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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 11 — Jun. 2, 2014
  • pp: 13308–13313
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On the absorption and electromagnetic field spectral shifts in plasmonic nanotriangle arrays

Sylvain Vedraine, Renjie Hou, Peter R. Norton, and François Lagugné-Labarthet  »View Author Affiliations


Optics Express, Vol. 22, Issue 11, pp. 13308-13313 (2014)
http://dx.doi.org/10.1364/OE.22.013308


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Abstract

The behavior of the electromagnetic field interaction with gold nanotriangles organized in bow-tie arrays is investigated. A side-by-side comparison between the measured absorbance of the array and the modelled integrated electric field resonances confined around the gold structures is presented and discussed to explain the spectral shift between both parameters. Finite difference time domain calculations and Raman measurements of gold triangles of different sizes and periodicity are systematically performed. Numerical calculations show that the spectral maximum of the electric field varies in distinct areas over the metallic structures.

© 2014 Optical Society of America

1. Introduction

Plasmonic structures have shown a large potential for a variety of applications in optics and spectroscopy [1

1. S. A. Maier, M. L. Brongersma, P. G. Kik, S. Meltzer, A. A. G. Requicha, and H. A. Atwater, “Plasmonics—a route to nanoscale optical devices,” Adv. Mater. 13(19), 1501–1505 (2001). [CrossRef]

, 2

2. P. L. Stiles, J. A. Dieringer, N. C. Shah, and R. P. Van Duyne, “Surface-enhanced Raman spectroscopy,” Annu Rev Anal Chem (Palo Alto Calif) 1(1), 601–626 (2008). [CrossRef] [PubMed]

]. The frequency of the surface plasmon is dependent on all opto-geometric parameters associated with the structure, including the shape, size, refractive index of the structure and the surrounding environment [3

3. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters (Springer, 1995).

]. One of the advantages of using surface plasmons is the ability to generate an intense electromagnetic field confined at the interface formed by a metal and its dielectric surrounding. This localized surface plasmon resonance (LSPR) directly influence the optical properties of the surrounding material such as its absorption and non-linear response [4

4. S. Vedraine, P. Torchio, D. Duche, F. Flory, J. J. Simon, J. Le Rouzo, and L. Escoubas, “Intrinsic absorption of plasmonic structures for organic solar cells,” Sol. Energy Mater. Sol. Cells 95(1), S57–S64 (2011). [CrossRef]

, 5

5. M. I. Stockman, D. J. Bergman, C. Anceau, S. Brasselet, and J. Zyss, “Enhanced second-harmonic generation by metal surfaces with nanoscale roughness: nanoscale dephasing, depolarization, and correlations,” Phys. Rev. Lett. 92(5), 057402 (2004). [CrossRef] [PubMed]

]. For surface-enhanced Raman spectroscopy (SERS) experiments [6

6. E. Le Ru and P. Etchegoin, Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects (Elsevier, 2008).

], the field enhancement originating from the LSPR can significantly increase the Raman signal from the molecules located in the vicinity of the metallic nanostructure. Noticeably, McFarland et al. show that the maximum SERS signal is observed when the EM enhancement is not coincident with the laser excitation wavelength [7

7. A. D. McFarland, M. A. Young, J. A. Dieringer, and R. P. Van Duyne, “Wavelength-scanned surface-enhanced Raman excitation spectroscopy,” J. Phys. Chem. B 109(22), 11279–11285 (2005). [CrossRef] [PubMed]

]. This implies that the maximum of the electric field induced by the irradiation does not occur at the same frequency as the experimentally determined absorption maximum of the nanostructure. It is therefore necessary to know the electric field intensity spectrum. In order to find the latter, a spectroscopic technique measuring the absorbance of a given structure is often used; this is not necessarily accurate since spectral shifts (usually a red shift) between absorbance and electric field may be observed. This red shift of the near-field peak energies with respect to the absorption has been investigated phenomenologically [8

8. B. M. Ross and L. P. Lee, “Comparison of near- and far-field measures for plasmon resonance of metallic nanoparticles,” Opt. Lett. 34(7), 896–898 (2009). [CrossRef] [PubMed]

]. Recently, J. Zuloaga et al. showed that the magnitude of this shift depends directly on the total damping of the system, whether it is intrinsic damping within the metal of the nanoparticle or radiative damping of the localized plasmon [9

9. J. Zuloaga and P. Nordlander, “On the energy shift between near-field and far-field peak intensities in localized plasmon systems,” Nano Lett. 11(3), 1280–1283 (2011). [CrossRef] [PubMed]

]. In the present study, we investigate this spectral shift using several approaches including side-by-side comparison of the experimental absorbance spectrum and calculated electric field intensity spectrum as well as SERS measurements for a series of gold nanotriangles organized in arrays of bow-tie assemblies.

