Description: Raman spectroscopy is often used for chemical fingerprinting. However, the Raman process is very inefficient and difficult to use for trace detection. Rough metal surfaces (usually Ag or Au) are used to increase the Raman signal of trace levels of molecules adsorbed on these surfaces. This SERS enhancement of molecules on the roughened metal surface is caused by local electromagnetic fields that are created by the laser excitation of surface plasmons at the metal surface. Enhancements of up to 8 orders of magnitude have been observed. NRL investigators have completed significant work using dielectric core/metal nanowire composites and nanoshell and NW arrays. It has been shown that this material exhibits high sensitivity due to local hot spots in the electric fields produced by crossings of the nanowires and by the nanowire interaction with the substrate. Furthermore, the surface plasmon resonance has been shown to be broad so that the nanowires are effective over a large wavelength range in the near UV to near IR. This allows a great flexibility in the choice of wavelengths for Raman excitation.

Advantages/Features Include:

  • Highly Selective: Raman spectral fingerprints unique to chemical identity.
  • Highly Sensitive: Metal/dielectric nanowire composites result in very high optical gain due to plasmonics. Gain >108 and trace and single molecule detection are possible.
  • Versatile: A variety of nanowires and arrays with unique characteristics can be obtained, such as broad surface plasmon resonance effective over a large wavelength (near UV to near IR) and precise diameter control for nanowire arrays.

Applications Include:

  • Enhance sensitivity of handheld Raman analyzers in the field
  • Identification of trace chemicals on surfaces
  • Forensics, including chemical fingerprinting
  • Tagging, tracking, and locating
  • Tamper detection
  • Drug screening

References:

  • "Surface-Enhanced Raman Spectroscopy of Dielectric/Metal Nanowire Composites," Applied Physics Letters 90 (2007).
  • "Effect of Crossing Geometry on the Plasmonic Behavior of Dielectric Core/Metal Sheath Nanowires," Applied Physics Letters 94 (2009).
  • "Dielectric-Substrate Interactions with Metal/Dielectric Nanowires Composites in Surface-Enhanced Raman Spectroscopy," Phys. Rev. B 80, 1, 2009.

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