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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 8 — Apr. 11, 2011
  • pp: 7274–7279
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Electric field enhancements around the nanorod on the base layer

Zhongyue Zhang, Zhidong Zhang, Lijie Zhang, Chengzhi Huang, and Zuhong Xiong  »View Author Affiliations


Optics Express, Vol. 19, Issue 8, pp. 7274-7279 (2011)
http://dx.doi.org/10.1364/OE.19.007274


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Abstract

Electric field (E field) distributions of the silver rod-film nanostructures are calculated by the finite difference time domain method and compared with those of the individual nanorods. For the rod-film nanostructure, the incident waves are reflected back by the base layer and the superposition of the E fields of the incident wave and the reflection wave works as the excitation for the transverse mode electron oscillations in the nanorod, which results in the much enhanced E fields around the lateral surface of the nanorod. In addition, we investigate how the structural parameters of the rod-film nanostructure affect the E fields along the nanorod. These results would be much helpful for designing larger intensity surface enhanced Raman scattering substrates.

© 2011 OSA

1. Introduction

The collective oscillation of the electrons within noble metal nanostructures enables the resonant excitation of light at a particular wavelength, which is called localized surface plasmon resonance (LSPR). At the resonant wavelength, enhanced local electric fields (E fields) occur at the surface of metal nanostructures. These enhanced E fields contribute to the major part of the surface enhanced Raman scattering (SERS) intensity [1

1. T. Vo-Dinh, “Surface-enhanced Raman spectroscopy using metallic nanostructures,” Trends Analyt. Chem. 17(8-9), 557–582 (1998). [CrossRef]

3

3. A. Campion and P. Kambhampati, “Surface enhanced Raman scattering,” Chem. Soc. Rev. 27(4), 241–250 (1998). [CrossRef]

]. The resonant wavelengths depend strongly on the topological shapes of the metal nanostructures and their surrounding environments [4

4. 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(3), 668–677 (2003). [CrossRef]

12

12. X.-M. Lin, Y. Cui, Y.-H. Xu, B. Ren, and Z.-Q. Tian, “Surface-enhanced Raman spectroscopy: substrate-related issues,” Anal. Bioanal. Chem. 394(7), 1729–1745 (2009). [CrossRef] [PubMed]

]. Usually, the metal nanostructures are put on the dielectric substrates [4

4. 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(3), 668–677 (2003). [CrossRef]

10

10. Z.-J. Yang, N.-C. Kim, J.-B. Li, M.-T. Cheng, S.-D. Liu, Z.-H. Hao, and Q.-Q. Wang, “Surface plasmons amplifications in single Ag nanoring,” Opt. Express 18(5), 4006–4011 (2010). [CrossRef] [PubMed]

]. The plasmonic properties of the metal nanostructures on the metal base layers have not been widely investigated.

Recently, aligned silver nanorod arrays have been prepared to obtain large SERS enhancements for biological or chemical detections [13

13. S. B. Chaney, S. Shanmukh, R. A. Dluhy, and Y.-P. Zhao, “Aligned silver nanorod arrays produce high sensitivity surface-enhanced Raman spectroscopy substrates,” Appl. Phys. Lett. 87(3), 031908–031910 (2005). [CrossRef]

16

16. H. V. Chu, Y.-J. Liu, Y.-W. Huang, and Y.-P. Zhao, “A high sensitive fiber SERS probe based on silver nanorod arrays,” Opt. Express 15(19), 12230–12239 (2007). [CrossRef] [PubMed]

]. For this SERS substrate, a base layer of silver film is first deposited onto the glass slides, and then silver nanorod arrays are deposited on the top of the silver film by the oblique angle deposition (OAD) method. Driskell et al. compared the SERS intensity from the rod-film substrate with that from the rod array on bare glass slides [15

15. J. D. Driskell, S. Shanmukh, Y.-J. Liu, S. B. Chaney, X.-J. Tang, Y.-P. Zhao, and R. A. Dluhy, “The use of aligned silver nanorod arrays prepared by oblique angle deposition as surface enhanced Raman scattering substrates,” J. Phys. Chem. C 112, 895–901 (2008). [CrossRef]

