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.
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. 5 Dilution of Ga2O3/Ag nanowire composites.