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FEATURE ARTICLE Electronic and Optical Properties of Chemically Modified Metal Nanoparticles and Molecularly Bridged Nanoparticle Arrays Wyatt P. McConnell, James P. Novak, Louis C. Brousseau III, Ryan R. Fuierer, Robert C. Tenent, and Daniel L. Feldheim* Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695 ReceiVed: March 9, 2000
Nanometer-sized metal particles (e.g., gold and silver) are certain to be important fundamental building blocks of future nanoscale electronic and optical devices. However, there are numerous challenges and questions which must be addressed before nanoparticle technologies can be implemented successfully. For example, basic capping ligand chemistrysnanoparticle electronic function relationships must be addressed in greater detail. New methods for assembling nanoparticles together into higher-order arrays with more complex electronic functions are also required. This review highlights our recent progress toward characterizing electron transport in gold nanoparticles as a function of capping ligand charge state. These studies have shown that single electron tunneling energies can be manipulated predictably via pH-induced charge changes of surfacebound thiol capping ligands. We also show that rigid phenylacetylene molecules are useful bridges for assembling gold and silver nanoparticles into arrays of two, three, and four particles with psuedo D∞h, D3h, and Td symmetries. These nanoparticle “molecules” interact electromagnetically in a manner qualitatively consistent with dipole coupling models.
Introduction Metal particles comprise a fundamentally interesting class of matter in part because of an apparent dichotomy which exists between their sizes and many of their physical and chemical properties. For example, gold and silver particles may be synthesized in diameters which span from the macroscopic (microns) down to the molecular scale (0.8 nm). Across almost this entire size regime, however, their optical and electronic behaviors are described with relatively simple classical equations, rather than the quantum mechanical concepts required to understand molecular entities. Thus, classical free electron theory is employed together with optical constants for bulk gold to model successfully the intense visible extinctions observed for gold nanoparticles.1 Classical capacitor charging expressions, rather than quantum mechanical HOMO-LUMO energies, account well for the unusually large number of charging events observed for gold particles compared to typical molecules (ca. 8 or more charging events per volt depending on particle size).2 Moreover, the optical and electronic properties of metal particles can be tuned considerably simply by adjusting the size, shape, or extent of aggregation of the particle; indeed λmax can shift by hundreds of nanometers and particle charging energies altered by hundreds of millivolts via particle size and shape. Recent experimental studies of metal particles have demonstrated yet another aspect of the sizesphysical property dichotomy: Since the surface chemistry of nanoscopic metal particles is similar to that of bulk metal surfaces, surface chemical modification is straightforward. In fact, with only slight * Corresponding author. E-mail:
[email protected].
deviation, it appears that the same principles for modifying macroscopic planar gold surfaces with polymers,3 alkanethiols (e.g., self-assembled monolayers),4 enzymes,5 etc. also apply to nanometer-sized gold particles. This enables the synthesis of particles that are soluble in a variety of media, or possess specific affinities for certain analyte species in solution.6 The properties of metal particles described abovestheir free electron behaviors and straightforward surface modifications are of potential utility in chemical sensing, linear and nonlinear optics, and in a variety of nanoscale electronic device schemes. Despite steady progress toward these applications in recent years, several key questions and challenges must be addressed before metal nanoparticles can become essential building blocks for advanced optical and electronic devices. These are in part the following: (I) Establishing electrical contacts to individual nanoparticles reliably and routinely; (II) assembling nanoparticles into symmetrically and spatially well-defined arrays and characterizing electromagnetic interactions between particles in the array; and (III) understanding surface chemistrysnanoparticle optical and electronic property relationships. The latter goal is important because surface bound molecules will undoubtedly be used to solve the two former challenges; that is, certain molecules (e.g., alkyldithiols, DNA) will likely be used to make interparticle or particle-electrode contacts. Moreover, surfacebound ligands are known to effect particle optical and electronic properties and therefore could potentially be employed as reversible device switches or recognition agents for nanoscale chemical sensors.7 This paper reviews our recent investigations of metal particle electronic and optical behaviors. We focus first on the electronic
10.1021/jp000926t CCC: $19.00 © 2000 American Chemical Society Published on Web 08/25/2000
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Figure 1. Illustration of an STM tip-metal nanoparticle-alkanethiol coated gold substrate double tunnel junction and corresponding equivalent circuit.
