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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 13 — Jun. 25, 2007
  • pp: 8309–8316
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Tuning the resonance frequency of Ag-coated dielectric tips

Xudong Cui, Weihua Zhang, Boon-Siang Yeo, Renato Zenobi, Christian Hafner, and Daniel Erni  »View Author Affiliations


Optics Express, Vol. 15, Issue 13, pp. 8309-8316 (2007)
http://dx.doi.org/10.1364/OE.15.008309


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Abstract

A finite element model was built to investigate how to optimize localized plasmon resonances of an Ag-coated dielectric tip for tip-enhanced Raman spectroscopy (TERS). The relation between the resonance frequency, the electric field enhancement and the optical constant of the dielectric tip was numerically investigated. The results show that increasing the refractive index of the dielectric tip can significantly red shift the localized plasmon modes excited on the Ag-coated dielectric tip, and consequently alter the field enhancement. Moreover, the influence of the width of the resonance on the Raman enhancement was also considered. When taking all the factors into account, we find that an Ag-coated low-refractive index dielectric tip provides the best Raman enhancement in the blue—green spectral range. This is consistent with our prior experimental results.

© 2007 Optical Society of America

1. Introduction

There are two possible solutions for matching the plasmon resonance of the Ag-coated dielectric tip with the wavelength of the excitation source: (1) scanning the wavelength of the light source; (2) tuning the resonance frequency of the tip. At a first sight, the first approach seems more straightforward. However, excitation wavelength scan is expensive and inconvenient for TERS: a multi-wavelength laser source is needed; notch filters for the different laser lines and a Raman spectrograph covering a wide spectral range are required. The second approach is more practical: only one fixed laser line and one notch filter are required. It is well known that the resonance of LPs on a metal nanostructure is sensitive to its shape and the optical properties of its surrounding materials [14–16

14. J. B. Jackson and N. J. Halas, “Surface-enhanced Raman scattering on tunable plasmonic nanoparticle substrates,” PNAS 101, 17930–17935 (2004). [CrossRef] [PubMed]

]. From this point of view, the mismatch between the laser source and the resonance frequency of the Ag-coated tip is caused by the improper choice of the tip shape and the underlying material. Therefore, the solution to the problem lies in tuning the resonance by changing the accessible parameters of the metallized tip.

Recently, it was reported that the Ag-coated dielectric tips with different refractive indices give different Raman enhancement [13

13. B. Yeo, T. Schmid, W. Zhang, and R. Zenobi, “Towards rapid nanoscale chemical analysis using tip-enhanced Raman spectroscopy with Ag-coated dielectric tips,” Anal. Bioanal. Chem. 387, 2655–2662 (2007). [CrossRef] [PubMed]

, 17

17. B. S. Yeo, W. H. Zhang, C. Vannier, and R. Zenobi, “Enhancement of Raman signals with silver-coated tips,” Appl. Spectrosc. 60, 1142–1147 (2006). [CrossRef] [PubMed]

]. Following this idea, the performance of Ag-coated AFM tips has been significantly improved by using low refractive index AFM tips. However, the experiment was only performed for a 488 nm illumination with a limited number of AFM tips. The details for this material dependent enhancement are still unclear. We therefore systematically investigate how the optical properties of the AFM tip influence the resonance properties of the Ag coating, such as the resonance frequency, the resonance width and the associated field enhancement (i.e. the resonance amplitude). Furthermore, the physics behind the simulation is also discussed.

2. Simulation

In this work, an Ag-coated axis-symmetric tip was simulated as shown in Fig. 1. A dielectric tip with a rounded end (10 nm diameter) was used to model the dielectric AFM tip. The shape and size correspond to the geometric parameters of commercial AFM tips. The Ag coating was modeled by a 20 nm thick homogeneous layer on the top of the dielectric tip. Since only the p-polarization component of electric field (along the tip axis) can be greatly enhanced [7

7. L. Novotny, R. X. Bian, and X. S. Xie, “Theory of nanometric optical tweezers,” Phys. Rev. Lett. 79, 645–648 (1997). [CrossRef]

, 8

8. A. V. Zayats, “Electromagnetic field enhancement in the context of apertureless near-field microscopy,” Opt. Commun. 161, 156–162 (1999). [CrossRef]

