Peña-Rodríguez and Pal Nanoscale Research Letters 2011, 6:279 http://www.nanoscalereslett.com/content/6/1/279 to obtain an opposite behavior between the inner and outer energy modes (one red-shifts when the other does the contrary). For this configuration, the optical response of the bimetallic nanostructures can be tuned to find an optimum relation between t1 and t3 yielding the maximum gain in intensity for a given red-shift. Moreover, the optical response can be analyzed across the full range where it can vary in a single simulation. Calculated values were compared with those obtained for the equivalent Au-only structures in both cases. Results and discussion The wavelength variations of extinction efficiency (Qext) for MDM and DCN structures of different compositions with a fixed thickness of 5 nm for all of its layers and embedded in water (n = 1.33) are shown in Figure 2. It can be seen that, with respect to the equivalent Au-only structures, the Au-Ag and Ag-only configurations have a more intense ω+ energy mode, but the ω− mode is − − considerably less intense and blue-shifted. Additionally, the last two configurations are not advantageous for Page 3 of 5 practical applications, due to the poor stability and bio- compatibility of silver. On the other hand, the Ag-Au configuration with a more intense ω− energy mode − retains the advantages of Au structures, due to outer Au layer. The only drawback of the latter structures is a slight blue-shift of ω− energy mode, which could easily − be corrected by adjusting their geometrical parameters [22,23]. Considering these findings, we restrict our further studies only to the gains obtained by using the configurations containing Ag and Au at the inner and outer metallic shell, respectively, instead of their Au- only counterparts. The theoretical extinction efficiencies, calculated for fixed dielectric layers (5 nm) and the simultaneous inverse variations of t1 and t3 (t1 = 2, ..., 14 nm; t3 = 16 nm - t1) are shown in Figures 3 and 4 for the MDM and DCN structures, respectively. The spectra obtained as a function of t3 can be roughly divided into three regions. The first one corresponds to a very thin outer layer (below 5 nm); for which the ω−i and ω−o modes are widely separated. In this case, the position of the bonding (antibonding) mode of the composite structures Figure 2 Simulated extinction efficiency for MDM and DCN structures of various compositions. Simulated extinction efficiency as a function of the wavelength for (a) MDM and (b) DCN structures with a fixed thickness of 5 nm in all of its layers (for the MDM r0 = t0 = 0). Black, red, blue, and green lines correspond to the Au-Au, Ag-Au, Au-Ag, and Ag-Ag compositions, respectively. Figure 3 Simulated extinction efficiency for Au-Au and Ag-Au MDM structures. Simulated extinction efficiency as a function of the wavelength for (a) Au-Au and (b) Ag-Au MDM structures, having t2 = 5 nm, while t1 and t3 are varied simultaneously (t1 = 2, ..., 14 nm; t3 = 16 nm - t1).