cyclic voltammograms (CVs) are typically very useful in investigating deposition mechanisms; however, CVs could not

provide critical data to understand our system involving the gradual change of composition and nature of the working electrode (e.g., Ti, Pd, and SWCNTs) over time. A highly informative metric for analyzing the time-dependent changes at the working electrode during electrodeposition is the steadystate potential (or the final potential) from each pulse as a function of deposition time. Plots of the steady-state potential versus deposition time for several 2.0 mA/cm2 depositions are shown in Figure 2b. The initial steady-state potential is quite low and quickly becomes more positive as Pd nucleates on the Ti at each pore bottom (region I). At approximately 30 s, the growth front of the nanowires begins to reach the embedded Fe layer from which the SWCNTs protrude (150 nm from the Ti), which can be seen in Figure 3e. Over the next 50 s, the potential decreases by 20 mV (region II), which is believed to

Controlled Decoration of SWCNTs with Pd Nanocubes correspond to a progressive contacting of the SWCNTs by the Pd nanowires and electrical charging of their quantum and double-layer capacitances for the nucleation of Pd.9 This gradual contact with the SWCNTs is consistent with the observed variation in Pd nanowire height within the pores.13 The region II decrease in potential was not observed for the pulsed growth of Pd nanowires in PAA when SWCNTs were absent, as reported in our previous work.10 Around 80 s, the potential upturns and increases by more than 50 mV over the next 50 s before tapering off again (region III). This large increase is due to the increase in the working electrode area from the addition of the SWCNTs. The SWCNTs can now also reduce Pd2+, generating additional cathodic currents, which results in a more positive potential to maintain the same level of total current. Note that the size of the Pd nanocubes nearly doubles and the nanowire height increases between the demarcation of regions II and III (75-s deposition, Figure 3b and e) and the final condition in region III (200-s deposition, Figure 3c and f). An interesting observation is the presence of Pd nanocubes on the SWCNTs before the Pd nanowires have contacted the SWCNTs, as shown in Figure 3a and d. As previously reported by Choi et al.,14 certain noble metals in salt solutions can spontaneously be reduced at SWCNT sidewalls without the presence of reducing agents or an applied potential. In order to verify that Pd was, in fact, spontaneously reduced, several samples were immersed in the 2 mM PdCl2 solution, and Pd nanocubes were subsequently observed on a portion of the SWCNTs (Figure 4b). The density and size of the nanocubes saturated within 30 s at a cube size of approximately 20 nm. Importantly, very few of the SWCNTs hosted spontaneous Pd nanocube formations, and the SWCNTs that did were the brightest when viewed in the FESEM. We hypothesize that the brighter SWCNTs on the insulating PAA surface are metallic whereas those that are dark are semiconducting. Figure 4c illustrates in an energy level diagram the difference between metallic and semiconducting SWCNTs in the spontaneous reduction process. With the SWCNT work function in the range of 4.8-5.2 eV,14-17 the SWCNT Fermi level would be located approximately 0.6 V from the standard hydrogen electrode (SHE). Therefore, m-SWCNTs would be at a more negative potential (higher energy) than the reduction potential of Pd2+ (the standard reduction potential of Pd2+ is 0.951 V), which would allow reduction of the Pd2+ ions to Pd without an applied potential (see Figure 4c). However, for s-SWCNTs, because of their energy band gap (assumed to be ∼0.8 eV in the present diagram with the Fermi level at mid-gap),17 electrons in the valence band are at a more positive potential (lower energy) than the Pd2+ reduction potential and therefore do not spontaneously transfer to support the Pd2+/Pd reduction. In scanning electron microscopy, conductors appear bright compared to insulators because of the ease with which secondary electrons can be emitted from them. When imaging SWCNTs on PAA, prior to any postsynthesis processing, a portion of the SWCNTs appear bright, whereas the rest are darker, as shown in Figure 4a. This unusual observation of distinct contrast differences among SWCNTs on an insulating surface might be specific to the present PAA surface. Brintlinger et al. reported that SWCNTs exhibit contrast differences in FESEM imaging based on their electrical connection to a local metal pad.18 However, in the present study, the SWCNTs exhibit such differences while supported solely by the porous alumina insulator, prior to any postsynthesis processing. In the experiments performed by Brintlinger et al., as with most other reports of SWCNTs imaged using FESEM, the SWCNTs were supported on a planar SiO2 substrate. Furthermore, Brintlinger et

