One-dimensional arrays of silver nanoparticles with a particle size of 10.5？nm and interparticle spacing of 8.5？nm were fabricated by depositing nanoparticles in gas phase on block copolymer self-assembled templates. The substrate showed a surface-enhanced Raman scattering (SERS) enhancement factor as high as with good reproducibility and stability. The dependence of the average enhancement factor and the SERS intensity on the packing density of the analyte molecules were investigated. For a tiny amount of analytes in the range of to ？mol/mm2, the SERS signal showed a linear dependence on the molecule packing density on a logarithmic scale, with a slope of about 1.25. The substrates are promising for quantitative detection of trace level molecules. 1. Introduction The last three decades have witnessed an ever-increasing interest in surface-plasmon-based analytical techniques, of which surface-enhanced Raman scattering (SERS) [1–5] is one of the most important applications. However, the rational design and facile fabrication of reproducible SERS substrates with large and stable enhancement of Raman signals remain a critical challenge [6, 7]. Despite that extensive efforts have been devoted to the local field enhancement and single molecule detection at the hot spots, less attention has been paid to the average enhancement factor , which has greater relevance for the design and optimization of SERS-based chemical sensors [8, 9]. Metal nanoparticle assemblies are important substrates for SERS-based molecule sensing, because of the light confinement between nanoparticles, which provides enhanced local electromagnetic fields and the tunability of localized surface plasmon resonances. To enable molecule detection in high SERS activity, stability and reproducibility, a knowledge of the arrangement of the analyte molecules and hot spots, and their effects to the average enhancement are important. To obtain a large average enhancement, it is crucial to provide a state of aggregation of the metal nanoparticles and to place the analyte molecules in the sites of the junctions of metal nanoparticles. In this sense, one- and two-dimensional ordered arrays of closely spaced metal nanoparticles can be excellent candidates for SERS substrates. They can provide not only giant field enhancement both through long range photonic coupling and the local electromagnetic near field confinement, but also high reproducibility because of the uniform and dense distribution of hot spots. Recently, block copolymer self-assembled nanopatterns were used as templates to fabricate high
References
[1]
M. Fleischmann, P. J. Hendra, and A. J. McQuillan, “Raman spectra of pyridine adsorbed at a silver electrode,” Chemical Physics Letters, vol. 26, no. 2, pp. 163–166, 1974.
[2]
D. L. Jeanmaire and R. P. Van Duyne, “Surface Raman spectroelectrochemistry: part I. heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode,” Journal of Electroanalytical Chemistry, vol. 84, no. 1, pp. 1–20, 1977.
[3]
M. G. Albrecht and J. A. Creighton, “Anomalously intense Raman spectra of pyridine at a silver electrode,” Journal of the American Chemical Society, vol. 99, no. 15, pp. 5215–5217, 1977.
[4]
M. Moskovits, “Surface roughness and the enhanced intensity of Raman scattering by molecules adsorbed on metals,” The Journal of Chemical Physics, vol. 69, no. 9, pp. 4159–4161, 1978.
[5]
M. Moskovits, “Surface-enhanced Raman spectroscopy: a brief perspective,” Topics in Applied Physics, vol. 103, pp. 1–17, 2006.
[6]
H. Ko, S. Singamaneni, and V. V. Tsukruk, “Nanostructured surfaces and assemblies as SERS media,” Small, vol. 4, no. 10, pp. 1576–1599, 2008.
[7]
M. Geissler and Y. Xia, “Patterning: principles and some new developments,” Advanced Materials, vol. 16, no. 15, pp. 1249–1269, 2004.
[8]
Y. Fang, N.-H. Seong, and D. D. Dlott, “Measurement of the distribution of site enhancements in surface-enhanced Raman scattering,” Science, vol. 321, no. 5887, pp. 388–391, 2008.
[9]
D. A. Genov, A. K. Sarychev, V. M. Shalaev, and A. Wei, “Resonant field enhancements from metal nanoparticle arrays,” Nano Letters, vol. 4, no. 1, pp. 153–158, 2004.
[10]
W. A. Lopes and H. M. Jaeger, “Hierarchical self-assembly of metal nanostructures on diblock copolymer scaffolds,” Nature, vol. 414, no. 6865, pp. 735–738, 2001.
[11]
Z. Shi, M. Han, F. Song, J. Zhou, J. Wan, and G. Wang, “Hierarchical self-assembly of silver nanocluster arrays on triblock copolymer templates,” Journal of Physical Chemistry B, vol. 110, no. 37, pp. 18154–18157, 2006.