], a TM mode incident light beam (E is parallel to the tip) was employed. An important issue for the simulation of Ag nanostructures is the optical constants, which can be different from those of the corresponding bulk material when their sizes are smaller than 5 nm [18

18. C. F. Bohren and D. R. Juffman, Absorption and scattering of light by small particles (John Wiley: New York, 1983).

, 19

19. U. Kreibig and M. Voller, Optical Properties of Metal Clusters (Springer: Berlin, 1995).

]. Since the thickness of the Ag layer in our model is relatively large (20 nm), measured data from bulk silver was directly used without further considering the negligible size-induced effect [20

20. R. W. C. P. B. Johnson, “Optical Constants of the Noble Metals,” Phys. Rev. B 6, 4370–4379 (1972). [CrossRef]

].

Fig. 1. (a). Dimension size and mesh of the tip used for the simulation. Here, r1=10 nm, r2=30 nm. (b) SEM picture from side-view. The scale bar denotes 100 nm.

For the numerical analysis, a finite element method (FEM) based tool integrated in the COMSOL Multiphysics package was employed to solve the Maxwell equations [21]. Because in of FEM simulations, any infinite structure has to be truncated by the boundary condition to obtain a tractable computation window, we terminated the support of the tip at the length of 550 nm. A significant reduction in computational costs and memory requirements is achieved by taking axis-symmetric boundary conditions into account. To get reliable data, the boundary conditions were carefully chosen: because FEM is a domain method requiring the discretization of the entire field domain, absorbing boundary conditions (ABCs) must be implemented for simulating an open space. In this work, the ABCs were applied with a 500 nm (larger than the half wavelength) distance from the tip to avoid an unphysical result caused by inefficient adsorption of the evanescent components of the scattering field. Scattering boundary conditions were then used on the top and bottom of the simulated space. To reduce spurious reflections, especially from the radial corners, fictitious perfectly matched layers (PMLs) were added proximately to the outer side of the scattering boundaries.

3. Results and discussion

Figure 2 depicts the calculated distribution of the electric field strength at the tip apex for p-polarized excitation. As expected, the p-component of the electric field is strongly enhanced as it has been also observed for pure Ag tips by other groups before [7

7. L. Novotny, R. X. Bian, and X. S. Xie, “Theory of nanometric optical tweezers,” Phys. Rev. Lett. 79, 645–648 (1997). [CrossRef]

, 22

22. I. Notingher and A. Elfick, “Effect of sample and substrate electric properties on the electric field enhancement at the Apex of SPM Nanotips,” J. Phys. Chem. B 109, 15699–15706 (2005). [CrossRef]

].

Fig. 2. Calculated electric field distribution of the p-component.
Fig. 3. Spectral responses of the field enhancement for an Ag coated Si3N4 tip, glass and AlF3 tip.

3.1 How does the refractive index influence the resonance behavior of an Ag-coated dielectric tip?

To study the resonance behavior of Ag-coated AFM tips, we calculated the spectral characteristics of the LP resonances for three different AFM tips (Si3N4, glass and AlF3 tips). Figure 3 shows the spectral responses of field enhancement for these three material systems. A complicated resonance structure is observed from the Ag-coated Si3N4 tip. Its most pronounced resonance peak is in the green range (about 550 nm), while there are other weak resonances in a wide spectral range. For the Ag coated glass and AlF3 tip, there are mainly two broad and strong resonances in the visible light range: 450–550 nm and 640 nm–near infrared. Comparing these three tips, the field enhancement of the Ag-coated glass and AlF3 tip is generally higher than that of the Si3N4 tip in the blue—green spectral range, and consequently, will provide a higher Raman enhancement in this spectral range.

Fig. 4. (a). Field enhancement against the refractive index and the wavelength (i.e. as two dimensional intensity plot) to reveal the resonance behaviors of the Ag-coated AFM tip. (b) The same plot for an Ag coated spherical particle. In these two plots, dark blue and deep red represent the lowest and highest intensities respectively.