2. Results and discussion

SERS spectra were measured on gold triangles functionalized with 4-nitrothiophenol (4-NTP). 1.5 mg of 4-NTP was dissolved in 10 mL of 100% ethanol. Each sample was placed in the solution during 24 hours, washed in ethanol and dried with nitrogen just prior to optical measurements. A 785 nm laser was focused with an intensity of 1 mW on the sample using a x100, N.A. 0.9 objective. The acquisition time was 10 s per spectrum. For every pattern, 5 measurements were done at several locations. The background was approximated with a polynomial law and subtracted. Figure 4(a)
Fig. 4 (a) Raman spectrum of 4-NTP on gold triangle G = 100 nm, and L = 80, 100, 120, 140 nm. (b) Measured normalized absorbance spectrum and (c) calculated electric field intensity of gold triangle G = 100 nm, and L = 80, 100, 120, 140 nm in air.
shows the Raman signal obtained on 4 different lengths, L: 80, 100, 120 and 140 nm, with G = 100 nm and a polarization along the X-axis. In order to keep a filling factor (FF) close for each pattern, we adopted the period: Px = Py = 400 nm for L = 80, 100 nm and Px = Py = 500 nm for L = 120, 140 nm, leading to FF of 4%, 5.5%, 5% and 6% for L = 80, 100, 120 and 140 nm, respectively. The FF corresponds to the percentage of the surface covered by gold. The Raman spectra of 4-NTP are consistent with literature [15

15. W. Xie, C. Herrmann, K. Kömpe, M. Haase, and S. Schlücker, “Synthesis of bifunctional Au/Pt/Au core/shell nanoraspberries for in situ SERS monitoring of platinum-catalyzed reactions,” J. Am. Chem. Soc. 133(48), 19302–19305 (2011). [CrossRef] [PubMed]

]. The main spectral features observed in Fig. 4(a) at 1077, 1107, 1333 and 1571 cm−1 (noted 1-4 on Fig. 4(a)) are assigned to ν7a coupled with C-S stretching, νϕ-N, νsNO2 and ν8a, respectively. Upon irradiation, the peaks at 1138 cm−1, 1388 cm−1 and 1430 cm−1 (noted 5-7 on Fig. 4(a)) correspond to C-N symmetric stretching, R-N = N-R stretching and C-H in-plane bending modes, respectively, and can be assigned to the formation of 4,4′-dimercaptoazobenzene (DMAB) possibly generated by plasmon mediated catalytical reaction [16

16. D. Y. Wu, L. B. Zhao, X. M. Liu, R. Huang, Y. F. Huang, B. Ren, and Z. Q. Tian, “Photon-driven charge transfer and photocatalysis of p-aminothiophenol in metal nanogaps: a DFT study of SERS,” Chem. Commun. (Camb.) 47(9), 2520–2522 (2011). [CrossRef] [PubMed]

]. Nevertheless, irradiation time and the laser intensity were always the same for each structure. It is noteworthy that 4,4′-DMAB is more easily observable in the case of a pattern with triangle of 120 nm of side length although 4-NTP peaks are still more intense. However, Fig. 4(b) shows absorbance measured on identical patterns of nanotriangles. The excitation at 785 nm and the spectral Raman range up to 900 nm that corresponds to ~1700 cm−1 are indicated. Based on this figure, the triangle with a size of 140 nm should lead to a better SERS activity because its absorbance is higher above 785 nm. Nevertheless, the calculation of the electric field intensity around the gold nanotriangles [Fig. 4(c)] predicts a higher intensity in the range of characterization (785 – 905 nm) for triangle with L = 120 nm. The strongest SERS enhancement occurs under conditions where the incident and Raman scattered photons are both strongly enhanced [7

7. A. D. McFarland, M. A. Young, J. A. Dieringer, and R. P. Van Duyne, “Wavelength-scanned surface-enhanced Raman excitation spectroscopy,” J. Phys. Chem. B 109(22), 11279–11285 (2005). [CrossRef] [PubMed]

]. For L = 120 nm, the maximum of the integrated electric field intensity is centered in the range of Raman characterization, which allows to fill this criterion, unlike for L = 140 nm. This correlates the observations on the Raman spectra: L = 120 nm lead to a better SERS platform at an excitation wavelength of 785 nm, showing again the gap between absorbance and electric field intensity.

