Article pubs.acs.org/JPCC
Novel Plasmonic Structures Based on Gold Nanobelts Courtney M. Payne,†,§ Lindsey J. E. Anderson,‡,§ and Jason H. Hafner*,†,‡,§ †
Department of Chemistry, Rice University, 6100 Main Street, Houston, Texas 77005, United States Department of Physics and Astronomy, Rice University, 6100 Main Street, Houston, Texas 77005, United States § Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, United States ‡
S Supporting Information *
ABSTRACT: Gold nanobelts are elongated nanostructures that exhibit plasmon resonant scattering that is tunable with the cross-sectional aspect ratio. Their surfactant-directed chemical synthesis produces uniform nanobelts as well as similar, yet more complex, structures. Tapered gold nanobelts have a shift in their cross-sectional aspect ratio along their length and, therefore, a shift in plasmon resonance wavelength. The tapered nanobelts are shown to act as mesoscale plasmonic spectrometers to probe enhanced emission from nearby quantum dots. Nanobelts also form dimers, either because of bifurcation during growth or because of alignment during deposition, that run side by side with sub-10 nm gaps which create large volumes of electromagnetic enhancement for surface-enhanced Raman spectroscopy.
■
INTRODUCTION One-dimensional nanomaterials extend unique nanometer-scale properties to macroscopic lengths and create new opportunities for basic research and applications.1 For example, the surface plasmon resonances supported by noble-metal nanostructures cause strong, tunable absorption, scattering, and near-field enhancement under optical illumination. These effects have been studied widely for applications in nanomedicine,2 near-field optics,3,4 surface-enhanced spectroscopy,5,6 and chemical and biological sensing.7 When extended in one dimension, noblemetal nanostructures can also transmit optical signals. Light that is tightly focused at one end of a gold or silver nanowire couples to surface-plasmon modes that propagate down the wire and then that scatter out at the other end.8−10 Plasmonic nanostructures transmit light with small-mode volumes, so they have the potential for strong interactions with nearby quantum emitters.11,12 In fact, rudimentary plasmonic modulators,13 wave plates,14 and logic networks15,16 have already been demonstrated with silver nanowires through microscopic imaging experiments.17 Sharp plasmon resonances are not required to detect plasmon propagation. In fact, most of these experiments were carried out on silver nanowires with sufficiently large diameters that they do not exhibit sharp plasmon resonances because of radiative damping and the simultaneous excitation of different surface plasmon modes. We have recently shown that sharp, tunable plasmon resonances can be observed in extended structures. Chemically synthesized gold nanobelts, with elongated cross-sectional geometries that are similar in size to gold nanorods, yet with lengths of 10s of micrometers, were found to exhibit strong plasmon resonances by dark field microspectroscopy.18 The plasmon resonances were due to an asymmetric excitation across © XXXX American Chemical Society
the nanobelt width and were tunable with cross-sectional aspect ratio (x-AR), defined to be the nanobelt width divided by its height. Extending plasmonic properties to structures that are micrometers in length may be useful for biosensing and nanomedicine because of increased scattering and absorption cross sections19 and for nanoscale optical devices because of the possibility of resonant plasmonic propagation.20 In addition to uniform gold nanobelts, the synthesis yields more complex nanobelt structures that split and that have varying aspect ratios along their length. Here, we describe these nanostructures and demonstrate potential applications.