One of the most prominent features in the plot [Fig. 4(a)] is that all the resonance modes are red shifted with an increasing refractive index of the AFM tip. However, the resonance structure shown in Fig. 4(a) is complicated due to the geometrical complexity of the tip. To exclude the intricacy induced by the geometrical reasons and further understand the origin of this material-dependent red-shift effect, we simplified our tip model to an Ag-coated spherical particle, which is ideal in terms of geometry, analytically accessible and well studied [18

18. C. F. Bohren and D. R. Juffman, Absorption and scattering of light by small particles (John Wiley: New York, 1983).

, 19

19. U. Kreibig and M. Voller, Optical Properties of Metal Clusters (Springer: Berlin, 1995).

]. We assume that the particle is much smaller than the excitation wavelength, and consequently, the quasi-static model is feasible. If the core diameter is R, and the thickness of the Ag coating is d, the static polarizability of this particle will be [18

18. C. F. Bohren and D. R. Juffman, Absorption and scattering of light by small particles (John Wiley: New York, 1983).

, 19

19. U. Kreibig and M. Voller, Optical Properties of Metal Clusters (Springer: Berlin, 1995).

]:

α=4π3ε0(R+d)3(εs1)(εc+2εs)+(RR+d)3(εcεs)(1+2εs)(εs+2)(εc+2εs)+(RR+d)3(εcεs)(2εs1)
(1)

Here εs and εc denote the permittivities of the shell and core materials, respectively. Using Eq. (1), a field-enhancement map was plotted again with R/(R+d) = 1/3 (a value corresponds to our tip model), as shown in Fig. 4(b). The LP mode set of this shell structure is explicit. There is only one resonance mode in the whole spectral range. It shifts to lower frequency (longer wavelength) with increasing the refractive index of the core material. This is exactly in accordance with the case of the Ag-coated dielectric tip shown in Fig. 4(a). Therefore, the core material induced resonance shift is rather a fundamental effect of such metal coated structures, no matter whether a structure is as simple as a spherical particle, or a much more complicated structure like the metal-coated dielectric tip.

Besides the resonance frequency, Raman enhancement is also dictated by the magnitude of the LPs excited on the Ag-coated tips. In Fig. 4(a), three resonance modes, marked by arrows, afford the strongest field enhancement. Mode 1 is the highest one. Unfortunately, it requires a high refractive index of the AFM tip, which is hard to be met by common materials. Mode 2 is also strong. However, the resonance is narrow (<20 nm), and consequently difficult to be matched to a fixed excitation laser line. Besides the difficulty of frequency-matching, a narrow resonance encounters other problems for TERS/SERS, which will be discussed below. Mode 3 is less enhancing comparing to modes 1 and 2, but it has some advantages: (1) the refractive index needed by this mode is around 1.4 – 1.5, which includes the value for glass; (2) its resonance is broad, over 40 nm, which is important for the aforementioned practical reasons.

The width of plasmon resonance is an important, but often neglected issue in area of TERS/SERS. In a large number of papers, the formula ARaman = A(vL)4 is used for estimating the Raman enhancement (ARaman is the enhancement of the Raman signal, and A(vL) denotes the enhancement of excitation field at a frequency vL). However, this is only an approximation of a more correct expression, ARaman = A(vL)2 A(vS)2 [23

23. A. Otto, I. Mrozek, H. Grabhorn, and W. Akemann, “Surface-enhanced Raman-Scattering,” J. Phys.: Condens. Matter 4, 1143–1212 (1992). [CrossRef]

, 24

24. M. Moskovits, “Surface-enhanced spectroscopy,” Rev. Mod. Phys. 57, 783–826 (1985). [CrossRef]

], with the assumption that A(vL)~A(vS) (A(vS) is the electric field enhancement of the scattering light with a Stokes shift of vS). In the field enhancement plot shown in Fig. 4(a), this assumption is not always correct. The mode 2 is a clear example. Its resonance width is less than 20 nm in its most highly enhanced spectral range (500 nm – 650 nm). Considering a light source of 500 nm, the wavelength of a Ramam band with a Stokes shift of 1000 cm-1 (a typical vibrational frequency) will be 526 nm, 26 nm from the laser line. In other words, the assumption A(vL)~A(vS) is no longer correct in this case, and the real Raman enhancement will be much lower than estimated by the formula A(vL)4.

Taking all these considerations into account, the most promising region of the whole field enhancement map for TERS applications is the low refractive index part (mode 3), where the magnitude of the resonance is relatively high and the width of the resonance is broad, covering the spectral range interesting for chemical analysis. This finding is in excellent agreement with what has been observed in the prior publication [17

17. B. S. Yeo, W. H. Zhang, C. Vannier, and R. Zenobi, “Enhancement of Raman signals with silver-coated tips,” Appl. Spectrosc. 60, 1142–1147 (2006). [CrossRef] [PubMed]

].