]. The results show that the absolute Raman intensity of trans-1,2-bis(4-pyridyl)ethane (BPE) from the rod-film substrate is three orders of magnitude stronger than that from the nanorod array without film. Although Zhou et al. found that the SERS intensity from the rod-film substrate increases linearly with the base layer reflectivity, the origin of the larger SERS intensity is not clearly known [17

17. Q. Zhou, Y.-J. Liu, Y.-P. He, Z.-J. Zhang, and Y.-P. Zhao, “The effect of underlayer thin films on the surface-enhanced Raman scattering response of Ag nanorod substrates,” Appl. Phys. Lett. 97(12), 121902 (2010). [CrossRef]

]. Because the enhanced E fields contribute to the major part of SERS intensity, there could be stronger E fields around the nanorods in the rod-film substrates than those around the individual nanorods.

When E fields interact with the metal film covered by a dielectric medium, the surface plasmon polariton (SPP) waves are excited and then propagate along the surface of the metal film [18

18. H. Raether, Surface Plasmons (Springer, Berlin, 1988).

]. For the rod-film nanostructure, the SPP waves could launch to the nanorod and subsequently to the top of the nanorod [19

19. A. Normatov, P. Ginzburg, N. Berkovitch, G. M. Lerman, A. Yanai, U. Levy, and M. Orenstein, “Efficient coupling and field enhancement for the nano-scale: plasmonic needle,” Opt. Express 18(13), 14079–14086 (2010). [CrossRef] [PubMed]

]. Do the SPP waves propagating along the nanorod contribute to the much enhanced E fields around the nanorods?

To prove the existence of the much enhanced E fields around the nanorods in the rod-film nanostructures and study the origin of these E fields, the E field distributions of the rod-film nanostructures are calculated by the finite difference time domain (FDTD) method and compared with those of the individual nanorods. The results show that the E fields around the nanorods in the rod-film nanostructures are much larger than those around the individual nanorods. These E fields are not mainly due to the SPP waves propagating along the nanorod but the superposition of the E fields of the incident wave and the reflection wave. In addition, we investigate how the height and the radius of the nanorod as well as the thickness of the base layer affect the E fields around the nanorod in the rod-film nanostructure. These results are of great relevance to design SERS substrates to obtain larger SERS intensities.

2. Simulation and method

Figure 1
Fig. 1 Schematics for the incident direction and the rod-film nanostructure.
illustrates the geometry of the rod-film nanostructure and the polarization configuration of the incident plane wave used in the calculations. For the rod-film nanostructure, the nanorod locates at the center of the rectangular film. The length (along x-axis) and the width (along y-axis) of the film are fixed at 4000 nm and 800 nm, respectively. The thickness (along z-axis) of the film is defined as t. The nanorod has a height of H and a radius of r. To compare the E fields around the nanorods in the rod-film nanostructures with those around the individual nanorods, the E field distributions of individual nanorods are also calculated. In all the calculations, plane waves are incident along –z direction with a horizontal polarization (along x-axis) as shown in Fig. 1. The wavelength of the excitation is fixed at λ = 785 nm. The cell size used in FDTD calculations is 5 nm. The permittivity of silver is given by the modified Debye model [21

21. H.-F. Gai, J. Wang, and Q. Tian, “Modified Debye model parameters of metals applicable for broadband calculations,” Appl. Opt. 46(12), 2229–2233 (2007). [CrossRef] [PubMed]

].