■
EXPERIMENTAL SECTION Gold Nanobelt Synthesis. Gold nanobelts were synthesized as described previously.18 In a typical synthesis, a surfactant solution was prepared by the addition of 650 μL of 50 mM cetyltrimethylammonium bromide and 500 μL of 10 mM sodium dodecyl sulfate to 3.45 mL of water. To this mixture, 100 μL of 10 mM HAuCl4 was added, and the solution was allowed to sit for approximately 5 min. Following this, the solution was reduced at room temperature with 300 μL of 100 mM L-ascorbic acid and was immediately transferred to a 27 °C water bath for 24 h. The gold nanobelt products formed were then further analyzed. Preparation of Quantum Dot Films. CdSe/ZnS core/shell quantum dots capped in octadecylamine were purchased from Ocean NanoTech. The quantum dot emission peak was centered at λem = 600 nm with a quantum yield of over 50% (emission profile shown in Figure S1 of the Supporting Information). The Received: August 11, 2012 Revised: January 25, 2013
A
dx.doi.org/10.1021/jp3080089 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
computational difficulties at sharp edges in the finite element method. A 1.1 μm circular simulation space surrounded the nanobelt cross section with a 200 nm thick perfectly matched layer (PML).
quantum dots were dispersed in toluene at a concentration of 10 mg/mL. Polystyrene solutions were prepared in toluene at 1% w/w and at 2% w/w. Finally, a mixture of 588 uL of the 2% polystyrene solution and 12 uL of the quantum dot solution was prepared. Gold nanobelts were deposited on glass slides by drying 2 μL of nanobelt solution and by rinsing with several 10 μL drops of methanol in a spin coater. An approximately 50 nm dielectric spacer layer of polystyrene (without quantum dots) was then added by spin-casting 30 μL of the 2% polystyrene solution. Next, a 20 nm silicon monoxide (SiO) dielectric film was deposited onto the substrate by thermal deposition and was monitored by a quartz crystal microbalance. The purpose of the SiO layer is to keep subsequent polystyrene deposition from dissolving the bottom polystyrene layer. Finally, a 10 nm layer quantum dot/polystyrene film was spin-cast from a 10 μL drop of the mixture described above. For each layer that was spin-cast, the spin coater was run at 6000 rpm for 1 min. Imaging and Microspectroscopy. Gold nanobelts were deposited on glass slides by drying 2 μL of nanobelt solution and by rinsing with several 10 μL drops of methanol in a spin coater. The samples were imaged by epi-illumination dark field microscopy with a 50×/0.5 NA objective (Zeiss Axiovert 200M) and 100 W quartz tungsten halogen (QTH) light source. For nanobelt spectra, the microscope was coupled to a 150 mm imaging spectrograph (Acton MicroSpec 2150i) and CCD detector (Princeton Instruments Photon Max). The wavelength positions on the CCD were calibrated with narrow band pass interference filters calibrated at visible and near-infrared wavelengths. The nanoparticle scattering spectra were corrected for the spectral efficiencies of the optical components (light source, objective lens, grating, and detector) with spectra from micrometer-scale silver colloidal particles which are assumed to scatter at all wavelengths equally. Fluorescence images were collected with the same microscope and objective as the dark field system described above but using a fluorescence filter set rather than a dark field illumination annulus and a Nikon D5000 camera for imaging. The quantum dot emission and filter set curves are shown in Figure S1 of the Supporting Information. Fluorescence images in counts were converted to emission enhancement by dividing the image counts by the average background counts from a region with no nanobelt. In this way, the emission enhancement of the background is 1 (no enhancement), and the image directly displays the enhancement above the nanobelts. Electron Microscopy and Atomic Force Microscopy. Atomic force microscopy (AFM) was carried out in ambient tapping mode with silicon tips on a VEECO Multimode AFM. Transmission electron microscopy (TEM) was performed with a JEOL JEM-2100F TEM operating at 200 kV. Scanning electron microscopy (SEM) was carried out with a FEI Quanta 400. Surface-enhanced Raman spectroscopy (SERS) measurements were performed using a Renishaw InVia Raman microscope with a 50×/0.55 NA objective with a laser excitation wavelength of 785 nm at 50mW. Electromagnetic Simulations. Numerical simulations of nanobelt and nanobelt dimer structures were performed by the finite element method using COMSOL Multiphysics. The dielectric function of gold was taken from Johnson and Christy21 with specific values determined by linear interpolation of the data (Figure S2 of the Supporting Information). The nanobelts were simulated as 2D cross sections with infinite length. The corners were rounded to have a 2 nm radius of curvature to avoid
■
RESULTS AND DISCUSSION Figure 1 displays a dark field optical image of gold nanobelts on a glass substrate. Colorful filamentous structures are observed as
Figure 1. Optical dark field image of gold nanobelts produced in a typical synthesis. A range of nanobelt lengths, plasmon resonant frequencies, and structures are produced by the synthesis.