Fig. 5. Spectral response of the field enhancement for an Ag coated dielectric tip with different conductivities. The refractive index of the tip is 3.48, the conductivities of these two tips are 10-12 S·m-1 and 1 S·m-1.

3.2 Does the conductivity of the AFM influence the tips performance?

3.3 Other considerations

To exclude unwanted shape dependencies, we fixed all the geometric parameters of the Ag-coated tip, such as the vertex angle of the tip, the thickness of the coating, the shape of the tip apex, etc. in our simulation. Nevertheless, it is expected that changing these parameters can modify the resonance behavior of Ag-coated tips to some extent. In fact, a similar effect has recently been observed, and is intensively studied in the context of nanoshell structures [25

25. H. Wang, D. W. Brandl, F. Le, P. Nordlander, and N. J. Halas, “Nanorice: A hybrid plasmonic nanostructure,” Nano Lett. 6, 827–832 (2006). [CrossRef] [PubMed]

].

4. Conclusion

We simulated the spectral features of Ag-coated dielectric tips with different optical properties. We found that the frequency, width and magnitude of the plasmon resonances of an Ag-coated tip can be significantly modified by the refractive index of the tip body. Considering all factors that may influence the TERS performance, an AFM tip with a low permittivity provides the best result.

Acknowledgments

We would like to thank the Electron Microscopy Centre at ETH Zurich (EMEZ), and Dr. Frank Krumeich for the SEM analyses. We would also like to acknowledge the Gebert-Rüf Foundation (grant no. P-085/03) for financial support of this project.

References and links

1.

R. M. Stöckle, Y. D. Suh, V. Deckert, and R. Zenobi, “Nanoscale chemical analysis by tip-enhanced Raman spectroscopy,” Chem. Phys. Lett. 318, 131–136 (2000). [CrossRef]

2.

N. Hayazawa, Y. Inouye, Z. Sekkat, and S. Kawata, “Metallized tip amplification of near-field Raman scattering,” Opt. Commun. 183, 333–336 (2000). [CrossRef]

3.

A. Hartschuh, E. J. Sanchez, X. S. Xie, and L. Novotny, “High-resolution near-field Raman microscopy of single-walled carbon nanotubes,” Phys. Rev. Lett.90, (2003). [CrossRef] [PubMed]

4.

C. C. Neacsu, J. Dreyer, N. Behr, and M. B. Raschke, “Scanning-probe Raman spectroscopy with single-molecule sensitivity,” Phys. Rev. B 73, 193406 (2006). [CrossRef]

5.

K. F. Domke, D. Zhang, and B. Pettinger, “Toward Raman fingerprints of single dye molecules at atomically smooth Au(111),” J. Am. Chem. Soc. 128, 14721–14727 (2006). [CrossRef] [PubMed]

6.

W. Zhang, B. Yeo, S. Thomas, and R. Zenobi, “Single molecule tip-enhanced Raman Spectroscopy with silver tips,” J. Phys. Chem. C 111, 1733–1738 (2007). [CrossRef]

7.

L. Novotny, R. X. Bian, and X. S. Xie, “Theory of nanometric optical tweezers,” Phys. Rev. Lett. 79, 645–648 (1997). [CrossRef]

8.

A. V. Zayats, “Electromagnetic field enhancement in the context of apertureless near-field microscopy,” Opt. Commun. 161, 156–162 (1999). [CrossRef]

9.

H. X. Xu, J. Aizpurua, M. Kall, and P. Apell, “Electromagnetic contributions to single-molecule sensitivity in surface-enhanced Raman scattering,” Phys. Rev. E 62, 4318–4324 (2000). [CrossRef]

10.

R. J. Hamers, “Scanned probe microscopies in chemistry,” J. Phys. Chem. 100, 13103–13120 (1996). [CrossRef]

11.

Y. Saito, T. Murakami, Y. Inouye, and S. Kawata, “Fabrication of silver probes for localized plasmon excitation in near-field Raman spectroscopy,” Chem. Lett. 34, 920–921 (2005). [CrossRef]

12.