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Figure 4. Cross-sectional FESEM images showing bright and dark SWCNTs on the PAA surface (a) before immersion in the Pd solution and (b) after 2 min of submersion with no applied bias (inset shows magnified view of spontaneously formed Pd nanocube). (c) Composite plot showing the potential of Pd2+ and schematic electronic band diagrams of metallic and semiconducting SWCNTs with respect to both SHE and the vacuum level.

al. observed a decrease in contrast and overall visibility of the SWCNTs for accelerating voltages greater than 2 kV and/or low scan speeds; in comparison, we used a 5 kV accelerating voltage and observed an increase in the contrast with increasing voltage as well as with lower scan speeds. Notably, lowmagnification FESEM images show that approximately onethird of the SWCNTs on the PAA are bright (see Figure 4a), consistent with the statistical yield of one-third metallic SWCNTs from CVD synthesis. When the SWCNT density in the PAA is increased, the SWCNTs begin to bundle, and eventually, all SWCNTs on the PAA surface appear bright, because each is a part of a bundle that has at least one m-SWCNT. In a self-consistent fashion, the spontaneous reduction of Pd solely at the bright SWCNTs supports the hypothesis that these SWCNTs are metallic. A variation of the m-SWCNT work function might also play a role because not all of the bright SWCNTs are spontaneously decorated. Furthermore, spontaneous reduction likely occurs at defect sites on the m-SWCNTs, so that the bright tubes without decoration

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Figure 5. (a) Steady-state potential from each current pulse as a function of deposition time at three different current densities. (b) FESEM image of Pd nanocubes on SWCNTs from a 2.0 mA/cm2 deposition for 200 s. Circled area shows a noncubic deposit formed on an SWCNT that likely remained bound to the PAA surface during electrodeposition.

Figure 6. (a-c) FESEM images of Pd nanocubes from electrodepositions for 200 s using different current densities (see Figure 5a), showing an improvement of nanocube smoothness with decreasing current density. (d-f) Cross-sectional FESEM images showing the representative Pd nanowire length for each deposition current density. The dotted line indicates the approximate location of the Fe catalyst layer from which the SWCNTs grow.

could also be defect-free, as previously observed for the spontaneous reduction of Pt on graphite.19 The optimal current density for forming Pd nanocubes is large enough to enable Pd nanowire nucleation and growth from each pore bottom yet small enough to obtain nanocubes without rough surfaces. Electrodeposition at current densities below 1.0 mA/ cm2 greatly reduces the nanowire density; current densities above 3.0 mA/cm2 produce nanocubes with excessively rough and spiky surfaces, as well as increased nucleation on many SWCNTs. Therefore, the current density was varied between 1.0 and 3.0 mA/cm2, and the plots of the steady-state potential as a function of deposition time are shown in Figure 5a. Note that the decrease in potential (∼20 mV) associated with the charging of the SWCNTs’ capacitances is independent of current

density, which is attributed to the consistent density of SWCNTs on each sample. However, whereas the magnitude of the potential shift is consistent between the depositions, the rate of change depends strongly on the current density. At low current density (1.0 mA/cm2), the rate of change in potential is low, corresponding to a low growth rate of the nanowires and, thus, the gradual contact of the SWCNTs by Pd nanowires. In contrast, deposition at 3.0 mA/cm2 has a very high rate of change as the nanowires quickly contact the available SWCNTs. After adding the SWCNTs to the working electrode, a deposition pulse causes Pd2+ to reduce at their surfaces. Figures 3a-c and 6a-c show nanocubes on SWCNTs from various depositions. All observed Pd nanocubes annularly encompass the SWCNTs as opposed to forming solely on their top portion

