Nanogaps for SERS applications
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Introduction The concept of nanogaps, which are nanoscale gaps between nanostructures, has become one of the cornerstones of the field of plasmonics due to the enormous electromagnetic (EM) field enhancement and strong confinement of the optical field that can be induced between closely adjacent metal nanostructures. The optical properties of nanogaps are very important for surface-enhanced Raman scattering (SERS),1–4 but they have also influenced other areas of nano-optics, including nonlinear plasmonics,5,6 nano-optical forces,7,8 optical nanoantennas,9,10 and nanolasers.11 Localized surface plasmons (LSPs) can be excited in single metal particles and produce enhanced local EM fields, but unless the single particle has a very sharp feature, these fields are typically much weaker than that in a nanogap. The nanogap effect occurs because individual nanostructures couple electromagnetically if they are brought close to each other. The EM coupling results in additional near-field enhancement in the nanogap, forming the so-called “hot spot,” and this effect can amplify both the driving field and the radiative emission rate of an emitter located in the gap.12,13 In many cases, this is the basic reason why SERS is such a sensitive spectroscopic tool for molecular analysis.14–17 In this article, we briefly review some important concepts and experimental results on nanogaps for SERS. (See the August 2013 issue of MRS Bulletin on SERS substrates and materials.)
The gap distance is one of the crucial parameters that determines the magnitude of the local EM field. The additional field enhancement in the gap only occurs if two metal nanoparticles (NPs) are close to each other. “Close” here refers to a distance comparable to the extension of the evanescent EM near-field induced through excitation of a plasmon resonance. This distance is typically much smaller than the vacuum wavelength and often of the same order as the characteristic dimension of the nanostructure. With a further decrease in separation, the strength of EM coupling increases sharply, leading to a rapidly increased field enhancement in the nanogap.18,19 The increase continues until the distance between the two metal surfaces becomes so small that electron spill-out and non-local effects become important, eventually leading to electronic tunneling and electrical shortcut. The dominant current view is that classical electrodynamics provides a good description down to gap distances of the order of one nanometer, after which quantum and non-local theory approaches have to be used.20–22
Dipole approximation and voltage division The strength of EM coupling between two metal NPs strongly depends on the gap distance, particle geometries, and excitation configurations.3,23–26 As illustrated in Figure 1, the optical properties of a nanoparticle dimer, which is the most
Lianming Tong, Chinese Academy of Sciences; [email protected] Hongxing Xu, Chinese Academy of Sciences; [email protected] Mikael Käll, Chalmers University of Technology; [email protected] DOI: 10.1557/mrs.20
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