[12]
S. B. Darling, “Directing the self-assembly of block copolymers,” Progress in Polymer Science, vol. 32, no. 10, pp. 1152–1204, 2007.
[13]
Q. Li, J. He, E. Glogowski et al., “Responsive assemblies: gold nanoparticles with mixed ligands in microphase separated block copolymers,” Advanced Materials, vol. 20, no. 8, pp. 1462–1466, 2008.
[14]
Y. Wang, M. Becker, L. Wang et al., “Nanostructured gold films for SERS by block copolymer-templated galvanic displacement reactions,” Nano Letters, vol. 9, no. 6, pp. 2384–2389, 2009.
[15]
J. Lu, D. Chamberlin, D. A. Rider, M. Liu, I. Manners, and T. P. Russell, “Using a ferrocenylsilane-based block copolymer as a template to produce nanotextured Ag surfaces: uniformly enhanced surface enhanced Raman scattering active substrates,” Nanotechnology, vol. 17, no. 23, pp. 5792–5797, 2006.
[16]
C. H. Xu, B. Xie, Y. J. Liu, L. B. He, and M. Han, “Optimizing surface-enhanced Raman scattering by template guided assembling of closely spaced silver nanocluster arrays,” European Physical Journal D, vol. 52, no. 1–3, pp. 111–114, 2009.
[17]
S. Hadano, H. Handa, K. Nagai, T. Iyoda, J. Li, and S. Watanabe, “Surface-enhanced Raman scattering (SERS) effect of hexagonally arranged gold nanoparticle array with 29-nm particles and 23-nm gaps using liquid-crystalline block-copolymer template,” Chemistry Letters, vol. 42, no. 1, pp. 71–73, 2013.
[18]
W. J. Cho, Y. Kim, and J. K. Kim, “Ultrahigh-density array of silver nanoclusters for SERS substrate with high sensitivity and excellent reproducibility,” ACS Nano, vol. 6, no. 1, pp. 249–255, 2012.
[19]
Y. Liu, Y. C. Gong, L. B. He et al., “Formation of periodic nanoring arrays on self-assembled PS-b-PMMA film under rapid solvent-annealing,” Nanoscale, vol. 2, no. 10, pp. 2065–2068, 2010.
[20]
H. Haberland, M. Mal, M. Mosele, Y. Qiang, Th. Reiners, and Y. Thurner, “Filling of micro-sized contact holes with copper by energetic cluster impact (ECI),” Journal of Vacuum Science & Technology A, vol. 12, no. 5, pp. 2920–2925, 1994.
[21]
M. Han, C. H. Xu, D. Zhu et al., “Controllable synthesis of two-dimensional metal nanoparticle arrays with oriented size and number density gradients,” Advanced Materials, vol. 19, no. 19, pp. 2979–2983, 2007.
[22]
T. L. Morkved, P. Wiltzius, H. M. Jaeger, D. G. Grier, and T. A. Witten, “Mesoscopic self-assembly of gold islands on diblock-copolymer films,” Applied Physics Letters, vol. 64, no. 4, pp. 422–424, 1994.
[23]
R. L. Birke, C. Shi, W. Zhang, and J. R. Lombardi, “A time-resolved SERS study of the adsorption and electrochemical reduction of 4-pyridinecarboxaldehyde and 4-(hydroxymethyl) pyridine,” Journal of Physical Chemistry B, vol. 102, no. 41, pp. 7983–7996, 1998.
[24]
N. Félidj, J. Aubard, G. Lévi et al., “Controlling the optical response of regular arrays of gold particles for surface-enhanced Raman scattering,” Physical Review B, vol. 65, no. 7, Article ID 075419, 9 pages, 2002.
[25]
S. Zou and G. C. Schatz, “Silver nanoparticle array structures that produce giant enhancements in electromagnetic fields,” Chemical Physics Letters, vol. 403, no. 1–3, pp. 62–67, 2004.
[26]
L. Qin, S. Zou, C. Xue, A. Atkinson, G. C. Schatz, and C. A. Mirkin, “Designing, fabricating, and imaging Raman hot spots,” Proceedings of the National Academy of Sciences of the United States of America, vol. 103, no. 36, pp. 13300–13303, 2006.
[27]
L. Qin, M. J. Banholzer, J. E. Millstone, and C. A. Mirkin, “Nanodisk codes,” Nano Letters, vol. 7, no. 12, pp. 3849–3853, 2007.
[28]
C. H. Xu, X. Chen, Y. J. Liu et al., “Enhanced thermal stability of monodispersed silver cluster arrays assembled on block copolymer scaffolds,” Nanotechnology, vol. 21, no. 19, Article ID 195304, 2010.