3. Results and discussion

Figure 2(a)
Fig. 2 (Color online) E field distributions of individual nanorod and rod-film nanostructure: (a) E 2 of the individual nanorod, (b) E 2 of the rod-film nanostructure, (c) E z 2 of the rod-film nanostructure, and (d) E x 2 of the rod-film nanostructure.
illustrates the E field distribution of the nanorod with a height of 600 nm and a radius of 40 nm. Light is incident along the long axis with a polarization perpendicular to the long axis. As shown in Fig. 2(a), Enhanced E fields occur around the lateral surface of the nanorod. These E fields are due to the transverse mode electron oscillations in the nanorod [5

5. Z.-Y. Zhang and Y.-P. Zhao, “Extinction spectra and electrical field enhancement of Ag nanorods with different topologic shapes,” J. Appl. Phys. 102(11), 113308 (2007). [CrossRef]

]. Figure 2(b) illustrates the E field distribution of the rod-film nanostructure for x-z plane at y = 0. For the rod-film nanostructure, the thickness of the base layer t is 80 nm and the nanorod has the same parameters as those in Fig. 2(a). As shown in Fig. 2(b), the E fields occur at the lateral surface and congregate to the top and 1/3 of the way from the bottom of the nanorod. The magnitude of the E fields of the rod-film nanostructure is much larger than that of the E fields of the individual nanorod. Therefore, the rod-film nanostructures, when used as SERS substrates, are more capable of capturing and detecting small amount of molecules.

In the rod-film nanostructures, when SPP waves propagate along the nanorod, the major part of the E fields would occur in z-axis direction. Therefore, we plot the E field distributions of E z and E x components. As shown in Fig. 2(c), the E fields of E z component congregate to the top of the nanorod. Therefore, only the enhanced E fields at the top of the nanorod are due to the SPP waves propagating along the nanorod. As shown in Fig. 2(d), the E fields of E x component has a similar distribution and magnitude as those in Fig. 2(b). The much enhanced E fields appear around the top and 1/3 of the way from the bottom of the nanorod. Therefore, the much enhance E fields in Fig. 2(b) are not a result of the SPP waves propagating along the nanorod. When light is incident along the individual nanorod with the polarization perpendicular to the long axis (transverse mode excitation), strong E fields occur around the lateral surface of the nanorod [5

5. Z.-Y. Zhang and Y.-P. Zhao, “Extinction spectra and electrical field enhancement of Ag nanorods with different topologic shapes,” J. Appl. Phys. 102(11), 113308 (2007). [CrossRef]

]. Therefore, the much enhanced E fields in Fig. 2(b) are due to the transverse mode electron oscillations in the nanorod. In order to clearly illustrate the E field distributions along the nanorods, the averages of E 2, < E 2>, around the nanorods are calculated. Figure 3
Fig. 3 (Color online) E field distributions (E 2) along the individual nanorod and the nanorod in the rod-film nanostructure.
illustrates the E field distributions along the nanorod as a function of z. z denotes the distance to the bottom of nanorod. Then z = H denotes the top of the nanorod. As shown in Fig. 3, for the individual nanorod, E fields almost evenly distribute around the lateral surface except at the two ends. For the rod-film nanostructure, the maximums of the E fields appear around z = 187 nm and z = 600 nm. Although the E fields do not evenly distribute along the nanorod, the rod-film nanostructure has more hot spots than does the individual nanorod.

In order to investigate how the radius r of the nanorod affects the E fields around the nanorod, r is increased from r = 25 nm to r = 80 nm with fixed t = 80 nm and H = 600 nm. Figure 5
Fig. 5 (Color online) E field distributions (E 2) along the nanorods in the rod-film nanostructures with different nanorod radius r.
shows the E fields along the nanorods with r = 30, 40, 50, 60, and 70 nm, respectively. For different r, the maximum E fields also occur at the positions around z = 187 nm and z = 600 nm. However, the magnitude of the maximum E fields increase with the increasing r. We performed FDTD calculations of the extinction spectra of individual nanorods with H = 600 nm. The results show that when r increases from 30 nm to 70 nm, the transverse dipole mode plasmon peak red shift from 417 nm to 562 nm, which is approaching the incident wavelength in this paper and thereby results in the increasing E fields in Fig. 5.