properties of individual gold particles, emphasizing the effects of surface chemistry on particle electronic properties, while highlighting potential applications of nanoscale electronic behaviors. New methods for coupling gold and silver particles into “nanoparticle molecules” will then be described, and coupled optical phenomena presented. Our major findings are (i) surface chemistry can be used to reversibly modulate single electron tunneling energies in gold nanoparticles, and (ii) electromagnetic coupling between silver and gold nanoparticles depends on symmetry and interparticle distance in a manner qualitatively consistent with recent dipole approximations.
Figure 2. Current vs voltage for a single galvinol-coated gold particle acquired in aqueous solution at pH 5. Inset shows an STM image of the sample. Tip was coated with Apiezon wax, gold substrate was insulated with hexanethiol. (See ref 10 for details.)
Electron Transport in Metal Nanoparticles Single Particle Electronic Properties. The realization that chemical synthesis is an ideal means of obtaining large numbers of potential nanoscale device components prompted intense research aimed at elucidating the electronic properties of individual metal particles. Much of this work has focused on gold and silver particles because synthetic methods for producing these particles with virtually any diameter are well developed.8 In most single particle measurements conducted to date, ligand-capped nanoparticles are cast onto metallic substrates and an STM tip is positioned directly over a single particle to form a metal (tip)/insulator (ligand)/nanoscopic metal (particle)/ insulator (ligand)/metal (substrate) double tunnel junction (Figure 1).9 Because gold particles with diameters as small as ca. 2 nm behave as free electron metals (e.g., contain a continuum of electronic states), this system can be treated as a simple series RC circuit. Staircase-shaped I-V curves are then expected with charging potentials of
V ) (Qo - 1/2)e/C2 + Voffset
(1)
and current steps of
I ) e/2R2CT
(2)
where Qo is the charge on the particle, C2 and R2 are the capacitance and resistance, respectively, of the most resistive junction (typically the particle-substrate junction), CT is the total particle capacitance and Voffset accounts for any initial misalignment in tip-particle or particle-substrate Fermi levels and any charged impurities residing near the particle.7 Equations 1 and 2 can be used to estimate particle capacitance and junction resistances experimentally as a function of particle surface chemistry. In theory, one would expect that surfacebound ligands could alter the current staircase via the capacitance or voltage offset terms in eq 1. We have tested this prediction by synthesizing particles capped with two pHresponsive alkanethiols, galvinol-gold10 and mercaptohexylamino-gold.10b The particles were cast on hexanethiol-coated gold substrates and I-V curves recorded on individual particles
Figure 3. Current vs voltage curves for galvinol-coated gold particles acquired in aqueous solution at the pH indicated (curves have been offset for clarity). The vertical line marks the position of the second voltage plateau at pH 5.
with an STM tip in aqueous solution under controlled pH and ionic strength conditions. I-V curves recorded at pH 5 for galvinol-gold across a wide bias range show several well-defined current steps indicative of single electron tunneling (Figure 2). These data are typical of the I-V curves acquired under all pH conditions, although peak-shaped steps were frequently observed at higher pH during negative-bias sweeps (vide infra). The effects of increasing the solution pH on nanoparticle I-V behavior are shown in Figure 3. First, the entire staircase shifted to positive bias upon converting the neutral galvinol monolayer to the anionic galvinoxide species; i.e., each successive voltage plateau occurred at a more positive bias potential. The second effect observed upon monolayer charging was a decrease in the average potential plateau width. Similar behavior was observed in I-V curves recorded for mercaptohexylamine-modified gold particles, although peakshaped steps were now most often observed at low pH. Converting the neutral amine to the ammonium cation also caused shifts in the current staircase (Figure 4). In contrast to galvinol-gold, however, the voltage plateau widths decreased upon lowering the pH. Finally, we note that the reproducibility in staircase position at each pH was within 15 mV for fifteen different particles for galvinol-gold and 20 different particles for amino-gold. The contrasting behavior in voltage plateau widths vs pH for galvinol-gold and amino-gold particles provides evidence against
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Figure 4. Current vs voltage curves for mercaptohexylamine-coated gold particles acquired in aqueous solution at the pH indicated.
