Surface-Enhanced Raman Scattering from Metallic Nanostructures: Bridging the Gap between the Near-Field and Far-Field Responses

Abstract

We present here a detailed study of the complex relationship between the electromagnetic near-field and far-field responses of ''real'' nanostructured metallic surfaces. The near-field and far-field responses are specified in terms of (spectra of) the surface-enhanced Raman-scattering enhancement factor (SERS EF) and optical extinction, respectively. First, it is shown that gold nanorod-and nanotube-array substrates exhibit three distinct localized surface plasmon resonances (LSPRs): a longitudinal, a transverse, and a cavity mode. The cavity mode simultaneously has the largest impact on the near-field behavior (as observed through the SERS EF) and the weakest optical interaction: It has a ''near-field-type'' character. The transverse and longitudinal modes have a significant impact on the far-field behavior but very little impact on SERS: They have a ''far-field-type'' character. We confirm the presence of the cavity mode using a combination of SERS EF spectra, electron microscopy, and electromagnetic modeling and thus clearly illustrate and explain the (lack of) correlation between the SERS EF spectra and the optical response in terms of the contrasting character of the three LSPRs. In doing so, we experimentally demonstrate that, for a surface that supports multiple LSPRs, the near-field and far-field properties can in fact be tuned almost independently. It is further demonstrated that small changes in geometrical parameters that tune the spectral location of the LPSRs can also drastically influence the character of these modes, resulting in certain unusual behavior, such as the far-field resonance redshift as the near-field resonance blueshifts.

Plasmon enhanced fluorescence studies from aligned gold nanorod arrays modified with SiO2 spacer layers

“…Modern nanofabrication methodologies enable the creation of a number of different nanostructures with precisely controlled shapes, sizes, and spacing [3][4][5][6][7][8][9][10]. This has extended to the creation of a range of different metallic nanostructures array designs that produce localized surface plasmon resonances (LSPRs) [8][9][10][11][12][13][14][15]. One such design is based on arrays of quasi self-standing metal nanorods [13][14][15].…”

“…Quasi self-standing metallic nanorod arrays possessing a fixed nanorod diameter and interrod separation have been widely applied for SERS [12]. A limited number of studies have demonstrated that such nanomaterials can be applied to SEF [33].…”

Application of AAO Matrix in Aligned Gold Nanorod Array Substrates for Surface-Enhanced Fluorescence and Raman Scattering

“…Metallic nanostructures that possess anisotropic symmetry, such as nanorods, are reported to produce LSPRs with high quality factors. [16][17][18][19] Nanorod array design enables choice over precise particle shape, length, width and inter-particle coupling. Such nanorod arrays have been reported to support negative refraction at visible to near-infrared wavelengths or provide near-field focusing for sub-wavelength imaging.…”

“…18,19 2D nanorod arrays have been applied to enable surface-enhanced Raman scattering (SERS) spectra from molecules adsorbed on the surface of vertically aligned gold nanorod arrays In addition, gold nanorods possess good chemical stability and biocompatibility under biological conditions. 17,21 Fluorescence spectroscopy is another well-known spectroscopic method playing a major role in selective detection of bioanalytes. 23 Similarly to the effect of increasing the Raman scattering of molecules, the presence of LSPRs on plasmonic metal nanostructures can enhance the fluorescence signal from locally situated fluorophores, a process known as metal enhanced (MEF) or surface enhanced fluorescence (SEF).…”

Plasmon management in index engineered 2.5D hybrid nanostructures for surface-enhanced Raman scattering

“…45 If the individual plasmonic nanostructures, pure metallic or metal-hybrid, come closer than the range (B30 nm) of the plasmonic E field, extremely intense local electromagnetic fields can be generated. 46 In contrast to the localized surface plasmons studied here, for pure metallic NWs, propagating surface plasmons with a guided character can also result in highsensitivity bio-sensing. 47 Furthermore, we are only using the classical electromagnetic theory for the calculation and interpretation of results; quantum mechanical effects or chemical enhancement effects have not been considered.…”