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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 27 — Dec. 19, 2011
  • pp: 26056–26064
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Large-area plasmonic hot-spot arrays: sub-2 nm interparticle separations with plasma-enhanced atomic layer deposition of Ag on periodic arrays of Si nanopillars

Joshua D. Caldwell, Orest J. Glembocki, Francisco J. Bezares, Maarit I. Kariniemi, Jaakko T. Niinistö, Timo T. Hatanpää, Ronald W. Rendell, Maraizu Ukaegbu, Mikko K. Ritala, Sharka M. Prokes, Charles M. Hosten, Markku A. Leskelä, and Richard Kasica  »View Author Affiliations


Optics Express, Vol. 19, Issue 27, pp. 26056-26064 (2011)
http://dx.doi.org/10.1364/OE.19.026056


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Abstract

Initial reports of plasmonic ‘hot-spots’ enabled the detection of single molecules via surface-enhanced Raman scattering (SERS) from random distributions of plasmonic nanoparticles. Investigations of systems with near-field plasmonically coupled nanoparticles began, however, the ability to fabricate reproducible arrays of such particles has been lacking. We report on the fabrication of large-area, periodic arrays of plasmonic 'hot-spots' using Ag atomic layer deposition to overcoat Si nanopillar templates leading to reproducible interpillar gaps down to <2 nm. These plasmonic 'hot-spots' arrays exhibited over an order of magnitude increase in the SERS response in comparison to similar arrays with larger interpillar separations.

© 2011 OSA

1. Introduction

2. Experimental

2.1 Sample preparation

2.2 SERS measurements

In order to monitor the effective EM field intensities within these arrays, SERS measurements from the SAM of thiophenol on the Ag PEALD surface of these nanopillar arrays was carried out. SERS measurements were performed using the 532, 633 and 785 nm modules of a DeltaNu ExamineR μ-Raman system. Acquisition times used depended upon the array being measured and the incident wavelength, but they ranged from 0.1 to 2 s, with laser powers ranging from 2.2 to 8.2 mW. The neat Raman spectra were collected using the liquid sample cell on the microscope at the same power and several acquisition times to ensure reproducibility. All enhancement factors were calculated via the method outlined in Ref. [21

21. J. D. Caldwell, O. J. Glembocki, F. J. Bezares, N. D. Bassim, R. W. Rendell, M. Feygelson, M. Ukaegbu, R. Kasica, L. Shirey, and C. Hosten, “Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors,” ACS Nano 5(5), 4046–4055 (2011). [CrossRef] [PubMed]

] using the intensity of the 998 cm−1 C-H wagging vibrational mode of the thiophenol molecule (marked by black arrow in Fig. 3
Fig. 3 Neat Raman spectra of thiophenol (black trace), SERS spectra collected from the Ag PEALD film without nanopillars (green trace), and on arrays of ~200 nm diameter nanopillars with interpillar gaps of 198 (blue trace), 52 (light-blue trace) and <2 nm (red trace) gaps. Each spectra was normalized to account for both the incident laser power and corresponding acquisition time, while the corresponding number of molecules probed in each measurement is shown in the legend. The arrow in the figure denotes the position of the 998 cm−1 mode (C-H wag) used in the enhancement factor calculations and in the SERS spatial plots presented in Fig. 4. Inset: Semi-logarithmic plot comparing the SERS spectra from the <2nm gap arrays and the neat spectra.
). This mode was chosen because the C-H groups are spatially dislocated from the sulfur, which bonds to the Ag surface, thereby ensuring that any change in molecular polarizability induced due to the bonding, would have minimal impact upon the calculated enhancement [21

21. J. D. Caldwell, O. J. Glembocki, F. J. Bezares, N. D. Bassim, R. W. Rendell, M. Feygelson, M. Ukaegbu, R. Kasica, L. Shirey, and C. Hosten, “Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors,” ACS Nano 5(5), 4046–4055 (2011). [CrossRef] [PubMed]

,26

26. K. T. Carron and L. G. Hurley, “Axial and azimuthal angle determination with surface-enhanced Raman spectroscopy - thiophenol on copper, silver and gold metal-surfaces,” J. Phys. Chem. 95(24), 9979–9984 (1991). [CrossRef]

,27

27. S. Li, D. Wu, X. Xu, and R. Gu, “Theoretical and experimental studies on the adsorption behavior of thiophenol on gold nanoparticles,” J. Raman Spectrosc. 38(11), 1436–1443 (2007). [CrossRef]

].