Figure 5. Current vs voltage curves for hexanethiol-coated gold nanoparticles acquired in aqueous solution at pH 5 and 12 (curves have been offset for clarity). The vertical line marks the position of the second voltage plateau at pH 5.
TABLE 1: Single Particle Capacitance as a Function of pH nanoparticle Gal-Au amino-Au C8-Au
medium H2O (pH 5) H2O (pH 8) H2O (pH 12) H2O (pH 5) H2O (pH 8) H2O (pH 12) H2O (pH 5) H2O (pH 12)
diameter (nm) 8 5
C (aF) 1.2 ( 0.12 1.8 ( 0.14 2.4 ( 0.20 0.70 ( 0.08 0.58 ( 0.011 0.54 ( 0.05 2.7 ( 0.30 2.5 ( 0.10
tip or substrate effects which could alter the current staircase independent of capping ligand chemistry (e.g., shifts in pzc, ionic strength effects). Further evidence against a tip or substrate effect was obtained through two control experiments: (i) particles containing pH-insensitive ligands (hexanethiol) were synthesized and I-V curves recorded vs pH (Figure 5), and (ii) I-V curves for galvinol-gold were recorded vs ionic strength, holding pH constant. In both control experiments relatively little change is observed in step potential (Table 1) or average step width. The observations above can be rationalized by considering the voltage offset term in eq 1, and the well-documented effects of monolayer charging on the capacitance of metal electrodes.9c Qualitatively, excess charge near the metal island of a single electron transistor causes a pseudo-capacitance which shifts the entire staircase to higher or lower bias depending on the sign of the charge. Typically, these charges are unwanted impurities in the substrate.11 In this work, the “impurity” charge has been designed in as a functionalized organic capping ligand. The decreases in voltage plateau widths upon converting the neutral galvinol and amine ligands to charged galvinoxide and
Figure 6. Current vs voltage curves for a gold particle-modified Pt/Ir tip at decreasing tipssubstrate distances (particle diameter was 5 nm).
ammonium ions quantitatively indicate an increase in particle capacitance (Table 1). For galvinol-coated gold particles, the increase in capacitance results from an increase in solution pH and corresponding conversion of the neutral galvinol to the anionic galvinoxide species. In amino-gold, decreasing solution pH charges the amine, causing the increase in particle capacitance. These observations are consistent with capacitance changes reported previously for similar monolayers assembled on macroscopic planar gold electrodes.12 Finally, the observation of a peak at a bias potential where a voltage plateau is expected is a common feature of I-V curves recorded for single molecules and metal and semiconductor nanoparticles. This so-called negative differential resistance (NDR) is often attributed to resonant tunneling through quantum states in the molecule or particle, but can in fact occur for many reasons including tunneling into surface traps or interaction with surface plasmons.9 While we typically observe NDR when the particle capping ligand is charged (at high pH for galvinoxide and low pH for ammonium), at this time we are unsure of the origin of NDR in these systems. We conclude from these data that single electron tunneling energies in gold nanoparticles can be manipulated by chemical receptors (e.g., ω-substituted thiols) bound to the nanoparticle. Conversely, if environmental sensitivity is not a preferred property, alkanethiols appear to prevent an electronic response to certain environmental changes (e.g., ionic strength, pH). Electron Transport in “Double Dot” Systems. In addition to probing electron transport in gold nanoparticles bound to metal substrates, we have found that nanoparticles may be attached directly to a Pt/Ir STM tip via hexanedithiol linkers. I-V curves recorded at a single location over a gold surface vs tip height confirm that SET behavior is observed with the particle-modified tip (Figure 6). As predicted by SET theory,7 a current staircase evolves with increasing current step heights as the tip approaches the substrate. The scanning single electron tunneling tip may then be rastered over the substrate while collecting staircase-shaped I-V curves (Figure 7). A scanning SET tip provides the ability to probe the sensitivity of single electron tunneling to molecules or other nanostructures bound to a substrate. For example, the particlemodified tip may be placed over a gold particle bound to the surface to explore electron transport in coupled metal nanostructures. As the tip approaches the particle, single electron tunneling through the coupled particle system generates a very sharp Coulomb blockade response (Figure 8, curves 1-3). With
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Figure 7. Current vs voltage curves for a gold particle-modified Pt/Ir tip at three different locations on a polycrystalline bare gold surface. Approximately 10% of I-V acquired with particle-modified tips contain current steps on the polycrystalline surface.