T. A. Yano, Y. Inouye, and S. Kawata, “Nanoscale uniaxial pressure effect of a carbon nanotube bundle on tip-enhanced near-field Raman spectra,” Nano Lett. 6, 1269–1273 (2006). [CrossRef] [PubMed]

13.

B. Yeo, T. Schmid, W. Zhang, and R. Zenobi, “Towards rapid nanoscale chemical analysis using tip-enhanced Raman spectroscopy with Ag-coated dielectric tips,” Anal. Bioanal. Chem. 387, 2655–2662 (2007). [CrossRef] [PubMed]

14.

J. B. Jackson and N. J. Halas, “Surface-enhanced Raman scattering on tunable plasmonic nanoparticle substrates,” PNAS 101, 17930–17935 (2004). [CrossRef] [PubMed]

15.

M. D. Malinsky, K. L. Kelly, G. C. Schatz, and R. P. Van Duyne, “Nanosphere lithography: Effect of substrate on the localized surface plasmon resonance spectrum of silver nanoparticles,” J. Phys. Chem. B 105, 2343–2350 (2001). [CrossRef]

16.

K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, “The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment,” J. Phys. Chem. B 107, 668–677 (2003). [CrossRef]

17.

B. S. Yeo, W. H. Zhang, C. Vannier, and R. Zenobi, “Enhancement of Raman signals with silver-coated tips,” Appl. Spectrosc. 60, 1142–1147 (2006). [CrossRef] [PubMed]

18.

C. F. Bohren and D. R. Juffman, Absorption and scattering of light by small particles (John Wiley: New York, 1983).

19.

U. Kreibig and M. Voller, Optical Properties of Metal Clusters (Springer: Berlin, 1995).

20.

R. W. C. P. B. Johnson, “Optical Constants of the Noble Metals,” Phys. Rev. B 6, 4370–4379 (1972). [CrossRef]

21.

http://www.comsol.com.

22.

I. Notingher and A. Elfick, “Effect of sample and substrate electric properties on the electric field enhancement at the Apex of SPM Nanotips,” J. Phys. Chem. B 109, 15699–15706 (2005). [CrossRef]

23.

A. Otto, I. Mrozek, H. Grabhorn, and W. Akemann, “Surface-enhanced Raman-Scattering,” J. Phys.: Condens. Matter 4, 1143–1212 (1992). [CrossRef]

24.

M. Moskovits, “Surface-enhanced spectroscopy,” Rev. Mod. Phys. 57, 783–826 (1985). [CrossRef]

25.

H. Wang, D. W. Brandl, F. Le, P. Nordlander, and N. J. Halas, “Nanorice: A hybrid plasmonic nanostructure,” Nano Lett. 6, 827–832 (2006). [CrossRef] [PubMed]

OCIS Codes
(000.4430) General : Numerical approximation and analysis
(240.6490) Optics at surfaces : Spectroscopy, surface
(300.6450) Spectroscopy : Spectroscopy, Raman

ToC Category:
Optics at Surfaces

History
Original Manuscript: May 11, 2007
Revised Manuscript: June 4, 2007
Manuscript Accepted: June 6, 2007
Published: June 18, 2007

Citation
Xudong Cui, Weihua Zhang, Boon-Siang Yeo, Renato Zenobi, Christian Hafner, and Daniel Erni, "Tuning the resonance frequency of Ag-coated dielectric tips," Opt. Express 15, 8309-8316 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-13-8309