3. Results and discussion

In an effort to quantify the impact that plasmonic coupling would have within such a large-area periodic structure, COMSOL calculations of the predicted SERS enhancement at the center of a semi-infinite array of 150 nm diameter Si nanopillars over-coated with a conformal and continuous 45 nm thick Ag film were carried out and are presented as a function of interpillar gap in Fig. 4(b). The values presented are all normalized to the calculated SERS enhancement (E4) for the array featuring the widest interparticle gap (210 nm). This normalization is provided to clearly illustrate the role of the near-field coupling effects. Similar to the experimental results, the simulations indicate relatively little change in the SERS enhancement as the interpillar gap is reduced. Upon decreasing the gap below 50 nm, a dramatic increase in the SERS enhancement is observed. This increase continues as the gap is reduced further down to 2 nm, which was the tightest gap simulated. Plotted along with the simulations is the corresponding, normalized experimental data collected from the arrays with 279 nm diameter nanopillars (post-Ag deposition). As shown, the qualitative agreement between the generalized behavior of the experimental data with the simulations illustrates the fact that the SERS enhancement reported from the arrays with the tightest gaps is clearly due to the large collective EM fields that are induced due to near-field coupling from neighboring nanopillars. However, in contrast to the experiments, almost three orders of magnitude increase in the SERS enhancement is predicted from the simulations, although it is likely that this discrepancy is due to the inability of the simulations to correctly model the complex Ag PEALD morphology and/or due to capillary effects limiting thiophenol from completely coating the Ag nanopillar walls within the arrays with the tightest interpillar gaps.

Another key property of interparticle plasmonic coupling is the red-shift in the peak of the LSPR in addition to the increase in the amplitude discussed previously [3

3. M. I. Stockman, L. N. Pandey, and T. F. George, “Inhomogeneous localization of polar eigenmodes in fractals,” Phys. Rev. B Condens. Matter 53(5), 2183–2186 (1996). [CrossRef] [PubMed]

,4

4. J. P. Kottmann and O. J. F. Martin, “Plasmon resonant coupling in metallic nanowires,” Opt. Express 8(12), 655–663 (2001). [CrossRef] [PubMed]

,8

8. For a recent review seeN. J. Halas, S. Lal, W.-S. Chang, S. Link, and P. Nordlander, “Plasmons in strongly coupled metallic nanostructures,” Chem. Rev. 111(6), 3913–3961 (2011). [CrossRef] [PubMed]

]. In order to maintain a constant resonant wavelength that is required to match the incident laser line for the optimal SERS response, any coupling-induced red-shift requires a compensating blue shift. Such a blue-shift can be attained through shrinking the nanopillar diameter. Thus, one would anticipate that at the onset of near-field plasmonic coupling that the optimal SERS response would shift to a smaller nanopillar diameter. This is observed in Figs. 5(a) and (b)
Fig. 5 Spatial plots of the SERS intensity measured as a function of nanopillar diameter and interpillar gap at (a) 532 and (b) 785 nm incident. The values plotted correspond to the average SERS intensity of the C-H wag mode of thiophenol (998 cm−1) from a given array after being normalized to account for the laser power and acquisition time. All values are presented in units of countsW−1s−1.
where spatial plots of the SERS intensity as a function of both post-Ag diameter and interpillar gaps are presented for 532 and 785 nm incident light, respectively, (633 nm incident measurements were omitted for brevity). In these plots, each pixel corresponds to a given 100x100 nanopillar array, with the interpolation leading to the smoothed plots provided through Microcal Origin. Two very different dependences are observed at these two wavelengths. For the 785 nm incident measurements, the largest enhancements are found at a Ag-coated nanopillar diameter between 326 and 353 nm for arrays where the interpillar gap was larger than 35 nm, however, as the gap was reduced into the realm where near-field plasmonic coupling was observed, this optimal SERS enhancement shifted towards smaller nanopillar diameters, consistent with a plasmonic-coupling induced red-shift. On the contrary, at 532 nm incident, in all cases the most intense SERS response was observed at the tightest gaps, as illustrated in Fig. 4(a) and no distinct diameter dependence is identified. Thus, from the two spatial plots presented in Figs. 5(a) and (b) it can be ascertained that at 532 nm, the LSP resonance occurs at a diameter outside of the range explored here, and therefore any red-shift in the LSPR due to near-field plasmonic coupling would have minimal impact on the detected SERS response and only the increase in the overall amplitude would be observed. Further, as the enhancements from the off-resonant 532 nm measurements approached that of the on-resonant 785 nm values, it is anticipated that careful optimization for an on-resonant structure within the near-field coupling regime, would lead to significantly larger enhancements within the green region of the EM spectrum.