Figure 8. Current vs voltage curves for a gold particle-modified Pt/Ir tip at decreasing tip heights over a surface-bound gold particle (curve 4 is the closest tip-particle distance). Inset at lower right shows an STM image acquired with the tip. The line section revealed particle heights of ca. 5 nm.
decreasing particle-particle distance, however, electronic coupling between the two particles becomes too large to observe SET behavior at room temperature. The nonlinear I-V behavior thus begins to appear ohmic again (curve 4). The single particle electron transport studies described above are helping to establish basic capping ligand chemistrys nanoparticle electronic function relationships. However, many nanoscale computing schemes will ultimately require the assembly of nanoparticles together into spatially and symmetrically well-defined arrays (e.g., quantum cellular automata).7 Certain optical applications of metal particles such as surfaceenhanced Raman spectroscopy13 and second harmonic generation14 also benefit greatly from nonspherical particle geometries. Below we illustrate our initial attempts at assembling silver and gold particles into arrays with pseudo- D∞h, D3h, and Td symmetries. Assembly of Molecularly Bridged Metal Nanoparticle Arrays Our strategy for assembling metal particle into symmetrically and spatially well-defined arrays has been to synthesize rigid thiol-functionalized phenylacetylenes for use as molecular
Figure 9. Transmission electron micrographs of (A) 30 nm diameter silver nanoparticle dimer bridged by phenylacetylene structure Ic in Scheme 1, (B) and (C) 8 nm diameter gold nanoparticle trimers bridged by phenylacetylene structures IIa and IIb, respectively, and (D) 8 nm diameter gold nanoparticles bridged by phenylacetylene structure III in Scheme 1.
bridges (Scheme 1).15 Slow addition (e.g., over 30 min) of a slight excess of particles with the linkers shown in Scheme 1 results in the bridged nanoparticle structures shown in Figure 9. The structures shown in Figure 9 are typical of those found over the entire TEM grid. Counting ca. 100 structures for each sample revealed yields of 50%, 30% and 10% for the dimers, trimers and tetramers, respectively. The remainder of the sample in each case consisted predominantly of uncoupled particles. These images suggest that the phenylacetylene molecule can dictate the symmetry of the resulting nanoparticle assembly. We are currently attempting to purify each sample by size exclusion chromatography, gel electrophoresis, and centrifugation. Electromagnetic coupling between gold and silver particles as a function of array symmetry and particle spacing is being investigated by visible spectroscopy and hyper-Rayleigh scattering (HRS) spectroscopy. Here we focus on the visible extinctions of coupled gold and silver particle dimers.15c Two gold particles linked by 5-unit phenylacetylene spacers show only modest redshifts and intensity increases of the dipolar plasmon band compared to the single-particle plasmon band (Figure 10). In contrast, visible extinctions of bridged 30 nm diameter silver particle dimers are markedly different from their
Feature Article
Figure 10. UV-visible spectra of 8 nm diameter gold nanoparticles (lower spectrum) and nanoparticles following the addition of phenylacetylene Ia in Scheme 1 (upper spectrum). Inset shows the time evolution of the absorbance at 535 nm.