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References

  1. R. M. Stöckle, Y. D. Suh, V. Deckert, and R. Zenobi, "Nanoscale chemical analysis by tip-enhanced Raman spectroscopy," Chem. Phys. Lett. 318, 131-136 (2000). [CrossRef]
  2. N. Hayazawa, Y. Inouye, Z. Sekkat, and S. Kawata, "Metallized tip amplification of near-field Raman scattering," Opt. Commun. 183, 333-336 (2000). [CrossRef]
  3. A. Hartschuh, E. J. Sanchez, X. S. Xie, and L. Novotny, "High-resolution near-field Raman microscopy of single-walled carbon nanotubes," Phys. Rev. Lett. 90, (2003). [CrossRef] [PubMed]
  4. C. C. Neacsu, J. Dreyer, N. Behr, and M. B. Raschke, "Scanning-probe Raman spectroscopy with single-molecule sensitivity," Phys. Rev. B 73, 193406 (2006). [CrossRef]
  5. K. F. Domke, D. Zhang, and B. Pettinger, "Toward Raman fingerprints of single dye molecules at atomically smooth Au(111)," J. Am. Chem. Soc. 128, 14721-14727 (2006). [CrossRef] [PubMed]
  6. W. Zhang, B. Yeo, S. Thomas, and R. Zenobi, "Single molecule tip-enhanced Raman Spectroscopy with silver tips," J. Phys. Chem. C 111, 1733-1738 (2007). [CrossRef]
  7. L. Novotny, R. X. Bian, and X. S. Xie, "Theory of nanometric optical tweezers," Phys. Rev. Lett. 79, 645-648 (1997). [CrossRef]
  8. A. V. Zayats, "Electromagnetic field enhancement in the context of apertureless near-field microscopy," Opt. Commun. 161, 156-162 (1999). [CrossRef]
  9. H. X. Xu, J. Aizpurua, M. Kall, and P. Apell, "Electromagnetic contributions to single-molecule sensitivity in surface-enhanced Raman scattering," Phys. Rev. E 62, 4318-4324 (2000). [CrossRef]
  10. R. J. Hamers, "Scanned probe microscopies in chemistry," J. Phys. Chem. 100, 13103-13120 (1996). [CrossRef]
  11. Y. Saito, T. Murakami, Y. Inouye, and S. Kawata, "Fabrication of silver probes for localized plasmon excitation in near-field Raman spectroscopy," Chem. Lett. 34, 920-921 (2005). [CrossRef]
  12. T. A. Yano, Y. Inouye, and S. Kawata, "Nanoscale uniaxial pressure effect of a carbon nanotube bundle on tip-enhanced near-field Raman spectra," Nano Lett. 6, 1269-1273 (2006). [CrossRef] [PubMed]
  13. B. Yeo, T. Schmid, W. Zhang, and R. Zenobi, "Towards rapid nanoscale chemical analysis using tip-enhanced Raman spectroscopy with Ag-coated dielectric tips," Anal. Bioanal. Chem. 387, 2655-2662 (2007). [CrossRef] [PubMed]
  14. J. B. Jackson, and N. J. Halas, "Surface-enhanced Raman scattering on tunable plasmonic nanoparticle substrates," PNAS 101, 17930-17935 (2004). [CrossRef] [PubMed]
  15. M. D. Malinsky, K. L. Kelly, G. C. Schatz, and R. P. Van Duyne, "Nanosphere lithography: Effect of substrate on the localized surface plasmon resonance spectrum of silver nanoparticles," J. Phys. Chem. B 105, 2343-2350 (2001). [CrossRef]
  16. K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, "The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment," J. Phys. Chem. B 107, 668-677 (2003). [CrossRef]
  17. B. S. Yeo, W. H. Zhang, C. Vannier, and R. Zenobi, "Enhancement of Raman signals with silver-coated tips," Appl. Spectrosc. 60, 1142-1147 (2006). [CrossRef] [PubMed]
  18. C. F. Bohren, and D. R. Juffman, Absorption and scattering of light by small particles (John Wiley: New York, 1983).
  19. U. Kreibig, and M. Voller, Optical Properties of Metal Clusters (Springer: Berlin, 1995).
  20. R. W. C. P. B. Johnson, "Optical Constants of the Noble Metals," Phys. Rev. B 6, 4370-4379 (1972). [CrossRef]
  21. http://www.comsol.com.
  22. I. Notingher, and A. Elfick, "Effect of sample and substrate electric properties on the electric field enhancement at the Apex of SPM Nanotips," J. Phys. Chem. B 109, 15699-15706 (2005). [CrossRef]
  23. A. Otto, I. Mrozek, H. Grabhorn, and W. Akemann, "Surface-enhanced Raman-Scattering," J. Phys.: Condens. Matter 4, 1143-1212 (1992). [CrossRef]
  24. M. Moskovits, "Surface-enhanced spectroscopy," Rev. Mod. Phys. 57, 783-826 (1985). [CrossRef]
  25. H. Wang, D. W. Brandl, F. Le, P. Nordlander, and N. J. Halas, "Nanorice: A hybrid plasmonic nanostructure," Nano Lett. 6, 827-832 (2006). [CrossRef] [PubMed]

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