Controlled Decoration of SWCNTs with Pd Nanocubes and further pinning them to the surface. We attribute this annular deposition to the weak binding of the SWCNTs to the PAA surface, as has recently been reported.20 Although there have been many reports of Pd-nanoparticledecorated SWCNTs, the present work is the first to observe Pd deposits of a selective and distinct shape. A combination of mechanisms could be contributing to the formation of Pd crystals with well-defined cubic shapes. First, deriving from previous reports of metal nanoparticle synthesis, equilibrium Pd nucleates in truncated octahedral shapes with {100} and {111} facets.21,22 Under our synthesis conditions, it appears that Cl- ions in the deposition solution serve as additives that selectively adsorb on the {100} planes, enhancing the stability of these planes. This preferential adsorption reduces the crystal growth rate along the 〈100〉 direction and makes {100} planes gain in area while other planes, with relatively higher growth rates along their normal directions, grow out of existence in the final morphology, resulting in cubic shapes. The effect of Cl- ions acting as a habit modifier has previously been observed in the electrochemical synthesis of Cu2O crystals.23 Second, the SWCNTs here are supported on PAA instead of a planar insulator (such as SiO2), and the lack of intimate contact between the SWCNTs and the PAA surface would allow the nanocubes to drift freely in the electrolyte, being tethered by SWCNTs that are bound only at one end by their nucleation sites within the pores.20 Therefore, the SWCNTs can be fully exposed to the solution for Pd deposition. Combined with the relatively low nucleation density of Pd on the SWCNTs, the fully exposed SWCNT surface allows each Pd nucleus to build a spherical diffusion layer around it, which promotes isotropic and uniform growth of cubic Pd crystals. A final factor that might contribute to the tendency toward cubic shapes is the lack of postsynthesis processing of our SWCNTs. Most prior SWCNTs decorated with metal nanoparticles used either SWCNTs that had been dispersed and/or purified in various solvents or CVD-grown SWCNTS that underwent lithographic processes in order to form metal contacts prior to electrodeposition. In contrast, our SWCNTs are transferred directly from the PECVD synthesis chamber to the electrolyte for electrodeposition, allowing the SWCNTs to maintain, as nearly as possible, their as-grown characteristics. After initial Pd nucleation on the SWCNTs (whether it occurs by spontaneous reduction or electrodeposition), further pulses cause reduction at the Pd nanocube surface at a mass-transfer rate proportional to the applied current density. Therefore, it is expected that higher current densities will yield nanoparticles with less stable morphologies having rough and patchy surfaces, as is evident with the nanocube in Figure 6c from a 3.0 mA/ cm2 deposition. As the current density is lowered, Pd is deposited at a lower rate, and the nanocube surface becomes smoother. We observed that a current density of 1.0 mA/cm2 is capable of both nucleating a high density of nanowires and forming smooth-surface nanocubes on the SWCNTs (Figure 6a). The increasing size of the nanocubes in Figure 6a-c is due to the increasing deposition current density for the same deposition time; a longer 1.0 mA/cm2 deposition (not shown) yielded larger nanocubes that still maintained the smooth surfaces. Within the resolution limits of the FESEM, the nanocubes appeared to be approximately uniform in size, although there is likely a small variation within 10 nm. We also emphasize that not all observed Pd nanoparticles are nanocubes; at higher current densities, various unstable shapes are formed. While lower current density depositions increase the percentage of Pd nanocubes, the disordered Pd nanoparticles are not completely eliminated. Many