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Figure 12. Silver particle dimer spectra calculated by Schatz et al. (adapted from ref 16a). Particle diameter was 60 nm. Key: O, monomers; 0, 30 nm separation; ), 4 nm separation; 4, 2 nm separation.
experimental and calculated spectra are likely due to particle size dispersity, the presence of excess monomers, and orientational averaging in solution not accounted for theoretically. Conclusions and Future Studies
Figure 11. UV-visible spectra for (from bottom to top): 30 nm diameter silver particles; dimers linked by structure Ic; dimers linked by structure Ib; dimers linked by structure Ia. Spectra were offset along absorbance axis for clarity.
corresponding monomeric particles (Figure 11). Solutions of unlinked silver particles contain a single extinction at 420 nm corresponding to the well-known silver plasmon resonance band. Addition of a 9-unit phenylacetylene bridge caused a slight redshift and absorbance increase of the silver plasmon band. At separation distances corresponding to 7 phenylacetylene units, an extinction at 450 nm was observed with a more well-defined shoulder at 420 nm. Both bands were more intense than the single-particle plasmon band. Further reduction in separation to 3 phenylacetylenes caused no further shift in the low-energy extinction; however, the high-energy shoulder blue shifted to 370 nm and both bands grew in intensity. In addition, a relatively weak extinction was observed at ca. 600 nm. The visible spectra described above for gold and silver particle dimers are consistent with recent calculations using dipole approximations.16 For example, Figure 12 presents spectra calculated for 60 nm diameter particle dimers as a function of particle separation distance.7 Note, despite the size difference, the behaviors are qualitatively in aggreement; that is, both data sets reveal a red-shift of the single-particle plasmon band, an increase in overall extinction efficiency, growth of a high-energy shoulder (ca. 370 nm), and subsequent appearance of a band at 600 nm as interparticle distance decreases. Disparities between
The sensitivity of a single electron transistor to individual charged impurities has been known for over a decade.7,9 However, because charged impurities cause drifts in single electron “turn-on” voltages, this sensitivity is often cited as a major impediment in the implementation of nanoscale transistors as logic and memory devices. There are many occasions in chemistry, however, in which single-molecule sensitivity is welcome. The detection of single molecules by fluorescence spectroscopy, for example, has deepened our understanding of the behaviors of individual molecules and is leading to the development of devices that can rapidly screen small sample volumes for DNA or molecules of environmental significance.17 The work described above demonstrates that a chemically modified single electron transistor will respond to only a handful of chemical transformations. In principle, as little as a single molecule binding event, or a single redox reaction involving a molecule bound the particle surface should shift the current staircase by a detectable amount. Work is currently in progress to determine the ultimate sensitivity and response time of single electron tunneling to various chemical events. Given the myriad of molecular receptors and polymeric films that may be attached to metal particles, single molecule detection with nanoparticle electronic devices could prove to be a new type of sensing platform. We have also demonstrated that metal particles can be assembled into spatially and symmetrically well-defined aggregates using molecular templates. These structures may prove to be useful in optical applications such as second harmonic generation (SHG). Since metal nanoparticles as small as ca. 2 nm in diameter are free electron metals, they satisfy the polarizability requirement for SHG. Synthesis of an appropriate molecular bridge ensures the assembly of particles into noncentrosymmetric point groups, the other requirement for SHG. Preliminary results confirm the hypothesis that gold particle trimers yield large first-order hyperpolarizabilities.18 Electronically, metal nanoparticle dimers are interesting mesoscale analogues of the molecular Cruetz-Taube bridged metal complex systems.19 Insofar as the analogy holds, redox splitting vs bridge length should provide further information about electron delocalization across the bridge and/or electro-
