SPR dependencies and colour

The angle at which surface plasmon resonance occurs mainly depends on the nature of the metal, the wavelength of the incident light and the refractive index of the medium on either side of the metal surface. Because the refractive index is sensitive to temperature, it is important to perform the measurements at defined temperatures. In some cases, this dependency can be exploited. The metal must have conduction band electrons capable of resonating with the incoming light at a suitable wavelength. Metals that satisfy this condition are silver, gold, copper, aluminum, sodium and indium. In addition, the metal surface must be free of oxides, sulphides and not react with other molecules on exposure to the atmosphere or liquid. Of the metals, indium is too expensive, sodium is too reactive, copper and aluminum are too broad in their SPR response and silver is too susceptible to oxidation. This leaves gold as the most practical metal. Gold is resistant to oxidation and other atmospheric contaminants, but it is compatible with a lot of chemical modification systems. The light source should be monochromatic and p-polarized (polarized in the plane of the surface) to obtain a sharp dip as shown in

Enhanced strength and toughness

The strength and toughness of both ceramics and metals can be enormously enhanced if they are made out of nanoscale crystallites rather than the usual micron-sized grains. This effect is already widely exploited to make superior ceramics and tungsten carbide-cobalt composites. Ceramics made from nanoscale TiO2 particles not only sinter together even at 600°C, but also possess enhanced strength and toughness. There is a similar effect in metal systems. Nanocrystalline copper, for example, is up to five times stronger than ordinary copper. In this case, the explanation relies upon the observation that deformation in metals is generally carried by lattice defects called dislocations, and nanoscale copper crystals are actually too small to even contain dislocations. Aluminum also shows the same effect. A process called as equiangular extrusion is used to refine the grain size of the aluminum into the nanoscale, so that it will be hard and lose the ductility. Similar behavior is exhibited by gold.