well as pointlike particles. The filaments are gold nanobelts that are 25−100 nm wide, 15−30 nm tall, and many micrometers long. Through correlated AFM and dark field microspectroscopy on individual nanobelts, we recently demonstrated that the colors are due to plasmon resonant scattering by an asymmetric mode across the nanobelt width.18 As with gold nanorods and other elongated structures,22 the plasmon resonances shift to longer wavelengths with increasing aspect ratio. For gold nanobelts, the x-AR sets the resonant wavelength, and it is independent of the nanobelt length. The six nanobelts chosen for the previous study had uniform aspect ratios along their length. However, as synthesized here (and as synthesized almost identically in the original report), most gold nanobelts are not uniform. On the basis of an analysis of many nanobelt images like Figure 1, approximately 10% of the gold nanobelts are uniform, and the rest show significant variations in color along their length. While most vary with no discernible pattern, about 10% of the nanobelts exhibit a uniform transition from green to red because of a smooth change in aspect ratio. Several examples of uniformly tapered nanobelts are displayed in Figure 2, where the width variation occurs over length scales of nanometers to micrometers. The transmission and scanning electron micrographs (Figure 2a and b, respectively) directly display the variation in width, while the dark field optical image (Figure 2c) shows it indirectly through the variation in color. A single tapered nanobelt was analyzed in detail by correlated AFM and microspectroscopy. The peak wavelength and x-AR are plotted in Figure 3 along with the previously reported points for uniform nanobelts.18 The tapered nanobelt peak wavelengths match those from uniform nanobelts and confirm that the shift in color is due to a change in x-AR. The spatial dispersion of the plasmon resonant wavelength enables the tapered nanobelts to act as mesoscopic spectrometers, or plasmometers, for plasmonic interactions. Here, we describe light emission from quantum dots near tapered gold B
dx.doi.org/10.1021/jp3080089 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
complicated dependences on the chromophore’s position and orientation relative to the nanoparticle, the nanoparticle’s electromagnetic near field enhancement pattern, and the spectral properties of both the plasmonic nanostructure and the chromophore. However, it is important to deconvolute their effects especially since excitation and emission contribute to different applications. This has been achieved by studying different nanoparticles at different plasmon energies,26−32 but tapered gold nanobelts may bring unique capabilities because of the spatial separation of a continuum of plasmon resonances. To demonstrate their spectroscopic capabilities, gold nanobelts were deposited on glass substrates and were covered with an approximately 50 nm polystyrene film followed by a 20 nm SiO film and finally by an approximately 10 nm film of polystyrene containing CdSe/ZnS quantum dots. This structure keeps the quantum dots at a set separation from the nanobelts. The spincoating process causes the quantum dots to segregate into domains on top of the film so that they are well separated from the nanobelts (see AFM images in Figure S3 of the Supporting Information).33 Figure 4 presents both dark field scattering and fluorescence microscopy images of three tapered nanobelts on the substrate. For each nanobelt, the dark field image reveals which end is resonant near the emission peak of the quantum dots (λem = 600 nm). The fluorescence images were converted to emission enhancement images as described in the Experimental Section. The enhancement values near the nanobelts (between 1 and 1.5) are similar to those reported for quantum dots near single silver nanoprisms (approximately 1.2 −1.5).27,28 Here, the enhancements are clearly more prominent at the red end of the nanobelt where the plasmon resonances match the quantum dot emission, which indicates an enhanced rate of emission rather than excitation. Also, if quenching were to occur off the emission resonance, it would appear as a dip in the fluorescence background, which is not observed. A second novel nanostructure regularly found in these samples is the nanobelt dimer. In this structure, two nanobelts run side by side with a sub-10 nm gap for lengths greater than one micrometer. Images of nanobelt dimers, displayed in Figure 5, suggest that these can form when a