  We report Au/Ag bimetallic nanocomposites as a highly sensitive plasmonic
material. A unit approach via a three-dimensional numerical modeling is
introduced to observe collective plasmon resonance in Au/Ag bimetallic
nanocomposites as well as Au mono-metallic nanoensembles. Au nanoensembles
provide consistently identical plasmon wavelength, independent of inter-unit
distance. In analogy with mono-metallic nanoensembles, Au/Ag bimetallic
nanocomposites distinctly feature converging dual plasmon resonance peaks to a
single plasmon resonance peak, strongly depending on the packing density and
the unit size. An effective unit size of bimetallic nanocomposites is below 2.5
nm in a subwavelength structure, which is small enough to feature bimetallic
nanocomposites. As a result, the Au/Ag bimetallic nanocomposites clearly show
exceptionally high sensitivity and figure-ofmerit (approximately 3 fold of
conventional plasmon sensitivity and 4.3 fold of conventional plasmon FOM),
resulting from coupled Au-Ag quadrupole bimetallic nanounits. This study
provides essential rationales for Au/Ag bimetallic nanocomposites serving as a
desirable and alternative plasmonic material for advanced nanoplasmonic sensing
technologies.