3. Conclusion

In this work, we have showed experimentally that a spectral shift between the maximum of absorbance and the maximum of the electric field intensity was generally observed in anisotropic metallic structures. The FDTD method can be used to predict the electric field enhancement spectrum which should be used primarily to select plasmon resonance with respect to a given excitation wavelength. This spectral shift can be due to destructive interferences between the scattered and the transmitted field.

Acknowledgments

The authors wish to gratefully acknowledge the Nanofabrication Facility at The University of Western Ontario. This research was funded by the Sciences and Engineering Research Council of Canada Discovery Grant and by the Canada Research Chairs program.

References and links

1.

S. A. Maier, M. L. Brongersma, P. G. Kik, S. Meltzer, A. A. G. Requicha, and H. A. Atwater, “Plasmonics—a route to nanoscale optical devices,” Adv. Mater. 13(19), 1501–1505 (2001). [CrossRef]

2.

P. L. Stiles, J. A. Dieringer, N. C. Shah, and R. P. Van Duyne, “Surface-enhanced Raman spectroscopy,” Annu Rev Anal Chem (Palo Alto Calif) 1(1), 601–626 (2008). [CrossRef] [PubMed]

3.

U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters (Springer, 1995).

4.

S. Vedraine, P. Torchio, D. Duche, F. Flory, J. J. Simon, J. Le Rouzo, and L. Escoubas, “Intrinsic absorption of plasmonic structures for organic solar cells,” Sol. Energy Mater. Sol. Cells 95(1), S57–S64 (2011). [CrossRef]

5.

M. I. Stockman, D. J. Bergman, C. Anceau, S. Brasselet, and J. Zyss, “Enhanced second-harmonic generation by metal surfaces with nanoscale roughness: nanoscale dephasing, depolarization, and correlations,” Phys. Rev. Lett. 92(5), 057402 (2004). [CrossRef] [PubMed]

6.

E. Le Ru and P. Etchegoin, Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects (Elsevier, 2008).

7.

A. D. McFarland, M. A. Young, J. A. Dieringer, and R. P. Van Duyne, “Wavelength-scanned surface-enhanced Raman excitation spectroscopy,” J. Phys. Chem. B 109(22), 11279–11285 (2005). [CrossRef] [PubMed]

8.

B. M. Ross and L. P. Lee, “Comparison of near- and far-field measures for plasmon resonance of metallic nanoparticles,” Opt. Lett. 34(7), 896–898 (2009). [CrossRef] [PubMed]

9.

J. Zuloaga and P. Nordlander, “On the energy shift between near-field and far-field peak intensities in localized plasmon systems,” Nano Lett. 11(3), 1280–1283 (2011). [CrossRef] [PubMed]

10.

B. C. Galarreta, E. Harté, N. Marquestaut, P. R. Norton, and F. Lagugné-Labarthet, “Plasmonic properties of Fischer’s patterns: polarization effects,” Phys. Chem. Chem. Phys. 12(25), 6810–6816 (2010). [CrossRef] [PubMed]

11.

A. Tavlove and S. Hagness, Computational Electrodynamics: The Finite-Difference Time (Artech House, 2005).

12.

W. M. Haynes, CRC Handbook of Chemistry and Physics (CRC Press, 2011).

13.

C. Awada, T. Popescu, L. Douillard, F. Charra, A. Perron, H. Yockell-Lelievre, A. L. Baudrion, P. M. Adam, and R. Bachelot, “Selective excitation of plasmon resonances of single Au triangles by polarization-dependent light excitation,” J. Phys. Chem. C 116(27), 14591–14598 (2012). [CrossRef]

14.

S. H. Lim, W. Mar, P. Matheu, D. Derkacs, and E. T. Yu, “Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles,” J. Appl. Phys. 101(10), 104309 (2007). [CrossRef]

15.

W. Xie, C. Herrmann, K. Kömpe, M. Haase, and S. Schlücker, “Synthesis of bifunctional Au/Pt/Au core/shell nanoraspberries for in situ SERS monitoring of platinum-catalyzed reactions,” J. Am. Chem. Soc. 133(48), 19302–19305 (2011). [CrossRef] [PubMed]

16.