8930 J. Phys. Chem. B, Vol. 104, No. 38, 2000 static coupling between particles. Because phenylacetylene bridges have been discussed as potential molecular device components, gold particle dimers could represent useful model systems of a single molecule bound to two gold wires. Finally, we note that the assembly of bridged metal particle, mixed metal particle, and metal/semiconductor particle nanostructures should be straightforward. Each of these structures should have a unique set of interesting electronic and optical properties. Acknowledgment. We thank partial support of this work by Research Corp., ACS-PRF, NSF (CHE-9900073), NSF (DMR-Career), The Arnold and Mable Beckman Foundation, and the Office of Naval Research. We thank Dr. Wallace Ambrose at the University of North Carolina for help acquiring transmission electron microscope images. Professors Joseph T. Hupp, George Schatz, Colby A. Foss, Jr., and Christopher B. Gorman are acknowledged for lengthy discussions. References and Notes (1) Van de Hulst, H. C. Light Scattering by Small Particles; Dover: New York, 1981. (2) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (3) (a) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C., III; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (b) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc 1999, 121, 462. (4) (a) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (b) Peschel, S.; Schmid, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1442. (c) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. ReV. Phys. Chem. 1998, 49, 371. (d) Henglein, A.; Giersig, M. J. Phys. Chem. B 1999, 103, 9355. (e) Porter, L. A., Jr.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewics, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378. (f) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (g) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. B.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359.
McConnell et al. (5) Hayat, M. A. Colloidal Gold: Principles, Methods, and Applications; Academic Press: San Diego, 1989. (6) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (7) Feldheim, D. L.; Keating, C. D. Chem. Soc. ReV. 1998, 27, 1. (8) (a) Brust, M.; Walker, M.; Bethell, D.; Schriffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466. (9) (a) Amman, M.; Wilkins, R.; Ben-Jacob, E.; Maer, P. D.; Jaklevic, R. C. Phys. ReV. B 1991, 43, 1146. (b) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (c) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (d) Radojkovic, P.; Schwartzkipff, M.; Enachescu, M.; Stefanov, E.; Hartmann, E.; Koch, F. J. Vac. Sci. Technol. B 1996, 14, 1229. (e) Xue, Y.; Datta, S.; Hong, S.; Reifenberger, R.; Henderson, J. I.; Kubiak, C. P. Phys. ReV. B 1999, 59, R7852. (10) (a) Brousseau, L. C., III; Zhao, Q.; Shultz, D. A.; Feldheim, D. L. J. Am. Chem. Soc. 1998, 120, 7645. (b) Galvinol has a pKa of ∼11.5 on a gold particle, mercaptohexylamine has a pKa of ∼10.0. (11) Dubois, J. G. A.; Verheijen, E. N. G.; Gerritsen, J. W.; van Kempen, H. Phys. ReV. B 1993, 48, 11260. (12) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc 1987, 109, 3559. (13) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. ReV. 1999, 99, 2957. (14) (a) Vance, F. W.; Lemon, B. I.; Hupp, J. T. J. Phys. Chem. B 1998, 102, 10091. (b) Sandrock, M. L.; Foss, C. A., Jr. J. Phys. Chem. 1999, 103, 11398. (15) (a) Brousseau, L. C., III; Novak, J. P.; Marinakos, S. M.; Feldheim, D. L. AdV. Mater. 1999, 11, 447. (b) Marinakos, S. M.; Brousseau, L. C., III; Jones, A.; Feldheim, D. L. Chem. Mater. 1998, 10, 1214. (c) Novak, J. P.; Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 3979. (16) (a) Jensen, T.; Kelly, L.; Lazarides, A.; Schatz, G. C. J. Cluster Sci. 1999, 10, 295. (b) Yang, W.-H.; Schatz, G. C.; Van Duyne, R. P. J. Chem. Phys. 1995, 103, 869. (17) Moerner, W. E. Acc. Chem. Res. 1996, 29, 563. (18) Vance, F. W. Ph.D. Dissertation, Northwestern University, 1998. (19) Sutton, J. E.; Taube, H. Inorg. Chem. 1981, 20, 3125.