J. Phys. Chem. C, Vol. 111, No. 37, 2007 13761 of these unstable formations likely derive from SWCNTs that are too strongly bound to the PAA surface and thus are not able to float in the solution to fully expose their surface area (see Figure 5b). For the 1.0 mA/cm2 depositions, the average fraction of Pd nanoparticles that are nanocubic is 70%, as determined by counting the nanoparticle formations in 500 µm2 areas from several low-magnification FESEM images. We believe that this percentage can be further increased by decreasing the current density at the onset of Pd nanoparticle formation on the SWCNTs. Conclusions We have presented a process for fabricating intrinsically contacted Pd nanocubes by controlling the electrodeposition of Pd on SWCNTs templated in PAA. Spontaneous reduction of Pd was observed on select SWCNTs under no applied bias, and these SWCNTs were deduced to be metallic by their bright appearance in electron microscope images compared to other SWCNTs. The selective spontaneous decoration of m-SWCNTs was further interpreted via energy level diagrams, which highlight the role of the band gap in hindering the spontaneous reduction process for s-SWCNTs. Various mechanisms for Pd nucleation on the SWCNTs were examined by measuring the steady-state electrode potential as a function of the deposition time for a variety of deposition conditions. Pd nanocubes with smooth and flat surfaces were obtained while also maintaining a high Pd nanowire density at a current density of 1.0 mA/cm2. In general, this process for obtaining Pd nanocubes of controlled size and consistent shape that are electrically contacted by SWCNTs provides an improved structure for a variety of nanoparticle applications from hydrogen detection and storage to chemical and biological sensing. Acknowledgment. The authors gratefully acknowledge Dr. Matthew R. Maschmann and Professor Michael S. Fuhrer for insightful discussions. We also acknowledge support from the NASA-Purdue Institute for Nanoelectronics and Computing and the Birck Nanotechnology Center. A.D.F. recognizes support from a National Science Foundation Graduate Research Fellowship. References and Notes (1) (a) Mubeen, S.; Zhang, T.; Yoo, B.; Deshusses, M. A.; Myung, N. V. J. Phys. Chem. C 2007, 111, 6321-6327. (b) Sun, Y.; Wang, H. H. Appl. Phys. Lett. 2007, 90, 213107-213109. (c) Kong, J.; Chapline, M. G.; Dai, H. AdV. Mater. 2001, 13, 1384-1386. (d) Mu, S.-C.; Tang, H.-L.; Qian, S.-H.; Pan, M.; Yuan, R.-Z. Carbon 2005, 44, 762-767. (2) Forzani, E. S.; Li, X.; Zhang, P.; Tao, N.; Zhang, R.; Amlani, I.; Tsui, R.; Nagahara, L. A. Small 2006, 2, 1283-1291. (3) Lim, S. H.; Wei, J.; Lin, J.; Li, Q.; KuaYou, J. Biosens. Bioelectron. 2005, 20, 2341-2346. (4) (a) Norimatsu, F. Y.; Mizokoshi, Y.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Chem. Lett. 2006, 35, 276-277. (b) Stacchiola, D.; Calaza, F.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2004, 126, 15384-15385. (c) Hunka, D. E.; Herman, D. C.; Lormand, K. D.; Jaramillo, D. M.; Land, D. P. Langmuir 2005, 21, 8333. (5) Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Chem. Eur. J. 2005, 11, 454-463. (6) Siegfried, M. J.; Choi, K.-S. AdV. Mater. 2004, 16, 1743-1746. (7) Quinn, B. M.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2005, 127, 6146-6147. (8) (a) Day, T. M.; Unwin, P. R.; Macpherson, J. V. Nano Lett. 2007, 7, 51-57. (b) Heller, I.; Kong, J.; Heering, H. A.; Williams, K. A.; Lemay, S. G.; Dekker, C. Nano Lett. 2005, 5, 137-142. (c) Day, T. M.; Unwin, P. R.; Wilson, N. R.; Macpherson, J. V. J. Am. Chem. Soc. 2005, 127, 1063910647. (d) Fan, Y.; Goldsmith, B. R.; Collins, P. G. Nat. Mater. 2005, 4, 906-911. (e) Khan, M.; Sood, A. K.; Mohanty, S. K.; Gupta, P. K.; Arabale, G. V.; Vijaymohanan, K.; Rao, C. N. R. Opt. Express 2006, 14, 424-429. (f) Guo, D.-J.; Li, H.-L. J. Colloid Interface Sci. 2005, 286, 274-279. (g) Quinn, B. M.; Lemay, S. G. AdV. Mater. 2006, 18, 855-859. (h) Kim,

13762 J. Phys. Chem. C, Vol. 111, No. 37, 2007 B.-K.; Park, N.; Na, P. S.; So, H.-M.; Kim, J.-J.; Kim, H.; Kong, K.-J.; Chang, H.; Ryu, B.-H.; Choi, Y.; Lee, J.-O. Nanotechnology 2006, 17, 496500. (9) Heller, I.; Kong, J.; Williams, K. A.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2006, 128, 7353-7359. (10) Maschmann, M. R.; Franklin, A. D.; Scott, A.; Janes, D. B.; Sands, T. D.; Fisher, T. S. Nano Lett. 2006, 12, 2712-2717. (11) Maschmann, M. R.; Franklin, A. D.; Amama, P. B.; Zakharov, D. N.; Stach, E. A.; Sands, T. D.; Fisher, T. S. Nanotechnology 2006, 17, 3925-3929. (12) Silly, F.; Castell, M. R. Phys. ReV. Lett. 2005, 94, 046103. (13) Franklin, A. D.; Maschmann, M. R.; DaSilva, M.; Janes, D. B.; Fisher, T. S.; Sands, T. D. J. Vac. Sci. Technol. B 2007, 25, 343-347. (14) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. Am. Chem. Soc. 2005, 124, 9058-9059. (15) Lovall, D.; Buss, M.; Graugnard, E.; Andres, R. P.; Reifenberger, R. Phys. ReV. B 2000, 61, 5683-5691.

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