4. Conclusions

In this paper, the optical properties of the rod-film nanostructures are calculated by the FDTD method to prove that the much enhanced E fields around the nanorod do exist and study the origin of the much enhanced E fields. The results show that the E fields around the nanorod in the rod-film nanostructures are much enhanced compared with those around the individual nanorod. The enhanced E fields are not mainly due to the SPP waves propagating along the nanorod but the superposition of the E fields of the incident wave and the reflection wave which works as the excitation for the transverse mode electron oscillations in the nanorod. The E field distributions around the nanorod do not depend obviously on the thickness of the base layer. However, they depend strongly on the structural parameters of the nanorod. These results would be much helpful for designing SERS substrates to obtain larger SERS intensities.

Acknowledgements

This work was supported by National Natural Foundation of China (Grant Nos. 11004160 and 10974157), the Natural Science Foundation of CQ CSTC (Grant No. CSTC2010BB4005), the Fundamental Research Funds for the Central Universities (Grant Nos. XDJK2009C078 and XDJK2009A001), and the Southwest University Research Foundation (Grant No. SWU109024).

References and links

1.

T. Vo-Dinh, “Surface-enhanced Raman spectroscopy using metallic nanostructures,” Trends Analyt. Chem. 17(8-9), 557–582 (1998). [CrossRef]

2.

Z.-Q. Tian, B. Ren, and D.-Y. Wu, “Surface-enhanced Raman scattering: from noble to transition metals and from rough surface to ordered nanostructures,” J. Phys. Chem. B 106(37), 9463–9483 (2002). [CrossRef]

3.

A. Campion and P. Kambhampati, “Surface enhanced Raman scattering,” Chem. Soc. Rev. 27(4), 241–250 (1998). [CrossRef]

4.

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(3), 668–677 (2003). [CrossRef]

5.

Z.-Y. Zhang and Y.-P. Zhao, “Extinction spectra and electrical field enhancement of Ag nanorods with different topologic shapes,” J. Appl. Phys. 102(11), 113308 (2007). [CrossRef]

6.

J. J. Mock, D. R. Smith, and S. Schultz, “Local refractive index dependence of plasmon resonance spectra from individual nanoparticles,” Nano Lett. 3(4), 485–491 (2003). [CrossRef]

7.

H.-Y. Lin, C.-H. Huang, C.-H. Chang, Y.-C. Lan, and H.-C. Chui, “Direct near-field optical imaging of plasmonic resonances in metal nanoparticle pairs,” Opt. Express 18(1), 165–172 (2010). [CrossRef] [PubMed]

8.

S. Iyer, S. Popov, and A. T. Friberg, “Transmission resonances in periodic U-shaped metallic nanostructures,” Opt. Express 18(17), 17719–17728 (2010). [CrossRef] [PubMed]

9.

H.-C. Tseng and C.-W. Chang, “High displacement sensitivity in asymmetric plasmonic nanostructures,” Opt. Express 18(17), 18360–18367 (2010). [CrossRef] [PubMed]

10.

Z.-J. Yang, N.-C. Kim, J.-B. Li, M.-T. Cheng, S.-D. Liu, Z.-H. Hao, and Q.-Q. Wang, “Surface plasmons amplifications in single Ag nanoring,” Opt. Express 18(5), 4006–4011 (2010). [CrossRef] [PubMed]

11.

G. Xu, M. Tazawa, P. Jin, S. Nakao, and K. Yoshimura, “Wavelength tuning of surface plasmon resonance using dielectric layers on silver island films,” Appl. Phys. Lett. 82(22), 3811–3813 (2003). [CrossRef]

12.

X.-M. Lin, Y. Cui, Y.-H. Xu, B. Ren, and Z.-Q. Tian, “Surface-enhanced Raman spectroscopy: substrate-related issues,” Anal. Bioanal. Chem. 394(7), 1729–1745 (2009). [CrossRef] [PubMed]

13.