nanobelt splits during synthesis leading to two parallel daughter nanobelts or when two nanobelts come together upon deposition. Nanobelt dimers may serve as an interesting nanostructure geometry for plasmon hybridization,34 and they may create unique modes for plasmon propagation along the nanobelts considering they concentrate the electromagnetic field in the space between the nanobelts similar to plasmonic slot waveguides.35,36 Nanobelt dimers may also serve as substrates for analytical SERS applications which rely on large volumes of well-defined electromagnetic field enhancement.37−39 To evaluate their potential for analytical SERS, nanobelts were deposited on a glass slide from their native mixed surfactant solution and were dried. A nanobelt dimer was located by AFM relative to alignment marks. SERS measurements were taken on the nanobelt dimer, on a single nanobelt, and on the bare glass substrate all of which were covered with surfactant. Figure 6a displays the AFM image of the nanobelt dimer and includes an optical view of the excitation laser spot location. The laser was polarized horizontally across the vertically aligned nanobelt dimer. The measurements on the glass substrate and normal nanobelt showed no significant SERS signal. The nanobelt dimer yielded several strong peaks between 900 and 1500 cm−1 which correspond to alkane chain modes of the surfactants.39
Figure 2. (a) TEM, (b) SEM, and (c) optical dark field images of tapered gold nanobelts that occur at different scales.
Figure 3. A tapered gold nanobelt characterized by (a) dark field microspectroscopy showing spectra taken as a function of position along the belt (corresponding dark field image shown as insert) and (b) AFM to determine x-AR at each location the spectra were taken. (c) The spectral peak wavelength along the tapered nanobelt (●) plotted against x-AR with the spectral peak for uniform belts from a previous study (○).18.
nanobelts as an example. Plasmonic nanostructures enhance the electromagnetic field near their surface, which can increase the absorption cross section and therefore enhance the excitation rate of a chromophore. The radiative decay rate may also be enhanced, and together these effects can increase the brightness of a nearby chromophore.23,24 The plasmonic nanostructure can also open new nonradiative decay channels resulting in quenched chromophore emission.25 These interwoven effects each have C
dx.doi.org/10.1021/jp3080089 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 4. Quantum dot emission enhancement near a tapered gold nanobelt. The dark field images illustrate the wavelength and spatial location of the plasmon resonances, while the fluorescence images show increased quantum dot emission at the red end of the taper.
Figure 5. (a) TEM image of a gold nanobelt dimer showing that the two branches of the nanobelt run together with a uniform gap and (b) SEM of a nanobelt as it splits. (c) AFM imaging can also identify nanobelt dimers.
To analyze the magnitude and spatial distribution of the field enhancement in the nanobelt dimer, the structure was simulated in COMSOL. Since the AFM image does not provide the precise nanobelt dimer geometry, a typical structure was simulated on the basis of our TEM observations: two 20 nm by 50 nm nanobelts separated by a 3 nm gap (see Figure S4 of the Supporting Information).39 The electromagnetic enhancement (g = E/Eo) was calculated at both the incident laser wavelength of 785 nm and a Raman shifted wavelength of 858 nm (which corresponds to a ∼1200 cm−1 shift typical of a hydrocarbon mode) which was polarized perpendicular to the gap. The mesh used for this simulation is shown in Figure S5 of the Supporting Information. The results, in Figure 6d, show a region between the nanobelts with field enhancements (g, g′ where the prime denotes the field at the scattered frequency) reaching 16 and 12 for the incident and scattered wavelengths, respectively, and yielding an overall enhancement factor (EF = |g × g′|2) of ∼37 000. The total enhancement is not especially high, but that is expected given the large separation (relative to typical hot spots) and the lack of curvature in one dimension. The enhancement is similar to those at the ends of gold nanorods that we observed in our recent analytical SERS study with this instrument (15 000− 84 000).39 Simulations of a single nanobelt generate a field enhancement at the corners of approximately 6 and an enhancement factor of only 1300 (shown in Figure S5 of the Supporting Information).
