Thursday, December 31, 2009

New Navy Nanowire Excites Significantly Enhanced Surface Enhanced Raman Scattering (SERS) and Fluorescence (SEF) Signals-Shows Sensitivity to Parts per Billion

FIG. 1 U.S. Naval Research Laboratory Ga2O3 core/Ag shell nanowire composite drawing and SEM image.

In U.S. Patent 7,639,356 (Washington, DC) U.S. Naval Research Laboratory (NRL) scientists Sharka M. Prokes and Orest J Glembocki reveal a new surface enhanced Raman spectroscopy (SERS)-active substrate consisting of dielectric/Ag nanowire composites, which show very enhanced sensitivities, in the parts per billion range. The reason for such high sensitivities can be that the nanowire crossings lead to very large electric field enhancements in large regions around the nanowires, allowing a larger volume of molecules of interest to be detected.

Furthermore, the wires can also be dispersed in a liquid, allowing for stand-off sensing applications. The nanowires can also serve as taggants.

U.S. Patent 7,639,356, awarded to the U.S. Navy,  describes metal/wide band gap semiconductor nanowire composites which exhibit significantly enhanced surface enhanced Raman scattering (SERS) and fluorescence (SEF) signal and which can be used for very efficient chemical or biological sensors.

In one embodiment, the fabrication technique can be applicable to Ga2O3 semiconductor nanowires and other metal oxide nanowires, with controlled size diameters ranging from about 5 to about 100 nm. The composite can consist of the nanowire core structure, which can be coated with a silver metallic shell on the order of 3-10 nm.

An apparatus comprising a substrate and at least two nanowires on the substrate, the nanowires comprising a core and a metal shell, wherein the core is selected from the group consisting of a semiconductor and a dielectric, thereby forming a nanowire-composite to allow plasmon coupling for enhancements of the electric fields and enhancements of the surface enhanced Raman signal (SERS) and enhancements of the chemical or biological specificity and sensitivity.

A method of making a SERS-active substrate comprising providing a substrate and affixing a plurality of nanowires on the substrate thereby forming a nano-composite, creating plasmon coupling leading to enhanced electric fields in the vicinity of the nanowires and enhancements of the surface enhanced Raman signal (SERS) and enhancements of the chemical or biological specificity and sensitivity.

Raman scattering is often used in the chemical identification of materials. Light scattered from various vibrational modes in a material is red- and blue-shifted by the frequency of the vibrational modes.

The information that is obtained from Raman scattering is complementary to that of IR spectroscopy, but with the advantage of being performed with visible light. Because the Raman cross-sections of most materials are very small, the intensity of the Raman signal is often 8 orders of magnitude lower than the intensity of the exciting laser. Thus, rapid acquisition of Raman scattering requires the use of intense laser light, limiting the equipment to table top lasers. The situation is even more serious for small amounts of a species adsorbed on a surface of a material.

In the surface enhanced Raman scattering (SERS) effect rough metal surfaces (usually Ag) are used to increase the Raman signal of species adsorbed on the metal. Enhancements of up to 8 orders of magnitude have been observed. The SERS enhancement of molecules adsorbed 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. Significant work has been done in SERS using various metals and geometry for the roughened features, including aggregate films, nano-particles, nano-shells and solid metal nanowire ordered arrays.

The two Navy scientists showed that local hot spots in the electric fields produced by localized plasmons excited in nanoparticles can produce large SERS effects. Furthermore, it has been suggested that using nanoparticles of appropriate size and geometry can lead to further enhancements by moving the plasmons absorption frequency close to that of the exciting laser. This adds resonant enhancement to the SERS process, further increasing the Raman signal.

FIG. 3 Comparison of SERS signal for Mesophotonics "Klarite" commercial substrate and NRL Ga2O3/Ag nanowire composites. 

FIG. 4 Schematic diagram showing how crossed nanowires can be used to construct a 3D array that may maximize crossing points, where the SERS enhancement may be greatest. 

FIG. 5 Dilution of Ga2O3/Ag nanowire composites.

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