4. Conclusion

In summary, we report on the application of the plasma-enhanced atomic layer deposition (PEALD) of Ag on silicon nanopillar templates to achieve large-area, periodic ‘hot-spot’ arrays featuring tightly spaced (<2 nm) near-field coupled plasmonic nanopillars. This technique has enabled the fabrication of structures up to 60 μm on a side with interparticle gaps ranging from 196 down to < 2 nm. Using surface-enhanced Raman scattering (SERS), we have illustrated that the plasmonic coupling between these tightly-spaced plasmonic nanopillars leads to over an order of magnitude increase in the SERS intensity in comparison to similar nanostructures with interpillar gaps beyond the near-field coupling regime (>20nm). These measurements have shown average enhancement factors up to 2x108, 7.4x107 and 1.7x108 from the best performing near-field coupled arrays at 532, 633 and 785 nm, respectively. To the best of our knowledge, this is the first demonstration of periodic arrays of plasmonic nanoparticles with consistent interparticle gaps down to 2 nm and based on the relaxed lithographic requirements for the fabrication outlined here, such structures should open the door to highly sensitive SERS and fluorescence sensors, as well as providing the potential for enhanced emitters and absorbers.

Acknowledgments

References and links

1.

M. Fleischmann, P. J. Hendra, and A. J. McQuillan, “Raman spectra of pyridine adsorbed at a silver electrode,” Chem. Phys. Lett. 26(2), 163–166 (1974). [CrossRef]

2.

D. L. Jeanmaire and R. P. van Duyne, “Surface Raman electrochemistry part I: heterocyclic, aromatic and aliphatic amines adsorbed on the anodized silver electrode,” J. Electroanal. Chem. 84(1), 1–20 (1977). [CrossRef]

3.

M. I. Stockman, L. N. Pandey, and T. F. George, “Inhomogeneous localization of polar eigenmodes in fractals,” Phys. Rev. B Condens. Matter 53(5), 2183–2186 (1996). [CrossRef] [PubMed]

4.

J. P. Kottmann and O. J. F. Martin, “Plasmon resonant coupling in metallic nanowires,” Opt. Express 8(12), 655–663 (2001). [CrossRef] [PubMed]

5.

T. Atay, J.-H. Song, and A. V. Nurmikko, “Strongly interacting plasmon nanoparticle pairs: from dipole-dipole interaction to conductively coupled regime,” Nano Lett. 4(9), 1627–1631 (2004). [CrossRef]

6.

K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Field, “Single molecule detection using surface-enhanced Raman scattering,” Phys. Rev. Lett. 78(9), 1667–1670 (1997). [CrossRef]

7.

S. Nie and S. R. Emory, “Probing single molecules and single nanoparticles by surface-enhanced Raman scattering,” Science 275(5303), 1102–1106 (1997). [CrossRef] [PubMed]

8.

For a recent review seeN. J. Halas, S. Lal, W.-S. Chang, S. Link, and P. Nordlander, “Plasmons in strongly coupled metallic nanostructures,” Chem. Rev. 111(6), 3913–3961 (2011). [CrossRef] [PubMed]

9.

K. Kneipp, H. Kneipp, and J. Kneipp, “Surface-enhanced Raman scattering in local optical fields of silver and gold nanoaggregates-from single-molecule Raman spectroscopy to ultrasensitive probing in live cells,” Acc. Chem. Res. 39(7), 443–450 (2006). [CrossRef] [PubMed]

10.

A. M. Michaels, J. Jiang, and L. Brus, “Ag nanocrystal junctions as the site for surface-enhanced Raman scattering of single Rhodamine 6G molecules,” J. Phys. Chem. B 104(50), 11965–11971 (2000). [CrossRef]

11.

C. E. Talley, J. B. Jackson, C. Oubre, N. K. Grady, C. W. Hollars, S. M. Lane, T. R. Huser, P. Nordlander, and N. J. Halas, “Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates,” Nano Lett. 5(8), 1569–1574 (2005). [CrossRef] [PubMed]

12.