D. Y. Wu, L. B. Zhao, X. M. Liu, R. Huang, Y. F. Huang, B. Ren, and Z. Q. Tian, “Photon-driven charge transfer and photocatalysis of p-aminothiophenol in metal nanogaps: a DFT study of SERS,” Chem. Commun. (Camb.) 47(9), 2520–2522 (2011). [CrossRef] [PubMed]

OCIS Codes
(180.4243) Microscopy : Near-field microscopy
(250.5403) Optoelectronics : Plasmonics
(240.6695) Optics at surfaces : Surface-enhanced Raman scattering

ToC Category:
Plasmonics

History
Original Manuscript: February 19, 2014
Revised Manuscript: April 28, 2014
Manuscript Accepted: May 19, 2014
Published: May 27, 2014

Citation
Sylvain Vedraine, Renjie Hou, Peter R. Norton, and François Lagugné-Labarthet, "On the absorption and electromagnetic field spectral shifts in plasmonic nanotriangle arrays," Opt. Express 22, 13308-13313 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-11-13308


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References

  1. S. A. Maier, M. L. Brongersma, P. G. Kik, S. Meltzer, A. A. G. Requicha, H. A. Atwater, “Plasmonics—a route to nanoscale optical devices,” Adv. Mater. 13(19), 1501–1505 (2001). [CrossRef]
  2. P. L. Stiles, J. A. Dieringer, N. C. Shah, R. P. Van Duyne, “Surface-enhanced Raman spectroscopy,” Annu Rev Anal Chem (Palo Alto Calif) 1(1), 601–626 (2008). [CrossRef] [PubMed]
  3. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters (Springer, 1995).
  4. S. Vedraine, P. Torchio, D. Duche, F. Flory, J. J. Simon, J. Le Rouzo, L. Escoubas, “Intrinsic absorption of plasmonic structures for organic solar cells,” Sol. Energy Mater. Sol. Cells 95(1), S57–S64 (2011). [CrossRef]
  5. M. I. Stockman, D. J. Bergman, C. Anceau, S. Brasselet, J. Zyss, “Enhanced second-harmonic generation by metal surfaces with nanoscale roughness: nanoscale dephasing, depolarization, and correlations,” Phys. Rev. Lett. 92(5), 057402 (2004). [CrossRef] [PubMed]
  6. E. Le Ru and P. Etchegoin, Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects (Elsevier, 2008).
  7. A. D. McFarland, M. A. Young, J. A. Dieringer, R. P. Van Duyne, “Wavelength-scanned surface-enhanced Raman excitation spectroscopy,” J. Phys. Chem. B 109(22), 11279–11285 (2005). [CrossRef] [PubMed]
  8. B. M. Ross, L. P. Lee, “Comparison of near- and far-field measures for plasmon resonance of metallic nanoparticles,” Opt. Lett. 34(7), 896–898 (2009). [CrossRef] [PubMed]
  9. J. Zuloaga, P. Nordlander, “On the energy shift between near-field and far-field peak intensities in localized plasmon systems,” Nano Lett. 11(3), 1280–1283 (2011). [CrossRef] [PubMed]
  10. B. C. Galarreta, E. Harté, N. Marquestaut, P. R. Norton, F. Lagugné-Labarthet, “Plasmonic properties of Fischer’s patterns: polarization effects,” Phys. Chem. Chem. Phys. 12(25), 6810–6816 (2010). [CrossRef] [PubMed]
  11. A. Tavlove and S. Hagness, Computational Electrodynamics: The Finite-Difference Time (Artech House, 2005).
  12. W. M. Haynes, CRC Handbook of Chemistry and Physics (CRC Press, 2011).
  13. C. Awada, T. Popescu, L. Douillard, F. Charra, A. Perron, H. Yockell-Lelievre, A. L. Baudrion, P. M. Adam, R. Bachelot, “Selective excitation of plasmon resonances of single Au triangles by polarization-dependent light excitation,” J. Phys. Chem. C 116(27), 14591–14598 (2012). [CrossRef]
  14. S. H. Lim, W. Mar, P. Matheu, D. Derkacs, E. T. Yu, “Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles,” J. Appl. Phys. 101(10), 104309 (2007). [CrossRef]
  15. W. Xie, C. Herrmann, K. Kömpe, M. Haase, S. Schlücker, “Synthesis of bifunctional Au/Pt/Au core/shell nanoraspberries for in situ SERS monitoring of platinum-catalyzed reactions,” J. Am. Chem. Soc. 133(48), 19302–19305 (2011). [CrossRef] [PubMed]
  16. D. Y. Wu, L. B. Zhao, X. M. Liu, R. Huang, Y. F. Huang, B. Ren, Z. Q. Tian, “Photon-driven charge transfer and photocatalysis of p-aminothiophenol in metal nanogaps: a DFT study of SERS,” Chem. Commun. (Camb.) 47(9), 2520–2522 (2011). [CrossRef] [PubMed]

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