Figure 6. SERS from a nanobelt dimer. (a) AFM of a nanobelt dimer with (inset) the optical image of the dimer in the Raman microscope. (b) The corresponding SERS of dried surfactant on the nanobelt dimer (blue), on a single nanobelt (green), and on the glass substrate (red). (c) A schematic of the gold nanobelt dimer, and (d) the theoretical electromagnetic enhancement (g, g′) of its cross section with a 3 nm gap at (top) 785 nm and (bottom) 858 nm.
■
CONCLUSIONS Here, we have reported two novel plasmonic nanostructures based on gold nanobelts. Tapered nanobelts have uniformly varying x-AR and peak plasmon frequency along their length.
This spatial dispersion creates mesoscopic plasmometers to study enhanced light emission or other plasmonic effects. This capability was demonstrated by the spatially varying emission D
dx.doi.org/10.1021/jp3080089 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
plasmons in metallic nanowires coupled to quantum dots. Nature 2007, 450 (7168), 402−406. (12) Chang, D. E.; Sorensen, A. S.; Hemmer, P. R.; Lukin, M. D. Quantum optics with surface plasmons. Phys. Rev. Lett. 2006, 97 (5), 053002. (13) Li, Z. P.; Zhang, S. P.; Halas, N. J.; Nordlander, P.; Xu, H. X. Coherent Modulation of Propagating Plasmons in Silver-NanowireBased Structures. Small 2011, 7 (5), 593−596. (14) Zhang, S. P.; Wei, H.; Bao, K.; Hakanson, U.; Halas, N. J.; Nordlander, P.; Xu, H. X. Chiral Surface Plasmon Polaritons on Metallic Nanowires. Phys. Rev. Lett. 2011, 107 (9), 096801. (15) Wei, H.; Li, Z. P.; Tian, X. R.; Wang, Z. X.; Cong, F. Z.; Liu, N.; Zhang, S. P.; Nordlander, P.; Halas, N. J.; Xu, H. X. Quantum Dot-Based Local Field Imaging Reveals Plasmon-Based Interferometric Logic in Silver Nanowire Networks. Nano Lett. 2011, 11 (2), 471−475. (16) Wei, H.; Wang, Z. X.; Tian, X. R.; Kall, M.; Xu, H. X. Cascaded logic gates in nanophotonic plasmon networks. Nat. Commun. 2011, 2. (17) Gramotnev, D. K.; Bozhevolnyi, S. I. Plasmonics beyond the diffraction limit. Nat. Photonics 2010, 4 (2), 83−91. (18) Anderson, L. J. E.; Payne, C. M.; Zhen, Y.-R.; Nordlander, P.; Hafner, J. H. A Tunable Plasmon Resonance in Gold Nanobelts. Nano Lett. 2011, 11 (11), 5034−5037. (19) Kennedy, L. C.; Bickford, L. R.; Lewinski, N. A.; Coughlin, A. J.; Hu, Y.; Day, E. S.; West, J. L.; Drezek, R. A. A New Era for Cancer Treatment: Gold-Nanoparticle-Mediated Thermal Therapies. Small 2011, 7 (2), 169−183. (20) Chang, D. E.; Sorensen, A. S.; Hemmer, P. R.; Lukin, M. D. Strong coupling of single emitters to surface plasmons. Phys. Rev. B 2007, 76 (3), 035420. (21) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6 (12), 4370−4379. (22) Nehl, C. L.; Hafner, J. H. Shape-dependent plasmon resonances of gold nanoparticles. J. Mater. Chem. 2008, 18 (21), 2415−2419. (23) Thomas, K. G.; Kamat, P. V. Making Gold Nanoparticles Glow: Enhanced Emission from a Surface-Bound Fluoroprobe. J. Am. Chem. Soc. 2000, 122 (11), 2655−2656. (24) Ming, T.; Chen, H. J.; Jiang, R. B.; Li, Q.; Wang, J. F. PlasmonControlled Fluorescence: Beyond the Intensity Enhancement. J. Phys. Chem. Lett. 2012, 3 (2), 191−202. (25) Dulkeith, E.; Morteani, A. C.; Niedereichholz, T.; Klar, T. A.; Feldmann, J.; Levi, S. A.