S. B. Chaney, S. Shanmukh, R. A. Dluhy, and Y.-P. Zhao, “Aligned silver nanorod arrays produce high sensitivity surface-enhanced Raman spectroscopy substrates,” Appl. Phys. Lett. 87(3), 031908–031910 (2005). [CrossRef]

14.

Y.-J. Liu, J.-G. Fan, Y.-P. Zhao, S. Shanmukh, and R. A. Dluhy, “Angle dependent surface enhanced Raman scattering obtained from a Ag nanorod array substrate,” Appl. Phys. Lett. 89(17), 173134 (2006). [CrossRef]

15.

J. D. Driskell, S. Shanmukh, Y.-J. Liu, S. B. Chaney, X.-J. Tang, Y.-P. Zhao, and R. A. Dluhy, “The use of aligned silver nanorod arrays prepared by oblique angle deposition as surface enhanced Raman scattering substrates,” J. Phys. Chem. C 112, 895–901 (2008). [CrossRef]

16.

H. V. Chu, Y.-J. Liu, Y.-W. Huang, and Y.-P. Zhao, “A high sensitive fiber SERS probe based on silver nanorod arrays,” Opt. Express 15(19), 12230–12239 (2007). [CrossRef] [PubMed]

17.

Q. Zhou, Y.-J. Liu, Y.-P. He, Z.-J. Zhang, and Y.-P. Zhao, “The effect of underlayer thin films on the surface-enhanced Raman scattering response of Ag nanorod substrates,” Appl. Phys. Lett. 97(12), 121902 (2010). [CrossRef]

18.

H. Raether, Surface Plasmons (Springer, Berlin, 1988).

19.

A. Normatov, P. Ginzburg, N. Berkovitch, G. M. Lerman, A. Yanai, U. Levy, and M. Orenstein, “Efficient coupling and field enhancement for the nano-scale: plasmonic needle,” Opt. Express 18(13), 14079–14086 (2010). [CrossRef] [PubMed]

20.

K. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans. Antenn. Propag. 14(3), 302–307 (1966). [CrossRef]

21.

H.-F. Gai, J. Wang, and Q. Tian, “Modified Debye model parameters of metals applicable for broadband calculations,” Appl. Opt. 46(12), 2229–2233 (2007). [CrossRef] [PubMed]

OCIS Codes
(160.4760) Materials : Optical properties
(240.6680) Optics at surfaces : Surface plasmons
(260.5740) Physical optics : Resonance

ToC Category:
Optics at Surfaces

History
Original Manuscript: March 8, 2011
Revised Manuscript: March 23, 2011
Manuscript Accepted: March 24, 2011
Published: March 31, 2011

Citation
Zhongyue Zhang, Zhidong Zhang, Lijie Zhang, Chengzhi Huang, and Zuhong Xiong, "Electric field enhancements around the nanorod on the base layer," Opt. Express 19, 7274-7279 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-8-7274