H. Xu, E. J. Bjerneld, M. Kall, and L. Borjesson, “Spectroscopy of single hemoglobin molecules by surface enhanced Raman scattering,” Phys. Rev. Lett. 83(21), 4357–4360 (1999). [CrossRef]

13.

J. C. Hulteen, D. A. Treichel, M. T. Smith, M. L. Duval, T. R. Jensen, and R. P. van Duyne, “Nanosphere lithography: size-tunable silver nanoparticle and surface cluster arrays,” J. Phys. Chem. B 103(19), 3854–3863 (1999). [CrossRef]

14.

J. C. Hulteen and R. P. van Duyne, “Nanosphere lithography: a materials general fabrication process for periodic particle array surfaces,” J. Vac. Sci. Technol. A 13(3), 1553–1558 (1995). [CrossRef]

15.

J. J. Mock, R. T. Hill, A. Degiron, S. Zauscher, A. Chilkoti, and D. R. Smith, “Distance-dependent plasmon resonant coupling between a gold nanoparticle and gold film,” Nano Lett. 8(8), 2245–2252 (2008). [CrossRef] [PubMed]

16.

H. Im, K. C. Bantz, N. C. Lindquist, C. L. Haynes, and S.-H. Oh, “Vertically oriented sub-10-nm plasmonic nanogap arrays,” Nano Lett. 10(6), 2231–2236 (2010). [CrossRef] [PubMed]

17.

C.-F. Chen, S.-D. Tzeng, H.-Y. Chen, K.-J. Lin, and S. Gwo, “Tunable plasmonic response from alkanethiolate-stabilized gold nanoparticle superlattices: evidence of near-field coupling,” J. Am. Chem. Soc. 130(3), 824–826 (2008). [CrossRef] [PubMed]

18.

D. A. Alexson, S. C. Badescu, O. J. Glembocki, S. M. Prokes, and R. W. Rendell, “Metal-Adsorbate hybridized electronic states and their impact on surface enhanced Raman scattering,” Chem. Phys. Lett. 477(1-3), 144–149 (2009). [CrossRef]

19.

S. M. Prokes, O. J. Glembocki, R. W. Rendell, and M. Ancona, “Enhanced plasmon coupling in crossed dielectric/metal nanowire composite geometries and applications to surface-enhanced Raman spectroscopy,” Appl. Phys. Lett. 90(9), 093105 (2007). [CrossRef]

20.

J. Dorfmüller, R. Vogelgesang, W. Khunsin, C. Rockstuhl, C. Etrich, and K. Kern, “Plasmonic nanowire antennas: experiment, simulation, and theory,” Nano Lett. 10(9), 3596–3603 (2010). [CrossRef] [PubMed]

21.

J. D. Caldwell, O. J. Glembocki, F. J. Bezares, N. D. Bassim, R. W. Rendell, M. Feygelson, M. Ukaegbu, R. Kasica, L. Shirey, and C. Hosten, “Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors,” ACS Nano 5(5), 4046–4055 (2011). [CrossRef] [PubMed]

22.

X. Chen and K. Jiang, “A large-area hybrid metallic nanostructure array and its optical properties,” Nanotechnology 19(21), 215305 (2008). [CrossRef] [PubMed]

23.

M. Kariniemi, J. Niinisto, T. Hatanpaa, M. Kemell, T. Sajavaara, M. Ritala, and M. Leskela, “Plasma-enhanced atomic layer deposition of silver thin films,” Chem. Mater. 23(11), 2901–2907 (2011). [CrossRef]

24.

A. Niskanen, T. Hatanpaa, K. Arstila, M. Leskela, and M. Ritala, “Radical-enhanced atomic layer deposition of silver thin films using phosphine-adducted silver carboxylates,” Chem. Vapor Deposit. 13(8), 408–413 (2007). [CrossRef]

25.

J. D. Caldwell, O. J. Glembocki, R. W. Rendell, S. M. Prokes, J. P. Long, and F. J. Bezares, “Plasmo-photonic nanowire arrays for large-area surface-enhanced Raman scattering sensors,” Proc. SPIE 7757, 775723, 775723 (2010). [CrossRef]

26.

K. T. Carron and L. G. Hurley, “Axial and azimuthal angle determination with surface-enhanced Raman spectroscopy - thiophenol on copper, silver and gold metal-surfaces,” J. Phys. Chem. 95(24), 9979–9984 (1991). [CrossRef]

27.