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Moller, M.; Gittins, D. I. Fluorescence Quenching of Dye Molecules near Gold Nanoparticles: Radiative and Nonradiative Effects. Phys. Rev. Lett. 2002, 89 (20), 203002. (26) Chen, Y.; Munechika, K.; Ginger, D. S. Dependence of Fluorescence Intensity on the Spectral Overlap between Fluorophores and Plasmon Resonant Single Silver Nanoparticles. Nano Lett. 2007, 7 (3), 690−696. (27) Munechika, K.; Chen, Y.; Tillack, A. F.; Kulkarni, A. P.; Plante, I. J. L.; Munro, A. M.; Ginger, D. S. Spectral Control of Plasmonic Emission Enhancement from Quantum Dots near Single Silver Nanoprisms. Nano Lett. 2010, 10 (7), 2598−2603. (28) Munechika, K.; Chen, Y. C.; Tillack, A. F.; Kulkarni, A. P.; Jen-La Plante, I.; Munro, A. M.; Ginger, D. S. Quantum Dot/Plasmonic Nanoparticle Metachromophores with Quantum Yields That Vary with Excitation Wavelength. Nano Lett. 2011, 11 (7), 2725−2730. (29) Bharadwaj, P.; Novotny, L. Spectral dependence of single molecule fluorescence enhancement. Opt. Express 2007, 15 (21), 14266−14274. (30) Chen, C. T.; Liu, C. C.; Wang, C. H.; Chen, C. W.; Chen, Y. F. Tunable coupling between exciton and surface plasmon in liquid crystal devices consisting of Au nanoparticles and CdSe quantum dots. Appl. Phys. Lett. 2011, 98 (26), 261918. (31) Ming, T.; Zhao, L.; Chen, H. J.; Woo, K. C.; Wang, J. F.; Lin, H. Q. Experimental Evidence of Plasmophores: Plasmon-Directed Polarized Emission from Gold Nanorod-Fluorophore Hybrid Nanostructures. Nano Lett. 2011, 11 (6), 2296−2303.
enhancement of quantum dots near tapered nanobelts. Nanobelt dimers with sub-10 nm gaps over micrometer-scale lengths were also described. These structures have sufficient electromagnetic field enhancement to yield SERS spectra from molecules in the gap and uniquely create large enhancement volumes since the gap runs through the entire excitation beam spot. These nanostructures are created by a simple chemical synthesis and can be easily identified by optical or AFM imaging making them widely available for further study and development.
■
ASSOCIATED CONTENT
S Supporting Information *
Optical specifications of the CdSe/ZnS quantum dots and filter sets used in this study, the dielectric constants and mesh spacing used for the COMSOL simulations, the height profile found by AFM of the polystyrene/quantum dot films, an additional TEM image of a nanobelt dimer, and a COMSOL simulation of the field enhancement of a single gold nanobelt. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation under Grant 103575 (J.H.H., C.M.P.) and the Robert A. Welch Foundation under Grant C-1761 (J.H.H., L.J.E.A.).
