1. Field of the Invention
The present invention relates to a new process and device design for producing substrates for highly sensitive surface-enhanced Raman spectroscopy and multimodal sensing. The resulting devices are potentially very useful for chemical sensing for a variety of applications. The large surface enhancement of electromagnetic fields may also have uses in nonlinear optics and plasmonic signal generation or routing.
2. Brief Description of the Related Art
Multifunctional sensors with single-molecule sensitivity are greatly desired for a variety of sensing applications, from biochemical analysis to explosives detection. Chemical and electromagnetic interactions between molecules and metal substrates are used in surface-enhanced spectroscopies to approach single molecule detection. See M. Moskovits, Rev. Mod. Phys. 57, 783 (1985). Electromagnetic enhancement in nanostructured conductors results when incident light excites local electronic modes, producing large electric fields in a nanoscale region, known as a “hot spot”, that greatly exceed the strength of the incident field. This local field enhancement is the mechanism responsible for a variety of “surface-enhanced” spectroscopies, including surface-enhanced Raman (SERS), surface-enhanced infrared adsorption (SEIRA), and surface-enhanced fluorescence (SEF). Hot spots can lead to particularly large enhancements of Raman scattering, since the Raman scattering rate is proportional to |E(ω)|2|E(ω′)|2 at the location of the molecule, where E(ω) is the electric field component at the frequency of the incident radiation, and E(ω) is the component at the scattered frequency. Still, substrates that give large Raman enhancements are often useful for SEIRA and SEF as well. Large local field enhancements are also useful for nonlinear optical processes, and have been discussed in the context of optical information processing (Chang et al., Nature Physics 3, 807-812 (2007)).
It has been an ongoing challenge to design and fabricate a substrate for systematic surface-enhanced Raman spectroscopy (SERS) at the single molecule level. Achieving SERS with single-molecule sensitivity was first clearly demonstrated using random aggregates of colloidal nanoparticles. K. Kneipp, et al., “Colloidal silver rhodamine 6 g fluorescence spectroscopy gold,” Phys. Rev. Lett. 78, 1667 (1997); S. Nie, S. R. Emory, Science 275, 1102 (1997); H. Xu, E. J. Bjerneld, M. Käll, L. Börjesson, Phys. Rev. Lett. 83, 4357 (1999); and A. M. Michaels, J. Jiang, L. Brus, J. Phys. Chem. B 104, 11965 (2000). While numerous other metal substrate configurations have been used for SERS, including engineered nanoparticles made chemically, nanostructures defined by bottom-up patterning and traditional lithographic approaches, the most sensitive substrate geometries rely on closely adjacent subwavelength nanoparticles or nanostructures. See J. Jackson and N. J. Halas, Proc. Nat. Acad. Sci. U.S. 101, 17930-17935 (2004); H. Wang, C. S. Levin and N. J. Halas, J. Am. Chem. Soc. 127, 14992-14993 (2005); C. L. Haynes, R. P. van Duyne, J. Phys. Chem. B 105, 5599 (2001); L. Qin, et al., Proc. Nat. Acad. Sci. U.S. 103, 13300 (2006); D. P. Fromm, et al., J. Chem. Phys. 124, 061101 (2006).
In this geometry, incident light may excite the collective resonance of the pair of coupled nanostructures, resulting in large field enhancements within the interparticle gap. See A. J. Hallock, P. L. Redmond, and L. E. Brus, Proc. Nat. Acad. Sci. U.S. 102, 1280-1284 (2005); P. Nordlander, C. Oubre, E. Prodan, K. Li, and M. Stockman, Nano Letters 4, 899-903 (2004). Fractal aggregates of nanoparticles can further increase field enhancements by focusing plasmon energy from larger length scales down to particular nanometer-scale hotspots. See Z. Wang, S. Pan, T. D. Krauss, H. Du, L. J. Rothberg, Proc. Nat. Acad. Sci. U.S. 100, 8638 (2003); K. Li, M. I. Stockman, D. J. Bergman, Phys. Rev. Lett. 92, 227402 (2003). However, precise and reproducible formation of such gaps and assemblies in predetermined locations has been extremely challenging. An alternative approach is tip-enhanced Raman spectroscopy (TERS), in which the incident light excites an interelectrode plasmon resonance localized between a sharp, metal scanned probe tip and an underlying metal substrate. See D. Richards, R. G. Milner, F. Huang, F. Festy, J. Raman Spectrosc. 34, 663 (2003); C. C. Neascu, J. Dreyer, N. Behr, M. B. Raschke, Phys. Rev. B 73, 193406 (2006). A similar approach was recently attempted using a mechanical break junction. See J.-H. Tian, et al., J. Am. Chem. Soc. 128, 14748 (2006). While useful for surface imaging, TERS requires feedback to maintain a few-nm tip-surface gap, and is not scalable or readily integrated with other sensing modalities.