S. Li, D. Wu, X. Xu, and R. Gu, “Theoretical and experimental studies on the adsorption behavior of thiophenol on gold nanoparticles,” J. Raman Spectrosc. 38(11), 1436–1443 (2007). [CrossRef]

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(160.3918) Materials : Metamaterials
(240.6695) Optics at surfaces : Surface-enhanced Raman scattering

ToC Category:
Optics at Surfaces

History
Original Manuscript: September 30, 2011
Revised Manuscript: November 12, 2011
Manuscript Accepted: November 19, 2011
Published: December 7, 2011

Virtual Issues
Vol. 7, Iss. 2 Virtual Journal for Biomedical Optics
March 7, 2012 Spotlight on Optics

Citation
Joshua D. Caldwell, Orest J. Glembocki, Francisco J. Bezares, Maarit I. Kariniemi, Jaakko T. Niinistö, Timo T. Hatanpää, Ronald W. Rendell, Maraizu Ukaegbu, Mikko K. Ritala, Sharka M. Prokes, Charles M. Hosten, Markku A. Leskelä, and Richard Kasica, "Large-area plasmonic hot-spot arrays: sub-2 nm interparticle separations with plasma-enhanced atomic layer deposition of Ag on periodic arrays of Si nanopillars," Opt. Express 19, 26056-26064 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-26056