■
REFERENCES
(1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. One-Dimensional Nanostructures: Synthesis, Characterization, and Applications. Adv. Mater. 2003, 15 (5), 353−389. (2) Lee, S. E.; Lee, L. P. Biomolecular plasmonics for quantitative biology and nanomedicine. Curr. Opin. Biotechnol. 2010, 21 (4), 489− 497. (3) Giannini, V.; Fernandez-Dominguez, A. I.; Heck, S. C.; Maier, S. A. Plasmonic Nanoantennas: Fundamentals and Their Use in Controlling the Radiative Properties of Nanoemitters. Chem. Rev. 2011, 111 (6), 3888−3912. (4) Halas, N. J.; Lal, S.; Chang, W. S.; Link, S.; Nordlander, P. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111 (6), 3913−3961. (5) Camden, J. P.; Dieringer, J. A.; Zhao, J.; Van Duyne, R. P. Controlled Plasmonic Nanostructures for Surface-Enhanced Spectroscopy and Sensing. Acc. Chem. Res. 2008, 41 (12), 1653−1661. (6) Golightly, R. S.; Doering, W. E.; Natan, M. J. Surface-Enhanced Raman Spectroscopy and Homeland Security: A Perfect Match? ACS Nano 2009, 3 (10), 2859−2869. (7) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111 (6), 3828−3857. (8) Ditlbacher, H.; Hohenau, A.; Wagner, D.; Kreibig, U.; Rogers, M.; Hofer, F.; Aussenegg, F. R.; Krenn, J. R. Silver Nanowires as Surface Plasmon Resonators. Phys. Rev. Lett. 2005, 95 (25), 257403. (9) Krenn, J. R.; Lamprecht, B.; Ditlbacher, H.; Schider, G.; Salerno, M.; Leitner, A.; Aussenegg, F. R. Non−diffraction-limited light transport by gold nanowires. Europhys. Lett. 2002, 60 (5), 663−669. (10) Krenn, J. R.; Weeber, J.-C. Surface plasmon polaritons in metal stripes and wires. Phil. Trans. R. Soc. A 2004, 362 (1817), 739−756. (11) Akimov, A. V.; Mukherjee, A.; Yu, C. L.; Chang, D. E.; Zibrov, A. S.; Hemmer, P. R.; Park, H.; Lukin, M. D. Generation of single optical E
dx.doi.org/10.1021/jp3080089 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
(32) Tam, F.; Goodrich, G. P.; Johnson, B. R.; Halas, N. J. Plasmonic Enhancement of Molecular Fluorescence. Nano Lett. 2007, 7 (2), 496− 501. (33) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Electroluminescence from single monolayers of nanocrystals in molecular organic devices. Nature 2002, 420 (6917), 800−803. (34) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures. Science 2003, 302 (5644), 419−422. (35) Dionne, J. A.; Lezec, H. J.; Atwater, H. A. Highly Confined Photon Transport in Subwavelength Metallic Slot Waveguides. Nano Lett. 2006, 6 (9), 1928−1932. (36) Dionne, J. A.; Sweatlock, L. A.; Atwater, H. A.; Polman, A. Plasmon slot waveguides: Towards chip-scale propagation with subwavelength-scale localization. Phys. Rev. B 2006, 73 (3), 035407. (37) Le Ru, E. C.; Meyer, M.; Blackie, E.; Etchegoin, P. G. Advanced aspects of electromagnetic SERS enhancement factors at a hot spot. J. Raman Spectrosc. 2008, 39 (9), 1127−1134. (38) Sackmann, M.; Materny, A. Surface enhanced Raman scattering (SERS) - a quantitative analytical tool? J. Raman Spectrosc. 2006, 37 (1− 3), 305−310. (39) Lee, S.; Anderson, L. J. E.; Payne, C. M.; Hafner, J. H. Structural Transition in the Surfactant Layer that Surrounds Gold Nanorods as Observed by Analytical Surface-Enhanced Raman Spectroscopy. Langmuir 2011, 27 (24), 14748−14756.
F
dx.doi.org/10.1021/jp3080089 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