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References

  1. T. Vo-Dinh, “Surface-enhanced Raman spectroscopy using metallic nanostructures,” Trends Analyt. Chem. 17(8-9), 557–582 (1998). [CrossRef]
  2. Z.-Q. Tian, B. Ren, and D.-Y. Wu, “Surface-enhanced Raman scattering: from noble to transition metals and from rough surface to ordered nanostructures,” J. Phys. Chem. B 106(37), 9463–9483 (2002). [CrossRef]
  3. A. Campion and P. Kambhampati, “Surface enhanced Raman scattering,” Chem. Soc. Rev. 27(4), 241–250 (1998). [CrossRef]
  4. 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(3), 668–677 (2003). [CrossRef]
  5. Z.-Y. Zhang and Y.-P. Zhao, “Extinction spectra and electrical field enhancement of Ag nanorods with different topologic shapes,” J. Appl. Phys. 102(11), 113308 (2007). [CrossRef]
  6. J. J. Mock, D. R. Smith, and S. Schultz, “Local refractive index dependence of plasmon resonance spectra from individual nanoparticles,” Nano Lett. 3(4), 485–491 (2003). [CrossRef]
  7. H.-Y. Lin, C.-H. Huang, C.-H. Chang, Y.-C. Lan, and H.-C. Chui, “Direct near-field optical imaging of plasmonic resonances in metal nanoparticle pairs,” Opt. Express 18(1), 165–172 (2010). [CrossRef] [PubMed]
  8. S. Iyer, S. Popov, and A. T. Friberg, “Transmission resonances in periodic U-shaped metallic nanostructures,” Opt. Express 18(17), 17719–17728 (2010). [CrossRef] [PubMed]
  9. H.-C. Tseng and C.-W. Chang, “High displacement sensitivity in asymmetric plasmonic nanostructures,” Opt. Express 18(17), 18360–18367 (2010). [CrossRef] [PubMed]
  10. Z.-J. Yang, N.-C. Kim, J.-B. Li, M.-T. Cheng, S.-D. Liu, Z.-H. Hao, and Q.-Q. Wang, “Surface plasmons amplifications in single Ag nanoring,” Opt. Express 18(5), 4006–4011 (2010). [CrossRef] [PubMed]
  11. G. Xu, M. Tazawa, P. Jin, S. Nakao, and K. Yoshimura, “Wavelength tuning of surface plasmon resonance using dielectric layers on silver island films,” Appl. Phys. Lett. 82(22), 3811–3813 (2003). [CrossRef]
  12. X.-M. Lin, Y. Cui, Y.-H. Xu, B. Ren, and Z.-Q. Tian, “Surface-enhanced Raman spectroscopy: substrate-related issues,” Anal. Bioanal. Chem. 394(7), 1729–1745 (2009). [CrossRef] [PubMed]
  13. S. B. Chaney, S. Shanmukh, R. A. Dluhy, and Y.-P. Zhao, “Aligned silver nanorod arrays produce high sensitivity surface-enhanced Raman spectroscopy substrates,” Appl. Phys. Lett. 87(3), 031908–031910 (2005). [CrossRef]
  14. Y.-J. Liu, J.-G. Fan, Y.-P. Zhao, S. Shanmukh, and R. A. Dluhy, “Angle dependent surface enhanced Raman scattering obtained from a Ag nanorod array substrate,” Appl. Phys. Lett. 89(17), 173134 (2006). [CrossRef]
  15. J. D. Driskell, S. Shanmukh, Y.-J. Liu, S. B. Chaney, X.-J. Tang, Y.-P. Zhao, and R. A. Dluhy, “The use of aligned silver nanorod arrays prepared by oblique angle deposition as surface enhanced Raman scattering substrates,” J. Phys. Chem. C 112, 895–901 (2008). [CrossRef]
  16. H. V. Chu, Y.-J. Liu, Y.-W. Huang, and Y.-P. Zhao, “A high sensitive fiber SERS probe based on silver nanorod arrays,” Opt. Express 15(19), 12230–12239 (2007). [CrossRef] [PubMed]
  17. Q. Zhou, Y.-J. Liu, Y.-P. He, Z.-J. Zhang, and Y.-P. Zhao, “The effect of underlayer thin films on the surface-enhanced Raman scattering response of Ag nanorod substrates,” Appl. Phys. Lett. 97(12), 121902 (2010). [CrossRef]
  18. H. Raether, Surface Plasmons (Springer, Berlin, 1988).
  19. A. Normatov, P. Ginzburg, N. Berkovitch, G. M. Lerman, A. Yanai, U. Levy, and M. Orenstein, “Efficient coupling and field enhancement for the nano-scale: plasmonic needle,” Opt. Express 18(13), 14079–14086 (2010). [CrossRef] [PubMed]
  20. K. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans. Antenn. Propag. 14(3), 302–307 (1966). [CrossRef]
  21. H.-F. Gai, J. Wang, and Q. Tian, “Modified Debye model parameters of metals applicable for broadband calculations,” Appl. Opt. 46(12), 2229–2233 (2007). [CrossRef] [PubMed]

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