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References

  1. M. Fleischmann, P. J. Hendra, and A. J. McQuillan, “Raman spectra of pyridine adsorbed at a silver electrode,” Chem. Phys. Lett.26(2), 163–166 (1974). [CrossRef]
  2. D. L. Jeanmaire and R. P. van Duyne, “Surface Raman electrochemistry part I: heterocyclic, aromatic and aliphatic amines adsorbed on the anodized silver electrode,” J. Electroanal. Chem.84(1), 1–20 (1977). [CrossRef]
  3. M. I. Stockman, L. N. Pandey, and T. F. George, “Inhomogeneous localization of polar eigenmodes in fractals,” Phys. Rev. B Condens. Matter53(5), 2183–2186 (1996). [CrossRef] [PubMed]
  4. J. P. Kottmann and O. J. F. Martin, “Plasmon resonant coupling in metallic nanowires,” Opt. Express8(12), 655–663 (2001). [CrossRef] [PubMed]
  5. T. Atay, J.-H. Song, and A. V. Nurmikko, “Strongly interacting plasmon nanoparticle pairs: from dipole-dipole interaction to conductively coupled regime,” Nano Lett.4(9), 1627–1631 (2004). [CrossRef]
  6. K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Field, “Single molecule detection using surface-enhanced Raman scattering,” Phys. Rev. Lett.78(9), 1667–1670 (1997). [CrossRef]
  7. S. Nie and S. R. Emory, “Probing single molecules and single nanoparticles by surface-enhanced Raman scattering,” Science275(5303), 1102–1106 (1997). [CrossRef] [PubMed]
  8. For a recent review seeN. J. Halas, S. Lal, W.-S. Chang, S. Link, and P. Nordlander, “Plasmons in strongly coupled metallic nanostructures,” Chem. Rev.111(6), 3913–3961 (2011). [CrossRef] [PubMed]
  9. K. Kneipp, H. Kneipp, and J. Kneipp, “Surface-enhanced Raman scattering in local optical fields of silver and gold nanoaggregates-from single-molecule Raman spectroscopy to ultrasensitive probing in live cells,” Acc. Chem. Res.39(7), 443–450 (2006). [CrossRef] [PubMed]
  10. A. M. Michaels, J. Jiang, and L. Brus, “Ag nanocrystal junctions as the site for surface-enhanced Raman scattering of single Rhodamine 6G molecules,” J. Phys. Chem. B104(50), 11965–11971 (2000). [CrossRef]
  11. C. E. Talley, J. B. Jackson, C. Oubre, N. K. Grady, C. W. Hollars, S. M. Lane, T. R. Huser, P. Nordlander, and N. J. Halas, “Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates,” Nano Lett.5(8), 1569–1574 (2005). [CrossRef] [PubMed]
  12. H. Xu, E. J. Bjerneld, M. Kall, and L. Borjesson, “Spectroscopy of single hemoglobin molecules by surface enhanced Raman scattering,” Phys. Rev. Lett.83(21), 4357–4360 (1999). [CrossRef]
  13. J. C. Hulteen, D. A. Treichel, M. T. Smith, M. L. Duval, T. R. Jensen, and R. P. van Duyne, “Nanosphere lithography: size-tunable silver nanoparticle and surface cluster arrays,” J. Phys. Chem. B103(19), 3854–3863 (1999). [CrossRef]
  14. J. C. Hulteen and R. P. van Duyne, “Nanosphere lithography: a materials general fabrication process for periodic particle array surfaces,” J. Vac. Sci. Technol. A13(3), 1553–1558 (1995). [CrossRef]
  15. J. J. Mock, R. T. Hill, A. Degiron, S. Zauscher, A. Chilkoti, and D. R. Smith, “Distance-dependent plasmon resonant coupling between a gold nanoparticle and gold film,” Nano Lett.8(8), 2245–2252 (2008). [CrossRef] [PubMed]
  16. H. Im, K. C. Bantz, N. C. Lindquist, C. L. Haynes, and S.-H. Oh, “Vertically oriented sub-10-nm plasmonic nanogap arrays,” Nano Lett.10(6), 2231–2236 (2010). [CrossRef] [PubMed]
  17. C.-F. Chen, S.-D. Tzeng, H.-Y. Chen, K.-J. Lin, and S. Gwo, “Tunable plasmonic response from alkanethiolate-stabilized gold nanoparticle superlattices: evidence of near-field coupling,” J. Am. Chem. Soc.130(3), 824–826 (2008). [CrossRef] [PubMed]
  18. D. A. Alexson, S. C. Badescu, O. J. Glembocki, S. M. Prokes, and R. W. Rendell, “Metal-Adsorbate hybridized electronic states and their impact on surface enhanced Raman scattering,” Chem. Phys. Lett.477(1-3), 144–149 (2009). [CrossRef]
  19. S. M. Prokes, O. J. Glembocki, R. W. Rendell, and M. Ancona, “Enhanced plasmon coupling in crossed dielectric/metal nanowire composite geometries and applications to surface-enhanced Raman spectroscopy,” Appl. Phys. Lett.90(9), 093105 (2007). [CrossRef]
  20. J. Dorfmüller, R. Vogelgesang, W. Khunsin, C. Rockstuhl, C. Etrich, and K. Kern, “Plasmonic nanowire antennas: experiment, simulation, and theory,” Nano Lett.10(9), 3596–3603 (2010). [CrossRef] [PubMed]
  21. J. D. Caldwell, O. J. Glembocki, F. J. Bezares, N. D. Bassim, R. W. Rendell, M. Feygelson, M. Ukaegbu, R. Kasica, L. Shirey, and C. Hosten, “Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors,” ACS Nano5(5), 4046–4055 (2011). [CrossRef] [PubMed]
  22. X. Chen and K. Jiang, “A large-area hybrid metallic nanostructure array and its optical properties,” Nanotechnology19(21), 215305 (2008). [CrossRef] [PubMed]
  23. M. Kariniemi, J. Niinisto, T. Hatanpaa, M. Kemell, T. Sajavaara, M. Ritala, and M. Leskela, “Plasma-enhanced atomic layer deposition of silver thin films,” Chem. Mater.23(11), 2901–2907 (2011). [CrossRef]
  24. A. Niskanen, T. Hatanpaa, K. Arstila, M. Leskela, and M. Ritala, “Radical-enhanced atomic layer deposition of silver thin films using phosphine-adducted silver carboxylates,” Chem. Vapor Deposit.13(8), 408–413 (2007). [CrossRef]
  25. J. D. Caldwell, O. J. Glembocki, R. W. Rendell, S. M. Prokes, J. P. Long, and F. J. Bezares, “Plasmo-photonic nanowire arrays for large-area surface-enhanced Raman scattering sensors,” Proc. SPIE7757, 775723, 775723 (2010). [CrossRef]
  26. K. T. Carron and L. G. Hurley, “Axial and azimuthal angle determination with surface-enhanced Raman spectroscopy - thiophenol on copper, silver and gold metal-surfaces,” J. Phys. Chem.95(24), 9979–9984 (1991). [CrossRef]
  27. S. Li, D. Wu, X. Xu, and R. Gu, “Theoretical and experimental studies on the adsorption behavior of thiophenol on gold nanoparticles,” J. Raman Spectrosc.38(11), 1436–1443 (2007). [CrossRef]

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